Reducing Greenhouse Gas Emissions from Arable Land and Grassland: The Case for Organic Farming—A Critical Review

Abstract

The contribution of agriculture to the emission of the main greenhouse gases, CO₂, N₂O, and CH₄, is estimated to be between 25 and more than 50% of the total emissions worldwide. These data indicate that in developed, industrialized countries, severe policies might be successful in strongly reducing greenhouse gas emissions by focusing on agriculture. However, despite its central importance, agriculture is not at the center of political debate or meaningful emission-reducing policies. In this scientific review, current knowledge of the factors affecting the emission of greenhouse gases, including carbon dioxide, nitrous oxide, and methane, from agriculture is critically discussed. The pathways through which the reduction in greenhouse gas emissions from agriculture can be achieved are evaluated. For this purpose, we list the main factors contributing to the emission of greenhouse gases from agriculture and evaluate the roles of agricultural intensification, industrialization, and organic farming in greenhouse gas emissions. If the present trajectory of agricultural development continues, industrialized, intensive conventional agriculture will become an increasing source of greenhouse gas emissions worldwide. Also, the increasing quantitative relevance of energy plants in agriculture will contribute to increasing greenhouse gas emissions. Organic agriculture may offer an alternative means to reduce greenhouse gas emissions by applying the following central boundary conditions: a. the omission of mineral nitrogen fertilizers produced by the Haber–Bosch process, b. the combination of crop and livestock production, and c. the application of nutrient recycling at a regional level. This kind of organic agriculture may combine relatively high and sustainable crop yields with low emissions of greenhouse gases. Industrialized agriculture, whether in its conventional or even its industrialized organic form, is an important source of greenhouse gases with increasing emissions worldwide. Under conditions of agricultural industrialization, industrialized organic agriculture will also contribute to increasing greenhouse gas emissions. At present, there are no political attempts in the countries of the industrialized Western hemisphere to address agriculture-related contributions to greenhouse gas emissions.

1. Introduction

The global land surface of the earth is covered by 31.5% grassland and 12.6% arable land with annual and perennial crops [1]. Thus, more than 40% of the earth’s surface is agricultural managed land. The main greenhouse gases emitted through anthropogenic activity are carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) [2].

Agriculture has a strong and increasing impact on greenhouse gas emissions. For example, Paustian et al. [3] noted that a drastic change in land use over the last two centuries has contributed to about fifty percent of the increase in CO₂ emissions, with the main portion stemming from agricultural land. In general, agriculture has a strong impact on the emissions of the three greenhouse gases—carbon dioxide, nitrous oxide, and methane [1,4,5,6,7].

After the declaration of the 4 per 1000 goal within the Paris Agreement, an increasing number of scientific papers have considered the role of increased carbon storage in soils for the mitigation of CO₂ atmospheric concentrations. Among others, Paustian et al. [3,8] made some early suggestions on how organic carbon storage in soils can be increased to mitigate the increase in CO₂ concentrations in the atmosphere. After the Paris Agreement of 2016, there were also some recommendations for cover crops and organic fertilizers which may increase soil organic carbon (SOC) content (including [5,9,10]).

The problem with these more recent recommendations is that they partly lack an adequate theoretical framework, partly make wrong assumptions, and are not precise. Three examples will be briefly outlined.

First, the de novo formation of organic matter in soil through chemical, biochemical, or biological reactions, called humic substances, is often ignored or even denied [11,12]. However, humic substances are the dominant stable part of soil organic matter [13,14,15] and are an essential fraction of soil organic matter that regulates many soil processes, as recently shown by Hayes and Swift [16].

Second, the recommendations for cover crops to increase the soil organic carbon content must differentiate between different types of cover crops: stubble seed, winter cover crops, and cover crops undersown in the previous culture. These three types differ significantly in their time of growth, duration, rooting ability, and quantity of roots and residues. For example, undersown cover crops may exceed the root residue quantities that are found under cereal cash crops [17] (p. 22). A simple recommendation to grow cover crops is not adequate for soil and plant management aimed at the accumulation of SOC in soils.

Third, the effect of organic fertilization on SOC contents strongly depends on the kind of organic fertilizers, with farmyard manure or composts being superior to cattle, pig, or poultry slurry [18].

Thus, a more detailed view of the agricultural production steps with respect to their effect on the emission of greenhouse gases CO₂, N₂O, and CH₄ is recommended.

This review will focus on the agricultural factors which determine the quantities of the emissions of the aforementioned greenhouse gases. For this purpose, recent scientific publications, as well as scientific knowledge which stands the test of time, were reviewed to establish a framework of agricultural factors influencing the emission of greenhouse gases.

The focus of this paper is to evaluate the actual development of intensive and partly industrialized agriculture with respect to its effect on the emission of the three main greenhouse gases. This form of agriculture is challenged by another emerging agricultural method, organic farming, which is also considered here with respect to the emission of greenhouse gases.

The main agricultural factors listed in

Table 1. Main factors influencing the emission of carbon dioxide, nitrous oxide, and methane in agriculture.

Crop Rotation Mixtures	Inclusion of Legumes Inclusion of Semi Perennial Alfalfa/Clover/Grass Inclusion of Cover Crops
Tillage regime	Conventional tillage Reduced tillage Zero tillage
Mineral fertilizers	Inorganic nitrogen fertilizers phosphate/potassium/sulfur fertilizers Soil pH correction by lime
Organic fertilizers	Liquid manure Rotted farmyard manure composted farmyard manure Straw as amendment Biochar External organic fertilizers
Farming systems	Conventional vs. Organic Low input vs. high input agriculture Industrialized vs. non-specialized agriculture

will be considered and contextualized.

2. Features of Intensification and Industrialization in Present Day Agriculture—Consequences for the Emission of Greenhouse Gases

Characteristic agricultural development pathways in Europe, the USA, and in other parts of the industrialized countries of the northern hemisphere are the intensification and industrialization of agriculture [1]. Their characteristics are a high-input agriculture with respect to nutrients, energy, and machines [14].

For this type of agriculture, several features can be summarized:

• High yields are the main goal of intensive farming.
• High levels of nutrient applications, especially nitrogen (N), are used for intensive farming.
• Industrialization includes a high level of specialization on a farm level, also leading to the separation of plant and animal production. This results on one hand in regions without husbandry livestock and, on the other hand, in regions with large scale industrialized livestock, operations with a tendency to overfertilize surrounding soils with organic manure, mainly liquid manure. Rotted farmyard manure or compost are not produced in these operations because of the lower rationalization level involved in manure production, treatment, and application compared to that of liquid manure.
• A high level of agricultural industrialization means the substitution of human labor for machines of increasing weight that have the tendency to induce soil compaction, also increasing the risks of wind and water erosion.

What are the effects of intensification and industrialization on greenhouse gas emissions?

3. The Effect of the Intensification and Industrialization of Agriculture on Greenhouse Gas Emissions Worldwide

More than 40% of the landscape worldwide is covered by cropland or different forms of grassland [1]. Soils contain by far more carbon than any other pool on the earth’s surface [19,20]. For example, soils contain more carbon than vegetation and the atmosphere combined [21].

Small changes in the carbon content of agricultural soils may strongly affect carbon dioxide concentration of the atmosphere. Half of the increase in CO₂ emissions by human activity comes from terrestrial ecosystems, mainly from agricultural soils [3,22,23]. Most of the soil carbon is organically bound carbon, i.e., soil organic carbon (SOC). Increasing SOC stocks can strongly mitigate or even reduce atmospheric CO₂ concentrations.

The most effective way to increase the SOC contents of temperate region soils is the application of rotted farmyard manure or its composts and the inclusion of semi-perennial alfalfa-clover-grass years in crop rotations [24]. But this kind of cultivation is not compatible with input-intensive, specialized agriculture. Industrialized agriculture produces liquid manure, mainly because of rationalization reasons. But the application of pig, cattle, or poultry slurry, even in combination with the straw remaining in the field, has a relatively small effect on SOC contents compared to farmyard manure and its composts [18].

The effect of farmyard manure or its composts, as well as semi-perennial alfalfa-clover-grass, on SOC contents has been well known for decades. More than 50 years ago, Ernst Klapp, probably one of the most influential German agronomists of the 20th century, summarized the relevant results. An annual application of 8–12 t of farmyard manure increased the humus (soil organic matter) content by about 0.022% each year [25] (p. 184). He also noted that based on various findings, the humus content of arable soils in Germany is often around 2 and 5% but it can be increased to about 7–8% by including alfalfa or clover-grass in crop rotations; thus, the soil organic matter content can increase by a factor of two by using an adapted crop rotation system [25] (p. 179).

The instruments mentioned so far to increase SOC and to mitigate CO₂ atmosphere concentrations are, however, abandoned if crop production and husbandry livestock production are separated from each other.

Among agricultural soils, grassland often has a higher SOC content than cropland, which makes permanent grassland a CO₂ sink [1]. However, grassland is to be considered in a context based on its use and its ability to store organic carbon. This ability depends on the frequency of mowing or grazing, e.g., by cattle or sheep, and the application of rotted or composted farmyard manure. The type of cultivation of grassland, and not simply the existence of grassland, is also decisive for the SOC content of such soils [26].

Another largely unanswered question is whether permanent grassland or semipermanent alfalfa-clover-grass mixtures accumulate more organic carbon when both cropland and permanent grassland are considered within a rotation system or when permanent grassland is replaced by semipermanent mixtures within a rotation system on a farm level. If permanent grassland is included in rotation, it will increase the contribution of semi-permanent alfalfa-clover-grass to the rotation of a single farm, assuming a constant cattle or sheep livestock density. Under this condition, the increase in the SOC contents in all soils of a single farm may be higher than the higher SOC stocks in the permanent grassland of the same farm.

Worldwide, the emission of CO₂ equivalents from soils may be very high compared to those from fossil fuel combustion [1,27]. The trend toward the increasing liberation of greenhouse gases from agricultural land is the result of agricultural intensification and industrialization. For example, between 2000 and 2005, Europe’s terrestrial greenhouse gas balance was almost neutral, but the trend toward intensive agriculture “is likely to make Europe’s land surface a significant source of greenhouse gases” [28]. This is relevant for all three greenhouse gases considered here, i.e., CO_2, N₂O, and CH₄.

The key question of the extent to which the soil SOC content can be increased under a different management approach is addressed in more detail in later section of this paper.

A rather new development is the cultivation of energy plants, e.g., maize for the production of biogas, or oil plants for the production of fuels.

It is widely accepted that the cultivation of energy plants is a driver of greenhouse gases emissions from agriculture (for a summary, see Oertel et al. [1]). Reay et al. [4], in this line, wrote: “…expanded bioenergy programs can, in turn, increase terrestrial carbon emissions globally… Increased production of first-generation energy crops may also increase N₂O emissions as large areas of these crops are fertilized to maximum production”. In summary, intensive agriculture will be even more intensified for the production of energy plants, with increased CO₂ and N₂O emissions.

A special situation exists for peaty or peat soils with more than 15% SOC by weight. These soils developed in an environment with a high level of water restricting the decay of soil organic matter. If, for example, the ground water levels of these soils are lowered to facilitate agricultural use involving the application of lime, nitrogen fertilization, and soil tillage, then the mineralization of SOC may be strongly accelerated, leading to increased CO₂ emissions from peat soils. To avoid this, the rewetting of peat soils is often recommended. This means that the agricultural use of these soils would be severely restricted. If, however, the thickness of the peat layer is lower than 1.5–1.7 m, then ploughing up to 2.0 m will cover the peat layer with a mineral layer of 0.2 m or more, which will strongly reduce the mineralization of the peaty organic matter and will provide soils which will become highly fertile (see also [29,30]).

Cover crops have been frequently recommended as an important tool to increase SOC stocks [5,9].

Nutrient shortage with respect to mineral nutrients may decrease plant yields and also the yields of plant residues which, in turn, reduces the input of organic carbon to soils, which may consequently reduce the SOC contents. This may be relevant for nitrogen but also for plant macronutrients such as phosphorus (P), potassium (K), sulfur (S), or magnesium (Mg). Any farming system, conventional or organic, aims to produce sufficient food and may also maintain high levels of SOC if properly managed. High overfertilization of macronutrients, however, may damage the sustainability of the respective production system. Therefore, low input-agriculture is not necessarily sustainable agriculture from the viewpoint of SOC accumulation. However, a sufficient supply of mineral plant nutrients to roots may also be favored by nutrient recycling which itself is a sustainable instrument of low input agriculture.

Agriculture is responsible for two-thirds of the total anthropogenic nitrous oxide emission worldwide [4,6,31]. The main proportion of N₂O-N comes from mineral N-fertilizers and animal manure application (Wang et al. [6] and references therein). Nitrous oxide is, on a mass basis, nearly 300 times more effective as a greenhouse gas compared to carbon dioxide and is responsible for 6% of global greenhouse gas emissions [6].

The formation of N₂O depends on two soil processes, nitrification and denitrification, i.e., the formation of nitrate from ammonium and the formation of N₂ and N₂O from nitrate [6,32,33]. Both processes require soluble N species, i.e., ammonium nitrate, in the soil solution. High concentrations of soluble N species in the soil solution, i.e., a high N intensity level, favors the formation and degassing of N₂O into the atmosphere. Furthermore, a high level of water-filled pore spaces also increases the production and release of N₂O. Soil compaction, e.g., by heavy machines, may favor a higher water filled pore space at a given water content and may favor N₂O production in soils.

For mineral fertilizers, N₂O emission factors are set to between 0.5 and 1.6% and to 0.5–0.6% for organic fertilizers [6]. These data show that from 100 kg of applied mineral N, 1.6 kg N are assumed to be liberated as N₂O. However, this value may be much higher if the water filled pore space is high [1,33].

A relatively low N nutrient efficiency of mineral N fertilizer also contributes to the formation and liberation of N₂O. Compared to cattle, pig, or poultry slurry, farmyard manure shows low N₂O emission factors, i.e., below 0.4% [34].

Agriculture is the main anthropogenic source of methane [35], accounting for about 40% of global methane emissions [36]. Enteric fermentation in ruminants (cattle, sheep), manure management, and rice production are the main methane sources [35]. Soils themselves. i.e., mainly forest soils and grassland, are sinks for methane. Cropland and also grassland with high N fertilizer levels represent small sinks for methane, since soluble N reduces the ability of soils to oxidize methane [1]. Even forest soils which receive high loads of atmospheric N and acid deposition show low methane oxidation rates and consequently a reduced potential to absorb methane [37,38].

Methane production in soils requires strictly anaerobic conditions [39] which may be a result of strong soil compaction and rice cultivation on paddy soils. Wetlands exhibit relatively high methane release rates [1].

4. Technical Solutions to Reduce Emission of Greenhouse Gases in Intensive Agriculture

Measures that may reduce the release of greenhouse gases without changing the intensive and partly industrialized farming system are described here.

Nitrogen fertilizer-intensive and industrialized agriculture are a major—if not the main—origin for increasing emissions of greenhouse gases. In the last two decades, several solutions had been recommended to lower the emissions of these gases. The basic assumption is that technical problems can be solved by technical solutions. In the case of greenhouse gas emissions from agriculture, it has been suggested that several technical developments could solve these problems.

The synthesis of ammonia by the Haber-Bosch process, an essential step in the fabrication of mineral N fertilizers, is an extremely energy-consuming process. About 1–2% of the energy produced worldwide and 3–5% of natural gas is consumed by the Haber-Bosch process [40,41].

Several scientific groups have been working on ways to substitute the Haber-Bosch process for less energy consuming processes to produce ammonia (Ghavam et al. [41] and reference therein).

Since N₂O is produced during nitrification and denitrification, nitrification inhibitors may retard [34] or inhibit [33] N₂O release from soils, even at high N application rates.

Also, “precision farming” has been proposed as a solution to avoid the environmental problems associated with intensive agriculture [33]. In this context, the increase of N-fertilizer use efficiency was recommended by Reay et al. [4].

Another suggestion is to use “disruptive technologies” to increase the carbon storage of soils [5]. However, without a precise meaning and connection to the problem, the term “disruptive technologies” is overly vague.

Various other proposals to solve the problem of greenhouse gas emissions from agriculture put the focus on human food consumption by introducing plant-based diets and avoiding food wastage [4,35]. The question is whether this is a relevant approach. For example, in Germany, the N balance has been strongly positive for decades, with an approximate, average surplus approaching 100 kg N/ha over the whole German agricultural area, despite scientific recommendations for decades to reduce this N surplus value. The N surplus is ultimately eluted to groundwater, lakes, and rivers as nitrate, or is emitted as ammonia, N₂, or N₂O. This leads to higher N₂O emissions and less CH₄ decomposition in agricultural soils. However, policies to restrict meat consumption must differentiate between the way livestock is treated and fed. This will be explained in more detail in the following sections.

The conclusion from these results is that agriculture itself, in its actual forms of development, represents a serious problem which will, as it becomes increasingly intensified and industrialized, be responsible for the release even more greenhouse gases in the future. According to Wang et al. [6], the release of N₂O from agriculture increases annually by 0.25% as a result of intensive agriculture.

The question is to find a way of agriculture that can reduce greenhouse gas emissions while at the same time providing relatively high yields. It is the question of finding sustainable ways to increase agricultural yields.

5. Biochar Application: A Technique to Increase Organic Carbon in Soil?

A now widely recommended tool is the application of biochar to soils. The aim of this is the addition of organic carbon to soils which can persist in soil and can help to mitigate or to reduce the increasing CO₂ concentration in the atmosphere. Also, increased soil fertility is postulated to be the effect of the application of biochar. These assumed effects of biochar on both soil fertility and carbon storage in soil have led to efforts to implement biochar application in political policies (e.g., Krull et al. [42]). The basis for the postulated role of biochar has its origin in results of pyrogenic (black) carbon in soils which were reported for soils worldwide, i.e., by Glaser et al. [43] for soils of the Brazilian Amazonian region, by Schmidt et al. [44] and Schmidt et al. [45] for European chernozemic soils, and by Skjemstad et al. [46] for fertile, arable soils in the USA. In these soils, the cited authors found high quantities of BC as part of the SOC. Furthermore, the cited authors related the high contents of pyrogenic carbon to the fertility of these soils. This has led to the application of fabricated pyrogenic carbon-biochar to soils as a measure to improve soil fertility and to increase the content of soil organic carbon.

However, it was noted some time ago that the most commonly used methods to determine BC in soil strongly overestimate the concentration of soil BC partly by a factor of 10–100 or more (Schmidt et al. [47]; Brodowski et al. [48]). To overcome this problem, modifications of the most widely applied method were developed (e.g., Kappenberg et al. [49]). But the suggested modifications did not solve the problem of the selective determination of pyrogenic carbon in soils (Gerke, [50]). The determination of BC is based on the determination of condensed organic compounds (CONAC) (see Goranov et al. [51]) or the determination of a mixture of CONAC and other aromatic compounds (Nelson and Baldock, [52]). The central assumption behind various methods to determine BC in soil is that CONAC is exclusively produced during pyrolysis, e.g., by wildfires, and that aromatic C from lignin can be distinguished from BC, e.g., by ¹³C-NMR spectroscopy [52]. According to this view, the existence of aromatic C and CONAC in humic substances is simply ignored [52] or it is assumed that only minor quantities of humic substances exist in soil compared to BC [11].

The overview of Zimmermann and Mitra [53] raised serious doubts regarding the frequently used methods for the determination of BC in soils, indicating a probably strong overestimation of soil BC. Gerke [50] summarized analytical results, showing that BC in soil is often severely overestimated by commonly used methods. Chang et al. [54] showed that the use of the benzene polycarboxylic acid (BPCA) marker method to determine BC mostly determined humic C. And more recently, Goranov et al. [51] showed that CONAC is chemically produced in soil by Fenton-like reactions induced by reactive oxygen species or UV-irradiation combined with Fe-salts. Goranov et al. [51] calculated that, worldwide, chemical Fenton-like CONAC production is much higher than pyrogenous CONAC-production. This means that the main part of CONAC comes from humic substances and is chemically or biochemically produced, rather than being the result of wildfires. The dramatic overestimation of soil pyrogenous C has led to misinterpretations of the role and relevance of pyrogenous C for soil fertility and the persistence of SOC and organic carbon in waters [50].

These misinterpretations were the basis for recommendations regarding the application of biochar to increase soil fertility and the persistence of soil organic carbon.

The basic conclusion from the reported results is that recommendations to apply biochar/manufactured pyrogenic carbon to soils in order to yield stable and soil fertility-friendly organic carbon are based on false assumptions concerting the contents of pyrogenic C in soils.

6. Organic Agriculture

In the 1920s and 1930s, the concept of organic agriculture (also more or less misleadingly defined as “biological” or “ecological” agriculture) was introduced in Germany and Switzerland.

Developments after these beginnings led to a system of agriculture that avoids the application of mineral N fertilizers produced by the Haber-Bosch-process, omits the application of organic pesticides, and encourages combined crop production and agricultural husbandry. These characteristics of organic farming define a farming system that is completely different from conventional farming. Mineral N is substituted with N₂ fixation through the use of legumes, which play a central role in organic farming. Especially semi-perennial legumes such as alfalfa and a variety of clover species exhibit a high N₂ fixation potential, leading to different crop rotations between conventional and organic farming systems. Mixed cropping and crop rotations can avoid the selection of harmful pathogens or undesirable plant species. The combination of livestock farming and crop production is therefore an essential part of organic farming which can reduce the yield gap between organic and conventional farming by introducing mixed cropping and rotations with more varied crops [55].

Recent developments in organic farming partly counteract these initial developments. Years of semi perennial alfalfa-clover-grass production within an organic rotation system limit the cultivation of cash crops, representing a severe economic restriction within organic farming. Therefore, the rules of organic farming, as they have been dealt with by state laws, have changed in central points. Organic fertilizers such as farmyard manure or slurry can legally be imported into an organic farm, even from conventional farms. Also, concentrated feed can be imported to a certain extent, depending on the respective national rules for organic farming. The result is that N fixed by legumes within a rotation is no longer essential for organic farms, since N can be imported via organic fertilizers from conventional farming or by concentrate feed. Then, semi-perennial or grain legumes are no longer the basis for organic rotations. Instead of the 25–33%, as recommended for organic rotations, alfalfa-clover-grass mixture often accounts for less than 20%, or even less than 5–10% of the rotation of farms without livestock. This has a strong impact on the SOC contents, CO₂ and N₂O emissions, and finally, on net CH₄ emissions from organically managed farms. Also, by allowing the import of concentrated feed into organic farms, significant quantities of N will be imported, the most prominent variant of which is protein sources, mainly consisting of soja bean compounds. These developments result in the global transport of farm products, formerly a characteristic of conventional industrialized farms.

Under these conditions that are supported by legislation in several western states and the European Union, organic farming is transformed into a variant of industrialized and intensified agriculture. Only restrictions on organic pesticides for organic farms remain as a difference between organic and conventional farms.

7. Back to the Roots: Organic Farming as a Complex System That Is Able to Combine High Sustainability and the Reduction of Greenhouse Gas Emissions from Agriculture

The initial rules of organic farming were as follows: no significant external N inputs, with the exception of N₂ fixation by legumes, no organic pesticides and, as a consequence, the connection of livestock farming and the cultivation of arable soils and grassland. Agriculture in temperate regions based on these principles can show a different pattern of greenhouse gas emissions compared to intensive, conventional agriculture.

Table 2. An example of a crop rotation of a well-managed organic farm in a temperate climate zone and the application of mineral and organic fertilizers.

Year	Crop	Harvest Products	Fertilizers
First	alfalfa/clover/grass	hay, silage, fresh forage	e.g., dolomite, raw P and K sources, gypsum
Second	alfalfa/clover/grass	hay, silage, fresh forage	see above
Third	winter wheat with undersown clover/grass	cereal grain
Fourth	oats with undersown clover/grass	cereal grain	e.g., 10 t/ha farmyard manure, compost
Fifth	potatoes	tuber	e.g., 30 t/ha rotted farmyard manure
Sixth	barley with undersown alfalfa/clover/grass	cereal grain	e.g., dolomite, 10 t/ha rotted farmyard manure

shows an example of a six-year crop rotation of an organic farm with cattle or sheep livestock using alfalfa-grass-clover as forage, including the production and application of farmyard manure, its rotted form, or compost. The cash crops in this example are wheat, barley, oats, and potato tubers. The demand of all macronutrients can be met by the application of fertilizers as recommended in Table 2 (the problem of limited resources of rock phosphate is not further considered here). The only exception is the N that is introduced by legumes or by recycling and the application of farmyard manure. The key question with respect to high crop yields is whether the extent of N₂ fixation by the legumes in the rotation satisfies the demand of the four non-legume species in this rotation. Anglade et al. [56] made similar calculations for a nine-year organic rotation with alfalfa accounting for the initial three years. This group found a slight net N surplus of about 27 kg N/ha over nine years of rotation. This indicated that the N demand for non-legume crops was met. The rotation described in Table 2 is similar to that of Anglade et al. [56], but it can be expected that two instead of three subsequent years of alfalfa may represent an advantage with respect to N₂ fixation, since residual alfalfa (similar to clover) N can retard N₂ fixation in subsequent years. This has been shown for perennial grass-legume mixtures, where the N₂ fixation by legumes can be lowered to less than 100 kg N/ha_*year [57], which is fairly low compared to the more than 450 kg N/ha_*year that was reported for semi perennial alfalfa by Anglade et al. [56]. The N₂-fixation by alfalfa was more than 200 kg N/ha_*year higher than that by red clover [57]. The authors explained this difference by the fact that red clover was not harvested but was treated as green manure, with the partly mineralized residues leading to high N_min levels which restricted further fixation of N [56].

Considering an organic crop rotation approach that is, despite legal regulations, still widely present in today’s organic agriculture (Table 2), we will discuss the extent of release of the three main greenhouse gases, CO₂, N₂O, and CH₄.

• Carbon dioxide

Gattinger et al. [58] showed that organically managed, arable soils bind more carbon than conventionally managed soils. They attributed their results to years with semi-perennial alfalfa-clover-grass mixtures within the rotation. Indeed, Klapp [25] (p. 179) noted that this semi-perennial cultivation system has a strong impact on increasing soil organic carbon. The extent of SOC accumulation depends on biomass, i.e., the rooting intensity and rooting depth of the plant species present in these mixtures. Notably, the ability of alfalfa and medic clover to produce relatively high root masses at depths below one meter is well known [17] (p. 22). Deep rooting may strongly increase the persistence of SOC in lower soil horizons.

In several long-term field trials, organic farming has been compared to conventional farming. Soil parameters such as SOC have been measured, even decades after the beginning of the field trials. However, the distinction among different factors which can affect SOC contents in these trials is not possible by simply comparing conventional vs. organic agriculture.

An exception to this conception was a field trial initiated by Prof. Dr. Cord Bäumer at the University of Göttingen, Germany, conducted from 1981 on. In this trial conventional and organic farming were compared in a different way. The main three factors that separate conventional and organic farming systems were varied independently, i.e., the application of mineral nitrogen and its quantity, the rotation with or without semipermanent legumes, in this case alfalfa, and the application of organic pesticides, with or without. Forstreuter [59] described the results regarding the SOC contents in this field trial and found that in the highly fertile soil used for the trial, the inclusion of alfalfa in the crop rotation led to about 10 t/ha more of SOC at a depth of 0–20 cm after 15 years compared to rotation without alfalfa. This net SOC gain through the cultivation of alfalfa can be much higher if the full rooting depth of the soil, e.g., up to depths of about one meter, is included in the measurements. Such a SOC accumulation is the result of a net binding of CO₂ by the soil through the cultivation of alfalfa.

This outstanding effect of alfalfa on SOC content was confirmed by Song et al. [60] under very different climate conditions; those authors found that alfalfa increased the SOC content by about 10 t/ha within 17 years in a semiarid environment.

Organic fertilizers also have a strong impact on SOC. Körschens et al. [18] demonstrated, by evaluating long-term, worldwide fertilization trials, that the application of cattle, pig, or poultry slurry did not increase SOC, whereas the long-term application of rotted or composted farmyard manure increased the SOC content in soils. Klapp [25] (p. 184) noted that the annual application of 8–12 t farmyard manure increased the content of soil organic matter (humus) by about 0.022% per year. Considering the upper 30 cm of the soil and assuming a soil density of about 1.5 [kg/dm³], this quantity of farmyard manure application led to an accumulation of approximately 1 [t] of organic matter per hectare per year.

Organic agriculture and its initial regulations recommended the combination of livestock and crop production, including the use of straw for animal welfare. The result of this is the production of farmyard manure, which, in its rotted or composted forms, strongly increases the SOC content. The value of farmyard manure by composting, as compared to rotting with or without the application of external compounds such as clay minerals, has remained a matter of debate within the organic farmers community.

Composting or rotting can form stable organic matter, partly consisting of humic substances which give the dark brown color to composts and rotted farmyard manure [15]. The importance of humic substances for the fertility and stability of composts has often been shown. There is a close relationship between compost ripening and humification, i.e., the formation of humic substances (see [61,62,63,64] among others). But the accumulation of SOC by the application of stable compounds (humic substances) via composts or rotted farmyard manure is not the only way to increase the SOC contents and, consequently, decrease the net CO₂ release from soils. Humic substances have the ability to bind, to incorporate, and to stabilize other “non-humic” organic molecules. This type of chemical protection against microbial degradation is probably of key importance for SOC conservation [15]. Among others, Paustian et al. [3] presented a model for the physical protection of SOC within soil aggregates. However, easily degradable organic molecules such as carbohydrates, amino acids, or peptides can also be protected chemically by binding or incorporation into humic substances [65,66,67].

These results are in direct contrast to the hypothesis of a maximum C binding capacity of soils [11] that was cited by the FAO [12].

For the stability of SOC in soils, adsorption sites on inorganic surfaces may be important [68,69]. Also, the apparent molecular weight, molecule size, and the proportion of polycyclic aromatic carbon, as well as the relationship between hydrophilic and hydrophobic sites in humic substances and its association with non-humic substances may be of strong relevance for the stability of SOC and its content in soils [69]. The chemical quality of soil humic substances is therefore decisive for the stability of SOC. In line with this concept, the group of Italian humic chemist A. Piccolo found that the initiation of polymerization of soil humic substances by adding oxidizing agents can further stabilize these organic compounds in soil and can contribute to a significant annual SOC accumulation in soils, i.e., by about 2.24–3.90 t SOC/ha [70]. High molecular weight humic substances are much more resistant to microbial decomposition than humic substances of lower molecular weight, which explains the results of this group. The results of Piccolo and co-workers show a way to achieve rather high SOC accumulation rates in soil. However, these results play no role in the recommendations of international organizations such as the FAO, despite the fact that the 4 per 1000 goal of the Paris agreement assumes increasing C binding rates by soils with increasing SOC contents. Under the assumption and only under the assumption that humic substances account for a major part of SOC, the 4 per 1000 goal has a scientific basis [24,69].

At present, it is astonishing that soil organic matter stability is mostly related to physical protection by soil aggregates and by adsorption to mineral surfaces. The chemical mechanisms by which SOC is stabilized in humic substances are widely ignored [69], even though they are at the core of SOC transformations in soils [16]. There is no doubt that agriculture that incorporates the production and application of rotted or composted farmyard manure and the cultivation of semi perennial alfalfa-clover-grass mixtures can increase the SOC and humic substances contents and consequently mitigate the emission of CO₂ worldwide to a significant extent. Ghabbour et al. [14] compared great numbers of conventional versus organic farms in the USA and found both higher SOC contents and higher humic substances contents in the soils of organic farms compared to those of conventional farms, with humic substances accounting for the majority of the SOC.

The inclusion of cover crops in rotations may be relevant in organic as well as conventional farming systems. Cover crops can supply organic matter to soil, some of which can be used to build stable organic matter in soils within both farming systems. However, what is different between organic and conventional farming is that undersown cover crops and winter cover crops can play a more prominent role in organic farming than in conventional farming. Because of the lower nitrogen availability in organic farming, the survival of undersown crops is favored. The much longer growth of undersown and winter cover crops compared to stubble cover crops produces higher residual biomass in the latter (Könnecke, [17]). Short-term growing stubble cover crops can mostly be mineralized afterwards, whereas the higher residual masses of undersown and winter cover crops can contribute to the establishment of more stable SOC.

• Nitrous oxide

Worldwide, agriculture is considered to be responsible for between 60 and 67% of total anthropogenic N₂O emissions [4,6]. Organic agriculture can strongly decrease N₂O emissions in several ways. In organic farming, N is introduced to farms by symbiotic N₂ fixation by legumes used for forage or grain production. This is a profound advantage considering the sustainability of farming systems and the reduction of N₂O emissions.

Nitrification and denitrification are N-transforming processes in soils where N₂O release occurs [33]. The transfer of legume N coming from N₂ fixation in soils and forage on organically managed farms avoids both nitrification and denitrification during this transfer [71]. A part of the N fixed by legumes will be included in fodder for cattle, pigs, and poultry and will finally be a part of the organic fertilizers of the farm. The N₂O emission factors of organic fertilizers are lower than those of mineral N fertilizers [6]. The IPCC set the emission factor for mineral N-fertilizers to 1.6% and that of organic fertilizers to 0.6% [6]. These rough estimates further depend on the organic N source and on the level of N fertilization via both organic and inorganic fertilizers. Wang et al. [6] showed a high variation of N₂O emission factors for both organic and inorganic sources. The N₂O emission factor for organic sources depends on the form of nitrogen. Thorman and Nicholson [34] reported an emission factor of 0.37 for farmyard manure, whereas the emission factor for slurry application was 0.72 under the same conditions of application. The ratio of N₂O:N₂ formation depends on the level of nitrate in the soil, since high nitrate concentrations inhibit the reduction of N₂O to N₂, thereby increasing the release of N₂O from soils at high N fertilization levels, both relatively and absolutely [6,72,73,74].

Since organic fertilizer forms often have lower contents of available N compared to mineral N, and release N at smaller rates, nitrification and denitrification processes may occur at a lower level compared to inorganic N fertilizers.

Finally, adapted organic farming avoids soil compaction e.g., by avoiding the use of heavy machines and using crop rotations with semi-perennials that can, via intensive, permanent, and deep rooting, improve soil structure and soil aeration. Both factors avoid reduced zones within the soil core where increased N₂O production and release into the atmosphere occur [71,75]. Anglade et al. [56] showed a successful rotation of organic farming in the Paris basin, France, in a nine year rotation with the first three years consisting of alfalfa. Alfalfa introduced high nitrogen quantities into the rotation. Small N losses over the whole rotation, as found by Anglade et al. [56], are an important factor for the successful cultivation of non-legumes under these conditions. Anglade et al. [56] calculated a slightly positive N balance over nine years of rotation, indicating that the moderate to high N demand of non-legumes was met within the rotation. Successful organic farming avoids high N losses and reduces N₂O liberation from soils compared to intensive farming. It can be concluded that high yielding organic farming and low-N₂O-release farming belong together. The same argument may also apply to nitrate and ammonia losses from agriculture. Small N losses and high N₂-fixation rates by legumes within an organic rotation are a precondition for successful organic farming.

Skinner et al. [76] found in a long-term field trial that organic farming released on average about 40% less N₂O/ha compared to conventional farming. If the N₂O emitted per harvested unit was calculated, there was only a slight advantage in terms of N₂O emissions of organic farming vs. conventional farming, pointing to the yield gap between organic and conventional farming as an important parameter for N₂O emissions from agriculture.

A different picture is present for organic farming in its industrialized, intensified form, which is characterized by high organic fertilizer input and the separation of crop production and livestock production. Under the conditions of industrialized organic agriculture, the release of N₂O can reach or even exceed the level of industrialized conventional agriculture.

• Methane

Agriculture is the main source for anthropogenic methane emissions, accounting for up to 40% of the methane emissions globally [36]. The principial sources are ruminants (cattle, sheep) with enteric fermentation and rice cultivation [35,36]. This result may lead to the conclusion that reducing meat consumption and cattle/sheep production may help to mitigate methane emissions and greenhouse gases emissions in general [4,35]. But the full picture of greenhouse gas emissions has to be considered.

Agriculture is not only a CH₄ source but also a sink, since the main CH₄ sink is soil [33]. High N fertilization reduces the oxidation of methane in cropland, which is the most important way to degrade methane [35]. Additionally, grassland and forest land are important methane sinks [1]. The input of ammonia into forest soils, mainly from overfertilized agricultural sites, can strongly reduce this methane sink [1], extending the effect of intensive agriculture on greenhouse gas emission.

At present, the conclusion for the emission of methane from agriculture could be that cattle and possibly sheep production increases net methane emissions from agriculture independently of the management system. However, a more comprehensive view leads to a different conclusion. With this in mind, no other single instrument is as effective as semi perennial legume-grass mixtures in terms of increasing SOC contents and consequently decreasing CO₂ emissions from soils. Also, high quantities of N₂ fixed by legumes, as demonstrated by Stagnari et al. [71] and Anglade et al. [56], help to decrease N₂O emissions from soils. This can also be the case if permanent grassland partly consists of perennial legumes, which may act as a substitute for mineral N fertilizers. Reinsch et al. [57] showed that in permanent grassland, the proportion of legumes decreases over time, but the N fixation by legumes of permanent pastures was as high as about 90 kg N/ha_*year, even with high N application in the form of sludge. Semi-perennial and perennial grassland can easily and exclusively be used by ruminants for the production of high-value proteins and can strongly contribute to human protein nutrition worldwide. Probably, for organically managed perennial and semi-perennial legume/grass mixtures such as fodder for ruminants, the benefits in terms of reducing CO₂ and N₂O emissions, as well as the activation of the methane sink effect of agricultural soils and probably forest soils, can strongly overcompensate for an increased methane emissions by ruminants on organic farms. Considering the broader picture, cattle or sheep on organic farms can therefore strongly help to mitigate greenhouse gas emissions. Another picture may arise if cattle is concentrated in industrialized farms receiving high quantities of concentrated feed, even in organic farming settings.

Unspecific recommendations to reduce meat consumption as an instrument to reduce greenhouse gas emissions are therefore not justified. Industrialized meat production can strongly increase CH₄ and N₂O emissions, but cattle production on non-specialized organic farms can strongly reduce CO₂ and N₂O emissions. Harvesting and feeding ruminants with legume-based fodder will increase N₂ fixation and will contribute to a lower energy demand in agriculture.

Thus, recommendations concerning the emission of greenhouse gases from agriculture have to consider the whole agricultural system in order to avoid reaching misleading conclusions which may be misused by political actors.

Quantitative estimates

To obtain quantitative information on the relevance of reducing greenhouse gas emissions from agriculture, the following approximate calculations may help.

According to Klap [15] (p. 184), annual farmyard manure application of 8–12 t/ha increases organic matter content by an average of 0.022% annually in soil. Considering a 30 cm upper horizon and a soil density of 1.5, one t of organic matter is added annually. With a factor of organic C to organic matter of 1.7, this results in an annual addition of about 0.6 t SOC/ha. Forstreuter [60] found that through the inclusion of 16.7% of alfalfa into a rotation, an annual increase of about 0.7 t SOC/ha is possible. Piccolo et al. [71] found an annual increase of SOC of between 2.2 and 3.9 t C/ha. Using agronomic measures such as farmyard manure, semipermanent alfalfa-clover-grass within a rotation, or the addition of catalysts which improve the polymerization of soil humic substances can result in an increase of the SOC stocks of about 0.6–3.9 t SOC per ha per year. Taking an average increase of 1.5 t C per ha per year for arable land and 1.5 billion ha of arable land (destatis.de) worldwide, an additional fixation of C in soil organic matter would account for about 2 billion t C/year. We may further assume that a similar annual accumulation of organic C in permanent grassland worldwide would lead to an additional C binding in agricultural soils of more than 5–6 billion t C/year. The liberation of carbon dioxide into the atmosphere is estimated to 10 billion t CO₂-C/year worldwide (destatis.de). Thus, C fixation by agricultural soils could potentially reach about 60% of the C liberation of carbon dioxide. This quantity will strongly mitigate greenhouse gas concentration increases by changing agricultural instruments and using measures which are inherent to agriculture.

The potential to mitigate increasing concentrations of nitrous oxide and methane are also promising, since 60% of nitrous oxide and 40% of the methane emissions come from agriculture. However, for these two gases, quantitative calculations are not reliable at present.

8. Summary and Conclusions

Agriculture is, worldwide, a strong contributor to the emission of greenhouse gases CO₂, N₂O, and CH₄. Worldwide, half of the increase of atmospheric CO₂ has been attributed to soils.

Intensive, industrialized agriculture is the cause for increasing greenhouse gas emissions from agriculture.

Energy plants cultivated to create “regenerative energy” are the drivers of increasing greenhouse gas emissions from agriculture.

Farmyard manure in its rotted or composted form can effectively increase soil organic carbon stocks. Liquid manure and straw application have comparatively little impact on soil organic carbon stocks.

Zero tillage or reduced tillage leads to higher SOC stocks in the upper 10–15 cm of the soil profile compared to conventional tillage. However, the carbon stocks are often higher under conventional tillage due to the higher SOC concentrations in deeper soil horizons, which may overcompensate for the lower SOC stocks in the upper soil.

The role of biochar in raising soil SOC stocks is unclear, and even the analytical determination of pyrogenous carbon in soils remains to be undertaken.

With respect to greenhouse gas emissions, organic agriculture may be an alternative to industrialized agriculture. Organic farming introduces nitrogen to farms via N₂ fixation by including legumes in crop rotations, making them a significant part of grassland yield.

The cultivation of semi perennial alfalfa-clover-grass mixtures and the production of farmyard manure and its composts in organic farming systems can increase the SOC content, decrease N₂O emissions, and improve CH₄ oxidation in arable and grassland soils.

If imported organic N fertilizers and concentrated feed account for an important part of the N that is imported to organic farms, this, combined with the specialization and industrialization of organic agriculture, will strongly increase the emission of greenhouse gases.