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Review

Soil Health Intensification through Strengthening Soil Structure Improves Soil Carbon Sequestration

by
Ryusuke Hatano
1,*,
Ikabongo Mukumbuta
2 and
Mariko Shimizu
3
1
Soil Science Laboratory, Research Faculty of Agriculture, Hokkaido University, Kita 9 Nishi 9, Kita-ku, Sapporo 060-8589, Japan
2
Golden Valley Agricultural Research Trust, Lusaka P.O. Box RW50834, Zambia
3
Civil Engineering Research Institute for Cold Region, Hiragishi 1-3-1-34, Toyohira-ku, Sapporo 062-8602, Japan
*
Author to whom correspondence should be addressed.
Agriculture 2024, 14(8), 1290; https://doi.org/10.3390/agriculture14081290
Submission received: 17 June 2024 / Revised: 1 August 2024 / Accepted: 2 August 2024 / Published: 5 August 2024
(This article belongs to the Special Issue Feature Review in Agricultural Soils—Intensification of Soil Health)
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Abstract

:
Intensifying soil health means managing soils to enable sustainable crop production and improved environmental impact. This paper discusses soil health intensification by reviewing studies on the relationship between soil structure, soil organic matter (SOM), and ecosystem carbon budget. SOM is strongly involved in the development of soil structure, nutrient and water supply power, and acid buffering power, and is the most fundamental parameter for testing soil health. At the same time, SOM can be both a source and a sink for atmospheric carbon. A comparison of the ratio of soil organic carbon to clay content (SOC/Clay) is used as an indicator of soil structure status for soil health, and it has shown significantly lower values in cropland than in grassland and forest soils. This clearly shows that depletion of SOM leads to degradation of soil structure status. On the other hand, improving soil structure can lead to increasing soil carbon sequestration. Promoting soil carbon sequestration means making the net ecosystem carbon balance (NECB) positive. Furthermore, to mitigate climate change, it is necessary to aim for carbon sequestration that can improve the net greenhouse gas balance (NGB) by serving as a sink for greenhouse gases (GHG). The results of a manure application test in four managed grasslands on Andosols in Japan showed that it was necessary to apply more than 2.5 tC ha−1 y−1 of manure to avoid reduction and loss of SOC in the field. Furthermore, in order to offset the increase in GHG emissions due to N2O emissions from increased manure nitrogen input, it was necessary to apply more than 3.5 tC ha−1y−1 of manure. To intensify soil health, it is increasingly important to consider soil management with organic fertilizers that reduce chemical fertilizers without reducing yields.

1. Introduction

Since the Industrial Revolution, between 1750 and 2011, 555 GtC of CO2 has been emitted [1]. This includes 375 GtC from fossil fuel combustion and 180 GtC from land use change. Meanwhile, global warming is steadily progressing, with the temperature rise above pre-industrial revolution levels expected to reach 1.5 °C by 2040, making heat waves more likely to occur, and increasing the frequency of droughts and heavy rains that can affect agriculture and natural ecosystems [2]. Due to conventional cultivation, cropland around the world has significantly lower soil carbon content than natural ecosystems [3]. To mitigate this, efforts are underway to add carbon to agricultural soils and return them to the level of natural ecosystems [4]. This has progressed with the proposal of the concept of soil health [5].
The United Nations’ Food and Agricultural Organization (FAO) defines soil health as “the ability of the soil to sustain the productivity, diversity, and environmental services of terrestrial ecosystems” [6]. For the assessment of soil health, efforts have been made to identify effective and measurable biological, chemical, and physical indicators that can be controlled through management [7]. Among them, soil organic matter (SOM) is the most widely used [8]. According to Beillouin et al. [9], 15,857 studies on soil organic carbon (SOC) have been reported in more than 150 countries. Of these, 6550 were related to SOC content in land use, land use change, and climate change. The remaining 9307 were studies on the relationship between SOC and soil chemistry, plant productivity, soil biology, greenhouse gases (GHGs), soil physical properties, and water quality. 50% of them were studies on fertility management. By country, the United States and China had the most, followed by Brazil and Canada, Australia, India, and some European countries (UK, Germany, Spain, and Italy), but some regions, such as Africa, have very scarce data available.
In a study on SOC stock, Georgio et al. [3] used data from a total of 1144 soil profiles from 78 papers published since 1960 [3]. They included SOC measurements from managed land (n = 559 profiles) and natural (n = 585 profiles) in all soil types (Alfisols, Andisols, Aridisols, Entisols, Gelisols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols), except Histsols, across a range of mineralogy and vegetation types, with mean temperatures ranging from −2.9 to 29 °C and annual precipitation ranging from 79 to 3806 mm y−1. These data also included data from the Land Use and Coverage Area Frame Survey (LUCAS) [10], which was conducted uniformly by 28 European Union (EU) Member States. LUCAS analyzed the physicochemical properties of the soil at 0–20 cm of the topsoil at a total of 22,000 sites with various land uses (cropland, grassland, forest) in EU countries three times in 2009, 2015, and 2018. The data are freely available from the EUROSTAT website. Mäkipää et al. [11] reported on soil structural degradation using the 2009 LUCAS data.
In an ecosystem, there is an input of organic matter through the net primary production of plants, and an output of organic matter through decomposition, leaching, erosion, and harvesting of organic matter. A positive difference between the input and output causes the accumulation of organic matter in the ecosystem. The accumulation of organic matter in ecosystems is both an effect of removing carbon from the atmosphere and a process of carbon sequestration, which captures carbon in plants and soil. In particular, soil carbon sequestration is enhanced by increasing soil carbon content and soil depth distribution [12].
In ecosystems, capture of carbon from the atmosphere occurs through photosynthesis by plants [13,14]. 80% of the carbon fixed in the ecosystem by plants through photosynthesis is released into the atmosphere through the plants’ own autotrophic respiration and the heterotrophic respiration of soil microorganisms, and the rest is retained in plant tissues [15]. In agricultural fields, plant carbon is added to the soil as residue. At the same time, organic carbon can also be added by applying organic fertilizers such as animal manure. Soil carbon loss occurs through heterotrophic respiration of soil microorganisms, which is the microbial decomposition of previously formed carbon contained in SOM [16].
When fresh organic matter is added to soil, it is decomposed by soil microorganisms and becomes microbial biomass, which combines with soil minerals to form organo-mineral complexes and aggregates to stabilize the soil structure [17,18]. Building soil structure increases field water capacity [19]. In the United States, a pedotransfer function has been proposed that regresses available water by soil texture and SOC [20]. Mineral-associated organic carbon (MOC) is difficult for microorganisms to decompose [21], and MOC is essential for improving carbon sequestration in soils. On the other hand, the decomposition process of organic matter produces GHGs such as CO2, CH4, and N2O [22]. If the emissions of these gases into the atmosphere exceed the consumption through plant CO2 uptake, microbial CH4 uptake, and microbial N2O reduction in ecosystems, global warming will be exacerbated. In agroecosystems, harvesting removes organic matter from the field, so there is no chance of it being retained as SOM. Harvesting therefore contributes to global warming. On the other hand, inputting organic fertilizers such as manure or slurry can increase soil organic matter, thereby mitigating global warming.
At the same time, plants need nutrients, especially nitrogen, to fix carbon. In agricultural ecosystems, crops are obtained by inputting nitrogen as fertilizer. Organic agriculture is considered a strategy to increase SOC in agricultural lands. However, a biogeochemical model estimates that organic agriculture without cover crops or manure (no nutrients at all) would reduce carbon inputs to soils worldwide by 40% and reduce SOC by 9% [23]. Cover crops increase SOC and crop yields, and a meta-analysis showed that the greatest increases in yield and SOC were achieved with legume cover crops and nitrogen applications [24]. A long-term experiment on the effects of combined crop rotation and fertilization in Prague-Ruzyně since 1955 showed that SOC was maintained with more rotations and manure applications [25]. Furthermore, a meta-analysis of 13,662 cases in China revealed that increased SOC significantly increased yields of all three crops, maize, wheat, and rice, up to a certain SOC value for each crop. However, the effect of increasing crop yield was only one-fifth of that of nitrogen fertilization [26]. However, excessive fertilizer input increases the outflow of nutrients into the hydrosphere, leading to eutrophication, and increases ammonia volatilization and N2O emissions to the atmosphere [27]. It has been pointed out that the nitrogen cycle has already exceeded the planetary boundaries that define the “safe area for human activity” [28]. It is desirable to reduce the amount of chemical fertilizer applied to agricultural soils. To address this issue, management techniques are being developed to reduce the amount of chemical fertilizers used by taking into account the amount of nutrients such as nitrogen that are mineralized as organic matter added through cover crops and compost decomposes [29,30]. Especially, soil N2O is emitted during the processes of nitrification and denitrification, and since denitrification is a reaction that accompanies the decomposition of organic matter, it is possible that organic matter application increases N2O emissions, and so proper management of nitrogen fertilizer is important.
SOM is central to soil function. Accumulation of SOM improves water retention, permeability, and aeration by building soil structure, and enhances acid buffering, carbon sequestration, and plant nutrient storage and supply. It can be said that it is the most important factor of soil health [8]. SOM is the product of the net ecosystem carbon budget, the difference between carbon inputs (photosynthesis, organic inputs such as manure, and deposition) and carbon outputs (respiration by plants and microorganisms, harvesting, and erosion). Therefore, maintenance of SOM in agricultural land strongly depends on ecosystem carbon and nutrient management practices. It is recommended that to improve soil health, the amount of chemical fertilizers applied is reduced by using nutrients provided by organic matter application.
Supplying organic matter to soil using conservation farming methods promotes the formation of MOC, improves soil structure, achieves soil carbon sequestration, improves soil nutrient and water supply, and directly reduces environmental impact (Figure 1).
Johannes et al. [31] clarified that the SOC/Clay ratio can be an indicator of soil structure status, and soil with an SOC/Clay ratio of 1/13 or less is considered to have a degraded soil structure. Based on this indicator, Mäkipää et al. [11] showed that agricultural soils in the EU were more degraded than grasslands and forests. Soils that have been degraded due to carbon loss need to be restored by applying conservation farming practices. However, the SOC/Clay ratio is an indicator of soil degradation, but does not specifically estimate the increase or decrease in SOC. Therefore, to know the effect of conservation farming practices on the improvement of soil health, carbon balance and GHG balance need to be measured independently.
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Figure 1. Effects of soil organic matter improvement managements [8] on soil carbon sequestration [4], nutrient and water supply improvement [32,33] and environmental impact reduction [34,35,36].
Figure 1. Effects of soil organic matter improvement managements [8] on soil carbon sequestration [4], nutrient and water supply improvement [32,33] and environmental impact reduction [34,35,36].
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In this paper, we first discuss recent findings on soil carbon pools and soil carbon accumulation, and the concept of the SOC/Clay ratio as an indicator of soil structure degradation based on the mechanism of soil organic matter accumulation. We then present actual examples of measuring carbon and GHG balances through fertility management with organic matter application and use them to evaluate the soil structure degradation indicator using the SOC/Clay ratio. We also discuss how to mitigate the impact of fertility management with organic matter application on N2O emissions. From the above, we clarify that intensification of soil health through strengthening soil structure by organic matter application improves soil carbon sequestration and GHG balance.

2. How Much Organic Matter Can Soil Store?

Ecosystems emit 8.9 GtC y−1 annually (fossil fuel combustion 7.8 GtC y−1, land use change 1.1 GtC y−1), and each year 4 GtC y−1 is added to the atmosphere as CO2 [1].
There are two types of SOC, mineral-associated organic carbon (MOC) and particulate organic carbon (POC). POC is non-mineral-associated organic carbon such as peat. MOC is known to be stable and difficult for microorganisms to degrade [21]. Wang and Kuzyakov [37] summarize the mechanism of MOC formation as follows: (i) ligand exchange, (ii) electrostatic attraction, (iii) hydrophobic partition, (iv) cation bridging, (v) coprecipitation, and (vi) physical trapping by microaggregates. Globally, MOC accounts for 34–51% of SOC, but in mineral soils it accounts for ~65% [38]. Georgiou et al. [3] found that soils containing mainly low-activity minerals, such as kaolinite, can adsorb up to 48 gC/kg clay + silt, and soils containing mainly high-activity minerals such as smectite can adsorb up to 86 gC/kg clay + silt. Using these values for the world’s soils to determine the MOC content, they showed that most of the world’s soils are unsaturated in MOC. According to the report, the SOC at a depth of 1 m in mineral soil, excluding tundra due to insufficient sample sizes, is 1401 GtC, of which the MOC is 899 GtC, and 50% of this exists within the top 30 cm surface layer (Table 1). The maximum amount of MOC (MOCmax), which is calculated from the amount and type of soil minerals, is estimated to be 4596 GtC, and clay minerals can still contribute 3680 GtC. However, if the amount of carbon input is set at the level of the current natural ecosystem, then the SOC content can only accumulate carbon up to the level of the current natural ecosystem, so the MOC that agricultural soils can accumulate is 433 GtC. However, this amount is equivalent to 78% of the ecosystem carbon loss of 555 GtC (375 GtC from fossil fuel combustion and 180 GtC from land use change) since the Industrial Revolution [1]. Furthermore, the deeper soil layers have a large potential for MOC, and it is possible to increase MOC by adding organic matter to the lower layers by deep tillage or cultivating crop varieties with deep root systems. No-tillage increases surface carbon but has been shown to reduce carbon in the subsoil [39]. On the other hand, using deep-rooted oats as a cover crop reduces the hardness of the subsoil, and the roots of the succeeding cash crop also become deeper [40]. The results of 103 carbon addition experiments around the world show that the carbon sequestration rate increases as the degree of MOC saturation (the ratio of MOC to potential) decreases; the rate in soils with only 10% SOC saturation is three times higher than soils with 50% [3].

3. Assessment of Soil Structure Based on Soil Organic Matter Accumulation Status

Mäkipää et al. [11] showed that there was no correlation between SOC and clay, silt, or sand content in the top 0–20 cm data of 21,859 European soils (LUCAS soil data, 2009 [10]). In cropland, grassland, and forest, the median SOC content was 14.2, 15.8, and 18.2 g kg−1, respectively, while the median clay content was 210, 150, and 70 g kg−1, respectively. Thus, grassland and forest contain more SOC than cropland, despite having less clay content.
Asano and Wagai [17] investigated the relationship between soil particle size and soil carbon content using allophanic Andosol in Japan. They found that more than 95% of the total C was present as >53 μm aggregates with wet sieving of the aggregates alone, and even after mechanical shaking to break up the water-resistant aggregates, the >53 μm and 2~53 μm fraction contained 37% and 41% of the total C, respectively. Furthermore, when these were completely dispersed by using ultrasound sonication energy, 63% of the total C was present in micro organo-mineral complexes of <2 μm. The C/N ratio of the organic matter was low at 6–10, and microbial decomposition products enriched in stable C and N isotopes (13C and 15N) were predominant. From these findings, it is thought that plant debris supplied to the soil, after being decomposed by microorganisms, preferentially binds to organo-mineral complexes and becomes occluded in aggregates, forming strong aggregates [18].
Beare et al. [42] compared the effects of conventional tillage (CT) and no-tillage (NT) on SOC content and aggregate stability, and found that the SOC content in top 5 cm soil was 18% higher in NT than in CT, and that aggregates larger than 250 μm were significantly more abundant in NT than in CT. In other words, it is thought that tillage increased decomposition of SOC, reduced the production of organo-mineral complexes, and inhibited the growth of aggregates.
Since organo-mineral complex formation is the mechanism of aggregate formation, clay content is an important soil quality. Dexter et al. [43] suggested that clay retains maximum SOC when the SOC/Clay ratio is 1/10. Following this idea, Johannes et al. [31] investigated the relationship between the soil structure condition and the SOC/Clay ratio of 161 Swiss agricultural soils. The soil structure condition was measured using the visual method of Ball et al. [44], and classified the condition as “very good”, “good”, “adequate” and “bad”, using the SOC/Clay ratio of 1/8, 1/10, and 1/13 as thresholds (Table 2). When determining the soil structure condition using the SOC/Clay ratio, grasslands tended to have a good structure of 1/8 or higher. On the other hand, in upland fields, fields with conventional tillage (CT) tended to have a bad level of less than 1/13, while fields with no-tillage (NT) showed a wide variation, averaging around an adequate level of 1/10.
Based on these results, it is reasonable to aim for a SOC/Clay ratio of 1/10 as a rational target for organic carbon for soil management. Using this index, Prout et al. [45] evaluated the soil structure condition from the SOC/Clay ratio in the surface layer 0–15 cm of England and Wales, covering 3809 sites under arable land, grassland and woodland, and found that 38.2, 6.6 and 5.6% of arable, grassland and woodland sites, respectively, were degraded.
Furthermore, Mäkipää et al. [11] evaluated the soil structure condition using the SOC/Clay ratio of 2009 LUCAS soil survey data. As a result, 51.0%, 15.7%, and 4.2% of cropland, grassland, and forest, which account for 44.6%, 21.8%, and 28.6% of the total land cover, have been degraded, respectively, and the degree of degradation was particularly strong in areas with dry summers.
Mäkipää et al. [11] examined the relationship between this deterioration of soil structure and soil carbon loss based on the monitoring data of the SOC stock changes in agricultural soils for the national greenhouse gas inventories [11,46]. However, no significant relationship was found between them, and it was inappropriate to use the deterioration index of soil structure as an index of soil carbon loss. However, the evaluation of soil structure condition by SOC/Clay ratio shows interesting trends when viewed by soil type. The results indicate that despite being rich in highly active clay minerals, Vertisols were the most degraded, which was likely due to cumulative loss of soil carbon due to intensive and long-term agricultural use. Next, Calcisol and Solonchak, found in degraded arid and semi-arid regions, are soils containing high concentrations of calcium and sodium salts, respectively, suggesting the effects of limited productivity in these soils. On the other hand, the podzols under forests usually have sandy soils that are not particularly rich in clay, but the degree of soil structure deterioration was small. This is because Podsols typically have a cool climate, low microbial activity, and a high supply of organic carbon from the forest to the soil. Soil structure deterioration is strongly influenced by SOC dynamics due to climate and land use.
Additionally, allophanic soils (i.e., Andosols) containing large amounts of amorphous minerals can form stable Al and Fe complexes and accumulate SOC [47,48]. Regarding organic matter accumulation, sheet silicates (clays-OH) have the strongest correlation with phenolics, Fe (Fe-O) and Al (Al-O) oxides have a correlation with polysaccharides, and these saccharides were found to capture carbon from microbial necromass, which increased with the addition of organic matter [49]. This indicates that Fe and Al oxides promote organo-organic interactions in soils, providing the basis for large organic matter accumulation in allophanic soils. Furthermore, it has been shown that Fe and Al oxides increase their positive charge as the pH decreases, thereby capturing more negatively charged carboxyl groups [37].

4. Suppressing the Consumption of Soil Organic Matter and Increasing Soil Organic Matter

To prevent SOC from being depleted and to recover and increase SOC from depletion, it is necessary to increase the amount of organic matter input to the soil and to suppress excessive organic matter decomposition.
No-tillage and the application of mulch are effective ways to prevent excessive organic matter decomposition. On the other hand, to increase the amount of organic matter input into the soil, it is effective to apply organic fertilizers and introduce cover crops. Effective methods include applying biochar, restoring degraded land, linking crop cultivation and livestock farming, planting trees, preserving urban ecosystems, and building complex systems for biofuel production. Conservation farming techniques such as organic farming, agroforestry, ecosystem restoration, and infrastructure development have been developed for these practices [8] and these were further important to maintain regional circular economies during global emergencies such as the COVID-19 pandemic which induced heavy disruption of the supply chain of agricultural materials like chemical fertilizers.
As mentioned earlier, the carbon sequestration rate increases as the MOC saturation (the ratio of MOC to the potential) decreases [3], and microbial biomass becomes the MOC through microbial necromass [18,49]. Therefore, inputting fresh organic matter to soils with a low SOC/Clay ratio is more effective in increasing MOC and improving soil structure. Furthermore, considering that the SOC/Clay ratio is lower in the subsoil, cultivation of deep-rooted crops is considered to be effective for increasing MOC.
The application of conservation farming methods has the effect of improving soil structure, sequestering carbon, supplying soil nutrients and water, and reducing environmental burden (Figure 1). By suppressing the depletion of soil organic matter, the SOC/Clay ratio is improved, improving the soil structure status. At the same time, this is nothing but an increase in the amount of soil carbon sequestration. According to Minasny et al. [4], soil carbon content at 30 cm worldwide is estimated to increase by more than 4 per 1000 (0.4%) in the first 20 years following introduction of conservation farming practices [4]. Based on this, they showed that increasing the carbon content by 4 per 1000 could absorb all 4 Gt of carbon that remains in the atmosphere each year. An increase in soil carbon of 4 per 1000 in the top 1 m of agricultural soil will result in SOC sequestration of 2–3 GtC y−1.
Increasing the input of organic fertilizers into the soil and efforts to improve the soil structure will improve the supply of nutrients and water from the soil to plants. Improving water supply improves the efficiency of rainfall use by improving water retention, water permeability, aeration, and rhizome development, as an increase in soil organic matter content improves soil structure [50]. Rooting reaches its maximum when the matric potential is around −10 kPa [32]; when it is wetter than that, there is a lack of air, and when it is drier than that, there is a lack of water and the soil hardness increases, making it impossible for roots to penetrate. Therefore, increased water retention improves rooting. In addition, nitrogen mineralization and nitrification reach their maximum at a water saturation level (WFPS) of 60%, mineralization and nitrification decrease as WFPS increases above this level, and denitrification becomes dominant when WFPS exceeds 80% [33]. Therefore, here too, the development of soil structure leads to an increase in nitrogen supply power. From the above, it is thought that the effects of applying organic fertilizers are more pronounced during droughts [51].
At the same time, the application of organic fertilizers is also expected to have the effect of reducing environmental impact. Nutrient supply from organic fertilizers occurs with mineralization, so the synchronization rate with plant nutrient requirements is higher than chemical fertilizers, and it is effective in suppressing nitrate leaching and N2O emissions [34]. Because chemical fertilizers are usually applied out of synchronization with plant growth, they easily cause nitrate leaching, ammonia volatilization [35], and N2O emissions [36]. High concentration of chemical fertilizers can also cause seedling injury [52] and acid disorders of crops [53]. On the other hand, application of organic fertilizer increases CEC, which in turn buffers against acids and prevents a decrease in pH [54].

5. Measurement and Evaluation of Soil Carbon Balance and Greenhouse Gas Balance

According to Mäkipää et al. [11], the SOC/Clay ratio indicates the state of soil structure degradation but does not indicate the dynamic of whether it is improving or degrading. Therefore, to show the effect of soil carbon management on soil health, it is essential to estimate the actual soil carbon balance and GHG balance.
Figure 2 shows a schematic diagram of net ecosystem carbon balance (NECB) and net GHG balance (NGB) regarding GHG emissions and uptakes, harvesting and organic fertilizer application.
NECB is a source of SOM and is obtained as net ecosystem production (NEP) [55] and can be written as follows [56]:
NECB = NEP + Cinput − Coutput
where Cinput is the amount of carbon input to the ecosystem, such as the application of organic fertilizers, and Coutput is the amount of carbon output through harvesting and erosion. NEP is the difference between net primary production (NPP) of plants and organic matter decomposition by soil microorganisms (Rh), as follows:
NEP = NPP − Rh
From the above, NECB can be obtained by measuring these parameters in the field. NEP can be estimated using a biometric method that measures plant production by cutting and the amount of SOC decomposed during that period using the chamber methods. At the same time, it can be estimated using micrometeorological methods such as the eddy correlation method [57], which measures and integrates the inflow and outflow of CO2 between the atmosphere and ecosystem. These can be combined to obtain the net plant root production and plant respiration rate [15].
Furthermore, the CH4 and N2O fluxes in the ecosystem are measured using the chamber method and converted into CO2eq (GWPCH4 and GWPN2O, respectively), and the negative NECB is converted into CO2eq as the ecosystem CO2 flux (GWPCO2), and the Global Warming Potential (GWP) is calculated by the sum of these three parameters as follows:
GWP = GWPCO2 + GWPCH4 + GWPN2O
The coefficients for CO2eq conversion of CO2, CH4, N2O fluxes are 1 for CO2, 28 for CH4, and 265 for N2O [1].
GWPCO2 = NECB × (44/12) × 1
GWPCH4 = CH4 × (16/12) × 28
GWPN2O = N2O × (44/28) × 265
Furthermore, negative values of GWP can be evaluated as the NGB of the ecosystem.
NGB = −GWP
Since CH4 is mainly produced under anaerobic conditions as the final step in the anaerobic decomposition of SOM, CH4 emissions primarily occur in oxygen-limited soils such as wetlands [58]. On the other hand, in well-drained upland soils, CH4 is normally consumed by oxidation, although it is affected by NH4 oxidation [59]. N2O emissions are strongly influenced by the application of nitrogen fertilizers, either chemical, inorganic, or organic fertilizers [60]. N2O is produced both under aerobic conditions due to nitrification and under anaerobic conditions due to denitrification [61]. How nitrogen fertilization and carbon inputs are managed is key to achieving adequate soil carbon sequestration and environmental conservation and ensuring sustainable crop production, and is detailed in the next section.

6. Proper Application of Organic Fertilizer and Its Effects on Carbon and GHG Balance

Crop growth is limited by the least abundant of the essential elements. Organic fertilizers do not provide an ideal balance of elements for crop growth. Generally, livestock manure has a high potassium content, and the mineralization rate of potassium is higher than that of phosphorus and nitrogen, so if we try to provide the required amount of nitrogen and phosphorus with manure alone, too much potassium will be added. Therefore, as a strict fertilization design for organic fertilizer, the amount of organic fertilizer to be applied should be such that the amount of mineralized nitrogen, phosphorus, and potassium from organic fertilizer is not excessive relative to the element requirements of plants. The missing elements are supplemented with chemical fertilizers [29].
Using this fertilization method, we investigated the effects of three organic fertilizers, manure, slurry, and digestive fluid, on crop growth and GHG emissions in a managed grassland of an Andosol in southern Hokkaido for three years [22]. The results showed that the chemical fertilizer nitrogen application rate in the organic fertilizer treatments was reduced by 10% for manure, 19.7% for slurry, and 29.7% for digestive fluid compared to chemical fertilizer only, but there was no significant difference in grass yield between the fertilizer treatments. Three years of NECB resulted in significantly less carbon loss with organic fertilizer treatment than with chemical fertilizer only. NGB when using organic fertilizers was reduced by 16.5% in slurry, 27.0% in digestive fluid, and 36.2% in manure compared to treatment with chemical fertilizers only. The main effect was due to carbon storage, that is, NECB reduction accounted for more than 90% of the NGB reduction. Manure and digestive fluid reduced N2O emissions compared to chemical fertilizer only, but slurry increased N2O. CH4 was so small that it could be ignored.
This fertilization method was applied to grasslands in four different climates from Hokkaido to Kyushu in Japan, and NECB, N2O, and CH4 emissions were measured for three years. As a result, annual NECB and NGB were found to have a significant positive correlation with the amount of carbon applied through manure as shown in Figure 3 [62]. In other words, the NECB was positive when the amount of manure carbon applied was 2.5 tC ha−1 y−1 or more, and grassland became a net carbon sink. However, the NGB only became positive when the amount of manure carbon applied reached 3.5 tC ha−1 y−1 or more. This is because applying manure increases the generation of N2O, so increasing the amount of manure carbon applied by an additional 1 tC ha−1 y−1 will offset the increase in N2O and contribute to suppressing global warming.
Figure 4 shows the breakdown of NECB in the four grasslands in Japan shown in Figure 3 for the manure plot and the chemical fertilizer-only plot (i.e., the amount of Manure applied is 0). Net ecosystem production (NEP) was positive in both the chemical fertilizer plot and the manure plot. This indicates that these grassland ecosystems accumulate carbon in their natural state. However, NECB was negative in chemical fertilizer plots. This shows that producing grass using only chemical fertilizers in these grasslands decreases carbon in the ecosystems. As seen earlier, in the manure plot, NECB could be made positive by applying more than 2.5 tC ha−1 y−1 of manure.
The SOC/Clay ratio of the cultivated soil in this field was 1/2.3, 1/1.5, 1/5.4, and 1/1.8 for NKS, SZN, NSS, and KBY, respectively. These soils are in a “very good” soil structure condition based on the SOC/Clay ratio index (SOC content was 5.30%, 2.55%, 3.45%, and 5.30%, respectively, and clay content was 12%, 3.7%, 18%, and 9.5%, respectively). However, if grass is produced using only chemical fertilizers and carbon is taken out by harvesting the grass, NECB becomes negative and soil carbon is depleted, decreasing the SOC/Clay ratio.
Table 3 shows the results of estimating how soil carbon would be lost in the four grassland soils if no manure was applied. The number of years that the SOC contained in 0–35 cm of soil will decrease due to NECB without adding the current manure is calculated as “until SOC disappears” and “until the SOC/Clay ratio is 1/8 (good)”, “until 1/10 (appropriate)”, and “until 1/13 (deterioration)”. However, the reduction in SOC was assumed to occur linearly at the current rate of carbon loss (NECB values at chemical fertilizer-only plots). As a result, soil carbon disappears in 55 to 78 years when cultivated without manure. During this period, the soil structure deteriorated, but it took 31 to 65 years for the SOC/Clay ratio to reach a level of 1/10, which is an acceptable level for agricultural soil. It is estimated that it will take 39 to 68 years to reach 1/13 (Table 3). It takes quite a long time for it to deteriorate, but once it starts to deteriorate, it appears to deteriorate rapidly.

7. Controlling N2O Emissions

It is necessary to increase carbon input to soil to offset N2O emissions, but increasing organic matter application may also exacerbate nutrient imbalances. In this sense, fertilization management that suppresses N2O emissions is important.
Soil N2O is produced as a by-product by nitrification processes and as an intermediate product by denitrification processes. Therefore, N2O emissions increase with increasing N application and N mineralization [63]. Nitrification requires aerobic conditions and NH4+, while denitrification requires anaerobic conditions and NO3− and organic carbon [64,65]. Through nitrification and denitrification, N2O is also produced as a by-product and intermediate product. Since nitrification and denitrification are affected by soil moisture [33], N2O production is also affected by soil moisture [61]. N2O production starts at 30% of WFPS, peaks at 60% to 70% of WFPS, and decreases when WFPS exceeds 80%, with N2 predominating.
In nitrification and denitrification, NO is also emitted along with N2O, but in the nitrification process, NO production is dominant, and the N2O-N/NO-N ratio is less than 1. In the denitrification process, N2O production is dominant, and the N2O-N/NO-N ratio is greater than 100 [66]. In actual fields, investigation of the behavior of NO3− is important to understand the production and emission of N2O. For example, in an onion field of structured clay soil, the above-ground parts and roots are supplied as residue during harvest. It has been reported that when the soil is moist due to rainfall during the harvest season, a large amount of N2O is emitted, accounting for up to 70% of the annual emissions [67]. At this time, N2O-N/NO-N exceeded 100, indicating that denitrification was predominant. This indicates that the supply of residual organic matter and rainfall in the onion fields caused denitrification, and the excess NO3-N left behind is thought to increase N2O emissions. Similarly, organic fertilizers can significantly increase N2O emissions if excess NO3−N remains. A comparison of chemical fertilizer treatment and cow dung manure application in grassland showed that N2O emissions increased in the manure application plot [68]. However, there was no significant difference in the relationship of N2O flux to soil temperature and soil moisture between the chemical fertilizer-only plot and manure plot, and a peak of N2O flux was observed at 18 °C of soil temperature or above 70–80% of WFPS. Also, since the N2O-N/NO-N ratio was higher when the N2O flux was high for both plots of chemical fertilizer only and manure, it was thought that denitrification was the main process for N2O emission in both plots. Furthermore, the amount of surplus mineral nitrogen (amount of mineral nitrogen applied + amount of mineralized nitrogen—amount of plant nitrogen uptake) showed a significant correlation with N2O emissions. These results suggest that the cause of N2O emissions was the residual amount of mineral nitrogen.
Manure releases nitrogen as organic matter decomposes, but in the case of cow manure, the annual decomposition rate of organic nitrogen is less than 20%. However, as mentioned earlier, manure contains potassium with a high release rate. Manure is applied in such an amount that the amount of NPK mineralized does not exceed the amount required by the plants, and the lack of mineralization is supplemented with chemical fertilizers. However, since mineralization occurs from organic fertilizers even after the second year, continuous application of manure can reduce the amount of chemical fertilizer supplements. Jin et al. (2010) reported that chemical nitrogen fertilizer supplements became zero after 4 years, and N2O emissions were lower in the manure plot than in the chemical fertilizer plot [54]. Additionally, in manure plots, soil pH rarely changes above 5. On the other hand, in the chemically fertilized plots, the soil pH dropped to 4 after three years.
Annual N2O emissions from grasslands and corn fields were negatively correlated with decreasing mean soil pH when nitrogen was applied to fields, regardless of whether or not manure was applied, when soil pH was less than 6 [36]. Incubation experiments have found that there is no N2O release at high limestone soil pH above 7 [69]. This is because the optimal pH of incomplete denitrifying bacteria, which causes N2O release, is lower than pH 6.5, which is the optimal pH of complete denitrifying bacteria. Therefore, to suppress N2O emissions, it is necessary to raise the soil pH to 6.5 or higher. Maintaining soil pH by applying organic fertilizers is one of the strategies to reduce N2O emissions and maintain soil health. In addition, crop residues and organic fertilizers with high C/N ratios had significantly lower N2O emissions per mineralized nitrogen (i.e., emission factors) [22,70,71,72]. Considering the use of organic fertilizers with high C/N ratios is also important from the perspective of soil carbon sequestration.
As mentioned above, organic matter application maintains SOC and contributes to maintaining good soil structure. The direct effect of improving soil structure is the improvement of nutrient and water supply (Figure 1). This improves the nitrogen balance and suppresses N2O emissions [68]. In addition, fertilization methods that reduce chemical fertilizers by applying organic matter also suppress N2O [22].

8. Conclusions

It has become clear that the maintenance effect of soil structure by soil organic matter is essential for soil health. Agricultural soil has clearly lost organic matter due to past cultivation, and the SOC/Clay ratio, which is an index of soil structure condition, is significantly more likely to fall below the deterioration limit of 1/13 in cropland than in grassland or forest. It has become clear that agriculture has been damaging the health of the soil. On the other hand, it was also shown that agricultural soils have the potential capacity to sequester 78% of the 555 GtC released into the atmosphere since the Industrial Revolution as soil organic matter. Soil management that increases rather than decreases soil carbon is necessary. To achieve this, the net ecosystem carbon balance (NECB) must be made positive. In four undegraded managed grasslands from north to south of Japan, it was necessary to apply more than 2.5 tC ha−1y−1 of compost carbon to maintain standard yields and achieve positive NECB. Although it is possible to reduce the amount of chemical fertilizer applied by applying compost, N2O is emitted as a result of fertilization, so it is necessary to achieve carbon sequestration to offset the greenhouse effect caused by this N2O emission. It was shown that this can be achieved by applying an additional 1 tC ha−1y−1 of compost. It has also been recognized that the continuous use of organic fertilizers has a large effect on reducing N2O emissions. It is increasingly important to consider soil management using organic fertilizers, which reduces the need for chemical fertilizers without reducing yields.

Author Contributions

Conceptualization, R.H.; writing—review and editing, R.H., I.M. and M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partly supported by a Japanese Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology (No. 11460028); a research grant provided by the Project entitled ‘Establishment of good practices to mitigate Greenhouse Gas emissions from Japanese grasslands’ funded by Racing and Livestock Association; and ‘Development of Mitigation Technologies to Climate Change in the Agriculture Sector’ run by the Ministry of Agriculture, Forestry and Fisheries of Japan.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Acknowledgments

We thank Yo Toma of Hokkaido University for kind suggestions regarding the interpretation of the data.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 14 01290 g002
Figure 2. Schematic illustration of net ecosystem carbon balance (NECB) and net greenhouse gas balance (NGB) in agroecosystem.
Figure 2. Schematic illustration of net ecosystem carbon balance (NECB) and net greenhouse gas balance (NGB) in agroecosystem.
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Agriculture 14 01290 g003
Figure 3. Relationship between manure application rate and net ecosystem carbon balance (NECB) and net greenhouse gas balance (NGB) in Japanese managed grasslands at Nakashibetsu (NKS), Shizunai (SZN), Nas-Shiobara (NSS) and Kobayashi (KBY) (produced from [62]).
Figure 3. Relationship between manure application rate and net ecosystem carbon balance (NECB) and net greenhouse gas balance (NGB) in Japanese managed grasslands at Nakashibetsu (NKS), Shizunai (SZN), Nas-Shiobara (NSS) and Kobayashi (KBY) (produced from [62]).
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Figure 4. Breakdown of net ecosystem carbon balance (NECB) in four grasslands in Japan: net ecosystem production (NEP), carbon imported through manure application (Manure), carbon exported through harvest (Harvest); Nakashibetsu (NKS), Shizunai (SZN), Nas-Shiobara (NSS) and Kobayashi (KBY) (Produced from [62]).
Figure 4. Breakdown of net ecosystem carbon balance (NECB) in four grasslands in Japan: net ecosystem production (NEP), carbon imported through manure application (Manure), carbon exported through harvest (Harvest); Nakashibetsu (NKS), Shizunai (SZN), Nas-Shiobara (NSS) and Kobayashi (KBY) (Produced from [62]).
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Table 1. Global carbon stocks (excluding tundra due to insufficient sample sizes). Mineral-associated organic carbon (MOC), soil organic carbon (SOC), mineralogical carbon capacity (MOCmax, which is calculated from the amount and type of soil minerals) and MOC deficit (to the mineralogical capacity and natural land average environmental limit) totaled over all given land-use categories (except tundra) globally [41].
Table 1. Global carbon stocks (excluding tundra due to insufficient sample sizes). Mineral-associated organic carbon (MOC), soil organic carbon (SOC), mineralogical carbon capacity (MOCmax, which is calculated from the amount and type of soil minerals) and MOC deficit (to the mineralogical capacity and natural land average environmental limit) totaled over all given land-use categories (except tundra) globally [41].
DepthMOCSOCMOCmaxMOC Deficit Relative to MOCmaxMOC Deficit Relative to Natural Land Average C Saturation
cmGtC
0–304487001443990286
30–10045170131532690147
0–100899140145963680433
Table 2. Expected structure quality as a function of the SOC/Clay ratio [31].
Table 2. Expected structure quality as a function of the SOC/Clay ratio [31].
SOC/ClayExpected Soil Structural Quality
>1/8Very good
1/10 < SOC/Clay < 1/8Good
1/13 < SOC/Clay < 1/10Adequate
<1/13Bad
Table 3. Estimated number of years until SOC disappears and reduces to the level of SOC/Clay ratio of 1/8, 1/10 and 1/13 by using Clay and SOC contents in 0–35 cm soil depth and net ecosystem carbon balance (NECB) in Japanese managed grasslands at Nakashibetsu (NKS), Shizunai (SZN), Nas-Shiobara (NSS) and Kobayashi (KBY).
Table 3. Estimated number of years until SOC disappears and reduces to the level of SOC/Clay ratio of 1/8, 1/10 and 1/13 by using Clay and SOC contents in 0–35 cm soil depth and net ecosystem carbon balance (NECB) in Japanese managed grasslands at Nakashibetsu (NKS), Shizunai (SZN), Nas-Shiobara (NSS) and Kobayashi (KBY).
Number of Years until the SOC Reduces to:
ClaySOCNECB0SOC/Clay Ratio
Site(0–35 cm)(0–35 cm) 1/81/101/13
t/hatC/hatC/ha/yYears
NKS272 120 −1.80 67 48 52 55
SZN126 87 −1.57 55 45 47 49
NSS720 138 −2.10 66 23 31 39
KBY282 157 −1.99 79 61 65 68
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Hatano, R.; Mukumbuta, I.; Shimizu, M. Soil Health Intensification through Strengthening Soil Structure Improves Soil Carbon Sequestration. Agriculture 2024, 14, 1290. https://doi.org/10.3390/agriculture14081290

AMA Style

Hatano R, Mukumbuta I, Shimizu M. Soil Health Intensification through Strengthening Soil Structure Improves Soil Carbon Sequestration. Agriculture. 2024; 14(8):1290. https://doi.org/10.3390/agriculture14081290

Chicago/Turabian Style

Hatano, Ryusuke, Ikabongo Mukumbuta, and Mariko Shimizu. 2024. "Soil Health Intensification through Strengthening Soil Structure Improves Soil Carbon Sequestration" Agriculture 14, no. 8: 1290. https://doi.org/10.3390/agriculture14081290

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