One-Time Deep Application of Nitrogen Fertilizer: A Potential Measure of Ammonia Mitigation in Grainland

Abstract

Ammonia (NH₃) is one of the main precursors of secondary inorganic aerosols. In 2018, the NH₃ emissions of China’s cereal production (rice, wheat and maize) were estimated to be 3.3 Mt NH₃-N. Numerous NH₃ mitigation strategies have been developed in agriculture to reduce the emissions and improve air quality. However, due to the cost and unfeasibility of some developed techniques, the application of these mitigation measures is relatively slow in cropland. Therefore, developing low-cost, easy-operation, and feasible mitigation measures is an important breakthrough to solve the pollution of ammonia emissions in grain fields. The one-time deep application of nitrogen fertilizer in crop growing season, referred to as one-time application, is a promising ammonia mitigation measure for grain fields. It is a low-cost mode of fertilizer application suitable for grain fields as it saves labor and reduces the input of agricultural machinery. Therefore, incentive policies should be formulated to promote it for wide-range application in the whole country, especially in the areas with serious ammonia pollution, in order to achieve the goal of green and sustainable agricultural production.

1. Introduction

In order to ensure the food security of 1.4 billion people, the arable land of grain crops (wheat, corn and rice) in China has reached 110 million hectares, accounting for 58% of the total arable land [1]. As an important agricultural input, chemical fertilizer usage increased from 1 to 53 Mt over the last 50 years, this can partially explain the four-fold increase in Chinese grain production [2]. The application of chemical fertilizers has also led to a series of environmental problems, such as ammonia emissions, greenhouse gas emissions, and air pollution. Ammonia (NH₃) is one of the main precursors of secondary inorganic aerosols. In 2018, the NH₃ emissions of China’s cereal production (rice, wheat and maize) were estimated to be 3.3 Mt NH₃-N. By 2050, NH₃ emissions will increase by 23−32% under different climate change scenarios [3]. Accordingly, such emissions have caused extensive concern due to their serious effects on human health, including their involvement in triggering strokes, cardiovascular disease, and shortening visibility [4]. The redeposited NH₃ also can lead to serious detrimental environmental effects, such as soil and ecosystem acidification, eutrophication, and biodiversity losses [5]. Numerous NH₃ mitigation strategies have been developed in agriculture to reduce the emissions and improve air quality.

Among these measures, the use of non-urea-based fertilizers, deep placement of fertilizers, irrigation, and mixing fertilizers with amendments (pyrite, zeolite and organic acids) were reported to potentially reduce NH₃ emissions by 35% to 95% [6,7,8]. However, due to the cost and unfeasibility of some of the developed techniques, the application of these mitigation measures is relatively slow in cropland [9]. In particular, in contrast to high-income vegetables or orchards, it is more difficult to apply these measures in low-income grain fields. For example, the proportional application of a substitute fertilizer with a higher price, such as nitro or compound fertilizer, are far lower in grain fields than in vegetable fields and orchards [9,10]. Additionally, a considerable part of corn is planted on dry land, the mitigation measures of irrigation or fertilization were also difficult use widely, as in vegetable fields, due to high-cost input. Therefore, developing low-cost, easy-operation, and feasible mitigation measures is an important breakthrough to solve the pollution of ammonia emissions in grain fields.

2. Essential Aspects of One-Time Application

The one-time deep application of nitrogen fertilizer in crop growing season, referred to as one-time application, is a promising ammonia mitigation measure for grain fields. Contrasted to conventional splitting nitrogen fertilizer, one-time application involves inputting all the required fertilizer at the beginning of the crop season combined with the mechanical sowing or tillage. Furthermore, slow or controlled-release fertilizers are often used to ensure the nutrient demand of crops in the later period [11]. Obviously, one-time application is a comprehensive strategy of ammonia mitigation, including mechanical deep application, slow-release fertilizer and optimal fertilizer, etc. It combines the advantages of each single technology while avoiding the corresponding defect, and thus, has a bright prospect.

3. Advantages and Mitigation Capacity

Deep application of nitrogen fertilizer is the most effective measure for ammonia mitigation because it sharply reduces the content of ammonium in the surface soil. It was reported that NH₃ emissions can be reduced to 75–96% or 87–98% by changing fertilizer application from surface broadcast to deep placement in maize or winter wheat, respectively [12]. Normally, topdressing is generally performed in the high-temperature period for suitable crop growth, when the weather is more likely to aggravate the loss of NH₃ emission. Furthermore, due to the cover of dense crops in fields at the topdressing period, nitrogen fertilization is generally performed by surface broadcasting, inevitably resulting in greater NH₃ loss. Concentrated one-time application of nitrogen fertilizer in at the sowing season can avoid the risk of ammonia loss from topdressing by broadcasting at higher temperature time. Therefore, it can be seen that one-time deep application could normally reduce the cumulative NH₃ emission compared to splitting fertilizer with equivalent amount of nitrogen fertilizer for all the three grain crops (Figure 1). Although in some cases, one-time application with concentrated nitrogen could lead to an increased NH₃ emission rate or an extension of emission time [13], through improving the depth of fertilization and replacing common nitrogen fertilizer by slow and controlled-release nitrogen fertilizers, the loss of nitrogen using one-time application was not higher than the treatments of splitting application [14].

In contrast to conventional broadcast nitrogen fertilizer application, the deep application of nitrogen fertilizer can optimize root configuration [15], increasing the efficiency of nitrogen uptake by roots because of the more favorable moisture conditions in deeper layers, especially in water-limited areas [16]. It was shown that nitrogen fertilizer deep placement resulted in 3.7% average higher yield for all the mentioned cereal crops than splitting and/or broadcasting nitrogen fertilizer application based on a global meta-analysis [17]. The data we collected also showed that the median yield increased by about 5.9%, 7.5% and 7.0% under one-time application for rice, corn and wheat, respectively (Figure 2). In addition, the one-time application is combined with mechanical sowing or tillage, simplifying the nitrogen fertilization procedure and reducing the economic cost of labor, which is in line with the development trend of urbanization and declining labor resources in China [18]. The demand for labor varies considerably for different crop seasons, meaning that, in some areas/seasons, the shortage of employment and/or available labor associated with manual surface broadcast nitrogen fertilizer could be a big problem. Therefore, although the one-time application will increase fuel cost by about 17% due to disturbing the deep soil [19], the profit will be significantly improved by reducing the labor cost or the number of mechanical operations.

4. Implementation Obstacles

There are also some problems in one-time nitrogen fertilization, which hinder its wide application in grain fields. (1) The price of slow and controlled-release nitrogen fertilizers is relatively expensive, for example, the prices of sulfur-coated fertilizers can be even two-fold higher, while polymer-coated fertilizers can be up to three times more expensive than conventional ones [20]. Moreover, its nutrient release curve cannot be completely consistent with the crop absorption dynamics. Especially for wheat, insufficient nutrient supply after one-time application was observed in the later stage due to the longer growing season under one-time application. (2) The existing tillage and sowing machinery lack the effective combination with one-time nitrogen fertilizer application, and furthermore, the fertilization operation cannot adapt to the needs of different soil conditions, different planting methods and different growth stages of crops. (3) There is a lack of analysis on the comprehensive environmental effect of one-time deep application in the field. For example, some studies found that concentrated deep application would increase the risk of nitrogen leaching and N₂O emission loss [21,22]. It is necessary to optimize the one-time fertilization technology to reduce the total loss of nitrogen and improve its utilization efficiency.

The application of one-time nitrogen fertilization is not only hindered by technical barriers, as in the above, but also related to farmers’ cognition and the land management mode of small farmers in China. Smallholder farmers are a heterogeneous group in terms of income, knowledge, and other factors. Therefore, transferring knowledge from the government, institutes, and colleges to smallholder farmers still faces great challenges [23]. A survey based on 86,000 farmers found that NH₃ emissions (kg ha⁻¹) on average decrease by 0.07% for each 1% increase in average farm size [24]. In fact, farm size is a platform for all technologies to be functioned and implemented. The larger farm size normally means the higher cost-effectiveness obtained from the farm adopting these innovative technologies, such as slow and control-released nitrogen fertilizer, as well as deep placement of fertilizers by machinery [25]. It is also the reason why large-holder farmers intend to invest more in these innovative technologies, which can increase the crop yield while reducing the NH₃ emission. However, currently, over 98% of Chinese croplands are small scale, which inhibits the adaptation of new technologies [26].

5. Perspective

One-time application has been widely used in grain fields in China. For example, it is gradually becoming the mainstream fertilization method, and more than 51.4% of farmers have adopted this technology in maize-growing areas (

Table 1. Proportion of farm holders applying one-time nitrogen fertilizer for grain production in the major provinces in China.

	Jiangsu	Hubei	Sichuan	Heilongjiang	Jilin	Liaoning	Average
Rice (%)	62.7	47.8	54.0	21.8	53.5	68.3	7.5
	Shandong	Hebei	Henan	Heilongjiang	Jilin	Liaoning
Maize (%)	62.7	47.8	54.0	21.8	53.5	68.3	51.4
	Shandong	Hebei	Henan
Wheat (%)	51.3	14.5	33.6				33.1

). Although the proportion of one-time fertilization in wheat and rice fields is still low, the measure has been gradually developed, improved and applied in the wider range [16,27]. However, the measure needs to be developed and improved in theory, materials and equipment, so as to make the technology more widely used, especially in wheat and paddy fields. For different crops, optimizing the mode of deep application based on the effectiveness of ammonia mitigation and crop uptake, including mixing nitrogen fertilizer into soil, strip application and hole application. As for the slow and controlled-release nitrogen fertilizers for one-time application, it is more accurate to control the release of fertilizer nutrients, especially for winter wheat, which ensures sufficient nutrient supply in the late growth period. The controlled-release or coating materials of fertilizers are degradable and more environmentally friendly. The specific fertilizer for different crops should be explored to apply in one-time fertilization, and the cost of fertilizer production was further reduced. As for the fertilizer applicator, combined seeders or transplanters with more accurate fertilizing amount and more reasonable applying position in soil layers should be developed to realize precise application of fertilizer in one time. In addition, a variety of fertilizer applicators needs to be developed, which are applicable to different terrain and field scales, and can apply different forms of fertilizers, including powder, large particles and liquid fertilizers.

Overcoming socio-economic barriers to implement one-time application, the media and agricultural science and technology extension departments can strengthen publicity to improve farmers’ awareness of environmental protection and food security [28,29]. Implementing regulations of ammonia abatement is also an effective strategy to guide farmers to adopt new fertilization methods. For example, the German Fertilizer Ordinance stated that urea is not allowed to be sprayed unless a urease inhibitor is added, or it is immediately incorporated into soil within 4 h, since February 2020. The UK also plans to implement regulations to ban the use of solid urea and limit the fertilization time from January 15 to March 31 as of 2022 [30]. Considering the increased cost of adopting separately alternative fertilizers or incorporation into soil to reduce ammonia emission, one-time application is a simple method which may be the direction for seeking new alternative technology in fertilizer application.

It was found that small farm size is the key driving force for high NH₃ emission and other environmental performance from croplands. Based on a survey in China, the amount of nitrogen fertilizer and NH₃ emission from crop production would be reduced by 30% and 50%, respectively, if the farm size could conform to the global average level [31]. Optimizing nitrogen fertilizer would generally reduce potential greenhouse gases in farmland [32,33]. Therefore, governments can potentially transfer the benefits of emission reduction to farmers depending on farm size, which facilitates the transfer of land to cooperatives for centralized management and, therefore, the promotion of investing in more ammonia mitigation technologies. At the same time, priority will be given to promoting one-time deep application technology in agricultural cooperatives and large family farms, especially by supporting them to upgrade to the new machines of deep application. Thus, it will encourage more farmers to adopt the NH₃ mitigation measures and improve the nitrogen fertilizer application technique.

6. Conclusions

In summary, one-time deep application of nitrogen fertilizer is an effective technology for ammonia mitigation in farmland. It is a low-cost mode of fertilizer application suitable for grain fields which saves labor and reduces the input of agricultural machinery. Therefore, incentive policies should be formulated to promote it for the wide application in the whole country, especially in the areas with serious ammonia pollution, to achieve the goal of green and sustainable agricultural production.