How does partial substitution of chemical fertiliser with organic forms increase sustainability of agricultural production?
Graphical abstract
Introduction
Sustainable food security requires high yields from a finite land area. Conventional intensive use of synthetic nitrogen (N) fertilisers has greatly increased crop yields and labour efficiency over recent decades (Tilman et al., 2002; Wu et al., 2018). Yet, globally only 47% of added reactive N is converted into harvested products (Lassaletta et al., 2014). More than half of N are lost to the environment, detrimentally altering soils, ecosystems and the global climate. Thus, the use of synthetic N has been recognised as the major barrier to agricultural sustainability (Goucher et al., 2017).
Meanwhile, the disposal of organic solid waste, such as livestock manure and municipal sludge has been considered as a stinging and widespread issue due to increased production with low resource utilization (Abdel-Shafy and Mansour, 2018; Bai et al., 2016). For example, global livestock manure N production is about 80–130 Tg N/year, equivalent to the total synthetic N fertiliser consumed (Bouwman et al., 2013). Specialization of farming activities and easy access to synthetic fertiliser has caused spatial segregation of livestock and crops, with as little as 20% of total N from livestock excretion being applied to cropland (Jin et al., 2020; Oenema et al., 2005). Together, increased N losses from croplands and non-recycled organic wastes have caused extensive land and water body eutrophication, greenhouse gas emissions and biodiversity losses with significant damage to both human and ecosystem health (Yu et al., 2019). Global food demand is expected to double by 2050, further exacerbating these problems (FAO, 2017; Godfray et al., 2010).
Substituting chemical N fertiliser with organic forms (partial organic substitution hereafter) has been proposed as a solution to above mentioned challenges to promote resource cycling and reduce undesirable impacts on ecosystems and humans (Eyhorn et al., 2019). Partial organic substitution can improve crop yields and productivity by providing exogenous carbon (C) inputs to maintain soil health (EI-Sheikha, 2016). Besides, partial organic substitution can alter the loss of various active N (N2O emission, N runoff and leaching) by affecting the conversion of N in soil (Xia et al., 2017; Zhou et al., 2017). However, a recent global meta-analysis only considered crop yields and addressed that 100% organic substitution reduced crop yields, resulting in reduced environmental improvements or even losses when expressed per unit product (Seufert et al., 2012). This yield divergence may vary with different forms of organic fertiliser, substitution rates and crop types. Despite substantial evidence of the effects of partial organic substitution on agro-ecosystem functioning, field studies to date have tended to focus on a specific agronomic or environmental indicator. Few previous studies have simultaneously examined the benefits and trade-offs of partial organic fertiliser substitution in different organic forms on agronomic processes and various reactive N losses, together with economic assessments based on both cropland area and per unit production costs.
Another knowledge gap is how partial organic substitution affects crop yields and reactive N releases from agricultural soils. Microbial communities, including functional communities involved in the N cycle, such as ammonia oxidizers and denitrifiers, are the primary drivers of conversion N between forms (Kuypers et al., 2018). These functional guilds have been shown to regulate soil N available to plants, and thus affect loss of reactive N from agricultural systems through leaching, runoff and greenhouse gas emissions (Norton and Ouyang, 2019). In addition to microbial action releasing N forms sensitive to loss, soil microbial communities also release unavailable complex N to plants by mineralization of soil organic matter and decomposition of fresh litter, and limit N losses by immobilization of inorganic N in soil organic compounds and microbial biomass (Risch et al., 2019). Establishing clear links between the communities responsible for N transformations and fertiliser sources should improve the mechanisms understanding of agricultural production. This will also help the design of more efficient and sustainable N management. Although several studies have documented changes in size or composition of soil microbial community due to organic fertiliser amendments, short term shifts of a highly resilient community structure or an overriding effect of time have been proposed (Riber et al., 2014; Suleiman et al., 2016). This highlights the need for multi-seasonal studies to assess the dynamics of microbial communities and provide a stronger explanation of how partial organic substitution affects crop yields and environmental impacts by shaping microbial communities.
Here, we use a 2-year vegetable growth field experiment in China to assess impacts of partial substitution of fertiliser N with organic material derived from animal and human waste (composted pig manure or sewage sludge at 25% and 50% N substitution) over eight rotations. In doing so, we expand on a previous study of this system (Tang et al., 2019) both in terms of temporal spread of samples and parameters assessed, focussing on the dynamics of the key nutrient N transformations and its associated microbial groups. We address the directly policy-relevant hypotheses: that partial substitution of chemical fertiliser with organic forms increases or maintains yields while building soil fertility and reduces detrimental environmental effects. This is associated with significant changes in microbial populations responsible for key transformations. We also assess the economic benefit and environmental cost of each system per unit area and per unit production.
Section snippets
Field experimental set-up, management, and sampling
The field experiment site is located in the Zhangxi catchment, Zhejiang Province, China (29°47′34″N, 121°21′48″E) a typical peri-urban zone on the fringes of Ningbo in the Yangtze River Delta region. The climate of this region is subtropical monsoon, with a mean annual rainfall and temperature of 1460 mm and 17.4 °C, respectively (data from Ningbo weather station). The site was originally cultivated using conventional chemical fertilisation and other agronomic practices with a rice-wheat
Yield, soil properties and nitrogen losses
After two years and eight consecutive rotations of vegetable cultivation, our field trials showed a significantly positive effect of fertilisation treatment on yield (P < 0.001). Overall, partial organic substitutions significantly increased yields compared to conventional chemical N fertilisation by 38–47% (Fig. 1a). By averaging the soil physicochemical properties after each vegetable harvest, significant changes were observed in the SOC, pH and TN contents of different fertilisation
Partial organic substitution improves crop productivity and soil property and reduces nitrogen losses
Crop production is the primary reason that humans manage agroecosystems. In our study, partial organic substitution significantly increased vegetable yields compared with conventional chemical N fertiliser treatment, which is consistent with the results of several recently published global meta-analysis (Luo et al., 2018; Zhang et al., 2020). The increase in vegetable yield might be attributed to the improved synchronization of nutrient supply and soil properties (Gai et al., 2018; Zhang et
Conclusions
Crop production in intensive agriculture currently relies too heavily on synthetic N fertiliser inputs, which is one of the largest drivers of global environmental degradation, especially in developing countries under pressure to feed large and growing populations. It is becoming critical to provide alternative ways to intensively farm land to offer a win-win approach to increase productivity while also reducing environmental impacts of agriculture. Partial substitution of chemical N fertiliser
Code availability
All computer code used in this analysis is available from the authors upon reasonable request.
Data availability
All raw data present in this publication is available from the authors upon reasonable request. Sequences generated have been deposited in the European Nucleotide Archive (accession number PRJEB42605).
CRediT authorship contribution statement
Quan Tang: Writing – original draft, Data curation, Software, Visualization, Writing – review & editing. Anne Cotton: Data curation, Writing – review & editing, Validation. Zhijun Wei: Data curation, Writing – review & editing, Validation. Yongqiu Xia: Writing – review & editing, Validation. Tim Daniell: Data curation, Conceptualization, Writing – review & editing, Methodology, Validation. Xiaoyuan Yan: Conceptualization, Supervision, Validation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge Sara Moeskjaer and Duncan Cameron for critical reading and suggestions. We further thank the China Scholarship Council (File No. 201904910376) grant for QT's research visit to the UK. This work was supported by the National Key Research and Development Program of China (2017YFD0200100) and the National Natural Science Foundation of China (No. 42061124001) and NERC (NE/N00745X/1 and NE/S009132/1).
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