Soil N-oxide emissions decrease from intensive greenhouse vegetable fields by substituting synthetic N fertilizer with organic and bio-organic fertilizers
Graphical abstract
Introduction
Nitrous oxide (N2O) and nitric oxide (NO) are two important atmosphere trace gases, directly or indirectly contributing to global climate and environmental changes (IPCC, 2013). N2O is a potent greenhouse gas and the dominant stratospheric ozone-depleting substance (Ravishankara et al., 2009). NO is a key precursor of tropospheric ozone (O3) and contributes to the formation of acid rain (Pilegaard, 2013). Agricultural soils are significant sources of N2O and NO, primarily due to the increased use of chemical or organic nitrogen (N) fertilizers, emitting about 80% and 10% of the total anthropogenic emissions of N2O and NO, respectively (Davidson, 2009, IPCC, 2013, United Nations Environment Programme (UNEP), 2019). Emissions of N2O and NO are predicted to increase in the future, as the global use of N fertilizers is forecast to increase threefold by 2050 to meet the doubling of global food demand (Alexandratos and Bruinsma, 2012, Mueller et al., 2012).
Vegetable cultivation under greenhouse conditions can lead to substantial emissions of soil N-oxides. Nitrogen fertilizer application rates in greenhouse vegetable fields are often several times higher than in other cereal grain cropping systems (Liu et al., 2013, Rashti et al., 2015). Because of frequent irrigation and high temperatures, greenhouse vegetable cropping systems are highly susceptible to N losses, with annual soil N2O and NO emissions are as high as 60.5 and 10.8 kg N ha−1, respectively (Yao et al., 2019a, Zhang et al., 2016). In agricultural soils, N2O and NO emissions are mainly produced as by-products through the biotic processes of nitrification, denitrification, and nitrifier denitrification (Firestone and Davidson, 1989, Pilegaard, 2013, Wrage-Mönnig et al., 2018), while their emissions are influenced by the same factors in different ways (Loick et al., 2017). There has been a much stronger focus on N2O than on NO or combined N2O + NO emissions from vegetable fields (e.g., De Rosa et al., 2018, Zhang et al., 2018). At the regional or country scale, N2O or NO estimates generally described using emission factors (EF) to quantify the amount of N2O-N or NO-N lost as a function of the N inputs, excluding background N2O or NO emissions (Hergoualc’h et al., 2019). However, studies have found that the N2O emission factors for upland crops may differ between the crop-growing season (EFgs) and the whole year (EFwy), especially for vegetables (Shang et al., 2020). Different water management practices in agricultural systems can also result in high variability in EFs (Cayuela et al., 2017). Besides, 5.5–20.6% of the annual NO emissions from greenhouse vegetables occurred in the non-growing stage (Yao et al., 2019a), suggesting the importance of annual measurement of NO emissions. Nevertheless, the EFgs has been commonly used to determine national N2O or NO inventories for vegetable fields (Rashti et al., 2015, Wang et al., 2011). Exploring the EFs of N2O and NO over an annual period would contribute to a more accurate estimate of N2O and NO emissions from intensively managed greenhouse vegetable cropping systems.
In the past decade, shifts in fertilization strategies have received increasing attention due to the potential for greenhouse gas mitigation. For example, to address the challenges associated with the use of chemical N fertilizer (e.g., soil quality degradation, soil acidification, and groundwater pollution), substitution of synthetic fertilizer with organic fertilizer is promoted in intensively managed cropping systems (Zhang et al., 2020, Sanz-Cobena et al., 2017). However, the side effects of this fertilization strategy on N2O and NO emissions are uncertain. Organic fertilizers play multiple roles in microbial-mediated N2O production, leading to stimulatory or inhibitory effects. For instance, the easily mineralizable carbon (C) supply from poultry manure can stimulate heterotrophic denitrification, resulting in larger N2O emissions than occur with chemical N fertilization (Hayakawa et al., 2009). In contrast, a higher denitrification rate as a result of organic fertilizer application may promote the production of N2, with no significant difference in N2O emissions between organically farmed soils and conventionally farmed soils (Kramer et al., 2006). Several studies have reported that substituting organic for chemical fertilizer can contribute to decreased NO emissions (Akiyama and Tsuruta, 2003a, Akiyama and Tsuruta, 2003b, Meijide et al., 2007, Vallejo et al., 2006). Additionally, compared with liquid organic fertilizers, solid and composted organic fertilizers have a low N2O and NO emission potential due to reduced mineral N concentrations, especially the NH4+ content (Aguilera et al., 2013, Bertora et al., 2008, Meijide et al., 2007). The combination of drip irrigation with organic fertilizer reduced N2O emissions by 28% but had no significant effect on NO emissions when compared with furrow irrigation (Sanchez-Martin et al., 2010). These scenarios could be highly dependent on climate, soil properties, site-specific management practices, as well as the biochemical quality of the organic materials. Nevertheless, emerging evidence suggests that the differences in the rate of N release from synthetic and organic fertilizers can have the potential to significantly influence soil N-oxide emissions (Prosser et al., 2020). This is mainly due to niche preference of ammonia-oxidizing bacteria (AOB) or archaea (AOA) (Hink et al., 2017, Hink et al., 2018, Stein, 2019).
Substituting chemical fertilizer with organic fertilizer may have a neutral or even negative effect on vegetable yield, especially when the replacement rate of synthetic N with organic N exceeds 75% (Xia et al., 2017). The release of available N from composted organic fertilizer is slow and may not be able to meet the high N demand of fast-growing and high-yield vegetables (Berry et al., 2002). Thus, a promising alternative strategy is the use of bio-organic fertilizers (a mixture of organic material with beneficial soil microorganisms). Trichoderma. spp, a biological control and plant growth-promoting agent, is commonly used in bio-organic fertilizers (Harman et al., 2004). Colonization of Trichoderma on the root surface was shown to promote the development of plant roots and improve N use efficiency (Shoresh et al., 2010). The use of Trichoderma enriched bio-organic fertilizer has been shown to enhance plant uptake of soil nutrients and stimulate plant growth when compared with organic or chemical fertilizer (Pang et al., 2017). However, the effect of bio-organic fertilizer on soil N-oxide emissions remains unclear. Meanwhile, the benefits or trade-offs from substituting synthetic fertilizer with organic or bio-organic fertilizers are not well‐known in vegetable fields. Therefore, it is necessary to evaluate the yield-scaled N2O + NO emission (expressed as N2O + NO produced per unit of crop yield) for balancing soil N losses and food security in agricultural ecosystems (Linquist et al., 2012).
In this study, we conducted an in situ field measurement over a 16-month period to quantify the annual N2O and NO fluxes under various fertilizer-N regimes and to quantify crop yield benefits on a cucumber monoculture under a drip irrigation system in the greenhouse. We hypothesized that i) the substitution of synthetic N by organic or bio-organic fertilizer would mitigate N-oxide emissions due to the slow N release from added organic fertilizers; and ii) that bio-organic fertilizer would decrease N2O and NO emissions and increase crop yields, thus resulting in lower yield-scaled N2O and NO emissions. Specifically, the objectives of our study were to i) quantify the seasonal and annual emission intensity of N2O and NO and their direct EFs under different fertilizer-N regimes; ii) evaluate the effects of synthetic and organic N management on yield-scaled emissions of N2O and NO; and iii) clarify the environmental driving factors that regulate N-oxides emissions.
Section snippets
Site description and experimental design
Field experiments were conducted from August 2015 to December 2016 at a research site (31°95′N, 118°83′E) of Nanjing Agricultural University in suburban Nanjing, Jiangsu province, China. The experimental site has been in continuous vegetable cultivation (e.g., Chinese cabbage, cabbage and cucumber) for > 5 years. The climate is characterized by a subtropical monsoon, with hot-rainy summers and mild-less rainy winters. From 2015 to 2016, the annual average temperature is 16.5 °C, and summer
Environmental variables
Soil temperature during the three cropping cycles was comparable, ranging from 10.7 to 33.5 °C (mean: 22.4 °C), with the highest value occurring in July 2016 (Fig. 1a). Soil WFPS ranged from 20.5 to 80.6% (mean: 40.6%) in the second cropping cycle and was higher than the average of first and third cropping cycles, ranging from 23.6 to 57.3% (mean: 28.4%). The main reason for this difference was that the field was subjected to heavy rainfall during the second fallow stage due to the
Responses of N-oxide emissions to synthetic and organic N fertilizer applications
Our results demonstrated that applying organic fertilizers instead of synthetic N fertilizer can significantly decrease soil N2O and NO emissions in greenhouse cucumber monocultures with a drip irrigation system. For comparison, we compiled data from previous studies as shown in Table 2 and found that the mitigation effects of organic substitution on soil N-oxide emissions also occurred in cereal (Meijide et al., 2007, Yan et al., 2015, Yao et al., 2019b) and other vegetable fields (Akiyama and
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.
Acknowledgments
This work was supported by the National Key Research and Development Projects of China (2017YFD0800200), the Fundamental Research Funds for the Central Universities (KYXK202002), and the National Natural Science Foundation of China (41877093, 42007072). J.W acknowledges multiyear support from The Startup Foundation for Introducing Talent of Nanjing Agricultural University (030/804028).
References (63)
- et al.
The potential of organic fertilizers and water management to reduce N2O emissions in Mediterranean climate cropping systems. A review
Agric. Ecosyst. Environ.
(2013) - et al.
Pig slurry treatment modifies slurry composition, N2O, and CO2 emissions after soil incorporation
Soil Biol. Biochem.
(2008) - et al.
Direct nitrous oxide emissions in Mediterranean climate cropping systems: Emission factors based on a meta-analysis of available measurement data
Agric. Ecosyst. Environ.
(2017) - et al.
N2O and CO2 emissions following repeated application of organic and mineral N fertiliser from a vegetable crop rotation
Sci. Total Environ.
(2018) - et al.
Management of pig manure to mitigate NO and yield-scaled N2O emissions in an irrigated Mediterranean crop
Agric. Ecosyst. Environ.
(2017) - et al.
N2O and NO emissions from an Andisol field as influenced by pelleted poultry manure
Soil Biol. Biochem.
(2009) - et al.
“ Hot spots ” of N and C impact nitric oxide, nitrous oxide and nitrogen gas emissions from a UK grassland soil
Geoderma
(2017) - et al.
Nitrogen oxide emissions from an irrigated maize crop amended with treated pig slurries and composts in a Mediterranean climate
Agric. Ecosyst. Environ.
(2007) - et al.
Evaluation of nitrate and ammonium as sources of NO and N2O emissions from black earth soils (Haplic Chernozem) based on 15N field experiments
Soil Biol. Biochem.
(2008) - et al.
Combination of drip irrigation and organic fertilizer for mitigating emissions of nitrogen oxides in semiarid climate
Agr. Ecosyst. Environ.
(2010)