Optimizing nitrogen fertilization rate to enhance soil carbon storage and decrease nitrogen pollution in paddy ecosystems with simultaneous straw incorporation
Introduction
As the largest carbon (C) reservoir in terrestrial ecosystems, soils (0–1 m layer) store about 1500 Pg of organic C (Scharlemann et al., 2014), which is 3–4 times more than that stored in the atmospheric and biotic pools (Lal, 2004a, 2010). Therefore, even small changes in soil organic C (SOC) storage would result in profound influences on global C cycles and climate change (Lal et al., 2004; van Groenigen et al., 2014). Great efforts have been made to enhance SOC storage through optimizing management practices (Moran et al., 2013; Smith et al., 2016), especially for cropland soils that hold approximately 10 % of global SOC storage (Lal, 2004b; Lal et al., 2004; Lugato et al., 2018), but support the food supply for the world population of nearly 7.5 billion (Godfray et al., 2010). Agricultural management practices are expected to affect the SOC storage in croplands through regulating the balance of soil C input and output (Smith et al., 2010; Kim et al., 2017).
Among the agricultural management practices, N fertilization (NF) is a key practice that increases aboveground plant biomass and consequently belowground C input through enhanced root exudates (LeBauer and Treseder, 2008). A meta-analysis conducted by Ni et al. (2017) demonstrated that NF significantly increased C input to the soil by 20 %, mainly due to the increase in net primary production (NPP). Similarly, Xia et al. (2014) found that long-term (22 years) NF significantly improved SOC storage in the 0–20 cm layer and NPP in a rice–wheat cropping system in southern China by 14.3 % and 115 %, respectively, compared with an unfertilized control. However, excessive NF can cause soil stresses, such as acidification and salinization (Guo et al., 2010; Zhang et al., 2012), which may impair crop growth and NPP, and consequently reduce the SOC sequestration potential.
However, NF can increase soil respiration because of the higher microbial activities associated with enhanced C substrate availability, which might even override the stimulatory effect on NPP (Chen et al., 2018). Chen et al. (2017) demonstrated that NF increased plant C input into soils by 0.08 Mg C ha−1 compared with zero N addition in a maize cropland in northeast China, but simultaneously stimulated soil carbon dioxide (CO2) emissions by 0.09 Mg C ha−1, thereby leading to a decrease in the SOC storage. However, Zhang et al. (2018) reported that soil microbial biomass or activities decreased significantly with increasing NF rate. High NF rates caused toxicity for soil microbial communities, such as increased soil acidity and osmotic stress (Averill and Waring, 2018), which could inhibit the microbial decomposition of soil organic matter (OM) and consequently benefit soil C sequestration (Janssens et al., 2010). Therefore, it is essential to investigate the responses of soil C input (NPP) and output (soil and ecosystem respiration) to increasing NF rates to fully evaluate the changes of soil C storage under N enrichment.
In contrast to NF alone, NF with simultaneous straw incorporation (SI) from the last crop season has been widely recommended as an effective management practice to increase SOC storage and mitigate greenhouse gas (GHG) emissions (Lal et al., 2004; Lal, 2010; Lugato et al., 2018). This is particularly the case in intensive agricultural regions with a high level of farm mechanization (e.g., the Yangtze River Delta region), where crop straw is simultaneously chopped and returned to the field when the grain is harvested (Xia et al., 2016b). A meta-analysis conducted by Xia et al. (2018) demonstrated that NF with simultaneous SI significantly increased the SOC content in the 0–15 cm soil layer by 14.9 % compared with single NF in global croplands. However, Xia et al. (2014) found that, in terms of global warming potential, long-term NF increased net GHG emissions from a rice–wheat cropping system with simultaneous SI in southern China. This is because the straw-induced increase in methane (CH4) emissions largely outweighed the increase in SOC sequestration. This highlights the importance of comprehensively investigating the response of SOC storage to soil C input (NPP) and output (CH4 emissions and ecosystem respiration) under NF with simultaneous SI (Kim et al., 2017), particularly in flooded rice-cropping systems.
Because of the great difficulty in directly measuring SOC storage changes over short-term periods (Powlson et al., 2008), the net ecosystem carbon budget (NECB) was developed as a tool to precisely estimate the soil C balance between soil C input and output (Smith et al., 2010; Kim et al., 2017). To date, only very few studies have evaluated the responses of NECB to NF in different rice-cropping systems with simultaneous SI (Yang et al., 2015; Hwang et al., 2017). For example, Hwang et al. (2017) reported positive NECB values ranging between 2117–3699 kg C ha−1 from a paddy field in South Korea that received NF with simultaneous green manure incorporation. Based on a 3-year field experiment in a rice–wheat cropping system in Yangtze River Delta region of China, Yang et al. (2015) found that simultaneous NF with SI led to a positive NECB value of 7510–7780 kg C ha−1 yr−1. However, no comprehensive studies have explored the response of NECB to different NF rates, which can greatly affect the soil C input through NPP and straw biomass incorporation. In this regard, it remains uncertain whether the potential of soil C storage in rice-cropping systems under SI can be further enhanced through optimizing NF rates, without threating crop production (NPP) and aggravating CO2 and CH4 emissions.
We conducted a two-year field experiment in a typical rice–wheat cropping system with simultaneous SI in the Yangtze River Delta region of China to: (1) investigate the responses of crop yield, soil C input (NPP and straw biomass), output (ecosystem respiration and CH4 emissions), and SOC storage changes (NECB) and N losses to different NF rates; and (2) identify an optimum NF rate that can result in greater SOC storage and lower N losses without compromising crop production and aggravating GHG emissions.
Section snippets
Study site
The two-year field experiment was conducted at Changshu Agroecological Experimental Station (31.53 °N, 120.68 °E) of the Chinese Academy of Sciences, Jiangsu Province, China. The research station was located on the Yangtze River Delta, where summer rice–winter wheat rotation is the dominant cropping system. This region has a subtropical monsoon climate with an annual mean temperature of 14 °C and mean precipitation of 1000 mm. The soil is classified as an Anthrosol developed from lacustrine
Soil C input components
Straw and grain were the two largest components of NPP, which contributed 42.6 %–45.8 % and 29.5 %–32.7 % to the rice NPP, and 34.5 %–38.3 % and 34.3 %–38.2 % to the wheat NPP, respectively (Fig. 2). The NPP varied from 6710 to 11,852 kg C ha−1 and 7841–13075 kg C ha−1 for the rice-growing season in 2013 and 2014, respectively, which was 26 %–119 % and 60 %–246 % higher than that of the wheat-growing season (Table 3). The annual NPP varied from 9773 to 19,580 and 10110–20487 kg C ha−1 yr−1 in
Increasing SOC storage through simultaneous NF and SI
Appropriate agricultural management practice is critical to increasing plant biomass productivity (NPP), thereby increasing the SOC storage in croplands (Triberti et al., 2008; Xia et al., 2014). As the most important agricultural practice to ensure food security, NF was reported to increase the NPP by 96.8 % in the rice–wheat cropping system in the Yangtze River Delta region, which resulted in a significantly increase in NECB by 2250 kg C ha−1 yr−1 (Ma et al., 2013). However, the increased
Conclusion
Based on a two-year field experiment in a typical rice–wheat cropping system in the Yangtze River Delta region of China, our results demonstrate that, compared with the local NF rate, optimizing the NF rate to 420 kg N ha−1 yr−1 could increase food security, and SOC storage by enhancing soil C input and decreasing C output, and decrease N pollution caused by Nr losses. Future studies should focus on the long-term effects of optimizing NF rate on these environmental benefits in the paddy
Declaration of competing interest
None.
Acknowledgements
The study was financially supported by the National Key R & D Program of China (2017YFD0200100), the National Science Foundation of China (41425005 and 41471238) and the Humboldt Postdoctoral Research Fellowship. We gratefully acknowledge the technical assistance provided by the Changshu Agroecological Experimental Station of the Chinese Academy of Sciences.
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