Impacts of nitrogen management and organic matter application on nitrous oxide emissions and soil organic carbon from spring maize fields in the North China Plain
Introduction
Agriculture plays an important role in not only food production but also emissions of greenhouse gases that contribute to climate warming. Agricultural soils have been proposed to represent a significant potential sink for atmospheric carbon dioxide (CO2) through soil organic carbon (SOC) sequestration (Lal, 2004). The global potential of C sequestration in agricultural soils was estimated to be 1.2 to 3.1 Pg (1015 g) C year−1 (Lal, 2011). In addition, agricultural soils are an important source of atmospheric nitrous oxide (N2O) that significantly contributes to global warming due to its high global warming potential (298 times that of CO2 at a time horizon of 100 years) (IPCC, 2013) and its ability to deteriorate the atmospheric environment by depleting the stratospheric ozone layer (Ravishankara et al., 2009). Globally, N2O released from agriculture is approximately 4.1 Tg (1012 g) N year−1 (Syakila and Kroeze, 2011; IPCC, 2013), which is largely attributable to the use of synthetic N fertilizers and organic manure (Davidson, 2009).
Both SOC changes and soil N2O emissions are regulated by numerous environmental factors (such as climate, soil physical properties, and availability of mineral N) and farming management practices (FMPs) (Smith et al., 2007; Butterbach-Bahl et al., 2013; Luo et al., 2017). Changing rates of SOC and N2O emissions are extremely variable in time and space due to the variability in these regulators (Bouwman et al., 2002; Stockmann et al., 2015). Furthermore, close relationships exist among SOC stocks, soil fertility, crop productivity, and N2O emissions (e.g., Li et al., 2005; Cha-un et al., 2017). Therefore, it is necessary to consider crop productivity, SOC stocks, and N2O emissions when developing optimum options for sustainable agriculture. However it is often difficult to observe these variables and include multiple treatments with different FMPs in field studies.
The North China Plain (NCP) is one of the most important agricultural regions in China. The cultivated land area in the NCP is approximately 18 million hectares, which accounts for approximately 19% of the nation's total agricultural area (Liu et al., 2001), and this area supplies more than 75% and 32% of the nation's total wheat and maize, respectively (China Statistics Bureau, 2011). FMPs applied for maize production are generally characterized as intensive in this region, and standard tillage operations and excessive N fertilizer application are common on most croplands (Ju et al., 2009; Cui et al., 2010; Tian et al., 2017). For example, on-farm investigations have indicated that the average application rate of N fertilizers has been over 250 kg N ha−1 year−1 for the maize system in North China, a value that largely exceeded maize requirements (Cui et al., 2010). In addition to applications of synthetic fertilizers, organic matter could be applied to maize systems in the NCP to improve soil fertility and reduce GHG emissions (Han et al., 2018).
To quantify GHG emissions from intensive maize production in the NCP and to develop FMPs to effectively mitigate GHG emissions, a number of field studies have been performed (e.g., Gong et al., 2009; Liu et al., 2011; Liang et al., 2012; Hu et al., 2013; Dikgwatlhe et al., 2014; Yang et al., 2015). Most of these studies, however, have been limited in their abilities to provide knowledge regarding the comprehensive impacts of alternative FMPs on net GHG emissions because they focused on only SOC changes or N2O emissions. For example, Liang et al. (2012) quantified the impacts of applications of manure and inorganic fertilizers on SOC changes for a wheat and maize rotational system in the NCP. Although these practices have also been regarded as important strategies regulating N2O emissions, they did not observe N2O emissions (e.g., Zhou et al., 2017). Hu et al. (2013) investigated optimum practices of managing manure and synthetic N fertilizers for a wheat and maize rotational system by considering CH4 uptake and N2O emissions, but did not include SOC changes in their analysis. In addition, side-by-side comparisons quantifying the efficiency of diverse management strategies for mitigating GHG emissions are still lacking in the NCP because limited treatments were often included in previous studies (e.g., Zhang et al., 2011; Gao et al., 2014). Due to the scarcity of studies that simultaneously measure crop yield, SOC stock, and N2O emission under diverse FMPs, substantial uncertainty exists in the quantification and mitigation of GHG emissions from maize fields in the NCP.
In this study, we conducted a multi-year experiment that quantified the impacts of alternative N fertilization and organic matter management on SOC stocks and N2O emissions. Our objectives were a) to investigate the impacts of different FMPs on crop yields, SOC changes, and N2O emissions and the relationship of these variables with environmental factors, and b) to identify effective GHG mitigation strategies by considering maize productivity, soil C sequestration and N2O emissions.
Section snippets
Field site and treatments
The study was conducted at the Key Experimental Station on Ecological Environment of Yanshan Mountain in Qianxi County, Hebei Province, China (118° 18′03″E, 40° 12′07″N). The field site has a semi-humid continental monsoon climate with an annual mean air temperature of 10.1℃ and an average annual precipitation of 732 mm from 2012 to 2014. The soil of the site is a calcareous fluvo-aquic soil (calcareous Cambisols according to the FAO Classification). Soil properties were observed before the
Temperature, precipitation, and soil moisture
The study site experienced hot summers and cold winters. The daily minimum and maximum air temperatures varied from −19.7 to 24.1 °C and -−.2 to 35.5 °C, −20.0 to 27.1 °C and −7.9 to 37.8 °C, and −14.9 to 25.4 °C and −2.8 to 36.3 °C in 2012, 2013, and 2014, respectively (Fig. 1a). Daily mean air temperatures varied from −13.2 to 28.3 °C, −13.1 to 31.2 °C, and −8.3 to 30.1 °C in 2012, 2013, and 2014, respectively. The average soil temperatures were 17.8, 18.6, and 21.2 °C during the maize
Impacts of different fertilization on N2O emissions
Urea application was identified as the most important factor regulating N2O emissions from the studied cropping system. The seasonal cumulative N2O emissions under the CK and M treatments without urea application were significantly (P < 0.05) lower than those under the FP, SFP, and MFP treatments with the conventional urea application rate (Table 3). In addition, a positive linear relationship existed between the seasonal total N2O emissions and the urea application rate (P < 0.05, Fig. 4).
Conclusions
The impacts on maize yields, SOC changes, and N2O emissions from FMPs with different fertilization regimes and organic matter management practices were quantified based on a multi-year experiment performed on a spring maize field in the NCP. The field measurements demonstrated that urea application and returned maize straw exerted positive impacts on N2O emissions. The application of organic manure or returned maize straw enhanced SOC storage. The average net GHG emission from 2012 to 2014 was
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgments
We thank three anonymous reviewers for their constructive comments. This study was gratefully supported by (2016YFE0101100, 2016YFD0201204, 2017YFD0201801, 2017YFF0211701, and 2017YFF0211702) and the Natural Science Fund project of China (31770486, 31270486). This paper was also supported by the China Scholarship Council (Personnel Training Project of Agricultural Ecosystem Carbon and Nitrogen Gas Exchange and Mechanism of Nonpoint Source Pollution Control in Typical Watershed, No.: 2015-7169).
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