Soil organic matter priming and carbon balance after straw addition is regulated by long-term fertilization
Introduction
Soil organic carbon (SOC) is of fundamental importance to soil quality and fertility. Sustainable agricultural practices improve soil quality, ensure food security, and contribute to climate change mitigation through increasing SOC sequestration (Schmidt et al., 2011). Of the 3.8 Pg of crop straw (aboveground biomass) produced annually worldwide as agricultural byproducts, appropriate management of straw has important implications for increased soil nutrient availability, sustainable agricultural productivity, and our ability to minimize environmental pollution (LaI, 2005; Zhang et al., 2017). Straw incorporation into the soil is generally recommended for C sequestration in agroecosystems (Freibauer et al., 2004; Lu et al., 2009; Liu et al., 2014). However, straw addition may accelerate SOC mineralization and exert negligible or even negative impacts on soil C sequestration (Fontaine et al., 2004; Wang et al., 2011; Shahbaz et al., 2017a, 2017b). Despite numerous studies on the response of soil C dynamics to straw addition, the understanding of how C is actually sequestrated in the soil remains elusive, thus a thorough examination is warranted before the widespread implementation of straw application (Fontaine et al., 2003; Shahbaz et al., 2017a; Fang et al., 2018; Li et al., 2018).
Crop straw, containing large portions of labile C, is an essential source of C and energy for microorganisms (Qiu et al., 2016; Schmatz et al., 2017). Straw incorporation into the soil stimulates microbial activity via nutrient mining, thus accelerating SOC decomposition and causing a positive priming effect (PE) (Fontaine et al., 2003; Bol et al., 2010; Kuzyakov, 2010). The accelerated SOC decomposition may partially offset or outweigh the overall positive effects of straw addition on soil C sequestration, resulting in small net gains or even net losses in soil C storage (Fontaine et al., 2004; Sayer et al., 2011; Schmatz et al., 2017; Yemadje et al., 2017; Shahbaz et al., 2018). Alternatively, straw addition could also prompt microorganisms to switch their energy and C source from poorly degradable SOC to the easily decomposable straw C, thus retarding SOC mineralization and resulting in a negative PE (Guenet et al., 2010; Shahbaz et al., 2017b). The true potential of soil C sequestration largely depends on the final balance between accelerated SOC mineralization and the precise rate of newly formed SOC derived from the added C (Fontaine et al., 2003). The PE intensity and fate of straw C should, therefore, be considered simultaneously when investigating soil C sequestration in response to straw addition.
SOC turnover and straw mineralization are regulated by multiple factors, with mineral and organic fertilization playing a crucial role (Fontaine et al., 2011; Chen et al., 2014; Ghafoor et al., 2017; Meng et al., 2017). For example, soil nutrients replenished by fertilization influence the net effects of straw incorporation on soil C sequestration (Kirkby et al., 2014; Qiu et al., 2016). Fertilization itself also modulates microbial activity and the subsequent SOC mineralization response to straw incorporation (Neff et al., 2002; Meng et al., 2017). Most previous studies have investigated the short-term fertilization impacts (i.e., single addition of nutrients) on SOC turnover in response to straw addition and the fate of added straw C (Meng et al., 2017; Fang et al., 2018). However, short-term fertilization studies are not real representation of cropland management practices, as fertilizers are supplied over decades and at least once on an annual basis. The C sequestration in short-term and long-term fertilized soils may respond differently to straw incorporation (Neff et al., 2002; Qiu et al., 2016; Meng et al., 2017). Long-term application of mineral and/or organic fertilizers does not only change the available nutrient contents, but may also alter the soil physicochemical properties (Zamanian et al., 2018) and lead to shifts in the composition and diversity of microbial communities (Mäder et al., 2002; Marschner et al., 2003; Fontaine et al., 2011; Li et al., 2018). Soil properties, including the chemical traits of SOC and soil pH, affect SOC mineralization and straw C turnover via altered microbial community structure and activity (Aye et al., 2017; Lian et al., 2017). Microbial community composition and enzyme activities mediate SOC turnover and the transformation of added straw C into SOC, thereby regulating the soil C sequestration response to straw addition (Bell et al., 2003; Blagodatskaya and Kuzyakov, 2008; Lehmann and Kleber, 2015). However, the existing SOC mineralization in response to straw incorporation and the fate of straw C in soil subjected to long-term mineral and organic fertilization remain unclear.
In the present study, we assessed the impacts of straw addition in agricultural soil subjected to long-term mineral and organic fertilization regimes in order to: (i) quantify the direction and intensity of PE in response to straw addition; (ii) evaluate the impacts of mineral and organic fertilization on the fate of straw C after its addition; and (iii) provide new insights into the soil C sequestration response to straw addition. We hypothesized that fertilization would lower PE intensity (because of the well balanced C: N ratio in soil) and increase the incorporation of straw C into SOC, thereby resulting in an overall increase in soil C sequestration.
Section snippets
Site description, soil sampling and straw preparation
The soil used in this study was sampled from a permanent maize (Zea mays L.) cultivation field subjected to long-term fertilization at the Institute of Red Soil in Jinxian County, Jiangxi Province, China (28°35′N, 116°17′E). This region has a typical subtropical monsoon climate, with a mean annual temperature of 18.1 °C and rainfall of 1537 mm. The soil of the sampling site, derived from Quaternary red material, is a Ferralic Cambisol according to the FAO classification. The texture of the soil
Soil properties
Relative to the unfertilized soil, the contents of SOC, total N, NO3−, total P, available P, and MBC increased in the soil subjected to mineral fertilization (NPK and 2 × NPK) and further increased in the M and NPKM soils (Table 1). Mineral fertilization decreased the soil pH from 5.2 in the unfertilized soil to 4.6 in NPK and 4.5 in 2 × NPK. Manure application markedly alleviated soil acidification (pH of 6.2 for M and 5.9 for NPKM).
Soil CO2 efflux and priming effect
Total CO2 efflux from the soil, being initially high after
The priming of SOC mineralization
Straw addition accelerated SOC mineralization and resulted in positive PE in all treatments, primarily because of increased microbial biomass and β-glucosidase and cellobiohydrolase activities (Fig. 2a, c and d; Fig. 4a) (Fontaine et al., 2003). Soil fertilized with 2 × NPK increased PE, while manure application lowered PE, as compared to the unfertilized soil, thus highlighting the importance of fertilization in controlling the PE intensity (Fig. S1d).
The significant relationship between
Conclusions
Straw addition largely accelerated SOC mineralization and resulted in positive PE, primarily because of increased microbial biomass and β-glucosidase and cellobiohydrolase activities. The magnitude of straw-induced PE was higher in 2 × NPK but lower in M relative to the unfertilized soil, emphasizing the importance of fertilization in controlling the PE intensity. Compared to the unfertilized soil, fertilization considerably increased straw-derived MBC and straw-derived SOC, primarily due to
Acknowledgments
The authors are grateful to Mr Yijun Zhong and Huicai Ye from the Institute of Red Soil who helped us with soil sampling, and to Dr. Kamlesh Jangid for his comments on the structure of the manuscript. This research was financially supported by the Fundamental Research Fund for the Central Universities (2662016PY098), the National Science Foundation of China (Grant Number: 41671253), the Fundamental Research Fund for Central Non-profit Scientific Institution (No. 1610132019013), and the
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