Soil organic matter priming and carbon balance after straw addition is regulated by long-term fertilization

Highlights

• Fertilization regulated the intensity of priming effect induced by straw addition.

• Fertilization increased straw C assimilated by microbial biomass and straw-derived SOC.

• Straw-derived SOC was closely correlated with straw-derived microbial biomass C.

• NPK fertilization decreased priming effect intensity and increased straw-derived SOC.

• NPK fertilization facilitates the SOC sequestration after straw addition.

Abstract

Straw incorporation is crucial to soil organic carbon (SOC) sequestration, thus improving soil fertility and mitigating climate change. The fate of straw C and the associated net SOC balance remain largely unexplored, particularly in soils subjected to long-term mineral and organic fertilization. To address this, soil (δ¹³C: –19‰) that had been continuously cropped with maize for 31 years and subjected to five long-term fertilization regimes, including (i) control (Unfertilized), (ii) mineral fertilizer (NPK) application, (iii) 200% NPK (2 × NPK) application, (iv) manure (M) application, and (v) NPK plus manure (NPKM) application, was incubated with or without addition of rice straw (δ¹³C: –29‰) for 70 days. Straw addition largely primed SOC mineralization. The priming effect (PE) was considerably higher in 2 × NPK (+122% of CO₂ from soil without straw addition) but lower in M (+43%) relative to the unfertilized soil (+82%), highlighting the importance of fertilization in controlling PE intensity. Fertilization increased the straw-derived microbial biomass C by 90–577% and straw-derived SOC by 34–68% compared to the unfertilized soil, primarily due to the increased abundance of Gram-negative bacteria and cellobiohydrolase activity. Straw-derived SOC was strongly positively correlated with straw-derived microbial biomass C, suggesting that dead microbial biomass (necromass) was a dominant precursor of SOC formation. Consequently, fertilization facilitated microbial utilization of straw C and its retention in soil, particularly in the M and NPKM fertilized soils. The amounts of straw-derived SOC overcompensated for the SOC losses by mineralization, resulting in net C sequestration which was highest in the NPK fertilized soil. Our study emphasizes that NPK fertilization decreases the intensity of the PE induced by straw addition and increases straw C incorporation into SOC, thus facilitating C sequestration in agricultural soils.

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.