Soil organic carbon (SOC) sequestration is critical for agriculture and the environment, particularly in soil health and food security (Lal, 2004). SOC stocks are governed by the balance between carbon (C) input and output, and strongly affected by soil management practices (Paustian et al. 2000). Adoption of plant residue retention methods are essential to maintain, or improve SOC content and the sustainability of agricultural systems (Lal 2004; Diacono et al. 2010). However, SOC dynamics induced by plant residue retention generally occurs slowly since the total SOC pool is too large to be affected in the short-term (Salinas-Garcia et al. 1997; Ding et al. 2012). In this context, SOC fractions with relatively higher turnover rates and/or reactivity can be used to quantify the effect of agricultural management on soil quality (Silveira et al. 2008). Specifically, soluble organic C and microbial biomass C (MBC) fractions respond more quickly to soil management activities than other C pools (Nieder et al. 2008; Lehmann and Kleber 2015).
Plant residues provide available substrate for soil microorganisms (Shahbaz et al. 2017a), and in turn play an important role in microbe-mediated biogeochemical processes (Ge et al. 2015; Haubensak et al. 2002). Plant residue can be preferentially utilized by microorganisms for biomass production during microbial growth processes (Liang et al., 2017). Microorganisms solubilize plant-derived C through depolymerization by extracellular enzymes, and any net increase in the soluble C fraction is driven by microbial death, exudation, or a decrease in microbial assimilation (Burns et al. 2013). Generally, the dynamics of microbial utilization of residue C and stabilization, for example, relative abundances of soluble organic C and MBC, are mainly mediated by plant residue quality (Chen et al. 2009; Prescott 2010; Liang et al., 2017). In the traditional view, root residue is relatively recalcitrant, and has a slower decomposition rate, thus more root C is incorporated into the stable SOC fractions than that from stem and leaf residues (Lian et al. 2016; Johnson et al. 2014). However, a prevailing counter hypothesis proposes that labile C compounds can contribute more to long-term soil C stability than highly recalcitrant chemical compounds such as lignin (Amelung et al., 2008; Schmidt et al., 2011; Lehmann and Kleber 2015). This is because above-ground residues (e.g. stem and leaf residues) contain more easily degradable C, which plays a significant role in labile organic C accumulation, and subsequently, may contribute more to SOC sequestration due to high microbial utilization efficiency (Don and Kalbitz 2005; Cotrufo et al. 2013, 2015). These new theories challenge our understanding of the mechanisms of how root vs. aboveground residues is involved in SOC formation. Therefore, given the important role of labile C in the SOC accumulation, it is necessary to strengthen our knowledge of the impacts of plant residue type on the microbial assimilation of exogenous C and the dynamics of soil labile C, i.e., soluble organic C and MBC.
Soil fertilization and total SOC are important factors controlling microbial assimilation of plant residue (Wang et al. 2014; Zhu et al. 2016; Marschner et al. 2001). The dynamics and distribution of residue C in soil depends upon soil fertility under long-term soil fertilization regimes because more residue C tends to accumulate in MBC in low fertility soil compared with high fertility soil (An et al. 2015a). However, the combined applications of plant residues (leaf, stems and roots) in soils may differentially affect proportional labile organic C accumulation, resulting in differences in residue C accumulation in the total SOC pool of the soils with different levels of SOC (Lian et al. 2016). Because microbial competition for residue C depends upon initial soil properties (An et al. 2015a), the application of organic fertilizers, which enhance soil fertility (Macci et al. 2013; Jin et al. 2018), is expected to affect microbial utilization of plant residues. In general, the addition of residue C to soil enhances soil labile organic pools (An et al. 2015a), but the additions of different quality residues in soils with different levels of SOC and nutrients determine its accumulation (Singh et al. 2007; Fang et al. 2018). The effect of soil fertilization on the incorporation of different types of maize residues into labile C pools has received far less attention.
The objectives of our research were: (1) to quantify the contribution of different types of maize residue to soluble organic C, measured as extractable organic C (EOC), and MBC and (2) to determine the differences in distribution and utilization of residue C in long-term soil fertilization among residue types. We added the 13C-labeled maize residues (leaf, stem and root) to unfertilized and organic-fertilized soils and incubated the soils for 360 days. The percentages of different residue C in EOC and MBC fractions were determined. The incubation study was designed to test the following hypotheses: (1) plant residue addition would increase the labile organic C, with its magnitude depending on the residue types, i.e., more aboveground residue C would be incorporated in labile organic C than root residue C; (2) soil fertilization would regulate the distribution of plant residue C in EOC and MBC, and the greater distribution of residue C in labile organic C would be in unfertilized soil because of C deficiency than organic-fertilized soil.