Long-term stabilization of crop residues and soil organic carbon affected by residue quality and initial soil pH
Graphical abstract
Introduction
Retention of crop residue has been increasingly recognized as an essential strategy to enhance soil organic carbon (SOC) storage and compensate for SOC losses due to decomposition (Wang et al., 2016, Whitbread et al., 2003). In the context of elevated atmospheric CO2 and global warming, the potential of soil to sequester the residue-derived carbon (C) has received intensive attention (Blair et al., 2005, Gentile et al., 2011). Increases in SOC content through increased C input from crop residue retention have been demonstrated (Dolan et al., 2006, Whitbread et al., 2000). Given that about 55% to 70% of residue C could have been mineralized and released as CO2 within one year after addition (Johnson et al., 2007, Zech et al., 1997), residue-derived C contributes significantly (~ 50%) to the total CO2 emitted from terrestrial systems (Schlesinger and Andrews, 2000). Residue management practices should aim to enhance long-term SOC sequestration and minimize the adverse impact on global climate change.
The rate of residue decomposition is influenced by residue quality as defined by C/N ratio, lignin and polyphenol concentrations. Generally, plant residues with higher C/N ratios, such as cereal crop residues, decompose more slowly than residues with lower C/N ratios such as those derived from legumes (Butterly et al., 2013, Hirobe et al., 2004, Teklay and Malmer, 2004). Significant correlations have been detected between residue N content and loss of residue biomass (Heal et al., 1997, Trinsoutrot et al., 2000). Residues with high contents of lignin and other polyphenol compounds that are considered resistant to microbial degradation can persist in the soil for years (Heal et al., 1997, Marschner et al., 2008). As decomposition proceeds, the quality of the remaining residue declines due to the accumulation of these refractory C (Baldock et al., 1992, Baldock et al., 1997). Early studies suggest that it is N content and then the amount of lignin and polyphenol that controls the early and late stages of decomposition, respectively (Taylor et al., 1989, Tian et al., 1992).
While the effect of residue quality on its decomposition has been extensively studied, little is known about the nature of the residue C stabilized in the soil. Lignin and other polyphenol compounds are believed to be major precursors of stable SOC (Stevenson, 1994). Nevertheless, other studies have revealed that labile C compounds such as polysaccharides and N-containing compounds can be long-preserved due to physical protection through either aggregation (Blanco-Canqui and Lal, 2004, Six et al., 1999) or adsorption by clay minerals (Christensen, 2001, Hassink, 1997). The intrinsic property of residue shows little correlation to the decomposition rates of organic C associated with clay particles, suggesting that physical protection or the interaction with soil minerals determines the SOC stability over long periods of time (Heim and Schmidt, 2007, Schmidt et al., 2011). The major SOC stabilization mechanisms could vary among soils with different texture, however, the long-term fate of different crop residues in sandy soils with little physical protection remains largely unknown.
Apart from soil texture, the decomposition of crop residue may also be affected by other soil properties such as pH. It has been well recognized that soil pH can impact on the decomposition of SOC by directly affecting its solubility or indirectly by influencing microbial growth, activity and community structure (Pietri and Brookes, 2009, Wang et al., 2016). Microbial growth and activity are sensitive to change in soil pH, especially in the pH range of 4.3 to 5.3 (Kemmitt et al., 2006, Pietri and Brookes, 2008). Additionally, the incorporation of crop residues per se can affect soil pH, which in turn may affect their decomposition. The increase in soil pH upon the addition of crop residues (liming effect) is mainly attributed to the decomposition of organic anions within the residues (Butterly et al., 2013, Tang and Yu, 1999). So far, the interaction between soil pH and decomposition of different residues on soil C dynamics has been seldom studied.
This study aimed to examine the effect of initial soil pH and residue type on the decomposition and stability of three different crop residues in two sandy soils. Changes in different SOC pools and chemical composition of SOC in residue-amended soils with time were quantified by mid-infrared (MIR) and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, respectively. We hypothesised that 1) the decomposition rate of crop residues would be slower in soil with lower initial soil pH, and 2) long-term stabilization of organic C could be affected by the quality of residues.
Section snippets
Field column experiment
A column experiment was conducted under field conditions. It consisted of two soils and four residue treatments with three replicates for each destructive harvest. The two soils were a Podosol and a Tenosol (Isbell, 1996) or Orsteinic Podzol and Chromic Eutric Cambisol (WRB, 2014) collected at a depth of 0–20 cm from Frankston (38°14′S, 145°22′E) and Sepparton (36°28′S, 145°36′E),Victoria, Australia, respectively. They had a similar texture (> 80% sand) and low SOC content (< 3.0 mg g− 1), but
Soil pH
Soil pH in the 0–10 cm depth layer of the Podzol increased by 0.36 and 0.47 units (P < 0.01) following canola- and chickpea-residue amendments, respectively (Fig. 2a), compared with the non-residue control. Incorporation of canola and chickpea residues also increased soil pH of the Cambisol by 0.18 and 0.31 units, respectively, at 48 months (P < 0.05) (Fig. 2b). The pH increase was also evident in the 10–20 cm layers (P < 0.05). The addition of wheat residue had little impact on soil pH, relative to
Effect of residue amendment on change in soil pH
Chickpea and canola residues were more effective in ameliorating topsoil acidity compared to wheat residues, and this effect persisted for at least 48 months. This was attributed to the relatively high alkalinity generated from the decomposition of chickpea and canola residues during the first 3 months, as reported by Butterly et al. (2013). The relationship between ash alkalinity content in residues and their liming potential had been well established (Butterly et al., 2011, Noble et al., 1996,
Conclusions
The decomposition of crop residues and their priming effects largely depended on residue type, initial soil pH and their interactions. Microbial decomposition of crop residue at the initial stage (9 months) was greatly retarded by poor residue quality (high C:N ratio) and lower initial soil pH. Long-term SOC sequestration in the Cambisol was less affected by residue quality. Efforts to increase total SOC content in soils with higher initial pH should rely on increases in total C input. The
Acknowledgments
This research was supported under Australian Research Council's Discovery Projects funding scheme (project DP120104100). We thank Dr Gary Clark for his comments on the manuscript.
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