Litter decomposition and soil organic carbon stabilization in a Kastanozem of Saskatchewan, Canada
Introduction
Mitigating atmospheric CO2 levels through sequestered soil organic C (SOC) was the predicate underlying the “4 per 1000” Soils for Food Security and Climate initiative, given the capacity of soils for net accumulation of SOC to offset global fossil fuel-derived CO2 emissions (Minasny et al. 2017). Net SOC accumulation rates are directly affected by plant residue input levels and agricultural practices (Campbell et al. 2007; Minasny et al. 2017), which control the balance between plant litter inputs and subsequent decomposition. Litter decomposition is regulated by litter quality and climatic factors that affect microbial activity (Fujii et al. 2020b). The stability of litter-derived C in soil is also regulated by residue transformation into physically protected soil organic matter (SOM), as well as chemical reactions (i.e., pH and clay; Wagai et al. 2009; Fujii et al. 2020a).
Litter additions to soil occur either freely or through occlusion within soil aggregates (light fraction and SOM), or may be associated with clay minerals (heavy fraction; Wagai et al. 2009). It has been postulated that microbial necromass and metabolites, as well as litter degradation byproducts, can be precursors of SOM (Kallenbach et al. 2016). Stabilization of SOC in the heavy fraction is supported by physical and chemical associations between SOM and clay minerals (Mikutta et al. 2006), which is facilitated by polyvalent cation (e.g., Ca2+) bridges between SOM and clays in Chernozem or Kastanozem soils (Golchin et al. 1997).
Net SOC accumulation rates are approximately 0.7% of net primary production at a pedogenetic time scale (global average; Schlesinger and Bernhardt 2013). However, net SOC accumulation rates can increase with increasing C inputs. Especially, the higher SOC stabilization efficiency (up to 21%) has been reported from the Kastanozem agricultural soil over a 12-year moist period; (Campbell et al. 2007). However, a temporal increase in SOC stocks cannot contribute directly to long-term SOC sequestration when the increased SOC is composed of free SOM in the light fraction, which is easily accessible to microbes. We hypothesized that (1) the semi-arid and cool climate limits microbial activity and hence decomposition of the light SOC fraction compared to the global dataset, (2) mineralization of the mineral-associated heavy fraction is slower than the light SOC fraction due to incorporation into aggregates, as well as increased chemical recalcitrance (Helfrich et al. 2006), and (3) SOC is transformed and stabilized in the heavy fraction, rather than in the light SOC fraction. To test these hypotheses, we examined the stability and characteristics of 13C-labeled litter applied to a semi-arid temperate soil after 4 years, by assessing its decomposition and the transformation of litter-derived C into either light or heavy SOC fractions using nuclear magnetic resonance (NMR) spectroscopy.
Section snippets
Study location
The experiment was established at the Semiarid Prairie Agricultural Research Centre, located near Swift Current, Saskatchewan, Canada (13 U 304973E 5,570,828 N) in a field that had been cropped previously to a fallow–wheat rotation with minimal fertilizer addition since 1922. The Swinton Association field soil is an Orthic Brown Chernozem (Canadian System of Soil Classification) and Kastanozem (World Reference Base for Soil Resources), developed on about 55 cm of silty loess overlying loamy
Physicochemical and microbiological properties of the test soil
The soil pH was based on neutral and exchangeable bases dominated by Ca2+; however, negligible carbonate was found in the surface soil Ap horizon (Table 1). The light SOC fraction in the top 0–5 cm of the soil corresponded to about 36% of the bulk soil C throughout the incubation (Table 2). This value was higher than that in the subsurface soil (10%; Table 2). The litter addition contributed to 11% of the initial SOC in the light fraction of the amended soil; however, changes in the light SOC
Factors regulating litter decomposability and soil C sequestration rates
In a previous study, at adjacent field sites, net SOC accumulation corresponded to 8% to 21% of the SOC of residue-C inputs over a 12-year moist period (Campbell et al. 2007), which is higher than the global average (0.7%; Schlesinger and Bernhardt, 1997). This is partly explained by the semi-arid and cool climate that limits microbial activity, which can decompose litter (Gregorich et al., 2007). Comparison of the maize litter decomposition rate constants (derived using single exponential
Conclusion
The relatively high SOC sequestration rates observed in a Kastanozem soil compared to the global dataset are supported by the lower decomposition rates under the cold climate. Four years after maize litter addition, litter-derived SOC was transformed and stabilized in the heavy SOC fraction (14% of the plant residue-C inputs), rather than in the light SOC fraction (7%). The litter-derived C remaining in the light fraction SOC was enriched in lipids, along with aromatic and carboxylic groups.
Declaration of Competing Interest
None.
Acknowledgments
The authors wish to thank the members of the Semiarid Prairie Agricultural Research Centre for assistance with soil sampling. We also thank the reviewers for their time and for providing critical feedback to improve the manuscript.
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