Elsevier

Geoderma Regional

Volume 23, December 2020, e00348
Geoderma Regional

Litter decomposition and soil organic carbon stabilization in a Kastanozem of Saskatchewan, Canada

https://doi.org/10.1016/j.geodrs.2020.e00348Get rights and content

Highlights

  • Efficiency of litter stabilized as soil organic C is high in the Kastanozem soil

  • Cold climate limits litter decomposition in the Kastanozem soil

  • Heavy fraction (>1.60 g cm−3) stabilize14% of litter C input after 4-year incubation

  • Lipid enrichment and physical protection of light fraction stabilize 7% of litter C input

  • Protection in resistant heavy fraction contribute to high stabilization efficiency of litter

Abstract

Soil organic C (SOC) accumulation is known to increase by practices of increasing plant residue C inputs, while the SOC stabilization efficiency relative to C inputs is variable. The SOC stabilization efficiency in a Kastanozem soil (up to 21% of C inputs) is much greater than the global average (0.7%). To test whether litter is preserved by transformation into the mineral-associated SOC pool in a Kastanozem soil, we conducted a four-year incubation study of 13C-labeled maize residue in Saskatchewan, Canada. We monitored whether litter residue inputs are transformed and stabilized into the mineral-associated heavy fraction (>1.60 g cm−3). The litter decomposition rates in our study were the lowest among the global dataset due to the limited microbial activities under cold and arid climate. Fractionation of SOC into light and heavy fractions (< or >1.60 g cm−3, respectively) and subsequent analysis using nuclear magnetic resonance spectroscopy indicated that both the light and heavy fraction pools of transformed maize litter were enriched by microbial lipids, along with aromatic and carboxylic groups. The heavy fraction exhibited the lower decomposition rates than the light fraction due to selective loss of cellulose. This leads to the greater SOC stabilization in the heavy fraction (14% of the litter input) after the 4-year incubation period, compared to the light fraction (7%). Global temperature-dependency of microbial activities can account for slow litter decomposition under semi-arid and cold prairie, while the high stabilization efficiency of the litter-derived C is supported by microbial transformation and the relatively high capacity of mineral association in Kastanozem soils.

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.

References (32)

  • C.A. Campbell et al.

    Quantifying short-term effects of crop rotations on soil organic carbon in southwestern Saskatchewan

    Can J Soil Sci

    (2000)
  • C.A. Campbell et al.

    Quantifying carbon sequestration in a conventionally tilled crop rotation study in southwestern Saskatchewan

    Can J Soil Sci

    (2007)
  • K. Fujii et al.

    Effects of land use change on turnover and storage of soil organic matter in a tropical forest

    Plant and Soil

    (2020)
  • K. Fujii et al.

    A comparison of lignin-degrading enzyme activities in forest floor layers across a global climatic gradient

    Soil Ecol Lett

    (2020)
  • A. Golchin et al.

    A Model Linking Organic Matter Decomposition, Chemistry, and Aggregate Dynamics

    (1997)
  • E.G. Gregorich et al.

    Turnover of soil organic matter and storage of corn residue carbon estimated from natural 13C abundance

    Can J Soil Sci

    (1995)
  • View full text