31-year contrasting agricultural managements affect the distribution of organic carbon in aggregate-sized fractions of a Mollisol

Abstract

Evaluation of soil organic carbon (SOC) dynamics is often limited by the complexity of soil matrix. Quantitative information on the distribution of SOC within aggregate hierarchy will help elucidate the carbon flow in soil matrix. However, this knowledge still needs to be documented. Soils were sampled from a surface Mollisol with plots under 100 years of continuous cropping, 31 years of simulated overgrazing to severely degraded bareland, and grassland restoration from cropped soil. A combined density and chemical fractionation procedure within water-stable aggregate was utilized to quantify the distribution of OC after long-term different land use patterns. Results showed that grassland significantly increased total SOC and mean aggregate associated OC compared to initial soil in 1985 with total SOC (g kg −1 soil) from 46.1 to 31.7 and mean aggregate associated OC (g kg −1 aggregate) from 31.6 to 44.7. Converting cropland to grassland also enhanced the formation of macroaggregates (>0.25 mm) (from 34.7% to 52.2%) and increased the OC concentrations in density and humic fractions by 48.3%-75.9% within aggregates. But the proportions of OC in density and humic fractions to SOC only increased in macroaggregates in grassland. Alternatively, converting cropland to bareland caused substantial depletion of total SOC, macroaggregates and their associated OC concentrations. The SOC (g kg −1 soil) and mean aggregate associated OC (g kg −1 aggregate) significantly decreased from 31.7 to 25.7 and from 31.6 to 26.2, respectively. While the OC concentration of density and humic fractions within aggregates in bareland did not show significant decreases. Principal component analysis demonstrated that the soils were developed by contrasting land use changes, with the grassland soil being more associated with labile OC fractions within macroaggregats and bareland soil more associated with recalcitrant OC fractions within microaggregates and silt-clay units. These findings highlighted the favorable preservation of plant-derived carbon within soil aggregates, particularly in the labile OC fractions within macroaggregates under high plant inputs with 31 years of grassland conversion. For the cropland and bareland soils without organic inputs, more OC was stabilized within fine aggregates via organo-mineral interactions, tending to be more recalcitrant. The processes governing the formation, transformation, and stabilization of soil organic carbon (SOC) are closely tied to soil aggregate architecture 1. Soil aggregates can physically protect SOC against decomposition and store SOC crucial to climate change mitigation 2. Also, the location of organic carbon within aggregate is a key factor for the stabilization of SOC 3 due to spatial arrangement of soil constituents and soil microbes 1,4. One important but poorly understood aspect of soil organic matter (SOM) decomposition and stabilization is the role of soil aggregates. Soil aggregates controlled the degree of hierarchical inaccessibility of SOC to decomposing organisms or catalyt...

Long‐term grassland restoration exerts stronger impacts on the vertical distribution of labile over recalcitrant organic carbon fractions in Mollisols

“…Moreover, soil tillage might also influence SOC changes. The cessation of tillage disturbance could affect soil aggregate‐protected carbon, nutrients availability, and microclimate, thus preventing SOM decomposition (M. Sheng et al., 2020; Six et al., 2000).…”

“…However, a recent study found that SOC in subsoil was more susceptible to land use and climate changes than the topsoil (Luo et al., 2020). Moreover, there is growing evidence that SOC dynamics in the subsoil were also affected by land use changes (Poeplau & Don, 2013; M. Sheng et al., 2020), while comprehensively studies regarding the response of SOC and its fractions in subsoil to land use changes remain scarce.…”

“…Soil carbon fractionation protocol can help unveil subtle changes in specific compartments of soil matrix where the overall effect may be hidden when bulk soil is considered (Duval et al., 2018; M. Sheng et al., 2020). Different physical or chemical fractions of SOC represent different source and stability of SOC under different land use patterns (Li et al., 2020).…”

“…The OC fractions with different availability and stability varied with land use regimes. Higher OC contents within macroaggregates (M. Sheng et al., 2020), humic fractions (Hurisso et al., 2013; Kotzé et al., 2016), organo‐mineral complex (Hou et al., 2010), and microbial residues (Ding et al., 2017) were generally found in perennial grassland or forest soils compared with cropland. However, to date, only a few studies have focused on the effects of long‐term contrasting land use patterns on the profile allocation of different OC fractions.…”

“…The soil degradation inevitably threatened the national food security. Natural restorations, such as conversion cropland into grassland or forestland, were proven to be effective measures in improving carbon stock and soil quality in the surface layer in this region (Ding et al., 2017; Hou et al., 2010; M. Sheng et al., 2020). A deeper investigation of OC distribution in whole‐soil profile after introduction of natural perennials and forest from cropping system needs to be carried out.…”