Effect of root litter quality on carbon turnover and soil microbiology in topsoil and subsoil horizons

Summary

Deployment of deep-rooted crops is suggested to contribute to climate change mitigation by stimulating soil organic carbon (SOC) storage in deep soil layers where turnover of SOC is generally slower than in topsoil. Yet, there are few systematic studies on underlying mechanisms and how to optimize SOC content through deep-rooted crops. The aims of this study were to investigate the role of root chemical composition on carbon (C) mineralization dynamics in topsoil (20 cm) and subsoil horizons (60 and 300 cm) of a cultivated sandy loam, and to test the influence of root and soluble C input on soil microbial and enzymatic activities in the different horizons. We firstly analyzed the chemical properties of root fractions from divergent plant species and selected specific samples for an incubation study to measure the turnover of root-derived C in topsoil and subsoils under controlled laboratory conditions at 20 °C during an incubation period of 20 weeks. During the incubation period, soil respiration (CO2 production rate), enzyme activity (β-glucosidase) and C substrate utilization (MicroRespTM) were measured. Our results substantiated that root-derived C mineralization was significantly higher in topsoil than subsoil horizons. However, notable C mineralization also occurred in the subsoil horizons after a lag period of 2-3 days. The initial C mineralization rate varied greatly among root materials in all three soil depths, not at least for CO2 production in subsoil horizons. This could be partly related to the root chemical quality, notably the nitrogen (N) content, which suggests that root N facilitated the microbial activity. Overall, the addition of root materials increased enzyme activities in all three soil depths after incubation for 1, 5 and 20 weeks compared to the initial stage of 2 h after incubation. When the response was tested to added soluble C substrates, i.e., glucose, N-acetyl-D-glucosamine (NADG) and vanillin, it was found that turn-over of at least glucose and NADG was stimulated in the subsoil horizons after longer time incubation with root materials. Specifically, the turnover dynamics of the N-containing substrate NADG supported that the C mineralization potential in deep soil horizons may generally be limited by low soil N content.