Root litter chemistry and soil nutrient availability affect subsoil carbon turnover

Carbon (C) turnover in subsoil was studied by Zhi Liang, PhD student of the Deep Frontier project. The results presents a framework for enhanced subsoil C stock through a deep-rooted cropping system as related to root nitrogen (N) and lignin contents, soil N availability, and microbial activity.

Photo: Excavated soil profile under cultivated grassland

Key research questions of the study of Zhi Liang relate to:

  • What is the role of root chemical composition on regulating C mineralization in topsoil and subsoil layers?
  • What is the essential factor(s) controlling C turnover in subsoil?

Results showed that root N and lignin content were important traits affecting microbial activity and eventual fate of organic C. Moreover, subsoil N availability affected microbial mineralization of fresh C inputs.

Deployment of deep-rooted crops in agricultural ecosystems has been suggested as a way of stimulating C sequestration and mitigating climate change by the plant uptake of carbon dioxide and transfer into roots and exudates, and thence into subsoil layers, where eventual C stabilization may be more operative than in the topsoil. However, there is a general lack of studies on how to increase soil organic C content through deep-rooted crops. The overall aim was to study the C sequestration potential from deep-rooted crops in subsoils as compared to topsoil.

“If we could find some key root traits and subsoil characteristics regulating the net root-induced mineralization, then we might be able to adapt crops and soil management practices which are beneficial for enhancing C sequestration,” Zhi Liang explains. 

Root litter chemistry and C mineralization in topsoil and subsoil

As part of her PhD project, Zhi Liang examined the effect of root litter chemistry on net C mineralization in topsoil and subsoil horizons. First, the chemical composition of roots isolated from seven plant species was determined and root samples with divergent chemical compositions were used for an incubation study to measure the root-induced C mineralization in topsoil from 20 cm depth and subsoils from 60 and 300 cm depth.

During an incubation period of 20 weeks, C mineralization was measured as cumulative root induced C loss. Also, β-glucosidase activity and C source utilization were determined (Paper I). Results showed that in topsoil, net C losses were strongly correlated to root lignin concentration, whereas in subsoils net C losses were more dependent on root N concentration, yet with increasing dependence on lignin concentration over time. This suggested that chemically recalcitrant substances in root materials could contribute to higher C retention, and moreover revealed than available N limited C mineralization in subsoils. 

Nutrient availability and C mineralization in topsoil and subsoil

For testing the hypothesis of “subsoil N limitation”, Zhi Liang conducted another incubation study where added glucose was microbially decomposed under different N, phosphorus and sulphur availabilities in topsoil (20 cm) and subsoils (60, 100, and 300 cm) during an incubation period of six weeks (Paper II). Results showed that in topsoil glucose was always completely decomposed, whereas in subsoils glucose decomposition depended on the presence or absence of added N. Specially, when subsoils were incubated with glucose in the absence of N, 59–92% of the added glucose was recovered after six weeks of incubation.

“These results suggest that selection of crop species and nutrient management regime could affect C sequestration in subsoil horizons, potentially offering management options for the sequestration of C from deep-rooted crops. This could be based on combined plant traits such as efficient nutrient uptake and chemically recalcitrant root fractions, at least for facilitating enhanced C storage of root-derived C fractions,”  Zhi Liang explains.

Please see following publications for details:

Paper I

Liang, Z., Elsgaard, L., Nicolaisen, M.H., Lyhne-Kjærbye, A., Olesen, J.E., 2018. Carbon mineralization and microbial activity in agricultural topsoil and subsoil as regulated by root nitrogen and recalcitrant carbon concentrations. Plant Soil 433, 65-82.

Paper II

Liang, Z., Olesen, J.E., Jensen, J.L., Elsgaard, L., 2019. Nutrient availability affects carbon turnover and microbial physiology differently in topsoil and subsoil under a temperate grassland. Geoderma 336, 22-30.