Organic carbon mineralization and microbiology in subsoil

Summary in the original language of the document

Deployment of deep-rooted crops in agricultural ecosystems has been suggested as a way of mitigating climate change by improving organic carbon (OC) inputs to the subsoil where carbon (C) sequestration may be more operative than in the topsoil. Yet, there are few systematic studies on how to optimize soil organic carbon (SOC) content through deep-rooted crops and on the underlying mechanisms. Moreover, microbial mineralization and fate of subsoil OC inputs are affected by the environmental conditions, including the soil nutrient availability, by mechanisms which are still elusive. The present PhD project examined the loss of root-derived C in relation to root chemical composition, soil nutrient availability and microbial physiology in the topsoil and subsoils of a temperate sandy loam soil under unfertilized grassland. The studies were done under controlled incubation conditions at 20°C.
First, the chemical composition of roots isolated from seven plant species was determined (by fibre digestion methods) 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 in subsoils from 60 and 300 cm depth. During an incubation period of 20 weeks, C mineralization was measured as carbon dioxide (CO2) evolution rates and cumulative rootinduced C loss. Also, β-glucosidase activity and carbon source utilization (CSU) were determined (Paper I). The results showed that the root-induced C mineralization varied greatly among roots with different chemical compositions as related to the concentrations of lignin (LIG) and total nitrogen (N). In topsoil, net C losses were strongly correlated to root LIG concentration, whereas in subsoils net C losses were more dependent on root N concentration, yet with increasing dependence on LIG concentration over time. These results suggested that chemically recalcitrant substances in root materials could contribute to higher C retention, and also revealed the potential N limitation mechanism, which may affect C turnover in subsoils.
Subsoil N limitation was further substantiated in an incubation study where added glucose (to mimic root-derived labile C) was decomposed under different N, phosphorus (P) and sulphur (S) availabilities in topsoil (20 cm) and subsoils (60, 100, and 300 cm) during an incubation period of six weeks (Paper II). Results showed that the net losses of added C were similar among topsoil nutrient treatments, whereas glucose mineralization in subsoils differed between nutrient treatments. Thus, in topsoil glucose was always completely decomposed whereas in subsoils glucose decomposition was dependent 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. This was first documented by measurement of CO2 evolution, but subsequently verified by direct measurement of remaining labile glucose by a colorimetric assay that was adapted and optimized for soil incubation studies (Paper III). Since glucose adsorption to clay minerals and soil organic matter was demonstrated to be negligible in the tested soils, these results confirmed that decomposition of glucose in subsoils was N dependent.
In both incubation studies, the importance of N availability for C mineralization was reflected in the microbial processes. During the 20-week incubation study (Paper I), the addition of root materials increased β-glucosidase activity at all three soil depths to an extent that was closely correlated to the root total N concentration. This pattern was consistent with the bacterial population growth, which was highest in the N-rich root treatment. When the CSU was tested using a microplate respiration assay (MicroResp), the turnover of the N-containing C source, Nacetyl- D-glucosamine, was consistently found to be higher than the turnover of two other C sources devoid of N (glucose and vanillin). Further, after the six-week incubation study (Paper II), the resulting stimulation of β-glucosidase activity and CSU by added glucose and nutrients was mainly related to the absence or presence of added N.
The method presented for soil glucose extraction and quantification (Paper III) was shown to be generally applicable for mineral soils with low to medium clay contents (<22%). Here, simple glucose extraction by hand-shaking for 0.5 min followed by enzymatic quantification at constant room temperature (20°C) was adequate. For soils with higher clay contents, glucose extraction by mechanical shaking (e.g., 5 min) should be considered. Further, it was documented that glucose assays with organic soils (histosols) deserve special attention due to potential inference from extractable humic substances.
Collectively, the incubation studies were synthesized in a conceptual paradigm for improved subsoil C storage through deep-rooted cropping systems as related to root total N and LIG concentrations, soil N availability, and microbial activity. This paradigm considered both C sequestration resulting from high proportions of recalcitrant C compounds (e.g., LIG) as well as C sequestration resulting from stabilized metabolites resulting from microbial turnover of labile C compounds. Based on the presented results achieved under controlled conditions, future in situ studies coupled with isotopic techniques are suggested to evaluate the C stabilization by deep-rooted crops and the resulting change in microbiology and soil structure that may contribute to long-term persistence of OC in subsoil.