Substrate quality of drained organic soils—Implications for carbon dioxide fluxes

Abstract

Background Peatlands only cover a minor fraction of the global terrestrial surface, but due to drainage, they are major contributors to carbon dioxide (CO2) emissions from soils. Previous studies have shown that hydrological conditions, nutrient availability and anthropogenic disturbance play an important role in the mineralisation of organic matter. Furthermore, microbial turnover depends on peat quality, which is determined by its botanical origin and degree of transformation under natural conditions. Aims The objective of this study was to shed light on the interdependence between mineralisation rates, secondary transformation of peat and chemical composition by examining the differences between bog and fen peat and between strongly degraded topsoil and well‐preserved subsoil. Methods Bog and fen peat from ten different peatlands under grassland use in Germany were analysed for their chemical composition using standard 13C nuclear magnetic resonance (NMR) spectroscopy and wet chemical extractions for fibre analysis. The radiocarbon age was determined as well. The results were combined with CO2 fluxes from a previous incubation study. Results Topsoils had higher shares of proteins and lipids, and lower shares of carbohydrates and aromatics than subsoils. Bog peat subsoils were characterised by higher shares of carbohydrates and lower shares of aromatics than fen peat subsoils. Topsoils were more similar to each other in their chemical composition than the subsoils. Considering all samples, aromatics and phenolics were negatively correlated with CO2 fluxes. Measured CO2 fluxes from topsoils were significantly higher than from subsoils. However, no influences of chemical composition on CO2 fluxes were detected when examining topsoils and subsoils separately. Even though aromatics and phenolics showed positive relationships with radiocarbon age, differences in age alone were unable to explain the higher amounts of these compounds in the subsoil. Conclusions The results imply that chemical composition of topsoil peat is not the reason for higher mineralisation rates compared to subsoil peat, but rather a consequence of decomposition and transformation. Thus, peat mineralisation of drained organic soils under agriculture might not slow down over time due to gradually decreasing peat quality but could increase further.

Underestimation of carbon dioxide emissions from organic-rich agricultural soils

“…Incubation studies with sieved, air-dried samples (7.6-52.6% OC) indicated that the susceptibility of OC to mineralisation increased with decreasing OC content in German organic soils 26 and similar results were reported for Finnish soils (3.1-49.4% OC) incubated under anoxic and physically disturbed conditions 27 . The present results, obtained with structurally intact organic soils, support the possibility of positive feedback whereby organic soils in advanced stages of decomposition (i.e., with low OC content) may show an increased mineralisation rate 26,33,34 . However, other studies have emphasised the importance of OC quality and intrinsic decomposability for rates of CO 2 emission with the assumption that lower rates would be maintained by soils in advanced stages of decomposition, where the more easily decomposed fractions would already be depleted [35][36][37] .…”

“…https://doi.org/10.1038/s43247-024-01459-8 phenolics, were not found to be enriched in drained organic topsoils under agriculture in Germany 33 , and simple chemical characteristics of OC as proxies for biochemical decomposability were unable to explain the variability in CO 2 emissions in a comprehensive study of drained organic soils in Switzerland 25 . In summary, the role of OC quality needs to be further clarified, but the present data do suggest that the remaining OC content as such is not a strong predictor of area-scaled CO 2 emission rates (Figs.…”

Underestimation of carbon dioxide emissions from organic-rich agricultural soils

“…Incubation studies with sieved, air-dried samples (7.6-52.6% OC) indicated that the susceptibility of OC to mineralisation increased with decreasing OC content in German organic soils 7 and similar results were reported for Finnish soils (3.1-49.4% OC) incubated under anoxic and physically disturbed conditions 8 . The present results, obtained with structurally intact organic soils, support the possibility of a positive feedback whereby organic soils in advanced stages of decomposition (i.e., with low OC content) may show an increased mineralisation rate 7,33,34 . However, other studies have emphasised the importance of OC quality and intrinsic decomposability for rates of CO2 emission with the assumption that lower rates would be maintained by soils in advanced stages of decomposition, where the more easily decomposed fractions would already be depleted [35][36][37] .…”

Greenhouse gas fluxes from drained peatland : Measurement techniques and management impacts

“…Journal of Soils andSediments, 18, 2500-2510. https Peatlands cover about 3% of the global terrestrial surface (Xu et al, 2018) and store around 598 Pg carbon (C), which represents about 21% of the global soil organic C sock estimated to 2800 Pg C (Leifeld and Menichetti, 2018;Jackson et al, 2017). Around 13% of the world's peatlands have been drained for agriculture, forestry or peat extraction (Säurich et al, 2021), turning peatlands from C sinks or near C neutrality into major C sources. Under current circumstances, i.e.…”