The C:N:P:S stoichiometry of soil organic matter

The formation and turnover of soil organic matter (SOM) includes the biogeochemical processing of the macronutrient elements nitrogen (N), phosphorus (P) and sulphur (S), which alters their stoichiometric relationships to carbon (C) and to each other. We sought patterns among soil organic C, N, P and S in data for c. 2000 globally distributed soil samples, covering all soil horizons. For non-peat soils, strong negative correlations (p < 0.001) were found between N:C, P:C and S:C ratios and % organic carbon (OC), showing that SOM of soils with low OC concentrations (high in mineral matter) is rich in N, P and S. The results can be described approximately with a simple mixing model in which nutrient-poor SOM (NPSOM) has N:C, P:C and S:C ratios of 0.039, 0.0011 and 0.0054, while nutrient-rich SOM (NRSOM) has corresponding ratios of 0.12, 0.016 and 0.016, so that P is especially enriched in NRSOM compared to NPSOM. The trends hold across a range of ecosystems, for topsoils, including O horizons, and subsoils, and across different soil classes. The major exception is that tropical soils tend to have low P:C ratios especially at low N:C. We suggest that NRSOM comprises compounds selected by their strong adsorption to mineral matter. The stoichiometric patterns established here offer a new quantitative framework for SOM classification and characterisation, and provide important constraints to dynamic soil and ecosystem models of carbon turnover and nutrient dynamics

Thermal analysis of mineral soils before and after oxidation with sodium hypochlorite

The nature of recalcitrant organic carbon (OC) in soil is a matter of great debate and various chemical treatments exist for its isolation. We compared calorimetric properties from silt+clay fractions of eight mineral soils by means of DSC before and after chemical oxidation with sodium hypochlorite (NaOCl) to find out, whether recalcitrance of soil organic matter against chemical oxidation coincides with its thermal stability. NaOCl oxidized around 75% of the OC, which corresponded well to a mean loss in heat of reaction of 80%. Peak temperatures and 50% burnoff temperatures did not change systematically after oxidation showing that the thermal stability of NaOCl residues was similar to that of untreated samples. Three samples revealed peaks at >520°C after oxidation indicative for the presence of pyrogenic carbon