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

Abstract

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.

Keywords: Carbon; Nitrogen; Phosphorus; Protein; Soil organic matter; Stoichiometry; Sulphur.

Fig. 1

Regressions of %N versus %C, %P versus %C, % S versus %C for all soils other than ombrotrophic peats. All trends are significant (p < 0.001). Panels a–c show all data, panels d–f show data for samples with identified soil horizons. The numbers of data per horizon (O, A, E, B, C) are: 86, 439, 26, 214, 95 for %N versus %C (panel d); 85, 414, 26, 212, 94 for %P versus %C (panel e); 38, 55, 0, 30, 28 for %S versus %C (panel f)

Fig. 2

Element ratios plotted against each other for all soils other than ombrotrophic peats. All trends are significant (p < 0.001)

Fig. 3

Schematic of the mixing model, logarithmic (panel a) and linear (panel b) versions. The y-axis is the fraction of NPSOM or NRSOM

Fig. 4

Element ratios to C versus %C for all soils other than ombrotrophic peats, fitted with the two-endmember mixing model. The left end of each solid line corresponds to NRSOM, the right end to NPSOM (see Fig. 3)

Fig. 5

Relationships between P:C (y-axis) and N:C (x-axis) for soils from different ecosystems. The significance indicators refer to power-law regressions; *p < 0.05, **p < 0.01, ***p < 0.001. The solid line shows the mixing model trend. Axis labels are omitted for clarity

Fig. 6

Relationships between S:C (y-axis) and N:C (x-axis) for soils from different ecosystems. The significance indicators refer to power-law regressions; *p < 0.05, **p < 0.01, ***p < 0.001. The solid line shows the mixing model trend. Axis labels are omitted for clarity

Fig. 7

Relationships between P:C and N:C, and between S:C and N:C, for ombrotrophic peat topsoils

Fig. 8

Overall picture of C, N, S and P in SOM. Data for temperate litter stoichiometry are from Trofymow et al. (1995), for tropical plant (tree) litter from Tripathi and Singh (1992), Thompson and Vitousek (1997), Chuyong et al. (2002), Hirobe et al. (2004) and Isaac and Nair (2005), and for microbial biomass from Fagerbakke et al. (1996), Cleveland and Liptzin (2007), Griffiths et al. (2012). The illustrative three-component mixture has a stoichiometry adjusted to coincide with that of NRSOM (see text)

Fig. 9

Variation of pool sizes of organic C, N, P and S with soil carbon concentration, calculated from the NPSOM–NRSOM mixing model and an assumed dependence of bulk density on %C (see text). The decreases in the NPSOM and total pools of C (top left panel) arise because of the modelled dependence of bulk density on %C