Stable soil organic matter: A comparison of C:N:P:S ratios in Australian and other world soils

Sequestering soil carbon (C) relies upon the availability of stabilising elements, nitrogen (N), phosphorus (P) and sulphur (S) which are known to be essential components of the stable organic C pool (Himes, 1998: Lal, 2008). The C:N:P:S ratios were investigated for a series of soils to test the hypothesis that the stable portion of the soil organic material (humus) has constant ratios of C:N:P:S. Constant ratios, if established, would provide an excellent tool to evaluate the feasibility, cost and strategies to sequester soil C in terrestrial ecosystems. Freshly-collected Australian soils cited in the literature were analysed for total C, N, P. organic P (OP) and S, and the ratios were compared with values for soils from numerous locations around the world, hereafter known as the International soils. Total N and S were highly correlated with C for the International and Australian soils and the relationships were similar for both sets. The correlation of C with P for Australian soils was not as strong as the correlations with N and S. however, a stronger relationship was found for OP than P with C. The correlation of OP with C for the International soils was not as strong as for the Australian soils probably due, in part, to the different methodologies used to analyse soil for OP in the International soils compared with the single method used for the Australian soils. The weaker relationship between OP and C for both sets of soils, compared with the relationship between N, S and C was probably also due, in part, to the wide variety of compounds in the soil OP pool which vary in their relationship with humus and the wide C:P ratio found in the soil microbial biomass. Overall, the C:N:OP:S ratios were constant for the stable portion of the soil organic material and these were consistent across a wide range of global soils and should provide a reliable basis with which to determine the level to which the availability of N. P and S may limit humus-C sequestration in terrestrial ecosystems although further research is needed to more accurately determine the amount of OP in humus. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.