Nutrient availability limits carbon sequestration in arable soils

Soils are the largest reservoir of global terrestrial carbon (C). Conversion from natural to agricultural ecosystems has generally resulted in a significant loss of soil organic-C (SOC, up to 50% or ~30-40tha-1) and 'restoring' this lost C is a significant global challenge. The most stable component of soil organic matter (SOM), hereafter referred to as fine fraction SOM (FF-SOM), contains not only C, hydrogen (H) and oxygen (O), but substantial amounts of nitrogen (N), phosphorus (P) and sulphur (S), in approximately constant ratios. The availability of these associated nutrients is essential for the formation of FF-SOM. Here we show, in short term (56 day) incubation experiments with 13C labelled wheaten straw added to four soils with differing clay content, that conversion of straw into "new" FF-SOM is increased by up to three-fold by augmenting the residues with supplementary nutrients. We also show that the loss of "old" pre-existing FF-SOM increased with straw addition, compared to soils with no straw addition, but that this loss was ameliorated by nutrient addition in two of our soils. This finding may illuminate why the build-up of SOC in some productive agricultural soils is often much less than expected from the amounts of C-rich residues returned to them because optimum C sequestration requires additional nutrients above that required for crop production alone. Moreover, it provides greater understanding of short-term dynamics of C turnover in soil, and in the longer term, may have important implications for global strategies aimed at increasing soil C sequestration to restore fertility and help mitigate climate change.