The nitrogen nutrition potential of arable soils.

Soils are an important source of nitrogen in many of the world's cropping systems. Especially in low-input production systems, nitrogen release from soil organic matter turn-over is the major part of the crop's nitrogen supply and research suggests that this process is significantly affected by changes in climate. The knowledge of the amount of nitrogen being accountable for crop nutrition is purely empirical in many production areas in the world and data as a foundation of global-scale climate change and food security assessments is scarce. Here we demonstrate that nitrogen mineralisation in general follows similar rules as for carbon, but with different implications for agricultural systems. We analysed 340 data sets from previously published incubation experiments for potential nitrogen mineralisation which covered a large range of soils and climate conditions. We find that under warm and all-year humid conditions the share of potentially mineralisable nitrogen in the soil's total nitrogen is significantly smaller than in dry or temperate environments. We conclude that - despite relatively high soil nitrogen stocks - soil-borne nitrogen supply for crop production is very low in tropical and humid subtropical environments, which is a critical piece of information for global assessments of agricultural production and food security.