Global patterns of soil nitrogen storage

The amount of nitrogen stored in soil is related to climate through biotic processes associated with productivity of vegetation and decomposition of organic matter. Other factors, particularly rainfall input, dry deposition input, nitrogen fixation and losses of inorganic nitrogen due to leaching contribute to the variability of nitrogen storage. Here we report that soil nitrogen storage ranges from 0.2 kg m^-3 in warm deserts to 2 kg m^-3 in rain tundra, with a peak of 1.6 kg m^-3 in subtropical wet forests. Soil carbon storage shows a similar pattern. The global nitrogen pool in the surface metre of soil comprises an estimated 9.5 × 10¹³ kg. Each soil profile examined was classified according to the Holdridge life zone¹ in which it was found. Soil carbon/nitrogen ratios range from <10 in tropical deserts to >20 in cool, wet forests or rain forests. We determined C/N ratios of 15-20 in cool life zones and 10-15 in warm life zones. These results indicate that: (1) Relatively large amounts of soil nitrogen in wet tropical regions are associated with recalcitrant humic materials in an advanced state of decay, with low C/N ratios; (2) the seasonal climate contrast in temperate regions, combined with variable litter quality due to the mix of coniferous and deciduous species, results in moderate carbon and nitrogen storage in soil and variable C/N ratios; and (3) slow decomposition in wet tundra regions results in high carbon and nitrogen storage, with high C/N ratios.