Precipitation, soils, NPP, and biodiversity: resurrection of Albrecht's curve

Climate and soils are widely recognized as major drivers of virtually all properties of ecosystems and communities. However, despite major advances in the understanding of soil formation and ecosystem dynamics, the effects of climate on soil properties are not widely appreciated. Understanding the effects of water availability on the rates of chemical and biological processes that affect soil formation can help clarify the global patterns of soil fertility, which affect agricultural and forest productivity, as well as biodiversity. Empirical tests of Albrecht's conceptual model of soil development and degradation using global climate and soil data sets and soil chronosequences confirm that soil total exchangeable bases (TEB), phosphorus, nitrogen, and other components of soil fertility, along with plant productivity generally decline on older soils and under wetter conditions as precipitation exceeds potential evapotranspiration. The basic pattern of soil fertility in relation to water availability is a unimodal curve, with a maximum near or below a water balance of zero (annual precipitation minus annual potential evapotranspiration). Analysis of global data by subregions reveals significant differences between temperate and tropical soil fertility distributions, as well as significant differences between continents. The low levels of soil nutrients (e.g., TEB, P, N) and plant productivity found on ancient soils or highly weathered soils in regions with high precipitation suggest that the positive effects of low productivity on plant diversity that have been observed at local and regional scales may also occur at the global scale.