Aridity threshold in affecting soil phosphorus availability in natural terrestrial ecosystems

Phosphorus (P) availability in soils controls critical functions and properties of terrestrial ecosystems. As global warming intensifies aridity and extreme precipitation, soil moisture regimes are undergoing significant shifts. However, the effects of increasing aridity on soil P availability remain poorly understood. Using a global database of Hedley P fractions from 802 (semi-natural) natural soils compiled from 96 published studies, this study revealed nonlinear patterns of five functional P fractions (available P, adsorbed P to secondary minerals, organic P, Ca-bound P, and occluded P) across the aridity index (AI) gradient in global terrestrial ecosystems, with two critical thresholds at approximately AI = 1.14 and 2.26. From AI 0 to 1.14, P availability increases, likely due to the dissolution of Ca-bound P driven by decreasing soil pH as AI increases. Between AI 1.14 and 2.26, P availability declines, possibly because occluded P becomes dominant, with clay content and occlusion processes playing a larger role in limiting available P. From AI 2.26 to 4, P availability increases again, likely due to a significant reduction in occluded P, which may be linked to anaerobic conditions and high organic C concentrations that enhance P availability. These findings highlight distinct mechanisms and controlling factors governing P biogeochemical cycling across different AI regimes and provide critical insights for improving predictions of nutrient cycling in terrestrial ecosystems under future climate change scenarios.