Soil organic carbon variation determined by biogeographic patterns of microbial carbon and nutrient limitations across a 3, 000-km humidity gradient in China

Abstract Soil extracellular enzyme stoichiometry has been used to characterize the acquisition strategies of soil microorganisms in obtaining carbon (C), nitrogen (N) and phosphorus (P). However, the variations in soil microbial resource limitation to changing precipitation regimes and edaphic factors remain poorly understood, particularly for a highly divergent soil organic C gradient along the transect from dry to wet areas. This study investigated soil microbial C and nutrient (P/N) limitations along a 3,000-km humidity gradient. The results revealed a downward unimodal-shaped relationship between the humidity index (HI) and soil microbial P/N limitation with a threshold of HI = 0.68 (corresponding mean annual precipitation ranged from 469 mm to 551 mm). Changes in microbial C limitation, and total- and available soil P contents along the humidity gradient further revealed the presence of this threshold. Soil microbial C limitation remained constant at a humidity level below HI = 0.68, and it increased above this threshold. Microbial P limitation decreased and N limitation increased as humidity increased to HI = 0.68. Above HI = 0.68, the microbial P limitation gradually elevated with an increase in humidity. We also found that humidity and soil nutrients are critical factors explaining the variations in microbial resource limitation, and soil nutrients control microbial resource limitation on either side of the HI = 0.68 threshold. These findings suggest that the acquisition of N and P by soil microorganisms stimulates the decomposition of soil organic matter, and future predictions of ecosystem C budgets should thus consider enzymatic processes.