Long-term throughfall exclusion decreases soil organic phosphorus associated with reduced plant roots and soil microbial biomass in a subtropical forest

Abstract Long-term droughts can significantly reduce forest productivity due to water deficiency and associated changes in the availability of soil nutrients. Phosphorus (P) is a critical but limited element in subtropical forest ecosystems. However, how soil P availability responds to drought and the underlying mechanisms in subtropical forests remain poorly understood. In this study, we investigated the effects of drought treatment on soil P fractions, soil available N, microbial biomass N (MBN), microbial biomass P (MBP), soil microbial phospholipid fatty acids (PLFAs), enzyme activities, fine root biomass (FRB), and fine root biomass P (FRBP) in a subtropical Castanopsis carlesii forest after 7.5 years of throughfall exclusion. The results showed that throughfall exclusion significantly decreased resin extractable P (Resin-P) and total extractable organic P, suggesting that drought induced a reduction in soil available P and organic P. Meanwhile, Resin-P was positively correlated to total extractable organic P but not with NaHCO3 extractable P (NaHCO3-P) and total extractable inorganic P, which indicated that the decline of available P in soil was mainly associated with the reduction of organic P. In addition, soil microbial biomass, MBN, MBP, FRB, and FRBP were reduced under the throughfall exclusion treatment. The MBP and FRBP were positively correlated with Resin-P and total extractable organic P. These results implied that the reductions of MBP and FRBP primarily contributed to the declines of soluble P and organic P. Such information is essential for understanding the roles of microbes and plant roots on soil P cycle in subtropical forests under drought.