Decreased soil organic P fraction associated with ectomycorrhizal fungal activity to meet increased P demand under N application in a subtropical forest ecosystem

Phosphorus (P) is an essential but limited nutrient in highly weathered subtropical or tropical forest soils; however, increased P demands owing to elevated nitrogen (N) deposition might further exacerbate the P limitation in these regions. How soil P fractions respond to N enrichment and the underlying mechanisms are not yet understood completely. To address this issue, we conducted an experiment with 3 years of N addition to a Castanopsis carlesii natural forest in Fujian, China; the soil P fractions, microbial biomass, enzyme activities, and phospholipid fatty acid (PLFA) levels were determined. We found that N addition did not significantly alter the microbial biomass C (MBC) and microbial biomass N (MBN), but significantly decreased microbial biomass P (MBP); further, the ratios of MBC:MBP and MBN:MBP increased significantly after N application, indicating that N enrichment enhanced the P demands of plants and soil microorganisms. Moreover, neither resin-P nor NaHCO3-P (extracted by resin and NaHCO3, respectively) was decreased, whereas organic P extracted by NaOH (NaOH-Po) was decreased after N addition, implying that NaOH-Po is a potentially significant source of available P. In particular, appositive correlation between abundance of ectomycorrhizal fungi and NaOH-Po and significant relationships between acid phosphomonoesterase activity (ACP) and labile P contents were noted, suggesting that ectomycorrhizal fungi can play a major role in the depletion of soil organic P, and these enzyme activities can mobilize the unavailable P to labile P in the investigated forest. Taken together, our results revealed a novel mechanism for the maintenance of soil labile P in a subtropical natural forest under chronic stress of N deposition.