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… Click to show full abstract
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.
               
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