Abstract
Elevated nitrogen (N) deposition may aggravate phosphorus (P) deficiency in forests in the warm humid regions of China. To our knowledge, the interactive effects of long-term N deposition and P availability on soil microorganisms in tropical replanted forests remain unclear. We conducted an N and P manipulation experiment with four treatments: control, N addition (15 g N m−2·yr−1), P addition (15 g P m−2·yr−1), and N and P addition (15 + 15 g N and P m−2·yr−1, respectively) in disturbed (planted pine forest with recent harvests of understory vegetation and litter) and rehabilitated (planted with pine, but mixed with broadleaf returning by natural succession) forests in southern China. Nitrogen addition did not significantly affect soil microbial biomass, but significantly decreased the abundance of gram-negative bacteria PLFAs in both forest types. Microbial biomass increased significantly after P addition in the disturbed forest but not in the rehabilitated forest. No interactions between N and P additions on soil microorganisms were observed in either forest type. Our results suggest that microbial growth in replanted forests of southern China may be limited by P rather than by N, and this P limitation may be greater in disturbed forests.
Highlights
Soil microorganisms play key roles in ecosystems and mediate many ecological processes that are crucial to ecosystem functioning, including decomposition processes[13] and nutrient cycling[14]
We found no short-term (30 months) effect of P addition on soil microbial biomass in these two forest sites[11], but based on litter decomposition studies we observed signs that microorganisms in human disturbed forests in tropical China are N limited[25,26]
There was no significant difference in the chemical parameters for soil between the disturbed and rehabilitated forests, except for Soil moisture content (SMC) and soil NO3––N concentrations (Table 1)
Summary
Soil microorganisms play key roles in ecosystems and mediate many ecological processes that are crucial to ecosystem functioning, including decomposition processes[13] and nutrient cycling[14]. Unlike the old-growth forest, which is an undisturbed forest with minimal direct human impacts, and has been protected by monks for more than 400 years, the disturbed forests (the mixed and pine forests) originated from clear-cuts in the 1930 s and subsequent pine plantation establishment These kinds of disturbed forests cover more than half of the total forest area in subtropical and tropical China[22,23]. We found no short-term (30 months) effect of P addition on soil microbial biomass in these two forest sites[11], but based on litter decomposition studies we observed signs that microorganisms in human disturbed forests in tropical China are N limited[25,26]. We hypothesised that N is a limiting factor for microbial growth in these reforested forests (disturbed and rehabilitated), and that P may limit microbial growth in the disturbed forest because of the continuous removal of P by harvesting
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