Neutral effect of nitrogen addition and negative effect of phosphorus addition on topsoil extracellular enzymatic activities in an alpine grassland ecosystem

Soil nitrogen (N) and phosphorus (P) are common limiting nutrients affecting plant primary productivity in alpine ecosystems due to the low decomposition rate, though anthropogenic activities have greatly increased their inputs into ecosystems. Little is known regarding the effects of increasing N and P availabilities on the functioning of belowground microbial communities. To determine how soil microorganisms respond to N and P addition, we measured plant primary productivity, soil microbial biomass, soil mineral N availability, soil respiration, and the activities of soil extracellular enzymes after two years of N- and P-addition in an alpine grassland ecosystem on the Tibetan Plateau. We observed no significant effect of N addition on plant biomass, soil microbial biomass, soil respiration, or the activities of soil extracellular enzymes. In contrast, P addition increased plant biomass but suppressed the activities of most labile-C-cycling enzymes at 0-10 cm of soil depth, although the effects on soil microbial biomass and soil respiration were minor. Moreover, there was no interaction between N and P addition on these variables. Overall, N addition does not appear to exert a significant effect on plant primary productivity and microbial activity, whereas P addition increases plant primary productivity and tends to suppress topsoil microbial activity after two years of nutrient application.