Alpine grassland ecosystems have high soil organic carbon (SOC) reserves and play an important role in regulating the soil carbon-climate change feedback. However, the responses of SOC dynamics (and the underlying mechanisms) in these ecosystems to nutrient (such as nitrogen (N) and phosphorus (P)) availability induced by climate change and human activities remain elusive. In this study, we examined soil chemistry, microbial biomass and enzyme activities, SOC and its two fractions (particulate organic carbon, POC, >53 μm; and mineral-associated organic carbon, MAOC, <53 μm) in 10 year-old nutrient-addition experimental plots in an alpine Kobresia humilis meadow on the Qinghai-Tibetan Plateau. The four nutrient addition treatments include Control (CK), N addition (N, 10 g N m-2 yr-1), P addition (P, 5 g P m-2 yr-1), and both nutrients addition (NP).
Results/Conclusions
Our results showed that all nutrient-addition treatments showed higher plant biomass compared to the control treatment. Soil total and available (mineral) N were not significantly altered by nutrient addition, due to large variations within treatment and the complex balance between plant uptake and soil loss. In contrast, soil total and available (Olsen) P were both highly elevated by P and NP addition, but not by N addition. Soil pH was lowered by nutrient addition, particularly by the NP treatment. Soil microbial biomass C and N were not affected by N addition, but inhibited by both P and NP addition. Soil enzymes were surprisingly resistant to nutrient addition. Although bulk soil SOC and MAOC were not altered by nutrient addition, POC and the POC:MAOC ratio were promoted by nutrient addition, particularly by the P and NP treatments. Such increase in the POC:MAOC ratio with nutrient addition may be jointly explained by the increase in plant biomass and the decrease in microbial biomass with nutrient addition, suggesting that both input (through plant litter) and loss (through microbial decomposition) may contribute to the dynamics of POC. Moreover, the mechanisms for the no change in MAOC with nutrient addition remain uncertain at this point, because both soil acidification and lower microbial biomass would lead to lower MAOC if stabilization of microbial necromass and products is the main pathway for MAOC stabilization. Ongoing work with microbial community structure (by PLFA and DNA-sequencing) and functional gene (by high-throughput Q-PCR) may further unravel the microbial mechanisms mediating the dynamics of MAOC with nutrient addition.