Effects of nitrogen and phosphorus fertilization on soil organic matter priming and net carbon balance in alpine meadows

Fertilizers-induced priming effects of soil organic matter (SOM) decomposition influences net carbon balance and nutrient release. We hypothesize that very strong limitation of plant productivity and microbial activities by nitrogen (N) and phosphorus (P), common in Tibetan meadows, retard SOM decomposition and turnover. Consequently, N and/or P fertilization will induce priming effects of SOM and have implications for carbon balance. Soils from a nine-year fertilization experiment (N alone, P alone, NP together, and control) from a Tibetan alpine meadow were used to investigate priming effect of SOM and carbon balance after addition of C-13 labeled glucose. N and/or P fertilization acidified soil by 0.5 pH unit, decreased SOM content, and increased total and available N, total P. Regardless of fertilization, glucose addition accelerated SOM decomposition with priming effects of 30-60 mu g C g(-1) soil during 78 days. Alleviation of N and P limitation by N and NP fertilization lowered the priming effect by 17% and 14%, respectively, but P fertilization increased priming effect by 67%. The negative correlation of priming effect intensity with SOM, nitrate or total N, and microbial biomass contents indicated that fertilization-induced differences in soil N and the microbial community are responsible for the priming effects. Positive correlation of carbon balance with total N and ammonium contents suggested that soil N accounts for carbon sequestration. Therefore, long-term N and/or P fertilization accelerate SOM decomposition and reduce SOM storage in alpine meadows, of which P fertilization induces the highest priming effect and the lowest SOM storage.