The effects of N and P additions on microbial N transformations and biomass on saline-alkaline grassland of Loess Plateau of Northern China.

摘要

Microbial nutrient transformation plays an important role in regulating nitrogen (N) and phosphorus (P) cycling in terrestrial ecosystems. Soil N and P contents also control microbial nutrient transformations. However, there is still dispute on how N and P additions affect microbial activity and N transformations. A field experiment was conducted to examine the effects of N and P on microbial N transformations and biomass in saline-alkaline grassland in Loess Plateau of northern China during growing season in 2009. N was added at a rate of 10 g N m-2 y-1 in the form of NH₄NO₃. P was added at a rate of 5 g P m-2 y-1 in the form of P₂O₅-. We measured the in situ net ammonification rate (R_amm), and nitrification rate (R_nit) once a month from May to October; we also measured potential soil microbial biomass carbon (MBC), nitrogen (MBN), and potential microbial respiration (MR) once a month in laboratory. Results: During the whole growing seasons, P addition significantly stimulated soil inorganic N pool, soil extractable C, soil extractable N pool, R_min, and the metabolic quotient (qCO₂) from the estimates of microbial respiration and microbial biomass carbon, and there was no effect on peak aboveground biomass, MBC, MBN and MR during the whole growing seasons in 2009. N addition significantly increased peak aboveground biomass, inorganic N pool, R_min, MBN, MR, and qCO₂, decreased soil extractable C and the ratio of MBC/MBN, and there was no effect on soil extractable N and MBC during the growing season in 2009. P addition increased the soil net N mineralization rate and N addition not only increased the soil net N mineralization rate but also increased microbial biomass N. We observed that P induced a decreased soil inorganic N pool, but N addition directly increased soil inorganic N pool, how to balance the quantity of N and P additions in agriculture system is an important technique in agriculture harvest in the future in Loess Plateau of Northern China.