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

Abstract 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 (Ramm), and nitrification rate (Rnit) 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, Rmin, 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, Rmin, 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.