Abstract:

The aim of this research was to explore the effects of fertilization with different amounts of nitrogen and phosphorus on the characteristics of soil microbes. To this end， we prepared artificial grasslands with annual oat （Avena sativa） and alfalfa （Medicago sativa） grown alone as monocultures or as a mixture （1∶1）. The experiment consisted of a control， N₀P₀ （CK） and eight treatments， namely N₀P_2.5， N₀P₅， N₅P₀， N₅P_2.5， N₅P₅， N₁₀P₀， N₁₀P_2.5， and N₁₀P₅. These treatments represented combinations of N fertilizer at three levels （elemental N at 0， 75 and 150 kg·ha^-1） and phosphate fertilizer at three levels （elemental P at 0， 37.5 and 75 kg·ha^-1）. We determined the carbon， N， and P contents in soil microbial biomass， the activities of extracellular enzymes in soil， and their ecological stoichiometric ratios in the three types of artificial grassland under the different levels of N and P fertilization. It was found that： 1） Addition of different levels of N and P significantly affected the nutrient metabolism characteristics of soil microorganisms. In the oat monoculture， the fertilization treatment with high N and low P （N₁₀P_2.5） had a significant inhibitory effect on the carbon and N contents of soil microbial biomass. In the alfalfa monoculture， combined application of low N and high P （N₅P₅） resulted in increased carbon and N contents in soil microbial biomass. In the oat-alfalfa mixture， the influence of fertilization treatments on soil microbial biomass was complex， but fertilization with high P （N₀P₅） had an inhibitory effect on soil microbial biomass. 2） The activities of β-1，4-glucosidase， leucine aminopeptidase， and alkaline phosphatase in the oat monoculture were greatly affected by the N and P fertilization treatments， while those in the alfalfa monoculture and the mixed oat-alfalfa grassland were relatively stable across all the treatments. The soil N∶P enzyme activity ratios were lower than 1 in all three grassland types， indicating that the P limitation for soil microorganisms at the experimental site was greater than the N limitation. In summary， for grasslands in the northern part of Shanxi Province， the combined application of low N and high P （N at 75 kg·ha^-1 and P at 75 kg·ha^-1） is optimal for oat and alfalfa as monocultures. However， for mixed oat-alfalfa sowings， low N and low P （N at 75 kg·ha^-1 and P at 37.5 kg·ha^-1） should be prioritized.

Key words: artificial grassland, nitrogen and phosphorus addition, microbial biomass carbon， nitrogen and phosphorus, soil extracellular enzymes, ecological stoichiometric ratio