Soil Microbial Biomass Carbon And Nitrogen In Different Grassland Types On The Western Slopes Of The Helan Mountains, China

Soil microorganisms play an important part in maintaining soil quality. Soil microbial biomass represents the labile pool and can be used as a bio-indicator for evaluating soil quality. In this study we analyzed patterns of changes in soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN) in grasslands at different altitudes along the middle Helan Mountain range in Alex, Inner Mongolia. By field investigation and laboratory analysis, we studied the factors affecting soil microbial biomass distribution including annual precipitation, temperature, plant cover and productivity, soil water content, soil organic carbon, soil total nitrogen(TN), soil bulk density and pH. The effects of seasonal change, grazing and exclusion on soil microbial biomass are also discussed. The results of this study are shown as follows:(1) The results indicate that SMBC and SMBN content declined with decreasing altitude in the following order: alpine meadow > mountainous steppe > desert steppe > steppe desert (P<0.05). SMBC in the 0-10cm soil layer ranged from 48.58mg/kg to 937.64mg/kg;in the 10-20cm layer from 34.32mg/kg to 407.46mg/kg. SMBN in the 0-1 Ocm layer ranged from 13.78mg/kg to 194.07mg/kg;in the 10-20cm layer from 5.32mg/kg to 86.78mg/kg. The patterns of changes in mineralization quotient and K₂SO₄-extractable N / SMBN were contrary to the SMBC. Microbial quotient and SMBN / TN showed a trend: alpine meadow < steppe desert < mountainous steppe and desert steppe(P<0.05).Microbial quotient in the 0-10cm layer ranged from 0.88% to 2.38%;from 0.45% to 2.37% in the 10-20cm layer. Over the different grassland types, SMBC and SMBN contents were much higher in the 0-1 Ocm layer than in the 10-20cm layer.(2) SMBC and SMBN were positively correlated with plant cover, plant productivity, soil water content, soil organic carbon content and total nitrogen (P<0.001)and negatively correlated with annual average temperature , soil pH and bulk density(P<0.001). The most important factors affecting SMBC in the 0-1 Ocm soil layer were annual average temperature, total nitrogen, plant cover and organic carbon. In the 10-20cm soil layer, SMBC was most strongly affected by annual average temperature and soil water content. The most important factors affectingSMBN in the 0-1 Ocm layer were annual average temperature, plant productivity and bulk density. In the 10-20cm layer, SMBN was most strongly affected by annual average temperature, plant productivity and cover. As a whole, annual average temperature is the key factor that affected SMBC and SMBN.(3) SMBC and SMBN contents in mountainous steppe, desert steppe and steppe desert showed different seasonal patterns. In the mountainous steppe, soil microbial biomass in the 0-1 Ocm layer was much higher in July than in April and September, in the 10-20cm layer, soil microbial biomass did not differ significantly between month. In the desert steppe, soil microbial biomass in the 0-1 Ocm layer;was much higher in April than in July and September, seasonal change of soil microbial biomass in the 10-2 Ocm layer was contrary to that in the surface layer. In the steppe desert, soil microbial biomass in the 0-1 Ocm layers did not differ significantly. The most important factors affecting the seasonal change of soil microbial biomass in the mountainous steppe and desert steppe were annual average temperature and soil water content.(4) Grazing and exclusion management affected significantly soil microbial biomass. After 5 years of exclusion in mountainous steppe compared with the pastures that had been freely grazed, SMBC increased 25% and 50% in the 0-1 Ocm layer and 10-20cm layer, respectively;SMBN increased by 12% and 37%. After 5 years of exclusion in steppe desert compared with the pastures that had been freely grazed, SMBC increased by 78% in the 0-1 Ocm layer, SMBC did not increase significantly in the 10-20cm layer. SMBN increased by 70% and 69% in the 0-1 Ocm layer and 10-20cm layer. However in July, SMBN in grazed grassland was higher than in fenced grassland. After 8 years of exclusion in steppe desert compared with the pastures that had been freely grazed, SMBC increased by 115% and 124% in the 0-1 Ocm layer and 10-20cm layer, respectively;but SMBN increased by 159% and 119%.