Abstract: Seven soils were collected from forestland, grassland and cropland in semi-arid area of Inner Mongolia. The soil aggregates of >5 mm, 5-2 mm, 2-1mm, 1-0.5 mm, 0.5-0.25 mm and <0.25 mm were obtained with a dry-sieving method. The total N (TN), microbial biomass N (MBN), K2SO4-extractable N (K2SO4-N) of the soil aggregates were separately determined. The results showed that the different inputs of organic materials and nitrogen fertilizer of the sampled soils significantly affect not only the contents of total N, microbial biomass N and K2SO4-extractable N, but also their distributions in soil aggregates. The total N, microbial biomass N and K2SO4-extractable N were in the ranges of 0.67-2.06 g·kg-1, 11.40-71.03 mg·kg-1and 1.28-4.60 mg·kg-1, respectively. The irrigated soil (No. 6) had the highest value, but the lowest value was in the grassland soil (No.7). Generally, the small aggregates contained significantly lower total N and microbial biomass N than the larger aggregates in the soils with low inputs of organic materials and N fertilizer. In contrast, the small aggregates contained much higher total N and microbial biomass N than the larger aggregates in the soils with high input. It is implied that, under natural condition, N was firstly cycled in the large aggregates. Thus, the N of the large aggregates may play much more important role in natural low nutrition condition than that of the small aggregates.

Key words: Soil aggregate, Microbial biomass N, Semi-arid area