Soil enzymes activities and microbial biomass have an important influence on nutrient cycling. The spatial distribution of soil enzymes activities and microbial biomass were examined along a latitudinal gradient in farmlands of Songliao Plain, Northeast China to assess the impact of climatic changes along the latitudinal transect on nutrient cycling in agroecosystems. Top soils (0-20 cm depth) were sampled in fields at 7 locations from north (Hailun) to south (Dashiqiao) in the end of October 2005 after maize harvest. The contents of total C, N, and P, C/N, available N, and available P increased with the latitude. The activities of invertase and acid phosphatase, microbial biomass (MB) C and N, and MBC/MBN were significantly correlated with latitude (P < 0.05, r2 = 0.198, 0.635, 0.558, 0.211 and 0.317, respectively), that is, increasing with the latitude. Significant positive correlations (P < 0.05) were observed between invertase activity and the total N and available P, and between acid phosphatase activity and the total C, C/N, available N, total P and available P. The urease, acid phosphatase, and dehydrogenase activities were significantly correlated with the soil pH and electrical conductivity (EC) (P < 0.05). MBC and MBN were positively correlated with the total C, C/N, and available P (P < 0.05). The MBC/MBN ratio was positively correlated with the total C, total N, C/N, and available N (P < 0.05). The spatial distribution of soil enzyme activities and microbial biomass resulted from the changes in soil properties such as soil organic matter, soil pH, and EC, partially owing to variations in temperature and rainfall along the latitudinal gradient.
The diversity and distribution patterns of soil nematode communities in phaeozem agroecosystems of Northeast China were assessed to evaluate nematode taxonomic diversity and functional diversity in relation to climatic condition and soil characteristics in human modified landscape.Along the latitudinal gradient,soil samples were collected from north(Hailun) to south(Gongzhuling) down to a depth of 100 cm with intervals of 0-20,20-40,40-60,60-80,and 80-100 cm.The nematode abundance and taxonomic diversity(generic richness) were lower at Hailun than at other sites,and higher values of evenness were observed at Hailun and Harbin than at Dehui and Gongzhuling.Nematode faunal analysis revealed that soil food web at Hailun was successionally more mature or structured,and the environment little disturbed,while at Harbin and Gongzhuling,the soil food web was degraded with stressed environment.The environmental variables relevant in explaining the patterns of nematode distribution and diversity in phaeozem agroecosystems,using canonical correspondence analysis(CCA),were the mean annual temperature,total nitrogen,electrical conductivity,mean annual precipitation,and other soil properties.Among these variables,the mean annual temperature was a relatively important factor,which could explain 29.05% of the variations in nematode composition.
