Abstract:

The seasonal changes of soil urease activity appeared to produce single-peaked curves in Leymus chinensis, Puccinellia tenuiflora and Chloris virgata communities in the Songnen meadow. The activity became gradually lower with increasing depth of soil layers. Soil urease activity of the 0-10 cm layer had a power-function correlation to rainfall (monthly mean value) and an exponential correlation to soil temperature, which showed that urease activity was deeply influenced by water and heat factors. Path analysis between urease activity and soil physical-chemical factors showed that urease activity was the result of interactions among soil physical-chemical factors, and each factor acted differently in the three communities. In the L. chinensis community, soil physical-chemicalfactors affecting urease activity decreased in the order of pH > organic matter > available N > C/N ratio > soil bulk density > total N > available P. In the P． tenuiflora community, the order was organic matter > C/N ratio > total N > soil bulk density > available P > available N > pH. In the C． virgata community, the order was total N >organic matter> available N > pH > C/N ratio > available P > soil bulk density. The dominant influencing factors were also found in the three communities through stepping multi-regression. They were organic matter and pH in L． chinensis community, organic matter and soil bulk density in P．tenuiflora community and total N in C．virgata community, which showed that organic matter was the base of soil urease activity. Principal component analysis of the soil fertility indicated that the contribution of or ganic matter, total N, available N, C/N ratio and soil bulk density accounted for the most part, while the contribution of urease activity was low. That is why the soil urease activity of the C．virgata community, which grew in the barren region, was higher than that of the L．chinensis community, which grew in the fertile region in the Songnen meadow. Although the soil urease activity was remarkably correlated with most of the soil physical-chemical factors, it only dealt with the nitrogen transformation among many soil nutritional element transformations. Therefore, soil urease activity does not completely reflect the conditions of soil fertility but is only an assistant index for evaluation.