Responses of soil intracellular and extracellular urease activities to carbon additions in chernozem

Urease in soil, as the most important enzyme catalyzing urea hydrolysis, plays an important role in nitrogen supply of grassland ecosystems. While the effects of different carbon additions on extracellular urease in grassland soil are well understood, their effects on soil intracellular urease remain unknow. Moreover, whether the responses of intracellular and extracellular urease to carbon additions are consistent need to be clarified. With a field experiment in Hulun Buir grassland of Inner Mongolia, we investigated soil intracellular and extracellular urease activities in chernozem under four treatments, including carbon-free (C0), 250 (C250), and 500 (C500) kg C·hm-2·a-1, using glucose as carbon source. Further, we analyzed their relationships with soil properties. Carbon additions significantly increased soil intracellular urease activity and its ratio to total urease activities and total urease activity, but did not affect soil extracellular urease activities. There was significant positive correlation between soil intracellular urease activity and microbial biomass, suggesting that the increases of soil intracellular urease activities were mainly caused by the increases of microbial biomass. Results of structural equation modeling (SEM) analysis showed that carbon additions indirectly increased soil intracellular urease activities by affecting microbial biomass.