Urease activity and its relation to soil organic matter, microbial biomass nitrogen and urea-nitrogen assimilation by maize in a Brazilian Oxisol under no-tillage and tillage systems

We studied the relationship between urease activity (UA) and soil organic matter (SOM), microbial biomass N (N-biom) content, and urea-N fertilizer assimilation by maize in a Dark Red Latosol (Typic Haplustox) cultivated for 9 years under no-tillage (NT), tillage with a disc plough (DP), and tillage with a moldboard plough (MP). Two soil depths were sampled (0-7.5 cm and 7.5-15 cm) at 4 different times during the crop cycle. Urea was applied at four different rates, ranging from 0 to 240 kg N ha(-1). The levels of fertilizer N did not affect the UA, SOM content, and N-biom content. No significant difference between the treatments (NT, DP, and MP) was observed for SOM during the experiment, probably because the major part of the SOM was in recalcitrant pools, since the area was previously cultivated (conventional tillage) for 20 years. The N-biom content explained 97% and 69% of the variation in UA in the upper and deeper soil layer, respectively. UA and biomass N were significantly higher in the NT system compared to the DP and MP systems. The highest maize productivity and urea-N recovery was also observed for the NT system. We observed that the increase in urea-N losses under NT, possibly as a consequence of a higher UA, was compensated for by the increase in N immobilized in the biomass.