Urease activity and its relation to soil organic matter, microbial biomassnitrogen 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 m atter (SOM), microbial biomass N (N-biom) content, and urea-N fertilizer as similation 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 f our different rates, ranging from 0 to 240 kg N ha(-1). The levels of ferti lizer N did not affect the UA, SOM content, and N-biom content. No signific ant 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 r ecalcitrant pools, since the area was previously cultivated (conventional t illage) for 20 years. The N-biom content explained 97% and 69% of the varia tion 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 sys tems. 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 incre ase in N immobilized in the biomass.