Effects of Urea Application on Soil Organic Nitrogen Mineralization and Nitrogen Fertilizer Availability in a Rice–Broad Bean Rotation System

Rice cultivation is facing a situation where rice production stagnates while nitrogen fertilizer (NF) application continues to increase. The effectiveness of the NF residues from the rice season on the growth of rotating broad beans is unclear. High NF use in rice cultivation also affects the nitrogen supply through soil organic nitrogen (SON) mineralization (SONM). However, the rules of SONM and the NF availability in the rice–broad bean rotation system (RBRS) are unknown. A field trial of the RBRS was conducted using 15 N-labeled urea (CO( 15 NH 2 ) 2 ) as the partial NF source ( 15 N accounted for 5.3% of the total pure nitrogen applied) for the rice and no NF for the broad bean. It was found that 33.0–38.1% of NF in the rice season was utilized. NF availability was low in the broad bean season (3.6–4.0%). SONM was the most important source, providing approximately 60% of the nitrogen for rice growth. The SONM into mineral nitrogen and the fixation of mineral nitrogen into SON occurred simultaneously, with SONM dominating in most cases. SON content decreased slowly in the rice season and dramatically in the broad bean’s podding stage with a 0.64 g kg −1 (24.1%) decrease. The high nitrogen application in rice season promoted SONM and aggravated groundwater pollution. Soil urease activity, rather than catalase, phosphatase, and invertase activities, can be the main monitoring object of SONM. Furthermore, fungal abundance (especially Aspergillaceae, Neuroceae, and unclassified_o__Helotiales), rather than bacteria, was the primary target for SONM monitoring. It is unreasonable to apply large amounts of NF in the rice season but not in the broad bean season in the RBRS. N1 (135 kg N ha −1 ) had the best comprehensive benefits regarding crop yield, nitrogen supply by SONM, NF utilization, and nitrogen loss on the environment in the RBRS.