Dynamics of soil microbial biomass and nitrogen availability in a flooded sice soil amended with different C and N sources

A greenhouse experiment was conducted to compare effects of different C and N sources applied to a flooded soil on soil microbial biomass (SMB) C and N, extractable soil organic N (N-ORG), and NH4+-N in relation to plant N ac cumulation of rice (Oryza sativa L.). In addition to a control without inpu ts (CON), four treatments were imposed receiving: prilled urea (PU), rice s traw (RS), RS and PU (RS + PU), or Sesbania rostrata as green manure (SES). Treatments were arranged according to a completely randomized design with four replicates and further consisted of pots with and without transplanted rice. While plant effects on the SMB were relatively small, the applicatio n of organic N sources resulted in a rapid increase in SMB until 10 days af ter transplanting (DAT) followed by a gradual decline until 73 DAT. Plant N accumulation data in these treatments clearly indicated that the SMB under went a transition from a sink to a source of plant-available soil N during the period of crop growth. Seasonal variation of the SMB was small in treat ments without amendment of organic material (CON, PU) presumably due to a l ack of available C as energy source. Extractable NORG was significantly aff ected by soil planting status and organic N source amendment, but represent ed only a small N pool with little temporal variation despite an assumed ra pid turnover. Among the three treatments receiving the same amount of N fro m different sources, the recovery efficiency of applied N was 58% for PU an d 28% For both RS + PU and SES treatments at 73 DAT. The N uptake of rice, however, was not driven try N availability alone, as most evident in the RS + PU treatment. We assume that root physiological functions were impeded a fter application of organic N sources.