摘要：

To determine the relationships between microbial biomass nitrogen (N), nitrate-nitrogen leaching (NO_3-N leaching) and N uptake by plants, a field experiment and a soil column experiment were conducted. In the field experiment, microbial biomass N, 0.5 mol L^<-1> K_2SO_4 extractable N (extractable N), NO_3-N leaching and N uptake by corn were monitored in sawdust compost (SDC: 20 Mg ha^<-1> containing 158 kg N ha^<-1> of total N [approximately 50% is easily decomposable organic N]), chemical fertilizer (CF) and no fertilizer (NF) treatments from May 2000 to September 2002. In the soil column experiment, microbial biomass N, extractable N and NO_3-N leaching were monitored in soil treated with SDC (20 Mg ha^<-1>)+rice straw (RS) at five different application rates (0, 2.5, 5, 7.5 and 10 Mg ha^<-1> containing 0, 15, 29, 44 and 59 kg N ha^<-1>) and in soil treated with CF in 2001. Nitrogen was applied as (NH_4)_2SO_4 at rates of 220 kg N ha^<-1> for SDC and SDC+RS treatments and at a rate of 300 kg N ha^<-1> for the CF treatment in both experiments. In the field experiment, microbial biomass N in the SDC treatment increased to 147 kg N ha^<-1> at 7 days after treatment (DAT) and was maintained at 60-70 kg N ha^<-1> after 30 days. Conversely, microbial biomass N in the CF treatment did not increase significantly. Extractable N in the surface soil increased immediately after treatment, but was found at lower levels in the SDC treatment compared to the CF treatment until 7 DAT. A small amount of NO_3-N leaching was observed until 21 DAT and increased markedly from 27 to 42 DAT in the SDC and CF treatments. Cumulative NO_3-N leaching in the CF treatment was 146 kg N ha^<-1>, which was equal to half of the applied N, but only 53 kg N ha^<-1> in the SDC treatment. In contrast, there was no significant difference between N uptake by corn in the SDC and CF treatments. In the soil column experiment, microbial biomass N in the SDC + RS treatment at 7 DAT increased with increased RS application. Conversely, extractable N at 7 DAT and cumulative NO_3-N leaching until 42 DAT decreased with increased RS application. In both experiments, microbial biomass N was negatively correlated with extractable N at 7 DAT and cumulative NO_3-N leaching until 42 DAT, and extractable N was positively correlated with cumulative NO_3-N leaching. We concluded that microbial biomass N formation in the surface soil decreased extractable N and, consequently, contributed to decreasing NO_3-N leaching without impacting negatively on N uptake by plants.

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