Relationships between microbial biomass nitrogen, nitrate leaching and nitrogen uptake by corn in a compost and chemical fertilizer-amended regosol

To determine the relationships between microbial biomass nitrogen (N), nitrate-nitrogen leaching (NO₃-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[superscript [-]1] K₂SO₄ extractable N (extractable N), NO₃-N leaching and N uptake by corn were monitored in sawdust compost (SDC: 20 Mg ha[superscript [-]1] containing 158 kg N ha[superscript [-]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 NO₃-N leaching were monitored in soil treated with SDC (20 Mg ha[superscript [-]1]) [+] rice straw (RS) at five different application rates (0, 2.5, 5, 7.5 and 10 Mg ha[superscript [-]1] containing 0, 15, 29, 44 and 59 kg N ha[superscript [-]1]) and in soil treated with CF in 2001. Nitrogen was applied as (NH₄)₂SO₄ at rates of 220 kg N ha[superscript [-]1] for SDC and SDC [+] RS treatments and at a rate of 300 kg N ha[superscript [-]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[superscript [-]1] at 7 days after treatment (DAT) and was maintained at 60-70 kg N ha[superscript [-]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 NO₃-N leaching was observed until 21 DAT and increased markedly from 27 to 42 DAT in the SDC and CF treatments. Cumulative NO₃-N leaching in the CF treatment was 146 kg N ha[superscript [-]1], which was equal to half of the applied N, but only 53 kg N ha[superscript [-]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 NO₃-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 NO₃-N leaching until 42 DAT, and extractable N was positively correlated with cumulative NO₃-N leaching. We concluded that microbial biomass N formation in the surface soil decreased extractable N and, consequently, contributed to decreasing NO₃-N leaching without impacting negatively on N uptake by plants.