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

Abstract

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⁻¹ K₂SO₄ extractable N (extractable N), NO₃-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 NO₃-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 (NH₄)₂SO₄ 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 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⁻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 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.

Key words:

• extractable nitrogen
• microbial biomass nitrogen
• nitrate leaching
• nitrogen uptake
• organic manure

ACKNOWLEDGMENTS

The authors would like to acknowledge the Ministry of Agriculture, Forestry and Fisheries of Japan for financial support. We thank Professor Minoru Yamauchi (National Agricultural Research Center for Western Region) for his valuable advice and discussions.