Organic amendment enhanced microbial nitrate immobilization with negligible denitrification nitrogen loss in an upland soil

Both soil microbial nitrate (NO3−-N) immobilization and denitrification are carbon (C)-limited; however, to what extent organic C addition may increase NO3−-N immobilization while stimulate denitrification nitrogen (N) loss remains unclear. Here, 15N tracing coupled with acetylene inhibition methods were used to assess the effect of adding glucose, wheat straw and peanut straw on NO3−-N immobilization and denitrification under aerobic conditions in an upland soil, in which NO3−-N immobilization has been previously demonstrated to be negligible. The organic C sources (5 g C kg−1 soil) were added in a factorial experiment with 100, 500, and 1000 mg N kg−1 soil (as K15NO3) in a 12-d laboratory incubation. Microbial NO3−-N immobilization in the 12-d incubation in the three N treatments was 5.5, 7.7, and 8.2 mg N kg−1 d−1, respectively, in the glucose-amended soil, 5.9, 4.2, and 2.4 mg N kg−1 d−1, respectively, in the wheat straw-amended soil, and 4.9, 5.1 and 4.4 mg N kg−1 d−1, respectively, in the peanut straw-amended soil. Therefore, under sufficient NO3−-N substrate, the higher microbial NO3−-N immobilization in the glucose than in the crop residue treatments was likely due to the slow decomposition of the latter that provided low available C. The 15N recovery in the N2O + N2 pool over the12-day incubation was <2% for all treatments, indicating negligible denitrification N loss due to low denitrification rates in the aerobic incubation in spite of increasing C availability. We conclude that external C addition can enhance microbial NO3−-N immobilization without causing large N losses through denitrification. This has significant implications for reducing soil NO3−-N accumulation by enhancing microbial NO3−-N immobilization through increasing C inputs using organic materials and subsequently mitigating nitrate pollution of water bodies.