Effect of fresh green waste and green waste compost on mineral nitrogen, nitrous oxide and carbon dioxide from a Vertisol

Abstract

Incorporation of organic waste amendments to a horticultural soil, prior to expected risk periods, could immobilise mineral N, ultimately reducing nitrogen (N) losses as nitrous oxide (N2O) and leaching. Two organic waste amendments were selected, a fresh green waste (FGW) and green waste compost (GWC) as they had suitable biochemical attributes to initiate N immobilisation into the microbial biomass and organic N forms. These characteristics include a high C:N ratio (FGW 44:1, GWC 35:1), low total N (<1%), and high lignin content (>14%). Both products were applied at 3t C/ha to a high N (plus N fertiliser) or low N (no fertiliser addition) Vertisol soil in PVC columns. Cumulative N2O production over the 28day incubation from the control soil was 1.5mg/N2O/m2, and 11mg/N2O/m2 from the control+N. The N2O emission decreased with GWC addition (P<0.05) for the high N soil, reducing cumulative N2O emissions by 38% by the conclusion of the incubation. Analysis of mineral N concentrations at 7, 14 and 28days identified that both FGW and GWC induced microbial immobilisation of N in the first 7days of incubation regardless of whether the soil environment was initially high or low in N; with the FGW immobilising up to 30% of available N. It is likely that the reduced mineral N due to N immobilisation led to a reduced substrate for N2O production during the first week of the trial, when soil N2O emissions peaked. An additional finding was that FGW+N did not decrease cumulative N2O emissions compared to the control+N, potentially due to the fact that it stimulated microbial respiration resulting in anaerobic micro sites in the soil and ultimately N2O production via denitrification. Therefore, both materials could be used as post harvest amendments in horticulture to minimise N loss through nitrate-N leaching in the risk periods between crop rotations. The mature GWC has potential to reduce N2O, an important greenhouse gas.