Humus, nitrogen and energy balances and greenhouse gas emissions of organic farming with biowaste compost fertilization

Abstract

Soil organic matter plays a key role for soil quality and holds a large carbon sequestration potential. Compost fertilization is a way to increase soil organic matter content. Here, humus, nitrogen and energy balances and greenhouse gas emissions were calculated for a 14-year field experiment using the model software REPRO. Humus balances showed that compost fertilization at a rate of 8 t ha^-1 y^-1 (C1) resulted in a positive balance of 115 kg C ha^‑1 y^-1. With 14 and 20 t ha-1 y^-1 compost (C2 and C3), respectively, humus accumulated at rates of 558 and 1021 kg C ha^-1 y^-1. With mineral fertilization at rates of 29 to 63 kg N ha^-1 y^-1 (N1, N2, N3), balances were moderately negative (‑169 to -227 kg C ha^-1 y^-1), while a clear humus deficit of -457 kg C ha^-1 y^-1 showed in the unfertilized control. Compared with measured soil organic carbon (SOC) data REPRO predicted SOC contents fairly well with the exception of the treatments with high compost rates. Here REPRO clearly overestimated SOC contents for this site. Nitrogen balance was about zero in the control and in treatment N1, 10 to 20 kg N ha^-1 y^-1 in C1, C2, N2 and N3, and 29 kg N ha^-1 y^-1 in C3. Energy efficiency, as described by the output/input ratio, was highest in the control, followed by C1. Mineral fertilization treatment N3 was most energy intensive. The REPRO GHG balance indicated net carbon sequestration already with medium compost rates (C2), and net carbon sequestration of 1700 kg CO₂-eq ha^-1 y^-1 in C3. Mineral fertilization yielded net GHG emissions of around 2000 kg CO₂-eq ha^-1 y^-1. The highest GHG emissions had the unfertilized control due to SOC decline. The findings underline that compost fertilization holds a potential for carbon sequestration and for the reduction of greenhouse gas emissions.

Keywords: modelling, REPRO, soil organic carbon, greenhouse gas emissions