Humus, nitrogen and energy balances, and greenhouse gas emissions in a long-term field experiment with compost compared with mineral fertilisation

Compost fertilisation is one way to close material cycles for organic matter and plant nutrients and to increase soil organic matter content. In this study, humus, nitrogen (N) and energy balances, and greenhouse gas (GHG) emissions were calculated for a 14-year field experiment using the model software REPRO. Humus balances showed that compost fertilisation at a rate of 8 t/ha. year resulted in a positive balance of 115 kg carbon (C)/ha. year. With 14 and 20 t/ha. year of compost, respectively, humus accumulated at rates of 558 and 1021 kg C/ha. year. With mineral fertilisation at rates of 29-62 kg N/ha. year, balances were moderately negative (-169 to -227 kg C/ha. year), and a clear humus deficit of -457 kg C/ha. year showed in the unfertilised control. Compared with measured soil organic C (SOC) data, REPRO predicted SOC contents fairly well with the exception of the treatments with high compost rates, where SOC contents were overestimated by REPRO. GHG balances calculated with soil C sequestration on the basis of humus balances, and on the basis of soil analyses, indicated negative GHG emissions with medium and high compost rates. Mineral fertilisation yielded net GHG emissions of similar to 2000 kg CO2-eq/ha. year. The findings underline that compost fertilisation holds potential for C sequestration and for the reduction of GHG emissions, even though this potential is bound to level off with increasing soil C saturation.