Nitrous oxide and carbon dioxide emissions during initial decomposition of animal by-products applied as fertilisers to soils

The recycling of organic wastes as soil amendments is notably promoted in sustainable agricultural systems. However, for many animal by-products approved by organic farming regulations little is known about their effects on the greenhouse gas balance of the soil, in particular on N2O emissions. In this work we report on the N2O and CO2 emissions from six animal-derived wastes (horn and hoof meal, blood meal, hydrolysed leather, meat bone meal, chicken manure and a commercial organic mixed fertiliser). We compared these emissions to those from a mineral fertiliser (calcium ammonium nitrate) in a sandy and a loam soil during a three month laboratory incubation study. N2O flux dynamics varied strongly with residue category and soil type. In the sandy soil, cumulative N2O emissions correlated with soil NO3− content. Although the mineral fertiliser produced the highest total N2O emissions (5.7 mg N2O–N kg− 1 soil), the commercial organic fertiliser mixture led to statistically similar emissions (5.3 mg N2O–N kg− 1 soil). The other by-products emitted between 1.3–3.0 mg N2O–N kg− 1 soil, and only blood meal emitted less than 1 mg N2O–N kg− 1 soil. In the loam soil, N2O emissions never exceeded 1.0 mg N2O–N kg− 1 soil, and did not correlate with NO3− in soil. With regard to CO2 emissions and C storage potential, chicken manure was the only residue that significantly accumulated C in both soils during the course of our experiment. The addition of an extra easily available source of C (glucose) or N (NO3−) influenced N2O emissions differently depending on the residue applied. Our results showed that despite the extra amount of C added with the organic amendments, N2O emissions from animal-derived wastes were never higher than those from the mineral fertiliser.