Combined effect of biochar, cover crop residues, and manure on greenhouse gas emissions: Insights from 60-day soil incubations

Cover crops and manure are popular climate-smart tools providing ecosystem services. However, their potential can be compromised by increased greenhouse gas (GHG) emissions during decomposition. Stable, carbon-rich biochar can potentially offset such effects. We investigated soil GHG emissions during the decomposition of different cover crops and poultry litter (PL) with/without cotton biochar. Soil samples from an organic cotton field were incubated in a 5 x 2 factorial design in 2022 and 2023. Different cover crops were collected from the field experiment and PL combinations, such as PL with oat (Avena sativa L.), Austrian winter pea (Pisum sativum) (AWP), and a mix of oat and AWP; only PL and a control were used as factor 1. Cotton residue biochar pyrolyzed at 350 degrees C was applied to field-collected soils at a rate of 1% w/w as the second factor. After 60 days of incubation, the lower C:N ratios of AWP and mixed species led to increased soil CO2 and N2O emissions than oats, irrespective of biochar addition for both years. Biochar reduced soil N2O emissions by 49% and 43% in 2022 and 2023, respectively. Although cover crops and manure did not affect soil CH4 uptake, biochar reduced soil CH4 uptake by 37.5% in 2023. Regardless of biochar addition, AWP and mixed-species cover crops with manure had greater soil total nitrogen at the end of incubations. On the other hand, biochar increased both soil total carbon and nitrogen at the end of incubations regardless of the residue treatments. Overall, biochar can enhance the agroecosystem services of cover crops and manure through reduced GHG emissions and increased carbon and nitrogen stocks.