Use of composted manure and pig slurry to replace mineral fertilisers in the basal fertilisation of maize: impacts on GHG emissions and yield

Portuguese farmers seek evidence that organic fertilisers, particularly manure-based ones, can be safely used as partial replacements for mineral fertilisers (MF), taking advantage of their nutrient and organic matter (OM) content. This study aimed to clarify the effects of applying organic fertilisers, especially under no-till practices in maize production. The experiment involved replacing 30% of basal mineral nitrogen (N) fertilisation with composted animal manure (CM) and pig slurry (PS) across three contrasting fields with varying soil characteristics, tillage techniques, and OM supplementation. Results indicated that site 1, which had the lowest clay and OM content, benefited the most from organic fertilisers, particularly in enhanced nutrient absorption in both leaves and grain. In this site, partial nutrient balance of N was significantly higher in the PS treatment (0.56 kg grain N exported kg N applied-1) compared with the treatment with only MF (0.44). The impacts on greenhouse gas (GHG) emissions depended on site characteristics. CM led to higher emissions at site 2 (CH4) and site 3 (N2O), while PS did not increase GHG emissions at any site. Under no-till practices at site 3, CM resulted in higher global warming potential (154 kg CO2-eq ha-1) compared with the PS (128 kg CO2-eq ha-1) and MF (109 kg CO2-eq ha-1) treatments. Overall, this study suggests that organic fertilisers, particularly pig slurry, can be a viable strategy for improving soil health and maintaining sustainable maize production in Portuguese agriculture, especially in soils with lower OM content and under no-till practices.