Characteristics of particulate and mineral-associated organic matter and soil microbiota in response to nitrogen-rich organic inputs

A significant amount of agricultural inputs, such as nitrogen fertilizers, is required due to the increased demand for food and fiber. Nitrogen applied to the soil through organic residues and compounds is more sustainable and environmentally friendly than through inorganic sources. This study evaluated the impact of organic nitrogen sources on carbon dynamics and microbial activity in a citrus orchard soil in northeastern Brazil. Five treatments (Control soil without fertilization, sheep manure + biofertilizer spray, sheep manure, castor bean cake, and green manure, thin branches from gliricidia cut from the plant) were tested. Fertilizers were applied under the plant canopy in 2021, 2022, and 2023 twice a year. Soil samples analysis showed that the distribution of soil organic carbon between particulate organic matter (POM) and mineral-associated organic matter (MAOM) was significantly influenced by the application of nitrogen-rich organic amendments. In the control treatment, the dominance of MAOM over POM followed a typical pattern observed in soils, reflecting the stable nature of MAOM and greater capacity for long-term carbon storage. However, the castor bean cake disrupted this balance by significantly increasing POM and, in some cases, reducing MAOM. In contrast, the green manure treatment did not change the POM and MAOM balance. The results suggest that green manure, with its higher lignin and polyphenol content, did not significantly enhance the labile carbon pool or disrupt carbon stabilization in the MAOM fraction. These findings underscore the role of residue composition in determining the fate of carbon in soil organic matter pools.