Impact of nitrogen fertilization and cropping systems on aggregate-bound soil organic carbon fractions in Southern India

Understanding soil organic carbon fractions in southern India is crucial for enhancing soil health, crop productivity, and sustainable land management. It is majorly affected by cropping systems, nitrogen levels, irrigation management, etc. Therefore, the present study aimed to investigate the effects of varying nitrogen doses [N₁: No nitrogen, N₂:100% of recommended dose of N (RDN), and N₃: 200% RDN] and different cropping systems (fieldbean, finger millet, and maize) on aggregate-associated soil organic carbon and its fractions, as well as their impact on aggregate stability under rainfed and irrigated conditions. Aggregates were separated into different classes and analyzed for organic carbon and its fractions (dissolved organic carbon, microbial biomass carbon, potassium permanganate oxidizable carbon, and non-labile organic carbon). Applying 200% RDN enhanced total organic carbon (TOC) and its fractions. Conversely, aggregate stability was not influenced by N levels as determined by mean weight diameter and tensile strength. Macroaggregates (> 250 μm) had higher total organic carbon and their fractions than microaggregates (< 250 μm). The effect of the cropping systems was significant and the maize cropping system had the highest content of TOC and other fractions, followed by fieldbean and finger millet cropping systems. Mean weight diameter (MWD) was significantly higher in finger millet-grown soils. The impact of nitrogen fertilizer and cropping systems on aggregate-associated organic carbon was more pronounced in irrigated than rainfed conditions, indicating the potential for carbon sequestration under irrigated conditions. Thus, the optimum level of nitrogen and the type of cropping system adopted influence the distribution pattern of aggregate-associated organic carbon, along with its fractions that play a pivotal role in carbon accumulation and stabilization.