Effect of long-term fertilizer application on soil aggregation and SOC stocks in semi-arid Alfisols of South India

A field experiment was initiated during kharif 1981 to study the long-term effects of fertilizers and manures application on soil aggregation and carbon sequestration in a rainfed Alfisols (Typic Haplustalfs) under groundnut (Arachis hypogaea L.) monocropping system at Regional Agricultural Research Station, Tirupati, Andhra Pradesh, India. After 42 years of experimentation, NPK+lime@100 kg ha^-1 treatment showed significant (p≤0.01) improvement in macro-aggregates (5-2mm) (8.0per cent), mean weight diameter (MWD) (0.81 mm), geometric mean diameter (GMD) (0.47 mm) compared to all other treatments under study. However, FYM @ 5 Mg ha^-1 and P @ 20 kg ha^-1treatments showed higher aggregate stability (AS) (1.66 and 1.65) and per cent water stable aggregates (%WSA) e” 0.25 mm (62.5 and 61.8per cent) compared to other treatments. Whereas, N or K fertilizers alone deteriorated the soil structure by decreasing MWD, AS and %WSA by 10–12, 22–26 and 11–15per cent, respectively compared to untreated control. Significantly higher soil organic carbon (SOC) stocks were recorded in NPK + lime@100 kg ha^-1 treatment (11.2 Mg ha^-1), representing a 250 per cent increase over initial SOC stocks (3.2 Mg ha^-1). In contrast, the lowest SOC stocks (8.6 Mg ha^-1) were recorded in the control treatment. (Similarly, the higher SOC stock buildup over control and build-up rate were also recorded in NPK + lime@100 kg ha^-1 treatment (30.5 per cent & 68 kg ha^-1 year^-1) whereas the lowest build-up % was recorded under K @ 25 kg ha^-1 alone applied treatment (3.63 per cent) compared to control. The present study demonstrates that the application of NPK+lime@100 kg ha^-1 is the best nutrient management strategy for enhancing soil aggregation and SOC stocks in rainfed Alfisols.