The interplay between external residue addition, and soil organic carbon dynamics and mineralization kinetics:...

Maintaining soil carbon is vital under changing climate. Conservation agriculture (CA) is reported to have potential to store soil organic carbon (SOC). The impact of carbon inputs on SOC dynamics and mineralization kinetics, and the priming effect of residue addition under long-term CA in subtropical regions, however, are not clear or adequately evaluated. Therefore, we studied these under a 12-year-old CA-based pigeon pea-wheat cropping system with permanent broad bed with residue (CA-PBB), permanent flatbed with residue (CA-PFB), permanent narrow bed with residue (CA-PNB), and conventional till (CT) treatments. Also, an incubation study was undertaken to understand better the processes involved. Results showed that CA treatments significantly enhanced the total SOC compared to CT practice, and, among them, the CA-PFB exhibited highest total SOC with 36.6% and 35.8% higher values at 0-5 and 5-15 cm depths, respectively. The CA-PFB followed by CA-PBB and CA-PNB had significantly higher carbon management index and carbon retention efficiency than CT. The CA-PFB also showed higher carbon sequestration rates of 68.4 and 188.8 kg ha^-1 year^-1, surpassing values of 8.4 and 52.9 kg ha^-1 year^-1 under CT at 0-5 and 5-15 cm depth, respectively. Furthermore, soil incubation study revealed that the CA systems had higher cumulative mineralization values at 0-5 cm soil layer but lower at 5-15 cm soil compared to CT, indicating a considerable improvement in SOC at 5-15 cm soil depth. On the contrary, the SOC decay rate was higher under CA than CT, and at 35 °C than at 15 °C. A positive priming effect was also observed, depending on the substrate type, pigeon pea residue exhibiting higher priming effect than wheat residue. Thus, these studies show that residue input increases cumulative mineralization and SOC decay rate vis-à-vis helps to sequester carbon in the recalcitrant fraction, leading to higher stable carbon in soil.