Soil organic carbon regulation by pH in acidic red soil subjected to long-term liming and straw incorporation

The manipulation of soil pH through liming and straw incorporation plays a pivotal role in influencing soil organic carbon (SOC) dynamics in acidic red soil. This study aimed to assess the impact of these practices on SOC and elucidate the relationship between SOC and pH. Over a 31-year field experiment, seven different fertilization treatments were implemented: unfertilized (CK), nitrogen and potassium fertilizers (NK), NK with lime (NKCa), nitrogen, phosphorous, and potassium fertilizers (NPK), NPK with lime (NPKCa), NPK with straw (NPKS), and NPKS with lime (NPKSCa). Results revealed that liming and straw incorporation significantly elevated soil pH by 0.13-0.73 units. Lime application boosted SOC and mineral-associated organic carbon (MAOC) by 20.2% and 28.7%, respectively, in NK treatment, whereas its impact on SOC in NPK and NPKS treatments were negligible. SOC witnessed a 17.1% increase with NPKS and a 15.2% increase with NPKSCa compared to NPK alone. Notably, NPKS and NPKSCa led to a significant surge in particulate organic carbon (POC) by 19.7% and 37.7%, respectively, albeit NPKSCa reduced MAOC by 14.9% relative to NPK. Linear regression analysis unveiled a positive correlation between POC and soil pH, while SOC and MAOC exhibited an initial rise at lower pH levels followed by stabilization as pH continuously increasing. A partial least squares path model showed two pathways through which pH influenced SOC: firstly, by positively affecting SOC through increasing Fe and Al oxides contents and enhanced aggregate stability, and secondly, by negatively influencing SOC through altered ratios of fungi/bacteria and Gram-positive bacteria/Gram-negative bacteria. In conclusion, the long-term effects of lime and straw application on SOC and MAOC were contingent upon soil pH, with more pronounced positive effects observed at lower pH levels. These findings underscore the importance of considering soil pH when implementing lime and straw strategies to mitigate acidification and regulate SOC in acidic red soil.