Response of organic carbon mineralization and bacterial communities to soft rock additions in sandy soils

Bacteria play a vital role in biotransformation of soil organic carbon (SOC). However, mechanisms of bacterium and organic carbon mineralization remain unclear during improvement of sandy soil using soft rock additions. In this study, four treatments with differing ratios of soft rock to sand of 0:1 (CK), 1:5 (C1), 1:2 (C2) and 1:1 (C3) were selected for mineralization incubation and high-throughput sequencing. The results showed that SOC, total nitrogen (TN), available phosphorus (AP), nitrate nitrogen (NO3 -N), and mass water content (WC) of sandy soil increased significantly after addition of soft rock (P < 0.05). Compared with the CK treatment, cumulative mineralization and potential mineralized organic carbon content of C1, C2 and C3 increased by 71.79%-183.86% and 71.08%-173.33%. The cumulative mineralization rates of organic carbon treated with Cl and C2 were lower, 16.96% and 17.78%, respectively (P > 0.05). The three dominant bacteria were Actinobacteria, Proteobacteria and Chlorofleaci, among which Proteobacteria was negatively correlated with mineralization of organic carbon (P < 0.01). The mineralization rate constant (k) was positively correlated and negatively correlated with Cyanobacteria and Nitrospirae, respectively. Under C2 treatment, Proteobacteria and Nitrospirae had the largest increase, and Cyanobacteria had the largest decrease. Compared with other treatments, C2 treatment significantly increased bacterial diversity index, richness index and evenness index, and the richness index had a negative correlation with k value. In conclusion, when the ratio of soft rock to sand was 1:2, the k of SOC could be reduced. In addition, the retention time of SOC can be increased, and resulting carbon fixation was improved.