Rice Cultivation Duration Is Associated with Changes in Potential CO2 Production, Q10, Enzyme Activity, and Microbial Diversity in Saline–Alkali Soils

Abstract

Saline–alkali soils are important reserve resources for agricultural production. In western Jilin Province, China, large areas of saline–alkali land have been reclaimed as paddy fields; however, their carbon cycling responses to long-term rice cultivation and warming remain unclear. Using a space-for-time chronosequence approach, a laboratory incubation experiment was conducted at 15 °C and 25 °C using soils collected from two soil layers (0–20 and 20–40 cm) in fields with 2, 5, 10, and 12 years of rice cultivation, together with an uncultivated reference field (CK). The results showed that cumulative potential soil carbon dioxide (CO2) production was generally higher in cultivated soils than in CK, peaking at 5 years under 15 °C and at 10 years under 25 °C. It subsequently declined in the 10- and 12-year treatments at 15 °C and decreased slightly at 12 years at 25 °C. Soils with longer rice cultivation histories generally exhibited relatively high apparent temperature sensitivity (Q10) at several stages of incubation, particularly in the 0–20 cm soil layer. Hydrolytic enzyme activities generally increased along the rice cultivation chronosequence, whereas polyphenol oxidase activity showed an overall declining trend, reflecting differences in potential enzymatic capacity for carbon transformation among cultivation duration treatments. Bacterial and fungal alpha diversity was generally higher in cultivated soils, particularly from the 5-year stage onward, and they also differed significantly among rice cultivation duration groups in both soil layers, although the fungal pattern in the 20–40 cm layer was partly influenced by heterogeneous within-group dispersion. Partial least squares path modeling showed that enzyme activity was strongly associated with cumulative potential soil CO2 production at 25 °C and Q10. These findings indicate that potential SOC mineralization across the rice cultivation chronosequence showed clear temperature-dependent patterns. This study provides insights into carbon cycling and its temperature response in reclaimed saline–alkali paddy soils and may inform their sustainable management under changing temperature conditions.