Changes in microbial carbon cycling functions along rice cultivation chronosequences in saline-alkali soils

Saline-alkali soils are widely distributed worldwide and pose significant threats to food security and environmental safety. Rice cultivation is an effective strategy for ameliorating saline-alkali soils, increasing grain productivity, promoting soil organic carbon (SOC) accumulation and sustaining soil health in the Songnen Plain of Northeast China. This study investigated changes in microbial carbon cycling functions along rice cultivation chronosequences at two saline-alkali sites, Daan and Qianan. Rice cultivation improved soil health of the saline-alkali soils by neutralizing the pH and increasing the SOC content. Initial soil nutrient levels differed significantly at the two sites, but the nutrient dynamics were consistent, e.g., available nitrogen (AN) gradually increased, while available phosphorus (AP) initially increased and then decreased during rice cultivation. The diversity of carbon cycling genes increased with the duration of rice cultivation, while the relative abundance of genes associated with carbon decomposition decreased, which could promote carbon accumulation. These changes were primarily driven by the reduction in soil pH, followed by nutrient availability. Interestingly, at the Daan site, where AP and AN levels were relatively low, these nutrients significantly influenced carbon cycling genes. Conversely, at the Qianan site, where AP and AN levels were higher, there was less impact of nutrients than SOC on carbon cycling genes. This suggests that nutrient stoichiometry may be regulating carbon cycling genes. The soil succession process can be divided into two stages: less than 15 years of rice cultivation and more than 15 years. With longer-term rice cultivation, first the nitrogen limitation and then the phosphorus availability constrained microbial carbon cycling functions, and nutrient availability became more important than the soil pH effect. The soil nutrient and pH dynamics, together with flooding conditions in rice paddies, may limit microbial carbon decomposition, thereby promoting SOC accumulation in saline-alkali soils.