The effect of afforestation type on soil nitrogen dynamics in the riparian zone of the upper Yangtze River of China

Abstract

Afforestation is increasingly important in nutrient cycling in riparian ecotones given that ecosystems in riparian zones are susceptible to anthropogenic activities induced by land use change. However, how land use change (e.g., afforestation) with different planting types influences nitrogen (N) dynamics in riparian zones remains unclear. Here, we examined soil N dynamics following afforestation with three types of plantations of pure willow (Salix babylonica), pure mulberry (Morus alba), and the mixed two species paired with adjacent maize croplands in the upper Yangtze River of China. Our results showed afforestation with the two pure species significantly reduced soil total N (TN) concentration. Soil NO3−-N concentration was significantly reduced by the willow and mixed-species afforestation, but soil NH4+-N concentration was significantly higher in the willow and mixed woodlands compared to the paired croplands. Soil N concentrations were tightly associated with the potential N transformation rates, which showed a roughly decreasing trend in N mineralization following afforestation. Soil properties, microbial biomass, and extracellular enzymes jointly explained a large proportion of the total variation in soil N concentrations, with soil enzymes largely contributing to N variation in the topsoil and soil properties primarily contributing to N variation in the subsoil. Overall, our results demonstrate that afforestation with different planting types had contrasting effects on soil N content in the riparian zone. These findings provide new insights into the management of afforestation types to retain soil N by mediating soil properties and microbial activities in the riparian zones under future land use change.