Impacts of species mixture on soil nitrogen stocks in the Loess Plateau of China

Abstract

Nitrogen (N) is one of the primary limiting elements in terrestrial ecosystems, and vegetation restoration has been regarded as an effective way to increase soil total nitrogen (STN) stocks. Despite compelling evidence indicating that mixed plantations can demonstrate higher vegetation productivity and carbon (C) sequestration capacity than monocultures, the effects of mixed plantations on STN stocks remain unclear. Therefore, we conducted a meta-analysis of 128 observations from 49 peer-reviewed articles across the Loess Plateau of China to evaluate the impact of different afforestation models (i.e., mixed plantations and monocultures) on the dynamics of STN stocks in mineral soil (0–30 cm). The results showed that, compared with monocultures, mixed plantations significantly increased STN stocks by 19.3%. For different mixed types, tree-shrub mixed plantations increased more STN stocks than tree-tree mixed plantations. However, the positive effect of mixed plantations on STN stocks was significant > 20 years after stand establishment. Moreover, for areas with steeper slope (>25°), lower elevation (<1200 m), and climate restricted (MAT < 8℃, MAP < 500 mm), mixed plantations had more advantages in increasing STN stocks than monocultures. Additionally, the response ratio of STN stocks was negatively correlated with the initial STN stocks, response ratios of soil pH, and bulk density (BD) but positively correlated with the response ratio of soil organic carbon (SOC) stocks. Our results demonstrate that increasing tree species richness is an important strategy to increase STN stocks.