2026-08-01 APPLIED SOIL ECOLOGY 2026 224(卷), null(期), (null页)
Afforestation is a key strategy for restoring degraded ecosystems, yet how it influences the dynamics of soil inorganic nitrogen (IN) and organic nitrogen (ON) pools remains unclear. Here, we investigated soil N fractions (ammonium, nitrate, dissolved organic N, and microbial biomass N) and N-cycling functional genes along a chronosequence (15, 25, 35, and 45 years) of Robinia pseudoacacia plantations on the Loess Plateau in China. Our results showed that 25-45 years of afforestation led to a 24.3%-36.2% increase in soil total N and a 10.3%- 25.6% rise in dissolved organic N, while decreasing IN, nitrate N, and ammonium N by 27.9%-38.4%, 39.6%- 41.9%, and 14.1%-32.0%, respectively, compared with 15 years of afforestation. Afforestation increased the abundance of N-cycling genes, with increases observed in narG (5.0%-47.1%), nifH (11.2%-45.5%), and nirS gene (9.4%-119.8%). Afforestation enhances soil N retention primarily by transferring N from inorganic to organic pools, thereby potentially reducing N losses and fostering sustainable soil fertility in restored ecosystems.