Effects of plantation age and precipitation gradient on soil carbon and nitrogen changes following afforestation in the Chinese Loess Plateau

Afforestation of degraded land significantly influences soil organic carbon (SOC) and total nitrogen (STN) sequestration. The interaction effects of plantation age and climate gradient on SOC and STN changes following afforestation are not well understood. In this study, five sites were selected along a precipitation gradient (410-600 mm yr(-1)) in the Loess Plateau. The SOC and STN stocks at a depth of 0-200 cm were measured in cropland and black locust (Robinia pseudoacacia L.) forests with different plantation ages, that is, young forest (<15 years), middle-aged forest (15-25 years), and old forest (>25 years). The SOC and STN stocks in the 0- to 200-cm profiles of young forest, middle-aged forest, and cropland increased significantly with mean annual precipitation (p < .05), whereas the increasing trend of the SOC stocks of old forest was not significant, indicating an age-dependent change in the SOC and STN stocks across the precipitation gradient. The SOC stock change ( increment SOC) following afforestation increased with mean annual precipitation in young forest, but it had a decreasing trend in middle-aged and old forests. The STN stock changes ( increment STN) in the three forests were negative at most sites, and they all decreased along the precipitation gradient. There were significant positive correlations between increment SOC and increment STN (p < .01), and 1-g STN stock accumulation was accompanied by 8.40-, 6.10-, and 10.48-g SOC accumulation for young forest, middle-aged forest, and old forest, respectively. The different patterns of SOC and STN stock changes should be incorporated into soil C and N modelling and estimation.