Long-term afforestation significantly improves the fertility of abandoned farmland along a soil clay gradient on the Chinese Loess Plateau

In China, almost half of the total land area is degraded, mainly due to the unreasonable use of land for agricultural production. During the past half-century, afforestation has become a popular management option to remedy degraded soils. In this paper, we studied the long-term effects of afforestation on the major properties of soils with different textures. We measured the changes in soil physical, chemical, and fertility properties after 10, 20, and 30years of afforestation with black locust (Robinia pseudoacacia) on abandoned farmland at five sites on the Loess Plateau, China, which is characterized by a temperate monsoon climate. The soil clay contents varied significantly among sites. The objectives were to understand how long-term afforestation affects soil properties and whether such effects depend on the soil texture. Averaged across the five sites and the depths of 0-10 and 10-20cm, afforestation resulted in significant increases in the proportion of macroaggregates (+22.5%), saturated hydraulic conductivity (+185.4%), organic matter content (+110.5%), and total nitrogen content (+110.5%) but significant decreases in the proportion of microaggregates (-21.9%) and bulk density (-10.8%). The silt+clay fraction, soil pH, and nitrate content were minimally affected by afforestation. The effects of afforestation were greater at 0-10cm than at 10-20cm, but they varied between sites. Furthermore, most soil metrics linearly responded to the time of afforestation, and such responses were dependent on the soil clay content. Our results suggest that long-term afforestation improves soil fertility better in soils with a clay content of approximately 20%.