Changes in soil organic carbon and total nitrogen stocks in response to walnut plantation in the southern region of the Loess Plateau, China

Abstract

Afforestation of croplands has been widely adopted on the semi-arid Loess Plateau in recent decades. However, knowledge of soil carbon and nitrogen changes after afforestation is still limited because of the complexity of afforestation approaches. Therefore, an experiment was conducted on two dominant arable soils (tableland soil and terrace soil) on the southern Loess Plateau to evaluate the impact of two afforestation approaches (walnut monocultures and walnut-based agroforestry) on the soil organic carbon (SOC) and soil total nitrogen (STN) content within 2 m soil profiles. In the tableland soil, compared with the continuous crop monoculture (135.90 t C ha−1), SOC stocks in the agroforestry and walnut monoculture system increased by 7.91% and 1.91%, respectively. While in the terrace soil, compared with crop monoculture (102.95 t C ha−1), SOC stocks in the agroforestry and walnut monoculture system decreased by 3.86% and 0.20%, respectively. The STN stock was lower in the tree monocultures and SOC content was significantly correlated with STN. Afforestation increased the SOC and STN stocks in the 0–10 cm soils, but decreased them in the 10–60 cm soils. In the deep soil layers (60–200 cm), SOC content increase was only found in agroforestry in the tableland soil, and no significant differences were found in the terrace soil. Converting cropland to agroforestry has advantages in terms of soil carbon sequestration.