Which slope aspect and gradient provides the best afforestation-driven soil carbon sequestration on the China's Loess Plateau?

Abstract

On China's Loess Plateau, where the distribution of soil organic carbon (soil − Corg) is mainly influenced by topography, afforestation is an important strategy to sequester carbon and reduce soil erosion. To explore the effects of topography on soil − Corg, soil samples were obtained at six depths (0–0.1 m, 0.1–0.2 m, 0.2–0.4 m, 0.4–0.6 m, 0.6–0.8 m, and 0.8–1.0 m) from locations on the Loess Plateau which had supported apricot (Prunus armeniaca L.) growth for 16 years and differed in slope aspect (sunny or semi-sunny vs. shady or semi-shady) and gradient (14° ≤ S < 20°, 20° ≤ S < 25°, or 25° ≤ S ≤ 31°). Across a depth of 1.0 m, the mean soil − Corg and total soil carbon storage (soil − Ctot) under the shady slope were 7.23 g kg−1 and 77.41 Mg ha−1, respectively; while under the sunny slope, these were 6.81 g kg−1 and 74.22 Mg ha−1, respectively. Both the soil − Corg and soil − Ctot increased with slope gradient, with soil − Corg rising from 6.68 g kg−1 to 7.01 g kg−1, and to 7.34 g kg−1, and soil − Ctot changing from 72.60 Mg ha−1 to 75.35 Mg ha−1, and to 79.40 Mg ha−1, respectively. Slope aspect and gradient interacted in influencing soil − Corg, but not soil − Ctot. Across both slope aspect and gradient, roughly half of soil − Ctot was found within the first 0.4 m depth of soil. Overall a steep shady slope (vs. sunny gentle slope) was more favorable to the preservation of soil − Corg, and would, accordingly, be of greater importance with respect to soil − Corg protection and implementation of afforestation policy on the Loess Plateau and similar fragile environments worldwide.