Conversion effects of farmland to Zanthoxylum bungeanum plantations on soil organic carbon fractions in the arid valley of the upper reaches of the yangtze river, china

Abstract

Farmland afforestation is an effective measure to sequester carbon (C) into soil, but the effects of different farmland conversion approaches on soil organic C (SOC) fractions in arid valley are still not clear. In this work, the concentrations of each SOC fraction in 0–100 cm soil depth, litter stock (LS) and fine root biomass (FRB) in farmland (FL), abandoned land (AL) and Zanthoxylum bungeanum plantations of different ages (with planting ages of 8, 15, 20 and 28 years, abbreviated as ZB8, ZB15, ZB20 and ZB28, respectively) were measured. The results showed that 1) the concentrations of each SOC fraction in the same soil layer increased after the farmland conversion, and these indices ranked as AL > ZB28 > ZB20 > ZB15 > ZB8 > FL, and all of them decreased with the soil depth; 2) the SOC stock, SOC sequestration and SOC sequestration rate increased by 120.1–158.9 Mg ha−1, 4.9–43.8 Mg ha−1 and 0.6–1.6 Mg ha−1 yr−1, respectively, after FL was converted to AL and Z. bungeanum plantations and ranked the same order as the SOC fractions with the land-use types; and 3) the C pool management index (CPMI), LS and FRB of different land-use types ranked as AL > Z. bungeanum plantations > FL, and LS and FRB were significantly (p < 0.05) correlated with each SOC fraction and CPMI, while CPMI was significantly (p < 0.05) correlated with each SOC fraction. This implies that the farmland conversion in arid valleys mediated by LS and FRB is helpful to improve the size and activity of soil C pools, and CPMI can be used as a useful index to estimate soil C pools in this region. In this sense, litter and roots should be emphasized in the process of farmland conversion to increase soil C sequestration.