Soil redistribution reduces integrated C sequestration in soil-plant ecosystems: Evidence from a five-year topsoil removal and addition experiment

Abstract

Soil redistribution is an important movement process of surface material that drastically affects the dynamic cycling of essential elements and soil productivity in the terrestrial ecosystem. Although a number of studies have focused on the soil organic carbon (SOC) pool under soil redistribution, few studies have comprehensively assessed the effects of soil redistribution on the ecosystem carbon pool. A five-year experiment of simulated topsoil redistribution (removal and addition, R-A) was conducted to explore the effects of soil redistribution on soil CO2 emissions, the SOC pool and the plant C pool in the semi-arid Loess Plateau of China. The soil properties, soil CO2 emissions, and crop parameters were measured after the topsoil removal (R20) treatment and topsoil addition (A20) treatment and in the undisturbed control (CK). Our results showed that 1) compared with the undisturbed control, the mean soil CO2 emissions of the R-A treatment decreased by 8% while the mean SOC pool increased by 3.0%; 2) the plant C pool was significantly reduced by 53.8% in 2015 and 5.8% in 2019 compared with the undisturbed control; 3) the ecosystem C pool (the sum of the SOC pool and plant C pool) of the R-A treatment was reduced by 10.3%, 3.7% and 1.1% compared with the ecosystem C pool of the undisturbed CK for 2015, 2016 and 2017, respectively, but increased by 0.6% and 1.9% from 2018 to 2019. During the first three experimental years, the decrease in the ecosystem C pool of the R-A treatment was caused by the reduced plant C pool in R20 exceeding the elevated plant C pool in A20. Over time, the soil bulk density (BD) decreased in R20 and the SOC, microbial biomass carbon (SMBC) and total nitrogen (TN) increased, which jointly improved the plant C pool in R20 and the ecosystem C pool by 2019. The gradual recovering pattern of the ecosystem C pool in the later years highlights the key role of plants in restoring soil properties and stabilizing ecosystem C cycling. Overall, our findings highlight that any attempt to assess the net impacts of soil redistribution, whether as a sink or source for atmospheric CO2, should comprehensively consider the changes in the SOC pool and plant C pool.