Effect of soil erosion in slope cultivated land of Longmenshan earthquake zone on lateral movement of soil organism carbon

Abstract: Soil erosion in the sloping farmland has been recognized as a major contributor that affects soil organic carbon (SOC) stocks and dynamics. However, understanding of the influence of accelerated soil erosion (water erosion and tillage erosion) on carbon dynamics is limited. In particular, little is known on the influence of earth quake-induced erosion and deposition on SOC stocks and dynamics in terraced field systems and steep sloping farmland in Longmenshan earthquake zone, China. In this study, we assessed the spatial variation of soil erosion and lateral movement of soil organic carbon (SOC) in toposequence of stone dike terraces as well as a steep sloping farmland of Longmenshan earthquake zone, China using 137Cs technique and field investigation. In this study area, effective 137Cs reference value of the yellow brown soil was estimated at 1 473 Bq/m2. Soil loss appeared over the upper parts of the slopes and deposition occurred towards the downslope boundary on each terrace, as well as soil loss at upper terraced fields and soil accumulation at lower terraced fields. Those results indicated that water erosion could transport soil from upper terrace to lower terrace due to lacking banks between two adjacent terraces. It should be noted that net soil erosion rate in the toposequence of the terraced fields was very low. Meanwhile, tillage erosion played an important role in transporting soil from upper slope positions to lower positions within a terrace. Soil erosion rates in the steep slope were higher than those in the toposequence of terraced fields. Besides water erosion, collapse of terrace resulting from earthquake and tillage erosion were also important soil erosion processes on the complex slope. In the steep sloping farmland, soil erosion rates were high at the summit and the lower slope with a high slope curvature. Soil accumulation appeared at lower slope with a low slope curvature and toe slope position. Discrete patterns of SOC inventories (mass per unit area) appeared over the whole terraced toposequence, while SOC inventories were low over the upper parts of the slopes but increased towards the downslope boundary on each terrace. For the steep slope farmland, SOC inventories were lower at the top of slope and at lower slope position with a high slope curvature. Soil organic matter inventories in the terraced filed series and steep slope farmland showed a similar pattern as the 137Cs inventories. Those patterns were consistent with redistribution of SOC with soil as coupling effect of tillage erosion, water erosion and collapse due to earthquake. Those results indicated that terraced fields played an important role in soil conservation and SOC sequestration in the Longmenshan earthquake zone, China. Although different soil erosion processes were observed between the terraced field series and steep sloping farmland, severe erosion due to anthropogenic activity after the Wenchuan Earthquake significantly changed spatial variations in SOC inventories. In other words, tillage erosion was also one of the important soil erosion processes within a sloping farmland. More attention should be paid to prevention of tillage erosion in this area. Our results demonstrated that terrace with stone dike can better conserve soil in this earthquake stricken area. Soil erosion caused by tillage was one of the major soil erosions in this region for the sloped land. Soil conservation practice must be taken into a consideration in agriculture production. Our results also showed that 137Cs can be used to successfully trace soil erosion and SOC dynamics in sloping farmland in the Longmenshan earthquake zone, China.