龙门山地震带坡耕地土壤侵蚀对有机碳迁移的影响

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

坡耕地土壤再分布对土壤有机碳(SOC,soil organic carbon)迁移的作用机制研究已成为土壤侵蚀学研究的热点,然而目前极少有研究关注地震后生态脆弱的龙门山地震带坡耕地土壤侵蚀机理及其导致的土壤有机碳再分布规律。该研究选择龙门山地震带内(都江堰市)一块陡坡耕地和一个梯田系列,采用137Cs法和野外调查,对比分析强震导致田埂垮塌和未受损情况下坡耕地土壤侵蚀空间变化特征和有机碳运移变化机理。结果表明,该区黄棕壤有效137Cs背景值为1 473 Bq/m2;坡度较小的坡式梯田内部上坡表现为侵蚀,下坡表现为沉积,同时,上部梯田的侵蚀速率高于下部梯田,但整个梯田系列净侵蚀量非常小,这表明梯田之间由于缺乏田埂的保护,水力也起着侵蚀、搬运上坡梯田土壤的作用,但是整个坡式梯田系列可以起到较好的保土作用,同时,坡式梯田内部主要以耕作侵蚀为主,是造成梯田上部坡位土壤流失严重的主要原因;陡坡耕地的地形为复合坡,由于田埂垮塌导致其土壤侵蚀速率显著高于坡式梯田系列,在整个坡面上,除了坡顶土壤侵蚀速率高之外,下坡坡度变大(曲率较大)的部位土壤侵蚀速率也非常高,同时,土壤沉积也发生在2个坡位(中下坡坡度较缓的部位和坡脚部位);在梯田系列和陡坡耕地上,SOC与土壤137Cs的空间变化规律较为一致。研究结果表明,在龙门山地震带,质量较好的石埂梯田仍然发挥着较好的土壤保持效果,同时,耕作侵蚀是该区坡耕地上一种重要的土壤侵蚀形式,在制定相应的土壤保持措施时,必须充分考虑耕作侵蚀的作用,才能有效地控制土壤侵蚀,此外,该研究结果还表明采用137Cs核素示踪技术可以比较科学地解释该区域的土壤侵蚀速率和SOC的空间变异规律。

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 soft 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 s.loping 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.