137Cs和210Pbex双核素示踪"三北"防护林区退耕前后坡地土壤侵蚀变化

为查明"三北"防护林建设前后农耕地和退耕地土壤保持效益变化，利用137Cs和210Pbex双核素示踪技术，选择了防护林建设较为成功的张家口坝上地区（风力侵蚀区）作为典型区，研究了农耕地以及退耕地土壤137Cs和210Pbex的剖面变化规律及其示踪的土壤侵蚀变化。结果表明：1）由于耕作的混匀作用，农耕地土壤剖面中137Cs和210Pbex均呈均匀态分布；退耕地土壤剖面中137Cs和210Pbex则表现为表层（0～5cm）浓度最高、下层（5～25cm）浓度均相对较低且分布相对均匀的形态，这表明退耕后坡地土壤137Cs和210Pbex剖面形态均会发生一定变化，退耕驱动土壤137Cs和210Pbex剖面变化导致运用土壤核素估算侵蚀模型在该区域难以适用；2）基于土壤137Cs和210Pbex剖面变化规律，利用210Pbex质量平衡方程，提出了退耕地土壤210Pbex土壤侵蚀估算模型；3）利用137Cs比例模型估算退耕地土壤侵蚀速率为27.94±11.92 (t/hm2·a)，农耕地侵蚀速率为29.11±14.42 (t/hm2·a)，而利用修正后的210Pbex转换模型估算得到"三北"防护林区退耕地造林前平均侵蚀速率为82.16±14.36 (t/hm2·a)，造林后平均侵蚀速率为-41.28±33.91 (t/hm2·a)；农耕地造林前平均侵蚀速率为68.55±22.11 (t/hm2·a)，造林后平均侵蚀速率-8.52±47.32 (t/hm2·a)。这表明137Cs示踪技术主要表征了1963年以来该区坡地土壤侵蚀和沉积的平均结果，而210Pbex示踪技术则可以较好地示踪防护林建成前后的土壤侵蚀变化。此外，研究结果也表明，相比于"三北"防护林建成之前，建成之后该区农耕地和退耕地的土壤侵蚀速率均呈显著下降趋势，且均由前期的风沙侵蚀转变成了风沙沉积。

137Cs and 210Pbex tracing of soil erosions on cultivated and reforested slope lands in Three North-Shelter Forest Region

Abstract: To explore the changes of soil retention in the cultivated land and reforested land due to the construction of the "Three North" shelterbelt, the 210Pbex and 137Cs measurements were used to trace variations in soil depth and soil erosion rates in the cultivated land and reforested land in the Bashang Region of Zhangjiakou City, where is a typical shelterbelt region and wind dominant region. Due to the mixing effect of tillage, both 137Cs and 210Pbex in the soil profile of cultivated land were uniformly distributed. For the reforested land, the 137Cs and 210Pbex concentrations in the 0-0.05 m soil layers were obviously higher than that in the other depth soil layers. The 137Cs and 210Pbex concentrations from 0.05 m to 0.15 m of the upper 0.15 m soil layer were roughly uniform. Those results indicated that the 137Cs and 210Pbex profile of the soil would change after reforestation and the use of 137Cs and 210Pbex to estimate variation in soil erosion rates under conditions of reforestation remains challenging. This result indicated that the soil erosion models of 137Cs and 210Pbex were difficult to be applied in this area. Based on the temporal change trends of 137Cs and 210Pbex profiles, a revised soil erosion model of 210Pbex for reforested land was proposed by using the 210Pbex mass balance equation. The mean soil erosion rates of reforested land and cultivated land, which were calculated by 137Cs proportional model, were 27.94±11.92 t/(hm2·a) and 29.11±14.42 t/(hm2·a), respectively. Before the Three-North shelter forest constructed, the soil erosion rate of reforested land and cultivated land, which was estimated by the revised 210Pbex conversion model, were 82.16±14.36 t/(hm2·a) and 68.55±22.11 t/(hm2·a), respectively. After the Three-North Shelter Forest constructed, the soil erosion rates of reforested land and cultivated land, which were estimated by the revised 210Pbex conversion model, were -41.28±33.91 t/(hm2·a) and -8.52±47.32 t/(hm2·a), respectively. Those results indicated that the 137Cs could characterize the average soil erosion rates and deposition rates on the slopes since 1963. However, the 210Pbex tracer technology could trace the variations in soil erosion rates before and after the shelter forest was constructed. In addition, those results showed that wind erosion rates of the cultivated land and reforested land obviously declined due to the construction of Three-North Shelter Forest and the cultivated land and reforested land have been transformed from erosion zone to deposition zone in the study area.