137Cs tracing of soil erosion and its impact on soil nutrients across subsidence slope induced by coal mining

Abstract: The variation in soil nutrients related to soil erosion is crucial to the understanding of land degradation in mining areas. In this study, a representative subsidence slope induced by coal mining in Jiaozuo mine area, China was selected as the research site. The slope is a three-degree slope, has a horizontal length of 100 m, and has experienced a subsidence period of 15 years. Soil erosion and its impacts on total organic carbon (TOC), water soluble organic carbon (WSOC), total nitrogen (TN) and phosphorus (TP), and available nitrogen (AN) and phosphorus (AP) were determined by using the 137Cs technique. The 137Cs reference inventory, i.e. the total 137Cs concentration held in natural soil profiles free of erosion and human disturbance, was (1645±34) Bq/m2 in the study area. Across the subsidence slope, it was found that 137Cs inventory gradually decreased from the summit to the lower slope positions and then sharply increased at the toe slope positions where the highest values occurred. The summit, upper, middle and lower positions of the subsidence slope were erosional, which was indicated by the lower 137Cs inventories in these positions than the 137Cs reference level. The most erosional positions were found in the middle slope and lower slope. On the other hand, the elevated 137Cs inventories in the toe slope positions revealed the occurrence of the deposition. It was evaluated that the subsidence slope suffers from a moderate erosion with erosion rate estimated at 3.75 kg/(m2·a) according to the Ministry of Water Resources criteria for soil erosion (SL190-2007). Soil clay content increased downslope on the subsidence slope, suggesting the selective transport after the water erosion. The contents of soil nutrients including TOC, TN, TP, AN, and AP were lower at the erosional sites than the depositional sites of the subsidence slope. Compared with the control field, i.e. a non-subsidence cropland near the subsidence slope, the erosional sites of the subsidence slope showed significantly lower TOC, WSOC, TN, TP, AN, AP contents (P<0.05). Noteworthily, the TOC pool was highly degraded in the erosional sites, with a depletion up to 25 t/hm2. However, the depositional sites of the subsidence slope had similar soil nutrient contents compared with the control field (P>0.05), except for the WSOC, which decreased significantly (P<0.05). In addition, within the erosional sites the contents of both TOC and WSOC decreased downslope and showed a pattern similar to 137Cs distribution, whereas the similarity was not observed between other nutrients and 137Cs. Both TOC and WSOC contents were closely correlated to 137Cs inventories within the erosional sites (r=0.77, P<0.01 between TOC and 137Cs; r=0.82, P<0.01 between WSOC and 137Cs). These results suggest that soil erosion by water is serious on the subsidence slopes induced by coal mining in Jiaozuo mine area since the subsidence slopes formed. The dynamics of soil carbon, nitrogen and phosphorous are associated with soil redistribution on the subsidence slopes. The contents of both TOC and WSOC decrease with the increasing of soil erosion. In this study, it was also found that the ratios of WSOC to TOC were significantly greater at the most erosional positions of the subsidence slope compared with the control filed, whereas the ratios of TOC to TN (i.e. C/N) and that of TOC to TP (i.e. C/P) were significantly smaller at the most erosional positions (P<0.05). It can be expected that available carbon, nitrogen and phosphorus would be at a risk of erosion at the intense erosion positions of the subsidence slopes. Effective land consolidation should be taken in the overlapped areas of crop and mineral production in order to combat the soil erosion induced by the mining subsidence and its adverse impacts on soil nutrient.