Effects of land use on soil organic and inorganic C and N at 137Cs traced erosional and depositional sites in mountain agroecosystems

Patterns of erosion, transport and deposition of soil particles not only affect the distribution of mineral fraction of the soil but their organic components. The spatial variability of soil nutrients in Mediterranean mountain agroecosystems that combine complex land uses and abrupt topography is poorly documented despite emerging concerns on nutrients fate require understanding the potential impact of soil erosion on nutrients redistribution across landscapes. This study aims to assess at catchment scale the spatial patterns of soil organic and inorganic carbon (SOC, SIC), soil organic carbon fractions and total nitrogen (TN) in relation to land uses and soil redistribution. To this purpose we selected a well characterized endorheic catchment located in NE Spain where soil erosion and deposition rates derived from Cs-137 measurements were previously documented and we mapped the distribution of cropland and rangeland areas. A grid of 100 x 100 m was established for collecting a total of 205 soil samples for nutrients and standard soil analyses. In both rangeland and cropland the patterns of SOC and TN stocks were clearly related to Cs-137 inventories thus significantly lower nutrient stocks (SOC: 34.1 +/- 18.6 Mg ha(-1), TN: 2.9 +/- 1.7 Mg ha (-1)) were found in erosional sites whereas higher stocks (SOC: 44.9 +/- 24.2 Mg ha (-1), TN: 3.9 +/- 2.3 Mg ha (-1)) were recorded in depositional areas. In cropland the mean nutrients contents (SOC: 1.1 +/- 0.8%, TN: 0.1 +/- 0.1%) were significantly lower than in rangeland (SOC: 3.2 +/- 2.1%, TN: 0.2 +/- 0.1%). The highest C/N ratios (29) were found in rangeland, while in cropland they occurred in depositional areas with their values below 10 in the erosional sites. In turn, the SIC content (range 0.4 to 10.2%) was slightly lower at the depositional sites (4.9 +/- 2.2%) but not significantly different from the erosional sites (5.1 +/- 2.2%). Cultivated and steep slopes led to loss of soil and nutrients due to intense erosion processes also related to tillage and crop productivity. Our findings provide evidence that the combined effect of land use and soil redistribution exert an important control on the spatial distribution of nutrients, highlighting a significant decrease of SOC and TN stocks with increasing soil erosion in croplands. The information gained achieves a better understanding of soil nutrients dynamics and further knowledge for effective land management of mountain agroecosystems in the Mediterranean region.