Spatial patterns of SOC, SON, 137Cs and soil properties as affected by redistribution processes in a Mediterranean cultivated field (Central Ebro Basin)

•Land use changes and runoff affect the distribution of soil nutrients and 137Cs.•Loss of SON, SOC and 137Cs is related to soil loss by water erosion and tillage.•The effect of soil redistribution on SOC dynamics is evidenced in the study field.•Distribution of grain size fractions is linked to SOC, SON and 137Cs patterns.•Reduced tillage practices favor the increase of soil nutrients in topsoils.

Mediterranean mountain agroecosystems are sensitive areas to soil degradation due to climatic conditions and anthropogenic pressure from agriculture and grazing practices. Soil redistribution processes affect the spatial variability of soil properties and nutrients, as soil organic carbon and nitrogen (SOC and SON) are linked to soil quality and soil functions. A representative cereal field of rain-fed agroecosystems in a Mediterranean mountain environment where conventional and conservation agricultural practices have been implemented was selected on the northern border of the Ebro valley (NE Spain) to gain a better understanding of the effect of soil redistribution processes on the spatial pattern of soil properties at field scale. One hundred and fifty-six bulk (30–50 cm) and 156 topsoil samples (5 cm) were collected on a 10 × 10 m grid. Furthermore, 21 soil samples (40 cm) were collected at a nearby undisturbed reference site. 137Cs massic activity was measured on bulk and reference soil samples, and SOC and SON were measured on all soil samples. The characterisation of SOC pools into active (ACF) and stable carbon fraction (SCF) was carried out to study SOC pool dynamics. The fallout 137Cs was used in order to identify areas of 137Cs loss and gain and that of soil within the study field. Similar spatial patterns and direct and significant correlations (p < 0.01) were found between 137Cs and soil nutrients. As much as 70% of the total study field had lower values of 137Cs inventory than the reference for the area. In addition, there was a generalised loss of soil nutrients. SOC gain was found in less than 1% of the study field and there was a large loss of SON compared to the undisturbed reference site. Higher and significant (p < 0.01) contents of soil nutrients were found in topsoil than in bulk samples. Furthermore, there was an enrichment of the relative contribution of ACF to total SOC in sampling points where there was a 137Cs gain in both bulk and topsoil samples as a consequence of the soil redistribution particles by tillage and runoff. Understanding patterns of soil nutrients can be useful for developing or implementing land management strategies to preserve soil quality in Mediterranean agricultural areas.