Soil quality in three range soils of the semi-arid Pampa of Argentina

Degradation of semi-arid rangelands in the Argentinean Pampa due to increasing grazing intensity has been a growing concern among ecologist. Many studies document the impact of grazing on vegetation structure but little information is available on soil properties in these environments. The present study aims to establish some terms of reference for these range soils. Field studies were carried out to test the validity of selected soil chemical, physical and biological parameters for their capacity to discriminate soils according to quality under rangeland use. The selected data set includes total soil organic carbon (SOC), particulate organic carbon (POC), total N and P contents, dry aggregate size distribution, water stable aggregates and proctor maximum bulk density. Three soils (sand, sandy loam and loam) corresponding to different topographical situations and natural vegetation structure were sampled in 6 cm depth intervals up to 18 cm in bulk density cylinders, with six replicates in each site. Sun and shade vegetation communities were sampled separately. Standing biomass was determined and dry matter was analysed for N, P, Ca and Mg content. A positive significant relation between clay+silt content and SOC was found (R2=0.53)(R2=0.53) in the 0–6 cm depth. Topsoil SOC was related to standing biomass (R2=0.21)(R2=0.21), and POC showed a strong correlation with SOC (R2=0.94)(R2=0.94). The highest values of SOC and POC were found in the sandy loam, while the loam had the highest POC/SOC ratio. Aggregate size distribution especially of >8 and <1 mm fractions reflected textural differences. Vegetation communities differed in their POC contents and POC/SOC ratios, with higher values found in the soils under shade community. It was concluded that SOC, POC, POC/SOC ratio and aggregate fractions >8 and <1 mm are sensitive soil quality parameters that reflect differences of soil texture and vegetation community structure in range soils of this region.