Climate, organic matter and clay content relationships in the Pampa and Chaco soils, Argentina

Temperature and precipitation have strong effects on soil processes. The Pampa and the Chaco are vast plains with soils mainly derived from loess. Our objective was to study the effects of temperature and precipitation of both regions on selected soil properties. Using data from soil surveys for ca. 65 Mha, we defined 40 geographic units of ca. 1.6 Mha each. Organic carbon content (g cm(-3)), solum thickness and clay content were averaged for each unit. Temperature, precipitation and potential evapotranspiration were obtained from climatic records. Carbon inputs to the different soil layers were estimated by calculating net primary productivity of ecosystems and partitioning coefficients of above and belowground biomass. Inputs were affected by a retention factor taking into account temperature effects on organic matter decomposition. Soil organic carbon increases with higher precipitation and decreases with higher temperature. Consequently, the organic carbon content in the top 0-50 cm soil layer is positively correlated with the precipitation/temperature ratio (potential model r(2) = 0.693, P < 0.001). Carbon inputs to the soil explained 72% of organic carbon variations. Mean turnover time of soil carbon was estimated to be 14 yr. Solum thickness and clay amount (g cm(-2)) are not associated with temperature but the ratio precipitation/potential evapotranspiration explained 85% (P < 0.001) of the variation in the former and 79% (P < 0.001) in the later. Soils are deeper and with higher clay contents where this ratio increases, a consequence of available water for pedogenic processes. Climate is a main soil forming factor in the Pampa and Chaco regions. Clay neoformation and possible future trends of organic carbon under different climatic global change scenarios were estimated. (C) 1998 Elsevier Science B.V.