CLIMATE, ORGANIC-MATTER AND CLAY CONTENT RELATIONSHIPS IN THE PAMPA AND CHACO SOILS, ARGENTINA

Temperature and precipitation have strong effects on soil processes. T he Pampa and the Chaco are vast plains with soils mainly derived from loess. Our objective was to study the effects of temperature and preci pitation 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 p otential evapotranspiration were obtained from climatic records. Carbo n inputs to the different soil layers were estimated by calculating ne t primary productivity of ecosystems and partitioning coefficients of above and belowground biomass. Inputs were affected by a retention fac tor taking into account temperature effects on organic matter decompos ition. Soil organic carbon increases with higher precipitation and dec reases with higher temperature. Consequently, the organic carbon conte nt in the top 0-50 cm soil layer is positively correlated with the pre cipitation/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 t hickness and clay amount (g cm(-2)) are not associated with temperatur e but the ratio precipitation/potential evapotranspiration explained 8 5% (P < 0.001) of the variation in the former and 79% (P < 0.001) in t he later. Soils are deeper and with higher clay contents where this ra tio increases, a consequence of available water for pedogenic processe s. Climate is a main soil forming factor in the Pampa and Chaco region s. Clay neoformation and possible future trends of organic carbon unde r different climatic global change scenarios were estimated. (C) 1998 Elsevier Science B.V.