Ž . Geoderma 83 1998 127–141 Climate, organic matter and clay content relationships in the Pampa and Chaco soils, Argentina R. Alvarez ) , R.S. Lavado Departamento de Suelos, Facultad de Agronomıa, UniÕersidad de Buenos Aires, ´ AÕ. San Martın 4453, 1417 Buenos Aires, Argentina ´ Received 24 September 1996; accepted 12 November 1997 Abstract 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 Ž y3 . content g cm , 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. Conse- quently, the organic carbon content in the top 0–50 cm soil layer is positively correlated with the Ž 2 . precipitationrtemperature ratio potential model r s0.693, P -0.001 . Carbon inputs to the soil explained 72% of organic carbon variations. Mean turnover time of soil carbon was estimated to Ž y2 . be 14 yr. Solum thickness and clay amount g cm are not associated with temperature but the Ž . ratio precipitationrpotential evapotranspiration explained 85% P -0.001 of the variation in the Ž . 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. q 1998 Elsevier Science B.V. Keywords: organic carbon; clay; climate; soil development; climatic change ) Corresponding author. Fax: q54-541-5221687r8395; e-mail: radio@ferlav.agro.uba.ar 0016-7061r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0016-7061 97 00141-9