CORRELATION OF SPECTROSCOPIC INDICATORS OF HUMIFICATION WITH MEAN ANNUAL RAINFALL ALONG A TEMPERATE GRASSLAND CLIMOSEQUENCE

The stable free radical concentration, as measured by electron spin re sonance spectroscopy, and the E-4/E-6 absorbance ratio were determined for a suite of humic acids extracted from uncultivated surface soils collected along a 620 km climosequence in the Argentine Pampa (mean an nual temperature, 14-15 degrees C; mean annual rainfall, 300-1000 mm), These two spectroscopic properties were interpreted as indicators of the degree of humification of soil organic matter, with free radical c oncentration expected to increase and the E-4/E-6 ratio to decrease wi th greater humification. Both the free radical concentration and the E -4/E-6 ratio were found to be significantly correlated with increasing mean annual rainfall, which was accompanied by an increase in the sta ble free radical concentration and a decrease in the E-4/E-6 ratio. Ou r results indicate that, in the sequence of temperate grassland soils investigated, organic matter humification increased with increasing ra infall. (C) 1998 Elsevier Science B.V.