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 resonance 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 annual 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 concentration expected to increase and the E(4)/E(6) ratio to decrease with 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 stable free radical concentration and a decrease in the E(4)/E(6) ratio. Our results indicate that, in the sequence of temperate grassland soils investigated, organic matter humification increased with increasing rainfall. (C) 1998 Elsevier Science B.V.