Soil organic matter dynamics and quality during a wheat-clover rotation in a semiarid haplustoll

Several studies have reported modifications in soil chemical and physical parameters under rotations with legume inclusion. This modification had the potential of improving soil fertility and grain production. The objective of this research was to evaluate changes in the distribution and quality of soil organic fractions during a 3 year wheat (Triticum aestivum) - 3 year red clover (Trifolium pratense) rotation. Soil samples at 0 - 0.15 m depth were taken in five of the six year rotation. Organic fractions, physically (particulate organic carbon, COP) and chemically (humic and fulvic acids, AH and AF, respectively) separated were determined and chemically and spectrometrically characterised. The quantity and distribution of the organic fractions were different along the rotation. The more labile soil organic matter fractions, COP and AF, were higher during legume phase and diminished during wheat phase. This decrease of labile relative to recalcitrant organic matter resulted in significative differences in COP:total organic carbon (COT) and AH:AF ratios between wheat and clover phase. The COP:COT ratios were lower than 19 under wheat and higher than 21 under clover. The AH presented higher nitrogen and lower sulphur contents during clover phase, while the opposite was observed during wheat phase. This compositional changes were clearly reflected by the N:S and C:S atomic ratios. Total acidity in AH was slightly increased during the legume phase, as a consequence of the significative increase in -OH phenolic groups. Infrared and fluorescence spectra of the AH from soils under wheat and clover were similar among them, probably due to the short rotation period that produce only minor changes in the AH molecular structure. Legume inclusion in the rotation increased labile organic matter fractions and modified the composition of the recalcitrant ones. Both changes would improve soil fertility and grain production.