The effect of management practices on soil potential for regulating the residual concentration of pesticides was examined in samples from a Calcic Haploxeralf in Toledo (central Spain). Sorption-desorption of alachlor and linuron was found to depend on inputs of lignocelullosic wastes or cattle manure for the past 16 years. For a given herbicide, the soil sorption capacity (K(f)) follows the order control < crop residues < manure, which is consistent with the organic C content in the soil samples. Some structural characteristics of the soil humic acid as revealed by visible and infrared spectroscopies and analytical pyrolysis were useful to forecast the sorption-desorption intensity. Simple and multiple linear correlation analyses illustrate enhanced sorption of alachlor and linuron in soil plots where slightly altered soil organic matter accumulated (positive correlations with the intensity of infrared lignin signature band and with the methoxyphenol yields after pyrolysis of the humic acids and negative correlation with the aromaticity as pointed out by the optical density at 465 nm). Linuron showed a preference for soils with humic acids of low molecular weight and low degree of internal cross-linking, as inferred from the positive correlation with the ratio between optical densities at 465 and 665 nm. Under the conditions of the present experiment, agricultural practices including organic amendments seem to have a beneficial effect in the control of leaching and sorption of pesticides.

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Humic matter in virgin and cultivated Vertisols and Arenosols from Pandamatenga (northern Botswana) has been studied by chemical fractionation and visible and infrared derivative spectroscopies. The in vitro soil respiration was also determined. Soils contained <15 g of total C kg(-1) and displayed scant mineralization activity. In Vertisols, cultivation has led to scarce significant changes in humus characteristics, pointing to a noteworthy resilience of the organic matter. Humic acids showed a very dark color, indicating a large concentration of aromatic structures and stable free radicals. Infrared spectra were featureless and alike. This strong structural stability is also suggested by Curie-point pyrolysis of humic acids, which failed to yield substantial amounts of diagnostic products, mainly in the case of Vertisols. Some unexpected similarities between Vertisols and Arenosols indicate that the influence of external factors on the humic acid formation processes prevails on that of the geological substrate. In Pandamatenga soils only small amounts of recalcitrant C and N forms are sequestered; they represent stable pools relatively independent from short- or medium-term climatic changes or management practices.

After 16 years of periodical applications of either farmyard manure or crop wastes at two levels of mineral N fertilization to a Calcic Haploxeralf in the semiarid central Spain, we found significant changes in chemical fertility levels and in the concentration, chemical composition, and carbon mineralization rates of soil organic matter (SOM). The changes in SOM quality were related to significant improvements of soil physical properties, mainly aggregate stability and water retention. Such changes were related to the increased concentration of humic colloids in soil, the mineral N dose, and the type of organic matter applied. When compared with the control plots, the organic matter accumulated in the amended plots tended to be less transformed, and its total concentration and humification degree decreased with increasing external N‐inputs. Humic acids from the amended plots showed a more marked aliphatic character (mainly after N addition) than those from control plots. Farmyard manure led to a significant improvement of soil physical properties, but had a comparatively small effect in promoting biodegradation and humification of crop wastes. This could be due to the high biological stability of the manure used which, in semiarid Mediterranean fields, usually leads to an accumulation of little transformed SOM.