Changes in soil properties and humic substances after long‐term amendments with manure and crop residues in dryland farming systems

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.