Pyrolytic descriptors responsive to changes in humic acid characteristics after long-term sustainable management of dryland farming systems in Central Spain

Abstract

Structural changes in humic acids from a semiarid soil of an experimental farm in Central Spain have been studied by Curie-point pyrolysis. Soil has received periodic inputs of farmyard manure or crop wastes for the last 16 years, and mineral nitrogen fertilisation in 50% of the plots. Analytical pyrolysis suggested substantial differences in terms of the effect of soil management on the accumulation mechanisms of the humic acid fraction. When compared with control plots, humic acids from plots amended with crop wastes displayed well-defined methoxyphenol assemblages, indicating that the diagenetic transformation of lignin could be a dominant mechanism of organic matter stabilisation. The greatest yields of methoxyphenols after organic inputs were obtained in plots receiving nitrogen fertilisation, what agrees with the expected higher performance of the lignin biodegradation in nitrogen-limited media. Increased yields of lignin-derived methoxyphenols were also observed in plots treated with manure, which released in addition a conspicuous series of alkyl compounds suggesting recalcitrant wax-derived lipids incorporated to the humic acids. Highest yields of pyrolytic fatty acids were observed in humic acids from manure-amended plots without nitrogen fertilisation. Alkylbenzene proportions were also pyrolytic descriptors responsive to mineral nitrogen inputs, showing very significant differences ( P<0.01) in both control and amended plots. Results suggested that continued organic inputs can not be considered to increase soil organic matter quality, since microbial reworking of lignins became the dominant mechanism of accumulation of humic acid-type substances, even when nitrogen inputs were applied.