Soil organic matter properties after 40 years of different use of organic and mineral fertilisers

Organic inputs can help maintain soil fertility by improving chemical and biological soil properties. The effects of 40 years of organic, mineral and mixed fertilisations on soil organic properties were evaluated in a continuous maize system. The following properties were analysed: total organic carbon (TOC), humic carbon (HC) and its molecular weight distribution, HC/TOC and the hormone—(auxin and gibberellin-like) activities of the humic substances and their effects on two key enzymes (nitrate reductase (NR) and glutamine synthetase (GS)) involved in nitrate assimilation in maize seedlings. Farmyard manure fertilisation sustained TOC in the top layers while mineral treatments alone or mixed with organic exhibited a minor influence on the organic matter evolution: over 40 years the average TOC depletion was 23% with liquid manure and mixed fertilisations, 43% with mineral fertilisers alone and 51% in the control. The different treatments also influenced the HC molecular complexity. Farmyard fertilisations improved the production of humus with a high degree of policondensation, a fraction usually linked to soil fertility; the absence of organic fertiliser inputs determined the opposite, with a higher percentage of non-complex and light-weight humus. The hormone-like and biochemical activities of humus substances were evident with the organic and mixed fertilisations. The humus extracted by the soil treated with farmyard manure exhibited the best gibberellin-like activity and the highest increases of NR (+42%) and GS (+49%) activities with respect to the control. The other treatments showed intermediate stimulations as in the case of mixed fertilisations (NR + 22% and GS + 24%), or no activities as in the case of mineral inputs.