Effet à long terme d'apports d'engrais minéraux et de fumier sur les teneurs en C et en N des fractions densimétriques et des agrégats du loam limoneux Le Bras

Abstract

Long-term mineral fertilizer applications could reduce organic matter (OM) levels in soil if coupled with crop rotations with low organic residues inputs. The main objective of this study was to evaluate the C and N contents in whole soil, in densimetric OM fractions and in different aggregate size fractions of a Le Bras silt loam (Humic Gleysol). The treatments were arranged in a split-plot design, with dairy cattle manure applied at 0 and 20 Mgha−1 as the main factor. The subplots consisted of six fertilizer treatments (NK, PK, NP, NPK, NPKMg and the unfertilized check). The four year rotation included silage corn, (Zea mays L.) silage corn, wheat (Triticum Aestivum L.) and barley (Hordeum vulgaris L.). Contrary to mineral fertilizer, long-term manure applications significantly increased the C content in whole soil and also in the light and heavy fractions of OM (Fl and Fd). Mineral fertilizer significantly increased the C and N contents only in Fl. Moreover, manure application also increased the weight of the 5–8 mm aggregate size fraction and the C and N content in the 5–8 mm, 2–5 mm, 1–2 mm and 0,25–1 mm aggregate size fractions, compared to mineral fertilizer alone. In manured plots, soil C biomass, microbial respiration (CO2) and N mineralization (NO3) levels increased by 30% compared with mineral fertilizer treatments. Results of this study demonstrate the important effect of manure applications on C and N enrichment in soil and also on soil macroaggregation and biological activity. To maintain optimal C and N levels in soil and to favour soil macroaggregation, long-term mineral fertilizer application should be combined with crops in rotations which ensure high organic residues returns to soils. Key words: Macroaggregation, microbial biomass, N mineralization, long-term, light fraction, heavy fraction