Tillage practices of a clay loam soil affect soil aggregation and associated C and P concentrations

International audienceUnder long-term cultivation, greater accumulations of soil organic matter (SOM) and phosphorus (P) are found in the surface soil layer under no-till (NT) versus mouldboard ploughing (MP) practices. Our objective was to evaluate the effects of NT and MP practices on concomitant SOM and P distribution and sorption characteristics among water-stable aggregates and non-aggregated particles. The study was conducted in Quebec, Canada, as part of a long-term corn and soybean rotation experiment (established since 1992) on a clay loam soil of the St-Blaise series (Dark Grey Gleysol). Soil samples were collected in the fall of 2007 in the 0–5 cm layer from plots under NT and MP receiving 35 kg P ha–1 and 160 kg N ha–1. Samples were separated into three water-stable aggregate-sized classes (macro, 2000–250 μm; meso, 250–180 μm; micro, 180–53 μm) and (silt + clay)-sized particles (< 53 μm) using wet-sieving. Macro aggregates made up 60.2 and 48.5% of total soil weight under NT and MP, respectively. In wet-sieved soils from NT plots, water-extractable P (Pw) concentration increased in the order (silt + clay)-sized particles < micro- < meso- < macro-aggregates; under MP, micro-, meso-, and macro-aggregate fractions had the same Pw concentration, while the (silt + clay)-sized particles showed the lowest Pw concentration. The hierarchy observed among aggregate-sized classes under NT in relation to Pw concentration was also observed for carbon content, indicating that Pw is influenced by soil aggregation as driven by SOM accumulation. The lower Pw concentration in (silt + clay)-sized particles could be explained by a greater retention of P by reactive oxides and highly disordered alumino-silicates present on (silt + clay)-sized particles, thereby reducing the soluble P released from these particles. One important aspect of this study is the contrasting P sorption characteristics of solid particles under NT and MP. The P sorption maxima (Smax value) of the (silt + clay)-sized particles was twice that of the water-stable aggregates. Sorption characteristics reflect the hierarchy observed under NT for organic C and Pw, indicating a closer link between SOM and P dynamics within soil aggregates in contrast with MP