Organic matter kept Al toxicity low in a subtropical no-tillage soil under long-term (21-year) legume-based crop systems and N fertilisation

Nitrogen-fixing crops and N fertilisation increase soil acidification, but few studies have attempted to evaluate the capacity of soil organic matter to alleviate the Al toxicity in acid no-tilled soils. This study was carried out in a 21-year-old experiment aiming to evaluate the effect of crop systems [fallow/maize (Zea mays L.), F/M; oat (Avena strigosa Schreb)/maize, O/M; oat + vetch (Vigna sativa L.)/maize, O+V/M; lablab (Dolichos lablab) + maize, L+M; and pigeon pea (Cajanus cajan L. Millsp.) + maize, P+ M] and mineral N fertilisation (0 and 149 kg/ha.year) on chemical attributes and Al speciation in the 0-0.05 and 0.05-0.10m layers of a no-tilled Paleudult of Southern Brazil. The original soil pH (5.8) decreased in all crop systems, declining 0.37-1.52 units in 21 years without re-liming. This decrease was larger in subsoil layers and, in general, was exacerbated by legume-based crop systems and by N fertilisation. The drop in soil pH increased Al content in solid phase (range 0.07-1.85 cmol(c)/kg) and in soil solution (range 0.01-0.06 mmol/L), and decreased base saturation on cation exchange capacity (range 12.5-61.2%). However, the Al3+ activity in the soil solution (1.03 X 10(-7) - 9.3 X 10(-8)) was kept below threshold values of toxicity to maize roots, primarily due to the formation of organometallic complexes at low pH, which was estimated as up to 90% of the total Al in solution, but also due to the increased ionic strength in this no-till soil (0.0026-0.0104). Our results highlight that, although legume cover crops and N fertilisation can accelerate soil acidification, Al toxicity is offset by increased organic matter in no-till subtropical soils.