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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 0.05–0.10 m 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 cmolc/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 (1.03×10−7–9.3×10−8) 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.

Cropping systems and N fertilisation affect soil acidification mainly due to the removal of alkaline plant material from the field and nitrate leaching. The study evaluated the acidification of a subtropical soil under no till cropping systems with different C and N addition rates for 19 years. The contributions of leguminous and non-leguminous crops (fallow/maize, black oat/maize, black oat + vetch/maize, black oat + vetch/maize + cowpea, lablab + maize, pigeon pea + maize, and digitaria) and mineral N fertiliser (0 and 180 kg N/ha.year as urea) to total acidification were estimated. Cropping systems and N fertilisation significantly affected soil pH, which ranged from 4.3 to 5.1. The presence of leguminous species and mineral N promoted greater decreases in soil pH and net soil acidification, which resulted in increases in exchangeable Al content and Al saturation. Black oat + vetch/maize with N fertilisation promoted the highest soil net acidification rate (2.65 kmol H+/ha.year), while digitaria had the lowest (1.07 kmol H+/ha.year). Leguminous species and N fertilisation increased soil acidification through changes in the C cycle associated with the removal of alkaline plant material by grains. Leguminous-based cropping systems promoted higher maize yields than those comprising essentially gramineous species, indicating an opportunity for a reduction in N fertiliser rates. With N application, however, maize yield did not differ among cropping systems, despite differences in soil pH and exchangeable Al.