Long-term acidification of a Brazilian Acrisol as affected by no till cropping systems and nitrogen fertiliser

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.