Understanding subsoil acidification: effect of nitrogen transformation andnitrate leaching

Nitrification and nitrate leaching have been suggested to be major causes o f soil acidification. However, it is unknown whether these processes cause subsoil acidification. Soil column experiments examined the effect of the a ddition of Ca(NO3)(2) or (NH4)(2)SO4 to the topsoil horizon on subsoil acid ification under nodulated lupins (Lupinus angustifolius L.) and subterranea n clover (Trifolium subterraneum L.). Nitrate leaching was achieved by addi ng excess water to the surface of the columns. Where basal nutrients were applied only to the top 10-cm layer, about 60-70 % of the total root length of lupin and over 50% of subterranean clover wer e distributed in that layer. Plants grown without added nitrate for 105 day s decreased soil pH at all layers but more significantly in the top 20 cm ( by up to 0.7 units); the decrease in pH correlated well with increased root length density of both species (r(2) = 0.98, n = 9). The addition of Ca(NO 3)(2) to the top 10-cm layer caused less acidification by about 0.1 pH unit s at all depths than the treatment without Ca(NO3)(2). Where basal nutrients were applied uniformly throughout the column, root le ngth density of lupin and subterranean clover tended to increase with depth . The addition of (NH4)(2)SO4 in the top 10 cm significantly increased NO3- concentration in all layers but NH4+ was mainly retained in the top 30-cm layer. Lupin and subterranean clover grown without added NH4+ for 82 days d ecreased soil pH by 0.3 units at all depths. Compared with the plants recei ving no (NH4)(2)SO4, lupin grown with (NH4)(2)SO4 at 0-10 cm depth in the c olumn caused more acidification by 0.05-0.2 pH units in the top 10 cm but l ess acidification by 0.15-0.17 units at 10-40 cm depth in the column; subte rranean clover grown with (NH4)(2)SO4 caused more acidification by 0.35-0.4 6 units in the top 10 cm and less acidification by 0.14-0.19 units in the 2 0-50 cm layer. The results suggest that the leaching of nitrate from topsoil is unlikely t o cause subsoil acidification. In contrast, the uptake of nitrate by the ro ots reduces net acid production in subsoil layers.