Soil acidity changes in bulk soil and maize rhizosphere in response to nitrogen fertilization

Abstract

The capacity of nitrogen (N) fertilizers to acidify the soil is regulated principally by the rate and N source. Nitrogen fertilizers undergo hydrolysis and nitrification in soil, resulting in the release of free hydrogen (H+) ions. Simultaneously, ammonium (NH4 +) absorption by roots strongly acidifies the rhizosphere, whereas absorption of nitrate (NO3 -) slightly alkalinizes it. The rhizosphere effects on soil acidity and plant growth in conjunction with N rate are not clearly known. To assess the impact of these multiple factors, changes in the acidity of a Typic Argiudol soil, fertilized with two N sources (urea and UAN) at two rates (equivalent to 100 and 200 kg N ha-1), were studied in a greenhouse experiment using maize as the experimental plant. Soil pH (measured in a soil-water slurry), total acidity, exchangeable acidity, and exchangeable aluminum (Al) were measured in rhizospheric and bulk soil. Plant biomass and foliar area (FA) were also measured at the V6 stage. Nitrogen fertilization significantly reduce the pH in the bulk soil by 0.3 and 0.5 units for low and high rates respectively. Changes in the rhizosphere (the "rhizospheric effect") resulted in a significant increase in soil pH, from 5.9 to 6.2. The rhizospheric effect x N source interaction significantly increased exchangeable acidity in the rhizosphere relative to bulk soil, particularly when UAN was added at a low rate. Only total acidity was significantly increased by the fertilizer application rate. In spite of the bulk soil acidification, no significant differences in exchangeable aluminum were detected. Aerial biomass and FA were significantly increased by the higher N rate, but N source had no effect on them. Although changes in acidity were observed, root biomass was not significantly affected.