Seasonal dynamics of soil pH and N transformation as affected by N fertilization in subtropical China: An in situ ¹⁵N labeling study

Nitrogen (N)-induced soil acidification has received much attention worldwide. Nitrification and soil N mineralization are two keyNcycle processes that affect soil acidification. However, the seasonal dynamics of soil pHunder their combined influence is unclear. We studied the effect of N fertilization on soil pH and N transformations using 15N tracing in field lysimeters with soils developed from different parent materials (Quaternary red clay, sandstone, and basalt). Maize was planted with 200 kg N ha(-1) yr(-1) N-15-labeled urea addition. During 7-45 days after fertilization, proton (H+) production due to nitrification of fertilizer N, nitrate (NO3-) leaching, and plant uptake exceeded H+ consumption by base cations mobilization and leaching, resulting in a significant soil pH decline. When nitrification activity decreased (after 45 days), due to exhausted ammonium (NH4+) availability, soil pH rose again. During the fallow period, acid neutralization due to base cation mobilization, and ammonification of soil organic N (SON) offset H+ production caused by nitrification of mineralized SON, leading to a sustained rise in soil pH. After the one-year experiment, no significant soil pH decreasewas observed in any of the soils. Parentmaterial had little effect on the seasonal dynamics of soil acidification, which appeared to be controlled by fertilization, environmental factors (temperature and moisture), and plant uptake. In subtropical regions, monitoring of soil pH on an annual basis may mask the effect of N fertilization on soil acidification. (c) 2021 Elsevier B.V. All rights reserved.