[Soil acidification induced by nitrogen addition and its responses to water addition in Inner Mongolia Temperate...

Soil acidification due to exogenous nitrogen (N) deposition has been received sufficient attention in the past several decades. Whether the N-induced soil acidification is modulated by the altering precipitation regimes remains unclear. In this study, based on a five-year (2013-2017) manipulative field experiment with factorial N addition (10 and 40 g N·m^-2·a^-1 ) and water addition (a total amount of 80 mm·a^-1 was added in terms of five intensities, i.e., 2 mm×40 times, 5 mm×16 times, 10 mm×8 times, 20 mm×4 times, and 40 mm×2 times) in a typical steppe in Inner Mongolia, I analyzed the effects of water addition on soil acidification induced by N addition. I found that soil acidification induced by 40 g N m^-2·a^-1 N addition occurred earlier, more significant, and with a greater year by year changed rate than that of 10 g N·m^-2·a^-1 N addition. After one year treatment, I observed significant soil acidification at soil profiles of 0-5, 5-10, and 10-20 cm in the treatment of 40 g N·m^-2·a^-1 N addition, while significant soil acidification in 10 g N·m^-2·a^-1 N addition was observed after 1, 4, and 5 year since N was added for the three layers, respectively. In general, the magnitude of soil acidification induced by N addition increased with the increasing period after N addition, and the slope of the regression line was steeper in the relatively higher dose of N addition (40 g N·m^-2·a^-1). Soil acidification induced by N addition could not be reversed by water addition, but water addition in terms of relatively lower intensities (i.e., all the treatments except 40 mm×2 times) could relieve the N-induced soil acidification to a certain extent in the treatment with lower dose of N addition (10 g N·m^-2·a^-1) in dry years. In wet years, however, water addition exacerbated soil acidification after N was added, especially in the topsoil. The leaching of soil inorganic N after higher intensity of water addition (10-40 mm) could explain the deteriorated N-induced soil acidification when water was added. There was no significance for the relief or deterioration to N-induced soil acidification by water addition. Our results had important implications for understanding grassland responses to N deposition and altered precipi-tation regime in the future.