Impacts of nitrogen fertilizer type and application rate on soil acidification rate under a wheat-maize double cropping system

Nitrogen (N) fertilizer-induced soil acidification in Chinese croplands is well-known, but insight in the impacts of different N fertilizer management approaches (fertilizer type and rate) on soil acidification rates is very limited. Here, we conducted a field experiment on a moderate acid soil to quantify soil acidification rates in response to N fertilization by different fertilizer types and N rates through monitoring the fate of elements (mainly nutrients) related to H⁺ production and consumption. Two N fertilizer types (urea and NH₄Cl) and three N rates (control, optimized and conventional, 0/120/240 kg N ha⁻¹ for wheat, 0/160/320 kg N ha⁻¹ for maize) were included. Nitrogen addition led to an average H⁺ production of 4.0, 8.7, 11.4, 29.7 and 52.6 keq ha⁻¹ yr⁻¹, respectively, for the control, optimized urea, conventional urea, optimized NH₄Cl and conventional NH₄Cl plots. This was accompanied with a decline in soil base saturation of 1–10% and in soil pH of 0.1–0.7 units in the topsoil (0–20 cm). Removal of base cations by crop harvesting and N transformations contributed ~70% and ~20% to the H⁺ production in the urea treated plots, being ~20% and ~75% in the NH₄Cl treated plots, respectively. The large NH₄⁺ input via fertilization in the NH₄Cl treated plots strongly enhanced the H⁺ production induced by N transformations. The low contribution of N transformations to the H⁺ production in the urea treated plots was due to the limited NO₃⁻ leaching, induced by the high N losses to air caused by denitrification. Increased N addition by urea, however, strongly increased H⁺ production by enhanced plant uptake of base cations, mainly due to a large potassium uptake in straw. Our results highlight the important role of optimizing fertilizer form and N rate as well as straw return to the field in alleviating soil acidification.