Impacts of long-term nitrogen fertilization on acid buffering rates and mechanisms of a slightly calcareous clay soil

•N fertilization for 25 years accelerated natural acidification of a calcareous soil.•Acidification caused a decline in soil carbonate and base cation contents and pH.•Soil was more significantly acidified by fertilizing with NH4Cl than with urea.•Carbonate dissolution and mineral weathering were the main acid buffer mechanisms.•Accelerated mineral weathering caused less decline in base saturation than expected.

Acidification of cropland soils is a serious problem in China that may cause long term pH decline, which threatens the sustainability of soil fertility and crop yields. The objective of this research was to investigate those rates and mechanisms for a slightly calcareous soil. The field data were based on selected treatments from a long-term (1991-2015) N fertilization experiment at a rice-wheat rotation on a slightly calcareous (pH 7.7), young and hardly weathered clay soil (Regosol), including two plots with urea and NH4Cl fertilization and a control plot (no N fertilization). Urea and NH4Cl were applied at annual rates of 300 kg (1991-1996) and 285 kg (1997-2015) N ha− 1. Soil pH and exchangeable Ca2 + and Mg2 + declined in the first 12 years for the urea treatments, but remained constant thereafter. For the NH4Cl and control treatment, the exchangeable base cations showed an oscillating decrease over the 25 years. Soil pH and exchangeable Ca2 +, and Mg2 + levels were significantly (P < 0.05) lower in the NH4Cl treatment plot than the control. After 25 years, soil pH in the plots receiving urea and NH4Cl were 0.9 and 2.0 units lower than the control, respectively, while the total exchangeable base cations were about 10% and 16% lower, respectively. Carbonate dissolution, base cation exchange and mineral weathering were the main H+ buffer pathways of the Regosol. Long-term N fertilization favored the formation of smectite and vermiculite and accelerated the weathering of mica, potassium feldspar, and illite. This was more evident in the NH4Cl treatment than in the urea treatment. Consequently, the soil acid-buffering capacity was slightly enhanced after year 13 in the order of urea > control > NH4Cl. The increased weathering of soil primary minerals and clay minerals was the main reason that soil pH and exchangeable base cations stayed constant after about 12 years.