Nitrification and acidification from urea application in red soil (Ferralic Cambisol) after different long-term fertilization treatments

Long-term manure applications can prevent or reverse soil acidification by chemical nitrogen (N) fertilizer. However, the resistance to re-acidification from further chemical fertilization is unknown. The aim of this study was to examine the effect of urea application on nitrification and acidification processes in an acid red soil (Ferralic Cambisol) after long-term different field fertilization treatments. Soils were collected from six treatments of a 19-year field trial: (1) non-fertilization control, (2) chemical phosphorus and potassium (PK), (3) chemical N only (N), (4) chemical N, P, and K (NPK), (5) pig manure only (M), and (6) NPK plus M (NPKM; 70 % N from M). In a 35-day laboratory incubation experiment, the soils were incubated and examined for changes in pH, NH4 (+), and NO3 (-), and their correlations from urea application at 80 mg N kg(-1)(-80) compared to 0 rate (-0). From urea addition, manure-treated soils exhibited the highest acidification and nitrification rates due to high soil pH (5.75-6.38) and the lowest in the chemical N treated soils due to low soil pH (3.83-3.90) with no N-treated soils (pH 4.98-5.12) fell between. By day 35, soil pH decreased to 5.21 and 5.81 (0.54 and 0.57 unit decrease) in the NPKM-80 and M-80 treatments, respectively, and to 4.69 and 4.53 (0.43 and 0.45 unit decrease) in the control-80 and PK-80 treatments, respectively, with no changes in the N-80 and NPK-80 treatments. The soil pH decrease was highly correlated with nitrification potential, and the estimated net proton released. The maximum nitrification rates (K (max)) of NPKM and M soils (14.7 and 21.6 mg N kg(-1) day(-1), respectively) were significantly higher than other treatments (2.86-3.48 mg N kg(-1) day(-1)). The priming effect on mineralization of organic N was high in manure treated soils. Field data have shown clearly that manure amendment can prevent or reverse the acidification of the red soil. When a chemical fertilizer such as urea is applied to the soil again, however, soil acidification will occur at possibly high rates. Thus, the strategy in soil N management is continuous incorporation of manure to prevent acidification to maintain soil productivity. Further studies under field conditions are needed to provide more accurate assessments on acidification rate from chemical N fertilizer applications.