The N transformation mechanisms for rapid nitrate accumulation in soils under intensive vegetable cultivation

Rapid soil degradation occurring under intensive vegetable cultivation, an increasingly common agricultural strategy in China, is characterized by soil acidification, salinity, and NO (3) (-) accumulation. However, to date, the reasons for rapid NO (3) (-) accumulation in soils at the nitrogen (N) levels have not been completely understood. In this study, we explored the underlying mechanisms for rapid NO (3) (-) accumulation in soils used for intensive vegetable cultivation and implications of altered gross N transformation rates on soil properties and sustainable cultivation. A N-15 tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water-holding capacity at 25A degrees C for 96 h after being labeled with (NH4NO3)-N-15 or NH (4) (15) NO3. Gross N transformation rates were calculated with a N-15 tracing model (Muller et al. 2007). Intensive vegetable cultivation substantially altered most model-quantified gross N transformation rates. In the VF soils with pH of > 5.6, the total gross rates of autotrophic and heterotrophic nitrification were 3.0-14.6 times higher than that of the WF soil, with maximum values of 47.5 mg N kg(-1) day(-1) in VF1 soil, which was dominated by heterotrophic nitrification (73.6%). Quantification of autotrophic nitrification rate was primarily a result of an increase in NH (4) (+) substrate from the gross rates of recalcitrant and labile organic N mineralization. Although the gross NO (3) (-) immobilization rate rose in response to increasing gross nitrification rate, the net NO (3) (-) production rate was still 2.3-5.8 times higher in VF soils than in WF soil. As a consequence, a rapid accumulation of NO (3) (-) was observed in the vegetable cultivation, in which large ammonium fertilizers was applied. The VF4 soil, which was cultivated for the longest time (10 year), experienced substantial acidification, reducing soil pH to 4.3 and inhibiting both autotrophic and heterotrophic nitrification rates, resulting in NH (4) (+) accumulation along with NO (3) (-) accumulation. Because of the increasing use of intensive vegetable cultivation in China and its importance in the national agronomic strategy, based on the data gathered from our study, it is clear that additional research should be devoted to this practice in order to effectively resolve the problem of NO (3) (-) accumulation, and develop environmentally sound practices for long-term sustainability.