Mineralization and denitrification in upland, nearly saturated and flooded subtropical soil - I. Effect of nitrate and ammoniacal nitrogen

The influence of fertilizer N applied through nitrate and ammoniacal sources on the availability of nitrate, supply of C, and gaseous N losses via denitrification (using acetylene inhibition technique) in a semiarid subtropical soil (Typic Ustochrepts) was investigated in a growth chamber simulating upland [60% water-filled pore space (WFPS)], nearly saturated (90% WFPS), and flooded (120% WFPS) conditions. The rate of denitrification was very low in the upland soil conditions, irrespective of fertilizer N treatments. Increasing water content to nearly saturated and flooded conditions resulted in four- to sixfold higher rates of denitrification within 2 days, suggesting that the denitrifying activity commences quickly. Results of this study reveal that (1) under restricted aeration, these soils could support high rates of denitrification (similar to 6 mg N kg(-1) day(-1)) for short periods when nitrate is present; (2) application of fertilizer N as nitrate enhances N losses via denitrification (similar to 10 mg N kg(-1) day(-1)) - however, the supply of available C determines the intensity and duration of denitrification; (3) when fertilizer N is applied as an ammoniacal form, nitrification proceeds slowly and nitrate availability limits denitrification in flooded soil; (4) the nearly saturated soil, being partially aerobic, supported greater nitrification of applied ammoniacal fertilizer N than flooded soil resulting in higher relative rates of denitrification; and (5) under aerobic soil conditions, 26 mg mineral N kg(-1) accumulated in control soil over a 16-day period, demonstrating a modest capacity of such semiarid subtropical soils, low in organic matter, to supply N to growing plants.