Measurement of gaseous emissions from denitrification of applied N-15 .2. Effects of temperature and added straw

Abstract

Gas emissions of applied ¹⁵N were measured beneath a soil cover daily following saturation of Vertisol and Alfisol soils repacked in pots to the original field bulk density and held at three temperatures (5, 15 or 30°C) with or without addition of wheat straw. Collective gas emissions over 57, 43 and 15 days at 5, 15 and 30 degrees C respectively were compared with the ¹⁵N loss determined by mass balance. Loss measured by gas emissions (¹⁵N₂ and ¹⁵N₂O) ranged from 36% to 152% of the denitrification loss as determined by ¹⁵N mass balance. In the absence of added straw, measurement by gas emissions was consistently less than loss by ¹⁵N balance. Where straw was added, ¹⁵N loss by gas emissions was overestimated, probably because of a smaller headspace (0.3 L) than considered desirable (1-1.5 L) for emission measurements. Potential denitrification rates, in the presence of added straw, were similar for the Vertisol and Alfisol. Decreasing temperature slowed potential rates of denitrification from similar to 2.5 kg ha^-1 day^-1 at 30 °C to 0.8 kg ha^-1 day^-1 at 15 °C and 0.4-0.5 kg ha^-1 day^-1 at 5 °C. Decreasing temperature prolonged the period of waterlogging following a saturating event. Thus, collective loss of ¹⁵N was considerable even at the lower rates of denitrification at 5 °C (52-76% over 57 days) or 15 °C (87-92% over 43 days). Straw addition (10.5 t ha^-1) to the Vertisol, which contained no visible plant residues from previous crops, more than doubled the losses of applied ¹⁵N. In the absence of straw, rates of denitrification and immobilization were similar in magnitude, 0.97, 0.26 and 0.16 kg ha^-1 day^-1 for 30, 15 and 5 °C respectively. Very rapid loss of appliedha^-1 day^-1N in the presence of added straw led to decreases in immobilization of applied ha^-1 day^-1N, highlighting the potential effects of the much higher maximum rates for denitrification than for immobilization. The N₂O emissions generally represented the smaller fraction (<25%) of denitrification emissions, becoming smaller as temperature was increased. As a proportion of emissions due to denitrification, N₂O emissions were very low (<0.5% Vertisol, <3% Alfisol) in the presence of added straw.