The acetylene inhibition technique to determine total denitrification (N₂ + N₂O) losses from soil samples: potentials and limitations

The loss of N2 from intensively managed agro-ecosystems is an important part of the N budget. The monitoring of N2 emissions at the field scale is impossible due to the high atmospheric background of 78%, which precludes the measurement of fluxes. The acetylene (C2H2) inhibition technique is a rather simple, albeit imperfect, method to determine N2 losses from entire soil cores. Despites serious limitations it is one among very few methodological options to estimate total denitrification at high temporal resolution and on small spatial scale, with limited workload and costs involved. A laboratory system with two different detection systems (photoacoustic IR spectroscopy and gas chromatography) is presented, which allowed parallel measurements of up to 7 intact soil cores in air-tight glass tubes in a temperature controlled cabinet (adjusted to field conditions) with an automated C2H2 injection. A survey of total denitrification losses (N2 + N2O) over 1.5 yr in soil from an intensively managed, cut grassland system in central Switzerland showed a lower bound loss in the range of 6 to 25 kg N ha−1 yr−1 (3–13% of added N), roughly 3.4 times higher than the N2O loss. However, several drawbacks of the C2H2 inhibition technique preclude a more precise determination of the total denitrification loss.

Fate of nitrogen in pig slurry applied to a New Zealand pasture soil

A 2-year lysimeter study was conducted to determine the fate of nitrogen in pig slurry applied to a moderately fertile, semi-free-draining pasture soil in the Canterbury Plains of New Zealand. Pig slurry was applied annually for 2 years in autumn, at 3 rates of 0, 200, and 400 kg N/ha to 12 large soil lysimeters (4 at each rate), 800 mm in diameter by 1200 mm deep. Slurry applied in Year 1 was labelled with 15N and a mass balance obtained at the end of the experiment. The mass balance showed that over the 2 years following application of 15N-labelled slurry, 8–19% was lost in the leachate, 20% was removed in the cut pasture, 15–26% was lost via volatilisation, 14–18% remained in the roots and soil, and approximately 30% was lost by denitrification. The high denitrification loss was attributed to (i) a large soil concentration of nitrate supplied from nitrification of the ammonium-N in the slurry; (ii) a readily oxidisable source of carbon supplied in the slurry; and (iii) transient anaerobic conditions produced by textural discontinuities and impeding layers within the soil profile. The fate of applied nitrogen between years was affected by the pattern of water inputs (rainfall and irrigation) and the resulting effect on drainage. Concentrations of inorganic nitrogen in the leachate from the 200 kg N/ha·year treatment were found to be consistently below 25 mg N/L, but those from the 400 kg N/ha·year treatment were considerably higher (c. 65 mg N/L) and persisted for a prolonged period. The latter N concentration represented a significant loss of nitrogen over the study period and may be of environmental concern.

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

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.

Reducing denitrification loss with nitrification inhibitors following presowing applications of urea to a cottonfield

The effects of the nitrification inhibitors nitrapyrin, acetylene (provided by wax-coated calcium carbide), and phenylacetylene on nitrogen (N) transformations and denitrification losses following presowing applications of urea were determined in a cottonfield in the Namoi Valley of New South Wales. The study used 0.05-m-diameter microplots to follow the changes in mineral N, and 0.15-m-diameter microplots fertilised with 15N-labelled urea (6 g N/ m2; 5 atom % 15N) to assess losses of applied N. When urea was applied in February (34 weeks before sowing), 84% of applied N was lost from the soil. Loss of applied N was reduced by addition of nitrapyrin and phenylacetylene, to 53 and 57%, respectively. In the absence of nitrification inhibitors, less N was lost (72% of that applied) from an application in May than from the February application. Addition of acetylene, phenylacetylene, and nitrapyrin reduced losses over the 24 weeks to sowing to 57, 52, and 48%, respectively. These experiments show that N loss from presowing applications of urea can be significantly reduced by the use of nitrification inhibitors, but that the losses of N are still substantial.