Factors affecting nitric oxide and nitrous oxide emissions from ...

from the deposition of NO2 on the plant surface and diffusion of NO2 through stomata
following oxidation of NO by O3 within the canopy. This adsorption of NOx onto the plant
canopy may reduce global NOx emissions to the atmosphere to as low as 13 Tg N yr -1
compared to 21 Tg N yr -1 of the net ecosystem emissions to the atmosphere (Davidson &
Kingerlee, 1997). Yienger and Levy (1995) suggested that canopy recapture of NO is an
important correction to be made to global estimates of soil NO emissions.
In addition to canopy effects on NOx emissions, plants add carbon (C) to the soil in the form
of root exudates which, when decomposed become part of the soil organic-C. These C inputs
may affect soil microbial N transformation processes such as denitrification (Firestone &
Davidson, 1989). During plant root respiration oxygen (O2) is consumed and CO2 is released
by plant roots. Plant roots have been shown to create anaerobic conditions in soils and
provided substrates such as easily decomposable root exudates for the soil microorganism
resulting in enhanced denitrification (Mahmood et al., 1997; Nykanen et al., 1995). Carbon
regulates NO production primarily due to its effect on denitrification (section 9.5). In
addition, soil C inputs increase denitrification under anaerobic conditions (Baumgartner &
Conrad, 1992; Drury et al., 1991)). For example, Schuster and Conrad (1992) showed that
glucose addition increased NO production under anaerobic conditions by a factor of 6.6
compared to a control soil. Venterea and Rolston (2000a) found that a positive correlation
was obtained between soil organic C and NO production in agricultural soils (sterile soils).
These workers showed that it was due to the reaction of nitrification derived NO2 - with H +
concentrations at low soil pH levels. Stevenson (1994) reported that reactions of HNO2 with
phenolic and other constituents of soil organic matter promoted NO production.
However, with respect to the urine patch, previous studies have shown that solubilization of
soil organic matter occurs as a result of the high soil pH induced following hydrolysis
reactions. This may assist in the production of NOx emissions if denitrification is a dominant
production pathway.
2.7 Research Objectives
In view of the shortcomings in the current understanding of urine induced NOx fluxes a
sequence of controlled laboratory studies were also conceived in order to examine the effects
of soil moisture and temperature, soil pH and urine-N rate on the subsequent NOx emissions.
Further in situ studies were also performed to examine urine-N rate and in situ seasonal
effects on NOx emissions from urine patches.
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