Influence of photosynthetically active radiation on diurnal N2O emissions under ruminant urine patches

Nitrous oxide (Nâ‚‚O) is a greenhouse gas and is also recognized as the single most important ozone depleting emission. Static chambers are commonly used to measure Nâ‚‚O fluxes from agricultural soils with fluxes measured over approximately 1 hour typically extrapolated to a daily flux, thus assuming a constant flux over 24 h. Diurnal variation in soil Nâ‚‚O fluxes may lead to inaccurate inventory calculations. The aim of this study was to assess the effect of diurnal changes in photosynthetically active radiation (PAR) on Nâ‚‚O emissions from planted and nonplanted soil exposed to bovine urine. The effects of varying PAR, over a 16 h day length, on Nâ‚‚O emissions were observed in a split-split plot factorial experiment conducted in the New Zealand Biotron under constant soil and air temperatures. Treatments included absence or presence of both bovine urine and a 1-month-old perennial ryegrass (Lolium perenne) sward. Diurnal Nâ‚‚O fluxes were determined every 1.5 h on days 6 and 19 after urine deposition when ammonium (NHâ‚„+) and nitrate (NO₃"¾) were expected to dominate the soil inorganic-N pool, respectively. On day 6, NHâ‚„+ dominated the soil inorganic-N pool and Nâ‚‚O-N fluxes were relatively low with diurnal amplitudes of 0.5-3.4. By day 19, the soil NO₃"¾ pool had increased as did Nâ‚‚O-N fluxes along with diurnal amplitudes (2.1-41.7). The highest Nâ‚‚O-N fluxes occurred in the soil plus urine treatment as a consequence of PAR altering soil temperature and not as a direct consequence of PAR effects on grass. Diurnal fluxes of Nâ‚‚O-N following ruminant urine deposition to pasture did not occur as a direct result of diurnal PAR cycles potentially affecting root exudation. It is assumed that other sources of C, such as solubilized organic matter, dominated as a microbial C source for denitrifiers.