Already at tlic ciid of tlic last century GAYOS 6 DUPETIT (1886) made tlic imprt:int obscrwtion tlial ccrl:iin soil bactcrin arc cap1)lc of rcducing nitrate to iiiolccular iiitrogcn iiiicl nitrous oside. Sincc tlien, .tlic qimlion lins bccn sliidicd with considcrablc iritcrcsl by microbiologists and agricultur:il scientists, and at prcscnt :in cstcnsiw literature is avai1al)lc in this tloniain. It soon I)ccamc clcnr that tlic recluction of nitr:ite usually appeared under conditions of : i limited oxygen supply, :ind that this cotild bc ascrilxd to facultntivcly :icrobic bactcrin lwlonging to wry clifrcrent groups: It tvns furtlicriiiorc undcrstood t1i:it sucli n nitrntc reduction lias : i cerl:iin conncction with proccsscs which hnvc an energetic significaricc to tlie micro-or~aiiisiiis. Now:idiiys it is consiclcrctl quite ccrt:iin t l u t in this case flic nitrate scrvcs as tlic hydrogen acceptor in cnzyniatic cleligclrogcrintio~is of the organic or inorganic substrafe ( I < L u Y~E~ 6 I)OSI;ER 1926). Nitrate rcduction in tlic lattcr scnsc Iias bccn cnllctl clenitrificntion, in ordcr to disfingiiisli this from thc nitrate rcdnction in conncction with nitrntc assiniil:ition. It is cliaracteristic for tlic denitrification process that it gcncrally lcads to gascons reduction protfucts of nitrate, sucli as nitrous oside and iiiolccirlar nitrogcn. l'lius clcnitrification cnuscs n dircct loss of plant-arail:ible nitrogcn in tlic soil.Wlicll~er denilrificntion is of any mijor iniportancc in nrablc soils is :I qimtion which I~i s aroused widely divergent opinions :imong rcsc:ircli \\.orlicrs. 'I'licrc h v e , liowcvcr, 1)ccn soiiic w r y :ilnriiiing reports, wlierc no iiiorc tli:in 40-50 per cent of tlic nitrogen introduccd by coxiiiiicrcinl fcrtilizers could iic tr:iccd :it tlic cnd of the cspcrimental period in the crops and in tlic experimcntal soil (Ahss & BARSES 19.51). Attciiipts I i n~e hecn madc to explain this hy lcncliing out of tlic nitrate tlirougli precipitation water. Sonic long-tcrm Iysinieter espcrimcnts Iiavc, Iiowcvcr, shown that only part of the total deficit in 13 -563002 Ada dgricultura, Scandinacicn
Using diluted phosphoric acid-and glycerol-treated polyurethane plastic foam discs as static a m m o n i a absorbers, the extent of a m m o n i a loss from applied urea was determined on forest soil under field conditions. The investigation, which primarily involved a comparison of urea materials of two different pellet sizes (2-4 mg and 2060 mg urea per pellet, respectively), extended over a period of 28 days. The urea was applied to the soil surface at a rate of 200 kg N per hectare.It was found t h a t the gaseous a m m o n i a loss from the large-pellet urea (tablets) was markedly lower than t h a t from the small-pellet urea during the first two weeks of exposure. On extending the observation period to four weeks, this difference in cumulative a m m o n i a loss decreased successively. After 28 days' exposure, at a daily mean temperature of 13°C, the volatile loss a m o u n t e d to 20 and 22 per cent, respectively. An addition of 5~o (w/w) of a solution of concentrated orthophosphoric acid, or a fine-crystalline ortho-boric acid to the large-pellet urea, resulted in a reduction of the loss to half the value. The possibilities of controlling the ammonia loss from urea by combining the large pellet size with the addition of urease inhibitors are discussed.
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