A. M. Blackmer

Inhibitory effect of nitrate on reduction of N2O to N2 by soil microorganisms

Abstract The ability of soils to reduce N 2 O to N 2 depends very largely on their NO 3 − content. Low concentrations of NO 3 − delay reduction of N 2 O to N 2 by soil microorganisms, and high concentrations of NO 3 − almost completely inhibit this process. The inhibitory effect of NO 3 − on N 2 O reduction increases markedly with decrease in soil pH. These observations account for the finding in previous work that accumulation of N 2 O during denitrification of NO 3 − in soils incubated in closed systems is favored by high NO 3 − concentration and by low pH. They also indicate that, even if increased N fertilization of soils does not lead to a significant increase in the amount of N volatilized from soils as N 2 and N 2 O through denitrification of NO 3 − , it may cause a substantial increase in the ratio of N 2 O to N 2 and thereby pose a threat to the stratospheric ozone layer.

Nitrogen isotope discrimination in denitrification of nitrate in soils

Abstract Nitrogen isotope discrimination during denitrification in soils of nitrate containing natural concentrations of 14N and 15N was studied by determining the amount and the 15N content of nitrate-N and (nitrate + nitrite)-N in nitrate-treated soils incubated under anaerobic conditions (He atmosphere) for various times after treatment with glucose to promote denitrification. Analyses performed showed that the nitrate-N lost on incubation of these soils could largely be accounted for as products of denitrification (nitrite, NO. N2O and N2). The studies reported show that marked discrimination between 14N and 15N occurs during denitrification of nitrate in soils and that significant N isotope effects occur both in reduction of nitrate to nitrite and in reduction of nitrite to gaseous forms of N. They also indicate that the overall N isotope effect during denitrification of nitrate in soil will depend upon the tendency of the soil to accumulate nitrite under conditions that induce denitrification. It is concluded that discrimination between 14N and 15N during denitrification in soils of nitrate containing natural concentrations of these isotopes is of sufficient magnitude to invalidate the use of N isotope-ratio analyses for assessment of the contributions of soil and fertilizer N to nitrate in surface or ground waters or to nitrous oxide in the atmosphere.

Formation of nitrous oxide and dinitrogen by chemical decomposition of hydroxylamine in soils

Soil properties affecting formation of nitrous oxide (N2O) and dinitrogen (N2) by chemical decomposition of hydroxylamine (NH2OH) in soils were studied using 19 soils selected to obtain a wide range in properties. It was found that production of N2O by chemical decomposition of NH2OH in soils is more rapid than production of N2 and that, except with calcareous soils, N2O production greatly exceeds N2 production. Studies of the correlations between various soil properties and formation of N2O and N2 by decomposition of NH2OH showed that production of N2O was very highly correlated with exchangeable and oxidized Mn in the soils studied, and that production of N2 was very highly correlated with pH, CaCO3 equivalent, exchangeable Ca2+, and oxidized Mn. Production of N2 in neutral and acidic soils was highly correlated with both exchangeable and oxidized Mn, and production of N2 in calcareous soils was significantly correlated with oxidized Fe. The deductions from these correlations that Mn compounds are involved in the reactions leading to formation of N2O and N2 by chemical decomposition of NH2OH in soils, and that CaCO3 and Fe compounds are involved in the reactions leading to formation of N2 in calcareous soils, were supported by studies of N2O and N2 production through reactions of Mn and Fe compounds with NH2OH in the presence and absence of CaCO3. Production of N2O via chemical decomposition of NH2OH in soils greatly exceeds production of N2O through chemical decomposition of nitrite (i.e. via chemodenitrification), and the amount of N2 produced by decomposition of NH2OH in most soils exceeds the amount produced by decomposition of nitrite. Work reported indicates that, if N2O is formed in soils through nonbiological transformations of NH2OH produced by soil microorganisms, very little of this gas is generated by the reaction of NH2OH with nitrite frequently postulated as a mechanism of N2O production in soils (NH2OH + HNO2 → N2O + 2H2O).

MECHANISMS OF NITROUS OXIDE PRODUCTION IN SOILS

Recent research by atmospheric scientists has created international concern that increased use of nitrogen fertilizers to aid world food production will increase emission of nitrous oxide (N2O) from soils to the atmosphere via denitrification of fertilizer-derived nitrate and thereby promote destruction of the stratospheric ozone layer protecting the biosphere from biologically harmful ultraviolet radiation from the sun (see Council for Agricultural Science and Technology, 1976; McElroy et al., 1977; Crutzen and Ehhalt, 1977; Liu et al., 1977). This threat has stimulated extensive research on factors affecting N2O emissions from soils and the effects of nitrogen fertilizers on these emissions. The purpose of this article is to report recent research in our laboratory relating to the mechanisms of N2O production in soils.