Microbial processes contributing to N₂O production in two sandy Scottish soils

The objective of the present work was to investigate the potential contribution of different microbial processes to N20 fluxes and the influence of the main environmental factors on these processes in two light textured Scottish soils. Two imperfectly drained brown forest soils of the Peffer Series, derived from fine beach sand, were studied. One was a sandy loam, sampled from a deciduous woodland and the other was a loamy sand, sampled from a nearby winter wheat field, at Gullane, East Lothian, Scotland. Both soils had slightly alkaline pEls. Field and laboratory studies demonstrated that more than one process was responsible for N20 emissions. Field results showed two different patterns of N20 emissions. Measurements with static manual chambers showed very low fluxes (1.4 1.5 g N 2 0 N ha' 1 d'1) throughout the year, though with some seasonal variation. These fluxes were not correlated with any environmental parameter measured. However, measurements with automatic chambers occasionally showed higher N20 fluxes (up to 44 g N2 0-N ha' 1 d'1). The latter were favoured by the presence of fresh organic matter and high concentrations of N H /-N (rather than N 0 3 '-N). This was shown to be true both in a fertilization experiment in the woodland and in an incubation experiment with woodland soil cores. The nature of the processes responsible for the N20 field emissions, e.g. denitrification, autotrophic and heterotrophic nitrification, are discussed and it is concluded that nitrification (either autotrophic or heterotrophic) was mainly responsible for the low N20 fluxes, whereas denitrification or heterotrophic nitrification-dénitrification was the main source of the higher fluxes. N20 emissions, determined in a laboratory experiment in which the soils were incubated with different sources of nitrogen, with or without glucose, and with 0 , 1 and 100 ml C2 H2 I'1, showed large differences in the rate of N20 production both between the two soils and between the different N treatments. The arable soil showed very low N20 emissions derived from reduced forms of N as compared with the N20 which was produced when the soil was provided with N 0 2' or N 0 3" and a C source, suggesting a very active denitrifier population. In contrast, the woodland soil showed a very low denitrification activity and a much higher N20 production derived from the oxidation of N H / and reduction of N 0 2' by some processes probably mediated by autotrophic or heterotrophic nitrifiers. In both soils, the highest N20 emissions were induced by N 0 2' addition. Those emissions were demonstrated to have a biological origin, as no significant N20 emissions were measured when the soil was autoclaved. Experiments carried out using 15N pool dilution and enrichment techniques and physiological block techniques for prokaryotic and eukaryotic microorganisms (streptomycin and cycloheximide, respectively) showed that a well established population of heterotrophic nitrifiers was present in the woodland soil. The balance between autotrophic and heterotrophic nitrification in the soil was influenced by the concentration of organic N. The heterotrophic activity increased from 18% to 56% of the total nitrification activity when the peptone concentration was increased from 70 to 280 pg N g '1. Low concentrations (0-2.5 mg g '1) of both antibiotics had no apparent biocidal and disruptive effect on the microbial biomass, in the first 48 h incubation, indicating only a selective action of protein synthesis inhibition, whereas at high concentration (7.5 mg g '1) cycloheximide had a marked biocidal effect on the overall population of nitrifiers, blocking completely any nitrification activity. Heterotrophic nitrification was completely blocked and autotrophic nitrification was reduced at 2 mg cycloheximide g '1, while streptomycin only slightly reduced both autotrophic and heterotrophic nitrification, even at 3.5 mg g '1. This suggested that fungi could have a dominant role in N 0 3‘ production from readily available organic-N in the woodland soil, even at slightly alkaline pH. The partial inhibition of autotrophic nitrification by low concentrations of cycloheximide indicate the possibility for another fungal pathway of N 0 3" production which might utilize an inorganic route. This possibility was also supported by the results with non-isotopic techniques, where the N20 fluxes induced by peptone addition were completely inhibited by low concentrations of cycloheximide ( 1 2 mg cycloheximide g '1) but also by 0 . 1 % ( 1 0 0 Pa) acetylene, suggesting a possible role of ammonia monooxygenase in an organicinorganic pathway of nitrification in fungal metabolism.