Nitric oxide emissions from agricultural soils
Thorman, Rachel E.
Nitric oxide (NO) plays a crucial role in photochemistry, particularly in the formation of tropospheric ozone. In soil the biogenic production of NO is primarily conducted by the microbial processes of nitrification and denitrification. The management of soils may, therefore, significantly impact on local atmospheric NO concentrations. The aim of this study was to investigate the influence of various agricultural practices on the magnitude of NO flux, specifically the role of tillage technique in an arable system and the comparative effect of organic wastes and inorganic fertilisers applied to a grassland system.Fluxes of NO from a sandy loam/silty clay loam soil cropped with spring barley, with and without the addition of NH4NO3 fertiliser (80 kg N ha'1), were measured using a static chamber method. The site was managed to compare the influence of 3 tillage regimes; conventional mouldboard ploughing, deep ploughing and direct drilling. There was a marked effect on the magnitude of NO fluxes fromboth the nitrogen and tillage treatment. Nitric oxide fluxes ranged between2-1deposition and emission from -2.6-49.5 jug NO-N m" h" (fertilised & ploughed) and -2.0-2.0 /xg NO-N m2 h"1 (unfertilised & direct drilled). Emissions of NO were significantly larger from the ploughed soils than from the direct drilled soils, primarily due to the increased water filled pore space stimulating denitrification and reducing NO emission. Of the fertiliser N added 0.002-0.011% was lost as NO.The flux of NO between ungrazed grassland (clay loam) and the atmosphere was measured following the application, at a target rate of 120 kg available N ha"1, of either cattle slurry, anaerobically digested sewage sludge, thermally dried sewage sludge pellets, mineral NPK fertiliser & Ficote 70® slow release fertiliser or no fertiliser addition. Nitric oxide emissions were stimulated by both organic wastes and NPK inorganic fertiliser, with cumulative fluxes markedly higher from the organic wastes, particularly from the sewage sludge pellets, which were 1.3-42.3 times larger than the other treatments. It was estimated that 0.0004-0.03% of the applied total N was released as NO.Complementary laboratory studies designed to investigate the influence of dominant environmental factors on NO emission from repacked soil cores under controlled conditions showed that NO emission was 2.2-23.5 times larger from soil amended with sewage sludge pellets. The magnitude of the flux was associated with a soil saprophytic fungus and incorporation of the pellets appeared to reduce the cumulative NO loss. In field and laboratory studies NO flux rate was strongly dependent on soil NH4+-N, soil NO3VN, soil water filled pore space and the pattern of precipitation, particularly around fertiliser application. The data suggest that NO was primarily produced by nitrification in the grassland soil and a combination of both denitrification and nitrification in the arable soil.The total flux from UK agricultural land was estimated as 0.007 Tg of NO-N. This is approximately 1.5% of the annual UK total NO-N production. Based on the evidence collected from the 2 field studies, therefore, the emission of NO from agricultural soils in the UK is not significant in terms of its contribution to the NO-N total. Flowever, agricultural soils may emit NO to the atmosphere and produce localised concentrations high enough (e.g. after fertiliser application) to generate harmful levels of tropospheric O3.