Effects of soil amendment on gas depth profiles in soil monoliths using direct mass spectrometric measurement

Land use and agricultural practices are known to influence the source and sink concentrations of various gases, including greenhouse gases (NO_x, CH₄ and CO₂), in soils. With everincreasing production of domestic sewage sludge and the prohibition of disposal at sea, pressure on waste disposal increases. Anaerobically digested domestic sewage sludge and/or lime were applied to an upland, Scottish soil and their effects on gas depth profiles monitored as indicators of microbial processes of the soil ecosystem. The concentrations of various gases (Ar, O₂, CO₂, CH₄, N₂, NO_x) were measured simultaneously at each depth using membrane inlet mass spectrometry (MIMS). This technique enables the direct measurement of multiple gas species throughout soil cores with minimal disturbance. Intact soil monoliths were collected from the sample site, following amendment, and maintained in a constant temperature, environmental growth chambers. Statistical analyses (one-way ANOVA and LSD tests) were conducted to identify the depths at which gas concentrations in amended cores were significantly different from those in control (un-amended) cores. Significant effects were observed on the concentration of CO₂, CH₄, NO_x and N₂ at certain depths. Average CH₄ concentration was consistently higher (>1 μM) in the upper horizon following application of sludge and sludge and lime together. N₂ and NO_x concentrations were elevated in cores treated with lime by approximately 100 and 32 μM, respectively, in much of the upper horizon. CO₂ concentration increased above control mean values, at certain depths, following application of either sludge or lime. Some explanation for the changes in soil gas concentration was provided by reference to the microorganism assemblages and the gases associated with biochemistry of nitrification, denitrification, methane oxidation and methanogenesis.