Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes

Abstract

Summary This review examines the interactions between soil physical factors and the biological processes responsible for the production and consumption in soils of greenhouse gases. The release of CO2 by aerobic respiration is a non‐linear function of temperature over a wide range of soil water contents, but becomes a function of water content as a soil dries out. Some of the reported variation in the temperature response may be attributable simply to measurement procedures. Lowering the water table in organic soils by drainage increases the release of soil carbon as CO2 in some but not all environments, and reduces the quantity of CH4 emitted to the atmosphere. Ebullition and diffusion through the aerenchyma of rice and plants in natural wetlands both contribute substantially to the emission of CH4; the proportion of the emissions taking place by each pathway varies seasonally. Aerated soils are a sink for atmospheric CH4, through microbial oxidation. The main control on oxidation rate is gas diffusivity, and the temperature response is small. Nitrous oxide is the third greenhouse gas produced in soils, together with NO, a precursor of tropospheric ozone (a short‐lived greenhouse gas). Emission of N2O increases markedly with increasing temperature, and this is attributed to increases in the anaerobic volume fraction, brought about by an increased respiratory sink for O2. Increases in water‐filled pore space also result in increased anaerobic volume; again, the outcome is an exponential increase in N2O emission. The review draws substantially on sources from beyond the normal range of soil science literature, and is intended to promote integration of ideas, not only between soil biology and soil physics, but also over a wider range of interacting disciplines.

Nitrous oxide generation, denitrification, and nitrate removal in a seepage wetland intercepting surface and subsurface flows from a grazed dairy catchment

“…The elevated rates we observed reflect environmental conditions conducive to denitrification, i.e. low DO (<1 mg O 2 /L; Table 3) plus a highly enriched organic matter sediment (Table 2) and optimum sediment-water contact time within the confined lysimeter environment (Hill 1996;Fennessy and Cronk 1997;Hoffmann et al 2000;Smith et al 2003). The observed high denitrification rates could be attributed to the elevated DEA values (Table 4) in the wetland soils (Groffman et al 1999).…”

“…Both surface water and subsurface water had low DO levels (<1 mg O 2 /L; Table 3), suggesting that anaerobic conditions favourable for denitrification (Smith et al 2003;Zaman et al 2008b) were prevalent in this wetland. Various workers (Tiedje 1988;Patrick and Jugsujinda 1992;Achtnich et al 1995;Blicher-Mathiesen and Hoffmann 1999) have reported that NO 3 À reduction occurs mainly when subsurface water DO falls below 0.5-1.6 mg O 2 /L.…”

“…If N 2 O is produced at sites just below the top sediment layer, it may readily diffuse into sediment-air water interface as N 2 O, instead of being reduced to N 2 . In contrast, N 2 O produced in subsoils may be entrapped in subsurface waters and subsequently be reduced to N 2 by sediment denitrifiers (Hefting et al 2003;Smith et al 2003). In addition to NO 3 À concentration and depth, sediment pH, C, and O 2 content and temperature may also affect the generation rates of N 2 O and N 2 (Del Grosso et al 2000;Dobbie and Smith 2001;Smith et al 2003;Zaman et al 2004Zaman et al , 2007Zaman et al , 2008b.…”

“…Nitrous oxide in wetlands is produced not only by denitrification but also by nitrification and DNRA (Stevens and Laughlin 1998;Hefting et al 2003;Smith et al 2003). Nitrous oxide produced by various processes might form 1 pool before being reduced to N 2 by nitrous oxide reductase, the enzyme involved in reduction of N 2 O to N 2 (Stevens and Laughlin 1998), which suggests that wetlands can act as a source or a sink of N 2 O (Blicher-Mathiesen and Hoffmann 1999; Well et al 2001).…”

“…A wide range of N 2 O : N 2 ratios has therefore been reported for wetlands, depending on factors (e.g. soil pH, carbon, anaerobicity, NO 3 À concentrations) that govern the nitrification-denitrification-DNRA processes (Groffman et al 2000Smith et al 2003). Thus, the use of wetlands in decreasing diffuse N pollution of streams and rivers may shift the potential environmental problem from water pollution to greenhouse gas emission (Well et al 2001;Hefting et al 2003).…”