Soil–atmosphere greenhouse gas exchange in a cool, temperate Eucalyptus delegatensis forest in south-eastern Australia

Abstract

Forests are the largest C sink (vegetation and soil) in the terrestrial biosphere and may additionally provide an important soil methane (CH₄) sink, whilst producing little nitrous oxide (N₂O) when nutrients are tightly cycled. In this study, we determine the magnitude and spatial variation of soil–atmosphere N₂O, CH₄ and CO₂ exchange in a Eucalyptus delegatensis forest in New South Wales, Australia, and investigate how the magnitude of the fluxes depends on the presence of N₂-fixing tree species (Acacia dealbata), the proximity of creeks, and changing environmental conditions. Soil trace gas exchange was measured along replicated transects and in forest plots with and without presence of A. dealbata using static manual chambers and an automated trace gas measurement system for 2 weeks next to an eddy covariance tower measuring net ecosystem CO₂ exchange. CH₄ was taken up by the forest soil (−51.8 μg CH₄-C m⁻² h⁻¹) and was significantly correlated with relative saturation (S_r) of the soil. The soil within creek lines was a net CH₄ source (up to 33.5 μg CH₄-C m⁻² h⁻¹), whereas the wider forest soil was a CH₄ sink regardless of distance from the creek line. Soil N₂O emissions were small (<3.3 μg N₂O-N m⁻² h⁻¹) throughout the 2-week period, despite major rain and snowfall. Soil N₂O emissions only correlated with soil and air temperature. The presence of A. dealbata in the understorey had no influence on the magnitude of CH₄ uptake, N₂O emission or soil N parameters. N₂O production increased with increasing soil moisture (up to 50% S_r) in laboratory incubations and gross nitrification was negative or negligible as measured through ¹⁵N isotope pool dilution.

The small N₂O emissions are probably due to the limited capacity for nitrification in this late successional forest soil with C:N ratios >20. Soil–atmosphere exchange of CO₂ was several orders of magnitude greater (88.8 mg CO₂-C m⁻² h⁻¹) than CH₄ and N₂O, and represented 43% of total ecosystem respiration. The forest was a net greenhouse gas sink (126.22 kg CO₂-equivalents ha⁻¹ d⁻¹) during the 2-week measurement period, of which soil CH₄ uptake contributed only 0.3% and N₂O emissions offset only 0.3%.

Introduction

Increases in the concentrations of greenhouse gases (GHG) such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), in the atmosphere contribute to global climate change. The cumulative effects of a rapid accumulation of GHG in the atmosphere has led to changes in the earth's energy balance and concerns that “most of the warming observed over the last 50 years is attributable to human activities” (Houghton et al., 2001). Management of fossil fuel emissions has the potentially biggest impact on the magnitude of global climate change. Forest management has an impact upon the exchange of greenhouse gases (CO₂, CH₄ and N₂O) between the terrestrial biosphere and the atmosphere. Whilst we do have a good understanding of the terrestrial C and CO₂ cycle, we know little about how other GHG are cycled through the terrestrial biosphere. A comprehensive GHG balance and climate change strategy needs to take a multiple greenhouse gas approach (Crutzen et al., 2007, Robertson et al., 2000) and therefore investigate the importance of GHG fluxes simultaneously. Forests provide the most important mechanisms of CO₂ exchange between the atmosphere and terrestrial biosphere (Schimel et al., 2001). Less well known is the role forests play in the exchanges of CH₄ and N₂O with the atmosphere and how these GHG impact on the total GHG balance.

Soil CO₂ respiration (soil, litter and root) can represent up to 70% to total ecosystem respiration in temperate forests (Raich and Schlesinger, 1992). However, photosynthetic C assimilation by the temperate forest canopy usually exceeds total respiration C losses such that the forest acts as a net CO₂ sink (Law et al., 1999, Leuning et al., 2005, Raich and Schlesinger, 1992). The main environmental factors controlling soil respiration are temperature and moisture, as soil respiration generally increases exponentially with increasing soil temperature when soil moisture is not limiting (Fang and Moncrieff, 2001, Rey et al., 2005).

Temperate forest soils are expected to be a small N₂O source because of the large C:N ratios in temperate forest litter and topsoil (Attiwill et al., 1996, Butterbach-Bahl and Kiese, 2005) responsible for between 2.3 and 13.5% of global non-anthropogenic N₂O emissions (IPCC, 2007). N₂O is produced through soil microbial nitrification under aerobic (oxidative) conditions, or denitrification under anaerobic (reductive) conditions (Conrad, 1996). These microbial processes can occur simultaneously in adjacent anaerobic and aerobic micro-sites within the soil profile (Firestone and Davidson, 1989, Potter et al., 1996). The main environmental factors controlling N₂O production in temperate forest soils are soil N status (NH₄⁺, NO₃⁻), soil moisture, aeration and temperature (Butterbach-Bahl and Kiese, 2005). Soil N₂O emissions from forest soils have highly spatial variability according to the variability in these controlling environmental variables.

At some successional stage N₂-fixing tree species are present in most Australian forests and represent the single most important natural input of nitrogen to forest soils (Attiwill et al., 1996, Guinto et al., 2000, O’Connell and Grove, 1996). They are either an integral component of the mature forest vegetation structure (Orchard and Wilson, 2001), or an integral component of early forest succession, in recovery from fire or harvest (Adams and Attiwill, 1984, Van Der Meer and Dignan, 2007). In Australian forests wildfires can release up to 500 kg N ha⁻¹ (Raison, 1979) and in the early years following wildfire N₂-fixation by acacias can provide up to 50 kg N ha⁻¹ year⁻¹ (May and Attiwill, 2003) and thus replace some of the lost nitrogen. In other forests and woodlands the presence of N₂-fixing trees has been shown to greatly increase soil N₂O production (Angoa-Perez et al., 2004, Dick et al., 2006, Erickson et al., 2001, Skiba et al., 1998). Furthermore, soil N₂O emissions are much greater in phosphorous limited forest soils after N addition than from forest soils not limited by P (Hall and Matson, 1999), and most Australian forests are more P than N limited (May and Attiwill, 2003, Pate and Uncovich, 1999).

Forest soils are the most important terrestrial CH₄ sink, through the CH₄ oxidation activity of methanotroph bacteria in well-drained, aerobic soils, (Castro et al., 1995, Conrad, 1996, Price et al., 2004, Steudler et al., 1989). Soils in the temperate region are responsible for between 30 and 50% of the terrestrial CH₄ sink (Ojima et al., 1993). The diffusivity of CH₄ through the soil profile is the primary limiting factor upon CH₄ oxidation and this is influenced by soil moisture and bulk density. Furthermore, soil N status (NH₄⁺ and NO₃⁻) can limit CH₄ oxidation directly by inhibiting/competing with the mono-oxygenase enzyme of methanotrophs (Castro et al., 1995). In natural forest soils, the main factor controlling CH₄ production or consumption is soil water content, as such there can be considerable spatial variation in soil–atmosphere CH₄ exchange according to position in the landscape, proximity to creek lines and micro-topography.

Trace gas emissions from Australian forest soils have rarely been studied (Dalal et al., 2008, Kiese et al., 2003), as such, we have little understanding of the spatial and temporal variability in N₂O and CH₄ exchange in Australian forest soils, the soil parameters and microbial processes that control this exchange and the importance of forest type or tree species. There is a high level of uncertainty as to the contribution (source or sink) of Australian forests to regional, continental and the global CH₄ and N₂O balance. Temperate, open eucalypt forests in south-eastern Australia cover around 26 million hectares, providing important ecosystem services such as biodiversity, recreation, water resources and wood products. This study investigates the magnitude and spatial variation in the exchange of GHG between the soil and atmosphere in an Australian cool, temperate eucalypt forest during the transition period from (cold/wet) winter to (warmer/wet) spring 2006. Peak N₂O emissions have been measured during this seasonal transition period in other Australian forest systems (Butterbach-Bahl et al., 2004, Kiese and Butterbach-Bahl, 2002, Kiese et al., 2003). The objectives were to (i) measure trace gas exchange in forest areas with and without N₂-fixing Acacia dealbata trees, (ii) investigate differences in trace gas exchange according to position in the landscape, i.e. distance from a creek line and (iii) continuously measure soil trace gas exchange for a 2 week period to investigate the response to changing soil environmental conditions with passing weather fronts and precipitation events using an automated trace gas measurement system, (iv) relate soil–atmosphere CO₂, N₂O and CH₄ exchange to total ecosystem CO₂ exchange from concurrent eddy covariance measurements.