Land use change and the impact on greenhouse gas exchange in north Australian savanna soils

[摘要] Savanna ecosystems are subjected to accelerating land use change as humandemand for food and forest products increases. Land use change has beenshown to both increase and decrease greenhouse gas fluxes from savannas andconsiderable uncertainty exists about the non-CO₂ fluxes from the soil.We measured methane (CH₄), nitrous oxide (N₂O) and carbon dioxide(CO₂) over a complete wet-dry seasonal cycle at three replicate sitesof each of three land uses: savanna, young pasture and old pasture(converted from savanna 5–7 and 25–30 yr ago, respectively) in theDouglas Daly region of Northern Australia. The effect of break of seasonrains at the end of the dry season was investigated with two irrigationexperiments.Land use change from savanna to pasture increased net greenhouse gas fluxesfrom the soil. Pasture sites were a weaker sink for CH₄ than savannasites and, under wet conditions, old pastures turned from being sinks to asignificant source of CH₄. Nitrous oxide emissions were generally verylow, in the range of 0 to 5 μg N₂O-N m⁻² h⁻¹, and underdry conditions soil uptake of N₂O was apparent. Break of season rainsproduced a small, short lived pulse of N₂O up to 20 μg N₂O-N m⁻² h⁻¹, most evident in pasture soil. Annual cumulative soilCO₂ fluxes increased after clearing, with savanna (14.6 t CO₂-C ha⁻¹ yr⁻¹) having the lowest fluxes compared to old pasture(18.5 t CO₂-C ha⁻¹ yr⁻¹) and young pasture (20.0 t CO₂-C ha⁻¹ yr⁻¹). Clearing savanna increased soil-based greenhouse gasemissions from 53 to ∼ 70 t CO₂-equivalents, a 30% increasedominated by an increase in soil CO₂ emissions and shift from soilCH₄ sink to source. Seasonal variation was clearly driven by soil watercontent, supporting the emerging view that soil water content is a moreimportant driver of soil gas fluxes than soil temperature in tropicalecosystems where temperature varies little among seasons.

[学科分类] 地球化学与岩石