Trace gas flux and the influence of short-term soil water and temperature dynamics in Australian sheep grazed pastures of differing productivity

• Published in: Plant and soil 2008-08, Vol.309 (1-2), p.89-103
• Main Authors: Livesley, S. J, Kiese, R, Graham, J, Weston, C. J, Butterbach-Bahl, K, Arndt, S. K
• Format: Article
• Language: English
• Subjects: Agricultural soils; Agronomy. Soil science and plant productions; Animal, plant and microbial ecology; Biological and medical sciences; Biomedical and Life Sciences; Carbon dioxide; Ecology; Emissions; Fertilizers; Fluctuations; Forest soils; Fundamental and applied biological sciences. Psychology; Grassland soils; Greenhouse gases; Gross nitrification; Legumes; Life Sciences; Methane; Moisture content; Nitrification; Nitrogen fixation; Nitrous oxide; Pasture; Pastures; Plant Physiology; Plant Sciences; Pollutant emissions; Productivity; Regular Article; Sheep; Soil microbial activity; Soil microorganisms; Soil moisture; Soil quality; Soil Science & Conservation; soil temperature; Soil water; Temperature; Trace gas; Water content
• ISSN: 0032-079X; 1573-5036
• DOI: 10.1007/s11104-008-9647-8

Temperate pastures are often managed with P fertilizers and N₂-fixing legumes to maintain and increase pasture productivity which may lead to greater nitrous oxide (N₂O) emissions and reduced methane (CH₄) uptake. However, the diel and inter-daily variation in N₂O and CH₄ flux in pastures is poorly understood, especially in relation to key environmental drivers. We investigated the effect of pasture productivity, rainfall, and changing soil moisture and temperature upon short-term soil N₂O and CH₄ flux dynamics during spring in sheep grazed pasture systems in southeastern Australia. N₂O and CH₄ flux was measured continuously in a High P (23 kg P ha⁻¹ yr⁻¹) and No P pasture treatment and in a sheep camp area in a Low P (4 kg P ha⁻¹ yr⁻¹) pasture for a four week period in spring 2005 using an automated trace gas system. Although pasture productivity was three-fold greater in the High P than No P treatment, mean CH₄ uptake was similar (-6.3 ± SE 0.3 to -8.6 ± 0.4 μg C m⁻² hr⁻¹) as were mean N₂O emissions (6.5 to 7.9 ± 0.8 μg N m⁻² hr⁻¹), although N₂O flux in the No P pasture did not respond to changing soil water conditions. N₂O emissions were greatest in the Low P sheep camp (12.4 μg ± 1.1 N m⁻² hr⁻¹) where there were also net CH₄ emissions of 5.2 ± 0.5 μg C m⁻² hr⁻¹. There were significant, but weak, relationships between soil water and N₂O emissions, but not between soil water and CH₄ flux. The diel temperature cycle strongly influenced CH₄ and N₂O emissions, but this was often masked by the confounding covariate effects of changing soil water content. There were no consistently significant differences in soil mineral N or gross N transformation rates, however, measurements of substrate induced respiration (SIR) indicated that soil microbial processes in the highly productive pasture are more N limited than P limited after >20 years of P fertilizer addition. Increased productivity, through P fertilizer and legume management, did not significantly increase N₂O emissions, or reduce CH₄ uptake, during this 4 week measurement period, but the lack of an N₂O response to rainfall in the No P pasture suggests this may be evident over a longer measurement period. This study also suggests that small compacted and nutrient enriched areas of grazed pastures may contribute greatly to the overall N₂O and CH₄ trace gas balance.