Soil CO<sub>2</sub> efflux in an old-growth southern conifer forests (<i>Agathis australis</i>) – magnitude, components, and controls
Abstract. Total soil CO2 efflux and its component fluxes, autotrophic and heterotrophic respiration, were measured in a native forest in northern Aotearoa-New Zealand. The forest is dominated by Agathis australis (kauri) and is on an acidic, clay rich soil. Soil CO2 efflux, volumentric soil water content and soil temperature were measured bi-weekly to monthly at 42 locations over 18 months. Trenching and regression analysis was used to partition the total soil CO2 efflux. The effect of tree structure was investigated by calculating an index of local contribution (Ic, based on tree size and distance to the measurement location) followed by correlation analysis between Ic and soil CO2 efflux, root biomass, litterfall and soil characteristics. The mean total soil CO2 efflux was 3.47 μmol m−2 s−1. Using uni- and bivariate models showed that soil temperature (< 40 %) and volumetric soil water content (< 20 %) were poor predictors of the temporal variation in total soil CO2 efflux. In contrast, a stronger temperature sensitivity (around 57 %) was found for heterotrophic respiration. Autotrophic respiration accounted for 25 (trenching) or 28 % (regression analysis) of total soil CO2 efflux. We found significant positive relationships between kauri tree size distribution (Ic) and soil CO2 efflux, root biomass and mineral soil CN ratio within 5–6 m of the measurement points. Using multiple regression analysis revealed that 97 % of the spatial variability in soil CO2 efflux in this kauri dominated stand was explained by root biomass and soil temperature. Our findings highlight the need to consider tree species effects and spatial patterns in soil carbon related studies.