Hungate et al. (2003) raised the importance of nutrient feedbacks on carbon (C) uptake by terrestrial ecosystems to determine associated additional C storage under future predicted carbon dioxide (CO2) concentrations in the atmosphere. Nevertheless, there is only little research investigating how rising CO2 levels affect soil nutrient status. To address this, here, I present my PhD research looking at effects of elevated CO2 (eCO2) on soil nitrogen and phosphorus cycling in a mature Eucalyptus woodland in Australia.
Effect of elevated carbon dioxide on soil N and P cycling in a P-limited Eucalyptus woodland
Free Air CO2 Enrichment (FACE) experiments have consistently demonstrated increased plant productivity in response to eCO2, with the magnitude of responses related to soil nutrient status. Most experiments to date have been carried out in the northern hemisphere where nitrogen (N) is the primary growth-limiting nutrient. Whilst understanding nutrient constraints on productivity responses to CO2 is crucial to predicting C uptake and storage, and thus terrestrial feedbacks on climate change, very little is known about how eCO2 affects nutrient cycling in the nutrient poor, P-limited ecosystems that dominate in many parts of the Southern Hemisphere. Our study investigates the effects of eCO2 on soil N and P dynamics at the recently established EucFACE experiment in a P-limited woodland, in western Sydney, Australia (Figure 1). Three ambient and three eCO2 (+ 150 ppm) FACE rings (25 m diameter) were installed in a native Cumberland Plain Eucalyptus woodland, and CO2 treatments were initiated in September 2012. Instantaneous measurements of soil extracts and soil solution chemistry showed seasonally-dependant effects of eCO2 on ammonium, phosphate and DOC concentrations, representing 23 %, 14 % and 21 % increases over ambient rings, respectively, over the 18 month study period, with significant CO2 x Time interactions (P < 0.05). Extractable and soil solution nitrate concentrations and soil enzyme activities were not affected by CO2 treatment. Integrating measures of nutrient concentrations (in situ incubation of ion exchange resin strips) showed significantly higher levels of plant accessible nitrate (+95 %) with a CO2 x Time interaction (P = 0.05), ammonium (+12 %, P = 0.06), and phosphate (+54 %) with a CO2 x Time interaction (P < 0.001) under eCO2. Elevated CO2 was also associated with faster rates of nutrient turnover in the early part of the experiment, with P mineralisation rates 211 % higher in eCO2 rings compared to ambient in the first six months of CO2 fumigation, although this difference did not persist. Taken together, these results demonstrate that CO2 fertilisation increases nutrient turnover and availability – particularly for phosphate – in strongly P-limited soils, likely via increased plant belowground investment in labile C and associated enhancement of microbial turnover of soil organic matter. Early evidence therefore suggests that nutrient mining may reduce constraints on productivity responses to eCO2 in P-limited woodland ecosystems, at least in the short term.
Hungate BA, Dukes JS, Shaw MR, Luo YQ, Field CB (2003) Nitrogen and climate change. Science, 302, 1512-1513.