Elevated carbon dioxide increases soil nitrogen and phosphorus availability in a phosphorus‐limited Eucalyptus woodland

Abstract

Free-Air CO 2 Enrichment (FACE) experiments have demonstrated increased plant productivity in response to elevated (e)CO 2 , with the magnitude of responses related to soil nutrient status. Whilst understanding nutrient constraints on productivity responses to eCO 2 is crucial to predicting carbon uptake and storage, very little is known about how eCO 2 affects nutrient cycling in phosphorus (P)-limited ecosystems. Our study investigates eCO 2 effects on soil N and P dynamics at the EucFACE experiment in Western Sydney over an 18 month period. Three ambient and three eCO 2 (+150 ppm) FACE rings were installed in a P-limited, mature Cumberland Plain Eucalyptus woodland. Levels of plant accessible nutrients, evaluated using ion exchange resins, were increased under eCO 2 , compared to ambient, for nitrate (+93%), ammonium (+12%) and phosphate (+54%). There was a strong seasonality to responses, particularly for phosphate, resulting in a relatively greater stimulation in available P, compared to N, under eCO 2 in spring and summer. eCO 2 was also associated with faster nutrient turnover rates in the first six months of the experiment, with higher N (+175%) and P (+211%) mineralisation rates compared to ambient rings, although this difference did not persist. Seasonally-dependant effects of eCO 2 were seen for concentrations of dissolved organic carbon in soil solution (+31%), and there was also a reduction in bulk-soil pH (-0.18 units) observed under eCO 2 . These results demonstrate that CO 2 fertilisation increases nutrient availability -particularly for phosphate -in P-limited soils, likely via increased plant belowground investment in labile carbon and associated enhancement of microbial turnover of organic matter and mobilisation of chemically-bound P. Early evidence suggests that there is the potential for the observed increases in P availability to support increased ecosystem Caccumulation under future predicted CO 2 concentrations.

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO2]

“…Delivery delay of the microelectrode impeded pH measurements during DEC. As sap pH was stable between treatments and campaigns (see below), we averaged sap pH to estimate sap [CO 2 * ] during DEC. To ensure that any potential difference in stem respiratory fluxes was not caused by a bias on pH estimation during DEC, a sensitivity analysis was performed to assess the influence of sap pH on sap [CO 2 * ] and F T . For this, we recalculated sap [CO 2 * ], F T and R S using sap pH values varying from 4.5 to 7.4, the reported range for several species (compiled by Teskey et al, ) and according to the relatively acidic soil at the EucFACE (soil pH below 5.5; Hasegawa et al, ).…”

ElevatedCO2concentrations reduce C4cover and decrease diversity of understorey plant community in aEucalyptuswoodland

“…Data on soil N and P availability at the study site were obtained for the period from September 2012 to February 2016 (September 2012 to April 2014 previously published in Hasegawa et al. ()). A full description of the methods, using ion exchange membranes (IEMs), can be found in Hasegawa et al.…”

“…A full description of the methods, using ion exchange membranes (IEMs), can be found in Hasegawa et al. (); briefly, eight sets of IEMs, each comprising two anion and one cation membrane, were installed in the top 0–10 cm within each ring. Following 1–2 months of in situ incubation, nitrate (‐N) and ammonium (‐N) were extracted using 2 M KCl and phosphate (‐P) using 0.03 M NH4F.…”

“…Ring and Year were used as random factors. Because the effects of eCO 2 on soil nutrients were seasonal, we used data from between November and February, the period when earlier eCO 2 ‐associated increases in P availability were observed (Hasegawa et al., ). C 4 :C 3 ratios and soil N and P availability were log e ‐transformed prior to analysis.…”

Interactive effects of elevated CO2 and nitrogen deposition accelerate litter decomposition cycles of invasive tree (Triadica sebifera)

“…Litter decomposition could also be regulated by changes in soil environmental factors caused by elevated CO 2 and N addition (e.g., acidity, nutrient levels or microbial community) (Carney et al, 2007;Finn et al, 2015;Hasegawa et al, 2015;Liu et al, 2015). However, except for the positive effects of N addition on litter mass remaining via soil effects, no soil effects of elevated CO 2 or its interaction with N were observed in our study (Table 2; Fig.…”