Soil resource availability largely controls ecosystem responses to elevated CO2 (eCO2), determining the ability of ecosystems to function as a C sink under climate change scenarios. In this context, we aimed at investigating how, under greenhouse conditions, water and phosphorous interact with eCO2 to determine changes in (1) plant biomass allocation and (2) ecosystem C fluxes. To do this, we grew a plant community using native soil containing the native seed bank. This soil was collected from a P-limited south-eastern Australian woodland. The experiment consisted in a factorial combination of two levels of P fertilization (0 or 30 kg P ha-1), two levels of water supply (50% and 100% of the soil water holding capacity), and two levels of CO2 (400 ppm and 570 ppm). CO2 flux measurements were consistently carried out every 2–4 weeks between 9 am to 12 pm. We used an open-path LI-7500 infrared gas analyser (LI-COR) for measuring [CO2]; net ecosystem exchange (NEE) was measured for 2 minutes. After this, an opaque cover was placed over the chamber to block light and eliminate photosynthesis (GPP) to measure respiration for another 2 minutes.
Results/Conclusions
Phosphorous addition stimulated total plant biomass, both above and belowground and, hence, net C uptake. Elevated CO2 significantly increased aboveground biomass and decreased the root:shoot ratio. Moreover, eCO2 resulted in higher net C uptake and this response was related to reduced respiration rates rather than to stimulation of photosynthesis. The decrease of respiration rates under eCO2 could be attributed to an inhibitory effect on plant respiration, which was confirmed after standardizing respiration per unit of plant biomass. This response could also be attributed to the lower biomass allocated to roots under eCO2, which could lead to a reduced interaction between plants and soil microbes. Plants allocated more biomass belowground under low water conditions, but watering treatments did not affect CO2 fluxes. Finally, a higher availability of the three resources manipulated (CO2, P and water) resulted in the highest net C uptake, suggesting that, under non limiting resource conditions, the ability of our study system to act as a C sink is strengthened. Taken together, these responses indicate a potential for Australian woodlands to function as a C sinks under eCO2, even under low soil resource availability. Further analysis will investigate where the extra C was allocated.