Belowground C allocation of tropical rainforests under drought: an ecosystem 13CO2 labeling experiment

Pulse labeling experiments remain an invaluable tool for tracing element allocation dynamics following environmental changes, ecological disturbance or extreme events. They are primarily used on the small scale, from single organisms to maximally the plot scale. Thus, upscaling of their outcomes is frequently challenging because of a lack of spatial representativeness and potentially missed interactions due to excluded ecosystem components. As we tackle a black box when studying belowground processes, the uncertainties of upscaling from small-scale labeling studies increase further. Therefore, we conducted a complete ecosystem pulse labeling drought stress experiment to explore the impact of extreme droughts on tropical rainforest’s belowground C allocation using the “ecosystem in a box model” of Biosphere 2 in Arizona. The atmosphere of the tropical forest was exposed to a 13CO2 pulse for several hours under ambient and drought conditions. Besides continuous monitoring (leaf, stem and soil 13CO2 respiration), we performed regular post-pulse soil sampling campaigns to trace ecosystem belowground C allocations and monitor C partitioning at the soil–microbe-root interface. We aimed to identify key drought-adaptation strategies such as i) increased C allocation into subsoil layers which were expected to have higher moisture than dried-out topsoils and ii) increased relative C investment into specific rhizodeposits and mycorrhizal fungi that both may foster plant nutrition even from dry soil. We observed a high allocation of assimilated 13C tracer into topsoil roots under drought, but this C allocation did not contribute to a higher root biomass. This suggests that tropical plants might modify their root composition by forming osmolytes or increasing lignin content to resist the high topsoil drought stress. The rhizodeposition (allocation of assimilated C into rhizospheres soil) increased mainly in the subsoil under drought, suggesting that trees aim to keep rhizo-microbial activity high in subsoils, where moisture was still available throughout the drought period. Whereas under ambient conditions the Gram-negative microorganisms - the most abundant rhizosphere inhabitants - profited most from the 13C allocated to the rhizosphere microbiome, we observed under drought conditions a high 13C allocation into the 18:2w6,9 biomarker representative for saprotroph and ectomycorrhizal fungi. This suggests trees invest C into their mycorrhizal partners most likely hoping for improved nutrient uptake via the small, drought-resistant fungi able to exploit not yet dried-out microhabitats in soils. Generally, we found pronounced plot- and thus plant-specific differences in belowground C allocation, suggesting species- or functional plant type specific drought response strategies belowground. In summary, quantification of ecosystem C belowground allocation patterns at the plant-microbe-soil interface enables us to disentangle distinct belowground drought response strategies of tropical rainforests. This is essential to support those ecosystem traits that increase tropical rainforest’s resistance and resilience to climate change.