Characterizing the Fluxes and Age Distribution of Soil Water, Plant Water, and Deep Percolation in a Model Tropical Ecosystem

Abstract

Recent field observations indicate that in many forest ecosystems, plants use water that may be isotopically distinct from soil water that ultimately contributes to streamflow. Such an assertion has been met with varied reactions. Of the outstanding questions, we examine whether ecohydrological separation of water between trees and streams results from a separation in time, or in space. Here we present results from a 9-month drought and rewetting experiment at the 26,700-m(3) mesocosm, Biosphere 2-Tropical Rainforest biome. We test the null hypothesis that transpiration and groundwater recharge water are sampled from the same soil volume without preference for old nor young water. After a 10-week drought, we added 66 mm of labeled rainfall with 152 parts per thousand delta H-2 distributed over four events, followed by background rainfall (-60 parts per thousand delta H-2) distributed over 13 events. Our results show that mean transit times through groundwater recharge and plant transpiration were markedly different: groundwater recharge was 2-7 times faster (similar to 9 days) than transpired water (range 17-62 days). The "age" of transpired water showed strong dependence on species and was linked to the difference between midday leaf water potential and soil matric potential. Moreover, our results show that trees used soil water (89% +/- 6) and not the "more mobile" (represented by "zero tension" seepage) water (11% +/- 6). The finding, which rejects our null hypothesis, is novel in that this partitioning is established based on soil water residence times. Our study quantifies mean transit times for transpiration and seepage flows under dynamic conditions. Plain Language Summary Recent studies suggest that plants use a type of water that is different to the water that recharges the ground, a phenomenon described as the two water worlds. It is unclear, however, whether these waters are segregated in space or in time. That is, do plants draw water from parts of the soil different to groundwater recharge, or do plant water withdrawals happen at a different time from groundwater recharge? Evidence from well-controlled experiments is badly needed because the two water worlds, if true, means that our understanding of the water cycle is incomplete. Here we perform a 9-month drought and rainfall experiment, taking fingerprints of the water molecule, to follow a raindrop from the moment it enters the ground through to its exit via plants or groundwater recharge. Results point to two main discoveries: (1) the travel time of water via root water uptake is much longer than the travel time of water leading to groundwater recharge and (2) the water taken by tree roots comes from parts of the soil that are different to the water leading to groundwater recharge. These discoveries show the segregation of these two components of the water cycle in space and in time.