Hydraulic redistribution affects modeled carbon cycling via soil microbial activity and suppressed fire

Authors: FU, C. - University Of Connecticut; WANG, G. - University Of Connecticut; BIBLE, K. - University Of Washington; GOULDEN, M.L. - University Of California; SALESKA, S.R. - University Of Arizona; Scott, Russell - Russ; WOFSY, S.C. - Harvard University; CARDON, Z.G. - Marine Biology Laboratory

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2018
Publication Date: 7/3/2018
Citation: Fu, C., Wang, G., Bible, K., Goulden, M., Saleska, S., Scott, R.L., Wofsy, S., Cardon, Z. 2018. Hydraulic redistribution affects modeled carbon cycling via soil microbial activity and suppressed fire. Global Change Biology. 24:3472-3485. https://doi.org/10.1111/gcb.14164.
DOI: https://doi.org/10.1111/gcb.14164

Interpretive Summary: Plant roots are active conduits for moving moisture upwards and downwards in the soil from wetter regions of the soil to drier ones. While many field studies have shown that this process has important hydrological and ecological consequences on the functioning of ecosystems, the current generation of Earth System models does not include this process of water redistribution by plants. This study examined the consequences of this water redistribution on ecosystem carbon and nutrient cycling across four sites from Washington State to the Amazon. Though modeling predicted that upward water redistribution stimulated total photosynthesis by 10-50% during the dry seasons, the extent of that stimulation was curtailed by nutrient limitation. Modeling further indicated water redistribution can increase fuel availability, but it can also moisten the root zone and upper soil layers and diminish the spread of fire. Fire is a major driver of ecosystem structure and function, and climate warming is expected to increase soil drought in the future. Our results indicate that under future drier conditions, water redistribution by plants has the potential to influence biogeochemical cycling, both through the interplay of plant and microbial processes, and, more dramatically, through altered fire regimes.

Technical Abstract: Hydraulic redistribution (HR) of water from moist to dry soils, through plant roots, occurs world wide in seasonally dry ecosystems. Addition of a representation of HR to the Community Land Model CLM4.5 was recently shown to improve the fit of modeled to measured evapotranspiration, Bowen ratio, and soil moisture profiles in seasonally-dry ecosystems from the Pacific Northwest to the Sonoran Desert. HR has also long been suspected of affecting carbon and nutrient cycling, leading to carbon gain or loss to the atmosphere depending on the balance of whole system production and respiration. Here, we examined that balance using combined measurement and modeling at four seasonally-dry Ameriflux sites from Washington state to the Amazon. Though modeling predicted that upward HR stimulated gross photosynthesis by 10-50% during dry seasons, the extent of that stimulation was curtailed by nutrient limitation, which occurred even though decomposition was also stimulated 7-20% by HR. Modeling further indicated that though HR increased modeled fuel availability at all four sites, upward HR also moistened the root zone and upper soil layers, diminishing the modeled spread of fire and associated large carbon fluxes to the atmosphere. Fire is a major driver of ecosystem structure and function, and climate warming is expected to increase soil drought in the future. Our results indicate that under future drier conditions, HR has the potential to influence biogeochemical cycling and biosphere-atmosphere carbon exchange strongly, both through the interplay of plant and microbial processes, and, more dramatically, through altered fire regimes.