Research

My work is broadly centered around understanding interactions between humans and the environment. Of particular interest to me are reciprocal relationships between human activities and ecosystem services. Despite their importance in sustaining our societies, these services exist mainly as externalities in many social and political settings. For my research, I have looked at how land-use/cover changes are affecting important ecosystem services in grassland ecosystems, such as carbon sequestration and groundwater recharge. In these systems, vegetation changes such as cultivation of crops or woody plant invasion can affect the large amounts of carbon stored in their soils or the limited renewable water they have. My goal is to quantify and value the changes to these services in order to help determine the optimal land-use changes under global environmental changes. For this, I have visited South Africa, Argentina and Central Great Plains in the US for field work. Some of my findings are summarized below.


Tradeoff between carbon and water

Plants fix carbon from the atmosphere, where it acts as a greenhouse gas, and store it in their biomass. Biomass eventually decomposes due to herbivory or death, and while most of carbon is released back into the atmosphere, some of it is stablized in the soil. Plants and soil make up the largest terrestrial storages and sinks for atmospheric carbon on earth. The process of fixing carbon (photosynthesis) requires plants to draw up water from the soil and transpire it into the atmosphere. Thus, there is an exchange of carbon and water between soil and the atmosphere through the plants (see diagram). For this reason, type of plants or vegetation can have large effects on the cycling of carbon and water.

For example, the grasslands I have been working in, and in other parts of the world, are experiencing prominent vegetation shifts. The most widespread is conversion of natural grassland to cultivated crops such as corn, soybean or wheat. This tends to result in less water used by the crops (more water available in the soil for groundwater recharge) compared to the grasslands but also reduced carbon stock (loss of grass biomass and soil carbon). Another common vegetation change in grasslands is planted or invading trees. Conversely to cultivation, this vegetation shift usually results in greater amount of water used by trees than by grasslands (less water for groundwater recharge) but increased carbon stock (increased biomass). We observe this tradeoff between carbon and water from land-use changes across our field sites in South Africa, Argentina, and US grasslands.


Effect of vegetation on groundwater recharge

In addition, I'm interested in how vegetation controls the amount and quality of the water moving through the soil down to groundwater table. Are there general differences between broad vegetation types such as croplands, grasslands,  and woodlands? And what kind of plant traits affect groundwater recharge most (leaves, roots, height, etc.)? To answer some of these questions, I conducted a literature review of recharge rates under different vegetation types globally as well as carrying out field campaigns for estimating recharge rates under paired land uses, such as crop cultivation, grasslands, and woodlands. In both cases, we see clear distinction between land uses or vegetation types in recharge. Croplands tend to have the highest recharge rates, grasslands have intermediate and woodlands have the lowest recharge. Land-use changes involving shifts between these vegetation types therefore have large potential to affect not only groundwater yield but also subterranean hydrology and groundwater quality.


Hydrological changes and biogeochemical cycling

Water not only acts as a medium for life on earth but also as a vessel for movement of energy and other materials. Many chemical cycles such as that of carbon, are coupled to the water cycle, with water acting as a reactant, product, or solvent. For example, increased water movement down to the ground water could result in leaching of solutes found in the soil column, which can degrade ground water and at the same time, represents loss of nutrients from the soil. What are some of overlooked solutes in the soil, and how are they affected by changes in recharge due to land-use changes?

Value of the ecosystem services

Ultimately, valuation of these changes in carbon storage and groundwater recharge rates with land-use changes is necessary to see whether these environmental changes are net cost of benefit to our society. These are two of the more marketable ecosystem services and also the most pertinent, given the pressing need to combat worsening effects of climate change and global water shortages.


Other projects

In addition, I've been working on a few collaborative projects.

With Alexia Stokes, Catherine Roumet, Ivan Prieto, Zhun Mao and others, I am examining the ecosystem services provided by roots in several ecological contexts. At our sites in France, Costa Rica and Laos, we are quantifying carbon sequestration, hydraulic lift, and soil fixation as provided by roots in different agroforestry and natural land uses. For more information about this project, please visit here.

With Catarina F. Moura, Maria B. Caldeira, and others, I am examining the effect of woody shrub encroachment into cork oak woodlands in Portugal. We are examining the ecosystem effects of this encroachment holistically, including changes in biodiversity, hydrology and biogeochemistry, all coupled to human land management and wildlife use (mainly deer and boar).

With Gervasio Piñeiro, Stefano Manzoni, and others, I am developing and testing models that better explain decay of carbon in plant and soil. This has important implications for models that forecast terrestrial carbon emissions and future atmospheric carbon concentrations, because currently popular models do not adequately model changes in decays rates over time.

With Cristina Armas and others, I designed a greenhouse study to show that hydraulic lift, a process by which plant roots passively move water from a wet area or soil to drier areas, can enhance uptake of nutrients for the plants. It seems that having deep roots that usually tap into zone of higher moisture below the roots can alleviate both water and nutrient limitations.

With Dushmantha Jayawickreme and others, I studied a chronosequence of croplands that were converted from woodlands in Argentina 7-90 years ago. We found evidences of higher soil water movement under croplands and gradual leaching of the salts and nutrients that have built up under the natural woodlands due to the limited soil water flux under these conditions--woodlands had effectively used up most of the precipitation, leaving negligible water to reach the water table (and to carry the salts out of the root zone). The influx of water + salts with crop cultivation could deteriorate the local groundwater quality but also for the hydrologically connected area to the east, which represent some of the largest and most productive croplands in the world. Salinization of these areas would be disastrous for the regional economy and its risk is something to be considered in face of large-scale deforestation occurring in our study area.