Plant Available Nitrogen
Across diverse ecosystems worldwide, rhizospheres (composed of plant roots and their associated microbiome) are known to extend beyond soil horizons into weathered bedrock. These deep rhizospheres drive water and carbon cycling meters below the base of soil, supporting forest function. Little is known, however, about how these deep rhizospheres drive nutrient cycling at depth. In her dissertation research, Kelsey combines stable isotope and other biogeochemical techniques to explore how nitrogen (N), the most limiting nutrient to plant growth in terrestrial ecosystems, is cycled throughout deep rooting profiles in the critical zone, asking: (1) how do the chemical forms and concentrations of biologically available N change year round; (2) what is the source and fate of this N; and (3) to what extent do dominant tree species utilize N stored within weathered bedrock?
Kelsey and her colleagues have sampled for plant available nitrogen throughout the entire profile of the unsaturated zone at Rivendell using the VMS for more than two years, including sampling of : dissolved organic N (DON), ammonium (NH4+) and nitrate (NO3-). She has also conducted multi-year monitoring of NH4+ and NO3- standing stock, net mineralization in soil and bulk carbon and nitrogen stable isotopes in soil to generate a baseline of N and C content at our site. Together, these data form the basis for assessing biogeochemical N dynamics throughout the entire unsaturated (i.e. vadose) zone at Rivendell. Kelsey also routinely samples the dominant canopy species for foliar N and C content and stable isotope analysis.
Analyses indicate that the majority of N found in the rock moisture zone is DON, and it is found at ecologically significant concentrations. Total N increases with depth and is dynamic across seasons. The covariation of TN, NH4+, NO3-, and C dynamics also suggest N cycling by plants and/or microbes in the weathered bedrock rhizosphere at Rivendell. Additional analyses are underway to better understand the fate of N in the rock moisture zone, and additional gas sampling and N isotope analysis of dissolved N compounds sampled from the VMS will be performed to take a closer look at nitrogen cycling.
Additionally, to investigate plant uptake of N from rock moisture held in weathered bedrock, in summer 2020 Kelsey conducted an experiment testing N uptake capacity of Douglas fir fine roots growing in soil vs weathered rock. Fine roots from Douglas fir in the soil and saprolite were collected and incubated in 15N-labelled NH4+, NO3- and organic N solutions. Differences in d15N values between the control and 15N treatment roots will be used to calculate N uptake per gram of root. Results from this experiment will inform future research into differential gene expression in roots growing in soil versus weathered bedrock.
In Mediterranean climates, the season of water availability (winter) is out of phase with the season of light availability and atmospheric moisture demand (summer). Percy and her colleagues studied seasonality of tree transpiration in Rivendell, the small (4000 m2), forested, steep (32o) watershed that is the most intensively instrumented site in Angelo. They analyzed 3 years of half-hourly measurements from 39 sap flow sensors in 26 trees, six depth profiles of soil moisture measured by total domain refractometry, and collected micro-meteorological data from five sites. Sap flow measurements showed different seasons of peak transpiration for different tree species that were within several meters of each other. Douglas firs (Pseudotsuga menziesii), were active in the wet Mediterranean winter, with peak transpiration in the spring, followed by a sharp decline in transpiration during the summer dry season. in contrast, Pacific madrones (Arbutus menziesii), and to a lesser extent other broadleaf evergreen species (interior live oaks Quercus wislizeni, tannoaks Notholithocarpus densiflorus, bay Umbellularia californica), transpired maximally in the summer dry season. The difference in transpiration seasonality arises from different sensitivities to atmospheric evaporative demand and root-zone moisture. The greater sensitivity of Douglas fir to water stress appears to suppress their dry season evapotranspiration at the regional scale. Percy Link completed her Ph.D. in Earth and Planetary Science at Berkeley and is now a Software Engineer at Tesla.
Vegetation-induced changes in the stable isotope composition of near surface humidity in a coastal Californian watershed
At the Rivendell watershed on the Angelo Reserve we’ve used the oxygen and hydrogen stable isotope composition of meteoric waters as well as the derived d-excess parameter from these data to reconstruct changes in atmospheric water pools (e,g. sources, origins and overall balance) and the climatic conditions that prevail during surface evaporation. The d-excess parameter in particular was valuable in helping us evaluate the influence of forest canopies on atmospheric humidity within the mixed evergreen forest that inhabits the site. We found that during the day, when tree transpiration was at a maximum, the d-excess of atmospheric water vapor suggested the predominance of transpired water (from the trees) within the background atmosphere over this ecosystem while at night when transpiration was minor, the d-excess of atmospheric water vapor suggest the predominance of an ocean derived water vapor source. Observed diurnal fluctuations around the d-excess of background water vapor provided strong evidence that during the day as the land surface warms and the boundary layer above the ecosystem grows, the plants alter the isotope composition of atmospheric humidity but in a non-steady state, non- equilibrium manner. In contrast, at night equilibrium among the water pools in the ecosystem dominate as the atmosphere stabilizes. These daytime and nighttime fluctuations around the d-excess of ocean derived water vapor highlight the importance of plant transpiration for the isotope hydrology of near surface humidity and subsequently for the isotope composition of condensate like dew, an important water input to this ecosystem. (see Simonin et al., Ecohydrology, 2013, DOI: 10.1002/eco.1420).