Ecohydrologic separation of water between trees and streams in a Mediterranean climate

Water movement in upland humid watersheds from the soil surface to the stream is often described using the concept of translatory flow 1,2 , which assumes that water entering the soil as precipitation displaces the water that was present previously, pushing it deeper into the soil and eventually into the stream 2 . Within this framework, water at any soil depth is well mixed and plants extract the same water that eventually enters the stream. Here we present water-isotope data from various pools throughout a small watershed in the Cascade Mountains, Oregon, USA. Our data imply that a pool of tightly bound water that is retained in the soil and used by trees does not participate in translatory flow, mix with mobile water or enter the stream. Instead, water from initial rainfall events after rainless summers is locked into small pores with low matric potential until transpiration empties these pores during following dry summers. Winter rainfall does not displace this tightly bound water. As transpiration and stormflow are out of phase in the Mediterranean climate of our study site, two separate sets of water bodies with different isotopic characteristics exist in trees and streams. We conclude that complete mixing of water within the soil cannot be assumed for similar hydroclimatic regimes as has been done in the past 3,4 . Links between plant water-use (transpiration) and hydrology have been examined quantitatively since the paired-watershed studies in 1921 (ref. 5). These watershed-scale experiments clearly demonstrated links between vegetation and streamflow. However, the paired-watershed approach can only infer the mechanisms behind these vegetation-streamflow interaction

Seasonality of nitrogen balances in a Mediterranean climate watershed, Oregon, US

We constructed a seasonal nitrogen (N) budget for the year 2008 in the Calapooia River Watershed (CRW), an agriculturally dominated tributary of the Willamette River (Oregon, U.S.) under Mediterranean climate. Synthetic fertilizer application to agricultural land (dominated by grass seed crops) was the source of 90% of total N input to the CRW. Over 70% of the stream N export occurred during the wet winter, the primary time of fertilization and precipitation, and the lowest export occurred in the dry summer. Averaging across all 58 tributary subwatersheds, 19% of annual N inputs were exported by streams, and 41% by crop harvest. Regression analysis of seasonal stream export showed that winter fertilization was associated with 60% of the spatial variation in winter stream export, and this fertilizer continued to affect N export in later seasons. Annual N inputs were highly correlated with crop harvest N (r2 = 0.98), however, seasonal dynamics in N inputs and losses produced relatively low overall nitrogen use efficiency (41%), suggesting that hydrologic factors may constrain improvements in nutrient management. The peak stream N export during fall and early winter creates challenges to reducing N losses to groundwater and surface waters. Construction of a seasonal N budget illustrated that the period of greatest N loss is disconnected from the period of greatest crop N uptake. Management practices that serve to reduce the N remaining in the system at the end of the growing season and prior to the fall and winter rains should be explored to reduce stream N expor

Seasonal and Elevational Variation of Surface Water δ¹⁸O and δ²H in the Willamette River Basin

Climate change is expected to dramatically alter the timing and quantity of water within the nation’s river systems. These changes are driven by variation in the form, location and amount of precipitation that will affect the temporal and spatial distribution of river source water over time. To manage the impact of climate change, we will need to understand how water sources for rivers are shifting over time. Yet methods for knowing where river water comes from within the drainage basin at various times of the year are not well developed. Because stable isotopes of precipitation vary geographically, variation in the stable isotopes of river water can indicate source water dynamics. We monitored the stable isotopes (18O and 2H) of river and stream water within the southern Willamette River basin in Western Oregon over two years. We sampled sites along the Willamette River, and up six major river tributaries to the Willamette, and eight small catchments along each tributary that spanned the elevation range in the tributary. All sites were sampled four times a year, with a selected set of sites being sampled eight times a year. Seventy-five percent of the isotopic variation in stream water from the small catchments could be explained by the mean elevation of the catchment. A decrease in catchment water isotope values with increasing elevation is caused by Raleigh distillation of precipitation where heavy isotopes fall first, and rain is progressively lighter isotopically as storms move eastward from the Coast Range, across the Willamette Valley and up the Cascade Mountains. Coast Range catchments did not have a clear elevation pattern in the water isotopes. Water within the lower Willamette River showed distinct isotopic seasonal patterns. Isotopic values were at their lowest during summer low flow and at their highest during Feb/March when snow was accumulating in the mountains. This seasonal variation likely comes from a change in source elevation for water in the river. During winter when rain occurs in the valley and snow is accumulating in the mountains, the river isotopic signal reflects the valley bottom rain sources. During the dry Mediterranean summer, valley soils are dry and the water comes from snow melt and high elevation spring water. Using our relationship between catchment elevation and water isotope values, we estimated that the mean elevation of the source water shifted upward approximately 350 m during the summer low flow period. Reliance on high-elevation snowmelt water during summer low flow highlight the vulnerability of this system to influences of climate change, where snowpacks in the Cascade Mountains are predicted to decrease in the coming yearsPresented at The Oregon Water Conference, May 24-25, 2011, Corvallis, OR

Prolonged suppression of ecosystem carbon dioxide uptake after an anomalously warm year

Terrestrial ecosystems control carbon dioxide fluxes to and from the atmosphere, through photosynthesis and respiration, a balance between net primary productivity and heterotrophic respiration, that determines whether an ecosystem is sequestering carbon or releasing it to the atmosphere. Global and site-specific data sets have demonstrated that climate and climate variability influence biogeochemical processes that determine net ecosystem carbon dioxide exchange (NEE) at multiple timescales. Experimental data necessary to quantify impacts of a single climate variable, such as temperature anomalies, on NEE and carbon sequestration of ecosystems at interannual timescales have been lacking. This derives from an inability of field studies to avoid the confounding effects of natural intra-annual and interannual variability in temperature and precipitation. Here we present results from a four-year study using replicate 12,000-kg intact tallgrass prairie monoliths located in four 184-m3 enclosed lysimeters. We exposed 6 of 12 monoliths to an anomalously warm year in the second year of the study and continuously quantified rates of ecosystem processes, including NEE. We find that warming decreases NEE in both the extreme year and the following year by inducing drought that suppresses net primary productivity in the extreme year and by stimulating heterotrophic respiration of soil biota in the subsequent year. Our data indicate that two years are required for NEE in the previously warmed experimental ecosystems to recover to levels measured in the control ecosystems. This time lag caused net ecosystem carbon sequestration in previously warmed ecosystems to be decreased threefold over the study period, compared with control ecosystems. Our findings suggest that more frequent anomalously warm years, a possible consequence of increasing anthropogenic carbon dioxide levels, may lead to a sustained decrease in carbon dioxide uptake by terrestrial ecosystems

Type I IFNs and CD8 T cells increase intestinal barrier permeability after chronic viral infection

Intestinal barrier leakage constitutes a potential therapeutic target for many inflammatory diseases and represents a disease progression marker during chronic viral infections. However, the causes of altered gut barrier remain mostly unknown. Using murine infection with lymphocytic choriomeningitis virus, we demonstrate that, in contrast to an acute viral strain, a persistent viral isolate leads to long-term viral replication in hematopoietic and mesenchymal cells, but not epithelial cells (IECs), in the intestine. Viral persistence drove sustained intestinal epithelial barrier leakage, which was characterized by increased paracellular flux of small molecules and was associated with enhanced colitis susceptibility. Type I IFN signaling caused tight junction dysregulation in IECs, promoted gut microbiome shifts and enhanced intestinal CD8 T cell responses. Notably, both type I IFN receptor blockade and CD8 T cell depletion prevented infection-induced barrier leakage. Our study demonstrates that infection with a virus that persistently replicates in the intestinal mucosa increases epithelial barrier permeability and reveals type I IFNs and CD8 T cells as causative factors of intestinal leakage during chronic infections

The CRAPome: A contaminant repository for affinity purification-mass spectrometry data

10.1038/nmeth.2557Nature Methods108730-73