The Pacific Northwest is projected to experience up to 5.6°C of warming by 2100 and shift to less frequent but more intense rainfall. It is unknown what the biogeochemical response will be. Washington experienced record October rainfall and warm fall temperatures in 2016 that offer insight into what can be expected in future years. Through establishing a mini-ecological observatory, we were positioned to comprehensively monitor local biogeochemical response and address this gap. Water, dissolved organic carbon (DOC), and dissolved nitrogen (DN) responses were targeted because of their importance within ecosystems and direct linkages to rainfall. We specifically sought to answer questions of how an ecosystem responds to extreme precipitation in field and conifer- and deciduous-dominated sites over autumn seasonal transition. We characterized our site by climate, soil, geology, and vegetation. Air temperature, wind, rain volume, and soil temperature and moisture were measured continuously. Soil lysimeters were deployed and precipitation, throughfall, soil water, stream water, and groundwater samples were collected weekly. Lysimeter data were composited for analysis and analyzed for DOC and DN. Stream and groundwater 2H and 18O ratios were measured. Soil CO2 flux was recorded weekly. Litterfall was collected weekly for dry mass and carbon and nitrogen analysis.
2016 marked the wettest October on record and an abnormally warm fall in western Washington. Relative to a 20-year average, there was 13cm additional October rainfall and 1.8°C warmer temperatures during the study. Precipitation was driven by storm intensity rather than frequency. Leaf-off, soil moisture, DOC/DN fluxes, streamflow and groundwater chemistry were impacted by these anomalies. Throughfall increased 15% in deciduous soil after leaf-off, with the upper 0-5cm holding 47% more water than coniferous soil. Soils saturated following an extreme rainstorm. δ2H and δ18O measurements indicated stream composition was 85% groundwater and the largest October rainstorm had abnormally high rain out. Forest standing biomasses were 285-289 Mg carbon ha-1, while fields held less than 2% that. DOC enriched in throughfall and organic soil before depleting in mineral soil. DOC correlated with precipitation. The field site showed 0.1-0.6 ppm sec-1 greater soil CO2 evasion than forest soils despite the smaller carbon pool. DN in precipitation was depleted in the canopy. DN enriched in organic soil following the large October storm but showed less correlation with mild precipitation. Leaf-off occurred rapidly following the large October storm. The data suggest extreme rainfall couples with leaf-off and soil saturation and increases DOC/DN production.