[1] Recent years have seen a growing interest in measuring and modeling soil CO 2 efflux, as this flux represents a large component of ecosystem respiration and is a key determinant of ecosystem carbon balance. Process-based models of soil CO 2 production and efflux, commonly based on soil temperature, are limited by nonlinearities such as the observed diurnal hysteresis between soil CO 2 concentration ([CO 2 ]) and temperature. Here we quantify the degree to which hysteresis between soil [CO 2 ] and soil temperature is controlled by soil water content in a montane conifer forest, and how this nonlinearity impacts estimates of soil CO 2 efflux. A representative model that does not consider hysteresis overestimated soil CO 2 efflux for the entire growing season by 19%. At high levels of soil water content, hysteresis imposes organized, daily variability in the relationship between soil [CO 2 ] and soil temperature, and at low levels of soil water content, hysteresis is minimized. Citation: Riveros-Iregui,

Excess nitrogen (N) impairs inland water quality and creates hypoxia in coastal ecosystems. Agriculture is the primary source of N; agricultural management and hydrology together control aquatic ecosystem N loading. Future N loading will be determined by how agriculture and hydrology intersect with climate change, yet the interactions between changing climate and water quality remain poorly understood. Here, we show that changing precipitation patterns, resulting from climate change, interact with agricultural land use to deteriorate water quality. We focus on the 2012-2013 Midwestern U.S. drought as a ''natural experiment''. The transition from drought conditions in 2012 to a wet spring in 2013 was abrupt; the media dubbed this ''weather whiplash''. We use recent (2010)(2011)(2012)(2013)(2014)(2015) and historical data to connect weather whiplash (drought-to-flood transitions) to increases in riverine N loads and concentrations. The drought likely created highly N-enriched soils; this excess N mobilized during heavy spring rains (2013), resulting in a 34% increase (10.5 vs. 7.8 mg N L -1 ) in the flow-weighted mean annual Biogeochemistry (2017) 133:7-15 DOI 10.1007 nitrate concentration compared to recent years. Furthermore, we show that climate change will likely intensify weather whiplash. Increased weather whiplash will, in part, increase the frequency of riverine N exceeding E.P.A. drinking water standards. Thus, our observations suggest increased climatic variation will amplify negative trends in water quality in a region already grappling with severe impairments.

[1] We investigated the spatial and temporal variability of soil CO 2 efflux across 62 sites of a 393-ha complex watershed of the northern Rocky Mountains. Growing season (83 day) cumulative soil CO 2 efflux varied from $300 to $2000 g CO 2 m À2 , depending upon landscape position, with a median of 879.8 g CO 2 m À2 . Our findings revealed that highest soil CO 2 efflux rates were observed in areas with persistently high soil moisture (riparian meadows), whereas lower soil CO 2 efflux rates were observed on forested uplands (98% of watershed area). Furthermore, upslope accumulated area (UAA), a surrogate measure of the lateral redistribution of soil water, was positively correlated with seasonal soil CO 2 efflux at all upland sites, increasing in explanatory power when sites were separated by the major aspects of the watershed (SE/NW). We used the UAA-soil CO 2 efflux relationship to upscale measured CO 2 efflux to the entire watershed and found watershed-scale soil CO 2 efflux of 799.45 ± 151.1 g CO 2 m À2 over 83 days. These estimates compared well with independent eddy covariance estimates of nighttime ecosystem respiration measured over the forest. We applied this empirical model to three synthetic watersheds with progressively reduced complexity and found that seasonal estimates of soil CO 2 efflux increased by 50, 58, and 98%, demonstrating the importance of landscape structure in controlling CO 2 efflux magnitude. Our study represents an empirical quantification of seasonal watershed-scale soil CO 2 efflux and demonstrates that UAA (i.e., landscape position) and drainage patterns are important controls on the spatial organization of large-scale ($km 2 ) soil CO 2 efflux, particularly in semiarid, subalpine ecosystems.

The spatial and temporal controls on soil CO 2 production and surface CO 2 efflux have been identified as outstanding gaps in our understanding of carbon cycling. We investigated both across two riparian-hillslope transitions in a subalpine catchment, northern Rocky Mountains, Montana. Riparian-hillslope transitions provide ideal locations for investigating the controls on soil CO 2 dynamics due to strong, natural gradients in the factors driving respiration, including soil water content (SWC) and soil temperature. We measured soil air CO 2 concentrations (20 and 50 cm), surface CO 2 efflux, soil temperature, and SWC at eight locations. We investigated (1) how soil CO 2 concentrations differed within and between landscape positions; (2) how the timing of peak soil CO 2 concentrations varied across riparian and hillslope zones; and (3) whether higher soil CO 2 concentrations necessarily resulted in higher efflux (i.e. did surface CO 2 efflux follow patterns of subsurface CO 2 )? Soil CO 2 concentrations were significantly higher in the riparian zones, likely due to higher SWC. The timing of peak soil CO 2 concentrations also differed between riparian and hillslope zones, with highest hillslope concentrations near peak snowmelt and highest riparian concentrations during the late summer and early fall. Surface CO 2 efflux was relatively homogeneous at monthly timescales as a result of different combinations of soil CO 2 production and transport, which led to equifinality in efflux across the transects. However, efflux was 57% higher in the riparian zones when integrated to cumulative growing season efflux, and suggests higher riparian soil CO 2 production.