[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,

[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.

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.

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.

Summary• Understanding controls over plant-atmosphere CO 2 exchange is important for quantifying carbon budgets across a range of spatial and temporal scales. In this study, we used a simple approach to estimate whole-tree CO 2 assimilation rate (A Tree ) in a subalpine forest ecosystem.• We analysed the carbon isotope ratio (d 13 C) of extracted needle sugars and combined it with the daytime leaf-to-air vapor pressure deficit to estimate tree water-use efficiency (WUE). The estimated WUE was then combined with observations of tree transpiration rate (E) using sap flow techniques to estimate A Tree . Estimates of A Tree for the three dominant tree species in the forest were combined with species distribution and tree size to estimate and gross primary productivity (GPP) using an ecosystem process model.• A sensitivity analysis showed that estimates of A Tree were more sensitive to dynamics in E than d 13 C. At the ecosystem scale, the abundance of lodgepole pine trees influenced seasonal dynamics in GPP considerably more than Engelmann spruce and subalpine fir because of its greater sensitivity of E to seasonal climate variation.• The results provide the framework for a nondestructive method for estimating whole-tree carbon assimilation rate and ecosystem GPP over daily-to weekly time scales.

[1] Soil CO 2 efflux is a large respiratory flux from terrestrial ecosystems and a critical component of the global carbon (C) cycle. Lack of process understanding of the spatiotemporal controls on soil CO 2 efflux limits our ability to extrapolate from fluxes measured at point scales to scales useful for corroboration with other ecosystem level measures of C exchange. Additional complications are introduced by the effects of soil water content seasonality and rainfall on the performance of measurement techniques. In this paper we present measurements of soil CO 2 efflux made at two contrasting sites within a characteristic subalpine forest of the northern Rocky Mountains. Comparison of measurements between the soil respiration chamber technique and the soil CO 2 profile technique over daily and seasonal time scales indicated that soil water content plays a major role in the magnitude and seasonality of soil CO 2 efflux, especially after snowmelt or following summer rainfall. Agreement between both techniques was limited during high soil water content conditions and after summer rainfall. Differences in diel hysteresis patterns of soil CO 2 efflux between sites were controlled by the effects of canopy cover and temporal differences in photosynthetic activity of vegetation. Our results indicate that an accurate parameterization of soil water content heterogeneity in space and time must be a critical component of realistic model representations of soil CO 2 efflux from heterogeneous landscapes.

Carbon dioxide (CO2) emissions to the atmosphere from running waters are estimated to be four times larger than the total carbon (C) flux to the oceans. However, these fluxes remain poorly constrained because of substantial temporal variability in dissolved CO2 concentrations. Using a global compilation of high frequency CO2 measurements, we demonstrate that nocturnal CO2 emissions are consistently larger, by an average of 27% (0.9 g C m -2 d -1 ), than those estimated from diurnal concentrations alone. Canopy shading is the principal control on observed diel (24 hr) variation, suggesting this nocturnal increase arises from daytime fixation of dissolved inorganic C by photosynthesis. Because contemporary global estimates of CO2 emissions to the atmosphere from running waters (0.65 -1.8 Pg C yr -1 ) rely primarily on discrete measurements of dissolved CO2 obtained during the day, they substantially underpredict the magnitude of this important flux. Accounting for night-time CO2 elevates global estimates of emissions from running waters to the atmosphere by 0.20-0.55 Pg C yr -1 .Carbon dioxide (CO2) emission from inland waters to the atmosphere is a major flux in the global carbon (C) cycle, and four-fold larger than the lateral C export to oceans 1 . Streams and rivers are hotspots for this flux, accounting for ~85% of inland water CO2 emissions despite covering <20% of the freshwater surface area 2 . Despite this importance, the magnitude of global CO2 emissions from streams and rivers remains highly uncertain with estimates revised upwards over the past decade from 0.6 to 3.48 Pg C yr -1 (3,4) . Changes to this estimate follow improvements in the spatial resolution for upscaling emissions 2,5 , as well as new studies from previously underrepresented areas such as the Congo 6 , Amazon 7 , and global mountains 8 . Further refinements have emerged from considering temporal variability in CO2 emission rates 9 . However, despite recent studies showing dramatic day-night changes in stream and river water CO2 concentrations 10-14 the significance of systematic sub-daily variation on overall CO2 emissions remains unexplored.Diurnal cycles in solar radiation impose a well-known periodicity on stream biogeochemical processes, creating diel (i.e., 24-hr period lengths) patterns for many solutes and gases, including nutrients, dissolved organic matter, and dissolved oxygen (O2) 15 . Indeed, diel variation in O2 arising from photosynthetic activity is the signal from which whole-system metabolic fluxes are estimated 16 . Photosynthetic production of O2 is stoichiometrically linked to the day-time assimilation of dissolved inorganic carbon (principally bicarbonate and dissolved CO2), lowering CO2 concentrations during the day. The resulting diel variation, with higher night-time CO2 concentrations when respiration reactions dominate, implies increased emissions at night. Despite the obvious connection between photosynthesis and CO2 consumption, the implications for total aquatic CO2 emissions has been neglected, most likely ...

Abstract:Variability in soil respiration at various spatial and temporal scales has been the focus of much research over the last decade aimed to improve our understanding and parameterization of physical and environmental controls on this flux. However, few studies have assessed the control of landscape position and groundwater table dynamics on the spatiotemporal variability of soil respiration. We investigated growing season soil respiration in a ¾393 ha subalpine watershed in Montana across eight riparian-hillslope transitions that differed in slope, upslope accumulated area (UAA), aspect, and groundwater table dynamics. We collected daily-to-weekly measurements of soil water content (SWC), soil temperature, soil CO 2 concentrations, surface CO 2 efflux, and groundwater table depth, as well as soil C and N concentrations at 32 locations from June to August 2005. Instantaneous soil surface CO 2 efflux was not significantly different within or among riparian and hillslope zones at monthly timescales. However, cumulative integration of CO 2 efflux during the 83-day growing season showed that efflux in the wetter riparian zones was ¾25% greater than in the adjacent drier hillslopes. Furthermore, greater cumulative growing season efflux occurred in areas with high UAA and gentle slopes, where groundwater tables were higher and more persistent. Our findings reveal the influence of landscape position and groundwater table dynamics on riparian versus hillslope soil CO 2 efflux and the importance of time integration for assessment of soil CO 2 dynamics, which is critical for landscape-scale simulation and modelling of soil CO 2 efflux in complex landscapes.