[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,

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.

[1] Vegetation water stress plays an important role in the movement of water through the soil-plant-atmosphere continuum. However, the effects of water stress on evapotranspiration (ET) and other hydrological processes at the watershed scale remain poorly understood due in part to spatially and temporally heterogeneous conditions within the watershed, especially in areas of mountainous terrain. We used a spatially distributed model to understand and evaluate the relationship between water stress and ET in a forested mountain watershed during the snow-free growing season. Vegetation water stress increased as the growing season progressed, due to continued drying of soils, and persisted late into the growing season, even as vapor pressure deficit decreased with lower temperatures. As a result, ET became decoupled from vapor pressure deficit and became increasingly dependent on soil moisture later in the growing season, shifting from demand limitation to supply limitation. We found water stress and total growing season ET to be distributed nonuniformly across the watershed due to interactions between topography and vegetation. Areas having tall vegetation and low topographic index experienced the greatest water stress, yet they had some of the highest evapotranspiration rates in the watershed.

[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.

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.