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[1] Northern peatlands are a diverse group of ecosystems varying along a continuum of hydrological, chemical, and vegetation gradients. These ecosystems contain about one third of the global soil carbon pool, but it is uncertain how carbon and water cycling processes and response to climate change differ among peatland types. This study examines midsummer CO 2 and H 2 O fluxes measured using the eddy covariance technique above seven northern peatlands including a low-shrub bog, two open poor fens, two wooded moderately rich fens, and two open extreme-rich fens. Gross ecosystem production and ecosystem respiration correlated positively with vegetation indices and with each other. Consequently, 24-hour net ecosystem CO 2 exchange was similar among most of the sites (an average net carbon sink of 1.5 ± 0.2 g C m À2 d À1 ) despite large differences in water table depth, water chemistry, and plant communities. Evapotranspiration was primarily radiatively driven at all sites but a decline in surface conductance with increasing water vapor deficit indicated physiological restrictions to transpiration, particularly at the peatlands with woody vegetation and less at the peatlands with 100% Sphagnum cover. Despite these differences, midday evapotranspiration ranged only from 0.21 to 0.34 mm h À1 owing to compensation among the factors controlling evapotranspiration. Water use efficiency varied among sites primarily as a result of differences in productivity and plant functional type. Although peatland classification includes a great variety of ecosystem characteristics, peatland type may not be an effective way to predict the magnitude and characteristics of midsummer CO 2 and water vapor exchanges.

a b s t r a c tWe analyzed net CO 2 exchange data from 13 flux tower sites with 27 site-years of measurements over maize and wheat fields across midcontinent North America. A numerically robust "light-soil temperature-VPD"-based method was used to partition the data into photosynthetic assimilation and ecosystem respiration components. Year-round ecosystem-scale ecophysiological parameters of apparent quantum yield, photosynthetic capacity, convexity of the light response, respiration rate parameters, ecological light-use efficiency, and the curvature of the VPD-response of photosynthesis for maize and wheat crops were numerically identified and interpolated/extrapolated. This allowed us to gap-fill CO 2 exchange components and calculate annual totals and budgets. VPD-limitation of photosynthesis was systematically observed in grain crops of the region (occurring from 20 to 120 days during the growing season, depending on site and year), determined by the VPD regime and the numerical value of the curvature parameter of the photosynthesis-VPD-response, VPD . In 78% of the 27 site-years of observations, annual gross photosynthesis in these crops significantly exceeded ecosystem respiration, resulting in a net ecosystem production of up to 2100 g CO 2 m −2 year −1 . The measurement-based photosynthesis, respiration, and net ecosystem production data, as well as the estimates of the ecophysiological parameters, provide an empirical basis for parameterization and validation of mechanistic models of grain crop production in this economically and ecologically important region of North America.

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In the northern Great Plains, most runoff transport of N, and P to surface waters has historically occurred with snowmelt. In recent years, significant rainfall runoff events have become more frequent and intense in the region. Here, we examine the influence of landscape characteristics on hydrology and nutrient export in nine tributary watersheds of the Assiniboine River in Manitoba, Canada, during snowmelt runoff and with an early summer extreme rainfall runoff event (ERRE). All watersheds included in the study have land use that is primarily agricultural, but with differing proportions of land remaining as wetlands, grassland, and that has been artificially drained. Those watersheds with greater capacity for storage of water in surface depressions (noneffective contributing areas) exhibited lower rates of runoff and nutrient export with snowmelt. During the ERRE, higher export of total P (TP), but not total N, was observed from those watersheds with larger amounts of contributing area that had been added through artificial surface drainage, and this was associated primarily with higher TP concentrations. Increasing or restoring the storage of water on the landscape is likely to reduce nutrient export; however, the importance of antecedent conditions was evident during the ERRE, when small surface depressions were at or near capacity from snowmelt. Total P concentrations observed during the summer ERRE were as high as those observed with snowmelt, and N/P ratios were significantly lower. If the frequency of summer ERREs increases with climate change, this is likely to result in negative water quality outcomes.

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Cold agricultural regions are important sites of global food production. This has contributed to widespread water quality degradation influenced by processes and hydrologic pathways that differ from warm region analogues. In cold regions, snowmelt is often a dominant period of nutrient loss. Freeze-thaw processes contribute to nutrient mobilization. Frozen ground can limit infiltration and interaction with soils, and minimal nutrient uptake during the nongrowing season may govern nutrient export from agricultural catchments. This paper reviews agronomic, biogeochemical, and hydrological characteristics of cold agricultural regions and synthesizes findings of 23 studies that are published in this special section, which provide new insights into nutrient cycling and hydrochemical processes, model developments, and the efficacy of different potentially beneficial management practices (BMPs) across varied cold regions. Growing evidence suggests the need to redefine optimum soil phosphorus levels and input regimes in cold regions to allow achievement of water quality targets while still supporting strong agricultural productivity. Practices should be considered through a regional and site-specific lens, due to potential interactions between climate, hydrology, vegetation, and soils, which influence the efficacy of nutrient, crop, water, and riparian buffer management. This leads to differing suitability of BMPs across varied cold agricultural regions. We propose a systematic approach ("CUPCAKE"), to achieve water quality objectives in variable and changing climates, which combines nutrient transport process Conceptualization, Understanding BMP functions, Predicting effects of variability and change, Consideration of producer input and agronomic and environmental tradeoffs, practice Adaptation, Knowledge mobilization, and Evaluation of water quality improvement.