Net photosynthesis is the largest single flux in the global carbon cycle, but controls over its variability are poorly understood because there is no direct way of measuring it at the ecosystem scale. We report observations of ecosystem carbonyl sulfide (COS) and CO 2 fluxes that resolve key gaps in an emerging framework for using concurrent COS and CO 2 measurements to quantify terrestrial gross primary productivity. At a wheat field in Oklahoma we found that in the peak growing season the flux-weighted leaf relative uptake of COS and CO 2 during photosynthesis was 1.3, at the lower end of values from laboratory studies, and varied systematically with light. Due to nocturnal stomatal conductance, COS uptake by vegetation continued at night, contributing a large fraction (29%) of daily net ecosystem COS fluxes. In comparison, the contribution of soil fluxes was small (1-6%) during the peak growing season. Upland soils are usually considered sinks of COS. In contrast, the well-aerated soil at the site switched from COS uptake to emissions at a soil temperature of around 15°C. We observed COS production from the roots of wheat and other species and COS uptake by root-free soil up to a soil temperature of around 25°C. Our dataset demonstrates that vegetation uptake is the dominant ecosystem COS flux in the peak growing season, providing support of COS as an independent tracer of terrestrial photosynthesis. However, the observation that ecosystems may become a COS source at high temperature needs to be considered in global modeling studies.carbonic anhydrase | LRU | ERU | flux partitioning | soil metabolism C arbonyl sulfide (COS) is an atmospheric trace gas that holds great promise for studies of carbon cycle processes at regional to continental scales (1, 2). The drawdown of atmospheric CO 2 over the continents reflects the difference between terrestrial photosynthesis and respiration fluxes that are both substantially larger than the net CO 2 fluxes. This limits our ability to obtain information on gross fluxes from measurements of atmospheric CO 2 alone. On the other hand, the drawdown of atmospheric COS over the continents is thought to largely reflect photosynthetic fluxes (1,(3)(4)(5)(6). At the global scale, the largest source of COS is the ocean, and uptake by leaves and soil are its largest sinks at 62% and 30% of the total sink, respectively (1).The uptake of COS in leaves is due to hydrolysis catalyzed by the enzyme carbonic anhydrase (CA), resulting in production of H 2 S and CO 2 (7). During leaf uptake, COS and CO 2 share the same diffusional pathway. The resulting close coupling of vegetation COS and CO 2 fluxes during photosynthesis (8-10) makes COS a promising tracer for gross carbon uptake where concurrent respiration precludes direct measurements of photosynthesis. For example, eddy covariance (EC) measurements of COS and CO 2 can be used to obtain independent estimates of gross primary productivity (GPP) at the ecosystem scale.COS-based estimates of GPP are derived from ecosystem COS fluxes and ...

openAccessArticle: FalsePage Range: 581-581doi: 10.1016/j.rse.2012.06.004Harvest Date: 2016-01-12 15:13:54issueName:cover date: 2012-09-01pubType

Grasslands and agroecosystems occupy one-third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of CO₂ exchange measurements to quantify gross primary productivity, respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration with the use of the light-response method (Gilmanov, T. G., D. A. Johnson, and N. Z. Saliendra. 2003. Growing season CO₂ fluxes in a sagebrushsteppe ecosystem in Idaho: Bowen ratio/energy balance measurements and modeling. Basic and Applied Ecology 4:167-183) from the RANGEFLUX and WORLDGRASSAGRIFLUX data sets supplemented by 46 sites from the FLUXNET La Thuile data set partitioned with the use of the temperature-response method (Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, M. Falk, T. Gilmanov, A. Granier, T. Grunwald, K. Havrankova, D. Janous, A. Knohl, T. Laurela, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J.M. Ourcival, D. Perrin, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. Seufert, F. Vaccari, T. Vesala, and D. Yakir. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11: 1.424-1439). Maximum values of the quantum yield (alpha = 75 mmol.mol(-1)), photosynthetic capacity (A(max) = 3.4 mg CO₂ . m(-2).s-1), gross photosynthesis (P-g,P-max = 1.16 g CO₂ . m(-2).d(-1)), and ecological light-use efficiency (epsilon(ecol) = 59 mmol . mol(-1)) of managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of nonforest ecosystems for uptake of atmospheric CO₂. Maximum values of gross primary production (8 600 g CO₂ . m(-2).yr(-1)), total ecosystem respiration (7 900 g CO₂ . m(-2).yr(-1)), and net CO₂ exchange (2 400 g CO₂ . m(-2).yr(-1)) were observed for intensively managed grasslands and high-yield crops, and are comparable to or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the nonforest sites were apparent sinks for atmospheric CO₂, with mean net uptake of 700 g CO₂ . m(-2).yr(-1) for intensive grasslands and 933 g CO₂ . m(-2).d(-1) for croplands. However, part of these apparent sinks is accumulated in crops and forage, which are carbon pools that are harvested, transported, and decomposed off site. Therefore, although agricultural fields may be predominantly sinks for atmospheric CO₂, this does not imply that they are necessarily increasing their carbon stock

The eddy covariance technique was employed with a tunable diode laser spectrometer to quantify methane flux from a prairie marsh dominated by Phragmites australis in north‐central Nebraska, USA. The observations spanned the entire growing season (April to October) and a wide range of weather conditions, allowing a quantitative assessment of the physical and biological controls of methane emission in this ecosystem. Diel patterns in methane emission varied markedly depending on plant growth stage. Prior to plant emergence above water, the rate of methane emission from the marsh was fairly constant throughout the day. After emergence above water, there was a gradual increase in methane emission after sunrise with a peak in late afternoon. Significant changes in diel patterns were observed after tillering. Then, the diel pattern was characterized by a mid‐ to late‐morning peak and a 2‐to 4‐fold increase in methane emissions from night to daytime. In early stages of plant growth, molecular diffusion through dead/live plants and the standing water column seemed to be the primary transport mechanism. After tillering, a transition occurred in the transport mechanism from a molecular diffusion to a convective throughflow, which is a rapid and active gas transport driven by pressure differences. The role of convective throughflow became less important as the plants senesced. Integrated methane emission over the six‐month measurement period (April–October) was about 64 g CH4 m–2. On an annual basis, we estimate the annual methane emission from this ecosystem to be ≈ 80 g CH4 m–2 and that about 80% of the total methane emission occurred between late April and late October.

1Climate, vegetation cover, and management create fine-scale heterogeneity in unirrigated 2 agricultural regions, with important but not well-quantified consequences for spatial and 3 temporal variations in surface CO 2 , water, and heat fluxes. We measured eddy covariance fluxes 4 in seven agricultural fields-comprising winter wheat, pasture, and sorghum -in the U.S. 5Southern Great Plains (SGP) during the 2001-2003 growing seasons. Land-cover was the 6 dominant source of variation in surface fluxes, with 50-100% differences between fields planted 7 in winter-spring versus fields planted in summer. Interannual variation was driven mainly by 8 precipitation, which varied more than two-fold between years. Peak aboveground biomass and 9 growing-season net ecosystem exchange (NEE) of CO 2 increased in rough proportion to 10 precipitation. Based on a partitioning of gross fluxes with a regression model, ecosystem 11 respiration increased linearly with gross primary production, but with an offset that increased 12 near the time of seed production. Because the regression model was designed for well-watered 13 periods, it successfully retrieved NEE and ecosystem parameters during the peak growing 14 season, and identified periods of moisture limitation during the summer. In summary, the effects 15 of crop type, land management, and water limitation on carbon, water, and energy fluxes were 16 large. Capturing the controlling factors in landscape scale models will be necessary to estimate 17 the ecological feedbacks to climate and other environmental impacts associated with changing 18 human needs for agricultural production of food, fiber, and energy.