Sources and sinks of carbonyl sulfide in an agricultural field in the Southern Great Plains

Abstract

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 ...

Seasonal fluxes of carbonyl sulfide in a midlatitude forest

“…OCS has been observed to be taken up by oxic soils, but the rate is notably slower on the ecosystem scale than OCS uptake by vegetation (30). Maseyk et al (8) attributed ∼29% of total OCS flux by winter wheat to nighttime OCS uptake by foliage, with only 1-6% due to soils, at the peak of the growing season (8). The results of these studies generally agree with our results during the growing season.…”

“…Using the mean LRU of 1.5 calculated for September, the total daily sum of GEP OCS and GEP CO2 agrees to within 3.5%, a good agreement given the ∼10% uncertainty estimated for GEP CO2 (23). However, this result depends on the value of LRU assumed (8,19): varying the LRU between 2 and 1 results in a 29% underestimation or a 36% overestimation, respectively (Fig. 4).…”

“…Leaf chamber studies reported LRU values of 1-4 over a large range of light conditions and tree species (15) or 1.3-2.3 (28) for a variety of tree species. A field study of wheat reported LRU values of 0.9-1.9 for various light conditions (8). We calculate a mean daytime f OCS :f GEP ratio for air temperatures above 14°C (i.e., times of full canopy) of 1.4 ± 0.3, within the range of the previous values.…”

“…In the absence of OCS emission from the ecosystem, the expected daytime net OCS flux due to hydrolysis by CA (based on June and August peak OCS ecosystem uptake) should have been around −30 pmol·m −2 ·s −1 , and hence, the observed net flux of +20 pmol·m −2 ·s −1 in late July could correspond to a maximum gross emission by the responsible mechanisms of 50 pmol·m −2 ·s −1 at midday. A recent study reported OCS emissions from temperaturestressed soils and senescent wheat at harvest time (8,20). The metabolism of sulfur containing amino acids, which increases with temperature and plant stress, may lead to OCS production (8) in a similar manner to CO (34) and CH 4 production (35) from thermal degradation.…”

“…Initial OCS ecosystem flux estimations were made using flask sampling followed by analysis via gas chromatography-mass spectrometry (GC-MS) (13, 16), but these studies did not have sufficient resolution to examine daily or hourly controls on the OCS flux. Laser spectrometers have been developed (17, 18) to enable direct, in situ measurement of OCS fluxes by eddy covariance, and measurements of OCS ecosystem fluxes have been reported, for periods of up to a few weeks, above arid forests (19) and an agricultural field (8,20).…”