Sylvia Toet

Carbon respiration from subsurface peat accelerated by climate warming in the subarctic

Among the largest uncertainties in current projections of future climate is the feedback between the terrestrial carbon cycle and climate. Northern peatlands contain one-third of the world’s soil organic carbon, equivalent to more than half the amount of carbon in the atmosphere. Climate-warming-induced acceleration of carbon dioxide (CO2) emissions through enhanced respiration of thick peat deposits, centuries to millennia old, may form a strong positive carbon cycle–climate feedback. The long-term temperature sensitivity of carbon in peatlands, especially at depth, remains uncertain, however, because of the short duration or correlative nature of field studies and the disturbance associated with respiration measurements below the surface in situ or during laboratory incubations. Here we combine non-disturbing in situ measurements of CO2 respiration rates and isotopic (13C) composition of respired CO2 in two whole-ecosystem climate-manipulation experiments in a subarctic peatland. We show that approximately 1 °C warming accelerated total ecosystem respiration rates on average by 60% in spring and by 52% in summer and that this effect was sustained for at least eight years. While warming stimulated both short-term (plant-related) and longer-term (peat soil-related) carbon respiration processes, we find that at least 69% of the increase in respiration rate originated from carbon in peat towards the bottom (25–50 cm) of the active layer above the permafrost. Climate warming therefore accelerates respiration of the extensive, subsurface carbon reservoirs in peatlands to a much larger extent than was previously thought. Assuming that our data from a single site are indicative of the direct response to warming of northern peatland soils on a global scale, we estimate that climate warming of about 1 °C over the next few decades could induce a global increase in heterotrophic respiration of 38–100 megatonnes of C per year. Our findings suggest a large, long-lasting, positive feedback of carbon stored in northern peatlands to the global climate system.

Denitrification in the periphyton associated with plant shoots and in the sediment of a wetland system supplied with sewage treatment plant effluent.

Seasonal variation in denitrification and major factors controlling this process were determined in sediment, microbial communities attached to plant shoots (periphyton) and in the water of a Phragmites and an Elodea-dominated stand of a constructed wetland system between May 1997 and February 1998. The wetland was supplied with effluent from a sewage treatment plant. The denitrification rate in periphyton on plants shoots (expressed per shoot area) was always considerably higher than in the sediment and varied with the chlorophyll-a content of the periphyton in the course of the year. The algae in the periphyton provided attachment surfaces and probably also organic compounds to the denitrifying bacteria. Decreases in periphyton biomass and denitrification rate in the Phragmites and Elodea-dominated stands during the growing season were associated with enhanced shading by Phragmites shoots or a floating layer of macro-algae and Lemna spp., respectively. Light availability and the denitrification rate of periphyton increased again after the Phragmites shoots were cut in October. Nitrate appeared to limit the denitrification rate in the sediment. Periphyton denitrification rates were mostly lower on Elodea shoots than on Phragmites shoots, in spite of the higher living algal biomass on Elodea shoots. This difference was associated with lower nitrate concentrations in the Elodea-dominated stand. In the two stands, the daily denitrification rates in periphyton on shoots of Phragmites australis (44.4–121 mg N m−2 stand area d−1) and Elodea nuttallii (14.8–33.1 mg N m−2 d−1) were clearly more important than rates in the sediment (0.5–25.5 mg N m−2 d−1) or the water (0.4–3.9 mg N m−2 d−1). The presence of few bacteria attachment sites or low organic carbon availability possibly resulted in low denitrification rates in the water. Denitrification appeared to be a major process in nitrate removal from the through-flowing water in this wetland system.

THE EFFECT OF HYDRAULIC RETENTION TIME ON THE REMOVAL OF POLLUTANTS FROM SEWAGE TREATMENT PLANT EFFLUENT IN A SURFACE-FLOW WETLAND SYSTEM

We evaluated the effect of four hydraulic retention times (HRT, 0.3, 0.8, 2.3, and 9.3 days) on pollutant removal in a surface-flow wetland system for polishing tertiary effluent from a sewage treatment plant (STP). The removal efficiency of pollutants at these HRTs was based on mass budgets of the water inputs and outputs in parallel ditches, which together with a presettling basin, made up the wetland system. Fecal coliform and N-removal efficiencies in the ditches were enhanced by increasing the HRT, with only little removal of fecal coliforms during spring-summer at a HRT of 0.3 days. A HRT of 4 days turned out to be required to meet the desired bathing water standard for fecal coliforms (103 cfu 100 ml−1) and the future standard of ammonium (1 mg N l−1) all year. An annual N-removal efficiency of approximately 45% can be accomplished in the ditches at this HRT, corresponding to an annual N mass loading rate of 150 g N m−2 yr−1. Annual P removal was not improved by increasing the HRT even up to 9.3 days, largely because of the still high P mass loading rate (14 g P m−2 yr−1) in combination with relatively low P input concentrations. Substantial P removal can probably only be achieved at HRTs longer than 15 days, which will not be feasible for the situation investigated because of the large land area that would be required to reach such long HRTs. The future P standard (1 mg P 1−1) can therefore only be met by additional chemical P removal. In a densely populated country such as the Netherlands, adequate polishing of tertiary STP effluent in surfaceflow wetlands with similar goals as for this wetland is restricted to small and medium-sized STPs. The simultaneous use of these treatment wetlands for other functions, such as nature conservation, recreation, and flood control, however, would permit the use of relatively larger land areas.

Nutritional controls on carbon dioxide and methane emission from Carex-dominated peat soils

Abstract Eutrophication of peatlands may have significant effects on emissions of carbon dioxide and methane. This study analyses the effects of nitrogen (28.7 mmol NH4+N, N), phosphorus (1.78 mmol PO43−P, P) and glucose (2.79 mmol glucose, G) additions on CO2 and CH4 emission from intact soil cores from a Carex-dominated peatland in the Netherlands. The cores (taken from the upper 10 cm of the peat profile) were brought to field capacity and aerobically incubated at 20°C for 6 wk. Nutrients were added in the following combinations: 0 (unfertilized control), N, P, NP, G, NG, PG, NPG. All treatments in which glucose was present (G, NG, PG, NPG) stimulated CO2 emission during the first 2 wk of the experiment, but did not lead to increased decay of organic matter. At the end of the experiment, all treatments which included N (N, NG, NP, NPG) showed reduced CO2 emission. This was probably due to pH effects, because the pH in the N fertilized treatments was 0.4–0.8 units lower than in the unfertilized control. Cumulative CO2 emission in the N treatment was lower than in the control, but in the treatments where glucose was added it was higher. There was no effect of P addition on CO2 emission. In all treatments, cumulative CH4 emission was higher than in the control due to an initial stimulation of CH4 emission. Compared within treatments, cumulative CO2C emission was 35–164 times higher than cumulative CH4C emission. From these observations we conclude that increased amounts of NH4+N supply lead to reduction of decay of organic matter in peat soils and thereby to a reduction of gaseous carbon loss from these soils. Nutrient or glucose additions lead only to a short-term increase in methane emissions from peat soils.

Links between methane flux and transcriptional activities of methanogens and methane oxidizers in a blanket peat bog

The relationship between biogeochemical process rates and microbial functional activity was investigated by analysis of the transcriptional dynamics of the key functional genes for methanogenesis (methyl coenzyme M reductase; mcrA) and methane oxidation (particulate methane monooxygenase; pmoA) and in situ methane flux at two peat soil field sites with contrasting net methane-emitting and -oxidizing characteristics. qPCR was used to quantify the abundances of mcrA and pmoA genes and transcripts at two soil depths. Total methanogen and methanotroph transcriptional dynamics, calculated from mcrA and pmoA gene : transcript abundance ratios, were similar at both sites and depths. However, a linear relationship was demonstrated between surface mcrA and pmoA transcript dynamics and surface flux rates at the methane-emitting and methane-oxidizing sites, respectively. Results indicate that methanotroph activity was at least partially substrate-limited at the methane-emitting site and by other factors at the methane-oxidizing site. Soil depth also contributed to the control of surface methane fluxes, but to a lesser extent. Small differences in the soil water content may have contributed to differences in methanogen and methanotroph activities. This study therefore provides a first insight into the regulation of in situ, field-level surface CH(4) flux at the molecular level by an accurate reflection of gene : transcript abundance ratios for the key genes in methane generation and consumption.

Nutrient removal through autumn harvest of Phragmites australis and Thypha latifolia shoots in relation to nutrient loading in a wetland system used for polishing sewage treatment plant effluent.

Abstract The efficacy and feasibility of annual harvesting of Phragmites australis and Typha latifolia shoots in autumn for nutrient removal was evaluated in a wetland system used for polishing sewage treatment plant (STP) effluent. Aboveground biomass and nutrient dynamics nutrient removal through harvest were studied in parallel ditches with stands of Phragmites or Typhathat were mown in October during two successive years. The inflow rate of STP effluent to the ditches was experimentally varied, resulting in pairs of ditches with mean hydraulic retention times (HRT) of 0.3, 0.8, 2.3, and 9.3 days, corresponding to N and P mass loading rates of 122–4190 g N m− 2 yr− 1 and 28.3–994 g P m− 2 yr−1. Nitrogen and P removal efficiency by harvest of Phragmites and Typha shoots in October increased with increasing HRT, despite the opposite HRT effect on N and P standing stocks. This removal through harvest appeared to be useful in treatment wetlands with N and P mass loading rates lower than approximately 120 g N m−2 yr−1 and 30 g P m−2 yr−1, corresponding to a HRT of roughly 9 days in the ditches of this wetland system. At the HRT of 9.3 days, the annual mass input to the ditches was reduced through the harvest by 7.0–11% and 4.5 –9.2% for N and P, respectively. At the higher nutrient mass loading rates, the nutrient removal through harvest was insignificant compared to the mass inputs. The vitality of Phragmites and Typha, measured as maximum aboveground biomass, was not affected by the annual cutting of the shoots in autumn over two years. The Typha stands yielded higher N and P removal efficiencies through shoot harvest than the Phragmites stands, which was largely the result of lower decreases in N and P standing stocks between August and October. This difference in nutrient standing stocks between the two species was caused by a combined effect of greater decreases in nutrient concentrations largely due to higher nutrient retranslocation efficiencies of Phragmitesplants and greater reductions in shoot Phragmites biomass because of leaf fall and mass resorption. Nutrient removal by harvesting Phragmites shoots can probably be doubled without a reduction in vitality of the stands by advancing the harvest date to mid-September, which would at least approach the nutrient removal by harvesting Typha shoots in October. Phragmites also may be more profitable in very low-loaded wetland systems because the vigor of Typha stands seemed to be more sensitive to a lower nutrient availability at N and P mass input rates lower than the range indicated.

Nitrogen supply effects on productivity and potential leaf litter decay of Carex species from peatlands differing in nutrient limitation

We investigated the effect of increased N-supply on productivity and potential litter decay rates of Carex species, which are the dominant vascular plant species in peatlands in the Netherlands. We hypothesized that: (1) under conditions of N-limited plant growth, increased N-supply will lead to increased productivity but will not affect C:N ratios of plant litter and potential decay rates of that litter; and (2) under conditions of P-limited plant growth, increased N-supply will not affect productivity but it will lead to lower C:N ratios in plant litter and thereby to a higher potential decay rate of that litter. These hypotheses were tested by fertilization experiments (addition of 10 g N m-2 year-1) in peatlands in which plant growth was N-limited and P-limited, respectively. We investigated the effects of fertilization on net C-fixation by plant biomass, N uptake, leaf litter chemistry and potential leaf litter decay. In a P-limited peatland, dominated by Carex lasiocarpa, there was no significant increase of net C-fixation by plant biomass upon enhanced N-supply, although N-uptake had increased significantly compared with the unfertilized control. Due to the N-fertilization the C:N ratio in the plant biomass decreased significantly. Similarly, the C:N ratio of leaf litter produced at the end of the experiment showed a significant decrease upon enhanced N-supply. The potential decay rate of that litter, measured as CO2-evolution from the litter under aerobic conditions, was significantly increase upon enhanced N-supply. In a N-limited peatland, dominated by C. acutiformis, the net C-fixation by plant biomass increased with increasing N-supply, whereas the increase in N-uptake was not significant. The C:N ratio of both living plant material and of dead leaves did not change in response to N-fertilization. The potential decay rate of the leaf litter was not affected by N-supply. The results agree with our hypotheses. This implies that atmospheric N-deposition may affect the CO2-sink function of peatlands, but the effect is dependent on the nature of nutrient limitation. In peatlands where plant growth is N-limited, increased N-supply leads to an increase in the net accumulation of C. Under conditions of P-limited plant growth, however, the net C-accumulation will decrease, because productivity is not further increased, whereas the amount of C lost through decomposition of dead organic matter is increased. As plant growth in most terrestrial ecosystems is N-limited, increased N-supply will in most peatlands lead to an increase of net C-accumulation.

Fungal polygalacturonase activity reflects susceptibility of carnation cultivars to Fusarium wilt

Carnation cultivars with different levels of partial resistance were inoculated with race 2 of Fusarium oxysporum f.sp. dianthi and monitored for accumulation of host phytoalexins, fungal escape from compartmentalization, production of fungal pectin-degrading enzymes and development of external disease symptoms. Accumulation of phytoalexins, assessed after 10 days in the first 5 cm above the inoculation site, was weakly (methoxydianthramide S) or not (hydroxydianthalexin B) correlated with resistance levels after 12 weeks. Fungal escape from compartmentalization, assessed after 3 weeks as percentages colonized plants at 8 cm above the inoculation site, was highly correlated with expression of susceptibility after 12 weeks. Polygalacturonase (PG) activity, assessed after 4 weeks in the first 5 cm above the inoculation site, was highly correlated to final disease development. Linear increases in disease severity were accompanied by quadratic increases in PG activity. In contrast to water-treated plants, that lacked any PG activity, inoculated plants contained two main groups of fungal PGs, the dominant forms of which had estimated pI values of 7.0 and minimally 9.5, respectively. Compared to those of the first group, enzymes of the second group were produced only in trace amounts in liquid media containing pectin or polygalacturonate as sole source of carbon. On these media, the fungus also produced a pectin methyl esterase (PME) with an estimated pI of 9.3. Besides PMEs of host origin, inoculated plants of susceptible cultivars contained the fungal PME while no more than traces were found in resistant ones.Assessment of phytoalexin production by the host during defense responses cannot replace monitoring of external symptoms as a resistance test. Assessment of fungal growth, whether by reisolations above the compartmentalization area or by measurement of PG activity, provides a both rapid and reliable prediction of disease development.

Moss responses to elevated CO2 and variation in hydrology in a temperate lowland peatland

We studied the effects of elevated CO2 (180–200 ppmv above ambient) on growth and chemistry of three moss species (Sphagnum palustre, S. recurvum and Polytrichum commune) in a lowland peatland in the Netherlands. Thereto, we conducted both a greenhouse experiment with both Sphagnum species and a field experiment with all three species using MiniFACE (Free Air CO2 Enrichment) technology during 3 years. The greenhouse experiment showed that Sphagnum growth was stimulated by elevated CO2 in the short term, but that in the longer term (≥1 year) growth was probably inhibited by low water tables and/or down-regulation of photosynthesis. In the field experiment, we did not find significant changes in moss abundance in response to elevated CO2, although CO2 enrichment appeared to reduce S. recurvum abundance. Both Sphagnum species showed stronger responses to spatial variation in hydrology than to increased atmospheric CO2 concentrations. Polytrichum was insensitive to changes in hydrology. Apart from the confounding effects of hydrology, the relative lack of growth response of the moss species may also have been due to the relatively small increase in assimilated CO2 as achieved by the experimentally added CO2. We calculated that the added CO2 contributed at most 32% to the carbon assimilation of the mosses, while our estimates based on stable C isotope data even suggest lower contributions for Sphagnum (24–27%). Chemical analyses of the mosses showed only small elevated CO2 effects on living tissue N concentration and C/N ratio of the mosses, but the C/N ratio of Polytrichum was substantially lower than those of the Sphagnum species. Continuing expansion of Polytrichum at the expense of Sphagnum could reduce the C sink function of this lowland Sphagnum peatland, and similar ones elsewhere, as litter decomposition rates would probably be enhanced. Such a reduction in sink function would be driven mostly by increased atmospheric N deposition, water table regulation for agricultural purposes and land management to preserve the early successional stage (mowing, tree and shrub removal), since these anthropogenic factors will probably exert a greater control on competition between Polytrichum and Sphagnum than increased atmospheric CO2 concentrations.

Vascular Plant Responses to Elevated CO2 in a Temperate Lowland Sphagnum Peatland

Vascular plant responses to experimental enrichment with atmospheric carbon dioxide (CO2), using MINIFACE technology, were studied in a Dutch lowland peatland dominated by Sphagnum and Phragmites for 3 years. We hypothesized that vascular plant carbon would accumulate in this peatland in response to CO2 enrichment owing to increased productivity of the predominant species and poorer quality (higher C/N ratios) and consequently lower decomposability of the leaf litter of these species. Carbon isotope signatures demonstrated that the extra 180 ppmv CO2 in enriched plots had been incorporated into vegetation biomass accordingly. However, on the CO2 sequestration side of the ecosystem carbon budget, there were neither any significant responses of total aboveground abundance of vascular plants, nor of any of the individual species. On the CO2 release side of the carbon budget (decomposition pathway), litter quantity did not differ between ambient and CO2 treatments, while the changes in litter quality (N and P concentration, C/N and C/P ratio) were marginal and inconsistent. It appeared therefore that the afterlife effects of significant CO2-induced changes in green-leaf chemistry (lower N and P concentrations, higher C/N and C/P) were partly offset by greater resorption of mobile carbohydrates from green leaves during senescence in CO2-enriched plants. The decomposability of leaf litters of three predominant species from ambient and CO2-enriched plots, as measured in a laboratory litter respiration assay, showed no differences. The relatively short time period, environmental spatial heterogeneity and small plot sizes might explain part of the lack of CO2 response. When our results are combined with those from other Sphagnum peatland studies, the common pattern emerges that the vascular vegetation in these ecosystems is genuinely resistant to CO2-induced change. On decadal time-scales, water management and its effects on peatland hydrology, N deposition from anthropogenic sources and land management regimes that arrest the early successional phase (mowing, tree and shrub removal), may have a greater impact on the vascular plant species composition, carbon balance and functioning of lowland Sphagnum–Phragmites reedlands than increasing CO2 concentrations in the atmosphere.