The influence of water chemistry on the enzymatic degradation of leaves in streams

1. Aquatic hyphomycetes degrade leaf litter in both softwater and hardwater streams. During growth on leaves, these fungi secrete an array of extracellular polysaccharidases that are differentially affected by pH. Hydrolytic enzymes exhibit acidic pH optima, whereas pectin lyases have neutral to alkaline pH optima. 2. Enzyme activities associated with microbial communities colonizing yellow poplar (Liriodendron tulipifera) leaves submerged in an acidic (pH 6.3), softwater stream were compared with those occurring in an alkaline (pH 8.2), hardwater stream. In addition to pH differences, the hardwater stream had higher nutrient concentrations and higher temperatures than the softwater stream. Conditions in the hardwater stream favoured greater microbial growth, fungal activity, rates of leaf breakdown and softening. However, activities of hydrolytic enzymes (xylanase, endocellulase, galacturonanase) were lower in the hardwater stream than in the softwater stream. Consequently, activities of these hydrolytic enzymes were not good indicators of leaf breakdown in these streams. 3. In contrast, the activities of pectin lyase were higher in the hardwater stream than in the softwater stream, corresponding to the greater rates of leaf breakdown and softening that occurred in the hardwater stream. These results support previous findings that pectin lyase is closely associated with the softening and maceration of leaf detritus and suggest that pectin degradation is a key process in the initial stages of leaf breakdown.     

Does leaf quality mediate the stimulation of leaf breakdown by phosphorus in Neotropical streams?

1. Lowland tropical streams have a chemically diverse detrital resource base, where leaf quality could potentially alter the effect of high nutrient concentrations on leaf breakdown. This has important implications given the extent and magnitude of anthropogenic nutrient loading to the environment. 2. Here, we examine if leaf quality (as determined by concentrations of cellulose, lignin and tannins) mediates the effects of high ambient phosphorus (P) concentration on leaf breakdown in streams of lowland Costa Rica. We hypothesised that P would have a stronger effect on microbial and insect processing of high‐ than of low‐quality leaves. 3. We selected three species that represented extremes of quality as measured in leaves of eight common riparian species. Species selected were, from high‐ to low‐quality: Trema integerrima > Castilla elastica > Zygia longifolia. We incubated single‐species leaf packs in five streams that had natural differences in ambient P concentration (10–140 μg soluble reactive phosphorus (SRP) L^?1), because of variable inputs of solute‐rich groundwater and also in a stream that was experimentally enriched with P (approximately 200 μg SRP L^?1). 4. The breakdown rate of all three species varied among the six streams: T. integerrima (k‐values range: 0.0451–0.129 day^?1); C. elastica (k‐values range: 0.0064–0.021 day^?1); and Z. longifolia (k‐values range: 0.002–0.008 day^?1). Both ambient P concentration and flow velocity had significant effects on the breakdown rate of the three species. 5. Results supported our initial hypothesis that litter quality mediates the effect of high ambient P concentration on leaf processing by microbes and insects. The response of microbial respiration, fungal biomass and invertebrate density to high ambient P concentration was greater in Trema (high quality) than in Castilla or Zygia (low quality). Variation in flow velocity, however, confounded our ability to determine the magnitude of stimulation of breakdown rate by P. 6. Cellulose and lignin appeared to be the most important factors in determining the magnitude of P‐stimulation. Surprisingly, leaf secondary compounds did not have an effect. This contradicts predictions made by other researchers, regarding the key role of plant secondary compounds in affecting leaf breakdown in tropical streams.     

Decomposition of Fire Exposed Eucalyptus Leaves in a Portuguese Lowland Stream

We compared fire exposed with normal abscised eucalyptus leaves incubated in a stream running through eucalyptus plantations in central Portugal, in terms of breakdown rates, microbial activity, diversity and macroinvertebrate abundance. Although leaves exposed to fire had lower nutritional value, mass loss was similar for both leaf types (k = 0.0089–0.0095 d^–1 for fire and k = 0.0084–0.00103 d^–1 for normal leaves). Fungal biomass was similar among treatments, whereas sporulation and microbial respiration were lower in fire exposed leaves. Both leaf types had similar aquatic hyphomycetes communities. Physical fragmentation was important in fire exposed leaves breakdown. Invertebrates colonized leaves in low numbers in both treatments. Alteration of leaf litter quality determined by fires in streams does not seem to determine changes in ecosystem functioning in a short term. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resource quality and stoichiometric constraints on stream ecosystem functioning

1. Resource quality and stoichiometric imbalances in carbon : nutrient ratios between consumers and resources can influence key ecosystem processes. In many streams, this has important implications for food webs that are based largely upon the utilization of terrestrial leaf‐litter, which varies widely among litter types in its value as a food source for detritivores and as a substrate for microbial decomposers. 2. We measured breakdown rates and macroinvertebrate colonization of leaf‐litter from a range of native and exotic plants of differing resource quality and palatability to consumers [e.g. carbon : nitrogen : phosphorus (C : N : P) ratios, lignin and cellulose content], in a field experiment. We also measured C : N : P ratios of the principal leaf‐shredding invertebrates, which revealed strong stoichiometric imbalances across trophic levels: C : N and C : P ratios typically differed by at least one order of magnitude between consumers and resources, whereas N : P imbalances were less marked. Application of the threshold elemental ratio approach, which integrates animal bioenergetics and body elemental composition in examining nutrient deficiency between consumers and resources, revealed less marked C : P imbalances than those based on the simpler arithmetic differences described above. 3. Litter breakdown rates declined as nutrient imbalances widened and resource quality fell, but they were independent of whether resources were exotic or native. The principal drivers of total, microbial and invertebrate‐mediated breakdown rates were lignin : N, lignin : P and fungal biomass, respectively. However, multiple regression using orthogonal predictors yielded even more efficient models of litter breakdown, as consumers responded to more than one aspect of resource quality. For example, fungal biomass and litter C : N both influenced invertebrate‐mediated breakdown. 4. Large stoichiometric imbalances and changes in resource quality are likely to have serious consequences for stream ecosystem functioning, especially when riparian zones have been invaded by exotic plant species whose chemical composition differs markedly from that of the native flora. Consequently, the magnitude and direction of change in breakdown rates and, thus, resource depletion, will be driven to a large extent by the biochemical traits (rather than taxonomic identity per se) of the resident and invading flora.     

Resource–consumer diversity: testing the effects of leaf litter species diversity on stream macroinvertebrate communities

1. Understanding relationships between resource and consumer diversity is essential to predicting how changes in resource diversity might affect several trophic levels and overall ecosystem functioning. 2. We tested for the effects of leaf litter species diversity (i.e. litter mixing) on litter mass remaining and macroinvertebrate communities (taxon diversity, abundance and biomass) during breakdown in a detritus‐based headwater stream (North Carolina, U.S.A.). We used full‐factorial analyses of single‐ and mixed‐species litter from dominant riparian tree species with distinct leaf chemistries [red maple (Acer rubrum), tulip poplar (Liriodendron tulipifera), chestnut oak (Quercus prinus) and rhododendron (Rhododendron maximum)] to test for additivity (single‐species litter presence/absence effects) and non‐additivity (emergent effects of litter species interactions). 3. Significant non‐additive effects of litter mixing on litter mass remaining were explained by species composition, but not richness, and litter‐mixing effects were variable throughout breakdown. Specifically, small differences in observed versus expected litter mass remaining were measured on day 14; whereas observed litter mass remaining in mixed‐species leaf packs was significantly higher on day 70 and lower on day 118 than expected from data for single‐species leaf packs. 4. Litter mixing had non‐additive effects on macroinvertebrate community structure. The number of species in litter mixtures (two to four), but not litter species composition, was a significant predictor of the dominance of particular macroinvertebrates (i.e. indicator taxa) within mixed‐species packs. 5. In addition, the presence/absence of high‐ (L. tulipifera) and low‐quality (R. maximum) litter had additive effects on macroinvertebrate taxon richness, abundance and biomass. The presence of L. tulipifera litter had both positive (synergistic) and negative (antagonistic) effects on invertebrate taxon richness, that varied during breakdown but were not related to litter chemistry. In contrast, the presence/absence of L. tulipifera had a negative relationship with total macroinvertebrate biomass (due to low leaf mass remaining when L. tulipifera was present and higher condensed and hydrolysable tannins associated with leaf packs lacking L. tulipifera). Macroinvertebrate abundance was consistently lower when R. maximum was present, which was partially explained by litter chemistry [e.g., high concentrations of lignin, condensed tannins, hydrolysable tannins and total phenolics and high carbon to nutrient (N and P) ratios]. 6. The bottom‐up effects of litter species diversity on stream macroinvertebrates and litter breakdown are different, which suggests that structural attributes of macroinvertebrate communities may only partially explain the effects of litter‐mixing on organic matter processing in streams. In addition, stream macroinvertebrates colonising decomposing litter are influenced by resource diversity as well as resource availability. Broad‐scale shifts in riparian tree species composition will alter litter inputs to streams, and our results suggest that changes in the diversity and availability of terrestrial litter may alter stream food webs and organic matter processing.     

Non‐additive effects of litter mixing are suppressed in a nutrient‐enriched stream

Investigations of how species compositional changes interact with other aspects of global change, such as nutrient mobilization, to affect ecosystem processes are currently lacking. Many studies have shown that mixed species plant litters exhibit non‐additive effects on ecosystem functions in terrestrial and aquatic systems. Using a full‐factorial design of three leaf litter species with distinct initial chemistries (carbon:nitrogen; C:N) and breakdown rates (Liriodendron tulipifera, Acer rubrum and Rhododendron maximum), we tested for additive and non‐additive effects of litter species mixing on breakdown in southeastern US streams with and without added nutrients (N and phosphorus). We found a non‐additive (antagonistic) effect of litter mixing on breakdown rates under reference conditions but not when nutrient levels were elevated. Differential responses among single‐species litters to nutrient enrichment contributed to this result. Antagonistic litter mixing effects on breakdown were consistent with trends in litter C:N, which were higher for mixtures than for single species, suggesting lower microbial colonization on mixtures. Nutrient enrichment lowered C:N and had the greatest effect on the lowest‐ (R. maximum) and the least effect on the highest‐quality litter species (L. tulipifera), resulting in lower interspecific variation in C:N. Detritivore abundance was correlated with litter C:N in the reference stream, potentially contributing to variation in breakdown rates. In the nutrient‐enriched stream, detritivore abundance was higher for all litter and was unrelated to C:N. Thus, non‐additive effects of litter mixing were suppressed by elevated streamwater nutrients, which increased nutrient content of all litter, reduced variation in C:N among litter species and increased detritivore abundance. Nutrients reduced interspecific variation among plant litters, the base of important food web pathways in aquatic ecosystems, affecting predicted mixed‐species breakdown rates. More generally, world‐wide mobilization of nutrients may similarly modify other effects of biodiversity on ecosystem processes.     

Elevated temperature may intensify the positive effects of nutrients on microbial decomposition in streams

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Nutrients stimulate leaf breakdown rates and detritivore biomass: bottom-up effects via heterotrophic pathways

Most nutrient enrichment studies in aquatic systems have focused on autotrophic food webs in systems where primary producers dominate the resource base. We tested the heterotrophic response to long-term nutrient enrichment in a forested, headwater stream. Our study design consisted of 2 years of pretreatment data in a reference and treatment stream and 2 years of continuous nitrogen (N) + phosphorus addition to the treatment stream. Studies were conducted with two leaf species that differed in initial C:N, Rhododendron maximum (rhododendron) and Acer rubrum (red maple). We determined the effects of nutrient addition on detrital resources (leaf breakdown rates, litter C:N and microbial activity) and tested whether nutrient enrichment affected macroinvertebrate consumers via increased biomass. Leaf breakdown rates were ca. 1.5 and 3× faster during the first and second years of enrichment, respectively, in the treatment stream for both leaf types. Microbial respiration rates of both leaf types were 3× higher with enrichment, and macroinvertebrate biomass associated with leaves increased ca. 2–3× with enrichment. The mass of N in macroinvertebrate biomass relative to leaves tended to increase with enrichment up to 6× for red maple and up to 44× for rhododendron leaves. Lower quality (higher C:N) rhododendron leaves exhibited greater changes in leaf nutrient content and macroinvertebrate response to nutrient enrichment than red maple leaves, suggesting a unique response by different leaf species to nutrient enrichment. Nutrient concentrations used in this study were moderate and equivalent to those in streams draining watersheds with altered land use. Thus, our results suggest that similarly moderate levels of enrichment may affect detrital resource quality and subsequently lead to altered energy and nutrient flow in detrital food webs. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Effects of nutrient enrichment on boreal streams: invertebrates, fungi and leaf-litter breakdown

1. The effect of nutrient enrichment on structural (invertebrate indices) and functional (leaf‐litter breakdown rates) characteristics of stream integrity was studied in nine boreal streams. 2. The results showed predicted changes in biotic indices and leaf‐litter breakdown along a complex (principal component) nutrient gradient. Biotic indices were better correlated with nutrient effects than leaf‐litter breakdown. 3. Fungal biomass and invertebrate densities in the litter bags were positively correlated with leaf‐litter breakdown, and both were also positively related to the nutrient gradient. 4. Invertebrate community composition influenced breakdown rate. High breakdown rates at one site were associated with the high abundance of the detritivore Asellus aquaticus. 5. This study lends support to the importance of invertebrate and fungi as mediators of leaf‐litter decomposition. However, our study also shows that study design (length of incubation) can confound the interpretation of nutrient‐induced effects on decomposition.     

Respiration and annual fungal production associated with decomposing leaf litter in two streams

1. We compared fungal biomass, production and microbial respiration associated with decomposing leaves in one softwater stream (Payne Creek) and one hardwater stream (Lindsey Spring Branch). 2. Both streams received similar annual leaf litter fall (478–492 g m^?2), but Lindsey Spring Branch had higher average monthly standing crop of leaf litter (69 ± 24 g m^?2; mean ± SE) than Payne Creek (39 ± 9 g m^?2). 3. Leaves sampled from Lindsey Spring Branch contained a higher mean concentration of fungal biomass (71 ± 11 mg g^?1) than those from Payne Creek (54 ± 8 mg g^?1). Maximum spore concentrations in the water of Lindsay Spring Branch were also higher than those in Payne Creek. These results agreed with litterbag studies of red maple (Acer rubrum) leaves, which decomposed faster (decay rate of 0.014 versus 0.004 day^?1), exhibited higher maximum fungal biomass and had higher rates of fungal sporulation in Lindsey Spring Branch than in Payne Creek. 4. Rates of fungal production and respiration per g leaf were similar in the two streams, although rates of fungal production and respiration per square metre were higher in Lindsey Spring Branch than in Payne Creek because of the differences in leaf litter standing crop. 5. Annual fungal production was 16 ± 6 g m^?2 (mean ± 95% CI) in Payne Creek and 46 ± 25 g m^?2 in Lindsey Spring Branch. Measurements were taken through the autumn of 2 years to obtain an indication of inter‐year variability. Fungal production during October to January of the 2 years varied between 3 and 6 g m^?2 in Payne Creek and 7–27 g m^?2 in Lindsey Spring Branch. 6. Partial organic matter budgets constructed for both streams indicated that 3 ± 1% of leaf litter fall went into fungal production and 7 ± 2% was lost as respiration in Payne Creek. In Lindsey Spring Branch, fungal production accounted for 10 ± 5% of leaf litter fall and microbial respiration for 13 ± 9%.     

Isopod effects on decomposition of litter produced under elevated CO2, N deposition and different soil types

The performance of Oniscus asellus (Isopoda) and its influence on litter mass loss and mineralization was assessed in a microcosm experiment, using beech (Fagus sylvatica) leaf litter that was produced on different soil types, contrasting atmospheric CO₂ concentrations, and different nitrogen deposition rates. Litter quality was significantly altered by these treatments, and many of the CO₂ and N effects differed between soil types. Litter quality affected subsequent litter mass loss rates, microbial respiration, and leaching of dissolved organic carbon (DOC) and nitrate. These effects were largely independent of the presence of isopods, even though isopods highly accelerated litter mass loss, stimulated microbial respiration by 37%, and increased nitrate leaching by 50%. Isopods did not change their relative rates of litter consumption and growth in response to litter quality. Isopod mortality, however, increased with increasing litter lignin/N ratios, and was significantly different between soil types, which may indicate long‐term effects on litter decomposition through altered isopod densities. Having the choice among the litter of three different species [maple (Acer pseudoplatanus), beech (Fagus sylvatica) and oak (Quercus robur)] grown at either ambient or elevated CO₂, isopods preferred maple to beech when all the litter was produced under elevated CO₂. This suggests that beyond changes in consumption, an altered food selection by isopods in a CO₂‐enriched atmosphere may affect the temporal and spatial composition of the litter layer in temperate forests. In contrast to previous findings, we conclude that isopods do not always increase their consumption rates, and hence do not differentially affect microbial decomposition in litter of poorer quality. Nevertheless changes in animal densities and/or shifts in their food preferences, could result in altered decomposition and carbon and nutrient turnover rates.     

The quality of organic matter mediates the response of heterotrophic biofilms to phosphorus enrichment of the water column and substratum

1. Heterotrophic biofilms are important drivers of community respiration, nutrient cycling and decomposition of organic matter in stream ecosystems. Both organic matter quality and nutrient levels have been shown to affect biofilm biomass and activity individually, but both factors have rarely been manipulated simultaneously. 2. To experimentally manipulate the organic matter quality and phosphorus (P) levels of both the substratum and water column, we first used cellulose cloth as a low‐quality organic material and enhanced its quality and P‐content by amending the underlying agar with maltose and P, respectively (Experiment I). To manipulate water column P, artificial substrata were incubated in low‐ and high‐P sites of a whole‐stream P‐enrichment in lowland Costa Rica. 3. Results from Experiment I suggest that heterotrophic biofilm respiration on cellulose cloth is co‐limited by carbon (C) and P. Biofilm respiration responded in an additive manner to combined effects of maltose and P‐enrichment of water column and synergistically to maltose and high‐P in substrata. 4. As decomposing organic matter that supports heterotrophic biofilms varies naturally in its labile C content along with other physical and chemical properties, we conducted a second experiment (Experiment II) in which we amended leaf discs from two species (Trema integerrima, a labile C source and Zygia longifolia, a recalcitrant C source) with maltose. We incubated the substrata in low‐ and high‐P sites of the P‐enrichment stream. 5. Results from Experiment II indicate that biofilm respiration on a labile C source (Trema) was not C‐limited, while biofilm respiration on a recalcitrant C source (Zygia) was C‐limited. Phosphorus stimulated the biofilm respiration and breakdown rate on Trema, but not on Zygia, supporting the hypothesis that the stimulatory effect of P‐enrichment is dependent on the availability of labile C in decomposing leaves. 6. Our results suggest that the interactive effects of organic matter quality and nutrient loading of streams can significantly increase microbial biofilm activity, potentially altering the trophic base of stream food webs. Researchers should consider both the organic matter quality and the enrichment of both water column and substrata to better predict the effects of anthropogenic nutrient loading to stream the ecosystems.     

Does nitrogen become limiting under high‐P conditions in detritus‐based tropical streams?

1. We examined effects of nutrients on leaf breakdown in interior forest streams at La Selva Biological Station, Costa Rica. We tested the hypothesis that dissolved inorganic nitrogen (DIN) becomes limiting when ambient phosphorus (P) concentration is high. We also compared the breakdown of relatively ‘low quality’ leaves (lower C : N, Trema integerrima) with that of ‘higher quality’ leaves (higher C : N, Ficus insipida) in a high‐P stream. 2. Litterbags were incubated in two streams: one enriched experimentally with P [target concentration 200 μg soluble reactive phosphorus (SRP) L^?1] and one control (naturally low P concentration approximately 10 μg SRP L^?1). Ammonium enrichment was achieved by adding fertiliser upstream of half of the litterbags in each stream. 3. Phosphorus addition stimulated leaf breakdown, microbial respiration, ergosterol and leaf %P. Leaf breakdown rate was consistent with those in La Selva streams with naturally high P concentration. 4. Nitrogen (N) addition had no effect on leaf breakdown, microbial respiration, ergosterol or leaf chemistry in either the P‐enriched or the reference stream, in spite of low N : P ratios. We conclude that N is probably not limiting in streams at La Selva that are naturally high in P. This may be due to moderately high ambient N concentration (>200 μg DIN L^?1) prevailing throughout the year. 5. The species with a lower C : N decomposed more rapidly and supported higher microbial activity than that with a higher C : N. Subtle differences in leaf N content, as well as dissolved P concentration, may be important in determining microbial colonisation and subsequent leaf breakdown.     

Litter decomposition in a Cerrado savannah stream is retarded by leaf toughness, low dissolved nutrients and a low density of shredders

1. To assess whether the reported slow breakdown of litter in tropical Cerrado streams is due to local environmental conditions or to the intrinsic leaf characteristics of local plant species, we compared the breakdown of leaves from Protium brasiliense, a riparian species of Cerrado (Brazilian savannah), in a local and a temperate stream. The experiment was carried out at the time of the highest litter fall in the two locations. An additional summer experiment was conducted in the temperate stream to provide for similar temperature conditions. 2. The breakdown rates (k) of P. brasiliense leaves in the tropical Cerrado stream ranged from 0.0001 to 0.0008 day⁻¹ and are among the slowest reported. They were significantly (F = 20.12, P < 0.05) lower than in the temperate stream (0.0046–0.0055). The maximum ergosterol content in decomposing leaves in the tropical Cerrado stream was 106 μg g⁻¹, (1.9% of leaf mass) measured by day 75, which was lower than in the temperate stream where maximum ergosterol content of 522 μg g⁻¹ (9.5% of leaf mass) was achieved by day 30. The ATP content, as an indicator of total microbial biomass, was up to four times higher in the tropical Cerrado than in the temperate stream (194.0 versus 49.4 nmoles g⁻¹). 3. Unlike in the temperate stream, leaves in the tropical Cerrado were not colonised by shredder invertebrates. However, in none of the experiments did leaves exposed (coarse mesh bags) and unexposed (fine mesh bags) to invertebrates differ in breakdown rates (F = 1.15, P > 0.05), indicating that invertebrates were unable to feed on decomposing P. brasiliense leaves. 4. We conclude that the slow breakdown of P. brasiliense leaves in the tropical Cerrado stream was because of the low nutrient content in the water, particularly nitrate (0.05 mgN L⁻¹), which slows down fungal activity and to the low density of invertebrates capable of using these hard leaves as an energy source.     

Fungal contributions to carbon flow and nutrient cycling during decomposition of standing Typha domingensis leaves in a subtropical freshwater marsh

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Correlation of foliage and litter chemistry of sugar maple, Acer saccharum, as affected by elevated CO2 and varying N availability, and effects on decomposition

Rising atmospheric carbon dioxide has the potential to alter leaf litter chemistry, potentially affecting decomposition and rates of carbon and nitrogen cycling in forest ecosystems. This study was conducted to determine whether growth under elevated atmospheric CO₂ altered the quality and microbial decomposition of leaf litter of a widely distributed northern hardwood species at sites of low and high soil nitrogen availability. In addition, we assessed whether the carbon–nutrient balance (CNB) and growth differentiation balance (GDB) hypotheses could be extended to predict changes in litter quality in response to resource availability. Sugar maple (Acer saccharum) was grown in the field in open‐top chambers at 36 and 55 Pa partial pressure CO₂, and initial soil mineralization rates of 45 and 348 μg N g^?1 d^?1. Naturally senesced leaf litter was assessed for chemical composition and incubated in the laboratory for 111 d. Microbial respiration and the production of dissolved organic carbon (DOC) were quantified as estimates of decomposition. Elevated CO₂ and low soil nitrogen resulted in higher litter concentrations of nonstructural carbohydrates and condensed tannins, higher C/N ratios and lower N concentrations. Soil N availability appears to have had a greater effect on litter quality than did atmospheric CO₂, although the treatments were additive, with highest concentrations of nonstructural carbohydrates and condensed tannins occurring under elevated CO₂–low soil N. Rates of microbial respiration and the production of DOC were insensitive to differences in litter quality. In general, concentrations of litter constituents, except for starch, were highly correlated to those in live foliage, and the CNB/GDB hypotheses proved useful in predicting changes in litter quality. We conclude the chemical composition of sugar maple litter will change in the future in response to rising atmospheric CO₂, and that soil N availability will exert a major control. It appears that microbial metabolism will not be directly affected by changes in litter quality, although conclusions regarding decomposition as a whole must consider the entire soil food web.     

Nutrients and temperature additively increase stream microbial respiration

Rising temperatures and nutrient enrichment are co‐occurring global‐change drivers that stimulate microbial respiration of detrital carbon, but nutrient effects on the temperature dependence of respiration in aquatic ecosystems remain uncertain. We measured respiration rates associated with leaf litter, wood, and fine benthic organic matter (FBOM) across seasonal temperature gradients before (PRE) and after (ENR1, ENR2) experimental nutrient (nitrogen [N] and phosphorus [P]) additions to five forest streams. Nitrogen and phosphorus were added at different N:P ratios using increasing concentrations of N (~80–650 μg/L) and corresponding decreasing concentrations of P (~90–11 μg/L). We assessed the temperature dependence, and microbial (i.e., fungal) drivers of detrital mass‐specific respiration rates using the metabolic theory of ecology, before vs. after nutrient enrichment, and across N and P concentrations. Detrital mass‐specific respiration rates increased with temperature, exhibiting comparable activation energies (E, electronvolts [eV]) for all substrates (FBOM E = 0.43 [95% CI = 0.18–0.69] eV, leaf litter E = 0.30 [95% CI = 0.072–0.54] eV, wood E = 0.41 [95% CI = 0.18–0.64] eV) close to predicted MTE values. There was evidence that temperature‐driven increased respiration occurred via increased fungal biomass (wood) or increased fungal biomass‐specific respiration (leaf litter). Respiration rates increased under nutrient‐enriched conditions on leaves (1.32×) and wood (1.38×), but not FBOM. Respiration rates responded weakly to gradients in N or P concentrations, except for positive effects of P on wood respiration. The temperature dependence of respiration was comparable among years and across N or P concentration for all substrates. Responses of leaf litter and wood respiration to temperature and the combined effects of N and P were similar in magnitude. Our data suggest that the temperature dependence of stream microbial respiration is unchanged by nutrient enrichment, and that increased temperature and N + P availability have additive and comparable effects on microbial respiration rates.     

Decomposition of tree leaf litters grown under elevated CO2: Effect of litter quality

Ash (Fraxinus excelsior L.), birch (Betula pubescens Ehrh.), sycamore (Acer pseudoplatanus L.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) leaf litters were monitored for decomposition rates and nutrient release in a laboratory microcosm experiment. Litters were derived from solar domes where plants had been exposed to two different CO₂ regimes: ambient (350 L L^-1 CO₂) and enriched (600 L L^-1 CO₂).Elevated CO₂ significantly affected some of the major litter quality parameters, with lower N, higher lignin concentrations and higher ratios of C/N and lignin/N for litters derived from enriched CO₂. Respiration rates of the deciduous species were significantly decreased for litters grown under elevated CO₂, and reductions in mass loss at the end of the experiment were generally observed in litters derived from the 600 ppm CO₂ treatment. Nutrient mineralization, dissolved organic carbon, and pH in microcosm leachates did not differ significantly between the two CO₂ treatments for any of the species studied. Litter quality parameters were examined for correlations with cumulative respiration and decomposition rates: N concentration, C/N and lignin/N ratios showed the highest correlations, with differences between litter types. The results indicate that higher C storage will occur in soil as a consequence of litter quality changes resulting from higher atmospheric concentrations of CO₂.Abbreviations CHO soluble carbohydrates - DOC dissolved organic carbon - HCel holocellulose - WTREM weight remaining     

Litter quality and decomposition in Danthonia richardsonii swards in response to CO2 and nitrogen supply over four years of growth

Litter quality parameters of Danthonia richardsonii grown under CO₂ concentrations of ≈ 359 & ≈ 719 μL L^{? 1} at three mineral N supply rates (2.2, 6.7 & 19.8 g m^{? 2} y^{? 1}) were determined. C:N ratio was increased in senesced leaf (enhancement ratios, R_e/c, of 1.25–1.67), surface litter (1.34–1.64) and root (1.13–1.30) by CO₂ enrichment. After 3 years of growth, nonstructural carbohydrate concentrations were reduced in senesced leaf lamina (avg. R_e/c= 0.84) but not in root in response to CO₂ enrichment. Cellulose concentrations increased slightly in senesced leaf (avg. R_e/c= 1.07) but not in root in response to CO₂ enrichment. Lignin and polyphenolic concentrations in senesced leaf and root were not changed by CO₂ enrichment. Decomposition, measured as cumulative respiration in standard conditions in vitro, was reduced in leaf litter grown under CO₂ enrichment. Root decomposition in vitro was lower in the material produced under CO₂ enrichment at the two higher rates of mineral N supply. Significant correlations between decomposition of leaf litter and initial %N, C:N ratio and lignin:N ratio were found. Decomposition in vivo, measured as carbon disappearance from the surface litter was not affected by CO₂ concentration. Arbuscular mycorrhizal infection was not changed by CO₂ enrichment. Microbial carbon was higher under CO₂ enrichment at the two higher rates of mineral N supply. Possible reasons for the lack of effect of changes in litter quality on in‐sward decomposition rates are discussed.     

Fungal importance extends beyond litter decomposition in experimental early‐successional streams

Fungi are important decomposers of leaf litter in streams and may have knock‐on effects on other microbes and carbon cycling. To elucidate such potential effects, we designed an experiment in outdoor experimental channels simulating sand‐bottom streams in an early‐successional state. We hypothesized that the presence of fungi would enhance overall microbial activity, accompanied by shifts in the microbial communities associated not only with leaf litter but also with sediments. Fifteen experimental channels received sterile sandy sediment, minimal amounts of leaf litter, and one of four inocula containing either (i) fungi and bacteria, or (ii) bacteria only, or (iii) no microorganisms, or (iv) killed microorganisms. Subsequently, we let water from an early‐successional catchment circulate through the channels for 5 weeks. Whole‐stream metabolism and microbial respiration associated with leaf litter were higher in the channels inoculated with fungi, reflecting higher fungal activity on leaves. Bacterial communities on leaves were also significantly affected. Similarly, increases in net primary production, sediment microbial respiration and chlorophyll a content on the sediment surface were greatest in the channels receiving a fungal inoculum. These results point to a major role of fungal communities in stream ecosystems beyond the well‐established direct involvement in leaf litter decomposition.