Effects of elevated CO2 on litter chemistry and subsequent invertebrate detritivore feeding responses.

Elevated atmospheric CO2 can change foliar tissue chemistry. This alters leaf litter palatability to macroinvertebrate detritivores with consequences for decomposition, nutrient turnover, and food-web structure. Currently there is no consensus on the link between CO2 enrichment, litter chemistry, and macroinvertebrate-mediated leaf decomposition. To identify any unifying mechanisms, we presented eight invertebrate species from aquatic and terrestrial ecosystems with litter from Alnus glutinosa (common alder) or Betula pendula (silver birch) trees propagated under ambient (380 ppm) or elevated (ambient +200 ppm) CO2 concentrations. Alder litter was largely unaffected by CO2 enrichment, but birch litter from leaves grown under elevated CO2 had reduced nitrogen concentrations and greater C/N ratios. Invertebrates were provided individually with either (i) two litter discs, one of each CO2 treatment ('choice'), or (ii) one litter disc of each CO2 treatment alone ('no-choice'). Consumption was recorded. Only Odontocerum albicorne showed a feeding preference in the choice test, consuming more ambient- than elevated-CO2 birch litter. Species' responses to alder were highly idiosyncratic in the no-choice test: Gammarus pulex and O. albicorne consumed more elevated-CO2 than ambient-CO2 litter, indicating compensatory feeding, while Oniscus asellus consumed more of the ambient-CO2 litter. No species responded to CO2 treatment when fed birch litter. Overall, these results show how elevated atmospheric CO2 can alter litter chemistry, affecting invertebrate feeding behaviour in species-specific ways. The data highlight the need for greater species-level information when predicting changes to detrital processing-a key ecosystem function-under atmospheric change.

Introduction

Global c<span class="Species">oncentrati<class="Species">span class="Species">ons of atmospheric carbon dioxide (CO2) could more than double by 2100 [1]. Typically, CO2 enrichment leads to increased plant photosynthesis, resulting in greater biomass and production [2]. Plant tissue chemistry is typically modified, with decreasing nitrogen concentrations and increasing carbon-nitrogen (C/N) ratios affecting herbivore life-history and feeding responses [3].

Approximately 90% of primary producti<span class="Species">on in forest ecosystems escapes herbivory and forms detritus [4], providing a crucial energy pool that underpins the trophic structure of soils and adjacent freshwaters [5]. The effect of elevated <class="Species">span class="Chemical">CO2 on the chemical composition of green foliar tissues reduces its palatability to detritivores when it falls as litter [6]. In particular, elevated CO2 can reduce litter resource quality by decreasing litter nitrogen content [7], [8], subsequently increasing C/N ratios [9], [10]. Increases in structural [6], [8], [9] and defensive [10], [11] compounds have also been reported, along with both increases and decreases in phosphorus concentrations [12], [13]. The potential for rising CO2 concentrations to alter litter chemical composition is established, but the consequences for invertebrate-mediated decomposition – an important ecosystem function – remain unclear [14].

Detritivorous macroinvertebrates are functiclass="Species">onally important in detritus-based ecosystems, as they are reclass="Species">sp<class="Species">span class="Species">onsible for both comminution and consumption of litter, releasing nutrients for other organisms, such as saprophagous fungi [15], [16]. To maintain optimal body nutrient concentrations, theoretical predictions and empirical evidence suggest that invertebrates can increase feeding rates of reduced-quality material (e.g. [17], [18]), a process known as ‘compensatory feeding’ (as defined by [19]). Despite this, poor quality litter has also been shown to increase handling times [20], while reducing nutrient assimilation, slowing development rates, and increasing mortality [6], [21]. These conflicting responses have resulted from studies focusing on a small number of species (e.g. [13], [18]), which also fail to incorporate aquatic and terrestrial invertebrates, despite differences in detrital accumulation and energy flow between these habitats [22]. A broad-scale study incorporating a range of invertebrate species from different habitats is essential to identify the unifying mechanisms that govern invertebrate feeding responses to elevated-CO2 litter.

We investigated the feeding preferences and cclass="Species">onsumpti<class="Species">span class="Species">on rates of eight detritivorous macroinvertebrate species presented with Alnus glutinosa (Linnaeus) Gaertner (common alder) and Betula pendula Roth (silver birch) leaf litter produced under ambient and elevated atmospheric CO2. We tested the hypotheses that: (1) CO2 enrichment will reduce leaf chemical quality and, given nitrogen-fixing ability in alder, responses will differ by tree species; (2) when presented with a choice between ambient and elevated CO2 litter, invertebrates will prefer ambient material due to its higher quality; (3) when given litter of one CO2 treatment only, consumption of elevated-CO2 litter will be greater, to compensate for its reduced quality.

Methods

Leaf Litter Preparation

Alder and birch litters were produced at the BangorFACE facility, Bangor, UK [23] (Fig. 1). Trees were grown in eight identical plots (four ambient-class="Chemical">CO2 and four elevated-<class="Species">span class="Chemical">CO2) to minimise infrastructure-induced artefacts. CO2 enrichment was carried out using high velocity pure CO2 injection, controlled using equipment and software modified from EuroFACE [24]. Elevated CO2 concentrations, measured at 1 min intervals, were within 30% deviation from the pre-set target concentration of 580 ppm CO2 (ambient +200 ppm) for 75–79% of the photosynthetically-active period (daylight hours from budburst until leaf abscission) of 2005–2008. Vertical profiles of CO2 concentration measured at 50 cm intervals through the canopy showed a maximum difference of +7% from reference values obtained at the top of the canopy [23]. From the beginning of leaf senescence, fallen leaf litter was collected weekly until all leaves had abscised (October to December). Litter within each CO2 treatment was homogenised and air-dried.

Figure 1

Overview of the experimental approach.

Litter was produced under ambient- and elevated-CO2 atmospheres at BangorFACE, UK. Half of the litter from each CO2 treatment was conditioned aquatically and half terrestrially. Chemical analyses of the conditioned litter were undertaken, and litter discs were presented to aquatic and terrestrial invertebrates in choice and no-choice tests. Only one tree and one invertebrate species have been shown for clarity. Not to scale.

Litter was produced under ambient- and elevated-<span class="Chemical">CO2 atmoclass="Species">spheres at BangorFACE, UK. Half of the litter from each <class="Species">span class="Chemical">CO2 treatment was conditioned aquatically and half terrestrially. Chemical analyses of the conditioned litter were undertaken, and litter discs were presented to aquatic and terrestrial invertebrates in choice and no-choice tests. Only one tree and one invertebrate species have been shown for clarity. Not to scale.

Initial chemical leaching and microbial colclass="Species">onisati<class="Species">span class="Species">on of litter (‘conditioning’) are crucial steps in making litter palatable to detritivorous macroinvertebrates [25], [26]. Prior to the start of the experiment, litter was conditioned in fine mesh bags (100 µm to permit microorganisms only) placed in plastic containers (29×29×10 cm; Fig. 1). For each tree species ×CO2 treatment combination, one bag was placed in aerated stream water that was inoculated with stream-collected litter of mixed-species origin (‘aquatic conditioning’); a second bag per tree species ×CO2 treatment combination was inserted between field-collected soil and mixed deciduous leaf litter (‘terrestrial conditioning’). Containers were maintained at 11±1°C with a 12∶12 h light-dark cycle and terrestrial containers were sprayed with deionised water every three days to maintain humidity (∼50%). These conditions were selected to represent natural conditioning processes in aquatic and terrestrial habitats in a controlled manner. After two weeks, leaf discs were cut using a 9 mm diameter cork-borer (avoiding the mid-vein), which were air-dried and weighed (±0.1 mg) prior to experimental use.

Litter samples allocated to chemical analyses (Fig. 1) were stored at –80°C before being oven-dried (50°C for 24 h) and ground into powder (120 s, 50 beats s–1; Pulverisette 23 ball mill, Fritsch GmbH, Idar-Oberstein, Germany). Each sample was composed of litter from three separate leaves. For class="Chemical">carbon, <class="Species">span class="Chemical">nitrogen and phosphorus analyses, five samples were processed per tree × CO2 treatment × conditioning type combination; for lignin analysis, four samples were used. The percentage leaf dry mass (% leaf DM) of carbon and nitrogen, and the carbon-nitrogen (C/N) ratio, were determined by flash combustion and chromatographic separation of ∼1.5 mg leaf powder using an elemental analyser (Elemental Combustion System 4010 CHNS-O Analyzer, Costech Analytical Technologies, Inc., Milan, Italy), calibrated against a standard (C26H26N2O2S). Phosphorus concentrations (% leaf DM) were quantified using X-ray fluorescence (see [27] for detailed methodology). The percentage Acetyl-Bromide-Soluble Lignin (% ABSL) was determined following the acetyl bromide spectrophotometric method [28]. Lignin-nitrogen (lignin/N) ratios were calculated for each tree species × CO2 treatment × conditioning treatment combination.

Invertebrates

Eight macroinvertebrate species were selected for study (Table 1), representing a taxclass="Species">onomic range of litter c<class="Species">span class="Species">onsumers found in temperate forest habitats [29], [30]. Aquatic species were collected from streams in the Brecon Beacons National Park, South Wales, UK (51°50′53″N, 3°22′16″W and 51°50′55″N, 3°33′43″W) and Roath Park, Cardiff, UK (51°30′00″N, 3°10′10″W); terrestrial species were collected from soil-litter interfaces in Bute Park, Cardiff, UK (51°48′49″N, 3°18′24″W). The National Park Authority granted general permission to access sites on common land in the Brecon Beacons National Park, South Wales, UK. Cardiff Council granted permission for access to sites in Cardiff, UK. No endangered or protected species were involved in collections from the field. All individuals were adults, apart from larval Odontocerum albicorne and Sericostoma personatum caddisflies. Individuals from within each species were selected for size similarity. Prior to experimental use, invertebrates were maintained for at least four weeks in single-species containers (11±1°C, 12∶12 h light-dark cycle) and were fed Fagus sylvatica Linnaeus (common beech) litter conditioned as for experimental litter, preventing habituation to experimental alder and birch litter. Feeding was ceased two days prior to the experiments to allow for gut clearance.

Table 1

Detritivorous macroinvertebrate species used in the study.

Habitat	Name	Authority	Order: Family
Aquatic	Asellus aquaticus	(Linnaeus 1758)	Isopoda: Asellidae
	Gammarus pulex	(Linnaeus 1758)	Amphipoda: Gammaridae
	Odontocerum albicorne	(Scopoli 1763)	Trichoptera: Odontoceridae
	Sericostoma personatum	(Kirby & Spence 1826)	Trichoptera: Sericostomatidae
Terrestrial	Blaniulus guttulatus	(Bosc 1792)	Julida: Blaniulidae
	Oniscus asellus	Linnaeus 1758	Isopoda: Oniscidae
	Porcellio scaber	Latreille 1804	Isopoda: Porcellionidae
	Tachypodoiulus niger	(Leach 1815)	Julida: Julidae

Experimental Arenas

All experiments were c<span class="Species">onducted in 11×16.5×3.5 cm lidded plastic arenas (Cater For You Ltd, High Wycombe, UK) lined with compacted sterilised aquarium gravel (Unipac, Northampt<class="Species">span class="Species">on, UK) and were maintained at 11±1°C with a 12∶12 h light-dark cycle. Aquatic microcosms were filled with 400 ml of filtered (100 µm mesh) stream water (circumneutral pH; collected from 51°50′53″N, 3°22′16″W) and aerated through a pipette tip (200 µl Greiner Bio-One) attached to an air-line. Terrestrial microcosms were sprayed with deionised water every three days to maintain moisture content and humidity (∼50%). All arenas were uniquely labeled (‘microcosm ID’). These standardised conditions were chosen to mimic natural habitats, while minimising the availability of supplementary organic material that could act as a confounding resource during the feeding trials.

For litter of each tree species, detritivores were presented with: (i) a choice between ambient- and elevated-class="Chemical">CO2 material, to provide a direct comparis<class="Species">span class="Species">on of detritivore preferences, and (ii) a no-choice situation with each CO2 treatment presented on its own, approximating litter consumption in current (ambient-CO2) and future (elevated-CO2) atmospheric conditions (Fig. 1). In each experiment, ten microcosms were set up for each invertebrate and tree species combination (n = 160). A single invertebrate was added to each arena and was placed in the end opposite the airline in aquatic arenas and equidistant to both discs in the choice test. In the choice test, one disc of each CO2 treatment was pinned to the centre of the arena, 4 cm apart. Discs were replenished when at least 50% of the existing disc had been consumed. In the no-choice test, half of the microcosms contained one ambient-CO2 disc and the other half one elevated-CO2 disc, pinned to the centre of the arena. Both experiments ended after 14 days, or when five (50%) of the individuals of a specific species consumed at least 50% of one disc (choice experiment only). For each invertebrate, the total mass of litter consumed was calculated (±0.1 mg). For choice experiment data, this value was divided by the number of days over which the test had taken place.

Additi<span class="Species">onally, c<class="Species">span class="Species">ontrol microcosms were set up to ensure that differences in mass loss between CO2 treatments were due to invertebrate activity alone. For each experiment, ten microcosms were set up for each habitat type × tree species combination. Controls for the choice test each contained one disc of each CO2 treatment; half of the no-choice control microcosms contained one ambient-CO2 disc and the other half contained one elevated-CO2 disc. Leaf discs were air-dried and weighed (±0.1 mg) after 14 days and their total mass loss calculated.

Data Analysis

Statistical analyses were performed separately for alder and birch litter using R versi<span class="Species">on 3.0.1 [31]. Data available from http://dx.doi.org/10.6084/m9.figshare.791634. were checked for normality and homogeneity of variance following Crawley [32]; reclass="Species">sp<class="Species">span class="Species">onse variables were transformed using Box-Cox power transformations when assumptions were not met (car package [33]). Significance was set at α = 0.05 for all analyses.

Two-way analysis of variance (ANOVA) was used to test the main and interactive effects of class="Chemical">CO2 treatment and microcosm type <class="Species">span class="Species">on each chemical variable (carbon, nitrogen, phosphorus and lignin concentrations, and C/N ratio). Planned contrasts (lsmeans package [34]) were used to compare the effects of CO2 treatments for each conditioning treatment.

The main and interactive effects of <span class="Chemical">CO2 treatment and microcosm type were tested <class="Species">span class="Species">on the mass loss of control discs. Linear mixed-effects models were used to analyse choice control data (nlme package [35]), where non-independence of discs sharing the same microcosm was accounted for by including microcosm ID as a random term. The same fixed terms were used to analyse control data from the no-choice test using two-way ANOVA.

In the choice test, litter c<span class="Species">onsumpti<class="Species">span class="Species">on per day was analysed using linear mixed-effects models (nlme package [35]) with the main and interactive effects of CO2 treatment and invertebrate species as fixed effects and microcosm ID as a random effect. Planned contrasts were performed to compare consumption of ambient- and elevated-CO2 discs within (i) each invertebrate species, and (ii) invertebrate species grouped by habitat of origin (contrast package [36]).

In the no-choice test, the main and interactive effects of class="Chemical">CO2 treatment and invertebrate class="Species">species <class="Species">span class="Species">on litter consumption were tested using two-way ANOVA. Planned contrasts were performed to test the effects of CO2 treatment on disc consumption within (i) each invertebrate species (lsmeans package [34]) and (ii) invertebrate species grouped by habitat of origin (gmodels package [37]).

Results

Litter Chemistry

<span class="Chemical">CO2 enrichment altered leaf litter chemistry, but effects differed between tree class="Species">species. For birch, <class="Species">span class="Chemical">CO2-enriched litter contained lower nitrogen concentrations, and higher lignin concentrations and C/N ratios than ambient-CO2 litter (Tables 2 and 3). Litter chemistry varied between conditioning types, with higher carbon concentrations in aquatically-conditioned litter and lower nitrogen concentrations in terrestrially-conditioned litter (Table 2). For both conditioning types, elevated-CO2 litter contained lower nitrogen concentrations (aquatic, estimate = 0.76% DM, P<0.001; terrestrial, estimate = 1.17% DM, P<0.001; Table 3) and higher C/N ratios (aquatic, estimate = 8.31, P<0.001; terrestrial, estimate = 10.28, P<0.001; Table 3). For alder litter, the effect of CO2 treatment was less predictable, with differential responses between conditioning types (Table 2). Elevated CO2 increased alder nitrogen concentrations when conditioned terrestrially (estimate = 0.29% DM, P = 0.036; Table 3), although there was no concurrent effect in aquatically-conditioned litter (estimate = 0.1% DM, P = 0.44; Table 3). No treatment or species effects on litter phosphorus concentrations were observed (Tables 2 and 3).

Table 2

ANOVA summary table of main and interactive effects of CO2 treatment (CO2) and conditioning type (CT) on litter chemistry.

		Carbon		Nitrogen		Phosphorus		Lignin		C/N
Tree species	Variables	F 1,16	P	F 1,16	P	F 1,16	P	F 1,12	P	F 1,16	P
Alder	CO2	0.6	0.435	1.1	0.305	2.8	0.117	0.04	0.543	1.3	0.271
	CT	0.3	0.577	4.1	0.059	0.2	0.684	0.2	0.673	3.8	0.071
	CO2 × CT	1.5	0.241	4.7	0.045	0.4	0.387	3.6	0.082	4	0.064
Birch	CO2	0.1	0.712	791	<0.001	3.1	0.098	4.8	0.048	605.3	<0.001
	CT	12.1	0.003	95	<0.001	0.04	0.848	1	0.331	62.5	<0.001
	CO2 × CT	3.6	0.077	36.4	<0.001	0.3	0.566	0.1	0.756	6.8	0.019

P values <0.05 are emboldened.

Table 3

Chemical composition of leaf litter (mean ±1 SEM).

			Chemical composition				Chemical ratios
Tree species	CT	CO2	Carbon(% DM)	Nitrogen(% DM)	Phosphorus(% DM)	Lignin(% ABSL)	C/N	Lignin/N
Alder	Aquatic	Ambient	48.61±0.37a	3.73±0.16a	0.074±0.009a	22.17±2.64a	13.11±0.16a	5.94
		Elevated	48.48±0.25a	3.63±0.091a	0.064±0.009a	19.56±2.74a	13.37±0.36a	5.38
	Terrestrial	Ambient	48.04±0.22a	3.35±0.016a	0.084±0.009a	19.16±1.01a	14.33±0.02a	5.71
		Elevated	48.68±0.40a	3.65±0.026b	0.062±0.01a	24.34±1.14a	13.35±0.10a	6.68
Birch	Aquatic	Ambient	51.22±0.13a	2.54±0.018a	0.09±0.008a	22.10±3.28a	20.17±0.11a	8.7
		Elevated	50.84±0.13a	1.79±0.004b	0.066±0.01a	27.76±1.69a	28.47±0.08b	15.55
	Terrestrial	Ambient	49.86±0.24a	3.08±0.017a	0.082±0.01a	25.09±2.07a	16.19±0.04a	8.15
		Elevated	50.44±0.41a	1.91±0.063b	0.07±0.006a	29.32±1.52a	26.47±0.74b	15.33

Abbreviations: percent dry mass (% DM), percent acetyl-bromide-soluble lignin (% ABSL), conditioning type (CT).

Different lowercase letters indicate significant differences (P<0.05) between CO2 treatments for each tree species × CT combination.

Abbreviati<span class="Species">ons: percent dry mass (% <class="Species">span class="Chemical">DM), percent acetyl-bromide-soluble lignin (% ABSL), conditioning type (CT).

Different lowercase letters indicate significant differences (P<0.05) between <span class="Chemical">CO2 treatments for each tree class="Species">species × CT combinati<class="Species">span class="Species">on.

Invertebrate Responses

For both tree species in the choice and no-choice c<span class="Species">ontrol arenas, disc mass loss in the absence of invertebrates was unaffected by <class="Species">span class="Chemical">CO2 treatment and conditioning type (P>0.05). Litter mass loss in the presence of invertebrates was therefore assumed to be a result of invertebrate feeding alone.

In the choice test, leaf palatability affected invertebrate feeding, but this was dependent class="Species">on tree class="Species">species. Birch litter c<class="Species">span class="Species">onsumption was higher for ambient- than elevated-CO2 discs overall (F 1,72 = 10.48, P = 0.002); there was no effect of CO2 on consumption of alder discs (F 1,72 = 187.21, P = 0.34). Consumption also varied between invertebrate species (alder, F 7,72 = 0.92, P<0.001; birch, F 7,72 = 30.05, P<0.001). The effect of CO2 on birch consumption varied by invertebrate species (F 7,72 = 3.44, P = 0.003), where O. albicorne preferred ambient-CO2 discs (estimate = 1.29 mg d−1, P<0.001; Fig. 2B). The effect of CO2 on litter preference did not vary between invertebrates feeding on alder (F 1,72 = 0.5, P = 0.83; Fig. 2A). When grouped, aquatic species preferred ambient-CO2 birch discs over those grown under elevated CO2 (estimate = 1.09 mg d−1, P = 0.008), but no other preferences were exhibited (aquatic species fed alder, estimate = 0.02 mg d−1, P = 0.585; terrestrial species fed alder, estimate = 0.03 mg d−1, P = 0.496; terrestrial species fed birch, estimate = 0.06 mg d−1, P = 0.061).

Figure 2

Effects of CO2 treatment on feeding responses of each invertebrate species.

The mean litter consumption (±1 SE) of each invertebrate species is shown for (A) alder and (B) birch in the choice test, and (C) alder and (D) birch in the no-choice test. Asterisks indicate significant differences between CO2 treatments within each invertebrate species (***P<0.001). Species are arranged by habitat of origin: aquatic species are Asellus aquaticus (Aa), Gammarus pulex (Gp), Odontocerum albicorne (Oa) and Sericostoma personatum (Sp); terrestrial species are Blaniulus guttulatus (Bg), Oniscus asellus (On), Porcellio scaber (Ps) and Tachypodoiulus niger (Tn).

The mean litter c<span class="Species">onsumpti<class="Species">span class="Species">on (±1 SE) of each invertebrate species is shown for (A) alder and (B) birch in the choice test, and (C) alder and (D) birch in the no-choice test. Asterisks indicate significant differences between CO2 treatments within each invertebrate species (***P<0.001). Species are arranged by habitat of origin: aquatic species are Asellus aquaticus (Aa), Gammarus pulex (Gp), Odontocerum albicorne (Oa) and Sericostoma personatum (Sp); terrestrial species are Blaniulus guttulatus (Bg), Oniscus asellus (On), Porcellio scaber (Ps) and Tachypodoiulus niger (Tn).

In the no-choice test, c<span class="Species">onsumpti<class="Species">span class="Species">on rates were higher when invertebrates fed on ambient- rather than elevated-CO2 birch discs (F 1,64 = 6.39, P = 0.014). The trend was consistent across all invertebrate species, but no individual species showed a significant response (CO2 treatment × invertebrate species: F 7,64 = 0.341, P = 0.932; Fig. 2D). This overall effect of CO2 did not occur in alder leaves (F 1,64 = 3.6, P = 0.062), but the effect of CO2 varied significantly between species (F 7,64 = 4.56, P<0.001); more of the elevated-CO2 discs were consumed by G. pulex (estimate = 2.89 mg, P = 0.002) and O. albicorne (estimate = 3.22 mg, P<0.001), while O. asellus consumed more of the ambient-CO2 discs (estimate = 2.86 mg, P = 0.0022; Fig. 2C). When grouped by habitat, aquatic invertebrates ate more elevated-CO2 than ambient-CO2 alder (estimate = 1.965 mg, P<0.001) but there was no effect on birch (estimate = 0.1 mg, P = 0.073). CO2 treatment had no effect on consumption by terrestrial species fed either alder (estimate = 0.22 mg, P = 0.306) or birch (estimate = 0.1 mg, P = 0.085).

Discussion

Elevated atmospheric <span class="Chemical">CO2 and microbial c<class="Species">span class="Species">onditioning type modified leaf litter chemistry, though effects differed between tree species (supporting Hypothesis 1). Individual invertebrate species varied in their responses, suggesting that caution has to be taken when extrapolating general trends from single-species studies.

Elevated atmospheric class="Chemical">CO2 reduced birch litter quality: the c<class="Species">span class="Species">oncentration of nitrogen decreased and the C/N ratio increased, regardless of conditioning type. Most species did not respond to this change; O. albicorne was the only species with behaviour that supported Hypothesis 2, showing a strong preference for ambient-CO2 litter. Prior work supports this response: Ferreira et al. [13] showed that low C/N ratios reduced birch litter consumption by the caddisfly Sericostoma vittatum Rambur, while Cotrufo et al. [17] found that the woodlouse P. scaber preferred high quality (lower C/N ratio and lignin concentration) Fraxinus excelsior Linnaeus litter grown under ambient CO2. Alder litter showed negligible chemical change as a result of elevated CO2, perhaps due to symbiosis with nitrogen-fixing bacteria that help maintain nutrient supplies [38]. Unexpectedly, a slight increase in quality (increased nitrogen concentration) under elevated CO2 occurred when alder litter was conditioned terrestrially, but this did not result in any feeding preferences. Effects of conditioning type on litter chemistry may have occurred due to differences in chemical leaching and microorganism activity between aquatic and terrestrial environments [39]. Our data indicate that CO2 enrichment will affect litter palatability to macroinvertebrate detritivores as a result of chemical change, though these effects will be plant and invertebrate species-specific.

In the no-choice test, invertebrates were expected to compensate for low-quality litter by increasing cclass="Species">onsumpti<class="Species">span class="Species">on relative to high-quality litter. In contrast to this expectation, compensatory feeding was not observed in either tree species. There was no clear pattern for alder; invertebrate responses were highly idiosyncratic, with O. asellus being the only species to consume more of the low-quality resource (terrestrially-conditioned alder litter contained lower nitrogen when grown under ambient-CO2). Hättenschwiler et al. [18] detected a similar compensatory response for O. asellus and another woodlouse, P. scaber: higher consumption rates were recorded on low-quality, CO2-enriched F. sylvatica litter (low nitrogen concentration, high C/N ratio). The current study showed that G. pulex and O. albicorne consumed more elevated-CO2 than ambient-CO2 alder, despite no observed chemical differences. It is possible that elevated CO2 reduced litter palatability by altering chemical constituents that were not quantified here, such as secondary metabolites. For example, phenolics and tannins have been shown to be affected by CO2 levels [40]. Birch litter responses appeared less idiosyncratic, with no individual species increasing consumption of elevated-CO2 litter. These results suggest that litter species identity determines the predictability of invertebrate feeding responses, but that compensatory feeding is not a unifying trend amongst detritivorous macroinvertebrates.

Feeding rates may have varied due to increased handling times associated with low quality birch litter (e.g. [20]), or because of differences in species’ body chemistry and their ability to cope with elemental imbalances with class="Chemical">CO2-enriched resources [41], [42]. Heterotrophs, such as the detritivores in our study, tend to maintain c<class="Species">span class="Species">onstant body elemental composition [43] and may alter feeding behaviour to achieve optimum chemical balance. Our results show that individual invertebrate species rarely demonstrated significant responses to CO2 treatments in either test. This suggests that although individual species responses appear idiosyncratic, when considered as a whole, the invertebrate community generally shows consistent and predictable behavioural and functional responses to litter chemical changes induced by elevated CO2.

Altered c<span class="Species">onsumpti<class="Species">span class="Species">on of litter by macroinvertebrates will affect energy release from detritus, in turn affecting secondary production, and food-web structure and functioning [5]. Specifically, on the basis of invertebrate responses in our study, mineralisation of carbon and nutrients could slow down in forests dominated by birch or other tree species with similar chemistry. This is reinforced by our observations of high lignin/N and C/N ratios of elevated-CO2 birch litter, which are predictors for slow decomposition rates [44]. Conversely, stands containing a lot of alder, or other species with lower C/N ratios, may show little response in terms of detrital processing and nutrient turnover. Differences between tree species make it difficult to predict overall decomposition rates, a task made more difficult by the prevalence of litter mixtures in temperate deciduous forests, which tend to exhibit non-additive decay [45].

Changes to litter quality as a result of elevated <span class="Chemical">CO2 may also affect invertebrate community compositi<class="Species">span class="Species">on, a potentially important determinant of decomposition rates [19]. This could be caused by changes to food selection [46] and increased patchiness of resource quality in litter mixtures on the forest floor [47]. Differential changes to feeding rates may alter competitive dynamics between invertebrate species, with advantages for species whose dietary breadth extends beyond leaf litter, such as G. pulex and S. personatum [48], [49].

Our study provides, to date, the broadest assessment of detritivorous invertebrate species’ feeding respclass="Species">onses to <class="Species">span class="Chemical">CO2-enriched litter, improving our mechanistic understanding of a key ecosystem process in temperate woodland ecosystems. Future elevations of atmospheric CO2 are predicted to affect the breakdown of detritus indirectly by reducing leaf litter quality for macroinvertebrate detritivores. The study highlights that this process is highly tree species-specific, and there will be strong responses in some forest stands and minimal effects in others. Identifying the mechanisms governing such ecosystem variation in functional responses to climate change is essential if we are to predict the consequences of elevated CO2 for forest carbon dynamics and nutrient cycling at regional and landscape-scales.