Recalcitrant Litter Research Articles

Snowmelt timing affects short‐term decomposition rates in an alpine snowbed

AbstractAlpine snowbed communities are characterized as having areas of longer lasting snow cover duration compared with the surrounding landscape. The predictable accumulation of deep and long‐lasting snow on lee side ridges drives a unique ecology, providing stable microclimatic conditions under the snow through winter, supplying meltwater in spring, and controlling many biological processes. The timing and rate of plant litter decomposition are key controls on the nutrient balance of snowbed communities, and are thought to be strongly driven by snow dynamics. However, little is known about how the patterns and timing of snowmelt affect decomposition, nor how long these effects last into the growing season. We investigated the influence of snowmelt timing on decomposition rates across an alpine snowbed community by burying standardized plant litter (rooibos and green tea), at three incubation times (whole year, winter+spring, and summer), across three snowmelt zones. Decomposition rate (as percent mass loss of tea) was significantly higher in early‐melting zones compared to late‐melting zones, particularly for the recalcitrant litter (rooibos tea). Decomposition was also affected by the season(s) of incubation and was greatest where tea was buried for the whole year, or only over summer, with winter + spring only incubations decomposing the least. However, decomposition was more strongly influenced by litter quality (type of tea) than either the timing of snowmelt or seasonality. These results provide further understanding about how changes to the timing of snowmelt may in turn transform these rare and unique plant communities.

Differential responses of litter decomposition in the air and on the soil surface to shrub encroachment in a graminoid-dominated temperate wetland

Graminoid-dominated wetlands have been subjected to widespread shrub encroachment, yet the effect of this shift in species composition on litter decomposition remains unclear, especially in the standing-dead stage. We collected labile (Deyeuxia angustifolia) and recalcitrant (Carex schmidtii) graminoid leaf litter from a freshwater wetland in northeast China, and used the litterbag method to characterize litter ash-free dry mass (AFDM) loss and nitrogen (N) release in the air and on the soil surface in open wetlands and two shrub islands (Salix floderusii producing phenol-poor litter and Betula fruticosa producing phenol-rich litter) over 360 days of decomposition. Litter decomposition in the air and on the soil surface responded differentially to shrub expansion. In the air, AFDM loss and N release for both labile and recalcitrant litter were often lower in shrub islands than in open wetlands. On the soil surface, labile litter decomposition was decelerated in the presence of shrubs with phenol-rich litter, but accelerated in the presence of shrubs with phenol-poor litter. Despite the absence of significant differences in N release, recalcitrant litter AFDM loss was lower in shrub islands than in open wetlands after 360 days of decomposition on the soil surface. Shrub encroachment retards litter decomposition in the air, but the effects on litter decomposition on the soil surface vary with shrub type and litter degradability in graminoid-dominated wetlands. Moreover, these findings emphasize that standing litter decomposition should be considered to enhance our understanding of shrub encroachment effects on litter decomposition in these wetlands.

Soil microbial functioning and organic carbon storage: can complex timber tree stands mimic natural forests?

Conversion of natural forest to anthropogenic land use systems (LUS) often leads to considerable loss of carbon, however, proper management of these LUS may reverse the trend. A study was conducted in a semi-deciduous forest zone of Côte d’Ivoire to assess soil microbial functioning and soil organic carbon (SOC) stocks in varying tree stands, and to determine whether complex tree stands can mimic the natural forest in terms of these soil attributes. Tree plantations studied were monocultures of teak (Tectona grandis) and full-sun cocoa (Theobroma cacao L.), and a mixture of four tree species (MTS) with Tectona grandis, Gmelina arborea, Terminalia ivoriensis and Terminalia superba. An adjacent natural forest was considered as the reference. Each of these LUS had five replicate stands where soil (0–10 cm depth) samples were taken for physico-chemical parameters and microbial biomass-C (MBC), microbial activities, MBC/SOC ratio and metabolic quotient (qCO2). SOC and total N stocks were also calculated. The C mineralization rate (mg C–CO2 kg−1) and mineral N concentration (mg kg−1) drastically declined in the monocultures of cocoa (154.9 ± 29.3 and 49.8 ± 9.8, respectively) and teak (179.6 ± 27.1 and 54.1 ± 7.3) compared to the natural forest (258.4 ± 21.9 and 108.7 ± 12). However, values in MTS (194.7 ± 24.6 and 105.4 ± 7.4) were not significantly different from those in the natural forest. Similarly, SOC stocks in MTS (28.8 ± 1.9 Mg ha−1) were not significantly different from those recorded in the natural forest (32.9 ± 1.7 Mg ha−1) whereas teak (25.4 ± 1.7 Mg ha−1) and cocoa (23.1 ± 3.4 Mg ha−1) exhibited significantly lower values. Despite the acidic soil and recalcitrant litter conditions, increased MBC/SOC ratio and decreased qCO2 were recorded in the monocrops, suggesting a probable increase in the fungi/bacteria ratio. The complex MTS stand was found to mimic the natural forest in terms of soil microbial activity and organic status, due to the provision of a diversity of litter quality, which may serve as a basis for developing a climate smart timber system in West and Central Africa.

Fertilization and Tree Species Influence on Stable Aggregates in Forest Soil

Background and objectives: aggregation and structure play key roles in the water-holding capacity and stability of soils and are important for the physical protection and storage of soil carbon (C). Forest soils are an important sink of ecosystem C, though the capacity to store C may be disrupted by the elevated atmospheric deposition of nitrogen (N) and sulfur (S) compounds by dispersion of soil aggregates via acidification or altered microbial activity. Furthermore, dominant tree species and the lability of litter they produce can influence aggregation processes. Materials and methods: we measured water-stable aggregate size distribution and aggregate-associated organic matter (OM) content in soils from two watersheds and beneath four hardwood species at the USDA Forest Service Fernow Experimental Forest in West Virginia, USA, where one watershed has received (NH4)2SO4 fertilizer since 1989 and one is a reference/control of similar stand age. Bulk soil OM, pH, and permanganate oxidizable carbon (POXC) were also measured. Research highlights: fertilized soil exhibited decreased macro-aggregate formation and a greater proportion of smaller micro-aggregates or unassociated clay minerals, particularly in the B-horizon. This shift in aggregation to soil more dominated by the smallest (<53 µm) fraction is associated with both acidification (soil pH) and increased microbially processed C (POXC) in fertilized soil. Intra-aggregate OM was also depleted in the fertilized soil (52% less OM in the 53–2000 µm fractions), most strongly in subsurface B-horizon soil. We also document that tree species can influence soil aggregation, as soil beneath species with more labile litter contained more OM in the micro-aggregate size class (<250 µm), especially in the fertilized watershed, while species with more recalcitrant litter promoted more OM in the macro-aggregate size classes (500–2000 µm) in the reference watershed. Conclusions: long-term fertilization, and likely historic atmospheric deposition, of forest soils has weakened macro-aggregation formation, with implications for soil stability, hydrology, and storage of belowground C.

Herb litter mediates tree litter decomposition and soil fauna composition

Leaf litter from forest understorey species has very different characteristics (e.g. a higher nutrient content) which are regarded to be more palatable and digestible for the decomposer community compared to litter from the overstorey trees. Consequently, the decomposition of tree litter may show different dynamics when mixed with litter from herb species. To study the effect of understorey litter on tree litter decomposition, we set up a litterbag experiment in a temperate mixed deciduous forest in Belgium. We investigated the effect of spring (Anemona nemorosa) and summer (Aegopodium podagaria) herb litter on the litter decomposition of five native tree species that differ in litter quality. The spring and summer withering herb litters were added above the tree litters. After 742 days of in situ incubation, mass loss (40%–100%) and the decomposition rate (0.27–1.68 yr−1) of the litter from the five tree species were positively correlated with litter quality. The addition of summer herb litter resulted in increased mass loss and decomposition rates (k) of the underlying recalcitrant tree litter. In addition, summer herb litter increased nutrient concentration in the short-term as well as the abundance of soil fauna during litter decomposition. In contrast, spring herb litter slightly inhibited tree litter decomposition. Structural equation models showed that the effect of understorey litter on decomposition rates of tree litter acted via nutrient leaching towards the tree litter layer rather than via the alteration of soil fauna (detritivores and omnivores) abundance. Our results show that litter from understorey vegetation can alter nutrient availability in recalcitrant litters (Fagus sylvatica and Quercus robur) via increased nitrogen (N), calcium (Ca), magnesium (Mg), phosphorus (P) and potassium (K) concentrations by 9%, 21%, 22%, 41%, and 315% respectively after the summer herb litter addition. The effect on tree litter decomposition strongly depends on herb species identity and future research should focus on the generality of the observed patterns. We conclude that herb litter has a major role in accelerating carbon sequestration and nutrient cycling in temperate forests.

Teatime in the Serengeti: macrodetritivores sustain recalcitrant plant litter decomposition across human-modified tropical savannahs

Background and aimsIntensification of savannah land-use is predicted to negatively influence soil biodiversity and functioning such as litter decomposition by detritivores. Loss of macrodetritivores, particularly termites, may be problematic in drier savannahs due to the capacity of macrodetritivores to sustain litter decomposition. Here we investigate how human land-use and spatiotemporal rainfall influence the contribution of macrodetritivores to plant litter decomposition.MethodsWe measured decomposition using globally standardized litter: labile green and recalcitrant rooibos tea litter. The contribution of macrodetritivores to litter decomposition was determined through exclusion using meshed litterbags. Litter decomposition was determined in agricultural land, pastureland and wildlife protected areas during both wet and dry seasons and in mesic and wet rainfall regions across the borders of the Serengeti National Park, Tanzania.ResultsMacrodetritivores consumed recalcitrant rooibos and mainly avoided labile green tea litter. On average macrodetritivores enhanced recalcitrant litter decomposition by 22%, but litter mass loss varied across land-uses, typically being higher on agricultural and pastureland compared to wildlife protected areas, and was sustained during periods of water scarcity. However, we observed instances of higher decomposition of recalcitrant litter by macrodetritivores in wildlife protected areas. In contrast, litter decomposition by microbes and microdetritivores was more constrained by seasonal and regional water availability with a minor influence of land-use.ConclusionWe found that moderate human-modification of savannahs is compatible with macrodetritivore litter decomposition. As savannahs become more intensely used by humans, raising ecological awareness among agropastoralist is required to ensure continued contribution of macrodetritivores to litter decomposition.

The consistency of home-field advantage effects with varying climate conditions

The home-field advantage (HFA) hypothesis predicts that plant litters decompose faster in the vicinity of the plant from which they are derived than away from ‘home’. Although we are aware that both litter quality and the efficiency of decomposers within soil communities (hereafter, “soil ability”) are important drivers of HFA effects, we know little on variation in HFA effects and underlying processes of these changes across years with varying climates. In order to address these questions, we repeated four years with varying climates (two mild years, 2016 & 2018 and two stressed years, 2017 & 2019) a reciprocal transplant experiment with four litter types, crossing vegetation structure and soil fertility contrasts of both litter and ecosystems (forest and grassland litters from both fertile and unfertile soils). We used the Decomposer Ability Regression Test for both types of years to search for evidence of HFA effects and conducted ANOVAs on the cross effects of vegetation structure and soil fertility contrasts to further identifying the underlying processes of changes in HFA across climates. HFA effects were only significant for the most recalcitrant litter of Pinus pinaster, and much more significant during the mild years. Litter quality was stable across species and years, whereas differences in soil ability among ecosystems of the four species were strongly affected by climate, with highly significant differences among ecosystems occurring only during the mild years. Changes in soil ability across climates (decreasing HFA effects during stressed years) were explained by an overall increase in litter decomposition in forests as compared to grasslands during the stressed years, due to the facilitation of soil decomposers by forest canopies. Our study showed that HFA effects vary across climates as a result of changes in soil ability and that forest canopies may buffer the negative effects on litter decomposition of increasing stress with climate change.

Temporal dynamics of litter quality, soil properties and microbial strategies as main drivers of the priming effect

The priming effect describes the change in the rate of soil organic matter (SOM) mineralisation due to the addition of fresh organic matter (FOM), and is thus central to the evaluation of carbon fluxes in terrestrial ecosystems. However, little is known about how litter quality interact with soil properties and microbial communities to explain the priming effect. Here, we provide new empirical evidence showing how litter, soil and microbial parameters control FOM and SOM decomposition. We monitored the kinetics of carbon fluxes over 202 days and at 23 dates after additions of two 13C-labelled plant litters of contrasted quality (nutrient-rich and labile litter [i.e., high quality] vs nutrient-poor and recalcitrant litter [i.e., low quality]) using four soil types originating from different ecosystems (forest, grassland, cropland and plantation). We found that the priming effect varied over time with: (i) an ‘immediate’ priming effect after the addition of high quality litter, which rapidly decreases when nitrogen is available; and (ii) a ‘delayed’ priming effect after the addition of low quality litter, which increases continuously when nitrogen is depleted. The ‘immediate’ priming was mainly related to the decomposition of accessible carbon compounds, whereas the ‘delayed priming’ was mainly related to the decomposition of recalcitrant carbon compounds. These results were linked to changes in microbial resource acquisition strategies as indicated by shifts in enzyme activities from hydrolytic to oxidative and nitrogen-related enzymes, therefore suggesting that two series of mechanisms act in succession at different time scales in relation to litter quality and nitrogen availability. Furthermore, our data demonstrate that incorporating carbon in the soil via the increase in soil microbial biomass can counterbalance carbon losses via priming effect. This highlights the importance to assess the balance between the carbon outputs and inputs to determine whether terrestrial ecosystems act as a sink or a source of carbon. We conclude that shifts in litter quality, nutrient availability and microbial resource acquisition strategies need to be taken into consideration for accurate assessment of the variability of the priming effect from the short to the long term.

Soil Microbial Community and Litter Quality Controls on Decomposition Across a Tropical Forest Disturbance Gradient

Industrial logging and agricultural expansion are driving rapid transformations of tropical ecosystems, modifying patterns in above-ground plant and below-ground microbial communities. However, the extent to which these changes in biodiversity drive modifications of ecosystem process rates such as leaf litter decomposition is poorly understood. To determine the relative effects of changes to the chemical quality of litter and shifts in microbial decomposers on leaf litter decomposition rates, we performed a controlled, reciprocal transplant, litter decomposition experiment across a tropical land-use disturbance gradient. Litter mixtures and soils were collected from old growth forest, moderately logged forest, heavily logged forest, and oil palm plantation in Sabah, Malaysia, and combined in a fully crossed, factorial microcosm experiment maintained under controlled environmental conditions. We found that whilst litter quality was the most important predictor of litter mass loss, soil origin was also significant, explaining between 5.17 and 15.43% of total variation. Microbial decomposer communities from old growth forest had greater functional breadth relative to those from logged forests and oil palm plantation as all litter types decomposed faster when combined with old growth soil. The most chemically recalcitrant litter (lowest N, highest lignin, lignin:N, and C:N ratio) from moderate logged forest decomposed faster when combined with its “home” soil (Home-Field Advantage) whilst the most labile litter from oil palm decomposed slowest when combined with its “home” soil. This was correlated with lower total soil microbial biomass. Taken together, these findings demonstrate that whilst litter quality regulated rates of litter decomposition across the disturbance gradient, soil microbial decomposer communities were functionally dissimilar between land uses and explained a significant proportion of variation. The impact of disturbance on soil, including microbial community structure, should be considered alongside changes to plant communities when assessing effects on crucial ecosystem processes such as decomposition.

Optical traits perform equally well as directly‐measured functional traits in explaining the impact of an invasive plant on litter decomposition

Abstract Functional traits can help elucidate and predict the impact of invasive plant species on ecosystem functioning. Yet, this approach requires comprehensive and labour‐intensive trait collection campaigns, covering intraspecific trait variation of both the invader and native species in the invaded community. One potential way to overcome these logistic constraints is using hyperspectral remote sensing technology to efficiently quantify functional trait values. Although such spectrally derived or ‘optical’ traits are known to closely link to directly measured functional traits, little research has explored how well these optical traits perform in assessing invader‐induced ecosystem impact. Here, we explored the trait‐mediated impact of the invasive Rosa rugosa on litter decomposition and evaluated whether optical traits perform equally well as directly measured traits in predicting litter decomposition variation. We collected data on species‐specific functional traits, leaf hyperspectral reflectance and standardized ‘tea bag index’ litter decomposition across 25 invaded and 25 uninvaded coastal grassland plots. The selected traits were all potentially related to litter decomposition and covered the leaf economics spectrum, additional leaf structural components and competitive ability. Optical traits were quantified through a combination of a physical radiative transfer model inversion and vegetation indices calculations. Invasion significantly increased the stabilization factor, i.e. the amount of resulting recalcitrant litter. Invader impact on litter decomposition could be entirely explained by changes it induced in the functional traits of the native community, rather than by the invader's traits itself. More specifically, the invader pushed the invaded community towards traits associated with high litter quality. Optical traits performed equally well as directly measured traits in explaining the invasion impact on the stabilization factor (R2 = 41.9% vs. 38.5%). Furthermore, the interpretation of the results based on optical traits resulted in a similar functional understanding of the invader impact. Synthesis. Our results indicate the potential of hyperspectral data to explain changes in ecosystem functioning. The combination of radiative transfer models and vegetation indices allowed to extract all relevant trait information from the hyperspectral data. This framework thus presents a practical shortcut to assess relevant leaf traits, requiring only a limited amount of field trait measurements.

The Falkland Islands’ palaeoecological response to millennial‐scale climate perturbations during the Pleistocene–Holocene transition: Implications for future vegetation stability in the southern ocean islands

ABSTRACTOceanic island flora is vulnerable to future climate warming, which is likely to promote changes in vegetation composition, and invasion of non‐native species. Sub‐Antarctic islands are predicted to experience rapid warming during the next century; therefore, establishing trajectories of change in vegetation communities is essential for developing conservation strategies to preserve biological diversity. We present a Late‐glacial‐early Holocene (16 500–6450 cal a bp) palaeoecological record from Hooker's Point, Falkland Islands (Islas Malvinas), South Atlantic. This period spans the Pleistocene‐Holocene transition, providing insight into biological responses to abrupt climate change. Pollen and plant macrofossil records appear insensitive to climatic cooling during the Late‐glacial, but undergo rapid turnover in response to regional warming. The absence of trees throughout the Late‐glacial‐early Holocene enables the recognition of far‐travelled pollen from southern South America. The first occurrence of Nothofagus (southern beech) may reflect changes in the strength and/or position of the Southern Westerly Wind Belt during the Late‐glacial period. Peat inception and accumulation at Hooker's Point is likely to be promoted by the recalcitrant litter of wind‐adapted flora. This recalcitrant litter helps to explain widespread peatland development in a comparatively dry environment, and suggests that wind‐adapted peatlands can remain carbon sinks even under low precipitation regimes. © 2019 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd.

Leaf litter identity alters the timing of lotic nutrient dynamics

Abstract The effects of resource quality on ecosystems can shift through time based on preferential use and elemental needs of biotic consumers. For example, leaf litter decomposition rates are strongly controlled by initial litter quality, where labile litter is processed and depleted more quickly than recalcitrant litters. We examined the effect of this processing continuum on stream nutrient dynamics. We added one of four different litter compositions differing in litter quality (cottonwood [Populus deltoides], labile; sycamore [Platanus occidentalis], recalcitrant; bur oak [Quercus macrocarpa], recalcitrant; and mixed [equivalent mixture of previous three species]) to 12 large (c. 20 m long, with riffle, glide and pool sections) outdoor stream mesocosms to assess the effect of litter species composition on whole‐stream nutrient uptake. Nutrients were dosed once weekly for 8 weeks to measure uptake of NH4–N, NO3–N, and PO4–P. We also measured changes in litter C, N, and P content on days 28 and 56 of the study. Nutrient uptake rates were highly variable, but occasionally very different among litter treatments (c. 5× between highest and lowest uptake rates by species). Uptake rates were generally greatest in cottonwood (labile) streams early in the study. However, during the last 4 weeks of the study, bur oak streams (recalcitrant) took up more nutrients than cottonwood streams, resulting in more cumulative NO3–N uptake in bur oak than in cottonwood streams. Cumulative NO3–N uptake was greater in mixed streams than expected (non‐additive) on two dates of measurement, but was generally additive. Changes in litter nutrient content largely corroborated nutrient uptake patterns, suggesting strong N immobilisation early in the study and some N mineralisation later in the study. P was strongly retained by most litters, but especially bur oak. Nutrient content of litter also largely changed additively, suggesting minimal evidence for non‐additive diversity effects on nutrient source/sink status. Our results demonstrate that litter species identity can have whole‐ecosystem effects on stream nutrient dynamics, with important implications for the form and fate of nutrients exported downstream. Further, diverse litter assemblages may serve as temporal stabilisers of ecosystem processes, such as nutrient sequestration, due to microbial nutrient requirements and differential decomposition rates, or the classic litter processing continuum.

Litter of the invasive shrub Rhododendron ponticum (Ericaceae) modifies the decomposition rate of native UK woodland litter

Invasive alien plants are a worldwide problem, causing substantial damage to biodiversity as well as economies. Recent studies suggest invasive plants may also alter fundamental ecosystem processes such as nutrient and carbon cycling in soil by depositing chemically distinct leaf litter. Here, we used laboratory microcosms to test whether the chemical properties of Rhododendron ponticum litter, an invasive shrub in Britain, lead to slower decomposition than that of native (or naturalised) species with labile litter (Acer pseudoplatanus and Fraxinus excelsior), but not relative to the recalcitrant litter of Quercus petraea. Leading from this, we hypothesised that the labile native litter decomposition rate is reduced when mixed with R. ponticum litter in non-additive responses, with the strength of these responses increasing with the proportion of R. ponticum in litter mixes (25%, 50% and 75% R. ponticum). Over the incubation period, the decomposition (measured as the microbial respiration rate) of unmixed R. ponticum litter was significantly lower than that of A. pseudoplatanus and F. excelsior, but not Q. petraea. When mixed with R. ponticum (50%), F. excelsior litter decomposition was slowed, whilst no effect was seen for Q. petraea. However, A. pseudoplatanus litter decomposition was enhanced, contrary to expectation. The strength of the non-additive decomposition responses did not vary with different proportions of R. ponticum to the other species, with only the 50% mixtures showing significant non-additive respiration rates. Litter chemical properties were highly associated with decomposition rates, with both phenolic content and C:N ratio negatively correlated with microbial respiration. To test the influence of phenolics on litter decomposition, leachates of R. ponticum litter with phenolics present or removed (via activated carbon) were added to microcosms containing the native species litter. Microbial respiration in F. excelsior microcosms was lower when R. ponticum leachate contained phenolics. For A. pseudoplatanus and Q. petraea litter, no effect of leachate treatment was observed. Our results show that invasive litter chemistry can alter the decomposition of native litter, with the impact varying between species. Altered decomposition rates could cause plant-soil feedbacks, leading to altered soil nutrient concentrations. The novel soil conditions may favour the invader, increasing its dominance, whilst negatively influencing native species possessing greater nutrient demands.

Solar UV-A radiation and blue light enhance tree leaf litter decomposition in a temperate forest

Sunlight can accelerate the decomposition process through an ensemble of direct and indirect processes known as photodegradation. Although photodegradation is widely studied in arid environments, there have been few studies in temperate regions. This experiment investigated how exposure to solar radiation, and specifically UV-B, UV-A, and blue light, affects leaf litter decomposition under a temperate forest canopy in France. For this purpose, we employed custom-made litterbags built using filters that attenuated different regions of the solar spectrum. Litter mass loss and carbon to nitrogen (C:N) ratio of three species: European ash (Fraxinus excelsior), European beech (Fagus sylvatica) and pedunculate oak (Quercus robur), differing in their leaf traits and decomposition rate, were analysed over a period of 7–10 months. Over the entire period, the effect of treatments attenuating blue light and solar UV radiation on leaf litter decomposition was similar to that of our dark treatment, where litter lost 20–30% less mass and had a lower C:N ratio than under the full-spectrum treatment. Moreover, decomposition was affected more by the filter treatment than mesh size, which controlled access by mesofauna. The effect of filter treatment differed among the three species and appeared to depend on litter quality (and especially C:N), producing the greatest effect in recalcitrant litter (F. sylvatica). Even under the reduced irradiance found in the understorey of a temperate forest, UV radiation and blue light remain important in accelerating surface litter decomposition.

Resource-ratio theory predicts mycorrhizal control of litter decomposition.

Ecosystems with ectomycorrhizal plants have high soil carbon:nitrogen ratios, but it is not clear why. The Gadgil effect, where competition between ectomycorrhizal and saprotrophic fungi for nitrogen slows litter decomposition, may increase soil carbon. However, experimental evidence for the Gadgil effect is equivocal. Here, we apply resource-ratio theory to assess whether interguild fungal competition for different forms of organic nitrogen can affect litter decomposition. We focus on variation in resource input ratios and fungal resource use traits, and evaluate our model's predictions by synthesizing prior experimental literature examining ectomycorrhizal effects on litter decomposition. In our model, resource input ratios determined whether ectomycorrhizal fungi suppressed saprotrophic fungi. Recalcitrant litter inputs favored the former over the latter, allowing the Gadgil effect only when such inputs predominated. Although ectomycorrhizal fungi did not always hamper litter decomposition, ectomycorrhizal nitrogen uptake always increased carbon : nitrogen ratios in litter. Our meta-analysis of empirical studies supports our theoretical results: ectomycorrhizal fungi appear to slow decomposition of leaf litter only in forests where litter inputs are highly recalcitrant. We thus find that the specific contribution of the Gadgil effect to high soil carbon : nitrogen ratios in ectomycorrhizal ecosystems may be smaller than predicted previously.

Diverse recalcitrant substrates slow down decomposition of leaf litter from trees in a serpentine ecosystem

Litter decomposition in serpentine ecosystems has been studied solely addressing heavy metals like Ni. We aimed to show how leaf nutrients, cell-wall components and lamina mass per area and microbial communities affect litter decomposition in a cool-temperate serpentine ecosystem. Reciprocal transplant leaf-litter decomposition was observed in three tree species (Clethra barbinervis, Magnolia obovata, and Quercus serrata) common to serpentine and non-serpentine ecosystems. Thirteen litter quality variables of the initial litter and microbial respirations from decomposed litter were assessed. Litter from the serpentine ecosystem decomposed slower than that from the non-serpentine ecosystem. Compared with all species, C. barbinervis showed high heavy metal concentration, cellulose content, and lamina dry mass per area; M. obovata showed low Ca concentration; and Q. serrata exhibited high lignin/N ratio in litter from the serpentine ecosystem. Litter placed in the serpentine site decomposed faster than that placed in the non-serpentine site, but basal microbial respiration rates from litter did not differ between decomposition sites. Litter from serpentine ecosystems adversely influenced decomposition rate owing to its diverse recalcitrant litter quality. Our study suggested the feasibility of litter quality evaluation based on multiple litter traits for better-understanding of litter decomposition in serpentine ecosystems.

Priming effect of litter mineralization: the role of root exudate depends on its interactions with litter quality and soil condition

Root exudation can prime microbial synthesis of additional exoenzymes and consequently accelerate organic carbon (C) and nitrogen (N) mineralization. Such exudate induced priming effect (EPE) has been hypothesized to depend on exudate rate and stoichiometry. Little is known about how EPE would affect litter decomposition. We employed a microcosm experiment to evaluate the influence of root exudate on litter nutrient release and microbial enzyme functions. Leaf litters of Pinus massoniana, Quercus variabilis and Robinia pseudoacacia were incubated under two soil conditions (fertile versus barren). Solutions of chemicals often found in root exudates with contrasting C:N ratios were inoculated frequently into the microcosms to simulate exudation. By comparing with a water control, exudate effect was determined. In barren soils, exudates with C:N ratio of 10 significantly decelerated C loss of R. pseudoacacia, all N-containing exudates significantly enhanced the N-cycling related enzymes in decomposing Q. variabilis, while C-only exudate accelerated N loss of P. massoniana. In fertile soils, C-only exudate promoted the N-cycling related enzymes in decomposing R. pseudoacacia. A stoichiometric C:N constraint on microbial utilization of exudates arose in decomposing recalcitrant litters in barren soil. EPE and its stoichiometric constraint depend on interactions with litter quality and soil condition. The findings arouse the consequences of exudate rate and stoichiometry changes in determining soil nutrient balance.

Site Type Effect on Litter Decomposition Rates: A Three-Year Comparison of Decomposition Process between Spoil Heap and Forest Sites

Research Highlights: Direct comparison of leaf litter decomposition rates between harsh soil conditions of degraded lands and adjacent “closer to natural” forest areas has not been done before. Background and Objectives: We aimed to fill this knowledge gap by determining the differences in amounts of carbon and nitrogen released by species-specific litter depending on decomposition rates in various stand and habitat conditions, which enables selection of the most ecologically and economically appropriate (for fast soil organic layer development) tree species for afforestation of reclaimed lands. Materials and Methods: The study was conducted on the external spoil heap of the “Bełchatów” lignite mine (Central Poland) and adjacent forests. In December 2013, we established a litterbag experiment beneath the canopies of birch and pine stands. We used litter of Alnus glutinosa (Gaertn.), Betula pendula (Roth), Pinus sylvestris (L.), and Quercus robur (L.) collected ex situ, which we installed (after oven-drying) beneath the canopies of eight stands. The experiment lasted for three years (with sampling of three-month intervals). Results: Harsh soil conditions of degraded lands are unfavorable for litter mineralization. It was found that 23%–74% of decomposed materials were mineralized in spoil heap stands, whereas in forest stands these amounts ranged from 35%–83%. Litter of Q. robur in birch stands on the spoil heap is predicted to take 12 years longer for total decomposition than in forest stands of the same species. This hinders organic carbon turnover and could result in elongation of the time for full biological and economic reclamation of degraded lands. On the other hand, decomposition of relatively fast decomposable litter (A. glutinosa and B. pendula) in pine stands on the spoil heap was faster than in pine stands in forest sites (17% and 13% faster, respectively). We did not observe this trend for decomposition of more recalcitrant litter types of P. sylvestris and Q. robur. Conclusions: The results show the value of selective choice of tree species for afforestation of post-mining areas to accelerate the development of technogenic soil substrates. We recommend introducing all tree species studied in the cluster form of admixtures as all of them could bring some profits in ecological and economical reclamation.

No “Gadgil effect”: Temperate tree roots and soil lithology are effective predictors of wood decomposition

AbstractThe “Gadgil effect” hypothesizes that root associations may slow down decomposition through pre‐emptive competition. In the context of recalcitrant litter decomposition, specifically coarse wood debris, it is uncertain as to what is the relative importance of soil communities associated with living roots when compared to those without roots. Here, it is hypothesized that the presence of live roots and active photosynthates will enhance wood decomposition. To test this hypothesis, the presence or absence of temperate tree roots was used in this study. Sugar maple (Acer saccharum) and white oak (Quercus alba) roots were manipulated at three sites of either limestone or shale parent rock residuum. At each site, wood substrate was placed in soils beneath the canopy of either A. saccharum or Q. alba, while in the presence of roots (root+). At the same time, wood substrate was placed in the same soil community, but live root exposure was eliminated by trenching (root−). This eliminated active photosynthate supply to the soil microbial community. Results determined that live root exposure promoted faster decomposition and greater mycelial colonization of wood substrate. Also, sites of shale parent rock residuum had higher rates of decomposition in comparison with limestone parent rock residuum. Although additional work is needed to determine the extent in which roots and lithology can facilitate wood decomposition, these findings suggest that living roots impact decomposers and provide a pathway towards humus and soil organic matter formation.

Effects of snow pack reduction and drought on litter decomposition in subalpine grassland communities

In subalpine grasslands, litter decomposition controls soil nutrient availability and is highly sensitive to increasing intensity and frequency of extreme climate events, potentially impacting grasslands diversity and functioning. Here, we assessed the effects of early snowmelt and summer drought on decomposition, and how these were modulated by agricultural management. In a common garden, conservative and exploitative assembled communities were submitted for two years to combined snow removal and drought, and to cutting and fertilization. We measured decomposition rates of standard and native leaf material from each plant communities on their respective soils. We observed relatively weak climatic stress effects on decomposition rates. Management increased decomposition rates and remaining litter N contents through leaf quality improvement rather than changed soil biotic activity. Climate events impacted the decomposer community and limited litter N immobilization in conservative communities. Less recalcitrant litter of exploitative species facilitated decomposition and counterbalanced the negative effects of climate stress. Our results suggest that drought and earlier snowmelt could decrease N availability in subalpine grasslands due to reduced litter biomass and decomposition. In the long-term climate change may shift subalpine grasslands towards more conservative plant composition and lower soil fertility.