The Effect of Phosphorus Availability on Decomposition Dynamics in a Seasonal Lowland Amazonian Forest

Once the weathering of parent material ceases to supply significant inputs of phosphorus (P), vegetation depends largely on the decomposition of litter and soil organic matter and the associated mineralization of organic P forms to provide an adequate supply of this essential nutrient. At the same time, the decomposition of litter is often characterized by the immobilization of nutrients, suggesting that nutrient availability is a limiting factor for this process. Immobilization temporally decouples nutrient mineralization from decomposition and may play an important role in nutrient retention in low-nutrient ecosystems. In this study, we used a common substrate to study the effects of native soil P availability as well as artificially elevated P availability on litter decomposition rates in a lowland Amazonian rain forest on highly weathered soils. Although both available and total soil P pools varied almost three fold across treatments, there was no significant difference in decomposition rates among treatments. Decomposition was rapid in all treatments, with approximately 50% of the mass lost over the 11-month study period. Carbon (C) and nitrogen (N) remaining and C:N ratios were the most effective predictors of amount of mass remaining at each time point in all treatments. Fertilized treatments showed significant amounts of P immobilization (P < 0.001). By the final collection point, the remaining litter contained a quantity equivalent to two-thirds of the initial P and N, even though only half of the original mass remained. In these soils, immobilization of nutrients in the microbial biomass, late in the decomposition process, effectively prevents the loss of essential nutrients through leaching or occlusion in the mineral soil.     

Intraspecific litter diversity and nitrogen deposition affect nutrient dynamics and soil respiration

Anthropogenic forces are concurrently reducing biodiversity and altering terrestrial nutrient cycles. As natural populations decline, genetic diversity within single species also declines. The consequences of intraspecific genetic loss for ecosystem functions are poorly understood, and interactions among intraspecific diversity, nitrogen deposition, and nutrient cycling are unknown. We present results from an experiment that simulated both a decline in biodiversity and an increase in nitrogen deposition. In soil microcosms, we tested effects of variation in intraspecific litter diversity and nitrogen deposition on soil respiration and nitrogen leaching. Increases in intraspecific litter diversity increased soil respiration overall, with the greatest increases in respiration occurring under high nitrogen deposition. Nitrogen deposition increased the amount of inorganic nitrogen leached, while the amount of dissolved organic nitrogen leached was correlated with initial litter chemistry (lignin concentration) and remained independent of litter diversity and nitrogen deposition treatments. Our results demonstrate the potential for losses in genetic diversity to interact with other global environmental changes to influence terrestrial nutrient cycles.     

不同水分条件下毛果苔草枯落物分解及营养动态

于2009年5月至2010年5月采用分解袋法,研究了三江平原典型湿地植物毛果苔草枯落物分解对水分条件变化的响应,探讨了典型碟形洼地不同水位下枯落物分解1a时间内的分解速率与N、P等营养元素动态。分解1a内,无积水环境下枯落物失重率为34.99%,季节性积水环境下为27.28%,常年积水环境下随水位增加枯落物失重率分别为26.99%与30.67%,表明积水条件抑制了枯落物的分解。枯落物的分解随环境变化表现出阶段性特征,分解0—122 d内随水位增加枯落物失重率分别为16.09%、24.25%、23.53%与26.60%,即生长季内积水条件促进了枯落物有机质的分解及重量损失。而随实验进行,分解122—360 d内随水位增加毛果苔草枯落物的失重率分别为18.90%、3.02%、3.46%、4.03%,即在非生长季土壤冻融期积水条件抑制了枯落物分解(P<0.05)。水分条件对毛果苔草枯落物N元素的影响表现为积水条件促进生长季内枯落物的N固定,水位最高处毛果苔草N浓度显著高于无积水环境(P<0.05)。但进入冻融期后积水环境下枯落物N浓度与含量降低;其中季节性积水限制了枯落物的N积累能力,至分解360d时与初始值相比表现出明显的N释放(P=0.01)。毛果苔草枯落物分解61d时P出现富集,其中积水条件下P的富集作用增强,但与水位不相关。分解1a后毛果苔草枯落物表现为P的净释放,不同水分条件下枯落物P元素损失没有明显差异(P>0.05)。     

Response of degradative enzymes to N fertilization during litter decomposition in a subtropical forest through a microcosm experiment

With the continuing increase in human activities causing accelerating rates of anthropogenic nitrogen deposition inputs into forests, there is considerable interest in understanding the effects of nitrogen deposition on litter decomposition. Two dominant litters were chosen from Zijin Mountain in China: Quercus acutissima from a broad-leaved forest and Pinus massoniana from a coniferous forest. The litters were incubated in microcosms and treated with a gradient of nitrogen fertilization. During a 6-month incubation, changes in chemical composition (i.e., lignin, total carbohydrate, and nitrogen), litter mass losses, soil pH values, and the activities of degradative enzymes were determined. Results showed that medium-nitrogen and high-nitrogen fertilization significantly accelerated litter decomposition rates of leaves, while only the high-nitrogen fertilization significantly accelerated litter decomposition rates of needles. The results also showed that cellulase and nitrate reductase were primarily responsible for litter decomposition in the broad-leaved forest, while catalase, cellulase, and acid phosphatase were primarily responsible for litter decomposition in the coniferous forest under conditions of no N fertilization; catalase, cellulase, and acid phosphatase were primarily responsible for litter decomposition in the broad-leaved forest, while catalase, cellulase, invertase, and nitrate reductase were primarily responsible for litter decomposition in the coniferous forest under conditions of N fertilization. Nitrogen fertilization-stimulated litter decomposition was due to the fact that the activities of enzymes, particularly cellulase, were accelerated.     

Controls on leaching from coniferous forest floor microcosms

Summary Studies were conducted with coniferous forest floor microcosms to examine the potential influence of acid precipitation, temperature changes, and plant uptake upon the chemistry of soil leachate solutions. The experimental design included two temperatures and three different simulated throughfall chemistry treatments. When the acidity of throughfall inputs to the microscosms increased, the forest floors exhibited increased leaching losses of calcium, magensium, potassium, and ammonium. The fact that aluminum losses did not incrase correspondingly suggested that there may be a kinetic lag in the mobilization and leaching of aluminum. When microcosms were exposed to warmer temperatures, percolates showed increased leaching losses of calcium, potassium, ammonium, sulfate, nitrate, and organic anions. Forest floor microcosms exposed to simulated average field conditions behaved very much like field plots under the same environmental conditions; however, there were predictable differences in leaching losses between laboratory and field systems for those ions which are strongly controlled by plant uptake. In general, the exclusion of plant uptake from microcosms resulted in increased leaching of potassium, nitrate, ammonium. and sulfate relative to field plots.     

Short-Term Disappearance of Foliar Litter in Three Species Before and After a Hurricane1

Litter disappearance was examined before (1989) and after (1990) Hurricane Hugo in the Luquillo Experimental Forest, Puerto Rico using mesh litterbags containing abscised Cyrilla racemiflont or Dacryodes excelsa leaves or fresh Prestoea montana leaves. Biomass and nitrogen dynamics were compared among: (i) species; (ii) mid- and high elevation forest types; (iii) riparian and upland sites; and (iv) pre- and post-hurricane disturbed environments. Biomass disappearance was compared using multiple regression and negative exponential models in which the slopes were estimates of the decomposition rates subsequent to apparent leaching losses and the y-intercepts were indices of initial mass losses (leaching). Cyrilla racemiflora leaves with low nitrogen (0.39%) and high lignin (22.1%) content decayed at a low rate and immobilized available nitrogen. Dacryodes excelsa leaves had moderate nitrogen (0.67%) and lignin (16.6%) content, decayed at moderate rates, and maintained the initial nitrogen mass. Prestoea montana foliage had high nitrogen (1.76%) and moderate lignin (16.7%) content and rapidly lost both mass and nitrogen. There were no significant differences in litter disappearance and nitrogen dynamics among forest types and slope positions. Initial mass loss of C. racemiflora leaves was lower in 1990 but the subsequent decomposition rate did not change. Initial mass losses and the overall decomposition rates were lower in 1990 than in 1989 for Dacryodes excelsa. Dacryodes excelsa and C. racemiflora litter immobilized nitrogen in 1990 but released 10-15 percent of their initial nitrogen in 1989, whereas P. montana released nitrogen in both years (25-40%). Observed differences in litter disappearance rates between years may have been due to differences in the timing of precipitation. Foliar litter inputs during post-hurricane recovery of vegetation in Puerto Rico may serve to immobilize and conserve site nitrogen.     

塔里木沙漠公路防护林地表凋落物分解对施肥的响应

为探讨沙漠公路防护林地表凋落物的分解速率和养分释放动态对施肥的响应,采用凋落物分解袋法,对塔里木沙漠公路防护林地乔木状沙拐枣(Calligonum arborescens)同化枝、梭梭(Haloxylon ammodendron)同化枝和多枝柽柳(Tamarix ramosissima)枝凋落物在施肥处理下的分解及养分释放特征进行研究。结果表明:经过420d的分解,3种凋落物质量残留率在对照(不施肥)、施用氮肥、施用磷钾复合肥处理间存在显著性差异(P0.05)。乔木状沙拐枣同化枝、梭梭同化枝和多枝柽柳枝在对照处理下的质量残留率分别为56.95%、31.32%和50.24%。施肥处理下3种凋落物均呈现出梭梭同化枝分解速率最快,多枝柽柳枝次之,乔木状沙拐枣同化枝分解最慢。施用磷钾复合肥极显著提高了3种凋落物的分解速率(P0.01);施用氮肥则促进多枝柽柳枝的分解,抑制乔木状沙拐枣和梭梭同化枝的分解。凋落物分解过程中,对照组3种植物凋落物的C、N、P和K元素均呈现净释放状态;施肥后凋落物的N、P和K元素呈现出富集-释放的模式。凋落物初始P含量和C/N、C/P比值是分解初期的主导因素,初始K、木质素、纤维素含量和C/N、木质素/N比值是分解后期的主要控制因素。研究表明,施肥显著影响沙漠公路防护林地表凋落物的分解,增加防护林地表凋落物的养分归还量,延后养分释放的时间,改善塔里木沙漠公路防护林地的土壤肥力。凋落物初始C/N比值是预测塔里木沙漠凋落物分解的重要因素,且不同分解时期影响凋落物分解的初始化学组成有所差异。     

Hydrologic control of litter decomposition in seasonally flooded prairie marshes

The effect of seasonal inundation on the decomposition of emergent macrophyte litter (Scolochloa festucacea) was examined under experimental flooding regimes in a northern prairie marsh. Stem and leaf litter was subjected to six aboveground inundation treatments (ranging from never flooded to flooded April through October) and two belowground treatments (nonflooded and flooded April to August). Flooding increased the rate of mass loss from litter aboveground but retarded decay belowground. Aboveground, N concentration decreased and subsequently increased earlier in the longer flooded treatments, indicating that flooding decreased the time that litter remained in the leaching and immobilization phases of decay. Belowground, both flooded and nonflooded litter showed an initial rapid loss of N, but concentration and percent of original N remaining were greater in the nonflooded marsh throughout the first year. This suggested that more N was immobilized on litter under the nonflooded, more oxidizing soil conditions. Both N concentration and percent N remaining of belowground litter were greater in the flooded than the nonflooded marsh the second year, suggesting that N immobilization was enhanced after water-level drawdown. These results suggest different mechanisms by which flooding affects decomposition in different wetland environments. On the soil surface where oxygen is readily available, flooding accelerates decomposition by increasing moisture. Belowground, flooding creates anoxic conditions that slow decay. The typical hydrologic pattern in seasonally flooded prairie marshes of spring flooding followed by water-level drawdown in summer may maximize system decomposition rates by allowing rapid decomposition aboveground in standing water and by annually alleviating soil anoxia.     

Stemflow as a nutrient source for nikau palm (Rhopalostylis sapida) in a New Zealand forest

The hypothesis that stemflow is important in contributing to the nutrient requirements of nikau palm Rhopalostylis sapida individuals is examined. Stemflow directs many times more sodium, potassium, calcium and magnesium to the base of the palm than is required for annual net production. There are three sources for nutrients found in stemflow: (1) atmospheric inputs (wet and dry deposition); (2) foliar leaching; and (3) alien litter leaching and decomposition. Sources I and 3 together, representing nutrient input, contribute substantially more nutrients than are required for net annual production in mature individuals.     

Differential Controls of Water Input on Litter Decomposition and Nitrogen Dynamics in the Patagonian Steppe

Studies of the effects of precipitation on litter decomposition and nitrogen mineralization in arid and semiarid environments have demonstrated contradictory results. We conducted a manipulative experiment with rainout shelters in the semiarid Patagonian steppe, aimed at assessing the direct effects of water availability on litter decomposition and net nitrogen mineralization while isolating the indirect effects. We created four levels of precipitation input: control and three levels (30, 55 and 80%) of precipitation interception and we examined litter decomposition and nutrient release of a dominant grass species, Stipa speciosa, inorganic soil nitrogen, and in situ net nitrogen mineralization over two consecutive years. Litter decomposition rates (k, year⁻¹) varied significantly (P < 0.001) among precipitation interception treatments and were positively correlated with incoming annual precipitation (APPT, mm/year) (k = 0.0007 × APPT + 0.137). In contrast, net N mineralization was not correlated with incoming precipitation. Soil NO₃⁻ significantly decreased with increasing precipitation input, whereas soil NH₄⁺ concentration did not differ among precipitation interception treatments. Controls of water input on litter decomposition appear to be different from controls on N mineralization in the semiarid Patagonian steppe. We suggest that although water availability affects both the mineralization of C and N, it differentially affects the movement and fate of the inorganic products. A consequence of the accumulation of inorganic N during dry episodes is that periods of maximum water and soil nutrient availability may occur at different times. This asynchrony in the availability of N and water in the soil may explain the observed lags in the response of primary production to increases in water availability.     

Potential macro-detritivore range expansion into the subarctic stimulates litter decomposition: a new positive feedback mechanism to climate change?

As a result of low decomposition rates, high-latitude ecosystems store large amounts of carbon. Litter decomposition in these ecosystems is constrained by harsh abiotic conditions, but also by the absence of macro-detritivores. We have studied the potential effects of their climate change-driven northward range expansion on the decomposition of two contrasting subarctic litter types. Litter of Alnus incana and Betula pubescens was incubated in microcosms together with monocultures and all possible combinations of three functionally different macro-detritivores (the earthworm Lumbricus rubellus, isopod Oniscus asellus, and millipede Julus scandinavius). Our results show that these macro-detritivores stimulated decomposition, especially of the high-quality A. incana litter and that the macro-detritivores tested differed in their decomposition-stimulating effects, with earthworms having the largest influence. Decomposition processes increased with increasing number of macro-detritivore species, and positive net diveristy effects occurred in several macro-detritivore treatments. However, after correction for macro-detritivore biomass, all interspecific differences in macro-detritivore effects, as well as the positive effects of species number on subarctic litter decomposition disappeared. The net diversity effects also appeared to be driven by variation in biomass, with a possible exception of net diversity effects in mass loss. Based on these results, we conclude that the expected climate change-induced range expansion of macro-detritivores into subarctic regions is likely to result in accelerated decomposition rates. Our results also indicate that the magnitude of macro-detritivore effects on subarctic decomposition will mainly depend on macro-detritivore biomass, rather than on macro-detritivore species number or identity.     

Interactive effects of substrate,hydroperiod, and nutrients on seedling growth of Salix nigra and Taxodium distichum

The large river swamps of Louisiana have complex topography and hydrology, characterized by black willow (Salix nigra) dominance on accreting alluvial sediments and vast areas of baldcypress (Taxodium distichum) deepwater swamps with highly organic substrates. Seedling survival of these two wetland tree species is influenced by their growth rate in relation to the height and duration of annual flooding in riverine environments. This study examines the interactive effects of substrate, hydroperiod, and nutrients on growth rates of black willow and baldcypress seedlings. In a greenhouse experiment with a split-split-plot design, 1-year seedlings of black willow and baldcypress were subjected to two nutrient treatments (unfertilized versus fertilized), two hydroperiods (continuously flooded versus twice daily flooding/draining), and two substrates (sand versus commercial peat mix). Response variables included height, diameter, lateral branch count, biomass, and root:stem ratio. Black willow growth in height and diameter, as well as all biomass components, were significantly greater in peat substrate than in sand. Black willow showed a significant hydroperiod–nutrient interaction wherein fertilizer increased stem and root biomass under drained conditions, but flooded plants did not respond to fertilization. Baldcypress diameter and root biomass were higher in peat than in sand, and the same two variables increased with fertilization in flooded as well as drained treatments. These results can be used in Louisiana wetland forest models as inputs of seedling growth and survival, regeneration potential, and biomass accumulation rates of black willow and baldcypress.     

Little bluestem litter dynamics in Minnesota old fields

Summary We studied the decay and nitrogen dynamics of little bluestem (Schizachyrium scoparium) litter in fertilized and unfertilized Minnesota old fields, using the litterbag technique. Annual decay rates and nitrogen leaching losses during the first month of decay were highly correlated with N content of litter and not with fertilizer additions. After the first month of decay, nitrogen was immobilized for at least 18 months. In contrast to decay rates and early N leaching losses, these immobilization rates were correlated with the amount of ammonium nitrate added in fertilizer rather than with initial %N. Therefore, litter quality and exogenous nitrogen supply appear to have different and independent effects on decay rates and N dynamics of little bluestem litter.     

Role of arthropod communities in bioenergy crop litter decomposition†

The extensive land use conversion expected to occur to meet demands for bioenergy feedstock production will likely have widespread impacts on agroecosystem biodiversity and ecosystem services, including carbon sequestration. Although arthropod detritivores are known to contribute to litter decomposition and thus energy flow and nutrient cycling in many plant communities, their importance in bioenergy feedstock communities has not yet been assessed. We undertook an experimental study quantifying rates of litter mass loss and nutrient cycling in the presence and absence of these organisms in three bioenergy feedstock crops—miscanthus (Miscanthus x giganteus), switchgrass (Panicum virgatum), and a planted prairie community. Overall arthropod abundance and litter decomposition rates were similar in all three communities. Despite effective reduction of arthropods in experimental plots via insecticide application, litter decomposition rates, inorganic nitrogen leaching, and carbon–nitrogen ratios did not differ significantly between control (with arthropods) and treatment (without arthropods) plots in any of the three community types. Our findings suggest that changes in arthropod faunal composition associated with widespread adoption of bioenergy feedstock crops may not be associated with profoundly altered arthropod‐mediated litter decomposition and nutrient release.     

Response of litter decomposition and related soil enzyme activities to different forms of nitrogen fertilization in a subtropical forest

With the continuing increase in the impact of human activities on ecosystems, ecologists are increasingly becoming interested in understanding the effects of nitrogen deposition on litter decomposition. At present, numerous studies have investigated the effects of single form of nitrogen fertilization on litter decomposition in forest ecosystems. However, forms of N deposition vary, and changes in the relative importance of different forms of N deposition are expected in the future. Thus, identifying the effects of different forms of N deposition on litter decomposition in forest ecosystems is a pressing task. In this study, two dominant litter types were chosen from Zijin Mountain in China: Quercus acutissima leaves from a late succession broad-leaved forest and Pinus massoniana needles from an early succession coniferous forest. The litter samples were incubated in microcosms with original forest soil and treated with four different forms of nitrogen fertilization [NH₄ ⁺, NO₃ ⁻, CO(NH₂)₂, and a mix of all three]. During a 5-month incubation period, litter mass losses, soil pH values, and soil enzyme activities were determined. Results show that all four forms of nitrogen fertilization significantly accelerate litter decomposition rates in the broadleaf forest, while only two forms of nitrogen fertilization [i.e., mixed nitrogen and CO(NH₂)₂] significantly accelerate litter decomposition rates in the coniferous forest. Litter decomposition rates with the mixed nitrogen fertilization were higher than those in any single form of nitrogen fertilization. All forms of nitrogen fertilization enhanced soil enzyme activities (i.e., catalase, cellulase, invertase, polyphenol oxidase, nitrate reductase, urease, and acid phosphatase) during the litter decomposition process for the two forest types. Soil enzyme activities under the mixed nitrogen fertilization were higher than those under any single form of nitrogen fertilization. These results suggest that the type and activity of the major degradative enzymes involved in litter decomposition vary in different forest types under different forms of nitrogen fertilization. They also indicate that a long-term consequence of N deposition-induced acceleration of litter decomposition rates in subtropical forests may be the release of carbon stored belowground to the atmosphere.     

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.     

Mechanisms of plant species impacts on ecosystem nitrogen cycling

Plant species are hypothesized to impact ecosystem nitrogen cycling in two distinctly different ways. First, differences in nitrogen use efficiency can lead to positive feedbacks on the rate of nitrogen cycling. Alternatively, plant species can also control the inputs and losses of nitrogen from ecosystems. Our current understanding of litter decomposition shows that most nitrogen present within litter is not released during decomposition but incorporated into soil organic matter. This nitrogen retention is caused by an increase in the relative nitrogen content in decomposing litter and a much lower carbon‐to‐nitrogen ratio of soil organic matter. The long time lag between plant litter formation and the actual release of nitrogen from the litter results in a bottleneck, which prevents feedbacks of plant quality differences on nitrogen cycling. Instead, rates of gross nitrogen mineralization, which are often an order of magnitude higher than net mineralization, indicate that nitrogen cycling within ecosystems is dominated by a microbial nitrogen loop. Nitrogen is released from the soil organic matter and incorporated into microbial biomass. Upon their death, the nitrogen is again incorporated into the soil organic matter. However, this microbial nitrogen loop is driven by plant‐supplied carbon and provides a strong negative feedback through nitrogen cycling on plant productivity. Evidence supporting this hypothesis is strong for temperate grassland ecosystems. For other terrestrial ecosystems, such as forests, tropical and boreal regions, the data are much more limited. Thus, current evidence does not support the view that differences in the efficiency of plant nitrogen use lead to positive feedbacks. In contrast, soil microbes are the dominant factor structuring ecosystem nitrogen cycling. Soil microbes derive nitrogen from the decomposition of soil organic matter, but this microbial activity is driven by recent plant carbon inputs. Changes in plant carbon inputs, resulting from plant species shifts, lead to a negative feedback through microbial nitrogen immobilization. In contrast, there is abundant evidence that plant species impact nitrogen inputs and losses, such as: atmospheric deposition, fire‐induced losses, nitrogen leaching, and nitrogen fixation, which is driven by carbon supply from plants to nitrogen fixers. Additionally, plants can influence the activity and composition of soil microbial communities, which has the potential to lead to differences in nitrification, denitrification and trace nitrogen gas losses. Plant species also impact herbivore behaviour and thereby have the potential to lead to animal‐facilitated movement of nitrogen between ecosystems. Thus, current evidence supports the view that plant species can have large impacts on ecosystem nitrogen cycling. However, species impacts are not caused by differences in plant quantity and quality, but by plant species impacts on nitrogen inputs and losses.     

Insect biological control accelerates leaf litter decomposition and alters short‐term nutrient dynamics in a Tamarix‐invaded riparian ecosystem

Insect herbivory can strongly influence ecosystem nutrient dynamics, yet the indirect effects of herbivore‐altered litter quality on subsequent decomposition remain poorly understood. The northern tamarisk beetle Diorhabda carinulata was released across several western states as a biological control agent to reduce the extent of the invasive tree Tamarix spp. in highly‐valued riparian ecosystems; however, very little is currently known about the effects of this biocontrol effort on ecosystem nutrient cycling. In this study, we examined alterations to nutrient dynamics resulting from beetle herbivory in a Tamarix‐invaded riparian ecosystem in the Great Basin Desert in northern Nevada, USA, by measuring changes in litter quality and decomposition, as well as changes in litter quantity. Generally, herbivory resulted in improved leaf litter chemical quality, including significantly increased nitrogen (N) and phosphorus (P) concentrations and decreased carbon (C) to nitrogen (C:N), C:P, N:P, and lignin:N ratios. Beetle‐affected litter decomposed 23% faster than control litter, and released 16% more N and 60% more P during six months of decomposition, as compared to control litter. Both litter types showed a net release of N and P during decomposition. In addition, herbivory resulted in significant increases in annual rates of total aboveground litter and leaf litter production of 82% and 71%, respectively, under the Tamarix canopy. Our finding that increased rates of N and P release linked with an increased rate of mass loss during decomposition resulting from herbivore‐induced increases in litter quality provides new support to the nutrient acceleration hypothesis. Moreover, results of this study demonstrate that the introduction of the northern tamarisk beetle as biological control to a Tamarix‐invaded riparian ecosystem has lead to short‐term stimulation of nutrient cycling. Alterations to nutrient dynamics could have implications for future plant community composition, and thus the potential for restoration of Tamarix‐invaded ecosystems.     

Decomposition of eucalypt litter on rehabilitated bauxite mines

The initiation of nutrient cycling is important in developing a self-sustaining ecosystem, where inputs of fertilizer are not required, on rehabilitated open-cut mines. The loss of dry weight, surface area and nutrients from senescent jarrah (Eucalyptus marginata) leaves enclosed in litterbags for 18 months were measured on 27 rehabilitated bauxite mines and in two jarrah forests on the Darling Plateau in Western Australia. Respiration and acetylene reduction by the litter were also determined. Linear trends were found between litter decomposition on rehabilitated mines and understorey cover density, litter cover and a measure of the effect of the revegetation on soil moisture. During decomposition, N was retained relative to litter dry weight and, in most cases, amounts of N increased. Losses of Ca and S were correlated with dry weight losses. Sodium, Cl, Mg and K were lost from the litter by leaching. Rehabilitation techniques, including sowing a legume understorey and replacement of the topsoil, should favour the development of nutrient cycling on mined areas.     

Long-term presence of tree species but not chemical diversity affect litter mixture effects on decomposition in a neotropical rainforest

Plant litter diversity effects on decomposition rates are frequently reported, but with a strong bias towards temperate ecosystems. Altered decomposition and nutrient recycling with changing litter diversity may be particularly important in tree species-rich tropical rainforests on nutrient-poor soils. Using 28 different mixtures of leaf litter from 16 Amazonian rainforest tree species, we tested the hypothesis that litter mixture effects on decomposition increase with increasing functional litter diversity. Litter mixtures and all single litter species were exposed in the field for 9 months using custom-made microcosms with soil fauna access. In order to test the hypothesis that the long-term presence of tree species contributing to the litter mixtures increases mixture effects on decomposition, microcosms were installed in a plantation at sites including the respective tree species composition and in a nearby natural forest where these tree species are absent. We found that mixture decomposition deviated from predictions based on single species, with predominantly synergistic effects. Functional litter diversity, defined as either richness, evenness, or divergence based on a wide range of chemical traits, did not explain the observed litter mixture effects. However, synergistic effects in litter mixtures increased with the long-term presence of tree species contributing to these mixtures as the home field advantage hypothesis assumes. Our data suggest that complementarity effects on mixed litter decomposition may emerge through long-term interactions between aboveground and belowground biota.