Soil fauna alter the effects of litter composition on nitrogen cycling in a mineral soil

Plant chemical composition and the soil community are known to influence litter and soil organic matter decomposition. Although these two factors are likely to interact, their mechanisms and outcomes of interaction are not well understood. Studies of their interactive effects are rare and usually focus on carbon dynamics of litter, while nutrient dynamics in the underlying soil have been ignored. A potential mechanism of interaction stems from the role fauna plays in regulating availability of litter-derived materials in the mineral soil. We investigated the role of soil fauna (meso, macro) in determining the effect of surface-litter chemical composition on nitrogen mineralization and on the micro-food web in mineral soils. In a field setting we exposed mineral soil to six types of surface-applied litter spanning wide ranges of multiple quality parameters and restricted the access of larger soil animals to the soils underlying these litters. Over six months we assessed litter mass and nitrogen loss, nitrogen mineralization rates in the mineral soils, and soil microbes and microfauna. We found evidence that the structure of the soil community can alter the effect of surface-litter chemical composition on nitrogen dynamics in the mineral soil. In particular, we found that the presence of members of the meso- and macrofauna can magnify the control of nitrogen mineralization by litter quality and that this effect is time dependent. While fauna were able to affect the size of the micro-food web they did not impact the effect of litter composition on the abundance of the members of the micro-food web. By enhancing the strength of the impact of litter quality on nitrogen dynamics, the larger fauna can alter nitrogen availability and its temporal dynamics which, in turn, can have important implications for ecosystem productivity. These findings contribute to evidence demonstrating that soil fauna shape plant litter effects on ecosystem function.     

Leaf litter decomposition in the pure and mixed plantations of <Emphasis Type="Italic">Cunninghamia lanceolata</Emphasis> and <Emphasis Type="Italic">Michelia macclurei</Emphasis> in subtropical China

Leaf litter decomposition of Cunninghamia lanceolata, Michelia macclurei, and their mixture in the corresponding stands in subtropical China was studied using the litterbag method. The objective was to assess the influence of native evergreen broadleaved species on leaf litter decomposition. The hypotheses were: (1) M. macclurei leaf litter with lower C/N ratio and higher initial N concentration decomposed faster than C. lanceolata litter, (2) decomposition rates in litter mixtures could be predicted from single-species decay rates, and (3) litters decomposed more rapidly at the site that contained the same species as in the litterbag. The mass loss of leaf litter was positively correlated with initial N concentration and negatively correlated with C/N ratio. The decomposition rate of M. macclurei leaf litter was significantly higher than that of C. lanceolata needle litter in the pure C. lanceolata stand. Contrary to what would be predicted, the litter mixture decomposed more slowly than expected based on the results from component species decomposing alone. There was no significant difference in litter decomposition rate between different habitats.     

Nonadditive effects of litter mixtures on decomposition and correlation with initial litter N and P concentrations in grassland plant species of northern China

We studied the occurrence of nonadditive effects of litter mixtures on the decomposition (the deviation of decomposition rate of litter mixtures from the expected values based on the arithmetic means of individual litter types) of litters from three plant species (i.e., Stipa krylovii Roshev., Artemisia frigida Willd., and Allium bidentatum Fisch. ex Prokh. & Ikonn.-Gal.) endemic to the grassland ecosystems of Inner Mongolia, northern China and the possible role of initial litter N and P on such effects. We mixed litters of the same plant species that differed in N and P concentrations (four gradients for each species) in litterbags and measured mass losses of these paired mixtures after 30 and 80 days under field conditions. We found the occurrence of positive, nonadditive effects of litter mixtures and showed that the magnitude of the nonadditive effects were related to the relative difference in the initial litter N and P concentrations of the paired litters.     

Determinants of litter mixing effects in a Swedish boreal forest

When the litter of a given species decomposes, it will often break down in the proximity of litters from other species. We investigated the effects of litters of 10 different species in a boreal forest of northern Sweden on each others' decomposition and N release rates; this was done through the use of litterbags containing two compartments separated by single mesh partition. Different litters could be placed on opposite sides of this mesh so that they were in contact with each other. Treatments consisted of all the possible pairwise combinations of the 10 species, with members of each pair placed in different compartments of the same litterbag. Litterbags were harvested after 1, 2 and 4 years in the field. Species differed significantly in their effects on decomposition and N loss rates of associated litters. Generally, litters from feather mosses and lichens showed the greatest promotion of decomposition on associated litters, while some vascular plant species, notably Empetrum hermaphroditum, showed the least. At year four, feather mosses also had the greatest positive effects on N loss from the litters of associated species. There were several instances in which litter of a given species decomposed at different rate when litter from its own species, rather than that of a different species, was placed in the adjacent litterbag compartment. This was particularly apparent in the second year, when across the entire data set, litters decomposed fastest when associated with their own litters. Generally, slowly decomposing litters had the greatest positive effects on decomposition of associated litters. It is proposed that in boreal forests slow decomposing litters (particularly those of feather mosses) may contribute to enhancing moisture attention in the litter layer, which in turn promotes the decomposition and N release of associated litters. Further, while litter mixing effects were clearly demonstrated in our study, they were also shown to be of secondary importance to the effects of species identity on decomposition.     

How browsing by red deer impacts on litter decomposition in a native regenerating woodland in the Highlands of Scotland

Herbivores can indirectly affect ecosystem productivity and processes such as nutrient cycling and decomposition by altering the quantity and quality of resource inputs into the decomposer subsystem. Here, we tested how browsing by red deer impacts on the decomposition of, and nutrient loss from, birch leaf litter (Betula pubescens), and tested whether effects of browsing on these measures were direct, via alteration of the quality of leaf litter, or indirect through long term impacts of deer browsing on soil biological properties. This was tested in a microcosm experiment using soil and litter taken from inside and outside three individual fenced exclosures located at Creag Meagaidh National Nature Reserve, Scotland. We found that litter of un-browsed trees decomposed faster than that from browsed trees, irrespective of whether soil was sourced from inside or outside exclosures. These findings suggest that effects of browsing on litter quality, rather than on soil biological properties, are the key determinant of enhanced decomposition in un-browsed areas of this ecosystem. Despite this, we found no consistent impact of browsing on litter C:N, a key indicator of litter quality; however, the rate of litter decomposition was linearly and negatively related to litter C:N when analysed across all the sites, indicating that this measure, in part, contributed to variation in rates of decomposition in this ecosystem. Our findings indicate that herbivores impact negatively on rates of decomposition in this ecosystem, ultimately retarding nutrient cycling rates, and that these effects are, in part, related to changes in litter quality.     

Assessing the effects of plant protection products on organic matter breakdown in arable fields—litter decomposition test systems

There is a need for plant protection products (PPPs) to be assessed for their effects on the breakdown of organic matter (OM), which is an important functional process in terrestrial ecosystems. Little information is available on to how to assess effects of PPPs on this complex system and formal guidelines for a standardised test method are lacking. We critically reviewed the literature to determine appropriate methods to investigate OM breakdown for the risk assessment of PPPs. Five methods appeared to be potentially suitable: namely the use of mini-containers or litter-bags to enclose OM, cotton-strip and bait-lamina assays which provide an artificial OM substrate, and stable isotopes to track the chemical decomposition of OM. These methods were compared on the basis of 10 suitability criteria, which included ecological relevance, ease of use and relevance to risk assessors. Each test method has limitations but the use of litter-bags, which is the most frequently used method, has distinct advantages over the other approaches. Accordingly, literature describing OM breakdown in litter-bags when applying PPPs are reviewed, gaps in the methodology are highlighted and recommendations for the development of a standardised and validated test method are proposed.     

Photodegradation effects on CO2 emissions from litter and SOM and photo-facilitation of microbial decomposition in a California grassland

Decomposition of soil organic matter (SOM) and plant litter has been shown to be affected by high solar radiation; this could partly explain why biogeochemical models underestimate decomposition in arid and semi-arid ecosystems. We set out to test the effect of using traditional PVC chambers for measuring soil gas fluxes versus quartz chambers that allowed passage of light during field measurements in a dry-land field in Davis, CA. Results showed that fluxes from quartz-top chambers were on average 29% higher than from opaque chambers. We also studied the effect of solar light exposure on decomposition of native grass litter and SOM in a field experiment where plots were shaded or left exposed for 157 days during summer; litter did not seem to be affected by exposure to light. However, we concluded that SOM decomposition was affected by light exposure since shaded soil had similar respiration to sunlight-exposed soil indicating that microbial respiration occurred under the shade while photo-degradation likely occurred under the sun. Additionally, ¹⁵N-labeled grass was placed in litter bags in the field with either clear filters to allow light or aluminum covers to block light; 3-month exposure caused a change in lignin degradability as indicated by the change in the Ad/Al ratio. Incubation of that litter showed 9.3% more CO₂ produced from litter in clear and aluminum bags than unexposed litter. This showed that photo-facilitation occurred although to a small degree and was a result of light exposure and/or heat degradation. We attributed the similar respiration from clear- and aluminum-exposed litter to heat degradation of the aluminum-exposed litter. In conclusion, our results show that in hot dry ecosystems conventional PVC chambers underestimate measured CO₂ flux rates; sunlight exposure changes litter chemistry and appears to affect the degradation of soil organic matter, but the magnitude of degradation depends on an interaction of factors such as soil temperature and moisture.     

Changes in chemical composition of Pinus sylvestris needle litter during decomposition along a European coniferous forest climatic transect

The objective of this investigation was to assess the changes in chemical composition (lignin, cellulose, hemicelluloses, non-structural compounds, N, and ash) of decomposing litter. Standard Pinus sylvestris needle litter, originating from southern Sweden, was incubated in litterbags at 15 sites selected from the Netherlands to south Spain. The changes in chemical composition of this litter were determined using near infrared reflectance spectroscopy. The hypothesis was that standard (chemically uniform) litter decomposing under a range of climates would show different dynamics of accumulation and loss of C-fractions, N, and ash, relative to mass loss. It was shown that, for a given mass-loss value (10, 20, 30, 40, or 50%), the proportion of lignin, cellulose, hemicelluloses, non-structural compounds, N, and ash in the decomposing pine needles differed between sites. Lignin concentration in the litter residue at 50% mass loss ranged from approximately 26 to 43%, cellulose from 19 to 27%, hemicelluloses from 7 to 11%, non-structural compounds from 19 to 25%, N from 0.7 to 1.3%, and ash content from 1.4 to 10.1%. Lignin concentrations showed the highest range of variation. Lignin concentrations during decomposition were positively related to moisture factors as significant correlations were found with actual evapotranspiration and were improved in multiple regressions by the mean annual precipitation or the water surplus. Cellulose was degraded further at sites with high precipitation whereas hemicellulose degradation was related to temperature. This leads to the conclusion that the remaining organic matter produced by standard litter decomposition within the studied climatic range of variations tended to be more recalcitrant under wet and warm climatic conditions than under cold or dry climate.     

Comparison of litter decomposition in a natural versus coal‐slurry pond reclaimed as a wetland

Decomposition is a key function in reclaimed wetlands, and changes in its rate have ramifications for organic‐matter accumulation, nutrient cycling, and production. The purpose of this study was to compare leaf litter decomposition rates in coal‐slurry ponds vs. natural wetlands on natural floodplain wetlands in Illinois, USA. The rate of decomposition was slower in the natural wetland vs. the coal pond (k = 0·0043 ± 0·0008 vs. 0·0066 ± 0·0011, respectively); the soil of the natural wetland was more acidic than the coal pond in this study (pH = 5·3 vs. 7·9, respectively). Similarly, higher organic matter levels were related to lower pH levels, and organic matter levels were seven‐times higher in the natural wetland than in the coal pond. The coal slurry pond was five years old at the time of the study, while the natural oxbow wetland was older (more than 550 years). The coal‐slurry pond was originally a floodplain wetland (slough); the downstream end was blocked with a stoplog structure and the oxbow filled with slurry. The pattern of decomposition for all species in the coal pond was the same as in the natural pond; Potomogeton nodosus decomposed more quickly than Phragmites australis, and both of these species decomposed more quickly than either Typha latifolia or Cyperus erythrorhizos (k = 0·0121±0·0008, 0·0051 ± 0·0006, 0·0024 ± 0·0001, 0·0024 ± 0·0004, respectively). Depending on how open or closed the system is to outside inputs, decomposition rate regulates other functions such as production, nutrient cycling, organic‐layer accumulation in the soil, and the timing and nature of delivery of detritus to the food chain. Copyright © 2004 John Wiley & Sons, Ltd.