Low-quality Litter Research Articles

Changes in litter quality caused by simulated nitrogen deposition reinforce the N‐induced suppression of litter decay

Rates of nitrogen (N) deposition are increasing in industrialized and rapidly developing nations. Simulated N deposition suppresses plant litter decay rates, in particular for low quality (high lignin) litter. Litter quality is a primary driver of litter decomposition; however, it is not clear how changes in litter quality caused by long‐term ecosystem exposure to chronic N additions interact with altered soil N‐availability to influence litter decay dynamics. To document the effects of simulated N deposition on litter quality, we conducted a meta‐analysis of available litter nutrient data from simulated N deposition experiments in temperate forests. To directly test whether changes in litter quality caused by N deposition affect decay rates, we also conducted a reciprocal litterbag study in an existing N addition experiment, where a northern hardwood forest has been exposed to simulated N deposition for more than 20 years. The experiment enabled us to disentangle the effects of N additions, litter quality, and their interactions on litter decay dynamics. We measured litter mass loss and extracellular enzyme activities after one and two years of decomposition. The meta‐analysis demonstrated that simulated N deposition causes decreases of leaf Al, B, Ca, Mg, Mn, P, and Zn concentrations, and increased leaf N concentrations. Moreover, higher cumulative amounts of simulated N deposition result in greater decreases of leaf Ca and Mn concentrations. In the field experiment, litter originally collected from N‐enriched plots exhibited similar N‐induced nutrient changes as observed in our meta‐analysis and also had lower lignolytic enzyme activities and decay rates than litter collected from control plots. The decreased litter decay rates observed with simulated N deposition were a result of the combined effects of the N fertilization itself and the historical effects of N deposition on tree litter chemistry. The data suggest that changes in litter quality caused directly by long‐term N enrichment reinforce the negative effects of simulated N deposition on litter decay, particularly for high lignin species. The slowed decay associated with changes in litter quality caused by N enrichment itself may partly explain the accumulation of organic matter previously observed at ours and at other simulated N deposition experiments.

Soil microbial community and its interaction with soil carbon and nitrogen dynamics following afforestation in central China

Afforestation may alter soil microbial community structure and function, and further affect soil carbon (C) and nitrogen (N) dynamics. Here we investigated soil microbial carbon and nitrogen (MBC and MBN) and microbial community [e.g. bacteria (B), fungi (F)] derived from phospholipid fatty acids (PLFAs) analysis in afforested (implementing woodland and shrubland plantations) and adjacent croplands in central China. Relationships of microbial properties with biotic factors [litter, fine root, soil organic carbon (SOC), total nitrogen (TN) and inorganic N], abiotic factors (soil temperature, moisture and pH), and major biological processes [basal microbial respiration, microbial metabolic quotient (qCO2), net N mineralization and nitrification] were developed. Afforested soils had higher mean MBC, MBN and MBN:TN ratios than the croplands due to an increase in litter input, but had lower MBC:SOC ratio resulting from low-quality (higher C:N ratio) litter. Afforested soils also had higher F:B ratio, which was probably attributed to higher C:N ratios in litter and soil, and shifts of soil inorganic N forms, water, pH and disturbance. Alterations in soil microbial biomass and community structure following afforestation were associated with declines in basal microbial respiration, qCO2, net N mineralization and nitrification, which likely maintained higher soil carbon and nitrogen storage and stability.

Integrating plant litter quality, soil organic matter stabilization, and the carbon saturation concept.

Labile, 'high-quality', plant litters are hypothesized to promote soil organic matter (SOM) stabilization in mineral soil fractions that are physicochemically protected from rapid mineralization. However, the effect of litter quality on SOM stabilization is inconsistent. High-quality litters, characterized by high N concentrations, low C/N ratios, and low phenol/lignin concentrations, are not consistently stabilized in SOM with greater efficiency than 'low-quality' litters characterized by low N concentrations, high C/N ratios, and high phenol/lignin concentrations. Here, we attempt to resolve these inconsistent results by developing a new conceptual model that links litter quality to the soil C saturation concept. Our model builds on the Microbial Efficiency-Matrix Stabilization framework (Cotrufo etal., 2013) by suggesting the effect of litter quality on SOM stabilization is modulated by the extent of soil C saturation such that high-quality litters are not always stabilized in SOM with greater efficiency than low-quality litters.

Litter mass loss and nutrient release influenced by soil fauna of Betula ermanii forest floor of the Changbai Mountains, China

The objective of this study was to examine the litter mass loss of single and mixed plant species, and the nutrient release influenced by soil fauna on the Betula ermanii Cham. (BE) forest floor of the Changbai Mountains, China. A 24 month experiment of litter decomposition was conducted using litterbags with two mesh sizes (0.01 and 2mm), for three litter species incubated separately or in 1:1 and 1:1:1 combinations. Following the 24 month experiment, it was found that the faunal effect on the litter mass loss varied between 8.91 % and 19.9 %. Parasenecio komarovianus (Pojark.) Y.L. Chen (PK) litter decomposed fastest, however, B. ermanii had the highest faunal effect. During the experiment, C loss was found in all litter species and their mixtures, while N accumulation was found in B. ermanii, Rhododendron aureum Georgi (RA), and BE+RA litterbags of both mesh sizes only after 6 months. Soil fauna limited release of C was also found in all litter species and their mixtures after 6 months. By the 24th month of the experiment, soil fauna limited the release of C and N in the R. aureum litterbags, but the C and N release was enhanced in all other litter incubations. Our results illustrate that the mixed–species litter accelerated the mass loss for the low-quality litter, but reduced the soil fauna effect on the mass loss when it was mixed with the high-quality litter. The soil fauna contributed to the C and N release in all litter incubations, both single and mixtures, during the 24 month experiment, with the exception of R. aureum. The rate of litter mass loss, and the nutrient release attributable to the soil fauna, depended on the composition of the litter. The mechanism can be explained by litter heterogeneity, as well as by soil faunal interactions.

Elevated ozone and nitrogen deposition affect nitrogen pools of subalpine grassland

In a free-air fumigation experiment with subalpine grassland, we studied long-term effects of elevated ozone (O3) and nitrogen (N) deposition on ecosystem N pools and on the fate of anthropogenic N. At three times during the seventh year of exposure, N pools and recovery of a stable isotope tracer (15N) were determined in above- and belowground plant parts, and in the soil. Plants were much better competitors for 15N than soil microorganisms. Plant N pools increased by 30–40% after N addition, while soil pools remained unaffected, suggesting that most of the extra N was taken up and stored in plant biomass, thus preventing the ecosystem from acquiring characteristics of eutrophication. Elevated O3 caused an increase of N in microbial biomass and in stabilized soil N, probably resulting from increased litter input and lower litter quality. Different from individual effects, the interaction between the pollutants remained partly unexplained.

Optimization of Nitrogen Fertilization Input on <i>Zea mays</i> L. Cultivation through the Biological Inhibition of Nitrification

Introduction: Nitrogen (N) fertilizer is the most widely used fertilizer (300 - 350 Kg/Ha) on the corn plant, mean while it has mobile character, so it becomes the lowest utilization (40% - 50%) compared to other nutrient elements. Aims: The aim of this research is to examine the effect of various qualities and dose of litter on the dynamics of N mineral soil and soil nitrification potential on maize cultivation system. Method: The treatment is set by Randomized Completely Block Design (RCBD). Four kinds of litter is chosen to represent the groups of low quality (Tectona grandis and Pterocarpus indicus) and the groups of high quality (Oryza sativa and Eupatorium inulifolium) are given on four doses: 4, 8, 12 and 16 mg/Ha. All treatments are fertilized with urea 150 kg/Ha. Variables measured include the soil concentration of NH4- and NO3-, potential nitrification conducted in 2, 4, 6, 8 and 10 weeks after planting the corn. Result: The research shows that the addition of low-quality litter is influenced very significantly by release of NH4-, formation of NO3- and potential nitrification of soil. Others results show that the content of litter (lignin, polyphenol and C/N ratio) has the most powerful influence as regulator of NH4- released (p NO3- (p < 0.01) and potential nitrification of soil (p < 0.01) rather than the content of lignin, polyphenol and C/N ratio of litter separately. Conclusion: The conclusion of this research is that the management of litter quality input can be applied in the field to regulate the process of nitrification in soil and potential to increase N used efficiency of corn plant.

Interaction between decomposing litter and soil fauna of theBetula ermaniiforest floor of the Changbai Mountains, China

Soil fauna play a key role in litter decomposition as they influence the litter mass loss rate in terrestrial ecosystems. However, the interaction between decomposing litter and soil fauna has not been adequately addressed. We examine the interaction between different types of decomposing litter and soil fauna on the Betula ermanii Cham. (BE) forest floor of the Changbai Mountains, China, by measuring the mass loss of six litter species groups using litterbags with two sizes of mesh (4 mm and 0.01 mm) during a yearlong experiment. Soil fauna were identified at the order level. We found that soil fauna have a limited effect on litter mass loss at the initial stage of the experiment. Its positive effect became apparent at month 12 of the experiment. After 1 year, soil fauna increased the litter mass loss rate of the high-quality litter of Parasenecio komarovianus (Pojark.) Y.L. Chen (PK) by 7.02% and of the low-quality litter of Rhododendron aureum Georgi (RA) by 25.26%. BE + PK litter was associated with a significantly higher abundance of soil fauna at months 8 and 10 of the experiment and also with a significantly higher richness of soil fauna at month 10 of the experiment. At the end of the experiment, however, the Shannon–Wiener diversity index of soil fauna was not necessarily higher in mixed-species litter. Litter mixing did promote the abundance, richness, and diversity of soil fauna during the warm season in the high-quality litter substrate of BE + PK. Our results illustrate that the impact of soil fauna on the litter mass loss of both single- and mixed-species litterbags ranges from a limited impact to a positive impact as litter mass loss advances. The soil fauna contribute more to the litter mass loss of the low-quality litter with higher C to N ratios than to those with a low C to N ratio. The promoting effect of litter mixing on the soil faunal community composition is only short term and is dependent on substrate quality.

Micro-environmental conditions affect grass and shrub seedling emergence in denuded areas of the arid Patagonian Monte, Argentina

Harsh micro-environments in bare soil patches generated by disturbance may affect the emergence of plant species in desert ecosystems. We evaluated the effects of litter addition on the emergence of perennial grass and evergreen shrub species under different conditions of UV radiation and soil water content in disturbed patches of bare soil. We sowed seeds of perennial grasses (Nassella tenuis and Poa ligularis), and shrubs (Atriplex lampa, Larrea divaricata and Schinus johnstonii) in microcosms containing blocks of upper soil (28cm depth) subjected to different levels of three factors: litter (bare soil without litter cover, high quality litter cover from undisturbed plant patches, and low quality litter cover from disturbed plant patches), UV radiation (near ambient and attenuated UV radiation), and soil water (5–15% and 15–25% of volumetric soil water content). The patterns of seedling emergence in relation to litter, UV radiation, and soil water content varied among species independently of life form. Seedling emergence of all species increased under exposure to UV radiation at all combinations of litter and soil water factors. High soil water content had an important positive effect on the emergence of N. tenuis and L. divaricata in combination with UV radiation and litter. In contrast, soil water content did not affect or affected negatively the emergence P. ligularis and A. lampa in combination with litter and UV radiation. The emergence of S. johnstonii occurred only under exposure to UV radiation mostly at high soil water content, independently of litter levels. These results highlight the importance of the combined effects of the presence and type of litter, soil water content, and UV radiation on seedling emergence of perennial grass and shrub species and consequently on the vegetation reestablishment processes at disturbed areas.

Disturbance–diversity relationships for soil fauna are explained by faunal community biomass in a salt marsh

Disturbance–diversity relationships have long been studied in ecology with a unimodal relationship as the key prediction. Although this relationship has been widely contested, it is rarely tested for soil invertebrate fauna, an important component of terrestrial biodiversity. We tested disturbance–diversity relationships for soil meso- and macrofauna in a salt marsh where periodic sea water inundation and cattle grazing occur as stressors. We hypothesized a unimodal inundation frequency–diversity relationship, whereas we expected grazing to overrule the effects of inundation frequency due to its large effects on the habitat of soil fauna. We found a negative relationship between inundation frequency and diversity at the ungrazed sites and no relationship at the grazed sites. Moreover, we found a negative relationship between community biomass and diversity for soil fauna that may have caused this negative disturbance–diversity relationship. Community biomass at the intermediate inundation frequency increased due to the dominance of Orchestia gammarellus (a macro-detritivore species), which could exploit low quality litters at the ungrazed sites. We highlight that the negative relationship between faunal community biomass and faunal diversity may influence disturbance–diversity relationships and illustrate that total biomass distribution of feeding guilds of soil fauna can improve our understanding of the soil fauna response to stressors in salt marshes.

Carbon–nitrogen interactions during afforestation in central China

We conducted a field study in Danjiangkou Reservoir region of central China to evaluate soil C and N dynamics following afforestation by comparing soil organic C and N (SOC and SON), soil net N mineralization and nitrification, and inorganic N concentrations in the plant rhizosphere and open areas in the forest, shrubland and adjacent cropland. Afforestation increased SOC but did not significantly affect SON in the plant rhizosphere. Due to large quantity of low-quality litter (with high C:N ratios) inputs, afforestation enhanced soil C recalcitrant indexes (RIC) but decreased soil N recalcitrance indexes (RIN) in the plant rhizosphere. Both SON and RIN significantly decreased following afforestation in the open areas. Afforestation decreased inorganic N concentrations and net N mineralization. Soil net N mineralization were negatively correlated with soil C:N ratios across land use types. These results suggest that afforestation could increase SOC stocks resulting from large low-quality litter input, but over the long-term, this increase was likely limited due to decreased soil N availability.

No apparent effects of soil inoculum on green frog (Lithobates clamitans Latreille) tadpole performance

Invasions by nonnative plant species are transforming plant communities across the globe. An important challenge for ecologists is to understand how animals will respond to these changes. One way that plant invasions could affect aquatic animals is by changing the rate at which soil communities decompose litter, which could alter the flow of energy and nutrients from plant litter to aquatic communities. In this study, we measured larval amphibian responses to soil conditioned by either introduced or native genotypes of Phragmites australis L. (common reed) in northeastern North America. We collected soil from adjacent stands of introduced and native P. australis at three sites in central New York and inoculated outdoor aquatic mesocosms with soil extracts. Mesocosms contained six Lithobates clamitans Latreille (green frog) tadpoles and either low- or high-quality native P. australis americanus litter. We found that litter decomposition differed based on soil inoculum, and we observed a significant interaction between litter quality and soil inoculum; higher-quality litter tended to decompose faster when exposed to inocula from introduced P. australis, while lower-quality litter tended to decompose faster when exposed to inocula from native P. australis americanus. Tadpoles raised with high-quality litter developed faster and achieved greater body size, but soil inocula had no apparent effect on tadpoles. Our results suggest that plant invasions may alter microbial communities, causing subtle changes in litter decomposition rates, but these changes do not appear strong enough to influence larvae of a widespread amphibian.

Do non-native Platanus hybrida riparian plantations affect leaf litter decomposition in streams?

In forest headwater streams where the riparian canopy limits autochthonous primary production, leaf litter decomposition is a key process controlling nutrient and carbon cycling. Any alteration of the riparian vegetation may influence litter decomposition and detrital food webs. We evaluated the effect of non-native Platanus hybrida riparian plantations on leaf litter decomposition in Mediterranean streams. The experiment was conducted in six headwater streams; three lined by native riparian vegetation and three crossing P. hybrida plantations. We have characterized the processing rates of alder leaves and the assemblages of shredder macroinvertebrates and fungi. Litter decomposition was significantly faster in the P. hybrida than in the reference streams. Although the dissolved inorganic nitrogen concentration was higher in P. hybrida, no significant effect was observed in decomposition rates. Differences in decomposition rates reflected the macroinvertebrate and shredder colonization in alder litter, with higher abundance and richness in the P. hybrida streams. However, aquatic hyphomycete sporulation rate was higher in reference streams, suggesting that the variation in decomposition rates is a direct consequence of shredder abundance. Our findings support part of the substrate quality-matrix quality (SMI) hypothesis, which expects that high-quality litter will show increased decomposition rates in a low-quality litter matrix.

Litter quality as driving factor for plant nutrition via grazing of protozoa on soil microorganisms

Plant residues provide a major source of nitrogen (N) for plant growth. Litter N mineralization varies with litter carbon-to-nitrogen (C-to-N) ratio and presence of bacterial-feeding fauna. We assessed the effect of amoebae, major bacterial feeders in soil, on mineralization of litter of low (high quality) and high C-to-N ratio (low quality) and evaluated consequences for plant growth. We used stable isotopes to determine plant N uptake from litter and plant C partitioning. Stable isotope probing of phospholipid fatty acids was used to follow incorporation of plant C into microorganisms. Amoebae increased plant N uptake independent of litter quality and thereby the biomass of shoots and roots by 33% and 66%, respectively. Plant allocation of total (13)C to roots in low (42%) exceeded that of high-quality litter treatments (26%). Amoebae increased plant allocation of (13)C to roots by 37%. Microbial community structure and incorporation of (13)C into PLFAs varied significantly with litter quality and in the low-quality litter treatment also with the presence of amoebae. Overall, the results suggest that in particular at low nutrient conditions, root-derived C fosters the mobilization of bacterial N by protozoa, thereby increasing plant growth when microorganisms and plants compete for nutrients.

Afforestation with Norway spruce on a subalpine pasture alters carbon dynamics but only moderately affects soil carbon storage

There is a strong trend toward reforestation of abandoned grasslands in alpine regions which may impact the carbon balance of alpine ecosystems. Here, we studied the effects of afforestation with Norway spruce (Picea abies L.) on an extensively grazed subalpine pasture in Switzerland on soil organic carbon (SOC) cycling and storage. Along a 120-year long chronosequence with spruce stands of 25, 30, 40, 45, and >120 years and adjacent pastures, we measured tree biomass, SOC stocks down to the bedrock, natural 13C abundances, and litter quality. To unravel controls on SOC cycling, we have monitored microclimatic conditions and quantified SOC decomposability under standardized conditions as well as soil respiration in situ. Stocks of SOC were only moderately affected by the afforestation: in the mineral soil, SOC stocks transiently decreased after tree establishment, reaching a minimum 40–45 years after afforestation (−25 %) and increased thereafter. Soils of the mature spruce forest stored the largest amount of SOC, 13 % more than the pasture soils, mainly due to the accumulation of an organic layer (23 t C ha−1). By comparison, C accumulated in the tree biomass exceeded the SOC pool by a factor of three in the old forest. In contrast to the small impact on C storage, afforestation strongly influenced the composition and quality of the soil organic matter (SOM). With increasing stand age, δ13C values of the SOM became consistently more positive, which can be interpreted as a gradual replacement of grass- by spruce-derived C. Fine roots of spruce were enriched in 13C, in lignin and had a higher C/N ratio in comparison to grass roots. As a consequence, SOM quality as indicated by the lower fraction of readily decomposable (labile) SOM and higher C:N ratios declined after the land-use change. Furthermore, spruce plantation induced a less favorable microclimate for microbial activity with the average soil temperature during the growing season being 5 °C lower in the spruce stands than in the pasture. In situ soil respiration was approximately 50 % lower after the land use conversion, which we primarily attribute to the colder conditions and the lower SOM quality, but also to drier soils (−25 %) and to a decreased fine root biomass (−40 %). In summary, afforestation on subalpine pastures only moderately affected SOC storage as compared to the large C sink in tree biomass. In contrast, SOC cycling rates strongly decreased as a result of a less favorable microclimate for decomposition of SOM, a lower C input by roots, and a lower litter quality.

Predator/Prey-Interactions Promote Decomposition of Low-Quality Detritus

Predation on detritivores is expected to decelerate detritivore-mediated decomposition processes. In field mesocosms, we studied whether the decomposition of leaf and needle litter of live oak (Quercus virginiana) and loblolly pine (Pinus taeda), respectively, was affected by saltmarsh detritivores (Gastropoda: Littoraria irrorata and Melampus bidentatus) and predacious omnivores (Decapoda: Armases cinereum) and their interactions. Both crabs and snails alone increased decomposition (mass loss) rates of oak litter, while a combination of both resulted in the same mass loss as in animal-free controls, probably due to crabs feeding on snails rather than litter. Neither crabs nor snails alone affected mass loss of pine litter, but a combination of both significantly increased decomposition rates. Irrespective of the litter type, crabs significantly increased mortality of the snails but gained biomass only on pine litter and only when detritivorous snails were present. Our findings suggest that unidirectional facilitation of omnivorous semi-terrestrial crabs by their detritivorous prey (saltmarsh snails) promotes the decomposition of low-quality (pine) litter. On high-quality (oak) litter, by contrast, negative effects of the predator prevail, resulting in a drop of decomposition rates when crabs were present, probably owing to predation on detritivorous snails. Thus, the effects of predator/prey-interactions on decomposition processes are context-dependent and are controlled by resource quality.

How does litter quality affect the community of soil protists (testate amoebae) of tropical montane rainforests?

Litter quality and diversity are major factors structuring decomposer communities. However, little is known on the relationship between litter quality and the community structure of soil protists in tropical forests. We analyzed the diversity, density, and community structure of a major group of soil protists of tropical montane rainforests, that is, testate amoebae. Litterbags containing pure and mixed litter of two abundant tree species at the study sites (Graffenrieda emarginata and Purdiaea nutans) differing in nitrogen concentrations were exposed in the field for 12months. The density and diversity of testate amoebae were higher in the nitrogen-rich Graffenrieda litter suggesting that nitrogen functions as an important driving factor for soil protist communities. No additive effects of litter mixing were found, rather density of testate amoebae was reduced in litter mixtures as compared to litterbags with Graffenrieda litter only. However, adding of high-quality litter to low-quality litter markedly improved habitat quality, as evaluated by the increase in diversity and density of testate amoebae. The results suggest that local factors, such as litter quality, function as major forces shaping the structure and density of decomposer microfauna that likely feed back to decomposition processes.

Allochthonous inputs from grass-dominated wetlands support juvenile salmonids in headwater streams: evidence from stable isotopes of carbon, hydrogen, and nitrogen

We used dual-isotope mixing models (δ13C/δ15N and δ2H/δ15N) in a Bayesian framework to partition allochthonous and autochthonous energy sources for salmonids in 2 headwater streams in the Kenai Lowlands, Alaska (USA). Our 1st objective was to estimate the production base for juvenile coho salmon (Oncorhynchus kisutch) and Dolly Varden (Salvelinus malma). We hypothesized that consumers would be reliant on both autochthonous (filamentous algae and periphyton) and allochthonous sources, but that autochthonous sources would dominate because of the open canopy and lower-quality litter inputs provided by the riparian wetland vegetation, primarily bluejoint grass (Calamagrostis canadensis). Our 2nd objective was to evaluate the utility of stable H isotopes for tracing energy pathways in a northern-latitude ecosystem. We hypothesized that δ2H-based models would provide more precise estimates of source partitioning than δ13C-based models because of greater source separation. Allochthonous source contributions consistently exceeded autochthonous sources for all fish species and size classes at both study sites. However, diet shifted during ontogeny, and larger Dolly Varden relied more on autochthonous sources than did smaller individuals of both species. Last, we found good correspondence and similar levels of precision between the δ13C- and δ2H-based models despite greater source separation by δ2H. Our results highlight the importance of allochthonous sources in headwater streams, and we suggest that litter inputs from grasses may be an under-appreciated subsidy to salmon production. Stable H isotopes can be an effective foodweb tracer in northern-latitude streams, but source partitioning results were not sufficiently different from stable C isotope models for us to recommend unequivocally using them to replace or enhance δ13C in similar studies.

Decomposition and nitrogen transformation rates in a temperate grassland vary among co-occurring plant species

Decomposition of organic matter varies depending upon interactions between the composition of the organic matter and the source of the microbial community, with differences in these interactions among vegetation types leading to the Home Field Advantage (HFA) hypothesis whereby decomposition of litters is faster in soils previously conditioned by them. It is possible that HFA operates on smaller scales within plant communities with ecosystem processes responding to subtle changes of plant community dominance. Using field measurements and laboratory incubations, we found a strong plant species effect on nitrogen availability and transformations and the relative importance of autotrophic and heterotrophic processes to nitrification. We found that the origin of the soil microbial community had little influence on litter decomposition when litter quality was high but was important with low-quality litter, most of which was root material. Our results demonstrate that plant species identity has a substantial impact on both litter decomposition and N cycling even within a single vegetation type and on an extremely local scale via both litter chemistry and specificity of the associated soil microbial community. Therefore, changes in botanical composition could alter decomposition and nutrient release altering ecosystem productivity and carbon sequestration potential.

Soil fauna in sheep-grazed hill pastures under organic and conventional livestock management and in an adjacent ungrazed pasture

Abstract Organic pasture management includes a focus on mixed livestock grazing, restrictions on nutrient inputs and livestock pest control. These are all factors which influence the environment of soil invertebrates. In this study, soil macrofauna, mesofauna and microfauna were collected from duplicate 11 and 20 year old organic and conventional legume-based sheep-grazed pasture systems. Pastures in both systems had received the same annual input of reactive phosphate rock and elemental sulphur and were stocked with the same numbers of sheep. The major difference between the two systems was absence of the chemical control of livestock pests on the organic system, resulting in lower sheep liveweights. Two ungrazed pastures which had not received any nutrient inputs for 20+ years were also sampled as part of the study to provide an additional contrast. No significant differences in either the diversity or abundance of the soil invertebrate community were found between the organic and conventional systems, despite the use of chemicals in the conventional system. The lack of difference suggests that many of the observed and reported responses to organic management reflect altered nutrient inputs and grazing management (which were the same in the current study), rather than the cessation of chemical controls in the organic system. In contrast, the invertebrate community in the unfertilised, ungrazed pasture was distinctly different from both the grazed systems. The ungrazed pasture had a higher diversity with more New Zealand endemic species than the grazed pasture. This included twice as many large Oribatida as soil pore size increased. The lower litter quality from the lack of nutrient inputs in the ungrazed pasture was reflected in both lower earthworm abundance and the Nematode Channel Ratio (indicative of a higher proportion of fungal- than bacterial-feeding nematodes), than the grazed pasture. We conclude that organic management when limited to a comparison of livestock pest control is not beneficial to soil invertebrates. Management practices such as stocking rates and fertiliser regime, by altering the soil physical environment and food resources, are more important in influencing invertebrate populations in this soil. Some would argue these altered management practices are an integral part of an organic production system.

What is more important for invertebrate colonization in a stream with low-quality litter inputs: exposure time or leaf species?

The objective of this study was to evaluate the influences of detritus from the leaves of different species, and of exposure time on invertebrate colonization of leaves in a shaded Cerrado stream. We hypothesized that the exposure time is the main factor that influences the colonization of leaves by invertebrates. We used leaves of five tree species native to the Brazilian Cerrado: Protium heptaphyllum and Protium brasiliense (Burseraceae), Ocotea sp. (Lauraceae), Myrcia guyanensis (Myrtaceae), and Miconia chartacea (Melastomataceae), which are characterized by their toughness and low-nutritional quality. Litter bags, each containing leaves from one species, were placed in a headwater stream and removed after 7, 15, 30, 60, 90, and 120 days. The dominant taxon was Chironomidae, which comprised ca. 52% of all organisms and ca. 20% of the total biomass. The taxonomic richness of colonizing organisms did not vary among the leaf species. However, the density and biomass of the associated organisms varied differently among the kinds of detritus during the course of the incubation. The collector-gatherers and shredders reached higher densities in the detritus that decomposed more rapidly (Ocotea sp. and M. guyanensis), principally in the more advanced stages of colonization. The collector-filterers reached higher densities in the detritus that decomposed more slowly (P. heptaphyllum, P. brasiliense, and M. chartacea), principally in the initial stages of incubation. A cluster analysis divided the detritus samples of different leaf species according to the exposure time (initial phase: up to 7 days; intermediate phase: 7–30 days; advanced phase: 30–120 days), suggesting some succession in invertebrate colonization, with differences in taxon composition (indicator taxa analysis). These results suggest that regardless of the leaf-detritus species, exposure time was the main factor that influenced the colonization process of aquatic invertebrates.