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.     

Reciprocal transfer of carbon and nitrogen by decomposer fungi at the soil-litter interface

We have investigated whether decomposer fungi translocate litter-derived C into the underlying soil while simultaneously translocating soil-derived inorganic N up into the litter layer. We also located and quantified where the translocated C is deposited within the soil aggregate structure. When ¹³C-labeled wheat straw was decomposed on the surface of soil amended with ¹⁵N-labeled inorganic N, we found that C and N were reciprocally transferred by fungi, with a significant quantity (121-151 μg C g⁻¹ whole soil) of litter-derived C being deposited into newly formed macroaggregates (>250 μm sized aggregates). Fungal inhibition reduced fungal biomass and the bidirectional C and N flux by approximately 50%. The amount of litter-derived C found in macroaggregates was positively correlated with litter-associated fungal biomass. This fungal-mediated litter-to-soil C transfer, which to our knowledge has not been demonstrated before for saprophytic fungi, may represent an important mechanism by which litter C enters the soil and becomes stabilized as soil organic matter within the macroaggregate structure.     

Characterisation of cold-tolerant fungi from a decomposing High Arctic moss

Given that changes in the patterns of vegetation and size of carbon (C) pools in the Arctic are likely to be profound by the end of this century, it is necessary to characterise the identity and ecological groupings, in terms of temperature response and C substrate utilisation, of saprotrophic (decomposer) fungi in organic matter in Arctic soils. Thus, the aims of this study were: (1) to identify the fungi isolated from standing-dead material of Schistidium apocarpum, as an example of a High Arctic moss, (2) to determine mycelial extension rates of these fungi at a range of temperatures (4, 10 and 25 °C), and (3) to characterise the functional potential, defined by C substrate utilisation at 6 °C, of fungal taxa.Fungi were isolated at 4 °C from standing-dead material of S. apocarpum from an area of polar semi-desert (79 degrees N), close to Ny-Ålesund, Svalbard, in the High Arctic. From a collection of 662 isolates, 43 pure cultures were identified by DNA extraction, amplification, and sequencing. Phoma sclerotioides, previously known as a temperate snow mould, was isolated most frequently. The ecology of fifteen fungal isolates was characterised in detail. In terms of temperature response, two groups were apparent, one of truly psychrophilic/psychrotrophic fungi and one of more mesophilic fungi which were generally less frequently isolated. In terms of carbon substrate utilisation in semi-defined solid media, most fungi could utilise a variety of carbon substrates (degradation of casein, cellulose and starch was widespread), except for tannic acid (degraded by only two of the five P. sclerotioides isolates and Scytalidium lignicola) and lignin and chitin (not decomposed by any isolate). The majority of isolates had been recorded previously from polar environments and/or as being able to survive at low temperatures.Fungi in tundra ecosystems, therefore, have significant potential to mineralise C at temperatures below 10 °C. A better understanding of the ecology of these fungi will allow us to improve predictions of C dynamics in arctic biomes in the future.     

Differential production of oxalate by mycorrhizal fungi in arid ecosystems

Oxalate crystals and elements binding to the surfaces of mycorrhizal fungal hyphae were examined using scanning electron microscopy coupled with X-ray analysis of elemental composition. Mycorrhizae from the arid zone vegetation types in southern California were examined including chaparral, riparian oak woodlands, coastal sage, grasslands, and deserts. Only mat-forming ectomycorrhizal hyphae, such as Hysterangium separabile, were found to produce oxalate crystals. None of the arbuscular mycorrhizal fungal hyphae (Glomus spp. and Acaulospora elegans) examined had crystal structures associated with them. The hyphae of Hysterangium separabile without crystals did not show the Ca peaks that were present when the crystals existed nor did the arbuscular mycorrhizal fungal hyphae have the Ca peaks. The elimination of arbuscular mycorrhizae using benomyl did not affect soil P or oxalate. These data indicate that there are some fundamental differences in chemical exudation between mycorrhizal fungi that could affect P uptake and cycling in arid ecosystems. Received: 7 December 1994     

Evidence for gluconic acid production by Enterobacter intermedium as an efficient strategy to avoid protozoan grazing

The effect of pure gluconic acid and of gluconic-acid-producing bacteria on the activity of three protozoan species, Colpoda steinii (a ciliate), Vahlkampfia sp. (an amoeba) and Neobodo designis (a flagellate), was determined in vitro and in soil microcosms. Pure gluconic acid was shown to mediate disappearance of active cells, due to encystment and/or death of protozoa, at 0.15 mM in saline medium. Similarly, the presence of gluconic acid inhibited excystment of the three protozoa tested. Enterobacter intermedium 60-2G (Wt), a gluconic acid-producing rhizobacterium, elicited the same effects on protozoa when co-cultured in the presence of 5 g L⁻¹ glucose. However, the effect was not observed when glucose was omitted from the medium. Similarly, a pqqA isogenic mutant strain, unable to produce gluconic acid from glucose, exhibited a reduced effect on protozoan activity. Rhizosphere-microcosm studies performed with wheat (Triticum aestivum L.) confirmed the reduced ability of the pqqA mutant to limit protozoa reproduction compared to the Wt strain. Since the sodium salt of gluconic acid did not cause any significant stress to protozoa and considering that addition of 50 mM Tris-Cl (pH 7.2) abolished the deleterious effect of gluconic acid, acidification of the medium appeared as the key factor that induced encystment/death of protozoa. We propose that production and excretion of gluconic acid should be considered an efficient mechanism evolved by bacteria to escape, tolerate or defend themselves against protozoan grazing in rhizosphere environments.