Robert P. Griffiths

Soil solution chemistry of ectomycorrhizal mats in forest soil

Abstract Survival and productivity of Douglas-fir [ Pseudotsuga menziesii (Mirb.) Franco] depend on close association between host trees and ectomycorrhizal fungi. Two of these fungi, Hysterangium setchellii (Fischer) and Gautieria monticola (Harkness), form extensive hyphal mats with the roots of Douglas-fir and other conifers in the surface of the ‘A’ horizon, often at the interface between mineral oil and litter. The fungal mat alters the chemistry and mineral nutrition of the soil microenvironment within the rhizosphere, producing conditions that favor increased tree growth by increasing nutrient availability. Forest soils with or without obvious ectomycorrhizal mats were sampled at two locations in the Pacific Northwest. Cation and anion chemistry, dissolved organic carbon (DOC) and oxalate anions were analyzed. Mean concentrations of DOC, oxalate, PO 4 , SO 4 , H, Al, Fe, Cu, Mn and Zn were significantly higher in mat than in non-mat soil solutions in both mat types and locations and on both sampling dates. Significant statistical correlations between DOC or oxalate and PO 4 indicate that organic acids influence weathering and solubility of PO 4 in the mat soils. Mean oxalate concentrations were significantly lower in soil solutions from Hysterangium mat soils than in those from Gautieria mat soils. Organic acids released to the rhizosphere by G. monilcola and H. setchellii may provide a local weathering environment that increases availability of PO 4 , SO 4 and trace nutrients.

Effects of temperature and moisture on carbon respired from decomposing woody roots

Controls of temperature and moisture on root decomposition have not been well studied despite their direct relevance to climate change impacts on root carbon flux. The main objective of this laboratory study was to examine the respiration response of Sitka spruce, Douglas-fir, western hemlock, ponderosa pine, and lodgepole pine decomposing roots (1‐3 cm in diameter) to temperature and moisture change. Roots of Sitka spruce, Douglas-fir and western hemlock, and ponderosa pine and lodgepole pine were collected from Cascade Head, H.J. Andrews, and Pringle Falls site, respectively. Dead root respiration increased with temperature and reached the maximum at 30‐408C, and then decreased. Analysis of covariance indicated that the Q10 of root decomposition rate was influenced significantly (p<0.01) by incubation temperature range 5‐ 408C, but not by species, decay class or the direction of temperature change. At 5‐108C, Q10 averaged 3.99 and then decreased to 1.37 at 30‐408C. Over a range of 5‐608C, Q10 could be predicted by a single-exponential model using temperature as the independent variable. Analysis of variance showed that the respiration rate of dead roots was significantly (p<0.01) influenced by root moisture, species, and decay class as well as temperature. Dead root respiration increased with root moisture, reached the optimum range when moisture was between 100 and 275% and then decreased. Moreover, there were apparent interactions of root moisture and temperature on root respiration. Our study showed the direction of temperature and moisture change did not significantly influence root respiration, indicating that hysteresis may not occur for the temperature and moisture ranges examined. To better model global climate warming effects on root carbon flux, we suggest a temperature dependent Q10 function should be incorporated into current root dynamics models. The short-term laboratory incubation approach provided a good way to examine temperature and moisture controls on root decomposition, although we are cautious about long-term mass-loss extrapolations based on these short-term results. # 2000 Elsevier Science B.V. All rights reserved.

Comparison of direct vs fumigation incubation microbial biomass estimates from ectomycorrhizal mat and non-mat soils

Abstract Direct estimates of microbial biomass were compared to chloroform fumigation incubation estimates of microbial biomass using samples collected from mesic forest stands. Paired soil samples were collected from ectomycorrhizal mats, which contain visible amounts of fungal material, and from non-mat areas immediately adjacent to the mats but which contain no visible fungal material. As much as 30–50% of the dry wt of soil collected from ectomycorrhizal mats can be comprised of strictly fungal biomass. Direct estimates of microbial biomass from both mat and non-mat soils were 10–300 times higher than biomass estimates obtained using the fumigation incubation method. Fumigation incubation estimates of microbial biomass showed little seasonal variation, while direct estimates revealed that microbial biomass peaked during both the spring and fall when rainfall and temperatures were optimal and were lowest during the dry summer. We compared our values to ones reported for shortgrass prairies and in Jenkinsons original fumigation incubation paper. Fumigation incubation estimates indicated that microbial biomass carbon was the same in both prairie and forest soils, about 0.5 mg C g −1 soil. Direct estimates showed that microbial biomass was actually greater, by factors of 10–300, in these forest soils. Some forest soils, for which fumigation incubation indicated very low microbial biomass, contained visible amounts of fungal hyphae. Our conclusion is that fumigation incubation does not necessarily measure microbial biomass, and that the error can be extremely high when soils contain high quantities of fungal biomass.

Soil enzyme response to vegetation disturbance in two lowland Costa Rican soils

Abstract Conversion of forests to intensive agriculture often leads to degradation of weathered soils. The effects of two intensities of vegetation management on soil β-glucosidase (β-GLC) and phosphomonoesterase (PME) activities were studied on two river-terrace soils of differing fertility in Costa Rica. After approximately four years of annual harvest or continuous vegetation removal to bare soil, soil organic matter carbon (SOM-C), microbial biomass carbon (Mb-C), β-GLC and PME activity were reduced. Effects of continuous cropping to bare soil on Mb-C, β-GLC and PME were greater in the more weathered, acidic, Al-rich, P-limited upper-terrace soil than in the more neutral, base cation-rich lower-terrace soil. In contrast, more SOM-C was lost in the lower terrace. The annual harvest treatment produced intermediate decreases in SOM-C, Mb-C, β-GLC and PME on upper-terrace soils, intermediate reduction in lower terrace β-GLC, and no significant effect on lower terrace Mb-C or PME. β-GLC activity was the most sensitive indicator of treatment effect and may be a suitable alternative to Mb-C or SOM-C as a measure of change in soil health.

Hyphal mats formed by two ectomycorrhizal fungi and their association with Douglas-fir seedlings: A case study

The ectomycorrhizal fungi Gautieria monticola and Hysterangium setchellii both form dense hyphal mats in coniferous forest soils of the Pacific Northwest. We recently observed that all Douglas-fir seedlings found under the canopy of a maturing 60–75 year stand were associated with mats formed by ectomycorrhizal fungi. The significance of these mat communities in relation to seedling establishment and survival is discussed.

Microbial characteristics of ectomycorrhizal mat communities in Oregon and California

SummarySpecialized ectomycorrhizal fungi form dense mats in forest soils that have different enzyme levels, higher respiration rates, more biomass, different soil fauna, and different soil chemistry compared with adjacent soils not obviously colonized by these mats. In this study, mats formed by two genera of fungi collected in three locations were compared with a wide range of measurements. Per cent moisture, pH, chloroform fumigation-flush C, anaerobic N mineralization, exchangeable ammonium, and respiration, N2 fixation, and denitrification rates were compared between soils or litter colonized by ectomycorrhizal mat-forming fungi and adjacent non-mat material. Significant differences were observed between the two genera of mat-forming fungi and also between mats formed primarily in mineral soil and those formed in litter. These differences suggest that different mat-forming fungi perform different functions in forest soils and that these fungi function differently in mineral soil compared with litter.

Fatty acid esterase production by ectomycorrhizal fungi

Several reports have indicated the stimulating effeet of fatty acid-containing materials on the growth of ectomycorrhizal fungi. Growth of dif? ferent ectomycorrhizal fungi can be stimulated by lipids (Schisler and Volkoff, 1977; Fries et al, 1985), Tween 80 (Straatsma and Bruinsma, 1986), or certain free fatty acids (Lindeberg and Lin? deberg, 1974). Palmer and Hacskaylo (1970) re? ported no growth stimulation using the synthetic lipid triacetin, but this may have been the result of acetate inhibition (Lindeberg and Lindeberg, 1974) or the inability to use acetate as a sole carbon source.

Spatial distribution of ectomycorrhizal mats in coniferous forests of the Pacific Northwest, USA

Ectomycorrhizal mats in forest soils have a wide global distribution and have been noted as potentially important elements in forest soil nutrient cycling. To elucidate the relationship between ectomycorrhizal mats and their environment, we undertook field studies and spatial analyses of mat distributions at different spatial scales.We used two experimental approaches to study mat-forming ectomycorrhizal fungi in coniferous forests of the Pacific Northwest in the United States. In the first approach, ectomycorrhizal mats and other forest floor features were mapped in 2 × 10 m plots and digitized into a geographical information system (GIS) for spatial pattern analysis. In order to examine larger-scale phenomena, a second approach involving other sites was taken; soil cores were taken along 30-m transects, and distance to the closest living potential host tree was calculated for each core.Mat patterns were studied at two scales: (1) within-stand level (i.e. variability attributed to distribution of other mat species, forest floor attributes, and understory vegetation); and (2) stand level (i.e. variability expressed along a successional gradient). Mat distribution was influenced by: (1) the proximity of one mat to another; (2) the distance from the mat to the closest living tree; (3) the density of living trees in a stand; and (4) the successional stage of the stand.Although GIS analysis indicated that mats of different morphologies did not physically overlap, there was a tendency for clustering of mats. No apparent correlations were observed between forest floor features and mats located within the 2 × 10 m grids. On the scale of tens of meters, mats decreased with distance from the closest potential host tree. Spatial patterns of mat distributions in harvested sites suggest that these mats may persist at least 2 years after their host trees have been cut. For Gautieria mats, total mat area, size, and frequency differed with stand age.This study has demonstrated the importance of both spatial scaling and forest stand age when the natural distribution of mycorrhizal fungi is examined. Results suggest the need for mat research directed at higher-order scales (e.g. stand and watershed) that will provide accurate information for managing forests to ensure their survival and normal function. ei]J H Graham

Chemistry and microbial activity of forest and pasture riparian-zone soils along three Pacific Northwest streams

Throughout the United States, agricultural practices are responsible for large quantities of nutrients entering lakes and streams. Previous studies have shown that forested riparian areas can filter nutrients from surface runoff and groundwater that may potentially contaminate lakes and streams. This study examined seasonal differences in soil chemistry and soil microorganisms in paired mixed-forest riparian and pasture systems, the aim being to gain understanding of the sequestering of N and P. The forest soils retained higher levels of organic C and N, mineralizable N, extractable P, and fungal biomass, and had higher respiration rates than pasture soils. These findings suggest that forested riparian zones have a greater capacity than pasture soils to sequester C and retain nutrients. In past studies, fungal biomass has been shown to be less than bacterial biomass in grassland soils, but in this study, fungal biomass was greater than bacterial biomass throughout the year in both forest and pasture soils.

Acetylene reduction in conifer logs during early stages of decomposition

Acetylene reduction was examined periodically for as long as 68 months in the outer and inner bark, sapwood, and heartwood of decaying logs of western hemlock [Tsuga heterophylla (Raf.) Sarg.] western redcedar (Thuja plicata D. Don), Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], and Pacific silver fir (Abies amabilis Dougl. ex Forbes) in the western Oregon Cascade Mountains. Tissues from freshly cut logs from sound trees were unable to reduce acetylene. However, after 18 months of decomposition, acetylene reduction was found in all log tissues except heartwood. Over the 68-month study period, no significant relationship between reduction rate and tissue moisture was found. Acetylene reduction rates differed significantly among tissues, log species, and time of exposure to decomposers. Although acetylene reduction generally showed a steady increase with time, tissues of some species showed a more complex, nonlinear pattern of change. Although the amount of nitrogen fixed is low compared to the total present in decaying logs, it might be an important source of readily available nitrogen for the microbiota responsible for decomposition.