Surface Organic Horizons Research Articles

Changes in forest-floor chemistry caused by a birch admixture in Norway spruce stands

The aim of this study was to determine how soil chemistry and the distribution of fine roots (<1 mm) in the organic and upper mineral soil horizons were affected by an admixture of birch ( Betula pendula Roth and B. pubescens Ehrh.) in Norway spruce ( Picea abies (L.) Karst) stands. The surface organic horizons (LF and H) and mineral soil were characterized to a depth of 10 cm on three sites in southern and central Sweden. On these sites, replicated plots had been established that contained either ca. 30-year-old birch growing as a shelter over similar-aged spruce (mixed plots) or spruce only. The treatments had been created 8–11 years before this study was done. A fourth site, with plots containing ca. 90-year-old spruce or birch/spruce, and a fifth site, with 30-year-old spruce and a low admixture (12% by basal area) of birch, were also included in the study. Concentrations of Ca and Mg and pH in the LF layer were significantly higher in plots with a birch admixture. In the H-horizon, concentrations of K, Ca and Mg were significantly higher in mixed plots than in plots with pure spruce. Consequently, base saturation was higher in mixed plots than in pure spruce plots. A shelter of birch decreased the total amount of spruce fine roots (<1 mm), as revealed at one of the sites. Total fine root biomass (birch + spruce) in the organic and mineral soil horizons (to 10 cm) did not differ significantly between the pure spruce stands and the spruce stands with a birch shelter.

Do plants drive podzolization via rock-eating mycorrhizal fungi?

Weathering and supply of nutrients derived from minerals to plants is known to be stimulated by plant symbiotic mycorrhizal fungi. Nutrients are generally thought to pass the bulk soil solution before plant uptake. Jongmans et al. [Jongmans, A.G., van Breemen, N., Lundström, U.S., van Hees, P.A.W., Finlay, R.D., Srinivasan, M., Unestam, T., Giesler, R., Melkerud, P.-A., Olsson, M., 1997. Rock-eating fungi. Nature, 389, 682–683] showed that (ectomycorrhizal) fungi drill innumerable narrow cylindrical pores (diameter 3–10 μm) into weatherable minerals in podzol E horizons. The fungi probably form micropores by exuding strongly complexing low-molecular weight organic acids at their hyphal tips, causing highly local dissolution of Al silicates. Micropores occurred in all thin sections of podzols under Pinus sylvestris and Picea abies available from Sweden (3), Finland (2), Switzerland (2), Denmark (2) and the Netherlands (3), but not in the few available thin sections of non-podzolic soils under broadleaves. Many weatherable minerals in the podzol E horizon appeared to be perforated, as opposed to few if any in the abruptly underlying B horizon, suggesting a link to podzolization. High concentrations of Al and Si in organic surface horizons under boreal and temperate conifers can be explained by transfer by hypha of weathering products from the minerals to mycorrhizal roots in the O horizon, followed by release of weathering products that are not taken up by the plants. Rock-eating ectomycorrhizal fungi suggest a more direct role for plants in podzolization than hitherto realized, providing tight coupling between podzolization and mineral weathering. Preliminary observations, however, indicate that mycorrhizal fungi do not play a role in podzolization under Kauri ( Agathis australis) in New Zealand.

Atmospheric deposition and watershed nitrogen export along an elevational gradient in the Catskill Mountains, New York

Cumulative effects of atmospheric N deposition may increase N export from water- sheds and contribute to the acidification of surface waters, but natural factors (such as forest productivity and soil drainage) that affect forest N cycling can also control watershed N export. To identify factors that are related to stream-water export of N, elevational gradients in atmo- spheric deposition and natural processes were evaluated in a steep, first-order watershed in the Catskill Mountains of New York, from 1991 to 1994. Atmospheric deposition of SO 2 , and probably N, increased with increasing elevation within this watershed. Stream-water concentrations of SO 2 increased with increasing ele- vation throughout the year, whereas stream-water concentrations of NO 3 decreased with increasing elevation during the winter and spring snowmelt period, and showed no relation with elevation during the growing season or the fall. Annual export of N in stream water for the overall watershed equaled 12% to 17% of the total atmospheric input on the basis of two methods of estimation. This percentage decreased with increasing elevation, from about 25% in the lowest subwatershed to 7% in the highest subwatershed; a probable result of an upslope increase in the thickness of the surface organic horizon, attributable to an elevational gradient in temperature that slows decomposition rates at upper elevations. Balsam fir stands, more pre- valent at upper elevations than lower elevations, may also affect the gradient of subwatershed N export by altering nitrification rates in the soil. Variations in climate and vegetation must be considered to determine how future trends in atmospheric deposition will effect watershed export of nitrogen.

Sediment enrichment ratios after mechanical site preparation for Pinus radiata plantation in the Basque Country

The enrichment of organic matter, total nitrogen, phosphorus, and exchangeable calcium, magnesium, sodium, and potassium in runoff sediment was studied by means of sediment traps in two steep slopes (25–34°) for a period of 18 months after site preparation (scalping and down-slope ripping) for Pinus radiata plantation. Total nitrogen concentration was significantly higher in the source soils than in their sediments. Phosphorus, and exchangeable calcium, magnesium, and potassium concentrations were higher in the sediments, although not significantly. No relation was found between sediment enrichment ratios and total rainfall and rainfall intensity. Steep slopes and hydraulic soil-surface conditions favourable to runoff production are suggested to govern the high erosive power of runoff, which results in a nonselective displacement of soil particles. In spite of this lack of enrichment of the sediments, surface soil samples taken 18 months after site preparation showed significantly lower concentrations of organic matter, total nitrogen, phosphorus, and exchangeable magnesium, sodium, potassium, and aluminium, and higher aluminium saturation of the exchange complex in relation to samples taken before site preparation. Removal of the nutrient-rich organic and mineral surface horizons by scalping, rather than soil water erosion taking place afterwards, is the process responsible for most of the decrease in soil nutrient concentration occurring as a result of site preparation.

Site variation in methane oxidation as affected by atmospheric deposition and type of temperate forest ecosystem

Factors controlling methane oxidation were analyzed along a soil acidity gradient (pH(H2O) 3.9 to 5.2) under beech and spruce forests in Germany. Mean annual methane oxidation ranged from 0.1 to 2.5 kg CH4 ha−1 yr−1 and was correlated with base saturation (r2 = 0.88), soil pH (r2 = 0.77), total nitrogen (r2 = 0.71), amount of the organic surface horizon (r2 = 0.49) and bulk density of the mineral soil (r2 = 0.43). At lower pHs the formation of an organic surface horizon was promoted. This horizon did not have any methane oxidation capacity and acted like a gas diffusion barrier, which decreased the methane oxidation capacity of the soil. In contrast, on sites at the higher end of the pH range, higher burrowing activity of earthworms increased macroporosity and thereby gas diffusivity and methane oxidation. Gas diffusivity was also affected by litter shape: broad beech leaves reduced methane oxidation more than spruce needles. An increase in methane oxidation of most soil samples following sieving indicates that diffusion is the main limiting factor for methane oxidation. However, this “sieving effect” was less in soils with a pH below 5 than in soils with a pH above 5, which we attribute to a direct effect of soil acidity. We discuss our results using a hierarchical concept for the “short‐term” and “long‐term” controls on methane oxidation in forest ecosystems.

Comparison of microbial properties measured by O 2 consumption and microcalorimetry as bioindicators in forest soils

Microbial biomass C was determined in surface organic horizons and in the top 15 cm of the mineral soil of five European coniferous forests. Both oxygen-consumption and heat production following glucose addition were used to measure microbial biomass C. The undisturbed soils showed a marked gradient down the profile with regard to organic C and total N content and microbial biomass. Average values of microbial biomass decreased from the fresh fallen litter in the L horizon (range in five soils) to the B horizon in all five mineral soils from 6667 to 227 μg g −1 DW (O 2-consumption) and from 7555 to 206 μg g −1 DW (heat production), respectively. Correlation analysis between results of O 2-consumption and heat production was carried out using the data from the analysis of complete soil profiles with that from single horizons. The results of both methods for measuring microbial biomass C were, in general, highly correlated ( P<0.01). Significant correlations were also found between the results of both methods for measuring microbial activity. The correlations between soil chemical properties (pH CaCl 2 , organic C, total N and C-to-N ratio) and microbial biomass C indicated very similar relationships for both methods. There were significant relationships ( P<0.05) of microbial biomass C to Ph in F- and H-layers, and in the mineral horizons of microbial biomass to pH, soil organic C and total N. The specific activity of microbial biomass was calculated as the metabolic quotient (μg O 2 mg −1 C mic h −1 [C mic detected by O 2-consumption]) and the caloric quotient (μW mg −1 C mic [C mic detected by heat production]). Specific activity showed different patterns with depth and between soils. Metabolic quotient and caloric quotient were poorly correlated and indicate the limitations of specific activity as a bioindicator in forest soils.

SPECTROSCOPIC EVIDENCE FOR CONDENSED DOMAINS IN SOIL ORGANIC MATTER

Soil organic matter (SOM) has previously been proposed as a dual-mode sorbent for hydrophobic organic compounds (HOC), i.e., partitioning takes place in both expanded (rubbery) and condensed (glassy) domains, whereas competitive sorption takes place only in condensed domains. The objective of this paper was to provide some spectroscopic evidence of condensed domains in SOM. Six humic acids (HA) extracted from different depths of a single soil profile were analyzed using X-ray diffraction (XRD) and solid-state [sup 13]C nuclear magnetic resonance (NMR) with cross-polarization (CP) and magic angle spinning (MAS) techniques. The XRD results showed a strong peak at 0.35 nm for the condensed aromatic structure of the HA from mineral horizons and a dominant peak at 0.43 nm for less tightly packed side chains of the HA from surface organic horizons. The [sup 13]C CP/MAS NMR results indicated further that HA becomes more aromatic from surface organic to mineral horizons. This increase of aromatic structure in HA was confirmed by the increase of nonprotonated carbon signals at 130 ppm using a dipolar-dephasing technique and by the increase of atomic C/H and C/O ratios. These spectroscopic and elemental data corroborate further the dual-mode model for the sorption of HOC in SOM.

Microbial transformations of C and N in a boreal forest floor as affected by temperature

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding 15 N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH4+, NO3−, 15NH4+ and 15 NO3−. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH4+ mineralization and NH4+ immobilization increased. In the H layer, however, both gross NH4+ mineralization and NH4+ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate 15N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability.

Armillaria root disease, stand characteristics, and soil properties in young lodgepole pine

Thirty six lodgepole pine stands in west-central Alberta, ranging in age from 9 to 28 yr of age, were surveyed for Armillaria root disease mortality. Selected stand characteristics and edaphic factors were recorded in each stand to determine if there was a relationship between these factors and Armillaria root disease. Data was analysed using a multivariate linear regression. A statistically significant relationship was found between sand content, ammonium (NH 4 +) concentration and Armillaria root disease. The incidence of Armillaria root disease increased significantly with increasing sand content of the surface mineral horizon and decreasing NH 4 + concentration of the surface organic horizon (LFH). Stand age, density, elevation, soil pH, depth of the surface organic horizon (LFH), calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S) did not have a significant effect on the amount of mortality.

Runoff from Simulated Rainfall in 2 Montane Riparian Communities

Riparian ecosystems are the final terrestrial zone before runoff water enters a stream. They provide the last opportunity to decrease non-point source pollution delivery to streams by removing sediments from overland water flow from uplands and roads. To quantify processes of sediment transport, filtration and deposition, it is necessary to determine runoff characteristics for the area. A rotating boom rainfall simulator was used to evaluate the effects of 3 vegetation height treatments (control, 10-cm stubble height, and clipped to the soil surface) in 2 montane riparian plant communities (grass and sedge) on runoff characteristics. Each rainfall simulation event consisted of 2 phases, a dry run of about 60 min followed by a wet run approximately 30 min later. There were no differences in time to runoff initiation for either dry or wet runs that could be attributed to vegetation height treatments for either plant community. It usually required more time for runoff to be initiated in the sedge community compared to the grass community. Generally, there were lower equilibrium runoff percentages from dry runs in the sedge community compared with the grass community. These differences were less during wet runs. Several runoff parameters had characteristics of runoff from water repellent soils. The organic layer on the soil surface exhibited signs of water repellency that reduced the water infiltration rate during the initial stages of a rainfall simulation. These results indicate that runoff and infiltration processes in the surface organic horizon of riparian zones may not respond in the classical manner. This characteristic has important implications if criteria developed in areas with less organic matter on the soil surface are used to manage overland flow in the zone. Additional studies are needed to fully describe infiltration and runoff processes in riparian plant communities.

Regional Soil Organic Carbon Storage Estimates for Western Oregon by Multiple Approaches

AbstractSoil is an important factor in regional and global C budgets because it serves as a reservoir of large amounts of organic C. In our study, we compared six approaches of estimating soil organic C (kg C m−2, not including the surface organic horizon, hereafter called soil C) and its spatial pattern in the mountainous, largely forested western Oregon region. The approaches were (i) USDA NRCS pedons, (ii) other pedons, (iii) the State Soil Geographic Data Base (STATSGO), (iv) the United Nations Soil Map of the World, (v) the National Soil Geographic Data Base (NATSGO), and (vi) an ecosystem‐complex map. Agreement between approaches varied with scale. For the entire region (105 km2), estimates of average soil C varied from 4.3 to 6.8 kg C m−2 for the 0‐ to 20‐cm depth and from 12.1 to 16.9 kg C m−2 for the 0‐ to 100‐cm depth. At the subregional scale (≈104 km2), all approaches indicated higher soil C in the coastal area than in the inland southern area, but relative amounts in other subregions varied among the approaches. At the subsubregional scale (≈103 km2), soil C was consistent between individual STATSGO map units and NRCS pedons within those map units, but there was less agreement with other pedons. Rigorous testing of soil‐C maps requires data from pedons that are located by objective criteria, in contrast to the subjectively located pedons now available. The uncertainty associated with regional soil‐C amounts and spatial patterns should be considered when soil‐C maps are integrated into regional or global assessments of physical and biotic processes because simulation‐model outputs may be sensitive to soil C.

Effects of soil compaction and chipped aspen residue on aspen regeneration and soil nutrients

The effects of soil compaction and depth of chipped aspen residue on aspen regeneration, plant community development, decomposition, and nutrient cycling were tested on an Orthic Gray Luvisol in the Lower Foothills of Alberta. Twenty-seven 100-m × 15-m plots were established in 1992. Three depths of chipped aspen residue (0, 1–5 cm and &gt;10 cm) and three levels of compaction (no compaction, 6 and 16 skidder passes) were applied following whole-tree harvesting in 1993. Sixteen skidder passes and chip residue depths &gt;10 cm resulted in reduced plant cover and aspen sucker densities for at least 2 yr following treatment. Nutrient concentrations of the chip residue and surface organic horizon (LFH) were similar for the three compaction treatments. The initial total nitrogen (N) concentration of the chipped aspen residue was 3.6 g kg−1. Ammonium (NH4+-N) concentrations in the LFH of the chip residue treatments, particularly at the &gt;10-cm residue depth, were lower compared with the no-residue treatment. However, adequate N apparently was available to the aspen suckers and understory vegetation as the NH4+-N concentration remained higher than in the LFH before harvesting. Compaction and woody residue applications had only short-term (3 yr) effects on soil properties, plant community development and aspen suckering at this site when clear-cut harvesting was done in the fall with low soil moisture content. Key words: Ammonium, aspen regeneration, wood residue, compaction

Silvicultural treatments, microclimatic conditions and seedling response in Southern Interior clearcuts

Post-harvest levels of soil disturbance and vegetation regrowth strongly influence microclimate conditions, and this has important implications for seedling establishment. We examined the effects of blading (scalping), soil loosening (ripping) and vegetation control (herbicide), as well as no soil disturbance, on growing season microclimates and 3-yr seedling response on two grass-dominated clearcuts at different elevations in the Southern Interior of British Columbia. Warmer soil temperatures were obtained by removing surface organic horizons. Ripping produced somewhat higher soil temperatures than scalping at the drier, lower-elevation site, but slightly reduced soil temperatures at the wetter, higher-elevation site. Near-surface air temperatures were more extreme (higher daily maximums and lower daily minimums) over the control than over exposed mineral soil. Root zone soil moisture deficits largely reflected transpiration by competing vegetation; vegetation removal was effective in improving soil moisture availability at the lower elevation site, but unnecessary from this perspective at the higher elevation site. The exposed mineral surfaces self-mulched and conserved soil moisture after an initial period of high evaporation. Ripping and scalping resulted in somewhat lower near-surface available soil water storage capacities. Seedling establishment on both clearcuts was better following treatments which removed vegetation and surface organic horizons and thus enhanced microclimatic conditions, despite reducing nutrient supply. Such treatments may, however, compromise subsequent stand development through negative impacts on site nutrition. Temporal changes in the relative importance of different physical (microclimate) and chemical (soil nutrition) properties to soil processes and plant growth need to be considered when evaluating site productivity. Key words: Microclimate, soil temperature, air temperature, soil moisture, clearcut, seedling establishment

Forest floor mass and nutrients in two chronosequences of plantations: Jack pine vs. black spruce

The purpose of this study was to determine whether change of forest cover had an effect on the development of the organic surface horizons, particularly on those variables that influence nutrient cycling and forest productivity. Jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana [Mill.] B.S.P.) plantations were selected from among the youngest to oldest (2–16 yr) within a 100 km2 area in southeastern New Brunswick. Natural forests were also included as benchmark sites. The forest floor and tree foliage was sampled and trees measured on 0.05-ha plots. The forest floor samples were used to determine organic mass, nutrient contents and pH. In pine plantations, organic matter accumulated rapidly during the period of exponential tree growth, but leveled off at about 45 Mg ha–1. This was within the range of benchmark sites with mixed conifer-hardwood cover. In spruce plantations, the forest floor mass ranged upward to 77 Mg ha–1. Development was strongly influenced by the nature of the previous forest. Spruce forest floors were on average more acid and had lower nutrient concentrations, particularly N and Ca. The observed differences suggest that nutrients are recycled more rapidly in the pine plantations, partly explaining the superior growth of the latter. Key words: Forest floor, Kalmia angustifolia L., Picea mariana (Mill.) B.S.P., Pinus banksiana Lamb., nutrient cycling, plantation forest

Influence of precipitation composition on the chemistry of streams draining from peat examined using Na:Ca:Mg ratio

It is hypothesised that, since precipitation chemistry regulates the constitution of the base cations occupying the cation exchange sites of peat soils, and the latter in turn directly regulates the relative abundance of the corresponding base cations in drainage waters, the ratios of base cations in precipitation and drainage waters from peats should be very similar. The hypothesis is confirmed using data for 45 streams in western Scotland sampled under baseflow conditions, and for three streams in northeastern Scotland sampled along their length under baseflow conditions and also throughout a sequence of three successive storms. During the latter, as the soils became progressively wetter, and flow through or over surface organic horizons started to make an increasing relative contribution to total discharge, the difference between cation ratios in precipitation and river water was reduced. The effects of changing hydrological pathway in the three catchments were confirmed by changes in Al:Ca:Mg ratios between baseflow and storm water samples.

Soil Changes and Tree Seedling Response Associated with Site Preparation in Northern Idaho

Abstract Conifer regeneration in western North America is often hampered by low soil moisture, poor soil nutrient status, and competing vegetation. Three site preparation techniques were evaluated at two different elevations in northern Idaho as potential remedies for these problems: (1) soil mounds without control of competing vegetation, (2) soil mounds with herbicidal control of competing vegetation, and (3) scalping (removal of soil surface organic horizons and mineral topsoil). Treatments were evaluated for effects on soil nutrient levels, soil physical properties, and the growth of Douglas-fir (Pseudotsuga menziesii var. glauca) and western white pine (Pinus monticola) seedlings. Both species generally grew best when planted in the mounded treatment with competing vegetation removed and worst after scalping. Mounding with herbicide application resulted in the lowest bulk density, best seedling growth, and increased water availability. Mounding may be a viable site preparation method in the Inland Northwest on less productive sites that have severe competition. Scalping, especially when competition was not a problem, generally did not produce favorable seedling growth responses. Scalping may also reduce longer term seedling growth by removing surface organic matter. West. J. Appl. For. 12(3):81-88.

Soil Nutrient Dynamics in a Boreal Mixedwood Cutover Following the Application of Hexazinone

Large-scale human disturbances, primarily harvesting and site preparation, are recent and increasingly frequent activities in the Boreal Mixedwood, and potential impacts on long-term site productivity are unknown. A 7-yr experiment was carried out in a clear-cut Boreal Mixedwood forest to assess the effect of the herbicide hexazinone on soil nutrient dynamics. A completely randomized design with three herbicide treatments (0, 2, and 4 kg of active ingredient (a.i.) per hectare applied as Pronone 10G) and five replicates per treatment was established. The surface organic horizon (L-H) and three mineral horizons (Ah, Ae, and Btnj horizons) were sampled for five years: one year prior to the herbicide application and during the first, second, third, and sixth year after the herbicide was applied. Littertraps and lysimeters were installed and samples collected for the first three years of the study. Total biomass decreased and N concentrations increased in litterfall for at least 2 yr following the application of hexazinone. Other elemental concentrations were affected only in the year of application. Concentrations of ammonium (NH4+-N) and nitrate (NO3−-N) in the L-H of the hexazinone treatments were higher relative to the control. Extractable phosphorus (P), sulfur (S), and potassium (K) in the L-H of the herbicide treatments were lower by as much as 25%. These differences persisted for the duration of the study. No changes in extractable calcium (Ca) and magnesium (Mg) were found. Elevated concentrations of NH4+-N and K were measured in the Ah and Ae mineral horizons of the highest herbicide treatment, but total soil nutrient pools remained unaffected by the herbicide treatments. Reduced plant uptake of nutrients and the indirect impact of changes to the vegetation on soil mineralization–immobilization processes resulted in the differences in the available nutrient concentrations of the L-H horizon. The short-term results (6 yr) following the hexazinone application indicate that there was little change to the total nutrient pools and that the potential loss of nutrients at this site was minimal.

SOIL NUTRIENT DYNAMICS IN A BOREAL MIXEDWOOD CUTOVER FOLLOWING THE APPLICATION OF HEXAZINONE

Large-scale human disturbances, primarily harvesting and site preparation, are recent and increasingly frequent activities in the Boreal Mixedwood, and potential impacts on long-term site productivity are unknown. A 7-yr experiment was carried out in a clear-cut Boreal Mixedwood forest to assess the effect of the herbicide hexazinone on soil nutrient dynamics. A completely randomized design with three herbicide treatments (0, 2, and 4 kg of active ingredient (a.i.) per hectare applied as Pronone 10G) and five replicates per treatment was established. The surface organic horizon (L-H) and three mineral horizons (Ah, Ae, and Btnj horizons) were sampled for five years: one year prior to the herbicide application and during the first, second, third, and sixth year after the herbicide was applied. Littertraps and lysimeters were installed and samples collected for the first three years of the study. Total biomass decreased and N concentrations increased in litterfall for at least 2 yr following the application of hexazinone. Other elemental concentrations were affected only in the year of application. Concentrations of ammonium (NH4+-N) and nitrate (NO3−-N) in the L-H of the hexazinone treatments were higher relative to the control. Extractable phosphorus (P), sulfur (S), and potassium (K) in the L-H of the herbicide treatments were lower by as much as 25%. These differences persisted for the duration of the study. No changes in extractable calcium (Ca) and magnesium (Mg) were found. Elevated concentrations of NH4+-N and K were measured in the Ah and Ae mineral horizons of the highest herbicide treatment, but total soil nutrient pools remained unaffected by the herbicide treatments. Reduced plant uptake of nutrients and the indirect impact of changes to the vegetation on soil mineralization–immobilization processes resulted in the differences in the available nutrient concentrations of the L-H horizon. The short-term results (6 yr) following the hexazinone application indicate that there was little change to the total nutrient pools and that the potential loss of nutrients at this site was minimal.

LIVE AND DEAD ROOTS IN FOREST SOIL HORIZONS:CONTRASTING EFFECTS ON NITROGEN DYNAMICS

Roots are widely acknowledged to be major contributors to ecosystem nutrient cycles. However, live roots may have very different effects than dead roots, the quality of the soil surrounding the roots may itself affect the influence of the roots, and the nature of the soil profile may modify all of these interactions. We have used experimental, field-based mesocosms to dissect the differing effects of live and dead roots in organic and mineral horizons of spodosolic forest soils. Mesocosms were constructed in a simple ecosystem dominated by pitch pine (Pinus rigida Mill.) and blueberry (Vaccinium corymbosum L.) in the New Jersey Pinelands. They contained 5 cm deep surface horizons of either organic matter or an inert substitute (vermiculite), placed above 10 cm of mineral material from the E horizon, with defined amounts of live or dead roots added. Sampling was conducted over a 2-yr period. In mineral soil, live roots stimulated ammonification rates but had little effect on extractable N, and the stimulation occurred only if an organic surface horizon was present. In contrast, live roots in organic material reduced both ammonification rates and extractable N. Dead roots increased extractable N in the mineral material, but not the organic material. The presence or absence of an organic horizon had a dominating effect on all aspects of nitrogen dynamics in the mineral soil; not only extractable inorganic N, but also transformation rates, were higher in soils under organic matter. The results showed that the effects of roots on soil nutrients depended on (1) the ratio of live to dead root biomass, (2) the quality of the soil material in which the roots occurred, and (3) the structure of the soil profile. These findings necessitate a revision of the concept of “rhizosphere effect” in forest soils.

Planted conifer seedling growth under two soil thermal regimes in high-elevation forest openings in interior British Columbia.

A field trial was conducted investigating the single season growth response of 1+0 313 PSB Engelmann spruce (Picea engelmannii Parry ex Engelm.) and lodgepole pine (Pinus contorta Dougl. ex Loud.) seedlings planted into two different soil thermal regimes at three high-elevation locations spanning 200 km in the Engelmann Spruce-Subalpine Fir (ESSF) biogeoclimatic zone in the Cariboo Mountains of central British Columbia. Temperature treatments represented the extremes of soil temperature commonly found in high-elevation clear-cuts. A warm soil treatment (clear day, mid-afternoon soil temperature in mid-summer of 18 to 25 °C at −10 cm) consisting of a bare mineral soil hummock (average dimensions of 100 cm × 100 cm × 40 cm) was contrasted with a cool soil treatment (clear day, mid-afternoon soil temperature in mid-summer of 10 to 13 °C at −10 cm) comprised of organic forest floor overlying mineral soil. By the end of the growing season, seedlings of both species planted into the warm treatment generally exhibited greater root, stem, foliage, and total seedling biomass than cool treatment seedlings. Measurements of root growth at 30, 60, and 90 days after planting showed that total root number and total root length were consistently greater for warm treatment seedlings than for cool treatment seedlings. Root growth was greater from the bottom rather than from the side of the root plug for all seedlings. These results suggest that the effect of low soil temperatures on outplanted styroblock conifer seedlings is pronounced and may be limiting growth performance in high-elevation plantations in British Columbia. We recommend silvicultural treatments that secure natural regeneration, ensure that warmer microsites are always planted, and utilize seedling stocktypes able to make rapid lateral root growth into warmer surface organic horizons.