Mineral Soil Horizons Research Articles

Pedological Characterisation of Sites Along a Transect from a Primary Cadmium/Lead/Zinc Smelting Works

A pedological characterisation of seven sites along a transect from a smelter at Avonmouth, UK, was undertaken. Site locations comprised a mixture of both grassland (5 sites) and oak tree dominated (2 sites) areas. Geographically, sites were either low lying or on adjacent elevated ground. Across the transect, a severe gradient of metal concentrations was found with highest values for organic soil horizons from close to the factory. Comparisons with quality standards indicate that these metal levels are likely to be a major ecological stressor. In addition to the strong metal gradient, a number of other between site differences were also observed. These were firstly water regime, which at the low lying sites close to the smelter showed influence by both, groundwater and stagnant water, while at more distant sites indicated susceptibility to stagnant water only, and secondly soil pH, which showed large between site variations, although no consistent trend along the transect. Humus forms at sites more than 1.5 km from the smelter were characteristic for the soil conditions and land-use present, while those at sites within 1.5 km showed disturbed profiles. Indeed, the humus types present at these locations suggest that the activity of soil invertebrates may be inhibited. This is almost certainly due primarily to the high concentrations of metals that were present in mineral soil and organic horizons at the sites. However, the potential influences of water regime and pH could also be relevant factors influencing the results of any further studies conducted at these study sites.

Chemical and Seasonal Controls on the Dynamics of Dissolved Organic Matter in a Coniferous Old-growth Stand in the Pacific Northwest, USA

Soil organic matter (SOM) is the largest terrestrial C pool, and retention and release of dissolved organic matter (DOM) cause formation and loss of SOM. However, we lack information on how different sources of DOM affect its chemical composition, and how DOM chemical composition affects retention. We studied seasonal controls on DOM production and chemical controls on retention in soils of a temperate coniferous forest. The O horizon was not usually the dominant source for dissolved organic C (DOC) or N (DON) as has been reported for other sites. Rather, net production of both DOC and DON was often greater in the shallow mineral soil (0-10 cm) than in the O horizon. DOM production in the shallow mineral soil may be from root exudation as well as turnover of fine roots and microflora in the rhizosphere. In the field, the two acid fractions (hydrophobic and hydrophilic acids) dominated the soil solution at all depths. A major portion of net production and removal of total DOC within the soil column was explained by increases and decreases in these fractions, although a shift in chemical composition of DOM between the O and mineral soil horizons suggested different origins of DOM in these layers. A larger loss of the free amino fraction to deep soil water at this study site than at other sites suggested lower retention of labile DON. Field DOM removal measurements suggest that field-measured parameters may provide a good estimate for total DOM retained in mineral soil.

Non-native invasive earthworms as agents of change in northern temperate forests

Exotic earthworms from Europe and Asia are invading many northern forests in North America that currently lack native earthworms, providing an opportunity to assess the role of this important group of invertebrates in forest ecosystems. Research on earthworm invasions has focused on changes in soil structure and carbon (C) and nitrogen (N) cycling that occur following invasion. These changes include the mixing of organic and mineral soil horizons, decreases in soil C storage, and equivocal effects on N cycling. Less well studied are changes in the soil foodwebs that accompany earthworm invasion. Soils of north temperate forests harbor a tremendous diversity of microorganisms and invertebrates, whose distribution and abundance can be substantially altered by earthworm invasion. Furthermore, invasive earthworms can affect understory plant communities, raising concerns over the loss of rare native herbs in some areas. The ecological consequences of earthworm invasion are mediated through physical, geochemica...

SOIL CARBON DYNAMICS ACROSS A WINDTHROW DISTURBANCE SEQUENCE IN SOUTHEAST ALASKA

Few studies have examined the influence of natural disturbances, such as windthrow, on soil organic matter formation, stabilization, and loss in soils. In shallow (<1 m) mountain forest soils, windthrow activity may result in the redistribution and mixing of mineral and organic soil horizons down to bedrock. We studied the patterns of soil carbon, the dominant constituent of soil organic matter, in watersheds along a windthrow disturbance sequence in a mountainous temperate rain forest in southeast Alaska. Our objectives were (1) to evaluate the influence of windthrow and illuviation on the accumulation of soil organic carbon in mineral horizons and (2) to compare the forms of soil organic matter that have accumulated. Soils were described, and the thickness of the major organic and mineral horizons was measured, every 5 m along transects in three watersheds with contrasting windthrow histories. A subset of the soil description sites was randomly selected and then sampled to determine the quantity and qua...

Distributions of the capacity to take up nutrients by Betula spp. and Picea abies in mixed stands

We examined the distribution of nutrient uptake (i.e. uptake of radioactive isotopes of nutrients) in the forest floor and lower part of the rooting zone (mineral soil) for birch ( Betula pendula Roth and B. pubescens Ehr.) and Norway spruce ( Picea abies (L.) Karst) in mixed stands on different soils by applying radioactive 32 P , 86 Rb and 45 Ca around individual trees at two depths. One month after application the current year foliage of the tree around which the application had been done and of an adjacent tree was sampled and analyzed for radioactivity (Bq kg −1). Birch and spruce had a similar proportion of uptake of 32 P from the lower application depth. In two of the stands, application of 45 Ca and subsequent foliar analyses did not support the hypothesis that birch and spruce differ in their capacity to take up Ca from organic (forest floor) and upper mineral soil horizons. The analogue of potassium, 86 Rb was applied in the upper mineral soil on two different occasions (June, July/August) in one stand. The uptake of 86 Rb from the mineral soil by spruce did not differ when neighbouring trees consisted of birch and spruce (mixed neighborhood) or of spruce only (spruce neighborhood). The total uptake, from forest floor and mineral soil, of radioactive phosphorus ( 32 P / 33 P ) in current year foliage was in general higher for birch than for spruce indicating a higher demand for P by birch. The distribution of uptake of radiotracers did not match data on the distribution of fine roots at the investigated sites which indicated that spruce generally had a more shallow distribution of fine roots compared to birch. Our results do not support the hypothesis that broad-leaved tree species and conifers generally are complementary in nutrient uptake from the forest floor and mineral soil in mixed stands. Neither do the nutrient uptake from the mineral soil by spruce generally change with an admixture of a broad-leaved tree species.

Survey of mites in caves and deep soil and evolution of mites in these habitats

We studied adaptations to subterranean environments in Acari. Mite populations of two caves and of the mineral horizon of soil from three forest sites were analysed. In the soil, body length was limited by the pore size. The proportions of (i) predators, (ii) phoretic mites, and (iii) primitive oribatids were higher in caves. This was potentially explained by (i) polyphagy or predation on other groups and reduced interspecific competition, (ii) the fragmented structure and liability to flooding of the cave habitat, and (iii) the higher humidity in caves. Adaptations to dry habitats probably greatly influenced the evolution in Oribatida. It is proposed that numerous mites colonized caves directly from deep soil without a surface stage.

Assessing the use of delta(13)C natural abundance in separation of root and microbial respiration in a Danish beech (Fagus sylvatica L.) forest.

Our understanding of forest biosphere-atmosphere interactions is fundamental for predicting forest ecosystem responses to climatic changes. Currently, however, our knowledge is incomplete partly due to inability to separate the major components of soil CO(2) effluxes, viz. root respiration, microbial decomposition of soil organic matter and microbial decomposition of litter material. In this study we examined whether the delta(13)C characteristics of solid organic matter and respired CO(2) from different soil-C components and root respiration in a Danish beech forest were useful to provide information on the root respiration contribution to total CO(2) effluxes. The delta(13)C isotopic analyses of CO(2) were performed using a FinniganMAT Delta(PLUS) isotope-ratio mass spectrometer coupled in continuous flow mode to a trace gas preparation-concentration unit (PreCon). Gas samples in 2-mL crimp seal vials were analysed in a fully automatic mode with an experimental standard error +/-0.11 per thousand. We observed that the CO(2) derived from root-free mineral soil horizons (A, B(W)) was more enriched in (13)C (delta(13)C range -21.6 to -21.2 per thousand ) compared with CO(2) derived from root-free humus layers (delta(13)C range -23.6 to -23.4 per thousand ). The CO(2) evolved from root respiration in isolated young beech plants revealed a value intermediate between those for the soil humus and mineral horizons, delta(13)C(root) = -22.2 per thousand, but was associated with great variability (SE +/- 1.0 per thousand ) due to plant-specific differences. delta(13)C of CO(2) from in situ below-ground respiration averaged -22.8 per thousand, intermediate between the values for the humus layer and root respiration, but variability was great (SE +/- 0.4 per thousand ) due to pronounced spatial patterns. Overall, we were unable to statistically separate the CO(2) of root respiration vs. soil organic matter decomposition based solely on delta(13)C signatures, yet the trend in the data suggests that root respiration contributed approximately 43% to total respiration. The vertical gradient in delta(13)C, however, might be a useful tool in partitioning respiration in different soil layers. The experiment also showed an unexpected (13)C-enrichment of CO(2) (>3.5 per thousand ) compared with the total-C signatures in the individual soil-C components. This may suggest that analyses of bulk samples are not representative for the C-pools actively undergoing decomposition.

Isotopic estimates of new carbon inputs into litter and soils in a four-year climate change experiment with Douglas-fir

Because soil is a major reservoir of terrestrial carbon and a potential sink for atmospheric CO2, determining plant inputs to soil carbon is critical for understanding ecosystem carbon dynamics. We present a modified method to quantify the effects of global climate change on plant inputs of carbon to soil based on 13C:12C ratio (δ13C) analyses that accounts for isotopic fractionation between inputs and newly created soil carbon. In a four-year study, the effects of elevated CO2 and temperature were determined for reconstructed Douglas-fir (Pseudotsuga mensiezii (Mirb.) Franco) ecosystems in which native soil of low nitrogen content was used. The δ13C patterns in litter and mineral soil horizons were measured and compared to δ13C patterns in live needles, fine roots, and coarse roots. From regression analyses, we calculated the isotopic enrichment in 13C of newly incorporated soil carbon relative to needle and root carbon at 4‰ and 2‰, respectively. These enrichments must be considered when using shifts in soil δ13C to calculate inputs of plant carbon into the soil, and are probably a major factor in the progressive enrichment in 13C with increasing depth in soil profiles. Relative to the total carbon in each layer, the proportion of new carbon from recent photosynthate in each soil layer was 13–15% in the A horizon, 7–9% in litter layers, and 4% in the B2 and C horizons. New carbon in the A horizon was estimated at 370 g C m−2. Carbon concentrations and new carbon in A horizons were correlated (r 2=0.78, n=12), but with a slope of 0.356, indicating that about 36% of net carbon accumulation in the A horizon was from inputs via roots, root exudates or mycorrhizal fungi and 64% of carbon was derived from surface litter decomposition. Under the nitrogen-limited growth conditions used in this study, neither elevated CO2 nor temperature affected soil carbon sequestration patterns.

Soil microbiological monitoring of a pine forest after partial thinning for stand regeneration with beech seedlings

A 62 year old pine stand located in northeastern Germany (Brandenburg, Ost-Prignitz, Revier Beerenbusch) was studied with respect to a thinned vs. an unthinned variant. Samples of the litter layer and the top mineral horizon of an acid brown earth were collected from five plots at each variant in April and November during 3 years (2000–2002), respectively. Soil microbial biomass (Cmic and basal CO2 respiration were assayed using an infrared gas analysis system. Soil organic carbon (Core) and total nitrogen (Nt) contents were determined using a C/N elemental analyzer. In addition, cellulose decomposition in soil was assayed using a mini-container system. The contents of Corg, Nt, Cmic and basal respiration activity in the humus layer and in the mineral soil horizon did not differ significantly during 3 years shortly after thinning. Furthermore, no difference was detected comparing spring and autumn samplings of 3 years under study. On the other hand, cellulose decomposition in soil was slightly but not yet significantly increased in the thinned variant. Thus, 3 years after thinning no significant actual impacts on soil ecological properties were detected, but first evidence for increased soil biochemical activities in the thinned variant was found. Long-term monitoring and spatially more detailed analyzes are required to reveal lasting impacts of thinning. Special emphasis needs to be directed on the composition of soil microbial communities and on specific activities related to soil organic matter transformation.

Forest soil disturbance intervals inferred from soil charcoal radiocarbon dates

Forest soil disturbance intervals are usually too long to measure using plot-based studies, and thus they are poorly understood. The mean soil disturbance interval (MSDI) in an old-growth forest on the west coast of Vancouver Island was estimated from radiocarbon dates of charcoal from organic and mineral soil horizons. Two assumptions are required to estimate the MSDI: (1) charcoal from forest fires is deposited within the organic horizon and eventually mixed into deeper mineral horizons by soil disturbances, and (2) the probability of soil disturbance is spatially homogeneous and affected only by the time since the last fire or the last soil disturbance. The MSDI is then estimated by the rate at which the proportion of undisturbed sample sites (determined by the proportion of sites with charcoal from the most recent fire in the organic horizon) decreases with increasing time since the last fire. Soil charcoal evidence of time since fire was determined at 83 sites using 141 radiocarbon dates. The estimated MSDI was greater on slopes (ca. 2010 years) than on terraces (ca. 920 years). The long periods between soil disturbances, especially on slopes, are consistent with other evidence from the study area that suggests infrequent tree uprooting is the predominant mode of soil disturbance.

Natural &lt;sup&gt;15&lt;/sup&gt;N abundance in two nitrogen saturated forest ecosystems at Solling, Germany

This research deals with a comparative study of two different N-saturated forests: 1. beech forest and 2. spruce forest at the same locality of \solling, Central Germany. The present results show that &lt;sup&gt;15&lt;/sup&gt;N natural occurrence in the rainfall (both above and below canopy) at Solling site is similar (&amp;delta;&lt;sup&gt;15&lt;/sup&gt;N = &amp;ndash;15&amp;permil; to +19&amp;permil;) to other sites of the world (such as NITREX sites, USA etc.). Furthermore, &lt;sup&gt;15&lt;/sup&gt;N values in the soil water ranged from &amp;ndash;4.32 (&amp;plusmn; 2.09) to +5&amp;permil; (&amp;plusmn; 1.47), which also corresponds to NITREX sites and other sites of Europe and USA. In both forests, &amp;delta;&lt;sup&gt;15&lt;/sup&gt;N enrichment of both NH&lt;sub&gt;4&lt;/sub&gt;-N and NO&lt;sub&gt;3&lt;/sub&gt;-N showed a decreasing trend of their values from bulk precipitation to the upper soil layer, but increasing in the deeper soil layer again. An increase in the &lt;sup&gt;15&lt;/sup&gt;N enrichment of soil water from upper soil depth to lower soil depth was observed in our study and it is assumed to be due to the strong net nitrification taking place in the upper layer (organic surface layer) of soil. The soils at both sites showed characteristic low (negative) &amp;delta;&lt;sup&gt;15&lt;/sup&gt;N values in the upper organic layers, strongly increasing to positive &amp;delta;&lt;sup&gt;15&lt;/sup&gt;N values in the mineral soil. In the lower depths of mineral soil horizons of both stands, an increase in &amp;delta;&lt;sup&gt;15&lt;/sup&gt;N values was found to culminate at +3 to +5&amp;permil;. In contrast to the mineral soil horizon, in the organic soil horizon (0 to 6 cm depth) of both sites there was almost a similar or slight decrease in &amp;delta;&lt;sup&gt;15&lt;/sup&gt;N values with depth. This is attributed to the high nitrification rate in the organic soil horizon, resulting in excessive seepage water NO&lt;sub&gt;3&lt;/sub&gt;-output at both sites (especially at the spruce site).

Location and chemical composition of stabilized organic carbon in topsoil and subsoil horizons of two acid forest soils

The 14C age of soil organic matter is known to increase with soil depth. Therefore, the aim of this study was to examine the stabilization of carbon compounds in the entire soil profile using particle size fractionation to distinguish SOM pools with different turnover rates. Samples were taken from a Dystric Cambisol and a Haplic Podzol under forest, which are representative soil types under humid climate conditions. The conceptual approach included the analyses of particle size fractions of all mineral soil horizons for elemental composition and chemical structure of the organic matter by 13C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. The contribution of phenols and hydroxyalkanoic acids, which represent recalcitrant plant litter compounds, was analyzed after CuO oxidation. In the Dystric Cambisol, the highest carbon concentration as well as the highest percentage of total organic carbon are found in the <6.3 μm fractions of the B and C horizons. In the Haplic Podzol, carbon distribution among the particle size fractions of the Bh and Bvs horizons is influenced by the adsorption of dissolved organic matter. A relationship between the carbon enrichment in fractions <6.3 μm and the 14C activity of the bulk soil indicates that stabilization of SOM occurs in fine particle size fractions of both soils. 13C CPMAS NMR spectroscopy shows that a high concentration of alkyl carbon is present in the fine particle size fractions of the B horizons of the Dystric Cambisol. Decreasing contribution of O-alkyl and aromatic carbon with particle size as well as soil depth indicates that these compounds are not stabilized in the Dystric Cambisol. These results are in accordance with data obtained by wet chemical analyses showing that cutin/suberin-derived hydroxyalkanoic acids are preserved in the fine particle size fractions of the B horizons. The organic matter composition in particle size fractions of the top- and subsoil horizons of the Haplic Podzol shows that this soil is acting like a chromatographic system preserving insoluble alkyl carbon in the fine particle size fractions of the A horizon. Small molecules, most probably organic acids, dominate in the fine particle size fractions of the C horizons, where they are stabilized in clay-sized fractions most likely due to the interaction with the mineral phase. The characterization of lignin-derived phenols indicated, in accordance with the NMR measurements, that these compounds are not stabilized in the mineral soil horizons.

ICP-MS measurements of lead isotopic ratios in soils heavily contaminated by lead smelting: tracing the sources of pollution.

The Pb isotopic composition (206Pb/207Pb and 208Pb/206Pb) in smelter-impacted soils was measured using a quadrupole-based ICP-MS. Four forest/tilled soil profiles were sampled according to the distance from the lead smelter in Príbram (Czech Republic), prevailing wind direction, geological background and soil type. The results were compared with the Pb isotopic composition of bedrocks and waste materials from Pb metallurgy (smelting slags, air-pollution-control residues). The isotopic composition of soils confirms the predominant role of metallurgy on the general pollution in the area. The highly contaminated soils from the vicinity of the smelter contain up to 35,300 mg Pb kg(-1) and exhibit an isotopic composition close to that of car battery processing (206Pb/207Pb up to 1.177). A coupled concentration/isotopic study of soil profiles showed that the smelter-induced pollution had penetrated even to the mineral soil horizons, indicating an important vertical mobility of Pb contaminant within the soil profile. The calculated downward penetration rate of Pb in soils ranges from 0.3 to 0.36 cm year(-1).

Root turnover and root necromass accumulation of Norway spruce (Picea abies) are affected by soil acidity.

Fine root distribution and turnover were investigated in ca. 40-year-old pure Norway spruce (Picea abies Karst.) stands in Germany, growing on four sites that differed in soil acidity (Ebergötzen < Barbis < Fichtelgebirge = Harz). The density of fine root biomass and necromass in different soil horizons differed among the sites. At one of the most acidic sites (Harz), fine root density in the humus layer was more than twice that at the least acidic site (Ebergötzen). At the two most acidic sites, Fichtelgebirge and Harz, the ratio of biomass to necromass was significantly lower than at Ebergötzen and Barbis, particularly in the subsoil layer. In each stand, clear vertical gradients in fine root length density and root tip density were observed. Most of the roots and the root tips were in the humus layer and in the first mineral soil horizon (0-10 cm). There was a significantly different decrease in specific root length (cm gDM (-1)) and specific root tip density (root tips gDM (-1)) in the more acidified stands Fichtelgebirge and Harz compared with Ebergötzen and Barbis. Fine root production estimated by ingrowth cores and a net method was approximately twice as high in the more acidic stands Fichtelgebirge and Harz compared with Ebergötzen and Barbis. Rates of living fine root biomass turnover were higher at the Fichtelgebirge and Harz sites than at the Ebergötzen site. Rates of necromass turnover were similar at all sites. The results suggest that the accumulation of necromass was not due to a slower disappearance at the more acid sites, but to earlier root death. Roots contributed 46% to root + needle litter and 32% to root + total aboveground litter at the Harz site in 1997.

Spatial Analysis of Soils, Vegetation, Productivity, and Carbon Stored in Mountain Tundra Ecosystems, Khibiny Mountains, Russia

Data on the distribution of plant communities, their productivity, and major carbon stores held in mountain tundra vegetation and soils are presented. Extreme snow-bed communities had the lowest total biomass (662 g/m2), primarily in the main root portion in mineral soil horizons. Dwarf shrub-dominated heath on a gentle mountain slope had the highest total biomass (2406 g/m2), with woody perennial stems prevailing in the above-ground portion, and the main root portion concentrated in the organic horizon. Grass and sedge meadows situated in a hollow had the greatest total net primary production (215 g/m2), and roots were evenly distributed throughout the soil profile. The total amount of carbon stored in the ecosystem included carbon stored in above- and below-ground biomass, in the organic horizon, and in mineral soil horizons to a depth of 30 cm. Total carbon was lowest in snow-bed and lichen-dominated tundra (6183 g/m2 and 6337 g/m2, respectively), and highest in dwarf shrubs and grass-dominated tundra (25,261 g/m2 and 20,407 g/m2, respectively), the latter being comparable to total carbon stored in boreal forest ecosystems. Most of the carbon (95-97%) in tundra ecosystems was stored in soils (organic and mineral soil horizons).

Soil?Nitrogen Cycling in a Pine Forest Exposed to 5 Years of Elevated Carbon Dioxide

Empirical and modeling studies have shown that the magnitude and duration of the primary production response to elevated carbon dioxide (CO2) can be constrained by limiting supplies of soil nitrogen (N). We have studied the response of a southern US pine forest to elevated CO2 for 5 years (1997–2001). Net primary production has increased significantly under elevated CO2. We hypothesized that the increase in carbon (C) fluxes to the microbial community under elevated CO2 would increase the rate of N immobilization over mineralization. We tested this hypothesis by quantifying the pool sizes and fluxes of inorganic and organic N in the forest floor and top 30 cm of mineral soil during the first 5 years of CO2 fumigation. We observed no statistically significant change in the gross or net rate of inorganic N mineralization and immobilization in any soil horizon under elevated CO2. Similarly, elevated CO2 had no statistically significant effect on the concentration or flux of organic N, including amino acids. Microbial biomass N was not significantly different between CO2 treatments. Thus, we reject our hypothesis that elevated CO2 increases the rate of N immobilization. The quantity and chemistry of the litter inputs to the forest floor and mineral soil horizons can explain the limited range of microbially mediated soil–N cycling responses observed in this ecosystem. Nevertheless a comparative analysis of ecosystem development at this site and other loblolly pine forests suggests that rapid stand development and C sequestration under elevated CO2 may be possible only in the early stages of stand development, prior to the onset of acute N limitation.

Biological and chemical amelioration effects on the localities degraded by bulldozer site preparation in the Ore Mts. - Czech Republic

Biological and chemical (fertilising, liming) amelioration are among the principal tools used to restore degraded sites. These techniques were also used on the Ore Mts. plateau on plots prepared by bulldozing. We evaluated the impact of these amelioration techniques by measuring tree species primary production and humus form restoration. Begun in 1983&amp;ndash;1985, this project studied growth reaction of forest stands by measuring height and diameter increment, for the following species (blue spruce Picea pungens Engelm., European larch Larix decidua Mill., lodgepole pine Pinus contorta Dougl., Eastern white pine Pinus strobus L., alder Alnus incana Moench., European beech Fagus sylvatica L.) during the period 1994&amp;ndash;2000. The growth potential by species decreases in the following order: larch, alder, lodgepole pine, white pine, blue spruce. Beech was almost exterminated by red deer browsing and the harsh climate; pines suffered heavily from browsing and bark stripping. Holorganic horizons were measured and basic soil chemical and mechanical characteristics were determined &amp;ndash; pH, soil adsorption complex characteristics (using Kappen&amp;rsquo;s methodology), content of the humus and total nitrogen, exchangeable acidity and plant available macronutrients, granulometric composition of mineral soil horizons. Our results confirmed the relatively long-lasting effects of soil amendments, as well as the amelioration effects of alder, and the relatively inhibiting effects of larch and blue spruce.

Landscape Patterns of Net Nitrification in a Northern Hardwood‐Conifer Forest

The production of NO−3 in forest soils is an important process influencing the form of N available for plant uptake and the potential for off‐site N losses. We sampled mineral and organic soil horizons in 100 plots distributed across a 3160‐ha hardwood‐conifer forest in central New Hampshire to examine patterns of net nitrification rate (NR) and NO−3 concentration associated with physiographic features and vegetation abundances. Net NR and NO−3 concentrations each varied by a factor of 150 across the forest. Greater rates of net NO−3 production were associated with higher plot elevation, extent of southerly aspect, greater abundance of sugar maple (Acer saccharum Marsh.) and striped maple (Acer pensylvanicum L.), and lower abundances of coniferous trees. Regression models incorporating these factors accounted for 52% of the variance in NR across the entire valley. Higher soil water contents, N mineralization rates, and total N concentrations in higher elevation plots and higher N mineralization and respiration rates in more south‐facing plots contributed to the landscape patterns. Incorporation of measured soil C/N ratios together with landscape factors accounted for an additional 10% of the variation. Regression models incorporating vegetation, physiographic, and soils data explained 62 to 73% of the variance in soil C/N ratios across the valley. Our results demonstrate how multiple biotic and abiotic factors combine to generate wide variation in NO−3 production, and provide a quantitative basis for estimating how vegetation shifts and climate change may influence N cycling in heterogeneous landscapes characteristic of many temperate forest ecosystems.

Short-term increases in relative nitrification rates due to trenching in forest floor and mineral soil horizons of different forest types

The representation of NO3− dynamics within forest growth simulation models could improve forest management. An extensive literature review revealed an 88% probability of measuring a higher relative nitrification index (i.e. RNI = [NO3−] ÷ [NO3− + NH4+]) in mineral soil horizons than in forest floors, across a wide range of conifer and hardwood ecosystems. We then hypothesised that humus form and fine root density could be used as two crude variables to predict changes in in situ, potential and relative nitrification rates. Twenty-seven trench plots were established in 1999, across nine contrasting hardwood and coniferous stands in the Eastern Townships of Quebec. Forest floor and mineral soil samples were collected from each plot, and from a 1 m radius surrounding each plot, on three dates during summer 2000. In situRNI values increased significantly in trench plots as the season progressed. Potential nitrification rates (i.e. NO3− concentrations following incubation) were two orders of magnitude higher in forest floor than in mineral soil samples. RNI was significantly higher in mineral soil than in forest floor samples after incubations, but the relative increase in RNI due to trenching was higher in forest floor samples. Indices of available C were significantly higher in forest floor than mineral soil samples, and decreased only in forest floor samples during incubations. Likewise, trenching significantly reduced available C in forest floor samples only. Seasonal changes in soil temperature and fine root growth were the most plausible explanations for seasonal changes in NO3− dynamics, whereas other factors such soil acidity and moisture appeared less important in determining NO3− dynamics in this study. We conclude that crude assessments of humus form and fine root density have the potential to be used as calibration parameters for the simulation of NO3− dynamics in forest growth and yield models.

Landscape Patterns of Net Nitrification in a Northern Hardwood-Conifer Forest

The production of NO−3 in forest soils is an important process influencing the form of N available for plant uptake and the potential for off-site N losses. We sampled mineral and organic soil horizons in 100 plots distributed across a 3160-ha hardwood-conifer forest in central New Hampshire to examine patterns of net nitrification rate (NR) and NO−3 concentration associated with physiographic features and vegetation abundances. Net NR and NO−3 concentrations each varied by a factor of 150 across the forest. Greater rates of net NO−3 production were associated with higher plot elevation, extent of southerly aspect, greater abundance of sugar maple (Acer saccharum Marsh.) and striped maple (Acer pensylvanicum L.), and lower abundances of coniferous trees. Regression models incorporating these factors accounted for 52% of the variance in NR across the entire valley. Higher soil water contents, N mineralization rates, and total N concentrations in higher elevation plots and higher N mineralization and respiration rates in more south-facing plots contributed to the landscape patterns. Incorporation of measured soil C/N ratios together with landscape factors accounted for an additional 10% of the variation. Regression models incorporating vegetation, physiographic, and soils data explained 62 to 73% of the variance in soil C/N ratios across the valley. Our results demonstrate how multiple biotic and abiotic factors combine to generate wide variation in NO−3 production, and provide a quantitative basis for estimating how vegetation shifts and climate change may influence N cycling in heterogeneous landscapes characteristic of many temperate forest ecosystems.