Nitrogen acquisition and competitive ability of Kalmia angustifolia L., paper birch (Betula papyrifera Marsh.) and black spruce (Picea mariana (Mill.) B.S.P.) seedlings grown on different humus forms

Two species of boreal tree seedlings, paper birch (Betula papyrifera Marsh.) and black spruce (Picea mariana (Mill.) B.S.P.), and the ericaceous shrub Kalmia angustifolia L. were grown in pots with humus from a birch-dominated site and two spruce-Kalmia sites. Root systems interacted with humus form in controlling soil-N cycling as well as energy and nutritional deficiencies of soil microorganisms. In general, Kalmia seedlings affected microbial dynamics and N cycling differently than birch and spruce seedlings did. Birch and spruce seedlings reduced gross N mineralization and immobilization rates, soil mineral-N pools and the amounts of NH –N accreted on buried cation exchange resins in all three soils. Compared to birch and spruce seedlings, the growth of Kalmia resulted in significantly higher gross N mineralization rates, soil mineral-N pools and resin-NH accretion in soil from the fertile birch site. Gross N immobilization rates in all soils were generally higher with Kalmia than with spruce or birch seedlings. All three species of seedlings acquired N from the birch site soil, whereas only Kalmia seedlings acquired N from the two spruce-Kalmia site soils. Relative to control treatments, the amount of N mineralized anaerobically increased in the birch-site soil and decreased in the poor spruce-Kalmia site soil with all three species of seedlings. All seedlings increased the microbial biomass in the birch-site soil. Kalmia humus and Kalmia root systems increased microbial energy-deficiency and decreased microbial nutritional deficiency compared to the other humus and seedlings used. Results are discussed in terms of each species' nutrient acquisition mechanism and its competitive ability during secondary succession.     

Soil enzyme inhibition by condensed litter tannins may drive ecosystem structure and processes: the case of Kalmia angustifolia

Kalmia angustifolia is an ericaceous shrub that can rapidly spread on recently harvested boreal forest sites, causing a slow-down in soil nutrient cycling and reduced growth of spruce seedlings. It has been hypothesized that tannins released from Kalmia litter suppress soil enzyme activity, and are thus important in controlling ecosystem structure and processes. Here the effects of different concentrations of tannins extracted from both Kalmia and black spruce (Picea mariana) foliage were tested on enzyme activities of soil extracts. Then the effects of various Kalmia-black spruce litter mixtures on soil enzyme activity were investigated. Lastly, the correlation between Kalmia cover in the field and soil enzyme activity was measured. Both tannin types suppressed beta-glucosidase and acid phosphatase activities, and the magnitude of these effects was concentration-dependent. beta-glucosidase and amidase activity decreased linearly with an increasing Kalmia : spruce litter ratio added to soil. A field survey of 24 sites revealed a negative relationship between percentage Kalmia cover and beta-glucosidase activity. Collectively, results of the three experiments converge to support the claim that enzyme inhibition by litter tannins has evolved as an important mechanism controlling ecosystem processes and structure following Kalmia invasion on recently disturbed forest sites.     

Effects of Ledum groenlandicum amendments on soil characteristics and black spruce seedling growth

The effects of leaves and litter of the boreal forest understory shrub, Ledum groenlandicum, on soil characteristics and black spruce (Picea mariana) seedling growth were investigated. Organic and mineral soils, not previously associated with L. groenlandicum, were amended with leaves and litter of this species. The objectives of the present study were: (i) to determine the changes in soil characteristics after amending with L. groenlandicum, (ii) to determine the quantitative variation in the concentration of water-soluble phenolic allelochemicals in mineral and organic soil layers modified by L. groenlandicum and (iii) to study the growth response of black spruce in soils treated with different L. groenlandicum amendments. The amended organic and mineral soils were analyzed for pH, organic matter, PO₄, N, Ba, Cu, Zn, Fe, Mn, Ca, Na, K, Mg, Al and total phenolics equivalence. Results indicate that organic soils amended with L. groenlandicum leaves and litter were significantly different from unamended control soil for most of the chemical characteristics, while amended mineral soil was different from that of unmodified mineral soil for PO₄, organic matter, K and total phenolics equivalence. Water-soluble phenolics from L. groenlandicum and changes in nutrient availability are plausible causes of L. groenlandicum interference with black spruce seedling growth.     

Nutrient status of black spruce (Picea mariana [Mill.] BSP) forest soils dominated by Kalmia angustifolia L.

A study was conducted to determine soil chemistry in an uncut black spruce (Picea mariana) forest with and without the ericaceous understory shrub Kalmia angustifolia, as well as on a cut black spruce forest currently dominated by Kalmia. The organic (humus) and mineral (Ae, upper and lower B horizons) soils associated with Kalmia from uncut and cut forests, and non-Kalmia soils from uncut forest, were analyzed for selected soil properties. In general, mineral soils (B horizon) associated with Kalmia in uncut forest have lower values for organic matter (3.25%), organic nitrogen (0.66 mg·g⁻¹), Fe³⁺ (95.4 μg·g⁻¹) and Mn²⁺ (9 μg·g⁻¹), and higher values for pH (4.12) and Ca²⁺ (27 μg·g⁻¹) compared to non-Kalmia (organic matter, 3.43%; organic-N, 1.15 mg·g⁻¹; Fe³⁺, 431 μg·g⁻¹; Mn²⁺, 23.2 μg·g⁻¹; pH, 3.14; Ca²⁺, 15.6 μg·g⁻¹) and cut black spruce-Kalmia (organic matter, 3.74%; organic-N, 0.94 mg·g⁻¹; Fe³⁺, 379 μg·g⁻¹; Mn²⁺, 27 μg·g⁻¹; pH, 2.87; Ca²⁺, 25.2 μg·g⁻¹) forest. The high C:N ratio in Kalmia mineral soil from upper B (29.73) and lower B (identified as B+) (33.08) in uncut black spruce forest was recorded compared to non-Kalmia soils in B (18.17) and B+ (17.05) horizons in uncut black spruce forest. Phenolics leached out from Kalmia litter were lower in Kalmia associated soils than the non-Kalmia soils from the uncut forest, and Kalmia associated soils from the cut forest area. Results indicate that soils associated with Kalmia were nutrient poor particularly for nitrogen, phosphorus, iron and manganese, and provide some basis for the hypothesis that Kalmia has dominated microsites that were nutrient poor prior to Kalmia colonization.     

Differential effects of sugar maple, red oak, and hemlock tannins on carbon and nitrogen cycling in temperate forest soils

Tannins are abundant secondary chemicals in leaf litter that are hypothesized to slow the rate of soil-N cycling by binding protein into recalcitrant polyphenol–protein complexes (PPCs). We studied the effects of tannins purified from sugar maple, red oak, and eastern hemlock leaf litter on microbial activity and N cycling in soils from northern hardwood–conifer forests of the northeastern US. To create ecologically relevant conditions, we applied tannins to soil at a concentration (up to 2 mg g⁻¹ soil) typical of mineral soil horizons. Sugar maple tannins increased microbial respiration significantly more than red oak or hemlock tannins. The addition of sugar maple tannins also decreased gross N mineralization by 130% and, depending upon the rate of application, decreased net rates of N mineralization by 50–290%. At low concentrations, the decrease in mineralization appeared to be driven by greater microbial-N immobilization, while at higher concentrations the decrease in mineralization was consistent with the formation of recalcitrant PPCs. Low concentrations of red oak and hemlock tannins stimulated microbial respiration only slightly, and did not significantly affect fluxes of inorganic N in the soil. When applied to soils containing elevated levels of protein, red oak and hemlock tannins decreased N mineralization without affecting rates of microbial respiration, suggesting that PPC formation decreased substrate availability for microbial immobilization. Our results indicate that tannins from all three species form recalcitrant PPCs, but that the degree of PPC formation and its attendant effect on soil-N cycling depends on tannin concentration and the pool size of available protein in the soil.     

15N natural abundance in fruit bodies of different functional groups of fungi in relation to substrate utilization

Natural abundances of ¹⁵N and N concentrations of 34 fruit bodies from 24 species of ectomycorrhizal and saprophytic fungi were measured in a temperate Central European mixed forest stand. The fungi of the two life forms are known to be capable of utilizing different types of N sources (organic N compounds from the humus, inorganic N from the soil and N from litter or wood) differing by their ¹⁵N natural abundance values. Based on the two life forms and the three different N sources, four functional groups of fungi were distinguished: (1) ectomycorrhizal fungi capable of utilizing organic N from the humus; (2) ectomycorrhizal fungi known to depend on inorganic N compounds in the soil; (3) saprophytes capable of utilizing organic N from the humus; and (4) saprophytes utilizing N from dead wood or litter. Large differences were found between species in the δ¹⁵N values (−3.0 to 3.3‰) and in the N concentrations (0.84 to 6.61 mmol eq N g dw⁻¹) of the fruit bodies. In most cases fungi were more enriched in ¹⁵N than their respective bulk N source was. Fungi living in humus, and presumably having access to organic N compounds (groups 1 and 3), were significantly more enriched in ¹⁵N than fungi which are known to depend on inorganic N (e.g. Laccaria , group 2), or fungi living on litter or wood (group 4), irrespective of whether they were ectomycorrhizal or saprophytic species. Fungi living in humus had significantly higher N concentrations than fungi living on litter or wood.     

Nitrogen dynamics in humus and soil beneath Sitka spruce (Picea sitchensis (Bong.) Carr.) planted in pure stands and in mixture with Scots pine (Pinus sylvestris L.)

Sitka spruce planted on nutrient-poor soils in mixture with pine or larch, unlike pure spruce, does not become N deficient and does not require N fertilizer. To test the hypothesis that N availability in the soil is enhanced beneath mixed species, the seasonal changes in different N forms were compared in humus (L+F+H) and soil beneath 15-year-old Sitka spruce (SS) and mixed Sitka spruce-Scots pine (SS and SP) planted on a gleyed heathland soil. Amounts of mineral and organic N extracted from humus in spring were significantly (p < 0.05) higher in SS and SP than in SS. Larger amounts were measured in the underlying soil, which favoured the deeper-rooting spruce and pine in SS and SP plots. Annual net N mineralization, measured by in-situ incubation, was 32 and 47 kg N ha^-1 in the surface 10 cm beneath SS and (SS and SP), respectively. In spring, readily mineralized organic N (waterlogged incubation at 30°C) was higher in humus and soil from (SS and SP) than from SS by 15 kg N ha^-1. The larger N pools beneath (SS and SP) were consistent with the higher total N content of the humus beneath (SS and SP), 446 compared with 255 kg N ha^-1 beneath SS. This indicated that beneath (SS and SP) N had been transferred from the underlying soil.     

Root and needle litter decomposition responses to enhanced supplies of N and S in a Norway spruce forest in southwest Sweden

The effects of additions of ammonium sulfate (NS) on the decomposition of litter derived from Norway spruce roots (< 2 and 2 - 5 mm in diameter) in the humus and mineral soil layers (0 - 15 cm) of a Norway spruce stand in southern Sweden were investigated over a 6-year period. To this purpose, litterbags were incubated in the humus layer and in the mineral soil in June 1996, with roots collected from NS and control (C) plots incubated in the NS and C plots, respectively. The N concentrations in fine roots (< 2 mm) in the NS- plots were higher than those in 2 - 5 mm roots in both humus and mineral soil layers. In the humus layer, N concentrations in the fine roots in the C- and NS- plots were 12.8 and 15.7 mg g ^{? 1}, respectively. By the end of the fifth year the < 2 mm roots in humus layer had lost 48.5 and 50% of their mass in the C and NS plots, respectively, while the corresponding values for the 2 - 5 mm diameter class were 44 and 54%. The fresh root litter may be a sensitive indicator to responses to enhanced N and S deposition, although decomposition rates of both litter types are affected.     

Greater accumulation of litter in spruce (Picea abies) compared to beech (Fagus sylvatica) stands is not a consequence of the inherent recalcitrance of needles

Background and aims

Replacement of beech by spruce is associated with changes in soil acidity, soil structure and humus form, which are commonly ascribed to the recalcitrance of spruce needles. It is of practical relevance to know how much beech must be admixed to pure spruce stands in order to increase litter decomposition and associated nutrient cycling. We addressed the impact of tree species mixture within forest stands and within litter on mass loss and nutritional release from litter.

Methods

Litter decomposition was measured in three adjacent stands of pure spruce (Picea abies), mixed beech-spruce and pure beech (Fagus sylvatica) on three nutrient-rich sites and three nutrient-poor sites over a three-year period using the litterbag method (single species and mixed species bags).

Results

Mass loss of beech litter was not higher than mass loss of spruce litter. Mass loss and nutrient release were not affected by litter mixing. Litter decay indicated non-additive patterns, since similar remaining masses under pure beech (47%) and mixed beech-spruce (48%) were significantly lower than under pure spruce stands (67%). Release of the main components of the organic substance (C_org, N_tot, P, S, lignin) and associated K were related to mass loss, while release of other nutrients was not related to mass loss.

Conclusions

In contradiction to the widely held assumption of slow decomposition of spruce needles, we conclude that accumulation of litter in spruce stands is not caused by recalcitrance of spruce needles to decay; rather adverse environmental conditions in spruce stands retard decomposition. Mixed beech-spruce stands appear to be as effective as pure beech stands in counteracting these adverse conditions.     

Tannin impacts on microbial diversity and the functioning of alpine soils: a multidisciplinary approach

In alpine ecosystems, tannin-rich-litter decomposition occurs mainly under snow. With global change, variations in snowfall might affect soil temperature and microbial diversity with biogeochemical consequences on ecosystem processes. However, the relationships linking soil temperature and tannin degradation with soil microorganisms and nutrients fluxes remain poorly understood. Here, we combined biogeochemical and molecular profiling approaches to monitor tannin degradation, nutrient cycling and microbial communities (Bacteria, Crenarcheotes, Fungi) in undisturbed wintertime soil cores exposed to low temperature (0°C/−6°C), amended or not with tannins, extracted from Dryas octopetala . No toxic effect of tannins on microbial populations was found, indicating that they withstand phenolics from alpine vegetation litter. Additionally at −6°C, higher carbon mineralization, higher protocatechuic acid concentration (intermediary metabolite of tannin catabolism), and changes in fungal phylogenetic composition showed that freezing temperatures may select fungi able to degrade D. octopetala 's tannins. In contrast, negative net nitrogen mineralization rates were observed at −6°C possibly due to a more efficient N immobilization by tannins than N production by microbial activities, and suggesting a decoupling between C and N mineralization. Our results confirmed tannins and soil temperatures as relevant controls of microbial catabolism which are crucial for alpine ecosystems functioning and carbon storage.     

The interspecific competition presents greater nutrient facilitation compared with intraspecific competition through AM fungi interacting with litter for two host plants in karst soil

AM 真菌和枯落物互作下两种喀斯特植物种间竞争较种内竞争更能促进植物养分利用 枯落物是植物养分获取和土壤养分转化的关键载体。丛枝菌根(Arbuscular mycorrhizae, AM)对植物养分摄取的影响已被广泛认知。然而，在养分亏缺的喀斯特生境中，不同竞争方式的植物如何通过AM真菌和枯落物利用养分尚不清楚。本研究对两种喀斯特适生植物构树(Broussonetia papyrifera)和云贵鹅耳枥(Carpinus pubescens)进行种内竞争和种  间竞争种植处理，并通过幼套球 囊霉(Glomus etunicatum)接种或不接种处理，以及土壤中添加或不添加两物种叶片混合枯落物处理，测定了植物生物量以及氮、磷、钾浓度等指标，研究植物的生长和养分利用。研究结果表明，AM真菌对两种植物养分摄取影响不同，AM真菌显著提高了种内和种间竞争下构树的养分摄取量，但降低了云贵鹅耳枥的养分摄取量。种间竞争下接种AM真菌，枯落物添加促进了云贵鹅耳枥对氮的摄取，抑制了构树对氮的摄取。接种AM真菌和添加枯落物条件下，种间竞争的构树对氮、磷和钾的摄取量及云贵鹅耳枥对氮的摄取量均高于种内竞争；种间竞争下两物种养分竞争力呈现明显差异，即构树对磷和钾养分竞争力显著提高，对氮则不显著；云贵鹅耳枥仅对钾的养分竞争力显著降低，对氮和磷则无显著影响。这些结果说明，在AM真菌与枯落物相互作用下，两种喀斯特植物种间竞争较种内竞争更能促进植物养分利用。     

Anthropogenic nitrogen enrichment enhances soil carbon accumulation by impacting saprotrophs rather than ectomycorrhizal fungal activity

There is evidence that anthropogenic nitrogen (N) deposition enhances carbon (C) sequestration in boreal forest soils. However, it is unclear how free‐living saprotrophs (bacteria and fungi, SAP) and ectomycorrhizal (EM) fungi responses to N addition impact soil C dynamics. Our aim was to investigate how SAP and EM communities are impacted by N enrichment and to estimate whether these changes influence decay of litter and humus. We conducted a long‐term experiment in northern Sweden, maintained since 2004, consisting of ambient, low N additions (0, 3, 6, and 12 kg N ha^?1 year^?1) simulating current N deposition rates in the boreal region, as well as a high N addition (50 kg N ha^?1 year^?1). Our data showed that long‐term N enrichment impeded mass loss of litter, but not of humus, and only in response to the highest N addition treatment. Furthermore, our data showed that EM fungi reduced the mass of N and P in both substrates during the incubation period compared to when only SAP organisms were present. Low N additions had no effect on microbial community structure, while the high N addition decreased fungal and bacterial biomasses and altered EM fungi and SAP community composition. Actinomycetes were the only bacterial SAP to show increased biomass in response to the highest N addition. These results provide a mechanistic understanding of how anthropogenic N enrichment can influence soil C accumulation rates and suggest that current N deposition rates in the boreal region (≤12 kg N ha^?1 year^?1) are likely to have a minor impact on the soil microbial community and the decomposition of humus and litter.     

A comparative study of soil formation in primary stands of Scots pine (planted) and poplar (natural) on calcareous dune sands in the Netherlands

Initial soil formation under primary stands of Scots pine (planted) and European black poplar (natural) on calcareous dune sands was studied, paying particular attention to the humus forms and their spatial variability. The stands studied are both about 80 years old and are situated, at close distance, in the coastal dunes near Wassenaar (the Netherlands).Under Scots pine, soils with a mor-type humus form were observed, exhibiting slight podzolisation. Soil variability is rather slight and soil development is comparable to that under primary Scots pine stands on non-calcareous inland drift sands. Under poplar, mull-type humus forms occur which tend towards moder and exhibit a markedly stronger litter decomposition and bioturbation. In contrast to the soils under pine, soil variability is considerable. Results from chemical analyses of two representative soil profiles are in conformance with these trends.It is concluded that the observed trends in soil formation are in line with those described in the literature, and that a period of 80 years is sufficient for a strong vegetation related divergence in soil properties. Soil variability within the stands probably results from redistribution of litter by wind and/or gravity and will be rather site-dependent.     

The ectomycorrhizal flora of primary and secondary stands of Pinus sylvestris in relation to soil conditions and ectomycorrhizal succession

Abstract. Ectomycorrhizal species composition and sporocarp abundance in two 15–20-yr old primary stands of Pinus sylvestris in the central part of the Netherlands was compared with those in two 16- and 27-yr old secondary stands of P. sylvestris in the Northeast of the country. The trees of the primary stands were spontaneously seeded in a drifting sand area. Only thin litter and humus layers were present. The trees of the 16-yr old secondary stand were planted on podzolic sandy soil and those of the 27-yr old stand on non-podzolic sandy soil. In both secondary stands thick litter and humus layers had developed. The litter and humus partly originated from former stands at the same sites and partly from the present stands. In the secondary stands the thick litter and humus layers and herb vegetation were removed (‘sod-cutting’) in order to simulate the thin litter and humus layers in the primary stands. Control treatments were present. Surveys in 1991, 1992 and 1993 showed that sod-cutting enhanced both abundance and diversity of ectomycorrhizal fungi. However, species richness and diversity were higher in the primary stands than in the secondary ones, also even in the sod-cut plots. High species richness and diversity were associated with low concentrations of nitrogen and relatively high pH in the litter and humus layers, and in the mineral soil.     

Short- and Long-term Tannin Induced Carbon, Nitrogen and Phosphorus Dynamics in Corsican Pine Litter

Pine litter amended with either tannic acid (TA) or condensed tannins (CTs) was studied to assess the effects on C, N and P mineralization in relation to the fate of tannins by incubation experiments during various time intervals. TA induced a rapid short-term effect resulting in high C respiration and net N and P immobilisation. After one week of incubation, TA was decomposed and net C, N and P mineralization and net nitrification resembled that of the control (non-amended litter). CTs exhibited effects on net mineralization on longer terms, i.e. after several weeks of incubation until the end of the experiment (84 days). While net N and P mineralization were greatly reduced, net nitrification was only slightly affected. Most likely CTs formed complexes with organic N of the substrate thereby reducing net N mineralization, while such complexes were not involved in net nitrification processes. The reduction of net P mineralization is due to the lack of need for P by microbes when they cannot get access to N. The fact that decreasing amounts of extractable CTs were accompanied by increasing effects on mineralization processes with incubation time strongly suggests that CTs were incorporated into the litter in such a way that they were inextricable by the common solvents needed to measure tannins, such as for the Folin–Ciocalteu and HCl–butanol assays.     

Nitrogen Isotope Patterns in Alaskan Black Spruce Reflect Organic Nitrogen Sources and the Activity of Ectomycorrhizal Fungi

Global patterns in soil, plant, and fungal stable isotopes of N (δ¹⁵N) show promise as integrated metrics of N cycling, particularly the activity of ectomycorrhizal (ECM) fungi. At small spatial scales, however, it remains difficult to differentiate the underlying causes of plant δ¹⁵N variability and this limits the application of such measurements to better understand N cycling. We conducted a landscape-scale analysis of δ¹⁵N values from 31 putatively N-limited monospecific black spruce (Picea mariana) stands in central Alaska to assess the two main hypothesized sources of plant δ¹⁵N variation: differing sources and ECM fractionation. We found roughly 20% of the variability in black spruce foliar N and δ¹⁵N values to be correlated with the concentration and δ¹⁵N values of soil NH₄ ⁺ and dissolved organic N (DON) pools, respectively. However, ¹⁵N-based mixing models from 24 of the stands suggested that fractionation by ECM fungi obscures the ¹⁵N signature of soil N pools. Models, regressions, and N abundance data all suggested that increasing dependence on soil DON to meet black spruce growth demands predicates increasing reliance on ECM-derived N and that black spruce, on average, received 53% of its N from ECM fungi. Future research should partition the δ¹⁵N values within the soil DON pool to determine how choice of soil δ¹⁵N values influence modeled ECM activity. The C balance of boreal forests is tightly linked to N cycling and δ¹⁵N values may be useful metrics of changes to these connections.     

Spatial separation of litter decomposition and mycorrhizal nitrogen uptake in a boreal forest

Our understanding of how saprotrophic and mycorrhizal fungi interact to re-circulate carbon and nutrients from plant litter and soil organic matter is limited by poor understanding of their spatiotemporal dynamics. In order to investigate how different functional groups of fungi contribute to carbon and nitrogen cycling at different stages of decomposition, we studied changes in fungal community composition along vertical profiles through a Pinus sylvestris forest soil. We combined molecular identification methods with 14C dating of the organic matter, analyses of carbon:nitrogen (C:N) ratios and 15N natural abundance measurements. Saprotrophic fungi were primarily confined to relatively recently (< 4 yr) shed litter components on the surface of the forest floor, where organic carbon was mineralized while nitrogen was retained. Mycorrhizal fungi dominated in the underlying, more decomposed litter and humus, where they apparently mobilized N and made it available to their host plants. Our observations show that the degrading and nutrient-mobilizing components of the fungal community are spatially separated. This has important implications for biogeochemical studies of boreal forest ecosystems.     

Effects of understory removal, N fertilization, and litter layer removal on soil N cycling in a 13-year-old white spruce plantation infested with Canada bluejoint grass

Canada bluejoint grass [Calamagrostis canadensis (Michx.) Beauv., referred to as bluejoint below] is a competitive understory species widely distributed in the boreal region in North America and builds up a thick litter layer that alters the soil surface microclimate in heavily infested sites. This study examined the effects of understory removal, N fertilization, and litter layer removal on litter decomposition, soil microbial biomass N (MBN), and net N mineralization and nitrification rates in LFH (the sum of organic horizons of litter, partially decomposed litter and humus on the soil surface) and mineral soil (0–10 cm) in a 13-year-old white spruce [Picea glauca (Moench.) Voss] plantation infested with bluejoint in Alberta, Canada. Removal of the understory vegetation and the litter layer together significantly increased soil temperature at 10 cm below the mineral soil surface by 1.7 and 1.3°C in summer 2003 and 2004, respectively, resulting in increased net N mineralization (by 1.09 and 0.14 mg N kg⁻¹ day⁻¹ in LFH and mineral soil, respectively, in 2004) and net nitrification rates (by 0.10 and 0.20 mg N kg⁻¹ day⁻¹ in LFH and mineral soil, respectively, in 2004). When the understory vegetation was intact, nitrification might have been limited by NH₄ ⁺ availability due to competition for N from bluejoint and other understory species. Litter layer removal increased litter decomposition rate (percentage mass loss per month) from 2.6 to 3.0% after 15 months of incubation. Nitrogen fertilization did not show consistent effects on soil MBN, but increased net N mineralization and nitrification rates as well as available N concentrations in the soil. Clearly, understory removal combined with N fertilization was most effective in increasing rates of litter decomposition, net N mineralization and nitrification, and soil N availability. The management of understory vegetation dominated by bluejoint in the boreal region should consider the strong effects of understory competition and the accumulated litter layer on soil N cycling and the implications for forest management.     

Anthropogenic N Deposition Increases Soil C Storage by Decreasing the Extent of Litter Decay: Analysis of Field Observations with an Ecosystem Model

Recent meta-analyses of experimental studies simulating increased anthropogenic nitrogen (N) deposition in forests reveal greater soil carbon (C) storage under elevated levels of atmospheric N deposition. However, these effects have not yet been included in ecosystem-scale models of soil C and N cycling and it is unclear whether increased soil C storage results from slower decomposition rates or a reduced extent of decomposition (for example, an increase in the amount of litter entering slowly decaying humus pools). To test these alternatives, we conducted a meta-analysis of litter decomposition data. We then used the results from our meta-analysis to model C and N cycling in four sugar maple forests in Michigan using an ecosystem process model (TRACE). We compared model results testing our alternative hypotheses to field data on soil C storage from a 17-year N deposition experiment. Using data from published litter decomposition studies in forests, we determined that, on average, exogenous N inputs decreased lignin decomposition rates by 30% and increased cellulose decomposition by 9%. In the same set of litter decomposition studies increased exogenous N availability increased the amount of litter entering slowly decaying humus pools in a manner significantly related to the lignocellulose index of decaying litter. Incorporating changes to decomposition rates in TRACE did not accurately reproduce greater soil C storage observed in our field study with experimentally elevated N deposition. However, when changes in the extent of decomposition were incorporated in TRACE, the model produced increased soil C storage by increasing the amount of litter entering the humus pool and accurately represented C storage in plant and soil pools under experimental N deposition. Our modeling results and meta-analysis indicate that the extent of litter decay as humus is formed, rather than slower rates of litter decay, is likely responsible for the accumulation of organic matter, and hence soil C storage, under experimental N deposition. This effect should be incorporated in regional to global-scale models simulating the C balance of forest ecosystems in regions receiving elevated N deposition.     

Effects of Pinus sylvestris root growth and mycorrhizosphere development on bacterial carbon source utilization and hydrocarbon oxidation in forest and petroleum-contaminated soils

The hypothesis that Pinus sylvestris L. root and mycorrhizosphere development positively influences bacterial community-linked carbon source utilization, and drives a concomitant reduction in mineral oil levels in a petroleum hydrocarbon- (PHC-) contaminated soil was confirmed in a forest ecosystem-based phytoremediation simulation. Seedlings were grown for 9 months in large petri dish microcosms containing either forest humus or humus amended with cores of PHC-contaminated soil. Except for increased root biomass in the humus/PHC treatment, there were no other significant treatment-related differences in plant growth and needle C and N status. Total cell and culturable bacterial (CFU) densities significantly increased in both rhizospheres and mycorrhizospheres that actively developed in the humus and PHC-contaminated soil. Mycorrhizospheres (mycorrhizas and extramatrical mycelium) supported the highest numbers of bacteria. Multivariate analyses of bacterial community carbon source utilization profiles (Biolog GN microplate) from different rhizosphere, mycorrhizosphere, and bulk soil compartments, involving principal component and correspondence analysis, highlighted three main niche-related groupings. The respective clusters identified contained bacterial communities from (i) unplanted bulk soils, (ii) planted bulk PHC and rhizospheres in PHC-contaminated soils, and (iii) planted bulk humus and rhizosphere/mycorrhizosphere-influenced humus, and mycorrhizosphere-influenced PHC contaminated soil. Correspondence analysis allowed further identification of amino acid preferences and increased carboxylic/organic acid preferences in rhizosphere and mycorrhizosphere compartments. Decreased levels of mineral oil (non-polar hydrocarbons) were detected in the PHC-contaminated soil colonized by pine roots and mycorrhizal fungi. These data further support our view that mycorrhizosphere development and function plays a central role in controlling associated bacterial communities and their degradative activities in lignin-rich forest humus and PHC-contaminated soils.