| 1. |
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| 2. |
- Blasko, Robert, et al.
(författare)
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Shifts in soil microbial community structure, nitrogen cycling and the concomitant declining N availability in ageing primary boreal forest ecosystems
- 2015
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 91, s. 200-211
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Tidskriftsartikel (refereegranskat)abstract
- Plant growth in boreal forests is commonly limited by a low supply of nitrogen, a condition that may be aggravated by high tree below-ground allocation of carbon to ectomycorrhizal (ECM) fungi and associated microorganisms. These in turn immobilise N and reduce its availability to plants as boreal ecosystems develop. Here, we studied a boreal forest ecosystem chronosequence created by new land rising out of the sea due to iso-static rebound along the coast of northern Sweden. We used height over the ocean to estimate ecosystem age and examined its relationship to soil microbial community structure and the gross turnover of N. The youngest soils develop with meadows by the coast, followed by a zone of N2-fixing alder trees, and primary boreal conifer forest on ground up to 560 years old. The young soils in meadows contained little organic matter and microbial biomass per unit area. Nitrogen was turned over at low rates when expressed per area (m(-2)), but specific rates (per gram soil carbon (C)) were the highest found along the transect. In the zone with alder, the amounts of soil C and microbial biomass were much higher (bacterial biomass had doubled and fungal biomass quadrupled). Rates of gross N mineralisation (expressed on an area basis) were highest, but the retention of added labelled NH4+ was lowest in this soil as compared to other ages. The alder zone also had the largest extractable pools of inorganic N in soil and highest N % in plant foliage. In the older conifer forest ecosystems the amounts of soil C and N, as well as biomass of both bacteria and fungi increased. Data on organic matter C-14 suggested that the largest input of recently fixed plant C occurred in the younger coniferous forest ecosystems. With increasing ecosystem age, the ratio of microbial C to total soil C was constant, whereas the ratio of microbial N to total soil N increased and gross N mineralization declined. Simultaneously, plant foliar N % decreased and the natural abundance of N-15 in the soil increased. More specifically, the difference in delta N-15 between plant foliage and soil increased, which is related to relatively greater retention of N-15 relative to N-14 by ECM fungi as N is taken up from the soil and some N is transferred to the plant host. In the conifer forest, where these changes were greatest, we found increased fungal biomass in the F- and H-horizons of the mor-layer, in which ECM fungi are known to dominate (the uppermost horizon with litter and moss is dominated by saprotrophic fungi). Hence, we propose that the decreasing availability of N to the plants and the subsequent decline in plant production in ageing boreal forests is linked to high tree belowground C allocation to ECM fungi, a strong microbial sink for available soil N. (C) 2015 The Authors. Published by Elsevier Ltd.
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| 3. |
- Högberg, Mona N, et al.
(författare)
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Carbon–nitrogen relations of ectomycorrhizal mycelium across a natural nitrogen supply gradient in boreal forest
- 2021
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Ingår i: New Phytologist. - Chichester : Wiley-Blackwell. - 0028-646X .- 1469-8137. ; 232:4, s. 1839-1848
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Tidskriftsartikel (refereegranskat)abstract
- The supply of carbon (C) from tree photosynthesis to ectomycorrhizal (ECM) fungi is known to decrease with increasing plant nitrogen (N) supply, but how this affects fungal nutrition and growth remains to be clarified. We placed mesh-bags with quartz sand, with or without an organic N (15N-, 13C-labeled) source, in the soil along a natural N supply gradient in boreal forest, to measure growth and use of N and C by ECM extramatrical mycelia. Mycelial C : N declined with increasing N supply. Addition of N increased mycelial growth at the low-N end of the gradient. We found an inverse relationship between uptake of added N and C; the use of added N was high when ambient N was low, whereas use of added C was high when C from photosynthesis was low. We propose that growth of ECM fungi is N-limited when soil N is scarce and tree belowground C allocation to ECM fungi is high, but is C-limited when N supply is high and tree belowground C allocation is low. This suggests that ECM fungi have a major role in soil N retention in nutrient-poor, but less so in nutrient-rich boreal forests. © 2021 The Authors. New Phytologist © 2021 New Phytologist Foundation
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| 4. |
- Högberg, Mona N., et al.
(författare)
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Contrasting effects of nitrogen availability on plant carbon supply to mycorrhizal fungi and saprotrophs - a hypothesis based on field observations in boreal forest
- 2003
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Ingår i: New Phytologist. - : Wiley. - 1469-8137 .- 0028-646X. ; 160:1, s. 225-238
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Tidskriftsartikel (refereegranskat)abstract
- Soil microorganisms are considered C-limited, while plant productivity is frequently N-limited. Large stores of organic C in boreal forest soils are attributed to negative effects of low temperature, soil acidity and plant residue recalcitrance upon microbial activity. We examined microbial activity, biomass and community composition along a natural 90-m-long soil N supply gradient, where plant species composition varies profoundly, forest productivity three-fold and soil pH by three units. There was, however, no significant variation in soil respiration in the field across the gradient. Neither did microbial biomass C determined by fumigation-extraction vary, while other estimates of activity and biomass showed a weak increase with increasing N supply and soil pH. Simultaneously, a phospholipid fatty acid attributed mainly to mycorrhizal fungi declined drastically, while bacterial biomass increased. We hypothesize that low N supply and plant productivity, and hence low litter C supply to saprotrophs is associated with a high plant C supply to mycorrhizal fungi, while the reverse occurs under high N supply. This should mean that effects of N availability on C supply to these functional groups of microbes acts in opposing directions.
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| 5. |
- Högberg, Mona N., et al.
(författare)
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Contrasting patterns of soil N-cycling in model ecosystems of Fennoscandian boreal forests
- 2006
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Ingår i: Oecologia. - Berlin : Springer in cooperation with the International association for ecology (Intecol). - 0029-8549 .- 1432-1939. ; 147:1, s. 96-107
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Tidskriftsartikel (refereegranskat)abstract
- The low plant productivity of boreal forests ingeneral has been attributed to low soil N supply and lowtemperatures. Exceptionally high productivity occurs intoe-slope positions, and has been ascribed to influx of Nfrom surrounding areas and higher rates of soil Nturnover in situ. Despite large apparent natural variationsin forest productivity, rates of gross soil N mineralizationand gross nitrification have never beencompared in Fennoscandian boreal forests of contrastingproductivity. We report contrasting patterns of soilN turnover in three model ecosystems, representing therange in soil C-to-N ratios (19–41) in Fennoscandianboreal forests and differences in forest productivity by afactor close to 3. Gross N mineralization was seventimes higher when soil, microbial, and plant C-to-Nratios were the lowest compared to the highest. Thisprocess, nitrification and potential denitrification correlatedwith inorganic, total and microbial biomass N, butnot microbial C. There was a constant ratio between soiland microbial C-to-N ratio of 3.7±0.2, across wide ratiosof soil C-to-N and fungi-to-bacteria. Soil N-cyclingshould be controlled by the supplies of C and N to themicrobes. In accordance with plant allocation theory, wediscuss the possibility that the high fungal biomass athigh soil C-to-N ratio reflects a particularly high supplyof plant photosynthates, substrates of high-quality C, tomycorrhizal fungi. Methods to study soil N turnoverand N retention should be developed to take into accountthe impact of mycorrhizal fungi on soil N-cycling.
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| 6. |
- Högberg, Mona N, et al.
(författare)
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Does ectomycorrhiza have a universal key role in the formation of soil organic matter in boreal forests?
- 2020
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 140
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Tidskriftsartikel (refereegranskat)abstract
- Forest soil organic matter (SOM) is an important dynamic store of C and N, which releases plant available N and the greenhouse gases CO2 and N2O. Early stages of decomposition of recent plant litters are better known than the formation of older and more stable soil pools of N and C, in which case classic theory stated that selective preservation of more resistant plant compounds was important. Recent insights heighten that all plant matter becomes degraded and that older SOM consists of compounds proximally of microbial origin. It has been proposed that in boreal forests, ectomycorrhizal fungi (ECMF), symbionts of trees, are actively involved in the formation of slowly-degrading SOM.We characterized SOM in the mor-layer along a local soil N supply gradient in a boreal forest, a gradient with large variations in chemical and biological characteristics, notably a decline in the biomass of ECMF in response to increasing soil N supply.We found contrasting and regular patterns in carbohydrates, lignin, aromatic carbon, and in N-containing compounds estimated by solid-state C-13 and N-15 nuclear magnetic resonance (NMR) spectroscopy. These occurred along with parallel changes in the natural abundances of the stable isotopes C-13 and N-15 in both bulk SOM and extracted fractions of the SOM. The modelled "bomb-C-14" age of the lower layers studied ranged between 15 years at the N-poor end, to 70 years at the N-rich end of the gradient. On average half the increase in delta C-13 with soil depth (and hence age) of the mor-layer can be attributed to soil processes and the other half to changes in the isotopic composition of the plant C inputs. There was a decrease in carbohydrates (O-alkyl C) with increasing depth. This supports the classical hypothesis of declining availability of easily decomposable substrates to microorganisms with increasing soil depth and age. The observed increase in delta C-13 with depth, however, speaks against the idea of selective preservation of more resistant plant compounds like lignin. Furthermore, from the N-poor to the N-rich end the difference between N-15 in plant litter N and N in the deeper part of the mor-layer, the H-layer, decreased in parallel with a decline in ECMF.The latter provides evidence that the role of ECMF as major sink for N diminishes, and hence their potential role in SOM stabilization, when the soil N supply increases. At the N-rich end, where bacteria dominate over fungi, other agents than ECMF must be involved in the large build-up of the H-layer with the slowest turnover rate found along the gradient.
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| 7. |
- Högberg, Mona N, et al.
(författare)
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Quantification of effects of season and nitrogen supply on tree below-ground carbon transfer to ectomycorrhizal fungi and other soil organisms in a boreal pine forest
- 2010
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Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 187:2, s. 485-493
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Tidskriftsartikel (refereegranskat)abstract
- P>The flux of carbon from tree photosynthesis through roots to ectomycorrhizal (ECM) fungi and other soil organisms is assumed to vary with season and with edaphic factors such as nitrogen availability, but these effects have not been quantified directly in the field. To address this deficiency, we conducted high temporal-resolution tracing of 13C from canopy photosynthesis to different groups of soil organisms in a young boreal Pinus sylvestris forest. There was a 500% higher below-ground allocation of plant C in the late (August) season compared with the early season (June). Labelled C was primarily found in fungal fatty acid biomarkers (and rarely in bacterial biomarkers), and in Collembola, but not in Acari and Enchytraeidae. The production of sporocarps of ECM fungi was totally dependent on allocation of recent photosynthate in the late season. There was no short-term (2 wk) effect of additions of N to the soil, but after 1 yr, there was a 60% reduction of below-ground C allocation to soil biota. Thus, organisms in forest soils, and their roles in ecosystem functions, appear highly sensitive to plant physiological responses to two major aspects of global change: changes in seasonal weather patterns and N eutrophication.
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| 8. |
- Högberg, Mona N, et al.
(författare)
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The return of an experimentally N-saturated boreal forest to an N-limited state : observations on the soil microbial community structure, biotic N retention capacity and gross N mineralisation
- 2014
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Ingår i: Plant and Soil. - : Springer Netherlands. - 0032-079X .- 1573-5036. ; 381:1-2, s. 45-60
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Tidskriftsartikel (refereegranskat)abstract
- To find out how N-saturated forests can return to an N-limited state, we examined the recovery of biotic N sinks under decreasing N supply. We studied a 40-year-old experiment in Pinus sylvestris forest, with control plots, N0, three N treatments, N1-N3, of which N3 was stopped after 20 years, allowing observation of recovery. In N3, the N concentration in foliage was still slightly elevated, but the N uptake capacity of ectomycorrhizal (ECM) roots in N3 was no longer lower than in N0. Per area the amount of a biomarker for fungi, here mainly attributed ECM, was higher in N3 and N0 than in N1 and N2. Retention of labeled (NH4)-N-15 (+) by the soil was greater in the control (99 %) and N3 (86 %), than in N1 (45 %) and N2 (29 %); we ascribe these differences to biotic retention because cation exchange capacity did not vary. Gross N mineralisation and retention of N correlated, negatively and positively, respectively, with abundance of ECM fungal biomarker. The results suggest a key role for ECM fungi in regulating the N cycle. We propose, in accordance with plant C allocation theory, that recovery is driven by increased tree below-ground C allocation to ECM roots and fungi.
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| 9. |
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| 10. |
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| 11. |
- Högberg, Peter, et al.
(författare)
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Is the high 15N natural abundance of trees in N-loaded forests caused by an internal ecosystem N isotope redistribution or a change in the ecosystem N isotope mass balance?
- 2014
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Ingår i: Biogeochemistry. - : Springer Science and Business Media LLC. - 0168-2563 .- 1573-515X. ; 117, s. 351-358
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Tidskriftsartikel (refereegranskat)abstract
- High delta N-15 of tree foliage in forests subject to high N supply has been attributed to N-15 enrichment of plant available soil N pools after losses of N through processes involving N isotope fractionation (ammonia volatilization, nitrification followed by leaching and denitrification, and denitrification in itself). However, in a long-term experiment with high annual additions of NH4NO3, we found no change in the weighted average delta N-15 of the soil, but attributed the high delta N-15 of trees to loss of ectomycorrhizal fungi and their function in tree N uptake, which involves redistribution of N isotopes in the ecosystem (Hogberg et al. New Phytol 189:515-525, 2011), rather than a loss of isotopically light N. Here, we compare the effects of additions of urea and NH4NO3 on the delta N-15 of trees and the soil profile, because we have previously found higher delta N-15 in tree foliage in trees in the urea plots. Doing this, we found no differences between the NH4NO3 and urea treatments in the concentration of N in the foliage, or the amounts of N in the organic mor-layer of the soil. However, the foliage of trees receiving the highest N loads in the urea treatment were more enriched in N-15 than the corresponding NH4NO3 plots, and, importantly, the weighted average delta N-15 of the soil showed that N losses had been associated with fractionation against N-15 in the urea plots. Thus, our results in combination with those of Hogberg et al. (New Phytol 189:515-525, 2011) show that high delta N-15 of the vegetation after high N load may be caused by both an internal redistribution of the N isotopes (as a result of change of the function of ectomycorrhiza) and by losses of isotopically light N through processes fractionating against N-15 (in case of urea ammonia volatilization, nitrification followed by leaching and denitrification).
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| 12. |
- Högberg, Peter, et al.
(författare)
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Large differences in plant nitrogen supply in German and Swedish forests - Implications for management
- 2021
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Ingår i: Forest Ecology and Management. - : Elsevier BV. - 0378-1127 .- 1872-7042. ; 482
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Tidskriftsartikel (refereegranskat)abstract
- In European forests, plant N supply varies from regions where N deposition is negligible and a low natural N supply limits production to regions where high N deposition adds to a high natural N supply. Here, we ask if the differences in N supply are too large to make one system of management for wood production, continuous-cover forestry or rotational forestry, optimal across these conditions.We analyzed the C/N ratio inc. 8400 samples of surficial soil layers along a 2400 km long transect through Sweden and Germany to obtain a quantitative description of differences in plant N supply. We discuss the differences in relation to forest management, especially evidence that soil C/N ratios below 25 are associated with higher N supply, risks of leaching of nitrate, and gaseous losses of N2O, whereas ratios above 25 are associated with a tighter N cycle and an N limitation to tree growth.The percent soil with C/N ratios above 25 declines from 91 in N. Norrland in Sweden to 26 in Germany. Simultaneously, mor soils (with a distinct organic layer on top of the mineral soil) decline from 95% to 16%, while mull soils (in which organic matter and mineral particles are mixed) increase from 1% to 40%. However, low C/N ratios also occur in the north, where we find the largest width in C/N ratios from 16 in mull soils to 36 in mor soils, which compares with a variation in Germany from 17 to 27. Soils under conifers generally have higher C/N ratios than soils under broadleaves, but our survey data cannot support that the trees are the sole cause of this pattern. Very low C/N ratios occur in conifer-dominated forests in the north.The high incidence (74%) of C/N ratios below 25 indicates that forest management in Germany should use methods, which minimize the risk of N losses. Continuous-cover forestry may fulfill that objective. In the north with 9% of the soils below this threshold, risks of N losses are small. There, rotational forestry involving clear-felling alleviates the competition for soil N from larger trees allowing successful regeneration of tree seedlings. From the perspective of interactions between plant N supply and management of forests for wood production, no single management system seems optimal along this large gradient. We propose that research on forest management systems should address the importance of N supply.
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| 13. |
- Högberg, Peter, et al.
(författare)
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Large-scale forest girdling shows that current photosynthesis drives soil respiration
- 2001
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 411:6839, s. 789-792
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Tidskriftsartikel (refereegranskat)abstract
- The respiratory activities of plant roots, of their mycorrhizal fungi and of the free-living microbial heterotrophs (decomposers) in soils are significant components of the global carbon balance, but their relative contributions remain uncertain. To separate mycorrhizal root respiration from heterotrophic respiration in a boreal pine forest, we conducted a large-scale tree-girdling experiment, comprising 9 plots each containing about 120 trees. Tree-girdling involves stripping the stem bark to the depth of the current xylem at breast height terminating the supply of current photosynthates to roots and their mycorrhizal fungi without physically disturbing the delicate root-microbe-soil system. Here we report that girdling reduced soil respiration within 1-2 months by about 54% relative to respiration on ungirdled control plots, and that decreases of up to 37% were detected within 5 days. These values clearly show that the flux of current assimilates to roots is a key driver of soil respiration; they are conservative estimates of root respiration, however, because girdling increased the use of starch reserves in the roots. Our results indicate that models of soil respiration should incorporate measures of photosynthesis and of seasonal patterns of photosynthate allocation to roots.
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| 14. |
- Högberg, Peter, et al.
(författare)
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Nitrogen isotope fractionation during nitrogen uptake by ectomycorrhizal and non-mycorrhizal Pinus sylvestris
- 1999
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Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 142:3, s. 569-576
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Tidskriftsartikel (refereegranskat)abstract
- An experiment was performed to find out whether ectomycorrhizal (ECM) fungi alter the nitrogen (N) isotope composition, δ15N, of N during the transport of N from the soil through the fungus into the plant. Non- mycorrhizal seedlings of Pinus sylvestris were compared with seedlings inoculated with either of three ECM fungi, Paxillus involutus, Suillus bovinus and S. variegatus. Plants were raised in sand in pots supplied with a nutrient solution with N given as either NH4+ or NO3−. Fractionation against 15N was observed with both N sources; it decreased with increasing plant N uptake, and was larger when NH4+ was the source. At high ratios of Nuptake/Nsupplied there was no (NO3−), or little (NH4+), fractionation. There seemed to be no difference in fractionation between ECM and non-mycorrhizal plants, but fungal rhizomorphs were sometimes enriched in 15N (up to 5‰ at most) relative to plant material; they were also enriched relative to the N source. However, this enrichment of the fungal material was calculated to cause only a marginal decrease (−0.1‰ in P. involutus) in δ15N of the N passing from the substrate through the fungus to the host, which is explained by the small size of the fungal N pool relative to the total N of the plant, i.e. the high efficiency of transfer. We conclude that the relatively high 15N abundance observed in ECM fungal species should be a function of fungal physiology in the ECM symbiosis, rather than a reflection of the isotopic signature of the N source(s) used. This experiment also shows that the δ15N of plant N is a good approximation of δ15N of the available N source(s), provided that N is limiting growth.
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| 15. |
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| 16. |
- Högberg, Peter, et al.
(författare)
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Tamm review: on the nature of the nitrogen limitation to plant growth in Fennoscandian boreal forests.
- 2017
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Ingår i: Forest Ecology and Management. - : Elsevier BV. - 0378-1127 .- 1872-7042. ; 403, s. 161-185
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Forskningsöversikt (refereegranskat)abstract
- The supply of nitrogen commonly limits plant production in boreal forests and also affects species composition and ecosystem functions other than plant growth. These interrelations vary across the landscapes, with the highest N availability, plant growth and plant species richness in ground-water discharge areas (GDAs), typically in toe-slope positions, which receive solutes leaching from the much larger groundwater recharge areas (GRAs) uphill. Plant N sources include not only inorganic N, but, as heightened more recently, also organic N species. In general, also the ratio inorganic N over organic N sources increase down hillslopes. Here, we review recent evidence about the nature of the N limitation and its variations in Fennoscandian boreal forests and discuss its implications for forest ecology and management.The rate of litter decomposition has traditionally been seen as the determinant of the rate of N supply. However, while N-rich litter decomposes faster than N-poor litter initially, N-rich litter then decomposes more slowly, which means that the relation between N % of litter and its decomposability is complex. Moreover, in the lower part of themor-layer, where the most superficial mycorrhizal roots first appear, and N availability matters for plants, the ratio of microbial N over total soil N is remarkably constant over the wide range in litter and soil C/N ratios of between 15 and 40 for N-rich and N-poor sites, respectively. Nitrogen-rich and-poor sites thus differ in the sizes of the total N pool and the microbial N pool, but not in the ratio between them. A more important difference is that the soil microbial N pool turns over faster in N -rich systems because the microbes are more limited by C, while microbes in N-poor systems are a stronger sink for available N.Furthermore, litter decomposition in the most superficial soil horizon (as studied by the so-called litter-bag method) is associated with a dominance of saprotrophic fungi, and absence of mycorrhizal fungi. The focal zone in the context of plant N supply in N-limited forests is further down the soil profile, whereectomycorrhizal (ECM) roots become abundant. Molecular evidence and stable isotope data indicate that in the typical N-poor boreal forests, nitrogen is retained in saprotrophic fungi, likely until they run out of energy (available C-compounds). Then, as heightened by recent research, ECM fungi, which are supplied by photosynthate from the trees, become the superior competitors for N.In N-poor boreal soils strong N retention by microorganisms keeps levels of available N very low. This is exacerbated by an increase in tree C allocation to mycorrhizal fungi (TCAM) relative to net primary production (NPP) with decreasing soil N supply, which causes ECM fungi to retain much of the available soil N for their own growth and transfer little to their tree hosts. The transfer of N through the ECM fungi, and not the rate of litter decomposition, is likely limiting the rate of tree N supply under such conditions. All but a few stress-tolerant less N-demanding plant species, like the ECM trees themselves and ericaceous dwarf shrubs, are excluded.With increasing N supply, a weakening of ECM symbiosis caused by the relative decline in TCAM contributes to shifts in soil microbial community composition from fungal dominance to bacterial dominance. Thus, bacteria, which are less C-demanding, but more likely to release N than fungi, take over. This, and the relatively high pH in GDA, allow autotrophic nitrifying bacteria to compete successfully for the NH4+ released by C-limited organisms and causes the N cycle to open up with leaching of nitrate (NO) and gaseous N losses through denitrification. These N-rich conditions allow species-rich communities of N-demanding plant species. Meanwhile, ECM fungi have a smaller biomass, are supplied with N in excess of their demand and will export more N to their host trees. Hence, the gradient from low to high N supply is characterized by profound variations in plant and soil microbial physiologies, especially their relations to the C-to-N supply ratio. We propose how interactions among functional groups can be understood and modelled (the plant-microbe carbon-nitrogen model).With regard to forest management these perspectives explain why the creation of larger tree-free gaps favors the regeneration of tree seedlings under N-limited conditions through reduced belowground competition for N, and why such gaps are less important under high N supply (but when light might be limiting). We also discuss perspectives on the relations between N supply, biodiversity, and eutrophication of boreal forests from N deposition or forest fertilization. (C) 2017 The Authors. Published by Elsevier B.V.
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| 17. |
- Keel, Sonja G., et al.
(författare)
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Allocation of carbon to fine root compounds and their residence times in a boreal forest depend on root size class and season
- 2012
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Ingår i: New Phytologist. - Malden : Wiley-Blackwell. - 0028-646X .- 1469-8137. ; 194:4, s. 972-981
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Tidskriftsartikel (refereegranskat)abstract
- Fine roots play a key role in the forest carbon balance, but their carbon dynamics remain largely unknown. We pulse labelled 50 m2 patches of young boreal forest by exposure to 13CO2 in early and late summer. Labelled photosynthates were traced into carbon compounds of < 1 and 13 mm diameter roots (fine roots), and into bulk tissue of these and first-order roots (root tips). Root tips were the most strongly labelled size class. Carbon allocation to all size classes was higher in late than in early summer; mean residence times (MRTs) in starch increased from 4 to 11 months. In structural compounds, MRTs were 0.8 yr in tips and 1.8 yr in fine roots. The MRT of carbon in sugars was in the range of days. Functional differences within the fine root population were indicated by carbon allocation patterns and residence times. Pronounced allocation of recent carbon and higher turnover rates in tips are associated with their role in nutrient and water acquisition. In fine roots, longer MRTs but high allocation to sugars and starch reflect their role in structural support and storage. Accounting for heterogeneity in carbon residence times will improve and most probably reduce the estimates of fine root production.
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| 18. |
- Larson, Johannes, et al.
(författare)
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What happens to trees and soils during five decades of experimental nitrogen loading?
- 2024
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Ingår i: Forest Ecology and Management. - 0378-1127 .- 1872-7042. ; 553
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Tidskriftsartikel (refereegranskat)abstract
- High deposition of nitrogen was postulated to drive losses of NO3 - and nutrient base cations, causing soil acidification, nutrient deficiencies reducing tree growth and ultimately tree mortality. We tested these predictions in a uniquely long-term study involving three NH4NO3 addition treatments (N1-N3) in a boreal Pinus sylvestris forest. The lowest level (N1), 30 kg N ha− 1 yr− 1 was applied during 50 years. Twice this rate (N2) was added 38 years, followed by 12 years of recovery, while thrice this rate (N3) was added 20 years followed by 30 years of recovery. We compared tree growth, changes in foliar and soil chemistry among treatments including control plots without N additions. As predicted, the N treatments lowered soil pH and reduced soil base saturation by around 50 %. They also lowered foliar levels of Ca, Mg, K, P and B initially, but after 50 years only Ca and Mg remained lower than in the control. Lack of B motivated a single addition of 2.5 kg ha− 1 after ten years of N treatment. Tree stem growth became and then remained higher in N1 than in the other treatments through the 50 years of treatments. In N2 and N3, foliar δ15N increased during the N-loading phase, but declined during the recovery phase, indicating a return of ectomycorrhizal fungi and their role in tightening the N cycle in N-limited forests. In the terminated, initially highest N treatments, N2 and N3, the trees even show signs of returning to Nlimitation. In these treatments, the soil base saturation remains lower, while the pH was only lower at 0–10 depth in the mineral soil, but not in the 10–20 cm depth horizon or in the superficial organic mor-layer. Accurately documenting the effect of N additions on forest growth required a long-term approach, where reasonable rates of application could be compared with extreme rates. Such long-term experiments are necessary to support forest management in achieving goals for developing future forests as they shift in response to major, global-scale changes.
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| 19. |
- Näsholm, Torgny, et al.
(författare)
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Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests?
- 2013
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Ingår i: New Phytologist. - : Wiley. - 0028-646X .- 1469-8137. ; 198:1, s. 214-221
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Tidskriftsartikel (refereegranskat)abstract
- Symbioses between plant roots and mycorrhizal fungi are thought to enhance plant uptake of nutrients through a favourable exchange for photosynthates. Ectomycorrhizal fungi are considered to play this vital role for trees in nitrogen (N)-limited boreal forests. We followed symbiotic carbon (C)N exchange in a large-scale boreal pine forest experiment by tracing 13CO2 absorbed through tree photosynthesis and 15N injected into a soil layer in which ectomycorrhizal fungi dominate the microbial community. We detected little 15N in tree canopies, but high levels in soil microbes and in mycorrhizal root tips, illustrating effective soil N immobilization, especially in late summer, when tree belowground C allocation was high. Additions of N fertilizer to the soil before labelling shifted the incorporation of 15N from soil microbes and root tips to tree foliage. These results were tested in a model for CN exchange between trees and mycorrhizal fungi, suggesting that ectomycorrhizal fungi transfer small fractions of absorbed N to trees under N-limited conditions, but larger fractions if more N is available. We suggest that greater allocation of C from trees to ectomycorrhizal fungi increases N retention in soil mycelium, driving boreal forests towards more severe N limitation at low N supply.
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| 20. |
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| 21. |
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| 22. |
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| 23. |
- Persson, Jörgen, et al.
(författare)
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Nitrogen acquisition from inorganic and organic sources by boreal forest plants in the field
- 2003
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Ingår i: Oecologia. - : Springer Science and Business Media LLC. - 0029-8549 .- 1432-1939. ; 137:2, s. 252-257
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Tidskriftsartikel (refereegranskat)abstract
- A wide range of recent studies have indicated that organic nitrogen may be of great importance to plant nitrogen (N) nutrition. Most of these studies have, however, been conducted in laboratory settings, excluding important factors for actual plant uptake, such as competition, mycorrhizal associations and soil interactions. In order to accurately evaluate the importance of different N compounds to plant N nutrition, field studies are crucial. In this study, we investigated short- as well as long-term plant nitrogen uptake by Deschampsia flexuosa, Picea abies and Vaccinium myrtillus from 15NO3-, 15NH4+ and (U-13C, 15N) arginine, glycine or peptides. Root N uptake was analysed after 6 h and 64 days following injections. Our results show that all three species, irrespective of their type of associated mycorrhiza (arbuscular, ecto- or ericoid, respectively) rapidly acquired similar amounts of N from the entire range of added N sources. After 64 days, P. abies and V. myrtillus had acquired similar amounts of N from all N sources, while for D. flexuosa, the uptake from all N sources except ammonium was significantly lower than that from nitrate. Furthermore, soil analyses indicate that glycine was rapidly decarboxylated after injections, while other organic compounds exhibited slower turnover. In all, these results suggest that a wide range of N compounds may be of importance for the N nutrition of these boreal forest plants, and that the type of mycorrhiza may be of great importance for N scavenging, but less important to the N uptake capacity of plants.
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| 24. |
- Haas, Julia Christa, et al.
(författare)
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Microbial community response to growing season and plant nutrient optimisation in a boreal Norway spruce forest
- 2018
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Ingår i: Soil Biology and Biochemistry. - : Elsevier. - 0038-0717 .- 1879-3428. ; 125, s. 197-209
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Tidskriftsartikel (refereegranskat)abstract
- Interactions between Norway spruce trees and bacteria and fungi in nutrient limited boreal forests can be beneficial for tree growth and fitness. Tree-level effects of anthropogenic nutrient addition have been well studied, however understanding of the long-term effects on the associated microbiota is limited. Here, we report on the sensitivity of microbial community composition to the growing season and nutrient additions. Highthroughput sequencing of the bacterial 16S rRNA gene and fungal ITS1 region was used to characterise changes in the microbial community after application of a complete mineral nutrient mixture for five and 25 years. The experiment was conducted using the Flakaliden forest research site in northern boreal Sweden and included naturally low nutrient control plots. Needle and fine root samples of Norway spruce were sampled in addition to bulk soil during one growing season to provide comprehensive insight into phyllosphere and belowground microbiota community changes. The phyllosphere microbiota was compositionally distinct from the belowground communities and phyllosphere diversity increased significantly over the growing season but was not influenced by the improved nutrient status of the trees. In both root and soil samples, alpha diversity of fungal, in particular ectomycorrhizal fungi (EMF), and bacterial communities increased after long-term nutrient optimisation, and with increasing years of treatment the composition of the fungal and bacterial communities changed toward a community with a higher relative abundance of nitrophilic EMF and bacterial species but did not cause complete loss of nitrophobic species from the ecosystem. From this, we conclude that 25 years of continuous nutrient addition to a boreal spruce stand increased phylotype richness and diversity of the microbiota in the soil, and at the root-soil interface, suggesting that long-term anthropogenic nutrient inputs can have positive effects on belowground biodiversity that may enhance ecosystem robustness. Future studies are needed to assess the impact of these changes to the microbiota on ecosystem carbon storage and nitrogen cycling in boreal forests.
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| 25. |
- Högberg, Mona N
(författare)
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Can gas chromatography combustion isotope ratio mass spectrometry be used to quantify organic compound abundance?
- 2011
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Ingår i: Rapid Communications in Mass Spectrometry. - : Wiley. - 0951-4198 .- 1097-0231. ; 25, s. 2433-2438
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Tidskriftsartikel (refereegranskat)abstract
- Quantifying the concentrations of organics such as phospholipid fatty acids (PLFAs) and n-alkanes and measuring their corresponding (13)C/(12)C isotope ratios often involves two separate analyses; (1) quantification by gas chromatography flame ionisation detection (GC-FID) or gas chromatography/mass spectrometry (GC/MS), and (2) (13)C-isotope abundance analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC-C-IRMS). This requirement for two separate analyses has obvious disadvantages in terms of cost and time. However, there is a history of using the data output of isotope ratio mass spectrometers to quantify various components; including the N and C concentrations of solid materials and CO(2) concentrations in gaseous samples. Here we explore the possibility of quantifying n-alkanes extracted from sheeps' faeces and fatty acid methyl esters (FAMEs) derivatised from PLFAs extracted from grassland soil, using GC-C-IRMS. The results were compared with those from GC-FID analysis of the same extracts. For GC-C-IRMS the combined area of the masses for all the ions (m/z 44, 45 and 46) was collected, referred to as 'area all', while for the GC-FID analysis the peak area data were collected. Following normalisation to a common value for added internal standards, the GC-C-IRMS 'area all' values and the GC-FID peak area data were directly compared. Strong linear relationships were found for both n-alkanes and FAMEs. For the n-alkanes the relationships were 1: 1 while, for the FAMEs, GC-C-IRMS overestimated the areas relative to the GC-FID results. However, with suitable reference material 1:1 relationships were established. The output of a GC-C-IRMS system can form the basis for the quantification of certain organics including FAMEs and n-alkanes. Copyright (C) 2011 John Wiley & Sons, Ltd.
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| 26. |
- Högberg, Mona N
(författare)
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Fungal but not bacterial soil communities recover after termination of decadal nitrogen additions to boreal forest
- 2014
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 72, s. 35-43
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Tidskriftsartikel (refereegranskat)abstract
- The rate at which formerly nitrogen loaded forests will return to their natural nitrogen-limited state is of considerable scientific and societal interest. Yet the sensitivity of soil microorganisms to these putative changes is mainly unknown. We report effects on fungal and bacterial communities caused by two decades of chronic nitrogen fertilization and subsequent changes 14 years after termination of nitrogen load. We compare these changes in community composition with those observed in natural nitrogen supply and pH gradients using DNA fingerprinting methods and Sanger sequencing.Soil fungal ITS length-heterogeneity profiles correlated equally well to carbon-to-nitrogen ratios and pH. Sequencing results indicated a clear decrease in the relative abundance of amplicons ascribed to known ectomycorrhizal fungi in both natural and experimental high nitrogen conditions, and a recovery of species in the terminated nitrogen treatment. The dominant sequences in low nitrogen soils were identified as members of Piloderma spp. Terminal restriction fragment length profiles of the bacterial 165 rRNA gene were linked to carbon-to-nitrogen ratios and pH in the natural locations but to soil nitrogen in the nitrogen addition experiment that had low variability in pH. Sequencing revealed the dominance of Acidobacteria and Proteobacteria in all soils but also showed a marked increase in Bacteroidetes in high nitrogen treatment not evident in the natural high nitrogen and high pH environments. Proteobacteria sequences included described strains from high-organic and low-pH systems that are believed be involved in degradation of complex plant material.There were signs of recovery of fungal but not of bacterial communities in the sense that community's in terminated nitrogen addition plots did not differ significantly from those in control plots or from the low nitrogen stands in the natural nitrogen supply gradient. The need of further examination of the seemingly functionally redundant bacterial communities is stressed. (C) 2014 Elsevier Ltd. All rights reserved.
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| 27. |
- Högberg, Mona N
(författare)
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Soil bacteria and archaea change rapidly in the first century of Fennoscandian boreal forest development
- 2017
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Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 114, s. 160-167
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Tidskriftsartikel (refereegranskat)abstract
- Using a gradient of changing ground age, caused by glacial isostatic adjustment, we compared systems that spanned ages from 25 to 560-years-old. Illumina sequencing was applied to determine archaeal and bacterial composition, investigating how different phylogenetic groups change as ecosystems develop. Bacterial communities dramatically changed during early ecosystem development (p < 0.001), evidenced by significant compositional shifts between 25 and 115 year-old-soils. Although significant differences did occur in the three later aged sites, they did not change as much. This was consistent with vegetation that shifted from meadow (25 year) to alder dominated forest (115 year), to ecosystems containing spruce. Correlation networks revealed that the microbial communities became more interconnected in older age ecosystems with a two-fold increase in network density. Species richness had the opposite trend with a decreased number of species: 781 operational taxonomic units (OTUs) in the youngest ecosystem to 366 OTUs in the oldest ecosystem. The observed shifts in community composition are consistent with other reported ecosystem gradients, but here we show that not only does composition change, but as ecosystems age the network connectivity increases indicating potentially more social interactions among microbes or increasingly stringent plant-microbe-soil interactions. (C) 2017 Elsevier Ltd. All rights reserved.
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| 28. |
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| 29. |
- Högberg, Mona N
(författare)
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Termination of belowground C allocation by trees alters soil fungal and bacterial communities in a boreal forest
- 2009
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Ingår i: FEMS Microbiology Ecology. - : Oxford University Press (OUP). - 0168-6496 .- 1574-6941. ; 70, s. 151-162
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Tidskriftsartikel (refereegranskat)abstract
- The introduction of photosynthates through plant roots is a major source of carbon (C) for soil microbial biota and shapes the composition of fungal and bacterial communities in the rhizosphere. Although the importance of this process, especially to ectomycorrhizal fungi, has been known for some time, the extent to which plant belowground C allocation controls the composition of the wider soil community is not understood. A tree-girdling experiment enabled studies of the relationship between plant C allocation and microbial community composition. Girdling involves cutting the phloem of trees to prevent photosynthates from entering the soil. Four years after girdling, fungal and bacterial communities were characterized using DNA-based profiles and cloning and sequencing. Data showed that girdling significantly altered fungal and bacterial communities compared with the control. The ratio of ectomycorrhizal to saprobic fungal sequences significantly decreased in girdled treatments, and this decline was found to correlate with the fungal phospholipid fatty acid biomarker 18:26,9. Bacterial communities also varied in the abundance of the two dominant phyla Acidobacteria and Alphaproteobacteria. Concomitant changes in fungal and bacterial communities suggest linkages between these two groups and point toward plant belowground C allocation as a key determinant of microbial community composition.
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