| 1. |
- 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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| 2. |
- 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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| 3. |
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| 4. |
- 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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| 5. |
- 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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