Mona N. Högberg

Large-scale forest girdling shows that current photosynthesis drives soil respiration

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.

Boreal forest plants take up organic nitrogen

Plant growth in the boreal forest, the largest terrestrial biome, is generally limited by the availability of nitrogen. The presumed cause of this limitation is slow mineralization of soil organic nitrogen,. Here we demonstrate, to our knowledge for the first time, the uptake of organic nitrogen in the field by the trees Pinus sylvestris and Picea abies, the dwarf shrub Vaccinium myrtillus and the grass Deschampsia flexuosa. These results show that these plants, irrespective of their different types of root–fungal associations (mycorrhiza), bypass nitrogen mineralization. A trace of the amino acid glycine, labelled with the stable isotopes 13C and 15N, was injected into the organic (mor) layer of an old successional boreal coniferous forest. Ratios of 13C:15N in the roots showed that at least 91, 64 and 42% of the nitrogen from the absorbed glycine was taken up in intact glycine by the dwarf shrub, the grass and the trees, respectively. Rates of glycine uptake were similar to those of 15N-ammonium. Our data indicate that organic nitrogen is important for these different plants, even when they are competing with each other and with non-symbiotic microorganisms. This has major implications for our understanding of the effects of nitrogen deposition, global warming and intensified forestry.

15N abundance of surface soils, roots and mycorrhizas in profiles of European forest soils

Abstract15N natural abundances of soil total N, roots and mycorrhizas were studied in surface soil profiles in coniferous and broadleaved forests along a transect from central to northern Europe. Under conditions of N limitation in Sweden, there was an increase in δ15N of soil total N of up to 9% from the uppermost horizon of the organic mor layer down to the upper 0–5 cm of the mineral soil. The δ15N of roots was only slightly lower than that of soil total N in the upper organic horizon, but further down roots were up to 5% depleted under such conditions. In experimentally N-enriched forest in Sweden, i.e. in plots which have received an average of c. 100 kg N ha−1 year−1 for 20 years and which retain less than 50% of this added N in the stand and the soil down to 20 cm depth, and in some forests in central Europe, the increase in δ15N with depth in soil total N was smaller. An increase in δ15N of the surface soil was even observed on experimentally N-enriched plots, although other data suggest that the N fertilizer added was depleted in15N. In such cases roots could be enriched in15N relative to soil total N, suggesting that labelling of the surface soil is via the pathway: — available pools of N-plant N-litter N. Under N-limiting conditions roots of different species sampled from the same soil horizon showed similar δ15N. By contrast, in experimentally N-enriched forest δ15N of roots increased in the sequence: ericaceous dwarf shrubs<Scots pine<grass, suggesting increasing use of inorganic N along the sequence. Complementary studies at the major transect sites had shown that 90–99% of fine tree roots had ectomycorrhizas (ECMs). ECMs were 2% more enriched than corresponding non-mycorrhizal fine roots. Fungal sheaths stripped off ECMs were 2.4–6.4 enriched relative to the remaining root core. It is suggested that a flux of N through ECMs to aboveground parts in N-limited forests would leave 15N enriched compounds in fungal material, which could contribute to explain the observed δ15N profiles if fungal material is enriched, because it is a precursor of stable organic matter and recalcitrant N. This could act in addition to the previous explanation of the isotopically lighter soil surface in forests: plant uptake of 15N-depleted N and its redeposition onto the soil surface by litter-fall.

SOIL CHEMISTRY AND PLANTS IN FENNOSCANDIAN BOREAL FOREST AS EXEMPLIFIED BY A LOCAL GRADIENT

In Fennoscandian boreal forests, in which productivity in general is N lim- ited, there are regular, topographically related variations in forest productivity and plant community composition. Regional surveys have demonstrated strong correlations among soil pH, N content, and base saturation on the one hand and plant productivity and com- munity composition on the other, but the nature of these relationships is poorly understood. We studied in detail the variation in and controls of soil acidity, availability of N and P, and changes in community composition and plant nutrition along a short (only 90 m long) but extreme forest productivity gradient in northern Sweden, which ranged from a ground- water recharge area with low productivity to a very productive discharge area. The pH in the soil solution of the mor layer ranged from 3.5 in the recharge area to 6.4 in the discharge area, and it was strongly correlated with the base saturation of the exchange complex. Neither the acid strength of organic matter, the ionic strength of the soil solution, nor the quantity of acids could explain more than a minor part of this variation in pH. There were strong correlations between total N in the mor layer and soil solution pH (r = 0.97) and base saturation of the exchange complex (r = 0.88). At the poor end of the transect the concentration of inorganic N was very low in the mor, and plants with either ectomycorrhizae (ECM) or ericoid mycorrhizae (EM) dominated. With increasing pH, there was an increase in NH4 concentrations, while plants that potentially have arbus- cular mycorrhizae (AM) became prominent along with ECM and EM species. In the dis- charge area, which comprised only the last 10 m of the transect, NO3 dominated over NH4 in the soil solution, the soil had a high capacity for net nitrification, and the vegetation was totally dominated by potentially AM or nonmycorrhizal herbs, some of which had high foliar nitrate reductase activity. Foliar and root N concentrations increased steeply towards the discharge area, but foliar P/N ratios declined below critical levels at the end of the transect. Root 32P uptake bioassays also indicated a P deficiency in the discharge area, where the soil total P content was high, while the concentration of P04 in the soil solution was very low. The high capacity of the mor in the discharge area to adsorb P04, due to the presence of organically complexed Fe and Fe-oxihydroxides, may explain the low P04 concentrations. Our data indicate that the underlying factors influencing both productivity and com- munity composition are pH and supply of base cations. Fundamental differences in exchange characteristics of soil and soil water underlie other related nutrient supply features, in particular the amount and availability of N. Our study of a single short topographic transect supported a previous suggestion based on a regional survey in Norway that variability in soil pH and the supply of base cations affects plant productivity and community composition via effects on N supply. Our data also encompass the interrelations between soil pH, soil N turnover, and the mycorrhizal type of dominant plant species, which, according to Read (1991), occur along long latitudinal or altitudinal climatic gradients. Through millennia discharge areas like the one observed by us have probably provided a relatively stable environment for plants demanding high soil pH and N supply, at the same time as sur- rounding recharge areas have been acidified naturally through podzolization.

Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests

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 (13) CO(2) absorbed through tree photosynthesis and (15) N injected into a soil layer in which ectomycorrhizal fungi dominate the microbial community. We detected little (15) N 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 (15) N from soil microbes and root tips to tree foliage. These results were tested in a model for C-N 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.

Gross nitrogen mineralisation and fungi-to-bacteria ratios are negatively correlated in boreal forests

In terrestrial ecosystems, gross nitrogen mineralisation is positively correlated to microbial biomass but negatively to soil organic matter C-to-N ratios; the influence of the microbial community structure is less well known. Here, we relate rates of gross N mineralisation to fungi-to-bacteria ratios in three natural forest types of contrasting N availability and in a long-term N-loading experiment in a boreal forest. We report, for the first time, a strong negative correlation between gross N mineralisation and the fungi-to-bacteria ratio (

Forests trapped in nitrogen limitation – an ecological market perspective on ectomycorrhizal symbiosis

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Measurements of abundances of 15N and 13C as tools in retrospective studies of N balances and water stress in forests: A discussion of preliminary results

 = 0.91, P = 0.0005, N = 7). There was also a negative correlation between gross N mineralisation and the C-to-N ratio (