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Mona N Hogberg - One of the best experts on this subject based on the ideXlab platform.
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termination of belowground c allocation by trees alters soil fungal and bacterial communities in a boreal forest
FEMS Microbiology Ecology, 2009Co-Authors: Stephanie A Yarwood, David D Myrold, Mona N HogbergAbstract: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 18:2ω6,9. 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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Production of dissolved organic carbon and low-molecular weight organic acids in soil solution driven by recent tree photosynthate
Biogeochemistry, 2007Co-Authors: Reiner Giesler, Anders Nordgren, Mona N Hogberg, Andreas Richter, Bjarne W. Strobel, Peter HogbergAbstract:Dissolved organic carbon (DOC) is an important component in the terrestrial carbon cycle. Yet, the relative importance of different inputs of DOC to the soil solution remains uncertain. Here, we used a large-scale forest Girdling experiment to examine how the supply of recent photosynthate to tree roots and their mycorrhizal fungi affects DOC, in particular low-molecular weight organic acids (LMWOA). We also studied effects of tree Girdling on non-structural carbohydrates in microorganism, and examined the effects of freezing of soil and the presence of roots in the soil samples on soil solution DOC and LMWOA in this experiment. The concentration of DOC was reduced by 40%, while citrate was reduced by up to 90% in the soil solution by the Girdling treatment. Other LMWOA such as oxalate, succinate, formate and propionate were unaffected by the Girdling. We also found that Girdling reduced the concentrations of trehalose (by 50%), a typical fungal sugar, and of monosaccharides (by 40%) in microorganisms in root-free soil. The effect of freezing on DOC concentrations was marked in samples from control plots, but insignificant in samples from girdled plots. Release of DOC from cell lysis after freezing was attributed equally to roots and to microorganisms. Our observations suggest a direct link from tree photosynthesis through roots and their mycorrhizal fungi to soil solution chemistry. This direct link should impact solute transport and speciation, mineral weathering and C dynamics in the soil compartment. Importantly, our finding of a substantial photosynthate driven production of DOC challenges the paradigm that DOC is mainly the result of decomposition of organic matter.
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is microbial community composition in boreal forest soils determined by ph c to n ratio the trees or all three
Oecologia, 2006Co-Authors: Mona N Hogberg, Peter Hogberg, David D MyroldAbstract:In Fennoscandian boreal forests, soil pH and N supply generally increase downhill as a result of water transport of base cations and N, respectively. Simultaneously, forest productivity increases, the understory changes from ericaceous dwarf shrubs to tall herbs; in the soil, fungi decrease whereas bacteria increase. The composition of the soil microbial community is mainly thought to be controlled by the pH and C-to-N ratio of the substrate. However, the latter also determines the N supply to plants, the plant community composition, and should also affect plant allocation of C below ground to roots and a major functional group of microbes, mycorrhizal fungi. We used phospholipid fatty acids (PLFAs) to analyze the potential importance of mycorrhizal fungi by comparing the microbial community composition in a tree-Girdling experiment, where tree belowground C allocation was terminated, and in a long-term (34 years) N loading experiment, with the shifts across a natural pH and N supply gradient. Both tree Girdling and N loading caused a decline of ca. 45% of the fungal biomarker PLFA 18:2ω6,9, suggesting a common mechanism, i.e., that N loading caused a decrease in the C supply to ectomycorrhizal fungi just as tree Girdling did. The total abundance of bacterial PLFAs did not respond to tree Girdling or to N loading, in which cases the pH (of the mor layer) did not change appreciably, but bacterial PLFAs increased considerably when pH increased across the natural gradient. Fungal biomass was high only in acid soil (pH 38). According to a principal component analysis, the soil C-to-N ratio was as good as predictor of microbial community structure as pH. Our study thus indicated the soil C-to-N ratio, and the response of trees to this ratio, as important factors that together with soil pH influence soil microbial community composition.
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large scale forest Girdling shows that current photosynthesis drives soil respiration
Nature, 2001Co-Authors: Peter Hogberg, Anders Nordgren, Alf Ekblad, Mona N Hogberg, Mikaell Ottossonlofvenius, Nina Buchmann, Andy F S Taylor, Gert Nyberg, David ReadAbstract: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 uncertain1,2. 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.
Yongbiao Lin - One of the best experts on this subject based on the ideXlab platform.
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tree Girdling effect on bacterial substrate utilization pattern depending on stand age and soil microclimate in eucalyptus plantations
Applied Soil Ecology, 2012Co-Authors: Zhanfeng Liu, Lixia Zhou, Yongbiao LinAbstract:a b s t r a c t It has been increasingly recognized that soil microorganisms and plants are tightly linked, however, the effects of belowground plant-carbon allocation on soil microbial communities has not been fully under- stood, particularly in subtropical and tropical forest ecosystems. In the present study, we conducted a tree-Girdling experiment to explore the relationships between plant belowground carbon allocation and the functional diversity of soil microbial communities in two subtropical Eucalyptus plantations (2-year- old and 24-year-old). The functional diversity of soil culturable bacterial communities was characterized with community level physiological profile (CLPP) 1 year after tree Girdling. We found that carbon sub- strate utilization patterns of soil culturable bacterial communities were significantly affected by both tree Girdling and sampling season; but microbial metabolic activity and functional diversity were only affected by sampling season. Microbial metabolic activity and substrate utilization pattern, as well as soil moisture and temperature, did not show any apparent variations among stand ages. Pearson's correlation analysis indicated that both microbial metabolic activity of all 95 carbon sources (r = 0.869; P < 0.01) and the specific substrate metabolic activities (amides and amines: r = 0.838, P < 0.01; polymers: r = 0.884, P < 0.01; miscellaneous: r = 0.895, P < 0.01; carboxylic acids: r = 0.910, P < 0.01; amino acids: r = 0.806, P < 0.01; carbohydrates: r = 0.808, P < 0.01) were positively correlated with soil temperature. Metabolic diversity (r = 0.806, P < 0.01) and metabolic evenness (r = 0.819, P < 0.01) was positively correlated with soil temperature but metabolic richness (r = −0.517, P < 0.05) was negatively correlated with soil mois- ture. Metabolic evenness was positively correlated with soil temperature (r = 0.416, P < 0.05). Seasonal changes in soil bacterial community were greater than those induced by Girdling and stand age, and were tightly linked to seasonal soil microclimatic differences, such as soil moisture and temperature. The Girdling treatment exerted a greater effect on soil bacterial communities during the dry season than in the rainy season. Our study illustrated that the response of bacterial substrate utilization profiles to changes
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tree Girdling affects the soil microbial community by modifying resource availability in two subtropical plantations
Applied Soil Ecology, 2012Co-Authors: Dima Chen, Lixia Zhou, Joanna Hsu, Yongbiao LinAbstract:Abstract We used tree Girdling and phospholipid fatty acid (PLFA) analysis to evaluate the effect of nutrient availability and rhizodeposition on soil microbial community composition in two plantations ( Acacia crassicarpa and Eucalyptus urophylla ) in subtropical China. The magnitude of the Girdling effect was also evaluated as a function of tree species and time after Girdling (2 months vs. 9 months). In both plantations, tree Girdling reduced the concentration of fungal PLFAs and increased the concentration of bacterial PLFAs with a consequent decrease in the fungi/bacteria ratio, but did not affect the concentration of total PLFAs. Tree Girdling affected the concentration of gram-negative PLFAs and the ratio of gram-positive bacteria to gram-negative bacteria at 9 months but not at 2 months after Girdling. The ratio of cy17:0 to 16:1ω7 c was increased by Girdling of A. crassicarpa , indicating a stressful and nutrient-deficient habitat for soil microorganisms, but was inconsistent for Girdling of E. urophylla . In the A. crassicarpa plantation, responses to Girdling for most microbial groups were associated with changes in dissolved organic carbon (DOC), dissolved organic nitrogen (DON), the ratio of carbon to nitrogen, and soil pH. In the E. urophylla plantation, responses to Girdling were associated with changes in DON, DOC, and NO 3 − -N. These results confirm that (i) recent photosynthates allocated belowground affect soil C and N availability and therefore greatly affect microbial community composition in subtropical plantations; (ii) the magnitude of the tree Girdling effect increases with time after Girdling and differs between plant species; and (iii) soil microbial communities are closely linked to vegetation types and plant C allocation.
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changes in belowground carbon in acacia crassicarpa and eucalyptus urophylla plantations after tree Girdling
Plant and Soil, 2010Co-Authors: Dima Chen, Yongbiao Lin, Yang Zhang, Weixing ZhuAbstract:Limitations in the techniques used to separate root-derived and soil-organic-matter (SOM)-derived respiration have hampered the understanding of forest carbon cycling. Tree Girdling is considered to be a robust approach with little disturbance to the root–soil system. Using this approach, we tried to separate root-derived respiration from SOM-derived respiration under Acacia crassicarpa and Eucalyptus urophylla plantations in South China. We found that Girdling reduced soil respiration and temperature sensitivity of respiration (Q 10) under both plantations compared to controls, but the intensity of Girdling effects was species specific. Six months after Girdling, live fine root biomass was lower than the control in A. crassicarpa but not in E. urophylla. Soil microbial biomass (C mic) under A. crassicarpa was increased by Girdling 17 days after treatment, but decreased thereafter. In contrast, there was no difference in C mic between girdled and control treatments under E. urophylla. Girdling significantly decreased soil organic carbon (SOC) and dissolved organic carbon (DOC) under A. crassicarpa, but not under E. urophylla. We ascribe differences in Girdling effects on belowground carbon between the two species to differences in resprouting traits.
Lixia Zhou - One of the best experts on this subject based on the ideXlab platform.
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tree Girdling effect on bacterial substrate utilization pattern depending on stand age and soil microclimate in eucalyptus plantations
Applied Soil Ecology, 2012Co-Authors: Zhanfeng Liu, Lixia Zhou, Yongbiao LinAbstract:a b s t r a c t It has been increasingly recognized that soil microorganisms and plants are tightly linked, however, the effects of belowground plant-carbon allocation on soil microbial communities has not been fully under- stood, particularly in subtropical and tropical forest ecosystems. In the present study, we conducted a tree-Girdling experiment to explore the relationships between plant belowground carbon allocation and the functional diversity of soil microbial communities in two subtropical Eucalyptus plantations (2-year- old and 24-year-old). The functional diversity of soil culturable bacterial communities was characterized with community level physiological profile (CLPP) 1 year after tree Girdling. We found that carbon sub- strate utilization patterns of soil culturable bacterial communities were significantly affected by both tree Girdling and sampling season; but microbial metabolic activity and functional diversity were only affected by sampling season. Microbial metabolic activity and substrate utilization pattern, as well as soil moisture and temperature, did not show any apparent variations among stand ages. Pearson's correlation analysis indicated that both microbial metabolic activity of all 95 carbon sources (r = 0.869; P < 0.01) and the specific substrate metabolic activities (amides and amines: r = 0.838, P < 0.01; polymers: r = 0.884, P < 0.01; miscellaneous: r = 0.895, P < 0.01; carboxylic acids: r = 0.910, P < 0.01; amino acids: r = 0.806, P < 0.01; carbohydrates: r = 0.808, P < 0.01) were positively correlated with soil temperature. Metabolic diversity (r = 0.806, P < 0.01) and metabolic evenness (r = 0.819, P < 0.01) was positively correlated with soil temperature but metabolic richness (r = −0.517, P < 0.05) was negatively correlated with soil mois- ture. Metabolic evenness was positively correlated with soil temperature (r = 0.416, P < 0.05). Seasonal changes in soil bacterial community were greater than those induced by Girdling and stand age, and were tightly linked to seasonal soil microclimatic differences, such as soil moisture and temperature. The Girdling treatment exerted a greater effect on soil bacterial communities during the dry season than in the rainy season. Our study illustrated that the response of bacterial substrate utilization profiles to changes
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tree Girdling affects the soil microbial community by modifying resource availability in two subtropical plantations
Applied Soil Ecology, 2012Co-Authors: Dima Chen, Lixia Zhou, Joanna Hsu, Yongbiao LinAbstract:Abstract We used tree Girdling and phospholipid fatty acid (PLFA) analysis to evaluate the effect of nutrient availability and rhizodeposition on soil microbial community composition in two plantations ( Acacia crassicarpa and Eucalyptus urophylla ) in subtropical China. The magnitude of the Girdling effect was also evaluated as a function of tree species and time after Girdling (2 months vs. 9 months). In both plantations, tree Girdling reduced the concentration of fungal PLFAs and increased the concentration of bacterial PLFAs with a consequent decrease in the fungi/bacteria ratio, but did not affect the concentration of total PLFAs. Tree Girdling affected the concentration of gram-negative PLFAs and the ratio of gram-positive bacteria to gram-negative bacteria at 9 months but not at 2 months after Girdling. The ratio of cy17:0 to 16:1ω7 c was increased by Girdling of A. crassicarpa , indicating a stressful and nutrient-deficient habitat for soil microorganisms, but was inconsistent for Girdling of E. urophylla . In the A. crassicarpa plantation, responses to Girdling for most microbial groups were associated with changes in dissolved organic carbon (DOC), dissolved organic nitrogen (DON), the ratio of carbon to nitrogen, and soil pH. In the E. urophylla plantation, responses to Girdling were associated with changes in DON, DOC, and NO 3 − -N. These results confirm that (i) recent photosynthates allocated belowground affect soil C and N availability and therefore greatly affect microbial community composition in subtropical plantations; (ii) the magnitude of the tree Girdling effect increases with time after Girdling and differs between plant species; and (iii) soil microbial communities are closely linked to vegetation types and plant C allocation.
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effects of understory removal and tree Girdling on soil microbial community composition and litter decomposition in two eucalyptus plantations in south china
Functional Ecology, 2011Co-Authors: Jianping Wu, Lixia Zhou, Xiaoling Wang, Shenglei FuAbstract:Summary 1. Soil micro-organisms play important roles in ecosystems and respond quickly to environmental changes. We examined how understory removal and tree Girdling influence the composition of soil microbial community and the litter decomposition in two subtropical plantations. 2. Phospholipid fatty acids (PLFAs) analysis was used to characterize soil microbial community. Redundancy analysis and principal response curves (PRC) were used to investigate the relationships between soil microbial community and environmental factors. 3. Understory removal significantly reduced the amount of fungal PLFAs, the ratio of fungal to bacterial PLFAs, and the litter decomposition but did not affect bacterial PLFAs and total PLFAs. In contrast, tree Girdling did not affect the soil microbial characteristics. The changes in soil microbial community caused by understory removal were mainly attributed to the indirect effects such as increased soil temperature and soil NO3 ) -N availability. In addition, PRC analysis showed that the relative abundance of most PLFAs increased in response to understory removal in the 2-year-old plantation but decreased in the 24-year-old plantation. 4. We propose that understory plants are important components in subtropical forest ecosystems, and play different roles in maintaining soil microbial community and driving litter decomposition processes in young vs. old plantations. The functions of understory plants should be considered in forest management and restoration. The negligible effect of tree Girdling on the soil micro-organisms can be attributed to the resprouting trait and mycorrhizal interactions of Eucalyptus.
David D Myrold - One of the best experts on this subject based on the ideXlab platform.
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termination of belowground c allocation by trees alters soil fungal and bacterial communities in a boreal forest
FEMS Microbiology Ecology, 2009Co-Authors: Stephanie A Yarwood, David D Myrold, Mona N HogbergAbstract: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 18:2ω6,9. 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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is microbial community composition in boreal forest soils determined by ph c to n ratio the trees or all three
Oecologia, 2006Co-Authors: Mona N Hogberg, Peter Hogberg, David D MyroldAbstract:In Fennoscandian boreal forests, soil pH and N supply generally increase downhill as a result of water transport of base cations and N, respectively. Simultaneously, forest productivity increases, the understory changes from ericaceous dwarf shrubs to tall herbs; in the soil, fungi decrease whereas bacteria increase. The composition of the soil microbial community is mainly thought to be controlled by the pH and C-to-N ratio of the substrate. However, the latter also determines the N supply to plants, the plant community composition, and should also affect plant allocation of C below ground to roots and a major functional group of microbes, mycorrhizal fungi. We used phospholipid fatty acids (PLFAs) to analyze the potential importance of mycorrhizal fungi by comparing the microbial community composition in a tree-Girdling experiment, where tree belowground C allocation was terminated, and in a long-term (34 years) N loading experiment, with the shifts across a natural pH and N supply gradient. Both tree Girdling and N loading caused a decline of ca. 45% of the fungal biomarker PLFA 18:2ω6,9, suggesting a common mechanism, i.e., that N loading caused a decrease in the C supply to ectomycorrhizal fungi just as tree Girdling did. The total abundance of bacterial PLFAs did not respond to tree Girdling or to N loading, in which cases the pH (of the mor layer) did not change appreciably, but bacterial PLFAs increased considerably when pH increased across the natural gradient. Fungal biomass was high only in acid soil (pH 38). According to a principal component analysis, the soil C-to-N ratio was as good as predictor of microbial community structure as pH. Our study thus indicated the soil C-to-N ratio, and the response of trees to this ratio, as important factors that together with soil pH influence soil microbial community composition.
Peter Hogberg - One of the best experts on this subject based on the ideXlab platform.
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partitioning of soil respiration into its autotrophic and heterotrophic components by means of tree Girdling in old boreal spruce forest
Forest Ecology and Management, 2009Co-Authors: Peter Hogberg, Mikaell Ottosson Lofvenius, Anders NordgrenAbstract:Abstract Forests accumulate much less carbon than the amount fixed through photosynthesis because of an almost equally large opposing flux of CO2 from the ecosystem. Most of the return flux to the atmosphere is through soil respiration, which has two major sources, one heterotrophic (organisms decomposing organic matter) and one autotrophic (roots, mycorrhizal fungi and other root-associated microbes dependent on recent photosynthate). We used tree-Girdling to stop the flow of photosynthate to the belowground system, hence, blocking autotrophic soil activity in a 120-yr-old boreal Picea abies forest. We found that at the end of the summer, two months after Girdling, the treatment had reduced soil respiration by up to 53%. This figure adds to a growing body of evidence indicating (t-test, d.f. = 7, p
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Production of dissolved organic carbon and low-molecular weight organic acids in soil solution driven by recent tree photosynthate
Biogeochemistry, 2007Co-Authors: Reiner Giesler, Anders Nordgren, Mona N Hogberg, Andreas Richter, Bjarne W. Strobel, Peter HogbergAbstract:Dissolved organic carbon (DOC) is an important component in the terrestrial carbon cycle. Yet, the relative importance of different inputs of DOC to the soil solution remains uncertain. Here, we used a large-scale forest Girdling experiment to examine how the supply of recent photosynthate to tree roots and their mycorrhizal fungi affects DOC, in particular low-molecular weight organic acids (LMWOA). We also studied effects of tree Girdling on non-structural carbohydrates in microorganism, and examined the effects of freezing of soil and the presence of roots in the soil samples on soil solution DOC and LMWOA in this experiment. The concentration of DOC was reduced by 40%, while citrate was reduced by up to 90% in the soil solution by the Girdling treatment. Other LMWOA such as oxalate, succinate, formate and propionate were unaffected by the Girdling. We also found that Girdling reduced the concentrations of trehalose (by 50%), a typical fungal sugar, and of monosaccharides (by 40%) in microorganisms in root-free soil. The effect of freezing on DOC concentrations was marked in samples from control plots, but insignificant in samples from girdled plots. Release of DOC from cell lysis after freezing was attributed equally to roots and to microorganisms. Our observations suggest a direct link from tree photosynthesis through roots and their mycorrhizal fungi to soil solution chemistry. This direct link should impact solute transport and speciation, mineral weathering and C dynamics in the soil compartment. Importantly, our finding of a substantial photosynthate driven production of DOC challenges the paradigm that DOC is mainly the result of decomposition of organic matter.
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is microbial community composition in boreal forest soils determined by ph c to n ratio the trees or all three
Oecologia, 2006Co-Authors: Mona N Hogberg, Peter Hogberg, David D MyroldAbstract:In Fennoscandian boreal forests, soil pH and N supply generally increase downhill as a result of water transport of base cations and N, respectively. Simultaneously, forest productivity increases, the understory changes from ericaceous dwarf shrubs to tall herbs; in the soil, fungi decrease whereas bacteria increase. The composition of the soil microbial community is mainly thought to be controlled by the pH and C-to-N ratio of the substrate. However, the latter also determines the N supply to plants, the plant community composition, and should also affect plant allocation of C below ground to roots and a major functional group of microbes, mycorrhizal fungi. We used phospholipid fatty acids (PLFAs) to analyze the potential importance of mycorrhizal fungi by comparing the microbial community composition in a tree-Girdling experiment, where tree belowground C allocation was terminated, and in a long-term (34 years) N loading experiment, with the shifts across a natural pH and N supply gradient. Both tree Girdling and N loading caused a decline of ca. 45% of the fungal biomarker PLFA 18:2ω6,9, suggesting a common mechanism, i.e., that N loading caused a decrease in the C supply to ectomycorrhizal fungi just as tree Girdling did. The total abundance of bacterial PLFAs did not respond to tree Girdling or to N loading, in which cases the pH (of the mor layer) did not change appreciably, but bacterial PLFAs increased considerably when pH increased across the natural gradient. Fungal biomass was high only in acid soil (pH 38). According to a principal component analysis, the soil C-to-N ratio was as good as predictor of microbial community structure as pH. Our study thus indicated the soil C-to-N ratio, and the response of trees to this ratio, as important factors that together with soil pH influence soil microbial community composition.
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large scale forest Girdling shows that current photosynthesis drives soil respiration
Nature, 2001Co-Authors: Peter Hogberg, Anders Nordgren, Alf Ekblad, Mona N Hogberg, Mikaell Ottossonlofvenius, Nina Buchmann, Andy F S Taylor, Gert Nyberg, David ReadAbstract: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 uncertain1,2. 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.