Showing papers by "Peter Högberg published in 2007"

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

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

Environmental science: nitrogen impacts on forest carbon.

Abstract: Does the extra nitrogen input from anthropogenic sources mean that more carbon from the atmosphere is being locked up in boreal and temperate forests? 'Yes' is the answer to emerge from the latest analysis.

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

Abstract: 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 ( $$ R^{{\text{2}}}_{{{\text{adj}}}} $$ = 0.91, P = 0.0005, N = 7). There was also a negative correlation between gross N mineralisation and the C-to-N ratio ( $$ R^{{\text{2}}}_{{{\text{adj}}}} $$ = 0.89, P = 0.001, N = 7), but a weaker positive correlation between gross N mineralisation and soil pH ( $$ R^{{\text{2}}}_{{{\text{adj}}}} $$ = 0.64, P = 0.019, N = 7). Our analysis suggests that soil fungi-to-bacteria and C-to-N ratios are interrelated and that they exert strong influences on soil N cycling in boreal forests.

Production of dissolved organic carbon and low-molecular weight organic acids in soil solution driven by recent tree photosynthate

Abstract: 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.

Variation in the δ13C of foliage of Pinus sylvestris L. in relation to climate and additions of nitrogen: analysis of a 32‐year chronology

Abstract: We report an analysis of both the long- and short-term drivers of the carbon (C) isotope composition (δ¹³C) values of current year needles of Pinus sylvestris L. linked to changing atmospheric carbon dioxide (CO₂) concentrations (ca) and climate using data from a uniquely long-term nitrogen (N) fertilization experiment in the north of Sweden (consisting of three N dosage levels and a control treatment) from 1970 until 2002. N loading produced trees with less negative δ¹³C of foliage, by around 0.45[per thousand] on average, with the difference in δ¹³C between control and N treatments not dependant upon N dosage. The average δ¹³C values decreased at a rate of around 0.03[per thousand] yr⁻¹, even after accounting for the Suess effect (the decrease in the atmospheric CO₂δ¹³C due to anthropogenic emissions of isotopically light CO₂). This decrease is large enough to cause a significant, progressive change in the δ¹³C down through a soil profile. Modelled values of plant intrinsic water use efficiency (WUEi) and the ratio of leaf internal to external [CO₂] (ci/ca) showed that this was the result of ci increasing in parallel with ca (while ci/ca increased), thus causing little change in WUEi over the 32 years of study. The residuals from the relationships between year and δ¹³C were used to examine the impact of climate on the interannual variation of C isotope composition of needles. This included the use of a fire hazard index (FHI) model, which integrates climatic factors known to influence plant stomatal conductance and hence δ¹³C. The FHI produced the best fit with δ¹³C values when climate data were averaged over the whole growth season (for control plots) and for July for all the N treatments, explaining ca. 60% of the total interannual variation in δ¹³C. Further, trees from the N treatments appeared more susceptible to air-humidity-based climate parameters, as seen from higher correlation coefficients, than were control trees. Thus, our data suggest the possibility of increased susceptibility to drought conditions in ecosystems with moderate to high N deposition rates. Also, there is the possibility that, because there was no apparent change in WUEi of P. sylvestris in this ecosystem over the last 32 years, the rate of sequestration of C into boreal ecosystems may not increase with ca, as has been predicted.

No diurnal variation in rate or carbon isotope composition of soil respiration in a boreal forest

Abstract: Characterization of soil respiration rates and delta(13)C values of soil-respired CO(2) are often based on measurements at a particular time of day. A study by Gower et al. (2001) in a boreal forest demonstrated diurnal patterns of soil CO(2) flux using transparent measurement chambers that included the understory vegetation. It is unclear whether these diurnal patterns were solely the result of photosynthetic CO(2) uptake during the day by the understory or whether there were underlying trends in soil respiration, perhaps driven by plant root allocation, as recently demonstrated in Mediterranean oak savannah. We undertook intensive sampling campaigns in a boreal Picea abies L. Karst. forest to investigate whether diurnal variations in soil respiration rate and stable carbon isotope ratio (delta(13)C) exist in this ecosystem when no understory vegetation is present in the measurement chamber. Soil respiration rates and delta(13)C were measured on plots in which trees were either girdled (to terminate the fraction of soil respiration directly dependent on recent photosynthate from the trees), or not girdled, every 4 h over two 48-hour cycles during the growth season of 2004. Shoot photosynthesis and environmental parameters were measured concurrently. No diurnal patterns in soil respiration rates and delta(13)C were observed in either treatment, despite substantial variations in climatic conditions and shoot photosynthetic rates in non-girdled trees. Consequently, assessment of daily soil respiration rates and delta(13)C in boreal forest systems by single, instantaneous daily measurements does not appear to be confounded by substantial diurnal variation.