Soil carbon stock change following afforestation in Northern Europe: a meta-analysis

Northern Europe supports large soil organic carbon (SOC) pools and has been subjected to high frequency of land-use changes during the past decades. However, this region has not been well represented in previous large-scale syntheses of land-use change effects on SOC, especially regarding effects of afforestation. Therefore, we conducted a meta-analysis of SOC stock change following afforestation in Northern Europe. Response ratios were calculated for forest floors and mineral soils (0-10 cm and 0-20/30 cm layers) based on paired control (former land use) and afforested plots. We analyzed the influence of forest age, former land-use, forest type, and soil textural class. Three major improvements were incorporated in the meta-analysis: analysis of major interaction groups, evaluation of the influence of nonindependence between samples according to study design, and mass correction. Former land use was a major factor contributing to changes in SOC after afforestation. In former croplands, SOC change differed between soil layers and was significantly positive (20%) in the 0-10 cm layer. Afforestation of former grasslands had a small negative (nonsignificant) effect indicating limited SOC change following this land-use change within the region. Forest floors enhanced the positive effects of afforestation on SOC, especially with conifers. Meta-estimates calculated for the periods <30 years and >30 years since afforestation revealed a shift from initial loss to later gain of SOC. The interaction group analysis indicated that meta-estimates in former land-use, forest type, and soil textural class alone were either offset or enhanced when confounding effects among variable classes were considered. Furthermore, effect sizes were slightly overestimated if sample dependence was not accounted for and if no mass correction was performed. We conclude that significant SOC sequestration in Northern Europe occurs after afforestation of croplands and not grasslands, and changes are small within a 30-year perspective.

Sinks for nitrogen inputs in terrestrial ecosystems: a meta-analysis of 15N tracer field studies

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (< 1 week after 15N tracer application), total ecosystem 15N recovery was negatively correlated with fine-root and soil 15N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15N tracer application), total ecosystem 15N retention was negatively correlated with foliar natural-abundance 15N but was positively correlated with mineral soil C and N concentration and C:N, showing that plant and soil natural-abundance 15N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N x ha(-1) x yr(-1) above which most ecosystems showed net losses of applied 15N tracer in response to N fertilizer addition.

Carbon and nitrogen cycles in European ecosystems respond differently to global warming

The global climate is predicted to become significantly warmer over the next century. This will affect ecosystem processes and the functioning of semi natural and natural ecosystems in many parts of the world. However, as various ecosystem processes may be affected to a different extent, balances between different ecosystem processes as well as between different ecosystems may shift and lead to major unpredicted changes. In this study four European shrubland ecosystems along a north-south temperature gradient were experimentally warmed by a novel nighttime warming technique. Biogeochemical cycling of both carbon and nitrogen was affected at the colder sites with increased carbon uptake for plant growth as well as increased carbon loss through soil respiration. Carbon uptake by plant growth was more sensitive to warming than expected from the temperature response across the sites while carbon loss through soil respiration reacted to warming in agreement with the overall Q10 and response functions to temperature across the sites. Opposite to carbon, the nitrogen mineralization was relatively insensitive to the temperature increase and was mainly affected by changes in soil moisture. The results suggest that C and N cycles respond asymmetrically to warming, which may lead to progressive nitrogen limitation and thereby acclimation in plant production. This further suggests that in many temperate zones nitrogen deposition has to be accounted for, not only with respect to the impact on water quality through increased nitrogen leaching where N deposition is high, but also in predictions of carbon sequestration in terrestrial ecosystems under future climatic conditions. Finally the results indicate that on the short term the above-ground processes are more sensitive to temperature changes than the below ground processes.

Nitrogen cycling in a Norway spruce plantation in Denmark--a SOILN model application including organic N uptake

A dynamic carbon (C) and nitrogen (N) circulation model, SOILN, was applied and tested on 7 years of control data and 3 years of manipulation data from an experiment involving monthly N addition in a Norway spruce ( Picea abies, L. Karst) forest in Denmark. The model includes two pathways for N uptake: (1) as mineral N after mineralisation of organic N, or (2) directly from soil organic matter as amino acids proposed to mimic N uptake by mycorrhiza. The model was parameterised and applied to the data from the control plot both with and without the organic N uptake included. After calibration, the models performance was tested against data from the N-addition experiment by comparing model output with measurements. The model reproduced well the overall trends in C and N pools and the N concentrations in soil solutions in the top soil layers whereas discrepancies in soil-solution concentrations in the deeper soil layers are seen. In the control data, the needle-N concentration was well reproduced except for small underestimations in some years because of drought effects not included in the model. In the N-addition experiment, SOILN reproduces the observed changes; in particular, the changes in needle-N concentrations and the overall distribution within the ecosystem of the extra added 3.5 g N m(-2) year(-1) parallel the observations. When organic N uptake is included, the simulations indicate that in the control plot receiving c. 1.9 g N m(-2) year(-1), the organic N uptake in average supplies 35% of the total plant N uptake. By addition of an extra 35 kg N ha(-1) year(-1), the organic N uptake is reduced to 16% of the total N uptake. Generally, inclusion of the pathway for organic N uptake improves model performance compared with observations for both C and N. This is because mineral N uptake alone implies a larger mineralisation rate, leading to bigger concentrations of N in the soil and soil water, bigger N losses, and net loss of c. 100 kg C ha(-1) year(-1), thereby causing depletion of the organic soil layer.

Variations in heavy metal concentrations and speciation in two mining-polluted streams in central Norway

The dissolved (dialysis in situ), colloidal (filtered minus dissolved), and total concentrations of Cu, Zn, Cd, and Al were determined and compared with pH, Ca concentrations, and alkalinities through seasonal variations in water levels in two streams, Rugla and Naustebekken, in the Røros area of central Norway. The main objective was to monitor the dissolved fraction of the metals during the seasons and to establish the extent to which chemical parameters influencing metal toxicity were unfavorable during episodes of high total metal concentrations. The average dissolved fractions of Cu, Zn, Cd, and Al were lower in Rugla (55, 69, 64, and 17%, respectively) compared with Naustebekken (59, 82, 82, and 30%, respectively). This is probably due to sedimentation of metal particles in a small lake located in the path of Naustebekken. Alkalinity and Ca concentrations were generally low during the flood episodes, when the highest metal concentrations occurred. This is unfavorable since these parameters are assumed to counteract toxic effects of metals. On the other hand, the results indicate that toxic stress during metal concentration peaks was moderated by adsorption of dissolved metal species on colloids and/or particles.

Metal accumulation and metallothionein in brown trout, Salmo trutta, from two Norwegian rivers differently contaminated with Cd, Cu and Zn

In this work we have studied the accumulation of heavy metals in two brown trout (Salmo trutta) populations in their natural environment and the participation of metal binding to metallothionein (MT) in this process. Cd, Cu and Zn concentrations, total MT (including Cu MT) and Cd/Zn MT were measured in the gills, liver and kidney of trout inhabiting two rivers, one Cu-contaminated and the other Cd/Zn-contaminated, located at Røros, Central Norway. In both populations, high levels of Cu were found in the liver, whereas Cd was accumulated in liver and particularly in the kidney. The proportions of Cd/Zn MT and Cu MT in liver and kidney, but not in gills, reflected the accumulated and the environmental concentrations of these metals. The total Cu MT concentrations in the investigated tissues, however, were highest in trout from the river with the lowest ambient Cu concentration. It is suggested that MTs are of less importance in Cu-acclimated trout. The data also suggest that acclimation to a Cu-rich environment involves reduced Cu accumulation or increased Cu elimination. In trout from the Cd-rich environment, this metal was mainly bound to MT, whereas in trout from the Cu-rich environment Cd was also associated with non-MT proteins. These findings emphasize the importance to determine both Cd/Zn MT and Cu MT levels, when the participation of this protein in metal handling in trout tissues is investigated.

Metal accumulation and metallothionein in two populations of brown trout, Salmo trutta, exposed to different natural water environments during a run-off episode

Cd, Cu and Zn concentrations were measured in ambient water as well as in gills, liver and kidney tissues of two natural populations of brown trout (Salmo trutta) during a run-off episode in two rivers with different metal compositions due to mining pollution. Metallothionein (MT) was also measured in these tissues. The two rivers, Rugla (Cu contaminated) and Naustebekken (Cd and Zn contaminated), are located in two neighboring drainage basins separated by the topographic divide near the city of Røros in the County of Sør-Trøndelag, Norway. In Rugla, the Cu concentration increased from 15 µg/l at the low water level to 41 µg/l during the run-off episode. In Naustebekken, corresponding values for Cd were 90-170 ng/l and those for Zn were 49-91 µg/l. Gill concentrations of Cu and Cd/Zn MT in both populations of native trout clearly reflect the presence of these metals in the rivers during the run-off, in accordance with the hypothesis of protection caused by MT induction. When Rugla trout were transferred to Naustebekken and vice versa, both the amounts of MT itself and the Cu contents reflected the concentration of this metal in the new environment, indicating that MT induction also protects against acutely increased metal levels. The measured levels of MT in both native and transferred trout can account for all the Cd present in the tissues, but not for all of the Cu and Zn. The capacity of MT to regulate Cd and Cu in the trout populations in their natural habitat therefore seems clearly present. Our data also indicate that the MT I and II isoforms may bind metals selectively.

Concentrations of 50 major and trace elements in Danish agricultural crops measured by inductively coupled plasma mass spectrometry. 3. Potato (Solanum tuberosum folva)

The multi-element (Ag, Al, Au, Ba, Bi, Ca, Cd, Co, Cr, Cs, Cu, Dy, Er, Fe, Ga, Gd, Ho, In, Ir, La, Lu, Mn, Mo, Nb, Nd, P, Pb, Pd, Pr, Pt, Rb, Re, Rh, Ru, Sb, Sc, Sm, Sn, Sr, Ta, Tb, Th, Ti, Tl, Tm, U, V, Y, Yb, and Zn) concentrations (microg/kg, fresh weight) in potatoes (Solanum tuberosum, Folva) were investigated in this study. The potatoes were grown in two fertilization practices; one with pig slurry and one with calcium ammonium nitrate at three levels of N fertilization (0, 60, and 120 kg of N/ha). The experiment field was located at the Riso National Laboratory Agronomy Farms in Roskilde, Denmark. High-resolution-inductively coupled plasma mass spectrometry (HR-ICPMS) was used for analyses of the samples. The effect of three levels of N fertilization on elemental concentrations of the crop are evaluated by use of discriminant partial least-squares regression (PLS). The results provide useful biological and nutritional information on potatoes.

Spatial variability of throughfall fluxes in a spruce forest

The spatial variability of throughfall deposition of H(+), Ca(2+), Mg(2+), Na(+), K(+), Cl(-), NO(3)(-), NH(4)(+), O(4)(2-) to a Norway spruce (Picea abies (L.) Karst.) forest was intensively examined during the period October 1986 to October 1987. Large systematic spatial variability of the atmospheric deposition within the forest was observed. The flux of throughfall water was higher away from the trunk compared to the flux close to the trunk. In contrast to this, the deposition of all substances was considerably higher close to the trunk compared to the deposition at the periphery of the canopy. A linear decrease in deposition as a function of the distance from the nearest tree trunk was found. Further, the deposition varied quite dramatically between trees according to their size. The observed spatial variability in throughfall may be due to variabilities in the processes taking part in altering the distribution and composition of the precipitated water as it moves through the canopy. The influence of these processes of precipitation, wash-off, dry deposition and canopy exchange is discussed, and it is found that both increased dry deposition and canopy exchange in the tree tops contribute to the higher solute fluxes found close to the tree trunk.

Atmospheric deposition to the edge of a spruce forest in Denmark

Atmospheric deposition was measured during 1 year at the forest edge of a Norway spruce stand in Denmark. Inside the forest the deposition of H(+), Ca(2+), Mg(2+), Na(+), K(+), Cl(-), NO(3)(-), NH(4)(2) and SO(4)(2-) with canopy throughfall varies with the distance from the forest edge. The deposition at the edge is found to be 10-20 times as high as deposition to an open field and 2-8 times as high as deposition inside the stand. An exponential decrease in deposition as a function of the distance from the forest edge is found. Increased deposition of K(+) and non-sea salt Mg(2+), which mainly originates as a result of leaching from the needles may be explained by a larger leaf area index (LAI) at the forest edge. Deposition of particulate substances, especially Na(+), Cl(-), Mg(2+) and to some extent SO(4)(2-), NH(4)(+) and NO(3)(-) is increased much more than the LAI, which we believe to be caused by changes in wind movements at the forest edge.