Search

Temperate and boreal forest ecosystems contain a large part of the carbon stored on land, in the form of both biomass and soil organic matter. Increasing atmospheric [CO2], increasing temperature, elevated nitrogen deposition and intensified management will change this C store. Well documented single-factor responses of net primary production are: higher photosynthetic rate (the main [CO2] response); increasing length of growing season (the main temperature response); and higher leaf-area index (the main N deposition and partly [CO2] response). Soil organic matter will increase with increasing litter input, although priming may decrease the soil C stock initially, but litter quality effects should be minimal (response to [CO2], N deposition, and temperature); will decrease because of increasing temperature; and will increase because of retardation of decomposition with N deposition, although the rate of decomposition of high-quality litter can be increased and that of low-quality litter decreased. Single-factor responses can be misleading because of interactions between factors, in particular those between N and other factors, and indirect effects such as increased N availability from temperature-induced decomposition. In the long term the strength of feedbacks, for example the increasing demand for N from increased growth, will dominate over short-term responses to single factors. However, management has considerable potential for controlling the C store.

Recent investigations have highlighted the relative importance of the winter season for emissions of N(2)O from boreal soils. However, our understanding of the processes and environmental controls regulating these emissions is fragmentary. Therefore, we investigated the potential for, and relative importance of, N(2)O formation at temperatures below 0 degrees C in laboratory experiments involving incubations of a Swedish boreal forest soil. Our results show that frozen soils have a high potential for N(2)O formation and subsequent emission. Net N(2)O production rates at -4 degrees C equaled those observed at +10 to +15 degrees C at moisture contents >60% of the soil's water-holding capacity. The source of this N(2)O was found to be denitrification occurring in anoxic microsites in the frozen soil and temperature per se did not control the denitrification rates at temperatures around 0 degrees C. Furthermore, both net nitrogen-mineralisation and nitrification were observed in the frozen soil samples. Based on these findings we propose a conceptual model for the temperature response of N(2)O formation in soils at low temperatures.

The use of forest-derived biomass has steadily increased in Finland and Sweden during the past decades leading to more intensive forest management practices in the region, such as whole-tree harvesting, both above- and belowground. Stump harvesting results in a direct removal of stump and coarse-root carbon (C) from the stand and can cause extensive soil disturbance, which has been suggested to increase C mineralization. In this study, the effects of stump harvesting on soil C and nitrogen (N) mineralization, and soil surface disturbance were studied in two different clear-felled Norway spruce ( Picea abies ) sites in Central Finland. The treatments were whole-tree harvesting (WTH, removal of stems and logging residues), and WTH and stump harvesting (WTH + S). Both sites, Honkola (2 stands) and Haukilahti (6 stands) were mounded. In both treatments, soil samples were taken from different soil layers down to a total depth of 20 cm in the mineral soil from (i) mounds, (ii) undisturbed soil and (iii) pits. The sampling was performed 11–12 years after treatments. Soil C and N mineralization rates were determined in laboratory incubation experiments. In addition, total C and N pools (g m −2 ) were estimated for each disturbance class and soil layer. Soil C and N pools had a tendency to be lower following stump harvesting, but no statistically significant treatment effect was detected. Stump harvesting increased soil mixing as indicated by a significant decrease in C concentration in the mound disturbance class. There was no significant effect of stump harvesting on soil C mineralization rates. A combination of mineralization rates and soil pool data showed that field C mineralization (g CO 2 -C m −2 yr −1 ) did not significantly differ between stands where stumps were removed or were retained. Further, stump harvesting did not seem to have any stimulating effect on soil CO 2 efflux 11–12 years after treatment. [ABSTRACT FROM AUTHOR]

Highlights: [•] The impact of birch, pine and spruce on soil C and N pools and fluxes was studied. [•] C and N pools were larger under spruce than under birch and pine after 50years. [•] MRT of C was 9.5 (birch) to 31 (spruce) years in the Oe+Oa horizon. [•] High earthworm activity under birch explains high decomposition and low MRT. [ABSTRACT FROM AUTHOR]

Abstract: Tree girdling has been experimentally used to stop the allocation of carbohydrates from the canopy to tree roots and their associated ectomycorrhizal (EM) mycelia. We used three already established girdling experiments in northern Sweden, one in a Pinus sylvestris stand and two in Picea abies stands, to determine the effect of tree girdling on soil-living oribatid mites. These mites often feed on fungal hyphae, but it is not known to what extent they feed on EM or saprotrophic fungi. We hypothesised that a presumed decline in EM fungi after girdling would strongly reduce EM-feeding specialists and, correspondingly, reduce total abundance and species richness of oribatids in girdled plots. Tree girdling resulted in a significant decline in total abundance of oribatid mites in the two spruce stands, which was assumed to be linked to the decline in EM fungi, but not in the pine stand. Species richness decreased in girdled plots in one of the spruce forests. The decline in total abundance in girdled spruce stands was primarily dependent on a significant and consistent population decrease in Oppiella nova, which was the most abundant oribatid mite in the ungirdled spruce plots. Its abundance after girdling was only 8–18% of that in ungirdled spruce plots. In the pine stand, O. nova had much lower abundance than in the spruce stands, and despite a tendency to decline in number after girdling also in the pine stand, it had a minor effect on total abundance. These results suggest that O. nova may be dependent on EM fungi in spruce forest soils, whereas the dependence on EM fungi in pine forest soils is less evident. [Copyright &y& Elsevier]

A nitrogen (N) budget was constructed for a period of 6 years (1988–1993) in a Norway spruce stand with current deposition of 19 kg N and 22 kg S ha−1 year−1. The stand was fertilized annually by addition of 100 kg N and 114 kg S ha−1 (NS). Above and below ground biomass, litterfall, fine- root litter production, soil solution and net mineralization were measured to estimate pools, fluxes and accumulation of nitrogen. The average needle litterfall in control (C) and NS plots in 1993 was 2.2 and 2.5 ton ha−1 year−1, respectively. The fine root litter production prior to treatment (1987) was 4.4 ton ha−1 year−1 and after treatment (1993) it was 4.5 and 3.9 ton ha−1 year−1 in C and NS plots, respectively. Net N mineralization in the soil profile down to 50 cm was estimated to be 86 and 115 kg ha−1 year−1 in C and NS plots, respectively in 1992. During the treatment period the uptake of N in the needle biomass in C and NS plots was 29 and 77 kg ha−1 year−1, respectively. No N was accumulated in needles of C plot where the NS plots accumulated 34 kg ha−1 year−1. Of the annually added inorganic N to NS plots 47% was accumulated in the above and below ground biomass and 37% in the soil. N fluxes via fine-root litter production in the C plots were much higher (54 kg ha−1 year−1) than that via litterfall (29 kg ha−1 year−1). The corresponding values in the NS plots were 65 and 43 kg ha−1 year−1, respectively. Most of the net N mineralization occurred in the FH layer and upper mineral soil. It is concluded that fine root litter and litterfall play an important role in the cycling of N. Despite a high N uptake the losses of N in litterfall and fine root litter resulted in an incorporation of N in soil organic matter. [ABSTRACT FROM AUTHOR]

The aim of the study was to compare the allocation of applied 15NH4-N into soil microbial biomass and soil organic matter (SOM) in more humus samples from a N-poor and a N-rich plot inan experimental Scots pine (Pinus sylvestris L.) forest to clarify the N immobilization mechanisms. Intact humus cores were incubated for147 days in the laboratory after addition of 4, 20 and 100 mug N g-1 OM of 15N-labeled (NH4)2SO4. CO2-C evolution, extractable organic C, microbial C, labeled and unlabelled NH4-N, NO3-N,extractable organic N, microbial N, and N in soil organic matter (SOM) were determined. Initially, 15NH4-N was rapidly immobilized in all treatments. Microbial assimilation of 15N increased with increased 15N application rate in both the N-poor and N-rich substrate, indicating that NH4-N availability was limiting. No substrate effect was found. Recovery of15N in microbial biomass ranged from 0.6 to 25% of added 15N and was characterized by an increase in 15Nduring the first 14 days and a gradual decrease thereafter for nearly all treatments. The recovery of 15N in microbial biomassand SOM showed reciprocal trends after day 14, suggesting a considerable exchange of N between these pools. At the end of the incubation,36-74% of added 15N occurred in SOM, 2.4-25% was recovered as 15NH4-N, but almost no 15NO3-N was found, indicating no net nitrification. CO2-C evolution was almost constant during the incubation period and wasnot affected by substrate differences or 15N application rate. Microbial biomass C was highest in the N-poor substrate and wasnot affected by the 15N application rate. In conclusion, the study showed that lowering the C:N-ratios from37 to 24 did not significant [ABSTRACT FROM AUTHOR]