Three-source partitioning of soil respiration by <Superscript>13</Superscript>C natural abundance and its variation with soil depth in a plantation

Partitioning soil respiration into three components is vital to identify CO₂ sink or source and can help us better understand soil carbon dynamics. However, knowledge about the influences of soil depth and the priming effect on soil respiration components under field has been limited. Three components of soil respiration (root respiration, rhizomicrobial respiration and basal respiration) in a plantation in the hilly area of the North China were separated by the ¹³C natural abundance method. The results showed that the average proportions of rhizomicrobial respiration, root respiration and basal respiration at the 25–65 cm depths were about 14, 23 and 63 %, respectively. Three components of soil respiration varied with soil depth, and root respiration was the main component of soil respiration in deeper soil. The priming effect was obvious for the deep soil respiration, especially at the 40–50 cm depth. Thus, depth and priming effect should be taken into account to increase the accuracy of estimations of soil carbon flux.     

Natural abundance of <Superscript>15</Superscript>N in two cacao plantations with legume and non-legume shade trees

Natural abundance of ¹⁵N was sampled in young and mature leaves, branches, stem, and coarse roots of trees in a cacao (Theobroma cacao) plantation shaded by legume tree Inga edulis and scattered non-legumes, in a cacao plantation with mixed-species shade (legume Gliricidia sepium and several non-legumes), and in a tree hedgerow bordering the plantations in Guácimo, in the humid Caribbean lowlands of Costa Rica. The deviation of the sample ¹⁵N proportion from that of atmosphere (δ¹⁵N) was similar in non-legumes Cordia alliodora, Posoqueria latifolia, Rollinia pittieri, and T. cacao. Deep-rooted Hieronyma alchorneoides had lower δ¹⁵N than other non-N₂-fixers, which probably reflected uptake from a partially different soil N pool. Gliricidia sepium had low δ¹⁵N. Inga edulis had high δ¹⁵N in leaves and branches but low in stem and coarse roots. The percentage of N fixed from atmosphere out of total tree N (%N_f) in G. sepium varied 56–74%; N₂ fixation was more active in July (the rainiest season) than in March (the relatively dry season). The variation of δ¹⁵N between organs in I. edulis was probably associated to ¹⁵N fractionation in leaves. Stem and coarse root δ¹⁵N was assumed to reflect the actual ratio of N₂ fixation to soil N uptake; stem-based estimates of %N_f in I. edulis were 48–63%. Theobroma cacao below I. edulis had lower δ¹⁵N than T. cacao below mixed-species shade, which may indicate direct N transfer from I. edulis to T. cacao but results so far were inconclusive. Further research should address the ¹⁵N fractionation in the studied species for improving the accuracy of the N transfer estimates. The δ¹⁵N appeared to vary according to ecophysiological characteristics of the trees.     

Positive nitrogen balance of <Emphasis Type="Italic">Acacia mangium</Emphasis> woodlots as fallows in the Philippines based on <Superscript>15</Superscript>N natural abundance data of N<Subscript>2</Subscript> fixation

Nitrogen inputs from biological nitrogen fixation contribute to productivity and sustainability of agroforestry systems but they need to be able to offset export of N when trees are harvested. This study assessed magnitudes of biological nitrogen fixation (natural ¹⁵N abundance) and N balance of Acacia mangium woodlots grown in farmer’s fields, and determined if N₂ fixation capacity was affected by tree age. Tree biomass, standing litter, understory vegetation and soil samplings were conducted in 15 farmer’s fields growing A. mangium as a form of sequential agroforestry in Claveria, Misamis Oriental, Philippines. The trees corresponded to ages of 4, 6, 8, 10 and 12 years, and were replicated three times. Samples from different plant parts and soils (0–100 cm) were collected and analyzed for δ¹⁵N and nutrients. The B-value, needed as a reference of isotopic discrimination when fully reliant on atmospheric N, was generated by growing A. mangium in an N₂-free sand culture in the glasshouse. Isotopic discrimination occurring during N₂ fixation and metabolic processes indicated variation of δ¹⁵N values in the order of nodules > old leaves > young leaves > stems > litterfall and roots of the trees grown in the field, with values ranging from −0.8 to 3.5‰ except nodules which were enriched and significantly different from other plant parts (P < 0.0001). Isotopic discrimination was not affected by tree age (P > 0.05). Plants grown in N free sand culture exhibited the same pattern of isotopic discrimination as plants grown in the field. The estimated B-value for the whole plant of A. mangium was −0.86‰. Mature tree stands of 12 years accumulated up to 1994 kg N ha⁻¹ in aboveground biomass. Average proportion of N derived from N₂ fixation of A. mangium was 54% (±22) and was not affected by age (P > 0.05). Average yearly quantities of N₂ fixed were 128 kg N ha⁻¹ in above-ground biomass amounting to 1208 kg N fixed ha⁻¹ over 12 years. Harvest of 12-year old trees removed approximately 91% of standing aboveground biomass from the site as timber and fuel wood. The resulting net N balance was +151 kg N ha⁻¹ derived from remaining leaves, twigs, standing litter, and +562 kg N ha⁻¹ when tree roots were included in the calculation. The fast growing A. mangium appears to be a viable fallow option for managing N in these systems. However, other nutrients have to be replaced by using part of the timber and fuel wood sales to compensate for large amounts of nutrient removed in order for the system to be sustainable.     

Detection of nitrogen transfer from N<Subscript>2</Subscript>-fixing shade trees to cacao saplings in <Superscript>15</Superscript>N labelled soil: ecological and experimental considerations

Theobroma cacao seedlings were grown alone (TCA) or associated with saplings of N₂-fixing shade trees Gliricidia sepium and Inga edulis in 200 l of ¹⁵N labelled soil within a physical root barrier for studying direct nitrogen transfer between the trees and cacao. Root:shoot partitioning ratio for sapling total N was lower than biomass root:shoot ratio in all species. Sapling total ¹⁵N was partitioned between root and shoot in about the same ratio as total N in cacao and inga but in gliricidia much higher proportion of ¹⁵N than total N was found in roots. Thus, whole plant harvesting should be used in ¹⁵N studies whenever possible. Average percentage of fixed N out of total tree N was 74 and 81% for inga estimated by a yield-independent and yield-dependent method, respectively, and 85% for gliricidia independently of estimation method. Strong isotopic evidence on direct N transfer from trees to cacao was observed in two cases out of ten with both tree species. Direct N transfer was not correlated with mycorrhizal colonisation of either donor or receiver plant roots. Direct N transfer from inga and gliricidia to cacao is conceivable but its prevalence and the transfer pathway via mycorrhizal connections or via reabsorption of N-rich legume root exudates by cacao require further study. Competition in the restricted soil space may also have limited the apparent transfer in this study because the trees accumulated more soil-derived N than cacao in spite of active N₂ fixation.     

13C and 15N isotopic fractionation in trees, soils and fungi in a natural forest stand and a Norway spruce plantation

¹⁵N and ¹³C natural abundances of foliage, branches, trunks, litter, soil, fungal sporophores, mycorrhizas and mycelium were determined in two forest stands, a natural forest and a Norway spruce plantation, to obtain some insights into the role of the functional diversity of saprotrophic and ectomycorrhizal fungi in carbon and nitrogen cycles. Almost all saprotrophic fungi sporophores were enriched in ¹³C relative to their substrate. In contrast, they exhibited no or very little shift of δ¹⁵N. Judging from the amount of C discrimination, ectomycorrhizal fungi seem to acquire carbon from their host or from dead organic matter. Some ectomycorrhizal species seem able to acquire nitrogen from dead organic matter and could be able to transfer it to their host without nitrogen fractionation, while others supply their host with ¹⁵N-depleted nitrogen. Moreover ectomycorrhizal species displayed a significant N fractionation during sporophore differentiation, while saprotrophic fungi did not.     

Partitioning of applied <Superscript>15</Superscript>N fertilizer in a longleaf pine and native woody ornamental intercropping system

The cultivation of ornamentals to produce woody floral products—the fresh or dried stems that are used for decorative purposes—may be an attractive option for southeastern landowners looking to generate income from small landholdings. Since many shrubs native to the understory of the longleaf pine (Pinus palustris Mill.) ecosystem have market potential, one possibility is the intercropping of select species in the between-row spacing of young longleaf pine plantations. The objective of this study was to evaluate how interspecific competition affects the fate of ¹⁵N fertilizer when American beautyberry (Callicarpa americana L.), wax myrtle (Morella cerifera (L.) Small) and inkberry (Ilex glabra (L.) A.Gray) are intercropped with longleaf pine. Nitrogen derived from fertilizer (NDF), utilization of fertilizer N (UFN) and recovery of fertilizer N (RFN_soil) were compared between agroforestry and monoculture (treeless) treatments to assess the effects of competition. Results varied by species, with NDF being higher for C. americana foliage and lower for all M. cerifera tissues in the agroforestry treatment. No effect was observed for I. glabra. UFN was lower for all species in the agroforestry treatment. RFN_soil was higher in the agroforestry treatment for I. glabra, but no treatment effects were observed for C. americana or M. cerifera. Overall, while it is clear that interspecific competition was present in the agroforestry treatment, the inefficiency of fertilizer use suggests that nitrogen was not the most limiting resource. Management interventions, particularly those that address competition for water, will likely be critical to the success of this system.     

Effect of fire disturbance on active organic carbon of <Emphasis Type="Italic">Larix gmelinii</Emphasis> forest soil in Northeastern China

Active organic carbon in soil has high biological activity and plays an important role in forest soil ecosystem structure and function. Fire is an important disturbance factor in many forest ecosystems and occurs frequently over forested soils. However, little is known about its impact on soil active organic carbon (SAOC), which is important to the global carbon cycle. To investigate this issue, we studied the active organic carbon in soils in the Larix gmelinii forests of the Da Xing’an Mountains (Greater Xing’an Mountains) in Northeastern China, which had been burned by high-intensity wildfire in two different years (2002 and 2008). Soil samples were collected monthly during the 2011 growing season from over 12 sample plots in burned and unburned soils and then analyzed to examine the dynamics of SAOC. Our results showed that active organic carbon content changed greatly after fire disturbance in relation to the amount of time elapsed since the fire. There were significant differences in microbial biomass carbon, dissolved organic carbon, light fraction organic carbon, particulate organic carbon between burned and unburned sample plots in 2002 and 2008 (p < 0.05). The correlations between active organic carbon and environmental factors such as water content, pH value and temperature of soils, and correlations between each carbon component changed after fire disturbance, also in relation to time since the fire. The seasonal dynamics of SAOC in all of the sample plots changed after fire disturbance; peak values appeared during the growing season. In plots burned in 2002 and 2008, the magnitude and occurrence time of peak values differed. Our findings provide basic data regarding the impact of fire disturbance on boreal forest soil-carbon cycling, carbon-balance mechanisms, and carbon contributions of forest ecosystem after wildfire disturbance.     

Root activity of young <Emphasis Type="Italic">Acacia mangium</Emphasis> Willd trees: influence of stand density and pruning as studied by <Superscript>32</Superscript>P soil injection technique

Initial spacing and pruning are silvicultural strategies that influence the resource acquisition capabilities of trees. A field study was conducted in the humid tropics of peninsular India to test the assumptions that: (1) high stand density of Acacia mangium induces greater root uptake capacity close to the stem and from the subsoil; and (2) crown pruning stimulates greater root uptake capacity at proximal points. Root activity pattern of two-year-old A. mangium was evaluated as a function of three population densities (1,250, 2,500 and 5,000 stems ha⁻¹), with, and without 50% crown pruning, using ³²P soil injection. The label was placed at 25, 50 and 75 cm lateral distances and at 30 and 60 cm depth. Low density stands (1,250 stems ha⁻¹) generally showed higher ³²P recovery (P < 0.01), which was exaggerated by pruning. Pruned low density stands had 34% root activity at 25 cm, as against 23% for unpruned. The low density stands also showed higher root activity at 75 cm, signifying greater root spread. We suggest that high stem densities favour restricted spread of absorbing roots and may facilitate competitive downward displacement of roots. Pruning the lateral shoots at low stem densities may simulate this to some extent. The net outcome of interactions, however, will depend on trade-offs between stem density and tree management over time.     

Saprotrophic versus symbiotic strategy during truffle ascocarp development under holm oak. A response based on 13C and 15N natural abundance

? The development of truffles in the soil is not well understood. It is not known if a direct transfer of carbohydrates takes place between the host tree and the developing ascocarps through ectomycorrhizal structures or whether sporophores become independent from their hosts after several weeks or months and are able to use dead host tissues or soil organic matter as carbon (C) and nitrogen (N) sources.

? To study saprophytic or symbiotic capacities of truffle ascocarps the natural abundance of ¹⁵N and ¹³C in foliage, wood, fine roots, mycorrhizae, fungal sporophores and soil were determined in a truffle orchard.

? The processes of carbon and nitrogen allocation remained unchanged during the entire period of ascocarp development of Tuber melanosporum. From ¹³C and ¹⁵N natural abundance measurements, T. melanosporum, T. brumale and T. rufum did not exhibit saprotophic strategy during ascocarp development, which is contradictory to common statements found in handbooks regarding truffle cultivation.

     

The effects of experimental warming and CO<Subscript>2</Subscript> concentration doubling on soil organic carbon fractions of a montane coniferous forest on the eastern Qinghai-Tibetan Plateau

To explore the effects of elevated temperature (ET), elevated atmospheric CO₂ concentration (EC) and ET plus EC (ETC) on different fractions of soil organic carbon (SOC) is significant for understanding the interactions between SOC and environmental variables. Up to date, little information is available in montane forests of the eastern Qinghai-Tibetan Plateau, which is a key region for studying global climate change especially for high altitude areas. This work applied an automatic gas exchange system to investigate the responses of different fractions of SOC to these factors (ET: + 2.5 °C; EC: + 350 ppm; ETC: + 2.5 °C + 350 ppm) after two and a half years’ treatments. Results showed that both ET and EC significantly increased the average amount of SOC and not-readily oxidizable carbon (NROC), while significantly decreased the readily oxidizable carbon (ROC) in soil. The dramatic contrast between the content of NROC and ROC provided evidence that trees exposed to either ET or EC would employ more labile nutrient to meet their growth demands. Soil microbial biomass carbon (SMBC) was significantly related to the amount of ROC. Unlike ROC, the trends of particulate organic carbon (POC) and mineral-associated organic carbon (MOC) under different treatments were consistent with that of SOC. Under ET, EC and ETC, the alterations of %ROC were more obvious than that of %POC, and positive correlations were observed between SOC, POC, MOC and NROC, with the exception of ROC and SMBC. These results indicated that the biotical and chemical labile components were more sensitive than the physical active fraction of soil organic matter. Meanwhile, the physical protection and chemical recalcitrance effects were important protection mechanisms from the response to global climate change.     

Three-source partitioning of soil respiration by ~(13)C natural abundance and its variation with soil depth in a plantation

Partitioning soil respiration into three components is vital to identify CO_2 sink or source and can help us better understand soil carbon dynamics. However, knowledge about the influences of soil depth and the priming effect on soil respiration components under field has been limited. Three components of soil respiration(root respiration, rhizomicrobial respiration and basal respiration) in a plantation in the hilly area of the North China were separated by the 13 C natural abundance method. The results showed that the average proportions of rhizomicrobial respiration, root respiration and basal respiration at the 25–65 cm depths were about 14, 23 and 63 %, respectively. Three components of soil respiration varied with soil depth, and root respiration was the main component of soil respiration in deeper soil. The priming effect was obvious for the deep soil respiration, especially at the 40–50 cm depth. Thus, depth and priming effect should be taken into account to increase the accuracy of estimations of soil carbon flux.     

Evolution of soil organic matter composition and edaphic carbon effluxes following oak forest clearing for pasture: climate change implications

This research encompasses soil CO₂ emission studies from forest and pasture couplets located in northwestern Spain, where two mature oak forest ecosystems partially cleared for pasture 5 or 50?years ago were selected to monitor soil C effluxes during 2?years. The CO₂ exchanges in the soil–atmosphere interphase of forest and pasture plots were seasonally determined using portable infrared gas analysers. At the same time, soil samples from both forest and pasture ecosystems were used to carry out long-term incubations under laboratory conditions. Solid-state ¹³C-NMR with cross-polarization/magic angle spinning was applied to determine the deforestation effects on soil organic matter (SOM) composition. Pasture implantation caused a notable decline of the labile C pool and a decrease in the total soil C, with an increase in both the SOM humification and the relative concentration of phenolic and carboxyl C. After only 5?years, the deforestation caused a general decrease in the soil CO₂ emissions with reduced seasonal fluctuations, these effects being even more intense 50?years after clearing. The correlation observed in oak forests between the CO₂ measured in situ and the soil temperature, is masked in pasture ecosystems by the high summer soil dryness. After the partial disappearance of soil C stocks caused by deforestation, a new long-term C input/output equilibrium seems to be established, probably due to the joint adaptation of both SOM and microbial communities in the old pasture soil; however, the entire soil C retention capacity remains still degraded as compared with the original uncleared forest ecosystem.     

德国中部Soiling云杉-山毛榉混交林冠层组成对土壤CO2排放的影响

It was hypothesized that soil respiration can be affected by canopy composition. Hence, admixture of trees as a common forest management practice may cause significant change in the carbon cycling. This study was conducted in a mixed spruce-beech stand at Solling forest in central Germany to investigate the effect of canopy composition on soil respiration. The canopy cover was classified in four major canopy classes (pure beech, pure spruce, mixed and gap), and the area under each canopy class was identified as a sub-plot. Soil respiration in each sub-plot (n=4) was measured monthly from Jun 2005 to July 2006. Results show significant difference in annual soil respiration between the beech (359 g·m⁻²·a⁻¹ C) and gap (211 g·m⁻²·a⁻¹ C) sub-plots. The estimation of the total below-ground carbon allocation (TBCA) based on a model given by Raich and Nadelhoffer revealed considerably higher root CO₂ production in the beech sub-plot (231 g·m⁻²·a⁻¹ C) compare to the gap sub-plot (51 g·m⁻²·a⁻¹ C). The contribution of the root respiration to the total soil respiration was higher in the soil under the beech canopy (59%) compared with the soil in the gap (29%). The findings suggested that the condition under the beech canopy may cause more desirable micro-site for autotrophic respiration and consequently higher CO₂ release into the atmosphere.     

Seasonality of vertically partitioned soil CO<Subscript>2</Subscript> production in temperate and tropical forest

Soil CO₂ production seasonality at a number of depths was investigated in a temperate forest in Japan and in a tropical montane forest in Thailand. The CO₂ production rates were evaluated by examining differences in the estimated soil CO₂ flux at adjacent depths. The temperate forest had clear temperature seasonality and only slight rainfall seasonality, whereas the tropical montane forest showed clear rainfall seasonality and only slight temperature seasonality. In the temperate forest, the pattern of seasonal variation in soil respiration was similar at all depths, except the deepest (0.65 m–), and respiration was greater in summer and less in winter. The contribution of the shallowest depth (around 0.1 m) was more than 50% of total soil-surface CO₂ flux all year round, and the annual mean contribution was about 75%. CO₂ production mostly appeared to increase with temperature in shallower layers. In contrast, in the tropical forest, soil CO₂ production seasonality appeared to differ with depth. The CO₂ production rate in the shallowest layer was high during the rainy season and low during the dry season. Soil CO₂ production at greater depths (0.4 and 0.5 m–) showed the opposite seasonality to that in the shallower layer (around 0.1 m). As a result, the contribution from the shallow depth was greatest in the tropical forest during the rainy season (more than 90%), whereas it decreased during the dry season (about 50%). CO₂ production appeared to be controlled by soil water at all depths, and the different ranges of water saturation seemed to cause the difference in seasonality at each depth. Our results suggest the importance of considering the vertical distribution of soil processes, particularly in areas where soil water is a dominant controller of soil respiration.     

An alternative modelling approach to predict emissions of N<Subscript>2</Subscript>O and NO from forest soils

Emissions of N₂O from forest soils in Europe are an important source of global greenhouse gas emissions. However, influencing the emission rates by forest management is difficult because the relations and feedbacks between forest and soils are complex. Process-based models covering both vegetation and soil biogeochemical processes are frequently used to analyse emission patterns. Particularly, the simulation of soil C and N turnover processes driving N₂O production, consumption and emission from forest soils requires highly specific input data which renders their regional application difficult since at this scale, soil conditions are often not well understood. Therefore, a soil C and N model (DecoNit) has been developed which describes biogeochemical processes with a simplified structure compared to existing carbon/nitrogen models that nevertheless follows the basic physical and chemical laws involved and which allows to simulate N trace gas emissions. The DecoNit model was previously calibrated using an extensive dataset on decomposition rates of incubated plant materials, microbial dynamics and nitrification. The DecoNit model has now been embedded in a modular simulation environment (MoBiLE) where it is combined with soil water balance and forest process sub-modules. Here, we present the evaluation of MoBiLE-DecoNit with emission data of N₂O and NO from forest soils of 15 European sites and compare simulation results with a previous study in which a more complex model (PnET-N-DNDC) was used. Evaluation criteria were as follows: (1) precision of modelled annual average emission rates; (2) coherence of modelled and measured annual average and daily emissions; (3) a dynamic representation of emission rates that correspond with the observed variance of fluxes. The results show that MoBiLE-DecoNit captures average annual emission rates more precisely than the more complex model PnET-N-DNDC. Also the structural underestimation of N trace gas fluxes from forest soils was resolved. Moreover, we present evidence that the new modelling approach is also somewhat more adequate for describing inter-daily emission dynamics. The combined MoBiLE-DecoNit is therefore thought to be a promising approach to simulate forest development and greenhouse gas balances on site and regional scales.     

Stereochemistry and biosynthesis of (+)-lyoniresinol,a syringyl tetrahydronaphthalene lignan in <Emphasis Type="Italic">Lyonia ovalifolia</Emphasis> var. <Emphasis Type="Italic">elliptica</Emphasis> II: feeding experiments with <Superscript>14</Superscript>C labeled precursors

To clarify the biosynthetic pathway for syringyl lignans, especially syringyl tetrahydronaphthalene lignans and formation of the C2–C7′ linkage, production of (+)-lyoniresinol (LYR) and its predicted intermediates [syringaresinol (SYR), 5,5′-dimethoxylariciresinol (DMLR), and 5,5′-dimethoxysecoisolariciresinol (DMSLR)] in Lyonia ovalifolia var. elliptica was investigated by means of feeding experiments with radiolabeled precursors. Following individual administration of l-[U-¹⁴C]phenylalanine (Phe), [8-¹⁴C]sinapyl alcohol (SA), and [8,8′-¹⁴C]SYR to excised young shoots of L. ovalifolia and their subsequent metabolism, free [¹⁴C]lignans and [¹⁴C]lignan glycosides were extracted with methanol from stems and leaves and were divided into ethyl acetate-soluble fractions (lignans) and aqueous fractions (lignan glycosides), respectively. Using a combination of xylanase, cellulase, and β-glucosidase, the glycosides were hydrolyzed to liberate [¹⁴C]lignans as aglycones. l-[U-¹⁴C]Phe was incorporated into (+)-[¹⁴C]SYR [stem 0.38%, 8% enantiomeric excess (e.e.)], (−)-[¹⁴C]SYR (leaves 2.75%, 72% e.e.), (+)-[¹⁴C]DMLR (stem 0.07%, 18% e.e. and leaves 0.009%, 58% e.e.), (−)-[¹⁴C]DMSLR (stem 0.03%, 46% e.e. and leaves 0.05%, 20% e.e.), (+)-[¹⁴C]LYR (leaves 0.013%, 22% e.e.) and glycosides of (+)-[¹⁴C]LYR (stem 0.036%, 50% e.e.) in 24h. Based on the percent incorporation and enantiomeric composition of the lignans, the biosynthetic pathway of (8R,8′R)-(+)-LYR was proposed as follows: a nonselective dehydrogenative dimerization of sinapyl alcohol yields (±)-SYR, which is reduced with low specificity to give (8R,8′R)-(+)-DMLR. This is cyclized to directly give (+)-LYR as well as reduced again to (8R,8′R)-(−)-DMSLR. Although further transformation of (−)-DMSLR also leads to the formation of (+)-LYR, cyclization could be a main pathway for (+)-LYR biosynthesis. This report was presented at the IAWPS 2005 International Symposium on Wood Science and Technology, Yokohama, November 2005     

Effects of continuous nitrogen addition on microbial properties and soil organic matter in a <Emphasis Type="Italic">Larix gmelinii</Emphasis> plantation in China

Continuous increases in anthropogenic nitrogen (N) deposition are likely to change soil microbial properties, and ultimately to affect soil carbon (C) storage. Temperate plantation forests play key roles in C sequestration, yet mechanisms underlying the influences of N deposition on soil organic matter accumulation are poorly understood. This study assessed the effect of N addition on soil microbial properties and soil organic matter distribution in a larch (Larix gmelinii) plantation. In a 9-year experiment in the plantation, N was applied at 100 kg N ha^?1 a^?1 to study the effects on soil C and N mineralization, microbial biomass, enzyme activity, and C and N in soil organic matter density fractions, and organic matter chemistry. The results showed that N addition had no influence on C and N contents in whole soil. However, soil C in different fractions responded to N addition differently. Soil C in light fractions did not change with N addition, while soil C in heavy fractions increased significantly. These results suggested that more soil C in heavy fractions was stabilized in the N-treated soils. However, microbial biomass C and N and phenol oxidase activity decreased in the N-treated soils and thus soil C increased in heavy fractions. Although N addition reduced microbial biomass and phenol oxidase activity, it had little effect on soil C mineralization, hydrolytic enzyme activities, δ¹³C value in soil and C–H stretch, carboxylates and amides, and C–O stretch in soil organic matter chemistry measured by Fourier transform infrared spectra. We conclude that N addition (1) altered microbial biomass and activity without affecting soil C in light fractions and (2) resulted in an increase in soil C in heavy fractions and that this increase was controlled by phenol oxidase activity and soil N availability.     

The effect of forest management on trace gas exchange at the pedosphere–atmosphere interface in beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) forests stocking on calcareous soils

The effect of forest management (thinning) on in situ carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) trace gas exchange between soil and atmosphere was studied in three consecutive years at three beech forest sites, which differ in aspect [southwest (SW), northeast (NE), northwest (NW)]. At all sites adjacent thinning plots (“T”) and untreated control plots (“C”) were established. Measurements at the SW and NE sites covered the years 4–6 after thinning while at the NW site measurements covered the year before and the first 2 years after thinning. Mean N₂O fluxes were <3 μg N₂O–N m⁻² h⁻¹ at all plots except for the newly thinned NWT plot. CH₄ uptake was rather low, too. Very low CH₄ oxidation rates during dry periods are explained by physiological drought stress for CH₄ oxidizers. Heterotrophic litter decomposition constitutes the largest part of total soil respiration. On the whole, no significant positive or negative effects of the silvicultural treatment on the magnitude of CO₂-, CH₄- and N₂O-trace gas exchange could be observed at the SW site 4–6 years after thinning. Also at the NE site, no effects of thinning on CO₂ and N₂O fluxes could be demonstrated. However, at this site a significant moisture-induced lower CH₄ uptake could be shown. At the NW site forest management led to a dramatic increase in N₂O emissions in the first two summers after thinning and to distinct effects on CO₂ emissions and CH₄ uptake in the first year after the felling. The unambiguous effects of thinning at the NW site are mainly related to higher C input by dead residues leading to enhanced mineralization activity, to a shift in the competition for nutrients favoring microorganisms as compared to trees and to changes in the soil water availability at the thinned plot. Considering the data obtained from the NE and SW site we expect that with the development of an understorey vegetation at the NW site the observed effects on the magnitude of trace gas exchange due to thinning will continue to decline in the following years. Our results implicate that it is indispensable to take account of the effects of forest management in order to accurately calculate trace gas emission inventories for the investigated forest ecosystem in case thinning took place immediately before.     

Effects of previous forest types and site conditions on species composition and abundance of naturally regenerated trees in young <Emphasis Type="Italic">Cryptomeria japonica</Emphasis> plantations in northern Japan

Naturally regenerated trees in young Cryptomeria japonica plantations were investigated in 141 quadrats of 10 × 50 m within a watershed of 1,000 ha, and factors affecting their composition and abundance were examined. The species composition of naturally regenerated trees was classified into four types. Dominant species were Swida controversa, Magnolia hypoleuca and Pterocarya rhoifolia in Type A1, Betula maximowicziana, M. hypoleuca, Quercus crispula and Castanea crenata in Type A2, Q. serrata and C. crenata in Type B1, and Pinus densiflora in Type B2. The results of path analysis showed significant influences of previous forest type and geology among the factors that correlated with the species composition of naturally regenerated trees; previous forest type in particular showed a higher absolute path coefficient value. Species composition types of naturally regenerated trees corresponded to the previous forest types: Types (A1 and A2) corresponded to the natural forests composed of Thujopsis dolabrata var. hondai, Fagus crenata, Aesculus turbinata, etc., Type B1 corresponded to the secondary Q. crispula and Q. serrata forest, and Type B2 corresponded to the secondary P. densiflora forest, respectively. The abundance of naturally regenerated trees was strongly affected by geology; i.e., plantations on soft-sedimentary dacitic tuff contained great amounts of colonizing P. densiflora trees.