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1.
Studies on the combined effects of beech–spruce mixtures are very rare. Hence, forest nutrition (soil, foliage) and nutrient fluxes via throughfall and soil solution were measured in adjacent stands of pure spruce, mixed spruce–beech and pure beech on three nutrient rich sites (Flysch) and three nutrient poor sites (Molasse) over a 2-year period. At low deposition rates (highest throughfall fluxes: 17 kg N ha−1 year−1 and 5 kg S ha−1 year−1) there was hardly any linkage between nutrient inputs and outputs. Element outputs were rather driven by internal N (mineralization, nitrification) and S (net mineralization of organic S compounds, desorption of historically deposited S) sources. Nitrate and sulfate seepage losses of spruce–beech mixtures were higher than expected from the corresponding single-species stands due to an unfavorable combination of spruce-similar soil solution concentrations coupled with beech-similar water fluxes on Flysch, while most processes on Molasse showed linear responses. Our data show that nutrient leaching through the soil is not simply a “wash through” but is mediated by a complex set of reactions within the plant–soil system.  相似文献   

2.
Trees in farming systems can improve fertility of soils through mineralization of N in their litter. This study was to determine the quality parameters (i.e., chemical composition) of organic residues that are associated with N mineralization in soils under submerged and aerobic conditions, and to demonstrate that aeration conditions should be taken into account in categorization of organic residues as N sources in farming systems. Incubation experiments were conducted in Aeric Paleaquult soil under submerged and Oxic Paleustult soil under aerobic conditions. Treatments included litter and some fresh materials from trees as well as rice straw available in farming systems of Northeast Thailand. S. grandiflora and L. leucocephala (32 g kg−1 N) had the highest net N mineralization in both conditions. Some lower-quality (< 20 g kg−1 N) residues did exhibit low net N mineralization during the 16-week period under submerged conditions, but displayed almost no net N mineralization in aerobic conditions. Under submerged conditions, their net N mineralization was higher and more rapid. The nitrogen content of the residues was the most important factor controlling N mineralization under both conditions. Polyphenols exerted the highest negative influence on N mineralization in aerobic conditions, but exhibited no negative effect in submerged conditions. In categorizing organic residues for their effective use in soil fertility management, soil aeration conditions, as well as other environmental factors, should be taken into consideration in addition to residue quality.  相似文献   

3.
Nitrogen (N) limits productivity in many coniferous forests of the western US, but the influence of post-fire structure on N cycling rates in early successional stands is not well understood. We asked if the heterogeneity created by downed wood and regenerating pine saplings affected N mineralization and microbial community composition in 15-yr old lodgepole pine (Pinus contorta var. latifolia) stands established after the 1988 fires in Yellowstone National Park (Wyoming, USA). In three 0.25-ha plots, we measured annual in situ net N mineralization in mineral soil using resin cores (n = 100 per plot) under pine saplings, downed wood (legacy logs that survived the fire, and fire-killed trees that had fallen and were contacting or elevated above the ground), and in bare mineral soil. Annual in situ net N mineralization and net nitrification rates were both greater in bare mineral soil (8.4 ± 0.6 and 3.6 ± 0.3 mg N kgsoil−1 yr−1, respectively) than under pine saplings, contact logs, or elevated logs (ca. 3.9 ± 0.5 and 0.8 ± 0.1 mg N kgsoil−1 yr−1, respectively). Net nitrification was positively related to net N mineralization under all treatments except for elevated logs. In laboratory incubations using 15N pool dilution, NH4+ consumption exceeded gross production by a factor of two in all treatments, but consumption and gross production were similar among treatments. Contrary to our initial hypothesis, microbial community composition also did not vary among treatments. Thus, two- to three-fold differences in in situ net N mineralization rates occurred despite the similarity in microbial communities and laboratory measures of gross production and consumption of NH4+ among treatments. These results suggest the importance of microclimate on in situ annual soil N transformations, and differences among sites suggest that broader scale landscape conditions may also be important.  相似文献   

4.
Changes in soil N mineralization pathways occurring along a full rotation cycle have received little attention to date, while tree uptake for N may change during forest ageing. The aims of this study were (i) to characterize changes in potential net N mineralization and potential net nitrification within organic layers and the topsoil (organo-mineral horizon) along a 100-year chronosequence for a temperate oak–hornbeam forest and (ii) to reveal covariances between potential net N mineralization pathways and the properties of the humic epipedon (defined as the sum of organic layers and topsoil). For that purpose, a space-for-time substitution procedure and aerobic laboratory incubation method for 28 days at 28 °C in the dark were used. In addition, acetylene and captan were used to discriminate between autotrophic and heterotrophic (bacterial and/or fungal) nitrification. Several humic epipedon properties were determined, e.g. pH, exchangeable cation concentrations, effective cation exchange capacity, total C and N, dissolved organic C and N, fungal and microbial biomass N. Potential net N mineralization and nitrification pathways changed greatly along the mixed forest chronosequence. Potential net N mineralization in the organic layers increased with stand maturation whereas potential net nitrification in the topsoil decreased significantly. Selective inhibitors revealed changes in nitrification pathways along the chronosequence, i.e. potential net nitrification was autotrophic in the topsoil while it was mainly heterotrophic within the organic layers. In the organic layer, potential net nitrification was autotrophic at the onset of the chronosequence while it appeared heterotrophic during the aggradation phase and finally fungal in mature stands. A Co-Inertia Analysis was used to reveal covariances between N mineralization pathways and humic epipedon properties. The analysis showed two functional temporal shifts within N cycling along the chronosequence, one probably controlled by organic matter quality and high competition for available N resulting in the autotrophic versus heterotrophic nitrification shift in the organic layers and one mainly controlled by (i) fine organic matter abundance, allowing high N mineralization in the organic layers and (ii) acidity inhibited autotrophic nitrification in the topsoil.  相似文献   

5.
Lowland evergreen rainforests in southern Chile growing on highly productive soils and accessible sites have been subjected to traditional and industrial logging of valuable timber trees. Old-growth rain forests in this area are characterized by highly conservative N cycles, which results in an efficient N use of ecosystems. We hypothesize that different logging practices, by changing forest structure and species composition, can alter the quantity and quality (i.e. C/N ratio) of litterfall and soil organic matter and soil microbial processes that determine N storage and availability. To test this hypothesis we investigated chemical properties, microbial N transformations, N fluxes and N storage in soils of lowland evergreen rainforests of Chiloé Island after 10 years since industrial selective logging (ISL) and in stands subjected to traditional selective logging (TSL) by landowners in small properties. We compared them to reference unlogged old-growth stands (OG) in the same area. Tree basal area was more reduced in the stands subjected to ISL than to TSL. Litterfall inputs were similar in both logging treatments as in OG stands. This was due to greater biomass of understory species after logging. In TSL understory tree species determined a higher litterfall C/N ratio than ISL. We found higher soil N availability and content of base cations in surface soils of logged forests than in OG. The litter horizon of OG forest had significantly higher rates of non-symbiotic N fixation than logged forests. In the ISL treatment there was a trend toward increasing soil denitrification and significantly higher NO3–N/Nt ratio in spring waters, which led to a stronger δ15N signal in surface and deep soils. We conclude that massive understory occupation by the shade-intolerant native bamboo Chusquea quila in ISL led to enhanced litter quality (lower C/N ratios) relaxing the tightness of the N cycle, which increased soil N availability leading to a higher proportion of nitrate in spring waters and higher gaseous N losses. In contrast, under TSL a higher litterfall C/N ratio slowed decomposition and net N mineralization rates thus reducing the chances for N losses, and enhancing C and N storage in soil. We suggest that sustainable logging practices in these rain forests should be based on lower rates of canopy removal to enhance colonization of the understory by shade-tolerant trees, which are associated with a more efficient N cycle.  相似文献   

6.
To test effects of litter quality and soil conditions on N-dynamics, we selected seven forests in Luxembourg dominated by beech (Fagus sylvatica, L.) and hornbeam (Carpinus betulus L.), and located on acid loam, decalcified marl or limestone, and measured organic matter characteristics, microbial C and N and net N-mineralization in a laboratory incubation experiment. Organic layer characteristics were significantly affected by species, with lower litter decay and higher accumulation under the less palatable beech, even on limestone. However, beech and hornbeam did not show any differences in N-cycling at all. Instead of species, N-cycling was affected by site conditions, albeit different than expected. Microbial N generally increased from acid loam to limestone, but acid loam showed higher net N-mineralization, especially in the organic layer. Also, acid loam showed high instead of low efficiency of N-mineralization per unit microbe, in both organic layer and mineral topsoil. In addition, acid loam showed net consumption of DOC instead of release in both soil layers, which suggests that not N, but C was a limiting factor to decomposition. In contrast, limestone showed low net N-mineralization in the organic layer, despite high mass and well-decomposed organic matter, and low efficiency of N-mineralization per unit microbe in both organic layer and mineral topsoil. DOC was net released instead of consumed, which supported that not C, but N was a limiting factor. The general lack of differences in net N-cycling between species, but relatively clear site effects, is discussed in relation to different microbial strategies. Acid soil may have high net N-release despite low biological activity, because N-requirements of fungi are also low, while in calcareous soil, high bacterial N-demand may counteract high gross N-release. Thus, species producing litter that decomposes rapidly may be planted to improve soil conditions and plant biodiversity, but litter quality effects on N-availability may be less important than soil conditions.  相似文献   

7.
The objective of this study was to determine the rate of nitrogen (N) mineralization in response to various levels of canopy cover in red pine (Pinus resinosa Ait.) stands. Experimental plots consisted of various levels of canopy cover,i.e., clearcut, 25% (50% during first sampling year), 75%, and uncut in red pine plantations in northern Lower Michigan, USA. Net N mineralization and nitrification in the top 15 cm of mineral soil were examined during the first two growing seasons (1991–1992) following the canopy cover manipulations, using anin situ buried bag technique. Mean net N mineralization over the course of both growing seasons (May–October) ranged from 26.9 kg ha−1 per growing season in the clearcut treatment to 13.4 kg ha−1 per growing season in the uncut stand. Net N mineralization and nitrification increased significantly in the clearcut treatment compared to the uncut treatment during the second growing season only. However, net N mineralization and nitrification did not differ significantly between the partial canopy cover treatments and the uncut stand. Increased N mineralization and nitrification in the clearcut during the second growing season may be associated with increased soil temperature and changes of organic matter quality with time since canopy removal. This study was supported in part by the USDA Forest Service and Michigan Technological University.  相似文献   

8.
It was hypothesized that increasing air and/or soil temperature would increase rates of microbial processes including litter decomposition and net N mineralization, resulting in greater sequestration of carbon and nitrogen in humus, and consequently development in OH horizon （humus horizon）. To quantify the effect of temperature on biochemical processes controlling the rate of OH layer development three adjacent forest floors under beech, Norway spruce and mixed species stands were investigated at Soiling forest, Germany by an incubation experiment of OH layer for three months. Comparing the fitted curves for temperature sensitivity of OH layers in relation to net N mineralization revealed positive correlation across all sites. For the whole data set of all stands, a Q10 （temperature sensitivity index） value of 2.35-2.44 dependent on the measured units was found to be adequate for describing the temperature dependency of net N mineralization at experimental site. Species-specific differences of substrate quality did not result in changes in biochemical properties of OH horizon of the forest floors. Temperature elevation increased net N mineralization without significant changes in microbial status in the range of I to 15℃. A low Cmic /Corg （microbial carbon/organic carbon） ratio at 20℃ indicated that the resource availability for decomposers has been restricted as reflected in significant decrease of microbial biomass.  相似文献   

9.
In most temperate forest, nitrogen (N) is considered a limiting factor. This becomes important in extreme environments, as Nothofagus antarctica forests, where the antecedents are scarce. Thinning practices in N. antarctica forests for silvopastoral uses may modify the soil N dynamics. Therefore, the objective of this work was to evaluate the temporal variation of soil N in these ecosystems. The mineral extractable soil N, net nitrification and net N mineralization were evaluated under different crown cover and two site quality stands. The mineral N extractable (NH4 +–N + NO3 ?–N) was measured periodically. Net nitrification and net N mineralization were estimated through the technique of incubation of intact samples with tubes. The total mineral extractable N concentration varied between crown cover and dates, with no differences among site classes. The lowest and highest values were found in the minimal and intermediate crown cover, respectively. In the higher site quality stand, the annual net N mineralization was lower in the minimal crown cover reaching 11 kg N ha?1 year?1, and higher in the maximal crown cover (54 kg N ha?1 year?1). In the lower site quality stand there was no differences among crown cover. The same pattern was found for net nitrification. Thinning practices for silvopastoral use of these forests, keeping intermediate crown cover values, did not affect both N mineralization and nitrification. However, the results suggest that total trees removal from the ecosystem may decrease N mineralization and nitrification.  相似文献   

10.
We examined soil N dynamics, including inorganic N concentration, net N transformation rates, and estimated plant N uptake (EPNU) from soil N budgets, and litterfall inputs, in five Japanese cedar plantation stands of different ages (5, 16, 31, 42, and 89 years) in the Mt Gomadan Experimental Forest (GEF). Net soil N mineralization and nitrification rates did not differ significantly between the youngest and oldest stands; soil moisture and inorganic N concentration were higher in the youngest stand. The EPNU was highest in the 16-year-old stand and lowest in the 31-year-old stand, and had a significant negative correlation with litter C:N ratio. The oldest (89-year-old) stand had a higher soil C:N ratio, lower proportion of nitrification rate to mineralization rate (%NIT), and higher estimated plant NH4 + uptake than did the other stands, indicating that changes of soil organic matter quality can alter soil N dynamics. These results suggest that as a Japanese cedar plantation develops, soil N dynamics can be altered by the quantity and quality of input litter and soil organic matter, and can generate the imbalance between N supply from soil and N demand by plant.  相似文献   

11.
On fertile alluvial soils on the lakeshore plain of Malawi, maize (Zea mays L.) yields beneath canopies of large Faidherbia albida (synAcacia albida) trees greatly exceed those found beyound tree canopies, yet there is little difference in soil nutrients or organic matter. To investigate the possibility that soil nutrient dynamics contribute to increased maize yields, this study focused on the impact of Faidherbia albida on nitrogen mineralization and soil moisture from the time of crop planting until harvest. Both large and small trees were studied to consider whether tree effects change as trees mature.During the first month of the rainy season, a seven-fold difference in net N mineralization was recorded beneath large tree canopies compared to rates measured in open sites. The initial pulse beneath the trees was 60 g N g–1 in the top 15 cm of soil. During the rest of the cropping cycle, N availability was 1.5 to 3 times higher beneath tree canopies than in open sites. The total production of N for the 4-month study period was 112 g N g–1 below tree canopies compared to 42 g N g–1 beyond the canopies. Soil moisture in the 0–15 cm soil layer was higher under the influence of the tree canopies. The canopy versus open site difference grew from 4% at the beginning of the season to 50% at the end of the cropping season.Both N mineralization and soil moisture were decreased below young trees. Hence, the impact of F. albida on these soil properties changes with tree age and size. While maize yields were not depressed beneath young F. albida, it is important to realize that the full benefits of this traditional agroforestry system may require decades to develop.  相似文献   

12.
Despite the spatial significance of Canada's boreal forest, there is very little known about CH4 and N2O emissions from non-peatlands within it. The primary objective of this project was to study the atmosphere–soil exchange of CH4 and N2O at three sites in the boreal forest of central Saskatchewan. In the summers of 2006 and 2007, CH4 and N2O emissions were measured along transects in three different mature forest stands (aspen, black spruce and jack pine) using a sealed chamber method. At the aspen site, the gross rates of mineralization and nitrification, and the relative contribution of nitrification and denitrification to N2O emissions, were also measured using the 15N isotope dilution technique. Results indicated that the jack pine and black spruce sites were slight sinks of CH4 (−0.123 g CH4–C m−2 yr−1and −0.017 g CH4–C m−2 yr−1 respectively in 2006 and −0.095 g CH4–C m−2 yr−1and 0.045 g CH4–C m−2 yr−1 respectively in 2007), whereas the aspen site was a net source (4.40 g CH4–C m−2 yr−1 in 2006 and 19.60 g CH4–C m−2 yr−1 in 2007). The high CH4 emissions at the aspen site occurred at depressions that were water-filled due to above-average precipitation levels in 2005–2007. All three sites had very low cumulative N2O emissions, ranging from −0.002 to 0.014 g N2O–N m−2 yr−1 in both years. The 15N results indicated that N cycling at the aspen site was very conservative, allowing little N to escape the system as N2O; the emissions that did occur were due primarily to a nitrification-related process.  相似文献   

13.
We compared N fluxes in a 150-year-old Fagus sylvatica coppice and five adjacent 25-year-old plantations of Fagus sylvatica, Picea abies, Quercus petraea, Pinus laricio and Pseudotsuga menziesii. We measured net N mineralization fluxes in the upper mineral horizon (A1, 0–5 cm) for 4 weeks and gross N mineralization fluxes for two days. Gross rates were measured during the 48-h period after addition of 15NH4 and 15NO3. Mineralization was measured by the 15NH4 dilution technique and gross nitrification by 15NO3 production from the addition of 15NH4, and by 15NO3 dilution. Net and gross N mineralization was lower in the soil of the old coppice, than in the plantations, both on a soil weight and organic nitrogen basis. Gross nitrification was also very low. Gross nitrification measured by NO3 dilution was slightly higher than measured by 15NO3 production from the addition of 15NH4. In the plantations, gross and net mineralization and nitrification from pool dilution were lowest in the spruce stand and highest in the beech and Corsican pine stands. We concluded that: (1) the low net mineralization in the soil of the old coppice was related to low gross rate of mineralization rather than to the concurrent effect of microbial immobilisation of mineral N; (2) the absence of nitrate in the old coppice was not related to the low rate of mineralization nor to the absence of nitrifyers, but most probably to the inhibition of nitrifyers in the moder humus; (3) substituting the old coppice by young stands favours nitrifyer communities; and (4) heterotrophic nitrifyers may bypass the ammonification step in these acid soils, but further research is needed to check this process and to characterize the microbial communities.  相似文献   

14.
During 1992 and 1993, nitrogen dynamics and microbial activity were investigated in an agrisilvicultural system consisting of oats or barley cyltivated along the sides of a poplar plantation in Sweden. At each of three experimental sites (two silt loams and one silty clay loam), sampling for mineral nitrogen was carried out in three layers down to 90 cm at two distances from the trees, A (0.5–1.5 m) and B (4.0–5.0 m), two times each year (spring and autumn). Sampling of soil for organic amtter, carbon and nitrogen, potential nitrification, N ineralization, basic respiration and substrate-induced respiration was carried out in the 0–10 cm layer at three distances from the trees: A (0.5–1.5 m), B (2.5–3.5 m) and C (4.0–5.0 m).Significantly larger amounts of organic matter, total carbon and nitrogen at A than at B and C, indicated increased inputs from the trees through litter, decaying roots and root exudates. This could explain that the rates of nitrogen mineralization, potential nitrification and respiration were significantly higher at A than at B and C. The presence of trees resulted in a better utilization of nitrogen and moisture in the soil, reducing the potential for nitrate leaching and accumulating nitrogen close to the trees. The higher concentration of ammonium, lower concentration of nitrate and the consistently lower NO 3 – –N/NH 4 + –N-ratios observed at A than at C might be explained by a combined effect of increased nitrogen mineralization and efficient nitrate uptake by the trees.  相似文献   

15.
Carbon and nitrogen stocks and their medium-term and readily decomposable fractions in topsoils were compared in relation to soil microbial biomass and activity along sequences from coniferous to deciduous stands. The study was carried out in the Ore Mountains and the Saxonian lowland, representing two typical natural regions in Saxony, Germany. In accordance with current forest conversion practices, the investigation sites represent different stands: mature conifer stands of Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) (type A); Norway Scots spruce and pine with advanced plantings of European beech (Fagus sylvatica L.) or European beech/Common oak (Quercus petreae Liebl.) (type B); and mature deciduous stands of European beech and European beech/Common oak (type C). The investigated forest sites can be grouped into three silvicultural situations according to the development from coniferous stands to advanced plantings and finally mature deciduous forests (chronosequence A–B–C). The organic layer (L, F and H horizons) and uppermost mineral soil (0–10 cm) were analysed for potential C mineralisation, microbial biomass, concentrations of total C and N (TOC and TN) and for medium-term and readily decomposable C and N fractions, obtained by hot- and cold-water extraction respectively. The results showed an increase in organic layer thickness and mass as well as TOC and TN stocks along the forest sequences in the lowland. Yet, underplanted sites with two storeys revealed higher organic layer mass as well as TOC and TN stocks as compared to coniferous and deciduous stands. Stocks of hot- and cold-water-extractable C and N in relation to microbial biomass and its activity revealed a high turnover activity in deeper organic horizons of deciduous forests compared to coniferous stands. The stand-specific differentiation is discussed in relation to microbial biomass, litter quantity and quality and forest structure, but also with respect to the site-specific climatic factors and water budget as well as liming and fly-ash impacts. Results indicate higher dynamics in deciduous stands in the lowland especially during the initial turnover phase. The elevated microbial activity in deeper organic horizons of deciduous litter-influenced sites in spring is discussed as a specific indicator for long-term C sequestration potential as besides C mineralisation organic compounds are humified and thus, can be stored in the organic layer or in deeper soil horizons. Due to liming activities, stand-specific effects on organic matter turnover dynamics have evened out today in the Ore mountain region, but will presumably occur again once base saturation decreases. Here, the stand-specific effect on microbial biomass can currently be seen again as Cmic in the L horizon increased from spruce to beech. Our study sites in the lowland revealed no significant fly-ash impact. Differences between sites were evaluated by calculating the discriminance function. TOC and TN as well as medium-term degradable C and N were defined in this study as indicators for turnover dynamics along forest conversion sites.  相似文献   

16.
Karki  Himani  Bargali  Kiran  Bargali  S. S. 《Agroforestry Systems》2021,95(8):1603-1617

To access the process of nitrogen mineralization in soil, the buried-bag technique was used among traditional agroforestry systems in the Bhabhar belt of Kumaun Himalaya. The present study, determined the relationship between various parameters of N-mineralization with agroforestry systems, seasons and soil depths. Season and soil depth have significantly (p?<?0.001) affected the process of ammonification, nitrification and net N-mineralization. The soil ammonium-N pool was comparatively higher than the nitrate-N pool. Highest amount of ammonium and nitrate-N were recorded in the agri-horticulture (AH) system, and lowest in the agri-horti-silviculture (AHS) system. Among the systems, highest amount of inorganic-N (ammonium?+?nitrate) was recorded during rainy season while, lowest during winter season. The highest ammonification rate (6.47?±?1.47 mg kg?1 month?1) was observed in agri-silviculture system and lowest (5.67?±?1.68 mg kg?1 month?1) in AHS system, while nitrification value was maximum (2.53?±?0.40 mg kg?1 month?1) in AH system and minimum (2.23?±?0.37 mg kg?1 month?1) in AHS system. The values of net N-mineralization were ranged from 4.03?±?0.53 to 13.29?±?0.44 mg kg?1 month?1. The values of inorganic-N and net N-mineralization were significantly more (P?<?0.01) in the surface soil layer (0–20 cm) than the subsurface layers (20–40 cm and 40–60 cm). Nitrogen mineralization was negatively correlated with the soil pH and positively correlated with soil organic carbon and total soil nitrogen. Higher rate of N-mineralization in AHS system indicated rapid turnover of nitrogen due to soil management practices and suggested that the changes in agroforestry based land-use systems alter the process of net N-mineralization, nitrification and ammonification.

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17.
Seasonal changes in biomass, net primary productivity and turnover of dry matter of para grass (Brachiaria mutica) under a mixed tree stand and in an adjacent open stand in northern India are presented. Both stands attained peak values of live shoot biomass in September with a higher value under mixed tree stand (665 g m–2) than in the open stand (522 g m–2). The net aboveground production was 590 and 527 g m–2 yr–1 under mixed tree stand and in the open, respectively. The belowground net primary production was also greater under mixed tree stand (100 g m–2 yr–1) than in the open (76 g m–2 yr–1). Maximum aboveground and belowground net primary productions in both stands were obtained during the rainy season. The total net primary production for para grass was about 15% higher under mixed tree stand than in the open. The turnover rates of total plant biomass were greatest in the rainy season and the least during the summer season. The system transfer functions showed that the production of para grass on both stands was aboveground-oriented, accounting for 85–87% of annual total net primary production.  相似文献   

18.
The area of broadleaved forests is projected to increase in Denmark as well as in the rest of Europe. However, studies of the N leaching response to elevated N deposition have focused on coniferous stands and considerable uncertainty still remains on whether broadleaved and coniferous forests respond differently to elevated N. We studied N input–output relations for eight stands intensively monitored during 2002–2005 and literature data for 37 additional stands which together formed a comprehensive dataset on Danish forests including 26 broadleaved stands and 19 coniferous stands. Nitrate leaching was significantly higher in first generation stands on former arable land with mineral soil C/N ratios 10–15, but both low and high rates were observed independent of the N input. A net N loss was observed in some of these stands even though they are in the aggrading phase and accumulate N in the biomass. Broadleaved stands had significantly lower throughfall N deposition than coniferous stands and this seems to be the main process where forest type exerts an influence on the N cycle. Lower soil C/N ratios offset the effect of throughfall N deposition and thus N leaching did not differ between the two forest types. The best regression models for prediction of nitrate leaching included throughfall N deposition and C/N ratio, but only a minor part of the variability was explained. The C/N ratio of the upper mineral soil was more generally applicable than that of the organic layer. The N retention of the soil was reasonably well predicted above a C/N ratio of 25, but below this threshold the importance is not known. We suggest focusing future efforts on quantifying the relative retention functions (sink strength) of the vegetation and the soil organic matter to improve the predictions of N retention and N leaching.  相似文献   

19.
Monitoring of soil nitrogen (N) cycling is useful to assess soil quality and to gauge the sustainability of management practices. We studied net N mineralization, nitrification, and soil N availability in the 0 10 cm and 11 30 cm soil horizons in east China during 2006 2007 using an in situ incubation method in four subtropical evergreen broad-leaved forest stands aged 18-, 36-, 48-, and 65-years. The proper- ties of surface soil and forest floor varied between stand age classes. C:N ratios of surface soil and forest floor decreased, whereas soil total N and total organic C, available P, and soil microbial biomass N increased with stand age. The mineral N pool was small for the young stand and large for the older stands. NO 3 - -N was less than 30% in all stands. Net rates of N mineralization and nitrification were higher in old stands than in younger stands, and higher in the 0 10 cm than in the 11 30 cm horizon. The differences were significant between old and young stands (p < 0.031) and between soil horizons (p < 0.005). Relative nitrification was somewhat low in all forest stands and declined with stand age. N trans- formation seemed to be controlled by soil moisture, soil microbial bio- mass N, and forest floor C:N ratio. Our results demonstrate that analyses of N cycling can provide insight into the effects of management distur- bances on forest ecosystems.  相似文献   

20.
Chemical characteristics of forest soils subjected to long-term deposition of alkaline and acid air pollutants were analysed in spruce (Picea abies (L.) Karst.) stands in eastern Germany. Three forest sites along an emission gradient of 3, 6, and 15 km downwind of a coal-fired power plant were selected, representing high, intermediate, and low fly-ash input rates. Past emissions caused an accumulation of mineral fly-ash constituents in the organic layer, resulting in an atypically high mass of organic horizons of forest soils, especially in the F and H horizons. Total mass of organic layers at the site with heavy deposition loads was as high as 128 t ha–1, compared to 58 t ha–1 at the low input site. Fly-ash deposition significantly increased the pH values in the L, F and H horizons and mineral topsoil (0–10 cm). Significantly higher concentrations of NH4Cl-extractable cations (i.e. effective cation exchange capacities) and base saturations of >66% were found in the humic horizons at sites where the pH was increased due to the direct and indirect (i.e. higher proportions of deciduous trees) effects of fly-ash emissions. Stocks of basic cations were dominated by Ca2+ and decreased significantly along the fly-ash deposition gradient from 33.6 to 5.3 kmolc ha–1. Proportions of water-soluble basic cations out of the total potentially exchangeable (i.e. NH4Cl-extractable) basic cations generally increased in the forest soil with decreasing deposition loads following the cation exchange capacity and base saturation along the fly-ash gradient. Higher proportions of monovalent cations, such as K+ and Na+, were observed in the water extracts from fly-ash-affected forest soils, while the NH4Cl-extracts were dominated by bivalent cations, such as Ca2+ and Mg2+. These results suggest a greater leaching tendency for monovalent cations in these soils. Stocks of organic C and total N in the humus layer decreased from sites with high fly-ash deposition levels to sites with low levels, from 57.4 to 46.4 t C ha–1 and from 2.43 to 1.99 t N ha–1. The C/N ratios of the organic horizons varied from 22 to 25, revealing no distinct pattern along the fly-ash gradient. Measurements of hot-water-extractable and water-soluble organic C suggested a reduced availability or a faster decomposition of soil organic matter in soils with historically high fly-ash loads.  相似文献   

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