Impact of long-term nitrogen addition on carbon stocks in trees and soils in northern Europe

The aim of this study was to quantify the effects of fertiliser N on C stocks in trees (stems, stumps, branches, needles, and coarse roots) and soils (organic layer +0–10 cm mineral soil) by analysing data from 15 long-term (14–30 years) experiments in Picea abies and Pinus sylvestris stands in Sweden and Finland. Low application rates (30–50 kg N ha⁻¹ year⁻¹) were always more efficient per unit of N than high application rates (50–200 kg N ha⁻¹ year⁻¹). Addition of a cumulative amount of N of 600–1800 kg N ha⁻¹ resulted in a mean increase in tree and soil C stock of 25 and 11 kg (C sequestered) kg⁻¹ (N added) (“N-use efficiency”), respectively. The corresponding estimates for NPK addition were 38 and 11 kg (C) kg⁻¹ (N). N-use efficiency for C sequestration in trees strongly depended on soil N status and increased from close to zero at C/N 25 in the humus layer up to 40 kg (C) kg⁻¹ (N) at C/N 35 and decreased again to about 20 kg (C) kg⁻¹ (N) at C/N 50 when N only was added. In contrast, addition of NPK resulted in high (40–50 kg (C) kg⁻¹ (N)) N-use efficiency also at N-rich (C/N 25) sites. The great difference in N-use efficiency between addition of NPK and N at N-rich sites reflects a limitation of P and K for tree growth at these sites. N-use efficiency for soil organic carbon (SOC) sequestration was, on average, 3–4 times lower than for tree C sequestration. However, SOC sequestration was about twice as high at P. abies as at P. sylvestris sites and averaged 13 and 7 kg (C) kg⁻¹ (N), respectively. The strong relation between N-use efficiency and humus C/N ratio was used to evaluate the impact of N deposition on C sequestration. The data imply that the 10 kg N ha⁻¹ year⁻¹ higher deposition in southern Sweden than in northern Sweden for a whole century should have resulted in 2.0 ± 1.0 (95% confidence interval) kg m⁻² more tree C and 1.3 ± 0.5 kg m⁻² more SOC at P. abies sites in the south than in the north for a 100-year period. These estimates are consistent with differences between south and north in tree C and SOC found by other studies, and 70–80% of the difference in SOC can be explained by different N deposition.     

Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden

This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m⁻² year⁻¹). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m⁻² year⁻¹ in the north to +9 g C m⁻² year⁻¹ in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m⁻² year⁻¹. An erratum to this article can be found at     

Distribution of nitrogen-15 tracers applied to the canopy of a mature spruce-hemlock stand,Howland, Maine,USA

In N-limited ecosystems, fertilization by N deposition may enhance plant growth and thus impact C sequestration. In many N deposition–C sequestration experiments, N is added directly to the soil, bypassing canopy processes and potentially favoring N immobilization by the soil. To understand the impact of enhanced N deposition on a low fertility unmanaged forest and better emulate natural N deposition processes, we added 18 kg N ha⁻¹ year⁻¹ as dissolved NH₄NO₃ directly to the canopy of 21 ha of spruce-hemlock forest. In two 0.3-ha subplots, the added N was isotopically labeled as ¹⁵NH₄ ⁺ or ¹⁵NO₃ ⁻ (1% final enrichment). Among ecosystem pools, we recovered 38 and 67% of the ¹⁵N added as ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻, respectively. Of ¹⁵N recoverable in plant biomass, only 3–6% was recovered in live foliage and bole wood. Tree twigs, branches, and bark constituted the most important plant sinks for both NO₃ ⁻ and NH₄ ⁺, together accounting for 25–50% of ¹⁵N recovery for these ions, respectively. Forest floor and soil ¹⁵N retention was small compared to previous studies; the litter layer and well-humified O horizon were important sinks for NH₄ ⁺ (9%) and NO₃ ⁻ (7%). Retention by canopy elements (surfaces of branches and boles) provided a substantial sink for N that may have been through physico-chemical processes rather than by N assimilation as indicated by poor recoveries in wood tissues. Canopy retention of precipitation-borne N added in this particular manner may thus not become plant-available N for several years. Despite a large canopy N retention potential in this forest, C sequestration into new wood growth as a result of the N addition was only ~16 g C m⁻² year⁻¹ or about 10% above the current net annual C sequestration for this site.     

Belowground responses of <Emphasis Type="Italic">Picea asperata</Emphasis> seedlings to warming and nitrogen fertilization in the eastern Tibetan Plateau

The impacts of global climatic change on belowground ecological processes of terrestrial ecosystems are still not clear. We therefore conducted an experiment in the subalpine coniferous forest ecosystem of the eastern edges of the Tibetan Plateau to study roots of Picea asperata seedlings and rhizosphere soil responses to soil warming and nitrogen availability from April 2007 to December 2008. The seedlings were subjected to two levels of temperature (ambient; infrared heater warming) and two nitrogen levels (0 or 25 g m⁻²year⁻¹ N). We used a free air temperature increase from an overhead infrared heater to raise both air and soil temperature by 2.1 and 2.6°C, respectively. The results showed that warming alone significantly increased total biomass, coarse root biomass and fine root biomass of P. asperata seedlings. Both total biomass and fine root biomass were increased, but coarse root biomass was significantly decreased by nitrogen fertilization and warming combined with nitrogen fertilization. Warming induced a prominent increase in soil organic carbon (SOC) and NO₃ ⁻-N of rhizosphere soil, while nitrogen fertilization significantly decreased SOC and NH₄ ⁺-N of rhizosphere soil. The warming, fertilization and warming × N fertilization interaction decreased soil microbial C significantly, but substantially increased soil microbial N. These results suggest that nitrogen deposition combined with warmer temperatures under future climatic change possibly will have no effect on fine root production of P. asperata seedlings, but could enhance the nitrification process of their rhizosphere soils in subalpine coniferous forests.     

Are Swedish forest soils sinks or sources for CO<Subscript>2</Subscript>—model analyses based on forest inventory data

Forests soils should be neither sinks nor sources of carbon in a long-term perspective. From a Swedish perspective the time since the last glaciation has probably not been long enough to reach a steady state, although changes are currently very slow. In a shorter perspective, climatic and management changes over the past 100 years have probably created imbalances between litter input to soils and organic carbon mineralisation. Using extant data on forest inventories, we applied models to analyse possible changes in the carbon stocks of Swedish forest soils. The models use tree stocks to provide estimates of tree litter production, which are fed to models of litter decomposition and from which carbon stocks are calculated. National soil carbon stocks were estimated to have increased by 3 Tg yr⁻¹ or 12–13 g m⁻² yr⁻¹ in the period 1926–2000 and this increase will continue because soil stocks are far from equilibrium with current litter inputs. The figure obtained is likely to be an underestimation because wet sites store more carbon than predicted here and the inhibitory effect of nitrogen deposition on soil carbon mineralisation was neglected. Knowledge about site history prior to the calculation period determines the accuracy of current soil carbon stocks estimates, although changes can be more accurately estimated. This article has previously been published in issue 82/3, under DOI .     

Long-term effects of continuous direct seeding mulch-based cropping systems on soil nitrogen supply in the Cerrado region of Brazil

In the Cerrado region of Brazil conventional soybean monoculture is since the 1980s being replaced by direct seeding mulch-based cropping (DMC) with two crops per year and absence of tillage practices. The objective of this study was to assess the long-term impact of DMC on soil organic matter accumulation and nitrogen (N) mineralization. Measurements of soil organic carbon (C) content, soil total N content and soil N mineralization, both under laboratory conditions using disturbed soil samples and under field conditions using intact soil cores were conducted on a chronosequence of 2-, 6-, 9- and 14-year-old DMC fields (DMC-2, DMC-6, DMC-9 and DMC-14, respectively). The average increase of organic C in the 0–30 cm topsoil layer under DMC was 1.91 Mg C ha⁻¹ year⁻¹. Soil total N increased with 103 kg N ha⁻¹ year⁻¹ (0–30 cm). The potential N mineralization rate under laboratory conditions (28°C, 75% of soil moisture at field capacity) was 0.27, 0.28, 0.39 and 0.36 mg N kg soil⁻¹ day⁻¹ for, respectively, the DMC-2, DMC-6, DMC-9 and DMC-14 soils. The corresponding specific N mineralization rates were 0.16, 0.15, 0.22 and 0.17 mg N g N⁻¹ day⁻¹. There was no obvious explanation for the higher specific N mineralization rate of soils under DMC-9, given the similar soil conditions and land-use history before DMC was introduced. Results from the in situ N incubation experiments were in good agreement with those from the laboratory incubations. We estimated that soil N mineralization increases with about 2.0 kg N ha⁻¹ year⁻¹ under DMC. The increase was mainly attributed to the larger soil total N content. These results indicate that even in the medium term (10 years), continuous DMC cropping has limited implications for N fertilization recommendations, since the extra soil N supply represents less than 20% of the common N fertilization dose for maize in the region.     

Changes in soil carbon flux and carbon stock over a rotation of poplar plantations in northwest China

Forest soil is a major component of terrestrial ecosystems for carbon sequestration and plays an important role in the global carbon cycle. Soil carbon flux and soil carbon pools were investigated in a poplar plantation chronosequence over a rotation in northwest China. Based on continuous field observation in 2007, the results showed that mean soil CO₂ efflux rate was 5.54, 4.81, and 3.93 μmol CO₂ m⁻² s⁻¹ for stands of 2-, 8-, and 15-year-old, respectively, during the growing season. Significant differences in soil respiration of three age classes were mainly because soil temperature, carbon allocation, and fine root growth changed greatly with stand age. Multiple regression analysis suggested that soil temperature and fine root biomass in the upper layer could explain 78–85% of the variation in soil respiration. Mineral soil C stock at 0–40 cm depth was 55.77, 55.09, and 58.14 t ha⁻¹ in the 2-, 8-, and 15-year-old stands, respectively. The average rate of soil C sequestration was 0.13 t ha⁻¹ year⁻¹ following afforestation on former crop lands. Although the plantations had similar management practices and soil types since their establishment, many biotic and abiotic factors such as root biomass and turnover rate, soil condition of the plantations had undergone marked changes at different development stages, which could result in the remarkable differences in soil carbon flux and storage over a rotation. Our results highlight the importance of the development stage within a rotation of poplar plantation in assessment of soil carbon budget.     

Photosynthetic performance in Sphagnum transplanted along a latitudinal nitrogen deposition gradient

Increased N deposition in Europe has affected mire ecosystems. However, knowledge on the physiological responses is poor. We measured photosynthetic responses to increasing N deposition in two peatmoss species (Sphagnum balticum and Sphagnum fuscum) from a 3-year, north–south transplant experiment in northern Europe, covering a latitudinal N deposition gradient ranging from 0.28 g N m⁻² year⁻¹ in the north, to 1.49 g N m⁻² year⁻¹ in the south. The maximum photosynthetic rate (NP_max) increased southwards, and was mainly explained by tissue N concentration, secondly by allocation of N to the photosynthesis, and to a lesser degree by modified photosystem II activity (variable fluorescence/maximum fluorescence yield). Although climatic factors may have contributed, these results were most likely attributable to an increase in N deposition southwards. For S. fuscum, photosynthetic rate continued to increase up to a deposition level of 1.49 g N m⁻² year⁻¹, but for S. balticum it seemed to level out at 1.14 g N m⁻² year⁻¹. The results for S. balticum suggested that transplants from different origin (with low or intermediate N deposition) respond differently to high N deposition. This indicates that Sphagnum species may be able to adapt or physiologically adjust to high N deposition. Our results also suggest that S. balticum might be more sensitive to N deposition than S. fuscum. Surprisingly, NP_max was not (S. balticum), or only weakly (S. fuscum) correlated with biomass production, indicating that production is to a great extent is governed by factors other than the photosynthetic capacity.     

Biometric-based estimation of net ecosystem production in a mature Japanese cedar (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) plantation beneath a flux tower

Quantification of carbon budgets and cycling in Japanese cedar (Cryptomeria japonica D. Don) plantations is essential for understanding forest functions in Japan because these plantations occupy about 20% of the total forested area. We conducted a biometric estimate of net ecosystem production (NEP) in a mature Japanese cedar plantation beneath a flux tower over a 4-year period. Net primary production (NPP) was 7.9 Mg C ha⁻¹ year⁻¹ and consisted mainly of tree biomass increment and aboveground litter production. Respiration was calculated as 6.8 (soil) and 3.3 (root) Mg C ha⁻¹ year⁻¹. Thus, NEP in the plantation was 4.3 Mg C ha⁻¹ year⁻¹. In agreement with the tower-based flux findings, this result suggests that the Japanese cedar plantation was a strong carbon sink. The biometric-based NEP was higher among most other types of Japanese forests studied. Carbon sequestration in the mature plantation was characterized by a larger increment in tree biomass and lower mortality than in natural forests. Land-use change from natural forest to Japanese cedar plantation might, therefore, stimulate carbon sequestration and change the carbon allocation of NPP from an increment in coarse woody debris to an increase in tree biomass.     

Soil carbon storage, litterfall and CO2 efflux in fertilized and unfertilized larch (Larix leptolepis) plantations

This study evaluated the effects of forest fertilization on the forest carbon (C) dynamics in a 36-year-old larch (Larix leptolepis) plantation in Korea. Above- and below-ground C storage, litterfall, root decomposition and soil CO₂ efflux rates after fertilization were measured for 2 years. Fertilizers were applied to the forest floor at rates of 112 kg N ha⁻¹ year⁻¹, 75 kg P ha⁻¹ year⁻¹ and 37 kg K ha⁻¹ year⁻¹ for 2 years (May 2002, 2003). There was no significant difference in the above-ground C storage between fertilized (41.20 Mg C ha⁻¹) and unfertilized (42.25 Mg C ha⁻¹) plots, and the C increment was similar between the fertilized (1.65 Mg C ha⁻¹ year⁻¹) and unfertilized (1.52 Mg C ha⁻¹ year⁻¹) plots. There was no significant difference in the soil C storage between the fertilized and unfertilized plots at each soil depth (0–15, 15–30 and 30–50 cm). The organic C inputs due to litterfall ranged from 1.57 Mg C ha⁻¹ year⁻¹ for fertilized to 1.68 Mg C ha⁻¹ year⁻¹ for unfertilized plots. There was no significant difference in the needle litter decomposition rates between the fertilized and unfertilized plots, while the decomposition of roots with 1–2 mm diameters increased significantly with the fertilization relative to the unfertilized plots. The mean annual soil CO₂ efflux rates for the 2 years were similar between the fertilized (0.38 g CO₂ m⁻² h⁻¹) and unfertilized (0.40 g CO₂ m⁻² h⁻¹) plots, which corresponded with the similar fluctuation in the organic carbon (litterfall, needle and root decomposition) and soil environmental parameters (soil temperature and soil water content). These results indicate that little effect on the C dynamics of the larch plantation could be attributed to the 2-year short-term fertilization trials and/or the soil fertility in the mature coniferous plantation used in this study.     

Holocene Carbon Stocks and Carbon Accumulation Rates Altered in Soils Undergoing Permafrost Thaw

Permafrost soils are a significant global store of carbon (C) with the potential to become a large C source to the atmosphere. Climate change is causing permafrost to thaw, which can affect primary production and decomposition, therefore affecting ecosystem C balance. To understand future responses of permafrost soils to climate change, we inventoried current soil C stocks, investigated ∆¹⁴C, C:N, δ¹³C, and δ¹⁵N depth profiles, modeled soil C accumulation rates, and calculated decadal net ecosystem production (NEP) in subarctic tundra soils undergoing minimal, moderate, and extensive permafrost thaw near Eight Mile Lake (EML) in Healy, Alaska. We modeled decadal and millennial soil C inputs, decomposition constants, and C accumulation rates by plotting cumulative C inventories against C ages based on radiocarbon dating of surface and deep soils, respectively. Soil C stocks at EML were substantial, over 50 kg C m⁻² in the top meter, and did not differ much among sites. Carbon to nitrogen ratio, δ¹³C, and δ¹⁵N depth profiles indicated most of the decomposition occurred within the organic soil horizon and practically ceased in deeper, frozen horizons. The average C accumulation rate for EML surface soils was 25.8 g C m⁻² y⁻¹ and the rate for the deep soil accumulation was 2.3 g C m⁻² y⁻¹, indicating these systems have been C sinks throughout the Holocene. Decadal net ecosystem production averaged 14.4 g C m⁻² y⁻¹. However, the shape of decadal C accumulation curves, combined with recent annual NEP measurements, indicates soil C accumulation has halted and the ecosystem may be becoming a C source. Thus, the net impact of climate warming on tundra ecosystem C balance includes not only becoming a C source but also the loss of C uptake capacity these systems have provided over the past ten thousand years.     

Are Swedish forest soils sinks or sources for CO<Subscript>2</Subscript>—model analyses based on forest inventory data

Forests soils should be neither sinks nor sources of carbon in a long-term perspective. From a Swedish perspective the time since the last glaciation has probably not been long enough to reach a steady state, although changes are currently very slow. In a shorter perspective, climatic and management changes over the past 100 years have probably created imbalances between litter input to soils and organic carbon mineralisation. Using extant data on forest inventories, we applied models to analyse possible changes in the carbon stocks of Swedish forest soils. The models use tree stocks to provide estimates of tree litter production, which are fed to models of litter decomposition and from which carbon stocks are calculated. National soil carbon stocks were estimated to have increased by 3 Tg yr⁻¹ or 12–13 g m⁻² yr⁻¹ in the period 1926–2000 and this increase will continue because soil stocks are far from equilibrium with current litter inputs. The figure obtained is likely to be an underestimation because wet sites store more carbon than predicted here and the inhibitory effect of nitrogen deposition on soil carbon mineralisation was neglected. Knowledge about site history prior to the calculation period determines the accuracy of current soil carbon stocks estimates, although changes can be more accurately estimated.     

Shifts in litterfall and dominant nitrogen sources after expansion of shrub thickets

Woody encroachment into herbaceous ecosystems is emerging as an important ecological response to global change. A primary concern is alterations in C and N cycling and associated variations across a variety of ecosystems. We quantified seasonal variation in litterfall and litter N concentration in Morella cerifera shrub thickets to assess changes in litterfall and associated N input after shrub expansion on an Atlantic coast barrier island. We also used the natural abundance of ¹⁵N to estimate the proportion of litterfall N originating from symbiotic N fixation. Litterfall for shrub thickets ranged from 8,991 ± 247 to 3,810 ± 399 kg ha⁻¹ year⁻¹ and generally declined with increasing thicket age. Litterfall in three of the four thickets exceeded previous estimates of aboveground annual net primary production in adjacent grasslands by 300–400%. Leaf N concentration was also higher after shrub expansion and, coupled with low N resorption efficiency and high litterfall, resulted in a return of as much as 169 kg N ha⁻¹ year⁻¹ to the soil. We estimated that ∼70% of N returned to the soil was from symbiotic N fixation resulting in an ecosystem input of between 37 and 118 kg ha⁻¹ year⁻¹ of atmospheric N depending on site. Considering the extensive cover of shrub thickets on Virginia barrier islands, N fixation by shrubs is likely the largest single source of N to the system. The shift from grassland to shrub thicket on barrier islands results in a substantial increase in litterfall and foliar N concentration that will likely have a major impact on the size and cycling of ecosystem C and N pools. Increasing C and N availability in these nutrient-poor soils is likely to permanently reduce cover of native grasses and alter community structure by favoring species with greater N requirements.     

Production and retention of biogenic matter in the southeast Beaufort Sea during 2003�C2004: insights from annual vertical particle fluxes of organic carbon and biogenic silica

Regional variability in the annual fluxes of particulate organic carbon (POC) and biogenic silica (Si) at the periphery of the Mackenzie Shelf (Beaufort Sea) was investigated using eight long-term sediment traps moored at ~100-m depth. Relatively high autochthonous POC and Si fluxes were recorded in the Mackenzie Trough (4.1 and 8.9 g m⁻² year⁻¹ respectively) and off Cape Bathurst (6.6 and 79 g m⁻² year⁻¹), two areas where upwelling events are frequently observed. Diatomaceous new production was minimum on the mid-slope of the Mackenzie Shelf (2.8 g C m⁻² year⁻¹), moderate in the Mackenzie Trough (14.5 g C m⁻² year⁻¹), and highest off Cape Bathurst (128.7 g C m⁻² year⁻¹). High annual autochthonous POC flux corresponded to high diatom production. Among sites, the vertical attenuation of the POC flux increased with diatomaceous new production. Hence, the retention of autochthonous POC in the surface layer (<100 m) was highest (95%) at the highly productive site off Cape Bathurst, intermediate (72%) in the moderately productive Mackenzie Trough, and low (4%) at the unproductive mid-slope of the shelf. Our results indicate that, on Arctic shelves, upwelling and the production of diatoms increase the fraction of the POC which is retained in the surface layer and diverted to the pelagic food web. In the relatively unproductive waters of the Arctic Ocean, biological hot spots such as the one identified off Cape Bathurst where the food web promotes retention rather than vertical export could be disproportionately important as feeding grounds for higher trophic levels.     

Response of <Emphasis Type="Italic">Sphagnum</Emphasis> species mixtures to increased temperature and nitrogen availability

To predict the role of ombrotrophic bogs as carbon sinks in the future, it is crucial to understand how Sphagnum vegetation in bogs will respond to global change. We performed a greenhouse experiment to study the effects of two temperature treatments (17.5 and 21.7°C) and two N addition treatments (0 and 4 g N m⁻² year⁻¹) on the growth of four Sphagnum species from three geographically interspersed regions: S. fuscum, S. balticum (northern and central Sweden), S. magellanicum and S. cuspidatum (southern Sweden). We studied the growth and cover change in four combinations of these Sphagnum species during two growing seasons. Sphagnum height increment and production were affected negatively by high temperature and high N addition. However, the northern species were more affected by temperature, while the southern species were more affected by N addition. High temperature depressed the cover of the ‘wet’ species, S. balticum and S. cuspidatum. Nitrogen concentrations increased with high N addition. N:P and N:K ratios indicated P-limited growth in all treatments and co-limitation of P and K in the high N treatments. In the second year of the experiment, several containers suffered from a severe fungal infection, particularly affecting the ‘wet’ species and the high N treatment. Our findings suggest that global change can have negative consequences for the production of Sphagnum species in bogs, with important implications for the carbon sequestration in these ecosystems.     

Nitrous oxide emissions from drained organic forest soils––an up-scaling based on C:N ratios

Total emissions of N₂O from drained organic forest soils in Sweden were estimated using an equation linking the C:N ratio of the soil to N₂O emissions. Information on soil C:N ratios was derived from a national database. It was estimated that the emissions from Histosols amount to 2,820 tonnes N₂O a⁻¹. This is almost five times the value calculated for the same soils using the method suggested by the Intergovernmental Panel on Climate Change: 580 tonnes N₂O a⁻¹. The higher value in the present study can mainly be explained by improved accuracy of estimates of N₂O emissions from nutrient-rich soils, including former agricultural soils. In Sweden, in addition to 0.94 Mha of drained Histosols, there are 0.55 Mha of other types of drained organic soils. The annual emissions from these soils were estimated to amount to 1,890 tonnes of N₂O. The total emission value calculated for drained organic forest soils was thus 4,700 tonnes N₂O a⁻¹, which, if added, would increase the current estimate of the Swedish anthropogenic N₂O source strength by 18%. Of these emissions, 88% occur from sites with C:N ratios lower than 25. The exponential relationship between C:N ratio and N₂O emissions, in combination with a scarcity of data, resulted in large confidence intervals around the estimates. However, by using the C:N ratio-based method, N₂O emission estimates can be calculated from a variable that is readily available in databases. Also, the recent findings that there are exceptionally large emissions of N₂O from the most nitrogen-rich drained organic forest soils are taken into account. This article has previously been published in 84/2 under doi: .     

Soil N chemistry in oak forests along a nitrogen deposition gradient

Anthropogenic N deposition may change soil conditions in forest ecosystems as demonstrated in many studies of coniferous forests, whereas results from deciduous forests are relatively scarce. Therefore the influence of N deposition on several variables was studied in situ in 45 oak-dominated deciduous forests along a N deposition gradient in southern Sweden, where the deposition ranged from 10 to 20 kg N ha⁻¹ year⁻¹. Locally estimated NO ₋ ³ deposition, as measured with ion-exchange resins (IER) on the soil surface, and grass N concentration (%) were positively correlated with earlier modelled regional N deposition. Furthermore, the δ¹⁵N values of grass and uppermost soil layers were negatively correlated with earlier modelled N deposition. The data on soil NO ₋ ³ , measured with IER in the soil, and grass N concentration suggest increased soil N availability as a result of N deposition. The δ¹⁵N values of grass and uppermost soil layers indicate increased nitrification rates in high N deposition sites, but no large downward movements of NO ₋ ³ in these soils. Only a few sites had NO ₋ ³ concentrations exceeding 1 mg N l⁻¹ in soil solution at 50 cm depth, which showed that N deposition to these acid oak-dominated forests has not yet resulted in extensive leaching of N. The δ¹⁵N enrichment factor was the variable best correlated with NO ₋ ³ concentrations at 50 cm and is thus a variable that potentially may be used to predict leaching of NO ₋ ³ from forest soils.     

Indicators for nitrogen status and leaching in subtropical forest ecosystems,South China

The deposition of nitrogen (N) is high in subtropical forest in South China and it is expected to increase further in the coming decades. To assess effects of increasing deposition on N cycling, we investigated the current N status of two selected 40–45-year-old masson pine-dominated Chinese subtropical forest stands at Tieshanping (TSP, near Chongqing City) and Caijiatang (CJT in Shaoshan, Hunan province), and explored the applicability of several indicators for N status and leaching, suggested for temperate and boreal forest ecosystems. Current atmospheric N deposition to the systems is from 25 to 49 kg ha⁻¹ year⁻¹. The concentration of total N in the upper 15 cm of the soil is from as low as 0.05% in the B₂ horizon to as high as 0.53% in the O/A horizon. The concentration of organic carbon (C) varies from 0.74 (B₂) to 9.54% (O/A). Pools of N in the upper 15 cm of the soils range from 1460 to 2290 kg N ha⁻¹, where 25–55% of the N pool is in the O/A horizon (upper 3 cm of the soil). Due to a lack of a well-developed continuous O horizon (forest floor), the C/N ratio of this layer cannot be used as an indicator for the N status, as is commonly done in temperate and boreal forests. The net N mineralization rate (mg N g⁻¹ C year⁻¹) in individual horizons correlates significantly with the C/N ratio, which is from as high as 18.2 in the O/A horizon to as low as 11.2 in the B₂ horizon. The N₂O emission flux from soil is significantly correlated with the KCl extractable NH₄⁺–N in the O/A horizon and with the net nitrification in the upper 15 cm of the soil. However, the spatial and temporal variation of the N₂O emission rate is high and rates are small and often difficult to detect in the field. The soil flux density of mineral N, defined as the sum of the throughfall N input rate and the rate of in situ net N mineralization in the upper 15 cm of the soil, i.e., the combination of deposition input and the N status of the system, explains the NO₃⁻ leaching potential at 30 cm soil depth best. The seasonality of stream water N concentration at TSP and CJT is climatic and hydrologically controlled, with highest values commonly occurring in the wet growing season and lowest in the dry dormant season. This is different from temperate forest ecosystems, where N saturation is indicated by elevated NO₃⁻ leaching in stream water during summer.     

Carbon, nitrogen and phosphorus in volcanic soils following afforestation with native birch (Betula pubescens) and introduced larch (Larix sibirica) in Iceland

Afforestation has become an important tool for soil protection and land reclamation in Iceland. Nevertheless, the harsh climate and degraded soils are growth-limiting for trees, and little is know about changes in soil nutrients in maturing forests planted on the volcanic soils. In the present chronosequence study, changes in C, N and total P in soil (0–10 and 10–20 cm depth) and C and N in foliar tissue were investigated in stands of native Downy birch (Betula pubescens Enrh.) and the in Iceland introduced Siberian larch (Larix sibirica Ledeb.). The forest stands were between 14 and 97 years old and were established on heath land that had been treeless for centuries. Soils were Andosols derived from basaltic material and rhyolitic volcanic ash. A significant effect of tree species was only found for the N content in foliar tissue. Foliar N concentrations were significantly higher and foliar C/N ratios significantly lower in larch needles than in birch leaves. There was no effect of stand age. Changes in soil C and the soil nutrient status with time after afforestation were little significant. Soil C concentrations in 0–10 cm depth in forest stands older than 30 years were significantly higher than in heath land and forest stands younger than 30 years. This was attributed to a slow accumulation of organic matter. Soil N concentrations and soil P_tot were not affected by stand age. Nutrient pools in the two soil layers were calculated for an average weight of soil material (400 Mg soil ha⁻¹ in 0–10 cm depth and 600 Mg soil ha⁻¹ in 10–20 cm depth, respectively). Soil nutrient pools did not change significantly with time. Soil C pools were in average 23.6 Mg ha⁻¹ in the upper soil layer and 16.9 Mg ha⁻¹ in the lower soil layer. The highest annual increase in soil C under forest compared to heath land was 0.23 Mg C ha⁻¹ year⁻¹ in 0–10 cm depth calculated for the 53-year-old larch stand. Soil N pools were in average 1.0 Mg N ha⁻¹ in both soil layers and did not decrease with time despite a low N deposition and the uptake and accumulation of N in biomass of the growing trees. Soil P_tot pools were in average 220 and 320 kg P ha⁻¹ in the upper and lower soil layer, respectively. It was assumed that mycorrhizal fungi present in the stands had an influence on the availability of N and P to the trees. Responsible Editor: Hans Lambers.     

Plant and soil carbon accumulation following fire in Mediterranean woodlands in Spain

We measured plant and soil carbon (C) storage following canopy-replacing wildfires in woodlands of northeastern Spain that include an understory of shrubs dominated by Quercus coccifera and an overstory of Pinus halepensis trees. Established plant succession models predict rapid shrub recovery in these ecosystems, and we build on this model by contrasting shrub succession with long-term C storage in soils, trees, and the whole ecosystem. We used chronosequence and repeated sampling approaches to detect change over time. Aboveground plant C increased from <100 to ~3,000 g C m⁻² over 30 years following fire, which is substantially less than the 5,942 ± 487 g C m⁻² (mean ±1 standard error) in unburned sites. As expected, shrubs accumulated C rapidly, but the capacity for C storage in shrubs was <600 g C m⁻². Pines were the largest plant C pool in sites >20 years post fire, and accounted for all of the difference in plant C between older burned sites and unburned sites. In contrast, soil C was initially higher in burned sites (~4,500 g C m⁻²) than in unburned sites (3,264 ± 261 g C m⁻²) but burned site C declined to unburned levels within 10 years after fire. Combining these results with prior research suggests two states for C storage. When pine regeneration is successful, ~9,200 g C m⁻² accumulate in woodlands but when tree regeneration fails (due to microclimatic stress or short fire return intervals), ecosystem C storage of ~4,000 g C m⁻² will occur in the resulting shrublands.