Nitrate leaching from three afforestation chronosequences on former arable land in Denmark

In regions dominated by agricultural activities, nitrogen (N) is recognized as a major pollutant in aquatic environments. In north‐western Europe, afforestation of agricultural land is part of a strategy to improve water quality. In Denmark, former arable land has been afforested during the past 40–50 years. This study evaluated the effect of afforestation of former arable land on nitrate leaching, based on three afforestation chronosequences. Precipitation, canopy throughfall and soil water were collected and soil moisture was monitored at two Danish locations, Vestskoven (nutrient‐rich, medium deposition) and Gejlvang (nutrient‐poor, high deposition). Afforestation was performed using Norway spruce [Picea abies (Karst.) L.] and common oak (Quercus robur L.) at Vestskoven and Norway spruce at Gejlvang. The results suggest that afforestation of former arable land initially leads to lower nitrate leaching than that occurring under the former agricultural land use, and largely below the standard of 50 mg NO⁻₃ L⁻¹ for groundwater to be utilized as drinking water. Nitrate concentrations became almost negligible in forest stands of 5–20 years of age. However, after canopy closure (>20 years) nitrate concentrations below the root zone and nitrate leaching tended to increase. This was attributed to increased N deposition with increasing canopy development and decreased N demand once the most N‐rich biomass compartments had been built up. Nitrate leaching started to increase at a throughfall deposition level of about 10 kg N ha⁻¹ yr⁻¹. Compared with nutrient‐poor sandy soils, nutrient‐rich clayey soils appeared more vulnerable to disturbance of the N cycle and to increased N deposition, leading to N saturation and enhanced nitrate leaching. In approximately the first 35 years after afforestation, nitrate leaching below the root zone was generally higher below oak than below Norway spruce.     

Afforestation effects on SOC in former cropland: oak and spruce chronosequences resampled after 13 years

Chronosequences are commonly used to assess soil organic carbon (SOC) sequestration after land‐use change, but SOC dynamics predicted by this space‐for‐time substitution approach have rarely been validated by resampling. We conducted a combined chronosequence/resampling study in a former cropland area (Vestskoven) afforested with oak (Quercus robur) and Norway spruce (Picea abies) over the past 40 years. The aims of this study were (i) to compare present and previous chronosequence trends in forest floor and top mineral soil (0–25 cm) C stocks; (ii) to compare chronosequence estimates with current rates of C stock change based on resampling at the stand level; (iii) to estimate SOC changes in the subsoil (25–50 cm); and (iv) to assess the influence of two tree species on SOC dynamics. The two chronosequence trajectories for forest floor C stocks revealed consistently higher rates of C sequestration in spruce than oak. The chronosequence trajectory was validated by resampling and current rates of forest floor C sequestration decreased with stand age. Chronosequence trends in topsoil SOC in 2011 did not differ significantly from those reported in 1998, however, there was a shift from a negative rate (1998: ?0.3 Mg C ha^?1 yr^?1) to no change in 2011. In contrast SOC stocks in the subsoil increased with stand age, however, not significantly (P = 0.1), suggesting different C dynamics in and below the former plough layer. Current rates of C change estimated by repeated sampling decreased with stand age in forest floors but increased in the topsoil. The contrasting temporal change in forest floor and mineral soil C sequestration rates indicate a shift in C source‐sink strength after approximately 40 years. We conclude that afforestation of former cropland within the temperate region may induce soil C loss during the first decades followed by a recovery phase of yet unknown duration.     

Soil Microbial Biomass,Basal Respiration and Enzyme Activity of Main Forest Types in the Qinling Mountains

Different forest types exert essential impacts on soil physical-chemical characteristics by dominant tree species producing diverse litters and root exudates, thereby further regulating size and activity of soil microbial communities. However, the study accuracy is usually restricted by differences in climate, soil type and forest age. Our objective is to precisely quantify soil microbial biomass, basal respiration and enzyme activity of five natural secondary forest (NSF) types with the same stand age and soil type in a small climate region and to evaluate relationship between soil microbial and physical-chemical characters. We determined soil physical-chemical indices and used the chloroform fumigation-extraction method, alkali absorption method and titration or colorimetry to obtain the microbial data. Our results showed that soil physical-chemical characters remarkably differed among the NSFs. Microbial biomass carbon (Cmic) was the highest in wilson spruce soils, while microbial biomass nitrogen (Nmic) was the highest in sharptooth oak soils. Moreover, the highest basal respiration was found in the spruce soils, but mixed, Chinese pine and spruce stands exhibited a higher soil qCO₂. The spruce soils had the highest Cmic/Nmic ratio, the greatest Nmic/TN and Cmic/Corg ratios were found in the oak soils. Additionally, the spruce soils had the maximum invertase activity and the minimum urease and catalase activities, but the maximum urease and catalase activities were found in the mixed stand. The Pearson correlation and principle component analyses revealed that the soils of spruce and oak stands obviously discriminated from other NSFs, whereas the others were similar. This suggested that the forest types affected soil microbial properties significantly due to differences in soil physical-chemical features.     

Patterns and mechanisms of soil acidification in the conversion of grasslands to forests

Grassland to forest conversions currently affect some of the world's most productive regions and have the potential to modify many soil properties. We used afforestation of native temperate humid grassland in the Pampas with eucalypts as an experimental system to 1) isolate forest and grassland imprints on soil acidity and base cation cycling and 2) evaluate the mechanisms of soil acidification. We characterized soil changes with afforestation using ten paired stands of native grasslands and Eucalyptus plantations (10–100 years of age). Compared to grasslands, afforested stands had lower soil pH (4.6 vs.5.6, p < 0.0001) and 40% lower exchangeable Ca (p < 0.001) in the top 20 cm of the soil. At three afforested stands where we further characterized soil changes to one meter depth, soil became increasingly acidic from 5 to 35 cm depth but more alkaline below 60 cm compared to adjacent grasslands, with few differences observed between 35 and 60 cm. These changes corresponded with gains of exchangeable acidity and Na in intermediate and deeper soil layers. Inferred ecosystem cation balances (biomass + forest floor + first meter of mineral soil) revealed substantial vertical redistributions of Ca and Mn and a tripling of Na pools within the mineral soil after afforestation. Soil exchangeable acidity increased 0.5–1.2 kmol_c.Ha^–1.yr^–1 across afforested stands, although no aboveground acidic inputs were detected in wet + dry deposition, throughfall and forest floor leachates. Our results suggest that cation cycling and redistribution by trees, rather than cation leaching by organic acids or enhanced carbonic acid production in the soil, is the dominant mechanism of acidification in this system. The magnitude of soil changes that we observed within half a century of tree establishment in the Pampas emphasizes the rapid influence of vegetation on soil formation and suggests that massive afforestation of grasslands for carbon sequestration could have important consequences for soil fertility and base cation cycles.     

Production,standing biomass and natural abundance of <Superscript>15</Superscript>N and <Superscript>13</Superscript>C in ectomycorrhizal mycelia collected at different soil depths in two forest types

Nutrient uptake by forest trees is dependent on ectomycorrhizal (EM) mycelia that grow out into the soil from the mycorrhizal root tips. We estimated the production of EM mycelia in root free samples of pure spruce and mixed spruce-oak stands in southern Sweden as mycelia grown into sand-filled mesh bags placed at three different soil depths (0–10, 10–20 and 20–30 cm). The mesh bags were collected after 12 months and we found that 590±70 kg ha^–1 year^–1 of pure mycelia was produced in spruce stands and 420±160 kg ha^–1 year^–1 in mixed stands. The production of EM mycelia in the mesh bags decreased with soil depth in both stand types but tended to be more concentrated in the top soil in the mixed stands compared to the spruce stands. The fungal biomass was also determined in soil samples taken from different depths by using phospholipid fatty acids as markers for fungal biomass. Subsamples were incubated at 20°C for 5 months and the amount of fungal biomass that degraded during the incubation period was used as an estimate of EM fungal biomass. The EM biomass in the soil profile decreased with soil depth and did not differ significantly between the two stand types. The total EM biomass in the pure spruce stands was estimated to be 4.8±0.9×10³ kg ha^–1 and in the mixed stands 5.8±1.1×10³ kg ha^–1 down to 70 cm depth. The biomass and production estimates of EM mycelia suggest a very long turnover time or that necromass has been included in the biomass estimates. The amount of N present in EM mycelia was estimated to be 121 kg N ha^–1 in spruce stands and 187 kg N ha^–1 in mixed stands. The ¹³C value for mycelia in mesh bags was not influenced by soil depth, indicating that the fungi obtained all their carbon from the tree roots. The ¹³C values in mycelia collected from mixed stands were intermediate to values from pure spruce and pure oak stands suggesting that the EM mycelia received carbon from both spruce and oak trees in the mixed stands. The ¹⁵N value for the EM mycelia and the surrounding soil increased with soil depth suggesting that they obtained their entire N from the surrounding soil.     

Deposition of ammonia to the forest floor under spruce and beech at the Höglwald site

Laboratory and field measurements of the flux of ammonia to forest floor canopies of spruce and beech stands at the Höglwald site in southern Bavaria are reported. Measurements were performed with an open chamber method. A linearity between ammonia concentration and ammonia flux from the atmosphere to the ground floor canopy was detected. Deposition of ammonia showed no saturation even at air concentrations up to 50 g NH₃ m^–3 air. Temperature, water content and the moss layer of the ground floor canopy had a minor influence on the deposition velocity in laboratory experiments. Deposition velocity of ammonia was higher to the spruce (1.3 cm s^–1), and limed spruce ground floor canopy (1.17 cm s^–1) compared to the beech stand (0.79 cm s^–1). In field studies, a diurnal course of the deposition velocity was detected with highest velocities in midday and minor during night times, but not in the climatic chamber. The flux of ammonia to the ground floor canopy was estimated of app. 10 kg N ha^–1 yr^–1 for the soil under spruce, 9 kg N ha^–1 yr^–1 for the limed spruce and 6 kg N ha^–1yr^–1 for the soil under beech. The fluxes are interpreted as fluxes from the atmosphere to the ground canopies of the stands.     

Afforestation with Norway spruce on a subalpine pasture alters carbon dynamics but only moderately affects soil carbon storage

There is a strong trend toward reforestation of abandoned grasslands in alpine regions which may impact the carbon balance of alpine ecosystems. Here, we studied the effects of afforestation with Norway spruce (Picea abies L.) on an extensively grazed subalpine pasture in Switzerland on soil organic carbon (SOC) cycling and storage. Along a 120-year long chronosequence with spruce stands of 25, 30, 40, 45, and >120 years and adjacent pastures, we measured tree biomass, SOC stocks down to the bedrock, natural ¹³C abundances, and litter quality. To unravel controls on SOC cycling, we have monitored microclimatic conditions and quantified SOC decomposability under standardized conditions as well as soil respiration in situ. Stocks of SOC were only moderately affected by the afforestation: in the mineral soil, SOC stocks transiently decreased after tree establishment, reaching a minimum 40–45 years after afforestation (?25 %) and increased thereafter. Soils of the mature spruce forest stored the largest amount of SOC, 13 % more than the pasture soils, mainly due to the accumulation of an organic layer (23 t C ha^?1). By comparison, C accumulated in the tree biomass exceeded the SOC pool by a factor of three in the old forest. In contrast to the small impact on C storage, afforestation strongly influenced the composition and quality of the soil organic matter (SOM). With increasing stand age, δ¹³C values of the SOM became consistently more positive, which can be interpreted as a gradual replacement of grass- by spruce-derived C. Fine roots of spruce were enriched in ¹³C, in lignin and had a higher C/N ratio in comparison to grass roots. As a consequence, SOM quality as indicated by the lower fraction of readily decomposable (labile) SOM and higher C:N ratios declined after the land-use change. Furthermore, spruce plantation induced a less favorable microclimate for microbial activity with the average soil temperature during the growing season being 5 °C lower in the spruce stands than in the pasture. In situ soil respiration was approximately 50 % lower after the land use conversion, which we primarily attribute to the colder conditions and the lower SOM quality, but also to drier soils (?25 %) and to a decreased fine root biomass (?40 %). In summary, afforestation on subalpine pastures only moderately affected SOC storage as compared to the large C sink in tree biomass. In contrast, SOC cycling rates strongly decreased as a result of a less favorable microclimate for decomposition of SOM, a lower C input by roots, and a lower litter quality.     

The effect of afforestation with Scots pine (Pinus silvestris L.) of sandy post-arable soils on their selected properties. II. Reaction, carbon, nitrogen and phosphorus

Despite the extensive literature on the effect on soil properties of afforestation of former arable land, we still lack full understanding of whether the changes proceed in the same direction and at the same rate, and of how long is required to achieve a state of soil equilibrium typical of a natural forest ecosystem. Therefore, as part of a study comparing post-arable sandy soils (Dystric Arenosols) afforested with Scots pine (Pinus silvestris L.) with arable soils and soils of continuous coniferous forests, the range and direction of changes in pH, organic carbon (C_org), total nitrogen (N_tot), ammonium (N-NH₄) and nitrates (N-NO₃) in soil solution, total (P_tot) and available (P_av) phosphorus were determined. The studies were carried out in south-east Poland (51°30′-51°37′N, 22°20′-22°35′E). Ten paired sites of afforested soils (five with 14- to 17-year-old stands and five with 32- to 36-year-old stands) with adjacent cultivated fields, and five sites of continuous forest with present stands of ca. 130–150 years old were selected. Soil samples were taken from the whole thickness of master horizons and, in the case of the A horizon of the afforested soils, from three layers: 0–5 (A_0–5), 5–10 (A_5–10) and 10–20 cm (A_10–20). The cultivated soils in the Ap horizon showed higher pH (by ca. 1.0 unit), lower C_org and C:N, similar N_tot, lower N-NH₄, higher N-NO₃, higher P_tot and P_av contents compared with the Ah horizon of continuous forest soils. The results indicated decreased soil pH in the former plough layer of the afforested soils, with the greatest decrease observed in the 0–5 cm layer. In these soils, the C_org content was considerably higher in the A_0–5 layer, but lower in the two deeper layers and in the whole A horizon (0–20 cm) compared with the Ap horizon of the arable soils. The results indicate that the C_org content, after an initial phase of decline, again achieved a level characteristic of arable soils. The N_tot content in all layers of the A horizon of the afforested soils was lower than in the Ap horizon of the arable soils, and showed a reduction with stand age, especially in deeper layers. The C:N ratios in the mineral topsoil increased with stand age. N-NH₄ content increased and N-NO₃ decreased after afforestation. The P_tot and P_av contents in all layers and in the whole A horizon of the afforested soils, on stands of both ages, was lower than in the Ap of the cultivated soils. From the results, it could be concluded that, after more than 30 years of tree growth, the soils of the A horizon were still more similar to arable than to continuous forest soils with respect to C_org, P_tot and P_av. With respect to pH, N-NH₄ and N-NO₃, especially in the 0–5 cm layer, they were more similar to continuous forest soils than to cultivated soils, but with respect to N_tot and C:N ratio they were somewhere in between.     

Variation in the herb species response and the humus quality across a 200-year chronosequence of beech and oak plantations in Belgium

The present study aimed at exploring the long-term impact of pure and mixed beech Fagus sylvatica and oak Quercus robur stands on the forest floor by documenting changes in the herb species' behaviour and in humus index across a 200-yr chronosequence of forest stands. The research was conducted in central Belgium, in a 4383 ha beech-dominated forest. Analyses were carried out in stands which are replicated, of the same age, managed in the same way, and growing on the same soil type with the same land-use history. The results of this study indicate that stand aging is an important determinant of herb species occurrence in the studied area. Most of the species studied show a different response to stand age in pure compared to mixed stands. Our results clearly show a decrease of the humus quality with age in pure stands (beech as well as oak). On the other hand, we found that mixing beech and oak maintained or improved the humus status along the chronosequence according to the proportion of each tree. So the addition of some oak to the beech made it possible to keep a constant quality of the humus. We found that, even if the understory tree species is very scarce, it may be sufficient to maintain the humus status on the long term. In the present study, a cover of 1% oak in a beech stand was sufficient to show an effect of the minor species on these soils. This pattern contrasts with the widespread idea that substantial effects of the minor tree species on soils might not develop if the ratio of major/minor species is low.     

Microbial biomass C and N,and mineralizable-N,in litter and mineral soil under Pinus radiata on a coastal sand: Influence of stand age and harvest management

Microbial biomass C and N, and anaerobically mineralizable-N, were measured in the litter and mineral soil (0–10 cm and 10–20 cm depth) of Pinus radiata plantations in two trials on a nitrogen-deficient coastal sand. The trials comprised (a) stands of different age (1 to 33 years), with five of the seven stands studied being second rotation, and (b) a harvest-management trial, with stands established after different harvesting treatments of the first-rotation trees and understorey development controlled by manual weeding and chemical sprays. The harvest-management stands were sampled in the fifth year after the second-rotation establishment.In the stands of different age, the levels of microbial biomass C and N, and also mineralizable-N, in the litter and mineral soil showed no relationship with tree age and were similar to those in the oldest (33 years) stands of P. radiata. In the harvesting trial, five years after establishment of the second rotation, levels of microbial N and mineralizable-N in the litter and mineral soil were generally lowest where whole trees and the original forest floor had been removed; they were higher in associated plots in which the original forest floor had been removed but fertilizer N was regularly applied. No marked differences were then found between the other harvest treatments, viz. whole-tree harvest, stem-only harvest with slash remaining on site, and stem-only harvest plus extra added slash materials. In each trial, levels of microbial C and N and mineralizable-N were closely related to total C, and especially total N, in 0–10 cm depth mineral soil, but not generally in litter. Respiratory measurements strongly suggest that the microbial populations in mineral soil had a high metabolic activity.On an area basis in the harvest-management trial, total tree N and microbial N in the litter and mineral soil were lowest in stands where the original forest floor had been removed. In this particular treatment, microbial N in the litter plus mineral soil (0–20 cm depth) after five years of second-rotation growth comprised 7.3% of the total ecosystem N; values in the other treatments ranged between 5.6 and 6.0%.Our results emphasise the importance of slash and litter, and probably volunteer shrubs and herbaceous under-storey species, in conserving pools of potentially available N during the early stages of tree development.     

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.     

Relationship between decomposition of cellulose in the soil and tree stand characteristics in natural boreal forests

The relationship between the decomposition of cellulose placed on and buried in the forest floor and various tree stand characteristics was studied at sites with minimal anthropogenic influence. The 22 study sites, including both forested upland and peatland plots, were clustered in 4 catchments between 61°–69° N in Finland. The stands were 60 to 320 years old and composed of varying proportions of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies Karst.) and deciduous species (mainly t Betula spp.). Cellulose strips (softwood pulp) were placed on the forest floor surface and buried at four depths down to 5 cm for two 1-year periods and the weight loss measured. Decomposition did not significantly correlate with stand age, but was strongly and positively correlated with stand basal area, mean tree height and stem volume. This was valid at all depths, and even after differences due to climate between catchments had been taken into account. The stem volume of Scots pine dominated plots had the highest correlation. Our results showed that decomposition of organic matter on and in the forest floor is related to the stand characteristics. This relationship should be considered when comparing soil processes different stands, even when comparing stands of the same tree species composition.     

Increasing soil methane sink along a 120‐year afforestation chronosequence is driven by soil moisture

Upland soils are important sinks for atmospheric methane (CH₄), a process essentially driven by methanotrophic bacteria. Soil CH₄ uptake often depends on land use, with afforestation generally increasing the soil CH₄ sink. However, the mechanisms driving these changes are not well understood to date. We measured soil CH₄ and N₂O fluxes along an afforestation chronosequence with Norway spruce (Picea abies L.) established on an extensively grazed subalpine pasture. Our experimental design included forest stands with ages ranging from 25 to >120 years and included a factorial cattle urine addition treatment to test for the sensitivity of soil CH₄ uptake to N application. Mean CH₄ uptake significantly increased with stand age on all sampling dates. In contrast, CH₄ oxidation by sieved soils incubated in the laboratory did not show a similar age dependency. Soil CH₄ uptake was unrelated to soil N status (but cattle urine additions stimulated N₂O emission). Our data indicated that soil CH₄ uptake in older forest stands was driven by reduced soil water content, which resulted in a facilitated diffusion of atmospheric CH₄ into soils. The lower soil moisture likely resulted from increased interception and/or evapotranspiration in the older forest stands. This mechanism contrasts alternative explanations focusing on nitrogen dynamics or the composition of methanotrophic communities, although these factors also might be at play. Our findings further imply that the current dramatic increase in forested area increases CH₄ uptake in alpine regions.     

Does soil acidity reduce subsoil rooting in Norway spruce (Picea abies)?

Increasing evidence suggests that forest soils in central and northern Europe as well as in North America have been significantly acidified by acid deposition during the last decades. The present investigation was undertaken to examine the effect of soil acidity on rooting patterns of 40-year-old Norway spruce trees by comparing fine and coarse roots among four stands which differed in soil acidity and Mg (and Ca) nutrition. The coarse root systems of four to five 40-year-old Norway spruce trees per stand were manually excavated. The sum of cross sectional area (CSA) at 60 cm soil depth and below of all vertical coarse roots, as a measure of vertical rooting intensity, was strongly reduced with increasing subsoil acidity of the stands. This pattern was confirmed when 5 additional acidic sites were included in the analysis. Fine root biomass in the mineral soil estimated by repeated soil coring was strongly reduced in the heavily acidified stands, but increased in the humic layer. Using ingrowth cores and a screen technique, we showed that the higher root biomass in the humic layer of the more acidic stands was a result of higher root production. Thus, reduced fine root biomass and coarse root CSA in deeper soil layers coincided with increased root growth in the humic layer. Root mineral analysis showed Ca/Al ratios decreased with decreasing base saturation in the deeper mineral soil (20–40 cm). In the top mineral soil, only minor differences were observed among stands. In general, low Ca/Al ratios coincided with low fine root biomass. Calcium/aluminum ratios determined in cortical cell walls using X-ray microanalysis showed a similar pattern as Ca/Al ratios based on analysis of whole fine roots, although the amplitude of changes among the stands was much greater. Aluminum concentrations and Ca/Al ratios in cortical cell walls were at levels found to inhibit root growth of spruce seedlings in laboratory experiments. The data support the idea that Al (or Ca/Al ratios) and acid deposition-induced Mg (and possibly Ca) deficiency are important factors influencing root growth and distribution in acidic forest soils. Changes in carbon partitioning within the root system may contribute to a reduction in deep root growth.     

Soil Organic Carbon and Water Retention after Conversion of Grasslands to Pine Plantations in the Ecuadorian Andes

Tree plantations in the high elevations of the tropics constitute a growing land use, but their effect on ecosystem processes and services is not well known. We examined changes in soil organic carbon (C) and water retention in a chronosequence of Pinus radiata stands planted in páramo grasslands in Cotopaxi province, Ecuador. Water retention at 10, 33, and 1,500 kPa declined with stand age, with soils in the oldest pine stands retaining 39%, 55%, and 63% less water than grassland soils at the three pressures tested. Soil organic C in the 0–10-cm depth also declined with stand age, from 5.0 kg m^–2 in grasslands to 3.5 kg m^–2 in 20–25-year-old pine stands (P < 0.001); at greater depth in the A horizon, C contents decreased from 2.8 to 1.2 kg m^–2 (P = 0.047). There were no significant differences among age classes in the AC and C horizons (P = 0.15 and P = 0.34, respectively), where little or no weathering of the primary material has occurred. Inputs of C may be affected by the significantly higher carbon–nitrogen (C:N) ratio of the litter under older pine stands (P = 0.005), whereas outputs are influenced by substrate quality as well as soil environmental factors. Soil ratios at the 0–10 cm depth were significantly higher in grasslands and young pine stands (P < 0.001), whereas carbon–phosphorous (C:P) ratios at 0–10-cm depth followed a similar but not significant trend. However, there was no significant difference in short-term decomposition rates (P = 0.60) when the soils were incubated under uniform temperature and moisture conditions. In páramo ecosystems, where high soil moisture plays an important role in retarding decomposition and driving high C storage, the loss of water retention after afforestation may be the dominant factor in C loss. These results suggest that soil C buildup and water retention respond rapidly to changes in biota and need to be assessed with regard to implications for C sequestration and watershed management.     

Scarification,fertilization and herbicide treatment effects on planted conifers and soil fertility

The influences of soil surface modification (blade scarification and plastic mulching), fertilization and herbicide application on soil nutrient and organic carbon content and tree growth and foliar nutrient status were examined after seven years in a study located within the Great Lakes-St. Lawrence forest region of Canada. Plots had been planted with white pine (Pinus strobus L.) and white spruce (Picea glauca [Moench] Voss) seedlings. Light (PAR), soil moisture and temperature were monitored and recorded throughout the growing season. Forest floor and soil mineral (0–20 cm layer) samples were collected from all experimental plots, except those which had plastic mulching. Foliar samples were collected in autumn and analysed for N, P and K and storage compounds. Seedling mortality was 20% higher in unscarified plots. Combined silvicultural treatments increased productivity as much as 14 times, but scarification reduced soil carbon and nutrient capital 2–3 fold. Herbicide application reduced soil carbon by at least 20 %. Foliar nutrient, protein, starch and lipid contents in autumn were little affected by treatment. The future management of such stands in Canada probably will include more shelterwood harvesting and crop rotations, silvicultural systems that are more closely aligned with natural forest succession.     

Impact of common European tree species and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) on the physicochemical properties of the rhizosphere

Trees play a crucial role in nutrient cycling and ecosystem fertility, notably through rhizosphere processes. The aim of this study was to compare soil physicochemical properties between bulk soil and rhizosphere of several tree species, and to compare rhizosphere properties between fertilized and non-fertilized conditions. The soil sampling was performed in Breuil-Chenue forest (North-East of France) in seven stands: native forest (old beech (Fagus sylvatica L.) and oak (Quercus sessiliflora Smith) coppice with standards; CwS), beech, oak (Quercus petraea [Matt.] Liebl.), Douglas-fir and fertilised Douglas-fir, Norway spruce (Picea abies Karst.) and fertilised Norway spruce. Systematic soil sampling was performed at 0–3, 3–10, and 10–23 cm in 20 calibrated pits. The rhizosphere of the different species was generally enriched in C, N, Ca, Mg, and K. Interestingly, the same positive effect was observed in the fertilised plots. The rhizosphere effect varied between tree species for C, “base” cations, pH_water and cation exchange capacity. This study reveals that interactions between roots, microorganisms and soil can enrich the pool of nutrients in the rhizosphere compared to bulk soil whatever the soil fertility conditions, and that the magnitude of the rhizosphere effect depends on tree species.     

N uptake and N status in ponderosa pine as affected by soil compaction and forest floor removal

Soil compaction and forest floor removal influence fundamental soil processes that control forest productivity and sustainability. We investigated effects of soil compaction and forest floor removal on tree growth, N uptake and N status in ponderosa pine. Factorial combinations of soil compaction (non-compacted and compacted) and forest floor removal (forest floor present and no forest floor) were applied to three different surface soil textures. For studying N uptake, four trees from every treatment were ¹⁵N labeled with 130.6 mg m^–2 of ¹⁵N. Tree responses to compaction were dependent on the forest floor removal level. In loam and clay soils, non-compacted+no forest floor was beneficial to tree growth. Tree growth was depressed with compaction+no forest floor in clay soil. In sandy loam soil, compaction+no forest floor showed the best tree growth. No N deficiency was found in any soil type but a graphical method suggested correlation between N status and tree growth. In loam and clay soils, compaction+forest floor present increased N uptake. Nitrogen uptake was explained significantly by potential N mineralization in loam and clay soils. In sandy loam soil, the effects of compaction and forest floor removal were more complex, with the N uptake improved in the compaction+no forest floor treatment and reduced under compaction+forest floor present. Soil compaction may have influenced N tracer uptake because of improved unsaturated flow and root-soil contact. However, N immobilization may have restricted N uptake in compaction+forest floor present in the sandy loam soil. The study illustrates how soil properties and site preparation can potentially interact to affect N dynamics and forest productivity.     

Does Forest Continuity Enhance the Resilience of Trees to Environmental Change?

There is ample evidence that continuously existing forests and afforestations on previously agricultural land differ with regard to ecosystem functions and services such as carbon sequestration, nutrient cycling and biodiversity. However, no studies have so far been conducted on possible long-term (>100 years) impacts on tree growth caused by differences in the ecological continuity of forest stands. In the present study we analysed the variation in tree-ring width of sessile oak (Quercus petraea (Matt.) Liebl.) trees (mean age 115–136 years) due to different land-use histories (continuously existing forests, afforestations both on arable land and on heathland). We also analysed the relation of growth patterns to soil nutrient stores and to climatic parameters (temperature, precipitation). Tree rings formed between 1896 and 2005 were widest in trees afforested on arable land. This can be attributed to higher nitrogen and phosphorous availability and indicates that former fertilisation may continue to affect the nutritional status of forest soils for more than one century after those activities have ceased. Moreover, these trees responded more strongly to environmental changes – as shown by a higher mean sensitivity of the tree-ring widths – than trees of continuously existing forests. However, the impact of climatic parameters on the variability in tree-ring width was generally small, but trees on former arable land showed the highest susceptibility to annually changing climatic conditions. We assume that incompletely developed humus horizons as well as differences in the edaphon are responsible for the more sensitive response of oak trees of recent forests (former arable land and former heathland) to variation in environmental conditions. We conclude that forests characterised by a long ecological continuity may be better adapted to global change than recent forest ecosystems.     

Net nitrogen mineralization and net nitrification along a tropical forest-to-pasture chronosequence

Soil inorganic nitrogen pools, net mineralization and net nitrification rates were compared during the dry season along a chronosequence of upland (terra firme) forest, 3-, 9- and 20-year-old pastures in the western Brazilian Amazon Basin state of Rondônia to investigate the influence of forest conversion to pasture on soil nitrogen cycles. Surface soil (0 to 10 cm) from forest had larger extractable inorganic nitrogen pools than pasture soils. In the forest, NO ₃ ^– pools equaled or exceeded NH ₄ ⁺ pools, while pasture inorganic N pools consisted almost exclusively of NH ₄ ⁺ . Rates of net N mineralization and net nitrification in seven -day laboratory incubations were higher in the seven - day forest than in the pastures. Net N mineralization rates did not differ significantly among different-aged pastures, but net nitrification rates were significantly lower in the 20-year-old pasture. Higher net N mineralization and net nitrification rates were measured in laboratory and in situ incubations of sieved soil, compared with in situ incubations of intact soil cores. Rates calculated in seven-day incubations were higher than determined by longer incubations. Sieving may increase N mineralization and/or decrease N immobilization compared with intact cores. We concluded that 7-day laboratory incubation of sieved soil was the most useful index for comparing N availability across the chronosequence of forest and pasture sites. High net nitrification rates in forest soils suggest a potential for NO ₃ ^– losses either through leaching or gaseous emissions.