The effect of understory removal on microclimate and soil properties in two subtropical lumber plantations

Understory vegetation is an important component in forest ecosystems. However, the effects of understory on soil properties in subtropical forests are not fully understood. We thus conducted an experimental manipulative study in two young fast-growing plantations—Eucalyptus urophylla and Acacia crassicarpa—in southern China, by removing understory vegetation in both plantations, to estimate the effects of understory vegetation on microclimate, soil properties and N mineralization. Our data showed that, after 6 months, understory removal (UR) in both plantations had greatly increased soil surface luminous intensity (90–500 cd) and temperature (0.5–0.8 °C); soil moisture was reduced in the Eucalyptus plantation but not in the Acacia plantation. Understory removal also reduced soil organic matter (SOM), but had little impact on other soil chemical properties, including total phosphorus, C/N, pH, exchangeable cations (K, Ca, Mg), available P, ande extractable NH₄–N and NO₃–N. We found a significant decline of soil N mineralization and nitrification rates in the 0–5 cm soils of UR in both plantations. The decline of SOM in UR may contribute to the lower N transformations rates. This study indicates that a better understanding of understory vegetation effects on soil N cycling would be beneficial to forest management decisions and could provide a critical foundation for advancing management practices.     

Effect of six years of nitrogen additions on soil chemistry in a subtropical <Emphasis Type="Italic">Pleioblastus amarus</Emphasis> forest,Southwest China

Soil chemistry influences plant health and carbon storage in forest ecosystems. Increasing nitrogen (N) deposition has potential effect on soil chemistry. We studied N deposition effects on soil chemistry in subtropical Pleioblastus amarus bamboo forest ecosystems. An experiment with four N treatment levels (0, 50, 150, and 300 kg N ha^?1 a^?1, applied monthly, expressed as CK, LN, MN, HN, respectively) in three replicates. After 6 years of N additions, soil base cations, acid-forming cations, exchangeable acidity (EA), organic carbon fractions and nitrogen components were measured in all four seasons. The mean soil pH values in CK, LN, MN and HN were 4.71, 4.62, 4.71, and 4.40, respectively, with a significant difference between CK and HN. Nitrogen additions significantly increased soil exchangeable Al³⁺, EA, and Al/Ca, and exchangeable Al³⁺ in HN increased by 70% compared to CK. Soil base cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) did not respond to N additions. Nitrogen treatments significantly increased soil NO₃^?–N but had little effect on soil total nitrogen, particulate organic nitrogen, or NH₄⁺–N. Nitrogen additions did not affect soil total organic carbon, extractable dissolved organic carbon, incorporated organic carbon, or particulate organic carbon. This study suggests that increasing N deposition could increase soil NO₃^?–N, reduce soil pH, and increase mobilization of Al³⁺. These changes induced by N deposition can impede root grow and function, further may influence soil carbon storage and nutrient cycles in the future.     

pH and substrate regulation of nitrogen and carbon dynamics in forest soils in a karst region of the upper Yangtze River basin, China

Nitrogen (N) is the most limiting nutrient for forest development. In this study, using a natural pH gradient, N and carbon (C) mineralization was investigated and the effects of soil pH as well as the total C and N contents and the soil C/N ratio were evaluated in forest soils after mountain closure in a karst region. The N availability was poor based on the low N mineralization potential (N ₀) and the low active fraction of soil total N (N ₀/total N), while high microbial decomposition activity was indicated by a high mineralization rate constant (k _N). N ₀ was positively correlated with soil pH as well as the total C and N contents. Additionally, multiple regression analysis revealed that total CN (the product of the soil total C and total N contents) and the C/N ratio had more significant effects on N ₀ than soil pH. In contrast, the mineralization rate constants k _N and k _C were positively affected by soil pH. The results indicated that N availability was regulated by soil organic matter (SOM), while microbial activity was restricted by soil pH. Also, the lack of nitrification and the high C ₀/N ₀ ratio observed at soil pH <5.5 may be a strong indicator of alterations to the microbial composition prompted by severe soil acidification. Further research is required to determine the changes in soil microbial composition with the drop in soil pH and their effects on SOM decomposition and nutrient availabilities.     

Impacts of low-intensity prescribed fire on microbial and chemical soil properties in a <Emphasis Type="Italic">Quercus frainetto</Emphasis> forest

Prescribed fire is a common economical and effective forestry practice, and therefore it is important to understand the effects of fire on soil properties for better soil management. We investigated the impacts of low-intensity prescribed fire on the microbial and chemical properties of the top soil in a Hungarian oak (Quercus frainetto Ten.) forest. The research focused on microbial soil parameters (microbial soil respiration (R_SM), soil microbial biomass carbon (Cmic) and metabolic quotient (qCO₂) and chemical topsoil properties (soil acidity (pH), electrical conductivity (EC), carbon (C), nitrogen (N), C/N ratio and exchangeable cations). Mean annual comparisons show significant differences in four parameters (C/N ratio, soil pH, Cmic and qCO₂) while monthly comparisons do not reveal any significant differences. Soil pH increased slightly in the burned plots and had a significantly positive correlation with exchangeable cations Mg, Ca, Mn and K. The mean annual C/N ratio was significantly higher in the burned plots (28.5:1) than in the control plots (27.0:1). The mean annual Cmic (0.6 mg g^?1) was significantly lower although qCO₂ (2.5 µg CO₂–C mg Cmic h^?1) was significantly higher, likely resulting from the microbial response to fire-induced environmental stress. Low-intensity prescribed fire caused very short-lived changes. The annual mean values of C/N ratio, pH, Cmic and qCO₂ showed significant differences.     

The impact of six European tree species on the chemistry of mineral topsoil in forest plantations on former agricultural land

Influences on mineral topsoils of common European tree species (oak-Quercus robur L., lime-Tilia cordata Mill., ash-Fraxinus excelsior L., birch-Betula pendula Roth., beech-Fagus sylvatica L. and spruce-Picea abies (L.) Karst.) were studied in 30 to 40-year-old stands planted in adjacent plots on former arable land. Mineral soil samples from two depth layers (0–10 and 20–30 cm) under the different species were compared in terms of pH, base saturation, pools and concentrations of exchangeable macro- and micronutrients, total nitrogen and carbon. With the exception of pH (H₂O) and extractable Al and Fe, no significant differences between species were detected in the lower layer. The upper (0–10 cm) layer was, however, affected differently depending on tree species: significant differences in pH, base saturation, exchangeable base cations and other nutrients were observed. The most prominent differences were between lime and spruce. Lime had considerably higher pH, base saturation, base cation and boron pools compared to spruce, which had the most acidifying effect on the mineral topsoils. Among the deciduous species, beech had the most similar effect to spruce on the upper layer of mineral topsoils. Soil C, N and C/N ratios did not differ significantly among species.     

Influence of skidder traffic and canopy removal intensities on the ground flora in a clearcut-with-reserves northern hardwood stand in Minnesota,USA

We investigated the effects of skidder traffic intensity, soil disturbance intensity, and canopy removal intensity on the richness, diversity, composition, and cover of the entire ground flora (woody and herbaceous vegetation ≤2 m tall), various individual life forms, invasive/noxious species, and species with different requirements with respect to moisture, nutrients, heat, and light (synecological coordinates) in a mesic northern hardwood stand 6 years after a clearcut-with-reserves regeneration harvest removed 50–100% of the canopy. Skidder traffic was restricted to a network of trails and a global positioning system (GPS) tracked skidder movement to quantify the number of passes at pre-established sampling points along the anticipated soil disturbance gradient on and off skid trails. Soil disturbance intensity within the top 15 cm of soil was quantified by relativized resistance to penetration (RRP) compared to untrafficked plots; post-harvest increases in RRP ranged from 81 to 272%. Regression analysis and ordination revealed a pattern of increasing difference from pre-disturbance composition with increasing skidder traffic (i.e., forest floor disturbance), with increased RRP (i.e., soil compaction) and (less so) canopy removal intensity. The ground flora shifted from interior forest species such as Anemone quiquefolia, Aralia nudicaulis, Clintonia borealis, Maianthemum canadense, and Oryzopsis asperifolia to more ruderal, invasive/noxious, and disturbed-forest species such as Aster lateriflorus, Cirsium spp., Phleum pretense, Rubus idaeus, and Trifolium spp. The relative resistance of the initial ground flora to change (inverse of the distances between pre- and post-harvest samples in ordination space) was nonlinearly related to skidder traffic intensity and linearly related to RRP, indicating that the largest compositional changes occurred with the first few passes of the skidder. Mean plot scores for the synecological coordinates revealed that the post-harvest species were on average less demanding of water and nutrients; the opposite was true for light. Plots exposed to less skidder traffic and RRP had higher herb cover and higher nutrient scores; those with more skidder traffic and higher RRP levels had higher shrub cover and higher light scores. We conclude that protection of the ground flora from forest floor and soil disturbance requires careful planning of skid trail networks. Concentrating skidder traffic to a designated skid trail system can result in less area disturbed and spatially connected networks of larger, untrafficked remnant forest patches that may maintain species that are sensitive to forest floor and soil disturbance.     

Does soil organic matter variation affect the retention of NH4 and NO3 in forest ecosystems?

Human activities have fundamentally changed global nitrogen (N) cycling, leading to elevated N deposition in most parts of the world. The fate of deposited N, whether being retained to sustain plant growth or causing ecosystem N saturation, is critical to the global carbon (C) cycling and local environment. In a short-term laboratory experiment, we used ¹⁵N-labeled NH₄⁺ and NO₃⁻ to study the fate of N inputs in forest soils and what regulates N retention. Soils with a wide range of organic matter content and other attributes were collected from a 70-year-old plantation containing monotypic stands of Norway spruce (Picea abies), red pine (Pinus resinosa), sugar maple (Acer saccharum), and red oak (Quercus rubra), and separated into 0-5 cm and 5-15 cm layers. Nitrogen added to the soil was either immediately extracted (Time 0: T0) with K₂SO₄ solution, or incubated for 7 d (T7) and then extracted. Retention of ¹⁵N into the non-extractable soil pool at T0 was limited; but after the 7-d incubation, between 20 and 70% of the ¹⁵NH₄⁺ was retained. Nitrification transformed on average 50% of the ¹⁵NH₄⁺ into ¹⁵NO₃⁻ during the incubation while retention of ¹⁵NO₃⁻ at T7 remained low (7.40 ± 1.08%). Retention of ¹⁵NH₄⁺ into non-extractable soil at T7 was positively correlated to the percentage of soil organic matter (SOM) (r² = 0.323, P < 0.001), and was significantly higher (P < 0.001) in the high-SOM 0-5 cm layer than in the low-SOM 5-15 cm layer. Conversion of ¹⁵NH₄⁺ to ¹⁵NO₃⁻ during incubation significantly reduced the ¹⁵NH₄⁺ retention (P < 0.001). Our results suggest that the variations of SOM and other soil attributes play strong roles in the retention of newly deposited inorganic N and could affect forest ecosystem responses to chronic N deposition.     

The pH change in rhizosphere ofPinus koraiensis seedlings as affected by different nitrogen sources and its effect on phosphorus availability

Root mat method described by Kuchenbuch and Jungk was used to study the rhizosphere processes. The experiment was carried out on two years oldPinus koraiensis seedlings. Soil samples collected from the upper 20-cm soil layer in Changbai Mountain were treated with three different forms of nitrogen fertilizers: NO₃ ⁻−N, NH₄ ⁺−N and NH₄NO₃. The results showed that the soil pH and available P near the roots were all lower than in the bulk soil in control treatment. NH₄ ⁺−N application greatly decreased the soil pH near the roots compared to the control treatment and promoted the absorption of phosphorus, which led to a more remarkable depletion region of available P. On the contrary, the rhizosphere soil pH was higher than in the bulk soil in treatments with NO₃ ⁻−N and retarded the P absorption, which led to a nearly equal available P contents to the bulk soil. In treatment with NH₄NO₃, the rhizosphere soil pH was only a little lower than that in the control treatment and its effects on P absorption is mediate between the treatments with NH₄ ⁺−N and NO₃ ⁻−N. Foundation item: This paper was supported by National Natural Science Foundation of China (Grant No. 30170167). Biography: Chen Yong-liang (1969-), male, Ph. Doctor, lecture of Northeast Forestry University, Harbin 150040, Post-doctor in Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, P.R. China. E-mail: ylchin@sohu.com Responsible editor: Seng Funan     

Post-fire soil nitrogen content and vegetation composition in Sub-Boreal spruce forests of British Columbia's central interior,Canada

Forest fires are known to influence nutrient cycling, particularly soil nitrogen (N), as well as plant succession in northern forest ecosystems. However, few studies have addressed the dynamics of soil N and its relationship to vegetation composition after fire in these forests. To investigate soil N content and vegetation establishment after wildfire, 13 sites of varying age class were selected in the Sub-Boreal spruce zone of the central interior of British Columbia, Canada. Sites varied in time since the last forest fire and were grouped into three seral age classes: (a) early-seral (<14 years), (b) mid-seral (50–80 years) and (c) late-seral (>140 years). At each site, we estimated the percent cover occupied by trees, shrubs, herbs and mosses. In addition, the soil samples collected from the forest floor and mineral horizons were analyzed for the concentrations of total N, mineralizable N, available NO₃⁻-N and available NH₄⁺-N. Results indicated that soil N in both the forest floor and mineral horizons varied between the three seral age classes following wildfire. Significant differences in mineralizable N, available NO₃⁻-N and available NH₄⁺-N levels with respect to time indicated that available soil N content changes after forest fire. Percent tree and shrub cover was significantly correlated to the amount of available NH₄⁺-N and mineralizable N contents in the forest floor. In the mineral horizons, percent tree cover was significantly correlated to the available NH₄⁺-N, while herb cover was significantly correlated with available NO₃⁻-N. Moss cover was significantly correlated with total N, available NO₃⁻-N and mineralizable N in the forest floor and available NO₃⁻-N in the mineral horizons. We identified several unique species of shrubs and herbs for each seral age class and suggest that plant species are most likely influencing the soil N levels by their contributions to the chemical composition and physical characteristics of the organic matter.     

Vegetation community and soil characteristics of abandoned agricultural land and pine plantation in the Qinling Mountains,China

In order to determine the changes in the characteristics of the vegetation and soil following agricultural abandonment and compare the effects of different restoration approaches on ecosystem recovery, we studied the vegetation community and soil characteristics (nutrients, bulk density, water content and pH) of Pinus tabulaeformis plantations and abandoned croplands in different successional stages in the Foping National Nature Reserve, located in the Qinling Mountains, northwest China. The results indicated that natural vegetation and habitat could be restored via natural regeneration. These spontaneous restoration forests were characterized by high diversity, high soil fertility and rich unique species. The soil organic matter, total carbon, total nitrogen, available potassium, community cover, depth of litter, depth of humus and soil water content increased significantly with years after abandonment, while the total potassium, total phosphorus, available phosphorus, soil pH did not seem to change significantly with abandonment time. The soil mineral nitrogen (NH₄-N + NO₃-N) tended to increase during the first 50 years after abandonment and then decreased. The pine plantations tended to show a low level of biodiversity in tree and herb layer, but the shrub layer (including sapling) composition and diversity were similar to secondary forests. Although the pine plantation showed lower soil fertility, they did not seem to result in the habitat-degradation.     

Abundance response of western European forest species along canopy openness and soil pH gradients

In order to better understand the structure and composition of forest plant communities, we aimed to predict the abundance of understory herbaceous species locally at the stand level and according to different environments. For this, we seeked to model species distributions of abundance at a regional scale in relationship with the local stand structure (canopy openness) and regional soil resources (soil pH).Floristic inventories, performed in different light and soil conditions located in 1202 records of north eastern France, were used to analyze the combined effect of canopy openness and soil pH on the abundance of 12 common western European forest species: Anemone nemorosa, Deschampsia flexuosa, Festuca altissima, Hedera helix, Lamium galeobdolon, Lonicera periclymenum, Molinia caerulea, Oxalis acetosella, Pteridium aquilinum, Rubus fruticosus, Rubus idaeus, and Vaccinium myrtillus. Ordinal regression models relating species abundance responses to their environment were developed.For most species (eight out of 12), distribution was significantly affected by canopy openness and soil pH. Differences among low-abundance (i.e. cover <25%) and high-abundance (i.e. cover >25%) responses were noted for 11 species along the canopy openness gradient and four species along the pH gradient. The present study quantifies optimal light and soil nutrient requirements for high-abundance responses and quantifies light and soil nutrients tolerance conditions for low-abundance responses. The combination of both factors highlights the pre-eminence of pH conditions occurrence and canopy openness for species abundance.The models developed by this study may be used to define canopy openness thresholds in function of soil characteristics to control the development of species during forest regeneration. The species-specific reactions on local canopy openness along a regional soil gradient illustrate the need for a species-specific management approach.     

Silvopastoral use of Nothofagus antarctica in Southern Patagonian forests, influence over net nitrogen soil mineralization

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.     

Potential effects of management practices on nitrogen nutrition and long-term productivity of western hemlock stands

The FORCYTE-10 computer model, developed by J.P. Kimmins and K. Scoullar for Douglas-fir forests in British Columbia, was modified to simulate growth and nutrient cycling of coastal western hemlock stands. Initial calibration indicated that predicted yield was extremely sensitive to the rate of mineralization of soil organic matter (SOM), variation in SOM C:N ratio with site quality, the soil extractable NO₃⁻:NH₄⁺ ratio, and the decomposition rate and N mineralization pattern of large and medium-size roots and woody debris. The predictions suggested that yield and SOM remain stable under a management system consisting of six successive 90-year rotations. More intensive utilization (e.g., shorter rotations, whole-tree harvesting and commercial thinning) causes depletion of soil and forest floor nitrogen and a small decline in site productivity in later rotations.     

Soil microbiological properties and enzyme activity in Ginkgo–tea agroforestry compared with monoculture

Agroforestry practice is believed to be an effective means of maintaining and improving soil fertility, and is widely used by farmers around the world. To gain better understanding of the effects of agroforestry practice on soil fertility, the organic carbon content, total nitrogen content, microbial biomass, basal respiration, and activity of soil enzymes at three soil depths (0–10, 10–20, and 20–30 cm) of Ginkgo (Ginkgo biloba L.)–tea (Camellia sinensis (L.) O. Kuntze) agroforestry systems were investigated. Study plots were established in Yushan Farm in Changshu, Jiangsu Province, China. These involved two densities of Ginkgo trees mixed with tea (G₁ and G₂) and monoculture tea systems (G₀). The results showed that C, N, microbial biomass, and enzyme activity were higher in surface soil than in soil from the middle and lower layers whereas pH and metabolic quotient increased with soil depth. pH, microbial biomass C, N, basal respiration, and catalase and invertase activity in the 0–10 cm layer were significantly lower for G₀ than for G₁ and G₂. Polyphenoloxidase activity in the 0–10 cm layer was significantly lower for G₂ than for G₀ and G₁. Metabolic quotient in the 20–30 cm layer was significantly higher for G₀ than for G₂. The activity of soil enzymes, including catalase, dehydrogenase, urease, protease, and invertase, significantly and positively correlated with soil organic carbon and total nitrogen. The results of this study suggest that growing tea with Ginkgo could be regarded as good agroforestry practice which could enhance accumulation of organic matter in soil, improve the activity of soil enzymes, and maintain soil productivity and sustainability.     

Influence of tree species on gross and net N transformations in forest soils

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.     

Soil changes induced by Acacia mangium plantation establishment: Comparison with secondary forest and Imperata cylindrica grassland soils in South Sumatra,Indonesia

Acacia plantation establishment might cause soil acidification in strongly weathered soils in the wet tropics because the base cations in the soil are translocated rapidly to plant biomass during Acacia growth. We examined whether soils under an Acacia plantation were acidified, as well as the factors causing soil acidification. We compared soils from 10 stands of 8-year-old Acacia mangium plantations with soils from 10 secondary forests and eight Imperata cylindrica grasslands, which were transformed into Acacia plantations. Soil samples were collected every 5–30 cm in depth, and pH and related soil properties were analyzed. Soil pH was significantly lower in Acacia plantations and secondary forests than in Imperata grasslands at every soil depth. The difference was about 1.0 pH unit at 0–5 cm and 0.5 pH unit at 25–30 cm. A significant positive correlation between pH and base saturation at 0–20 cm depth indicated that the low pH under forest vegetation was associated with exchangeable cation status. Using analysis of covariance (ANCOVA), with clay content as the covariate, exchangeable Ca (Ex-Ca) and Mg (Ex-Mg) stocks were significantly lower in forested areas than in Imperata grasslands at any clay content which was strongly related to exchangeable cation stock. The adjusted average Ex-Ca stock calculated by ANCOVA was 249 kg ha⁻¹ in Acacia plantations, 200 kg ha⁻¹ in secondary forests, and 756 kg ha⁻¹ in Imperata grasslands at 0–30 cm. Based on a comparison of estimated nutrient stocks in biomass and soil among the vegetation types, the translocation of base cations from soil to plant biomass might cause a decrease in exchangeable cations and soil acidification in Acacia plantations.     

Microclimate and nutrient dynamics in a silvopastoral system of semiarid northeastern Brazil

There is little available information on nutrient cycling and the controls of ecosystem processes in land use systems of dry neotropical regions. In this study, we conducted field and glasshouse experiments to investigate the influence of Ziziphus joazeiro and Prosopis juliflora trees on microclimate and nutrient dynamics in pastures ofCenchrus ciliaris in semiarid northeastern Brazil. We found that soil moisture was lower under the crowns of P. juliflora trees during early season in comparison to patches ofC. ciliaris (grass patches), but the presence of Z. joazeiro had no effect on soil moisture. Soil and air temperatures were lower under Z. joazeiro in comparison to grass patches but P. juliflora had little effect on temperature. Losses of P from all litter types were lower under the crowns of Z. joazeiro trees, but losses of biomass and N from litter were not consistently affected by the presence of trees. Soil net N mineralized was greater under both tree species, in comparison to grass patches. However, net soil N mineralized per gram of total soil N was greater under the crowns of the potentially N₂ fixing P. juliflora, in comparison to grass patches, while there were no significant differences betweenZ. joazeiro crowns and grass patches. In the glasshouse study, plant biomass and nutrient uptake was greater in soil collected under the crowns of both tree species, in comparison to soil from grass patches. Our results indicate that the preservation of native trees or introduction of exotic tree species in C. ciliaris pastures in semiarid northeastern Brazil significantly affects microclimate and the dynamics of litter and soil nutrients, and may contribute to increases in the cycling rate of nutrients in these systems. Index terms: buffel grass, caatinga, nitrogen mineralization, Prosopis juliflora, Ziziphus joazeiro.This revised version was published online in November 2005 with corrections to the Cover Date.     

Gross N transformations were little affected by 4 years of simulated N and S depositions in an aspen-white spruce dominated boreal forest in Alberta, Canada

The effects of 4 years of simulated nitrogen (N) and sulfur (S) depositions on gross N transformations in a boreal forest soil in the Athabasca oil sands region (AOSR) in Alberta, Canada, were investigated using the ¹⁵N pool dilution method. Gross NH₄⁺ transformation rates in the organic layer tended to decline (P < 0.10, marginal statistical significance, same below) in the order of control (CK, i.e., no N or S addition), +N (30 kg N ha⁻¹ yr⁻¹), +S (30 kg S ha⁻¹ yr⁻¹), and +NS treatments, with an opposite trend in the mineral soil. Gross NH₄⁺ immobilization rates were generally higher than gross N mineralization rates across the treatments, suggesting that the studied soil still had potential for microbial immobilization of NH₄⁺, even after 4 years of elevated levels of simulated N and S depositions. For both soil layers, N addition tended to increase (P < 0.10) the gross nitrification and NO₃⁻ immobilization rates. In contrast, S addition reduced (P < 0.001) and increased (P < 0.001) gross nitrification as well as tended (P < 0.10) to reduce and increase gross NO₃⁻ immobilization rates in the organic and mineral soils, respectively. Gross nitrification and gross NO₃⁻ immobilization rates were tightly coupled in both soil layers. The combination of rapid NH₄⁺ cycling, negligible net nitrification rates and the small NO₃⁻ pool size after 4 years of elevated N and S depositions observed here suggest that the risk of NO₃⁻ leaching would be low in the studied boreal forest soil, consistent with N leaching measurements in other concurrent studies at the site that are reported elsewhere.     

Comparing indicators of N status of 50 beech stands (Fagus sylvatica L.) in northeastern France

We compared different potential indicators of nitrogen (N) availability across 50 beech forests growing on a wide range of soils in northeastern France. Among the 50 sites measured, high elevation acidic soils had the highest potential net N mineralization in the A horizon (PNM_0–5 cm), while low elevation neutral and calcareous soils had the lowest (PNM_0–5 cm). We found that (PNM_0–5 cm) was negatively correlated with soil pH (R² = 0.47***) and positively correlated with microbial C/N (R² = 0.34***). However, when high elevation sites were excluded from analyses, the relationship between PNM_0–5 cm and soil pH as well as microbial C/N became weaker (R² = 0.23*** for both variables). We found no relationship between PNM_0–5 cm and organic N concentration, soil C/N, or vegetation-based indices for N availability (Ellenberg N and Ecoplant C/N). Bivariate linear regression analyses showed that 69% of the variability in percent nitrification (%Nitrif) was explained by both soil pH (0–5 cm) and soil C/N. Percent nitrification was strongly correlated with vegetation-based indices for N availability. The Ellenberg N and R (pH index) values together explained 74% of the variation in %Nitrif. No relationship was found between %Nitrif and soil δ¹⁵N (natural abundance in ¹⁵N). Of the 76 plant species evaluated, the probability of presence of 61 plant species was significantly correlated with %Nitrif while the probability of presence of 27 plant species only was correlated with PNM_0–5 cm. From these results, we believe that the use of plant community composition or the combination of soil pH and C/N are robust indicators of N availability.     

Impact of plant species on the formation of carbon and nitrogen stock in soils under semi-desert conditions

The unique forest ecosystems investigated were created on the place of natural steppe biogeocoenoses 60?years ago. The aim of the study was to elucidate the effect of plant species on the formation of organic C and N stocks in soils and to estimate nitrogen availability for artificial wood plantation. For this purpose, 290 soil samples were taken from four forest monocultures (Quercus robur L., Pinus sylvestris L., Cotinus coggygria Scop., and Acer tataricum L.) and from virgin steppe ecosystem. The amounts and stocks of organic C, total and readily nitrified N, and seasonal dynamics of NO₃ ^? and NH₄ ⁺ ions activities were determined. It was shown that the species composition of the stands influenced the stock of organic C and N in soils. The storages of C and total N differed by 74 and 4.4?Mg/ha^?1, respectively, in the litter and upper horizons (0–40-cm layer) in the stands studied. The differences in distribution of stocks of these elements in virgin steppe and artificial forest ecosystems were found. Organic C and N stocks increased 1.6–6.6 times in the forest litter compared to the steppe one, while in 5–40-cm layer, the storages of C and N decreased by 20–35% compared to the virgin soil. The impact of litter on total N content in arid climate was limited in 0–5-cm layer. The deficit of mineral N compounds was observed in autumn in soil with low stock of total N.