Distribution of sulphur fractions in lysimeters previously treated with sulphuric acid,NPK fertilizer and a combination of acid and fertilizer

The retention of applied ³⁵S was studied in a lysimeter experiment, in which the lysimeters, consisting of soil columns with intact field and groundlayer vegetation, had previously been treated with either sulphuric acid, NPK or a combination of sulphuric acid and NPK.³⁵S was applied as Na₂³⁵SO₄ to all lysimeters in an amount corresponding to 2 kg S ha^?1, 3 months before the termination of the experiment. Of the applied ³⁵S, 4.7 ± 1.6% (the error term showing the standard error of the mean) was leached from the lysimeters; ³⁵SO₄^2?, was preferentially sorbed in the upper part of the B horizon, >61% being sorbed in the B horizon as a whole. The influence of previous treatments was seen in SO₄^2? content in the B horizon and also in the SO₄^2? concentrations in the leachates from the lysimeters. Sorption of ³⁵SO₄^2? was enhanced by the acid treatment in the A₀ and A₁/A₂ horizons. Ratios between water-extractable SO₄^2? and KCl-extractable SO₄^2? varied from 0.88 in the A₁/A₂ horizon to 0.46 in the lower part of the B horizon, indicating a preferential sorption of less available SO₄^2? fractions in that part of the soil.Significant positive correlations between soil organic carbon and various sulphur fractions, both labelled and unlabelled, indicated the importance of biological S transformations in the A₀ and A₁/A₂ horizons.Positive correlations between KCl-extractable Al and KCl-extractable SO₄^2? were found in the B horizon. Equilibrium calculations suggested that the SO₄^2? activity and Al³⁺ activity in leachates from control and acidified lysimeters were within the stability fields of basaluminite (Al₄(OH)₁₀SO₄), activities in leachates from the latter group of lysimeters, also being close to jurbanite (Al(OH)SO₄).     

Glyphosate application in forest—ecological aspects

The vegetation of four different forest soils in lysimeters was killed with glyphosate. New vegetation became established in some of the treated lysimeters the year after application. During this year, leachates from lysimeters without vegetation had nitrate concentrations up to 4.5 mM NO₃ ^? in the fall. A corresponding increase in leached cations was evident. The settlement of one raspberry or one willow herb in treated lysimeters effectively hindered nitrate leaching as soon as the plants were sufficiently large (in mid‐summer). As plants became established in treated lysimeters, the chemistry of these leachates approached those of untreated lysimeters. The absolute effect of even small plants in controlling water regime and water chemistry was evident.     

Stream water and soil solution responses to 5 years of nitrogen and sulfur additions at the Fernow Experimental Forest,West Virginia

To examine the effects of elevated N and S inputs on a central hardwood forest, a whole-watershed acidification experiment was initiated in 1989 on the Fernow Experimental Forest, West Virginia. Annual experimental additions of 40 kg S ha⁻¹ year⁻¹ and 35 kg N ha⁻¹ year⁻¹ as ammonium sulfate fertilizer were applied to a 34 ha watershed with a 25-year-old stand of central Appalachian hardwoods. An adjacent watershed served as the control. After 5 years of treatment (total additions of 275 kg S ha⁻¹ and 220 kg N ha⁻¹), stream water NO₃⁻, Ca²⁺, Mg²⁺ concentrations and export increased. Soil solution concentrations provide evidence that the treatment watershed is nitrogen-saturated, which was unexpected for such a young stand. No statistically significant changes in annual SO₄²⁻ export were observed, but peak stream water concentrations of SO₄²⁻ did increase during the treatment period. Changes in soil solution chemistry suggest that the treated watershed also may be approaching SO₄²⁻ saturation.     

Changes in soil acidity from 1927 to 1982–1984 in a forest area of south‐west Sweden

Reinvestigation of 90 soil profiles sampled for pH measurements in 1927 revealed a general decrease in pH with 0.3–0.9 units (measured electrometrically on field‐moist samples in water with the same ratio soil/water on both occasions). All soil horizons (A₀, A₂, B and the subsoil, C, at 70 cm depth) had become more acid beneath all types of canopy (beech, oak, spruce planted during different periods), but the spruce stands were on average more acid than the hardwoods. In the upper soil horizons (A₀ and A₂), old spruce stands were more acid than the young ones at both samplings, but this effect was small in the B horizon and absent in the C horizon. While the tree species effect and age effect in the spruce stands may be called biological acidification, the acidification of deeper horizons, now often below pH 4.5 and in the aluminium buffer range, seems difficult to explain without assuming an influence of acid deposition.     

Soil and microbial respiration in a loblolly pine plantation in response to seven years of irrigation and fertilization

Because soil CO₂ efflux or soil respiration (R_S) is the major component of forest carbon fluxes, the effects of forest management on R_S and microbial biomass carbon (C), microbial respiration (R_H), microbial activity and fine root biomass were studied over two years in a loblolly pine (Pinus taeda L.) plantation located near Aiken, SC. Stands were six-years-old at the beginning of the study and were subjected to irrigation (no irrigation versus irrigation) and fertilization (no fertilization versus fertilization) treatments since planting. Soil respiration ranged from 2 to 6 μmol m⁻² s⁻¹ and was strongly and linearly related to soil temperature. Soil moisture and C inputs to the soil (coarse woody debris and litter mass) which may influence R_H were significantly but only weakly related to R_S. No interaction effects between irrigation and fertilization were observed for R_S and microbial variables. Irrigation increased R_S, fine root mass and microbial biomass C. In contrast, fertilization increased R_H, microbial biomass C and microbial activity but reduced fine root biomass and had no influence on R_S. Predicted annual soil C efflux ranged from 8.8 to 10.7 Mg C ha⁻¹ year⁻¹ and was lower than net primary productivity (NPP) in all stands except the non-fertilized treatment. The influence of forest management on R_S was small or insignificant relative to biomass accumulation suggesting that NPP controls the transition between a carbon source and sink in rapidly growing pine systems.     

Nutrient use and uptake in Pinus taeda

We quantified nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) content, use (nutrient amount for one growth year), retranslocation (nutrients recycled before foliage senescence), uptake (use minus retranslocation), volume production per unit of uptake and fertilizer-uptake efficiency (percent applied taken up) in a 2 x 2 (nutrient and water) factorial experiment replicated four times in an 8-year-old loblolly pine (Pinus taeda L.) stand growing on a nutrient-poor sandy soil in Scotland County, North Carolina, USA. Over 14 years, we applied 1140, 168, 393, 168 and 146 kg ha(-1) of elemental N, P, K, Ca and Mg fertilizer, respectively, and an average of 710 mm year(-1) of irrigation. All plots received complete vegetation control. Fertilization about doubled tissue N, P, K and Mg contents at age 21, whereas irrigation resulted in smaller increases in nutrient contents. Maximum annual uptake was 101, 9.3, 44, 37 and 13 kg ha(-1) year(-1) and volume production per unit of nutrient uptake was 0.35, 3.5, 0.66, 1.1 and 3.1 m(3) kg(-1), for N, P, K, Ca and Mg, respectively. Irrigated plots had greater volume production per unit of N, P, K and Mg uptake than control plots, likely because irrigation allowed photosynthesis to continue during dry periods. Fertilized plus irrigated plots had less volume production per unit of these elements than the fertilized plots either because nutrient uptake exceeded the requirement for optimum growth or because available water (rainfall plus irrigation) was insufficient for the leaf area achieved with fertilization. At age 19, fertilizer-uptake efficiencies for N, P, K, Ca and Mg were 53, 24, 62, 57 and 39%, respectively, and increased with irrigation to 68, 36, 78, 116 and 55%, respectively. The scale of fertilizer uptake was likely a result of low native site nutrient availability, study longevity, measurement of all tissue components on site, a comprehensive assessment of coarse roots, and the 3-m rooting depth. Ecosystem nitrogen retention (applied nitrogen found in living plant material, litter fall and soil to 150-cm depth) was estimated at 79% at age 17, a value that would likely be greater when including soil nitrogen to rooting depth and calculating retention at age 21 when the study ended. The ecosystem retention value provides evidence that intensive site resource management can be accomplished with low likelihood of applied materials moving offsite.     

Estimation of evapotranspiration with drainage lysimeters in the Taihang Mountain area, China

In this research, the evapotranspiration (ET) of three native vegetation communities were measured using drainage lysimeters in the Taihang Mountain area, China. They are a local grass, Themeda japonica, a local shrub, Vitex negundo var. heterophylla Rehd. and a mixture of both communities. The ET was measured using level lysimeters (with a slope of 0°) and slope lysimeters (with a slope of 25°). In general, the measured ET was higher in the level lysimeters than in the slope lysimeters because of the water loss of surface runoff from the slope lysimeter. The total ETs over the growing season for the grass, shrub, and the mixture were 730.4, 742.0 and 790.7 mm, respectively in the level lysimeters, and 535.5, 504.1 and 540.1 mm, respectively in the slope lysimeters. In addition, the monthly ET peaked in August and had close linear relationship with leaf area index. The drainage lysimeter is an effective tool to estimate plant ET in mountain areas. The results from this research would provide scientific information for the vegetation recovery and sustainable development of forestry in the TM areas.     

黄河小浪底库区山地栓皮栎人工林土壤呼吸的季节动态

[目的]分离并量化土壤自养呼吸和异养呼吸,探讨各自贡献率及其随季节变化的动态特征。[方法]采用壕沟法和气体红外分析法,研究黄河小浪底库区山地栓皮栎人工林土壤总呼吸、自养呼吸和异养呼吸速率的季节动态变化、贡献率和环境影响因子。[结果]表明:栓皮栎人工林总土壤呼吸、自养呼吸和异养呼吸均呈夏季速率高、冬季速率低。栓皮栎土壤总呼吸、自养呼吸及异养呼吸速率与5 cm土壤温度均呈极显著指数相关,温度敏感性系数Q_(10)值大小为自养呼吸(3.40)异养呼吸(2.90)土壤总呼吸(2.45);栓皮栎土壤总呼吸、自养呼吸、异养呼吸速率与0 10 cm土壤体积含水量均显著线性相关;土壤总呼吸、自养呼吸速率与0 10 cm土壤电导率显著相关。土壤总呼吸和异养呼吸的温度敏感系数Q_(10)值均在冬季最大,夏秋季最小;而自养呼吸的Q_(10)值则呈相反的变化趋势。栓皮栎人工林自养呼吸和异养呼吸对土壤总呼吸的月贡献率为13.23%37.33%和62.67%86.76%,且自养呼吸的贡献率与土壤温度的季节变化规律相似。土壤总呼吸、异养呼吸与自养呼吸的CO2年通量分别为1 616.41、1 199.39、417.02 g·m~(-2)·a~(-1)。[结论]经过区分与定量化土壤总呼吸及其组分,确定异养呼吸为本研究区栓皮栎人工林土壤总呼吸的主要组分,作用于异养呼吸的生物与非生物因子均能显著影响整个森林生态系统表层CO_2总排放通量的大小,进一步为该研究区森林生态系统碳循环与能量流动的进一步量化研究提供参考。     

Prescribed burning and mechanical thinning effects on belowground conditions and soil respiration in a mixed-conifer forest,California

Soil respiration (R_S) is a major carbon pathway from terrestrial ecosystems to the atmosphere and is sensitive to environmental changes. Although commonly used mechanical thinning and prescribed burning can significantly alter the soil environment, the effect of these practices on R_S and on the interactions between R_S and belowground characteristics in managed forests is not sufficiently understood. We: (1) examined the effects of burning and thinning treatments on soil conditions, (2) identified any changes in the effects of soil chemical and physical properties on R_S under burning and thinning treatments, and (3) indirectly estimated the changes in the autotrophic soil respiration (R_A) and heterotrophic soil respiration (R_H) contribution to R_S under burning and thinning treatments. We conducted our study in the Teakettle Experimental Forest where a full factorial design was implemented with three levels of thinning, none (N), understory thinning (U), and overstory thinning (O; September to October 2000 for thin burn combination and June and July 2001 for thin only treatments) and two levels of burning, none (U) and prescribed burning (B; fall of 2001). R_S, soil temperature, soil moisture, litter depth, soil total nitrogen and carbon content, soil pH, root biomass, and root nitrogen (N) concentration were measured between June 15 and July 15, 2002 at each plot. During this period, soil respiration was measured three times at each point and averaged by point. When we assumed the uniform and even contribution of R_A and R_H to R_S in the studied ecosystem without disturbances and a linear relationship of root N content and R_A, we calculated the contributions of R_A to R_S as 22, 45, 53, 48, and 45% in UU, UO, BN, BU, and BO, respectively. The results suggested that after thinning, R_S was controlled more by R_H while after burning R_S was more influenced by R_A. The least amount of R_S variation was explained by studied factors under the most severe treatment (BO treatment). Overall, root biomass, root N concentration, and root N content were significantly (p < 0.01) correlated with soil respiration with correlation coefficients of 0.37, −0.28, and 0.29, respectively. This study contributes to our understanding of how common forestry management practices might affect soil carbon sequestration, as soil respiration is a major component of ecosystem respiration.     

施肥对云南松生长及其器官和土壤养分的影响

为探究配方施肥对云南松松针养分含量和土壤理化性质的影响,为云南松幼林抚育提供科学依据,以8年生云南松为试材,采用U*₁₅(15⁷)均匀设计开展复合肥、氮肥、磷肥、镁肥和硼肥配施的试验。处理组合的针叶长、烘干含水率、针叶全氮、全磷和全钾分别为23.90~27.34 cm、55.47%~60.88%、0.17%~0.57%、0.13%~0.33%和0.17%~0.82%,对照的则为24.14 cm、60.75%、0.35%、0.17%和0.29%。其中,全磷和全钾具有显著的差异。处理组合的土壤pH、烘干含水率、速效氮、速效磷、速效钾和胸径增长率分别为4.08~4.69、17.27%~34.59%、22.77~41.91、7.70~11.80、66.44~174.67 mg/kg和23.5%~50.3%,对照的则为4.42、17.27%、23.70、8.23、81.11 mg/kg和22.4%,处理组合间速效磷、速效钾和胸径增长率具有极显著的差异。土壤理化性质与因素具有复杂的相关性,硼肥与胸径增长率呈现显著的正相关;配施复合肥1.6 kg/3 cm、氮肥0.10 kg/3 cm、磷肥0.2 kg/3 cm,镁肥7.5 g/3 cm和硼肥1.5 g/3 cm可改善土壤理化性质,促进云南松幼树胸径生长。     

Influence of woody and herbaceous vegetation control on leaf gas exchange,water status,and nutrient relations of eastern white pine (Pinus strobus L.) seedlings planted in a central Ontario clearcut

The influence of herbaceous and woody vegetation control, either singly or in combination, on leaf gas exchange, water status, and nutrient relations of planted eastern white pine (Pinus strobus L.) seedlings was examined in a central Ontario clearcut over four consecutive growing seasons (GSs). Net carbon assimilation (A_n), leaf conductance to water vapour (G_wv), water use efficiency (WUE), and midday leaf water potential (ψ_m) were measured periodically during the second to fourth GSs of vegetation control treatments, while leaf nutrient relations were examined in GS five. Leaf A_n and G_wv were reduced (p ≤ 0.05) in the presence of herbaceous vegetation in GS two, by both herbaceous and woody vegetation in GS three, and only by woody vegetation (largely trembling aspen (Populus tremuloides Michx.)) in GS four. Leaf WUE was increased (p ≤ 0.05) in all three GSs in which herbaceous vegetation control was applied and where woody vegetation provided partial shading of planted white pine. Leaf water status was comparatively less responsive to vegetation control treatments, but leaf ψ_m was increased (p ≤ 0.05) in the presence of woody vegetation in GSs two and four, likely due to shading and reduced atmospheric evaporative demand of the white pine seedling environment. Within a given GS, the effects of vegetation control on A_n, G_wv, and ψ_m were strongly linked to treatment-induced changes in total vegetative cover, and light and soil moisture availability. Seedling height, diameter, and volume growth rates were positively correlated with A_n and WUE in GSs two and three, but less so in GS four. Vector analysis suggested that herbaceous competition induced foliar N, P, and K deficiencies in five-year-old white pine seedlings while competition from aspen resulted in foliar Ca deficiency.     

Effects of environmental factors on seedling growth and anthocyanin content in <Emphasis Type="Italic">Betula</Emphasis> ‘Royal Frost’ leaves

Determining the influences of environmental factors on seedling growth and leaf color may improve our understanding of the chromogenic mechanisms in leaves and the biosynthesis of anthocyanin. To determine the optimum conditions for Betula ‘Royal Frost’ seedling cultivation, we assessed seedling growth and anthocyanin content under natural conditions. The temperature had a significant influence on height but not on radial growth. Between June and September, the anthocyanin content was significantly positively correlated with light intensity and air moisture and significantly negatively correlated with temperature. In single-factor experiments to determine the variation in anthocyanin content in the greenhouse, anthocyanin content increased with increasing soil water content and at first increased, then decreased with increasing pH of the irrigation water and temperature, but decreased with increasing light intensity. K₂SO₄ can increase the anthocyanin content. So to maintain leaf color, seedlings should be cultivated with < 200 μmol m^?2 s^?1 light intensity at 20 °C and soil moisture content between 60 and 90%, and the irrigation water should be neutral or alkaline. Additionally, spraying with K₂SO₄ can be beneficial.     

Enhancement of Seed Germination in Trema orientalis (L.) Blume—Potential Plant Species in Revegetation of Mine Wastelands

Natural forest succession takes a long time to accumulate sufficient nutrients to support plant growth and enhance soil microbial activity. Human intervention in selecting native pioneer plant species is therefore required to accelerate sustainable restoration. Trema orientalis (L.) Blume, a fast growing pioneer plant species, has the ability to grow in nutrient deficient soils and proves to have reclamation potential in mine wastelands. However, its use has been limited due to low germination percentages and nonsynchronous seed germination. In the present study we tested the effect of sulphuric acid (H₂SO₄), hydrochloric acid (HCl), gibberellic acid (GA₃), and potassium nitrate (KNO₃) in varying concentrations and time durations on germination percentages and seed germination synchrony. We found that all treatments had a significant effect in predicting seed germination probabilities. Logistic regression analysis revealed that treatment solution and concentration had a significant effect on seed germination. Treatment with concentrated H₂SO₄ for 15 min increased germination up to 92% within 20 days with the least imbibition time (8 days) and highest Seedling Vigor Index (491). The scanning electron microscope images of seeds treated with H₂SO₄ showed complete dissolution of the honeycomb-like network of deposits on the seed coat surface removing the physical barrier and enhancing germination.     

Biogeochemical cycling and chemical fluxes in a managed northern forested wetland, Michigan, USA

Forest harvesting and subsequent regeneration treatments may cause changes in soil and solution chemistry that adversely affect forest productivity and environmental quality. The objective of this study was to assess soil carbon (C), nitrogen (N), and base cation pools and fluxes, and to construct a hydrogen ion (H⁺) mass balance to identify major processes controlling acidity production and consumption 14 years following whole-tree harvesting and regeneration in a northern forested wetland with underlying mineral soils derived from calcareous glacial drift. Results for soil elemental and nutrient pools in the harvested/regenerated stand were compared to an adjacent non-harvested stand and a riparian zone. The riparian zone had the highest soil total C, total N, and exchangeable calcium (Ca) and magnesium (Mg) pools; however, no difference in exchangeable potassium (K) was evident among stand types. Moreover, no differences between the harvested/regenerated and uncut stands were evident in any of the soil chemical pools.Net export of base cations was minimal and the H⁺ mass balance indicated that net cation exchange was not a significant process in H⁺ production or consumption in either the uncut or harvested/regenerated stands. The most striking differences in the H⁺ mass balance were (1) eight times the H⁺ consumption from sulfate (SO₄²⁻) reduction in the harvested/regenerated stand compared to that in the uncut condition and (2) nearly twice the H⁺ production due to N immobilization in the harvested/regenerated stand. However, both stand types were net H⁺ sinks and increases in H⁺ export due to whole-tree harvesting were not evident.The riparian zone was a net exporter of base cations. This finding was attributed to a combination of base cation exchange and carbonate mineral weathering; data suggested the importance of the latter. More research, however, is required to isolate the contributions of cation exchange and carbonate weathering on base cation export from the riparian zone. Stream chemistry was consistent with that of the riparian zone, indicating a strong linkage between the riparian zone stream chemistry, and whole-tree harvesting had no intermediate term (i.e., 14 years) effects on stream acidification in this managed northern wetland ecosystem.     

Dissolution of Phosphorus Fertilizers of Differing Solubility in Peat Soil: A Field Experiment on a Drained Pine Bog

A 2-yr field trial was conducted on a drained pine bog to measure the dissolution of five phosphorus (P) fertilizers of differing solubility. The rate of fertilizer dissolution was measured using a sequential fractionation which involved pre-extraction with 0.5 M NaCl/TEA followed by 1 M NaOH and a mixed acid (HNO₃+HCl+H₂SO₄) digestion. About 56% of Siilinjärvi apatite, a hard magmatic rock phosphate, dissolved over the 2 yrs. For a commercial P+K fertilizer manufactured from Siilinjärvi apatite and KCl (water-soluble P<0.1%), the corresponding value was 44%. For Moroccan phosphate rock, and a commercial fertilizer manufactured from Moroccan phosphate rock and KCl (water-soluble P=15.8%), the dissolution was about 44% and 58%, respectively. For superphosphate (water-soluble P=79%), the corresponding value was 93%.     

Soil greenhouse gas emissions from agroforestry and other land uses under different moisture regimes in lower Missouri River Floodplain soils: a laboratory approach

Changes in land use management practices may have multiple effects on microclimate and soil properties that affect soil greenhouse gas (GHG) emissions. Soil surface GHG emissions need to be better quantified in order to assess the total environmental costs of current and possible alternative land uses in the Missouri River Floodplain (MRF). The objective of this study was to evaluate soil GHG emissions (CO₂, CH₄, N₂O) in MRF soils under long-term agroforestry (AF), row-crop agriculture (AG) and riparian forest (FOR) systems in response to differences in soil water content, land use, and N fertilizer inputs. Intact soil cores were obtained from all three land use systems and incubated under constant temperature conditions for a period of 94 days using randomized complete block design with three replications. Cores were subjected to three different water regimes: flooded (FLD), optimal for CO₂ efflux (OPT), and fluctuating. Additional N fertilizer treatments for the AG and AF land uses were included during the incubation and designated as AG-N and AF-N, respectively. Soil CO₂ and N₂O emissions were affected by the land use systems and soil moisture regimes. The AF land use resulted in significantly lower cumulative soil CO₂ and N₂O emissions than FOR soils under the OPT water regime. Nitrogen application to AG and AF did not increase cumulative soil CO₂ emissions. FLD resulted in the highest soil N₂O and CH₄ emissions, but did not cause any increases in soil cumulative CO₂ emissions compared to OPT water regime conditions. Cumulative soil CO₂ and N₂O emissions were positively correlated with soil pH. Soil cumulative soil CH₄ emissions were only affected by water regimes and strongly correlated with soil redox potential.     

Response of spruce stands in national parks of southern Poland to air pollution in 1998–2005

The purpose of this study was to describe changes in the deposition of air pollutants and the response of spruce trees in the period of 1998–2005. The investigation was carried out in twelve sites, mainly national parks. Air pollution level, that is, SO₂, NO₂, and O₃ concentrations in the air, as well as throughfall and soil solution chemical composition was analysed. The reaction of trees was described based on a stand health parameter, that is, defoliation. A decrease in the concentration of SO₂ and NO₂ in the investigated period was observed. In most of the analysed sites, ozone concentrations in the growing season exceeded the level regarded as harmful to plants. Annual loads of nitrogen and sulphur reaching the forest soil exceeded 10 kg/ha. The data collected for the analysed period confirmed that the health status of forests at the investigated sites did not show a general tendency towards improvement. Also, no dependency between the air pollution level and defoliation rate was found.     

The effects of three regeneration harvest methods on plant diversity and soil characteristics in the southern Appalachians

We evaluated the effects of three regeneration harvest methods on plant diversity and soil resource availability in mixed-hardwood ecosystems. The study area is in the Wine Spring Creek watershed on the Nantahala National Forest of the Southern Appalachian Mountains in western North Carolina. The regeneration treatments were: an irregular, two-aged shelterwood cut (2A), with 5.0 m²/ha residual basal area; a shelterwood cut (SW), with 9.0 m²/ha residual basal area; a group selection cut (GS), with 0.10–0.20 ha openings and 25% overstory removal on area basis at first entry; fourth, the control, consisted of two uncut sites (UC). Each harvest treatment was replicated three times across the landscape in similar plant community types. Within each treatment area, permanent plots were marked and inventoried for overstory, midstory, and herbaceous layer plants. In each permanent plot, we collected soil samples in winter (December–March) to reduce temporal variation due to vegetation phenological stage and rainfall events. We analyzed soil samples for extractable calcium (Ca), magnesium (Mg), potassium (K), cation exchange capacity (CEC), pH, bulk density, A-horizon depth, total carbon (C), and nitrogen (N). Species diversity of overstory, understory, and herbaceous layer species was evaluated using species richness (S), Shannon–Wiener's index of diversity (H′), and Pielou's evenness index (E). We used direct gradient analysis (non-metric multidimensional scaling, NMS) to explore the changes in vegetation–site relationships among herbaceous layer abundance, and soil characteristics and overstory basal area between pre-harvest (1994) and post-harvest (2000). Twelve minor overstory species were cut from the 2A treatments and nine species were cut from the SW treatments. Thus, it is not surprising that S and H′ were reduced in the overstory on the heavily cut sites. However, most of these species sprouted from cut stumps and were substantially more abundant in the midstory layer after harvest than before. For the midstory, we found higher S and H′ on the harvested treatments than the control; however, H′ did not differ significantly among the harvest treatments. We measured an increase in herbaceous layer H′ on the more heavily cut treatments (2A and SW) after harvest. We found an increase in average distance in the NMS ordination among sites in 2000 compared to 1994, which suggests greater herbaceous species diversity after harvest. However, we did not see a clear separation among harvest treatments in the NMS ordination.     

Separating component parts of soil respiration under <Emphasis Type="Italic">Robinia pseudoacacia</Emphasis> plantation in the Taihang Mountains,China

Partitioning the respiratory components of soil surface CO₂ efflux is important in understanding carbon turnover and in identifying the soil carbon sink/source function in response to land-use change. The sensitivities of soil respiration components on changing climate patterns are currently not fully understood. We used trench and isotopic methods to separate total soil respiration into autotrophic (R _A ) and heterotrophic components (R _H ). This study was undertaken on a Robinia pseudoacacia L. plantation in the southern Taihang Mountains, China. The fractionation of soil ¹³CO₂ was analyzed by comparing the δ¹³C of soil CO₂ extracted from buried steel tubes with results from Gas Vapor Probe Kits at a depth of 50 cm at the preliminary test (2.03‰). The results showed that the contribution of autotrophic respiration (fR _A ) increased with increasing soil depth. The contribution of heterotrophic respiration (fR _H ) declined with increasing soil depth. The contribution of autotrophic respiration was similar whether estimated by the trench method (fR _A , 23.50%) or by the isotopic method in which a difference in value of ¹³C between soil and plant prevailed in the natural state (RC, 21.03%). The experimental error produced by the trench method was insignificant as compared with that produced by the isotopic method, providing a technical basis for further investigations.     

Carbon Isotope Discrimination, Gas Exchange and Stem Growth of Four Euramerican Hybrid Poplars under Different Watering Regimes

Seasonal changes in carbon isotope discrimination (Δ) and gas exchange traits were assessed in four Populus×euramericana clones differing in growth potential. Measurements were made during the second year after establishment in the field under two watering regimes, which were defined by the time-span between flood irrigations, hence resulting in different dry-down cycles: high irrigation (conservative schedule currently applied in the Ebro Valley, Spain) and low irrigation (equivalent to about a one-fourth reduction in water inputs). Net CO₂ assimilation rate (A), stomatal conductance (g_s), intrinsic water-use efficiency (A/g_s) and other related photosynthetic traits (leaf nitrogen concentration, leaf greenness and leaf mass per area) were measured prior to watering, and Δ was analysed in water-soluble leaf extracts (Δ_s) and bulk leaves (Δ_l). Stem growth was monitored over 3 years starting at the year of establishment (1998). Data were subjected to a repeated measures ANOVA over time for a randomised block split-plot design across watering regimes. Significant differences between watering regimes were detected using a long-term estimate of photosynthetic performance such as Δ_l, in agreement with changes in soil water status and evapotranspirative demand. However, the lack of significant genotype×watering regime interactions for gas exchange traits and Δ_s suggested that water shortage imposed by low irrigation was not sufficient to reveal physiological adaptations to drought. In this regard, the reduction in water inputs brought about by low irrigation did not reduce tree growth for any of the clones, suggesting that the current irrigation scheme employed in the region is superfluous to the water consumption needs of poplars. Genotypic variation was detected in gas exchange traits, Δ_s, Δ_l and stem growth under both watering treatments. Significant correlations with stem volume for Δ_s (r = −0.60, p<0.05) and A (r = + 0.61, p<0.05) suggested that growth was improved by higher water-use efficiency (the ratio of carbon fixed to water lost, as inferred by Δ_s) due to variation in A rather than in g_s. This observation corroborated the expectation derived from current theories that a lower Δ is related to higher stem volume, as a result of changes in net CO₂ assimilation rates.