Effects of a heather beetle attack on soil moisture and water balance at a Danish heathland

The effects of reclamation and high atmospheric nitrogen deposition have for long threatened the existence of heathlands in Denmark. A high nitrogen input increases the frequency and intensity of heather beetle attacks. However, any indirect effects of these attacks on the soil water balance are seldom investigated. In autumn 1994 a 2000-year old Danish inland heath was struck by a severe heather beetle attack and the effects on the soil moisture and the water balance were studied. Soil water content, gross precipitation and throughfall were measured continuously from 1993 to 1998 at the heath. The first signs of the attack on the water balance were seen in the dry summer of 1995 when the soil water content was relatively high. Four years after the beetle attack, new heather plants covered the area again and during summer the soil water seemed to be depleted to the same degree as before the beetle attack. In the years after the beetle attack a high coefficient of variation between individual soil moisture measurements was seen. It is proposed that the inhomogeneous wetting was caused by heterogeneous throughfall, water-repellent soil and break down of the structure of the organic top-horizons due to the beetle attack. The effect of the beetle attack was examined using a simple water balance model. Model simulations showed that evapotranspiration was reduced by respectively, 14, 29 and 5% in the three years following the beetle attack. From 1993 to 1998 percolation was on average 62% of precipitation with very little variation from year to year. Evapotranspiration was on average 38%, but in the years affected most by the beetle attack transpiration was relatively low whereas evaporation from soil was increased. A comparison between the modelled and measured throughfall, as well as percolation estimated by the chloride mass balance method, showed that the water balance parameters were estimated well in the two years which were most affected by the beetle attack, i.e. 1995 and 1996.     

Vkgetation analysis along a successional gradient from heath to oak forest

At Hjelm Hede in Denmark where an oak shrub is invading into a heathland the successional process has been studied on poor sandy outwashed plain. The study used a transect through a successional gradient from open heath to oak shrub where vegetation, major topsoil parameters and tree characteristics were analysed. The study enables us with spatial variation of key parameters to combine numerous detailed studies from the same site into an overall picture of changing vegetation and soil parameters reflecting the succession, which has been documented previously. The data were analysed by the use of Detrended Correspondence Analysis (DCA). Along the gradient from heath to forest the thick mor layer of the heathland soil decomposed, the CM ratio decreased from 25 to 15, pH increased and ammonium became available for the plants during a time span of approximately 120 years. The vegetation on the open heath was typical Danish inland heath vegetation dominated by Calluna vulgaris, Empetrum nigrum and Deschampsiaflexuosa. Under the young oak shrub the vegetation was dominated by Deschampsia Jexuosa, Trientalis europaea , and Maianthemum bijiolium. In the oldest and most forest like parts of the shrub a vegetation dominated by Holcus mollis, Anemone nemomsa and Stellaria holostea was found. Woody chamaephytes were replaced first by hemicryptophytes and later by geophytes. The vegetation's Ellenberg values indicated an increase in nitrogen availability and a decrease in acidity and light availability through secondary succession.     

Leaching of nitrate and ammonium in heathland and forest ecosystems in Northwest Germany under the influence of enhanced nitrogen deposition

To study the impact of high atmospheric nitrogen deposition on the leaching of NO₃^– and NH₄⁺ beneath forest and heathland vegetation, investigations were carried out in adjacent forest and heathland ecosystems in Northwest Germany. The study area is subjected to high deposition of nitrogen ranging from 15.9 kg ha^–1 yr^–1 in bulk precipitation to 65.3 kg ha^–1 yr^–1 beneath a stand of Pinus sylvestris L. with NH₄–N accounting for 70–80% of the nitrogen deposited. Considerable leaching of nitrogen compounds from the upper horizons of the soil, mostly as nitrate, occurred at most of the forest sites and below a mixed stand of Calluna vulgaris (L.) Hull. and Erica tetralix, but was low in a Betula pubescens Ehrh. swamp forest as well as beneath Erica tetralix L. wet heath and heath dominated by Molinia caerulea(L.) Moench. Ground water concentrations of both NO₃–N and NH₄–N did not exceed 1 mg L^–1 at most of the sites investigated.     

Long‐term influence of sod cutting depth on the restoration of degraded wet heaths

Temperate heaths have an unfavorable conservation status in most European biogeographical regions. Increasing nitrogen levels promote competitive grass species such as Molinia caerulea, which is a main threat to heathland conservation in Europe. This article investigates the long‐term influence of sod cutting and the resulting changes in soil properties on the heath composition, integrity, and structure. In 15 nature reserves across the northern half of Belgium, we used (1) a large number of plots (203); (2) a broad range of sod cut depths (2–40 cm), and (3) a temporal dimension that describes how long the effects of sod cutting persist (census up to 19 years after sod cutting). Multivariate analyses were used in order to explore the influence of sod cut depth and time after sod cutting on the soil and vegetation properties. There was a positive relationship between sod cut depth and soil pH and water level, and a negative relationship with Al³⁺, NH₄⁺, and total organic matter (TOM). However, only a limited number of typical (target) species appeared after sod cutting, and then only weakly. Most of the time they remained a minor component of the restored vegetation. Moreover, M. caerulea reappeared and its cover significantly increased during the years following sod cutting. Although we were able to show that sod cut depth has a differential effect on soil properties and vegetation recovery, it also appeared that sod cutting does not restore wet heaths in the long term when applied in regions with high nitrogen deposition.     

南方红壤侵蚀区不同植被恢复年限下芒萁叶功能性状对土壤因子的响应

目前关于我国南方红壤侵蚀区生态恢复植被—芒萁(Dicranopteris dichotoma)功能性状(Functional Traits)的研究鲜有报道。以该区域不同植被恢复年限下(包含0年未恢复地、6年、12年、18年、36年以及80年的次生林地)芒萁叶功能性状为研究对象,结合不同植被恢复年限下土壤因子的变化,采用样方调查分析法,探讨随植被恢复年限的增长芒萁叶功能性状的变化规律及其相互关系,以及芒萁叶功能性状对土壤因子的响应。结果表明,芒萁叶功能性状各指标随着植被恢复年限的增加变化差异明显(P0.05),并随植被恢复年限的增长表现出胁迫型、扩展型、竞争型不同功能性状组合;芒萁叶功能性状间的相关性也表现出显著的特征(P0.01);主成分分析(Principal Component Analysis, PCA)表明叶全氮含量、比叶面积和叶全磷含量主成分贡献排名高,可作为芒萁叶功能性状评价的主要指标。通过冗余分析(Redundancy Analysis, RDA)发现,土壤全碳含量、土壤全氮含量、土壤全磷含量对芒萁叶功能性状各项指标影响显著(P0.01)。芒萁伴随着植被恢复年限的增长,叶功能性状表现出不同的组合状况,且受土壤因子变化表现出相应的适应策略,反映了芒萁叶片随土壤因子变化形态可塑性强,作为重要的生态恢复植被有利于南方红壤侵蚀区植被的有效建植与恢复。     

Recolonization of coastal heath by Pseudomys novaehollandiae (Muridae) following sand mining

The regeneration of coastal heath after disturbance by mineral sand mining was studied on mid-seral stages from 4 to 11 years old at Hawks Nest, N.S.W. The main purpose was to gain some understanding of factors influencing recolonization by pioneer species of small mammals such as Pseudomys novaehollandiae. Changes in floristics and vegetation structure with time were studied as possible contributing factors together with environmental variables. An apparently linear relationship was found between plant species diversity (X₁) and P. novaehollandiae biomass which was also correlated with an index representing the proportion of heath species present (X₂). Both plant species diversity and P. novaehollandiae biomass showed a linear increase with regeneration age. A multiple regression analysis revealed a predictive equation explaining 96% of the variation in P. novaehollandiae biomass (Y): Y=-7.92 + 1.21X₁+3.92X₂ - 3.09X₃ The third variable (X₃) is a measure of soil hardness. A path diagram using contribution coefficients based on a partial correlation analysis included the effects of vegetation structure below 50 cm and regeneration age. On mid-seral stages after sand mining P. novaehollandiae is associated with areas having a variety of heath plants, with vegetation cover below 50 cm and softer substrates; its abundance increases with increasing regeneration age and with the total amount of vegetation present. A minimum of 20 years is seen to be necessary for both total amount of vegetation and P. novaehollandiae biomass to reach values encountered on control plots of undisturbed heath.     

Recovery of subalpine dwarf shrub heath after neighbour removal and fertilization

We tested if subalpine heath vegetation in northern Italy recovered after experimental perturbation of soil nutrient availability (fertilization) and species composition (removal of co-dominant dwarf shrubs). Species cover was assessed non-destructively before the start of the treatments (1995), at the end of the treatments (1999) and 4 years after the treatments ended (2003). Shrub removal had rather modest effects on heath vegetation, except for mosses which decreased significantly in removal plots. Fertilization decreased the cover of shrubs, mosses, and some graminoids but increased the cover of Festuca rubra. Fertilization converted heath to grassland, but the response of graminoid species was individualistic. Fertilization decreased vascular species richness and evenness, probably through negative effects of shading and litter accumulation on plant growth or recruitment. The vegetation had not recovered completely 4 years after the perturbations had stopped. This suggests that, in contrast to rapid responses to species removal and fertilization, recovery from these perturbations was rather slow, presumably because recovery was affected by long-term biotic interactions and species controls on ecosystem properties.     

基质和水氮处理对假茉莉水分利用率及生物量的影响

假茉莉(Clerodendrum inerme)是一种重要的半红树植物,是海岛及沿海防护林的重要组成部分,淡水、土壤是海岛植被恢复的重要生态因子。为了总结出最适合假茉莉生长发育的土壤及水肥条件,该文设置了不同基质、浇水频率及氮素处理,对不同处理下假茉莉的生物量及水分利用率进行了研究。结果表明:(1)在不同的基质中,含泥炭的混合基质对于假茉莉生物量的积累具有突出作用,S_4基质(珊瑚砂∶泥炭∶红壤土∶椰糠=12∶2∶3∶3)地下生物量(平方根)为1.66 g,极显著高于不含泥炭的基质,S_1基质(珊瑚砂∶泥炭=3∶2)总生物量(平方根)为4.54 g,极显著高于其他基质。(2)中、低浇水频率下的总生物量分别为4.02、4.23 g,极显著高于高浇水频率下的3.46 g,而不同氮素处理间生物量的差异并不显著,这表明假茉莉在中、低浇水频率,少施氮肥或不施氮肥的条件下仍然能够获得较高的生物量。(3)对于水分利用率而言,原珊瑚砂基质保水性较差,水分利用率较低,而混合基质在中、低浇水频率下水分利用率均有不同显著程度的提高。综上结果显示,假茉莉在含泥炭基质,中、低浇水频率,无氮或低氮条件下能够较好生长。     

Increased inorganic nitrogen leaching from a mountain grassland ecosystem following grazing removal: a hangover of past intensive land-use?

Heathlands and grasslands occur in montane regions, naturally or due to anthropogenic land-use. These are typically nutrient-poor but exposure to elevated nitrogen deposition and intensive livestock grazing causes large-scale ecological change. We studied the long-term implications of grazing removal on soil and drainage water biogeochemistry and the implications for nitrogen cycling in 50-year replicated grazing exclosures on a montane grassland exposed to high rates of ambient nitrogen deposition. Evidence of ‘ecosystem recovery’ represented by successional change from graminoid to shrub-dominance after cessation of grazing was not reflected in the soil biogeochemistry. Cessation of grazing had a negative impact, with increased soil extractable and soil solution nitrate concentrations; an apparent shift towards a more nitrogen-rich, bacterially dominated microbial community; and the acidification of soils and leachate. The increase in nitrate leaching appears to have been counterbalanced by a decrease in dissolved organic nitrogen leaching, approximately maintaining the overall nitrogen balance of the system, whilst apparently altering ecosystem functioning. High rates of organic matter cycling and inorganic nitrogen uptake in grazed grassland may have sustained ecosystem N limitation under elevated nitrogen deposition. Grazing removal caused long-term over-supply of nitrogen from mineralisation of enriched organic matter, exacerbated by continued high nitrogen deposition, exceeding the uptake demand of heath vegetation and resulting in nitrification and nitrate leaching. This disequilibrium between vegetation and soil following grazing removal has implications for restoration after periods of intensive grazing. Grazing may not simply leave a legacy of nutrient enrichment but its cessation may trigger nitrogen saturation and soil and freshwater eutrophication and acidification which counteract the immediate benefits of natural vegetation recovery. Long term, nitrogen saturation of abandoned grasslands is likely to reduce ecosystem resilience to invasion by nitrophilous species, pathogen attack and vulnerability to environmental pressures such as climate change. We conclude that partial and/or phased reduction in grazing levels may permit the more synchronised recovery of soils and vegetation, thereby avoiding imbalances between nitrogen supply and nitrogen demand and detrimental ecological effects.     

Carbon and nitrogen in soil and vegetation at sites differing in successional age

We studied vegetation and soil development during primary succession in an inland drift sand area in the Netherlands. We compared five sites at which primary succession had started at different moments in the past, respectively 0, 10, 43 and 121 years ago, and a site at which succession had not yet started. In the three younger sites the vegetation was herbaceous, whereas in the two older sites a pine forest had formed. Forest formation was accompanied by the development of an FH-layer in the soil, an increase in the amount of soil organic matter, and an increase in nitrogen mineralisation rate from 1.9 to 18 g N m^–2 yr^–1. Soil moisture content also increased, whereas pH showed a steady decrease with site age. The vegetation changed from a herbaceous vegetation dominated by mosses and lichens and the grass species Corynephorus canescens and Festuca ovina towards a pine forest with an understorey vegetation dominated by Deschampsia flexuosa and, at the oldest site, with dwarf shrubs Empetrum nigrum and Vaccinium myrtillus. At the same time the total amounts of carbon and nitrogen of the ecosystem increased, with a relatively stronger increase of the carbon pool. The establishment of trees during succession greatly affects the dynamics of the ecosystem, especially its carbon dynamics.     

Phosphorus availability and microbial respiration across different tundra vegetation types

Phosphorus (P) is an important nutrient in tundra ecosystems that co-limits or in some cases limits primary production. The availability of P is largely driven by soil characteristics, e.g., pH, organic carbon, and abundance of P-sorbing elements such as aluminium (Al) or iron (Fe). We tested how vegetation and soil properties relate to P availability across different tundra vegetation types. The different soil P fractions in the organic horizon were measured and plant foliar nitrogen (N) to P ratio and a plant bioassay was used as indicators of plant nutrient status. Microbial bioassays were used to study microbial respiration kinetics and in response to carbon, N, and P amendments. The distribution of P fractions differed significantly across vegetation types; labile fractions of P were less abundant in meadow sites compared to heath sites. Calcium-phosphates seemed to be an important P-fraction in meadows, but were only found in lower concentrations in the heath. There were only small differences in NaOH–extractable P between the vegetation types and this correlated with the distribution of oxalate-extractable Al. Plant N:P ratios and the plant bioassay indicated decreasing P availability from dry heath to mesic heath to mesic meadow. The microbial bioassay suggests that the heterotrophic microbial community is C-limited with N as a secondary limiting nutrient although there were indications that microbial P availability was lower in the meadow sites. Overall, we suggest that the observed variations in soil P across vegetation types are affecting both plant and microbial function although the differences seem to be relatively small.     

Carbon dynamics at frost-patterned tundra driven by long-term vegetation change rather than by short-term non-growing season warming

Frost-patterned grounds, such as mostly barren frost boils surrounded by denser vegetation, are typical habitat mosaics in tundra. Plant and microbial processes in these habitats may be susceptible to short-term warming outside the growing season, while the areal cover of barren frost boils has decreased during the past decades due to climate warming-induced shrub expansion. The relative importance of such short-term and long-term climate impacts on carbon (C) dynamics remains unknown. We measured ecosystem CO₂ uptake and release (in the field), microbial respiration (in the laboratory), as well as microbial biomass N and soil extractable N in frost boils and the directly adjacent heath in late spring and late summer. These habitats had been experimentally warmed with insulating fleeces from late September until late May for three consecutive years, which allowed us to investigate the direct short-term effects of warming and longer-term, indirect climate effects via vegetation establishment into frost boils. Non-growing season warming increased C uptake at the frost boils in late spring and decreased it in late summer, while the timing and direction of responses was opposite for the heath. Experimental warming had no effects on microbial or ecosystem C release or soil N at either of the habitats. However, C cycling was manifold higher at the heath compared to the frost boils, likely because of a higher SOM stock in the soil. Short-term climate change can thus directly alter ecosystem C uptake at frost-patterned grounds but will most likely not affect microbial C release. We conclude that the C dynamics at frost-patterned grounds under a changing climate depend most strongly on the potential of vegetation to encroach into frost boils in the long-term.     

Mycelial colonization by bradyrhizobia and azorhizobia

This study examines mycelial colonization of common soil fungi by bradyrhizobia and an azorhizobial strain, resulting in the forming of biofilms. The effects of the fungal exudates on a bradyrhizobial strain have also been investigated. Bradyrhizobia gradually colonized the mycelia for about 18 days, after which the biofilm structures collapsed with the release of the rhizobial cell clusters to the culture medium. The azorhizobial strain showed differential colonization of the mycelia. In general, there were no considerable mycotoxin effects of the fungal exudates on the bradyrhizobial strain used, instead the rhizobial strain utilized the exudates as a source of nutrition. This study indicates that the present microbial association with biofilm formation has important implications in the survival of rhizobia under adverse soil conditions devoid of vegetation. Further, it could have developed an as yet unidentified nitrogen fixing system that could have contributed to the nitrogen economy of soils.     

High arctic heath soil respiration and biogeochemical dynamics during summer and autumn freeze‐in – effects of long‐term enhanced water and nutrient supply

In High Arctic NE Greenland, temperature and precipitation are predicted to increase during this century, however, relatively little information is available on the role of increased water supply on soil CO ₂ efflux in dry, high arctic ecosystems. We measured soil respiration (R_soil) in summer and autumn of 2009 in combination with microbial biomass and nutrient availability during autumn freeze‐in at a dry, open heath in Zackenberg, NE Greenland. This tundra site has been subject to fully factorial manipulation consisting of increased soil water supply for 14 years, and occasional nitrogen (N) addition in pulses. Summer watering enhanced R_soil during summer, but decreased R_soil in the following autumn. We speculate that this is due to intensified depletion of recently fixed plant carbon by soil organisms. Hence, autumn soil microbial activity seems tightly linked to growing season plant production through plant‐associated carbon pools. Nitrogen addition alone consistently increased R_soil, but when water and nitrogen were added in combination, autumn R_soil declined similarly to when water was added alone. Despite several freeze‐thaw events, the microbial biomass carbon (C) remained constant until finally being reduced by ~60% in late September. In spite of significantly reduced microbial biomass C and phosphorus (P), microbial N did not change. This suggests N released from dead microbes was quickly assimilated by surviving microbes. We observed no change in soil organic matter content after 14 years of environmental manipulations, suggesting high ecosystem resistance to environmental changes.     

Sandblasting as a possible factor controlling the distribution of plants on a coastal dune system

Intensity of the abrasive effect of wind-borne sand – sandblasting – in addition to other environmental factors was measured at two vegetation zones on a sandy beach and one site at an inland area. One zone on the beach included foredunes sparsely vegetated by dune species such as Carex kobomugi and Calystegia soldanella. The other zone which was located ∼50 m inland from the first zone was flat grassland dominated by inland species such as Miscanthus sinensis and Imperata cylindrica var. Koenigii. The inland site consisted of short grassland located 3 km inland from the beach. Intensity of sandblasting was estimated by the whiteness of a transparent plastic sheet exposed to the air for 2 weeks. This sheet turned whitely opaque when it was abraded by wind-borne sand. The other environmental factors measured at the beach were intensity of salt spray, soil water content, soil salinity, and sand accumulation, while intensity of salt spray was the only additional factor measured at the inland site. Intensity of sandblasting was considerably higher at the foredune zone, while that at the grassland zone was as low as that at the inland site. Considerable salt spray was detected at the foredune and grassland zones. Differences in other environmental factors were small between the two zones on the beach. In order to compare the difference in tolerance to sandblasting, a jet of sand was applied to one ordinary species, C. kobomugi, from the foredune and two species, M. sinensis and I. cylindrica, from the grassland zone. The difference in tolerance was determined by the decrease in the area of green leaf after applying sandblasting with commercial sandblaster and/or spraying with sea water. M. sinensis and I. cylindrica lost much of the leaf area after sandblasting and salt spraying, while C. kobomugi lost little. These results indicated that one of the characteristic environmental factors of a foredune is the high intensity of sandblasting accompanied by salt spray, and that species found in the foredune are more tolerant to sandblasting than species distributing in more inland areas.     

Effects of topographic and edaphic conditions on alpine plant species distribution along a slope gradient on Mount Norikura,central Japan

This study examined the effects of topographic and edaphic conditions on alpine plant species distribution along a slope gradient on Mt. Norikura (3026 m a.s.l.), central Japan. Topographic and edaphic factors investigated at 40 plots were: slope inclination, ground surface texture, soil water content and soil inorganic nitrogen concentration (NO₃-N, NH₄-N). The topographic and edaphic factors changed with slope positions: slope inclination was steeper, soil texture was coarser, and soil water and inorganic nitrogen concentration decreased with increasing slope position. Five vegetation types were located along the slope gradient and related to two factor-groups: (1) changes in soil water, NH₄-N, slope inclination along the slope gradient, and (2) ground surface texture. A tall herbaceous plant community developed at the low slope position, near tarns, with fine soil surface texture, high soil water and NH₄-N, while Dicentra peregrina dominated on an unstable rubble slope near the ridge top. The distribution of each species was predictable from the two factor-groups. Although the five vegetation types were related to the two factor-groups, responses to the two factor-groups differed among species, even within the same vegetation type. Therefore, this study showed that the topography of the terrain largely regulated alpine plant distribution by affecting edaphic conditions, and that global warming may alter species composition by changing edaphic conditions.     

贡湖湾退圩还湖区水位高程下植被分布格局与土壤特征

为揭示水陆交错带水位高程下植被分布格局、多样性指数和土壤特征的变化,于2015年10月对贡湖湾湿地北部生态修复区不同水位高程下植物群落组成和土壤特征进行调查,结果表明:整个研究区有165种植物,隶属65科、142属,多为多年生植物且受人为影响严重;随水位高程增加,植物群落R、H、D、J呈现逐渐递增趋势,具体表现为常年水淹区G1季节性水淹区G2水淹区G3;土壤容重及机械组成中粉、黏粒含量随水位高程升高而增加;随土壤营养物渐增其植物群落多样性增加,多样性和优势度指数与土壤全氮(TN和有机质(OM)负相关。OM含量呈"V"型先减后增分布;TN呈逐渐增多的趋势;速效钾(EP)含量无明显波动,约为13.50 mg/kg;有效磷(AP)含量先减少后基本不变;4个多样性指数间正相关;TN与OM显著正相关(R~2=0.533);OM与EP负相关(R~2=-0.144;TN与丰富度、多样性、优势度正相关;AP与4个多样性指数均正相关;pH值与4个多样性指数负相关;回归分析可见,TN和OM是影响植物群落多样性的关键因子。科学的植被管理、全面调控土壤营养盐含量有助于水陆交错带植被恢复。     

Influence of soil and microclimate on species composition and grass encroachment in heath succession

Aims and Methods Mostly due to land use changes, European heathlands have become increasingly rare. In addition, the increasing amount of atmospheric nitrogen deposition has resulted in an encroachment of grasses and a loss in species diversity. Despite many investigations, information about the precise environmental parameters that determine the development and maintenance of heathland vegetation is still insufficient. In order to determine the environmental factors that control heath succession and grass encroachment, and to develop appropriate management schemes, we studied the influence of several soil and microclimate parameters on species composition and vegetation characteristics in five successional stages in a coastal heathland on the island of Hiddensee, north-east Germany, where the encroachment of Carex arenaria has become a major problem.Important findings We recorded the highest plant species richness in grey dune and birch forest plots, while the encroachment of C. arenaria let to a significant decline in plant species richness. The most important environmental factors influencing species richness and distribution of single species were microclimate, soil moisture, soil pH and the C/N ratio. While many studies reported the importance of differences in nutrient availability, we found no significant correlations between soil nutrient availability and vegetation pattern. Environmental conditions in dense C. arenaria stands, especially soil properties (e.g. soil pH), showed great differences in comparison to the other successional stages. However, no correlations between the encroachment of C. arenaria and single environmental factors were found. Our results show that not only soil nutrients are important abiotic factors in heaths but that also microclimate and soil moisture play an important role and that many factors are involved in heath succession and in the promotion of grass encroachment. Management plans for the conservation and restoration of heathlands should therefore focus on the specific site conditions and should take several abiotic and biotic factors into account.     

Nitrogen in interstitial waters in the Sahel; Natural Baseline, Pollutant or Resource?

Nitrate in the unsaturated zone between the soil surface and the water table was studied in agroforestry Parklands in north western Senegal by examination of samples obtained by hand auger. Depending on location, water tables existed at depths between 10 and 35m below ground. Previous studies of groundwater in this region had found that large concentrations of nitrate were unconnected with anthropogenic activity. The objective of this study was to determine whether nitrogen fixing vegetation had a role in groundwater nitrate accumulation and whether roots of trees were located deeply enough to access the nitrate. Accordingly, sample profiles were augered close to stems of nitrogen fixing trees, non-nitrogen fixing trees and also in adjacent areas that were unaffected by tree presence. These adjacent areas were typically open pasture or cultivated fields. Tree fine roots were quantified in the samples and examined for the presence of mycorrhizas. Similarly, sand/soil samples were examined and tested for the presence of nitrogen fixing rhizobia that were capable of forming functional nodules on appropriate host plants. Concentrations of nitrate were greatest in soils beneath nitrogen fixing trees and nitrate was more plentiful in profiles augered beneath nitrogen fixing crops than it was elsewhere suggesting that N-fixation was the source of the nitrate. The concentrrations of nitrate that were found in the unsaturated zone were greatly in excess of the WHO recommended limit for nitrate in drinking water. High NO₃-N/Cl ratios confirm insitu production of nitrate, and indicate that this is a natural baseline occurrence related to N-fixation. The nitrate is moving down the profile and impacts the groundwater unless it can be intercepted by plant roots. NO₃-N amounts in solution in the soil profile varied between 75 and 1000kg ha^–1 beneath trees and between 120 and 400kg ha^–1 in areas outwith tree crowns. Although these quantities of N occupy the lower end of the range of N values obtained in north American deserts, they comprise a considerable dryland resource where amounts of organic fertilizer are limited and where cost prohibits the use of commercial fertilizers. Roots of both nitrogen fixing and non-nitrogen fixing trees were deep enough to access the nitrate but the small amounts of available water at intermediate depths suggest that large scale uptake of nitrate will only be possible in the wetter zones located close to the water table. Shallow roots tended to be more heavily colonized by mycorrhizas than deeper roots but mycorrhizas were recovered from roots located 22m below ground. Tree roots and rhizobia had similar patterns of distribution. They were commonest close to the soil surface, less frequent at intermediate depths and tended to increase in frequency close to the water table.     

Feedback of carbon and nitrogen cycles enhances carbon sequestration in the terrestrial biosphere

The efforts to explain the ‘missing sink’ for anthropogenic carbon dioxide (CO₂) have included in recent years the role of nitrogen as an important constraint for biospheric carbon fluxes. We used the Nitrogen Carbon Interaction Model (NCIM) to investigate patterns of carbon and nitrogen storage in different compartments of the terrestrial biosphere as a consequence of a rising atmospheric CO₂ concentration, in combination with varying levels of nitrogen availability. This model has separate but closely coupled carbon and nitrogen cycles with a focus on soil processes and soil–plant interactions, including an active compartment of soil microorganisms decomposing litter residues and competing with plants for available nitrogen. Biological nitrogen fixation is represented as a function of vegetation nitrogen demand. The model was validated against several global datasets of soil and vegetation carbon and nitrogen pools. Five model experiments were carried out for the modeling periods 1860–2002 and 2002–2100. In these experiments we varied the nitrogen availability using different combinations of biological nitrogen fixation, denitrification, leaching of soluble nitrogen compounds with constant or rising atmospheric CO₂ concentrations. Oversupply with nitrogen, in an experiment with nitrogen fixation, but no nitrogen losses, together with constant atmospheric CO₂, led to some carbon sequestration in organismic pools, which was nearly compensated by losses of C from soil organic carbon pools. Rising atmospheric CO₂ always led to carbon sequestration in the biosphere. Considering an open nitrogen cycle including dynamic nitrogen fixation, and nitrogen losses from denitrification and leaching, the carbon sequestration in the biosphere is of a magnitude comparable to current observation based estimates of the ‘missing sink.’ A fertilization feedback between the carbon and nitrogen cycles occurred in this experiment, which was much stronger than the sum of separate influences of high nitrogen supply and rising atmospheric CO₂. The demand‐driven biological nitrogen fixation was mainly responsible for this result. For the modeling period 2002–2100, NCIM predicts continued carbon sequestration in the low range of previously published estimates, combined with a plausible rate of CO₂‐driven biological nitrogen fixation and substantial redistribution of nitrogen from soil to plant pools.