Effect of lipid removal on carbon and nitrogen stable isotope ratios in crustacean tissues

The analysis of tissue's naturally occurring stable carbon and nitrogen isotope ratios is a useful tool to delineate trophic relationships. However, the interpretation of δ¹³C and δ¹⁵N is complicated by the influence of multiple factors such as the tissue-specific lipid content. The aim of this work was to evaluate the effects of lipid extraction on δ¹³C and δ¹⁵N compositions in muscle, hepatopancreas and gonads of a marine decapod crustacean, the spider crab Maja brachydactyla. Samples were analyzed for stable isotopes before and after lipid removal, using a derived Soxhlet extraction method. Differences in δ¹³C and δ¹⁵N were measured among tissues before and after treatment. Lipid extraction of muscle did not have a significant effect on either δ¹³C or δ¹⁵N. By contrast, ecologically significant shifts for both carbon and nitrogen stable isotopes ratios (+ 2.9 ± 0.8‰ for δ¹³C, and + 1.2 ± 0.7‰ for δ¹⁵N) were noticed in the hepatopancreas. In regard to gonads, lipid extraction led to a shift only on δ¹³C (+ 1.3 ± 0.3‰). Finally, the derived Soxhlet extraction method removed the lipid influence for δ¹³C, and had an effect on δ¹⁵N composition for lipid-rich samples. We recommend this treatment for carbon stable isotope studies on decapod crustacean lipid-rich tissues.     

Root dynamics influence tree–grass coexistence in an Australian savanna

Both resource and disturbance controls have been invoked to explain tree persistence among grasses in savannas. Here we determine the extent to which competition for available resources restricts the rooting depth of both grasses and trees, and how this may influence nutrient cycling under an infrequently burned savanna near Darwin, Australia. We sampled fine roots <2 mm in diameter from 24 soil pits under perennial as well as annual grasses and three levels of canopy cover. The relative proportion of C₃ (trees) and C₄ (grasses) derived carbon in a sample was determined using mass balance calculations. Our results show that regardless of the type of grass both tree and grass roots are concentrated in the top 20 cm of the soil. While trees have greater root production and contribute more fine root biomass grass roots contribute a disproportional amount of nitrogen and carbon to the soil relative to total root biomass. We postulate that grasses maintain soil nutrient pools and provide biomass for regular fires that prevent forest trees from establishing while savanna trees, are important for increasing soil N content, cycling and mineralization rates. We put forward our ideas as a hypothesis of resource‐regulated tree–grass coexistence in tropical savannas.     

Estimating the relative nutrient uptake from different soil depths in Quercus robur, Fagus sylvatica and Picea abies

The distribution of fine roots and external ectomycorrhizal mycelium of three species of trees was determined down to a soil depth of 55 cm to estimate the relative nutrient uptake capacity of the trees from different soil layers. In addition, a root bioassay was performed to estimate the nutrient uptake capacity of Rb⁺ and NH₄⁺ by these fine roots under standardized conditions in the laboratory. The study was performed in monocultures of oak (Quercus robur L.), European beech (Fagus sylvatica L.) and Norway spruce [Picea abies (L.) Karst.] on sandy soil in a tree species trial in Denmark. The distribution of spruce roots was found to be more concentrated to the top layer (0–11 cm) than that of oak and beech roots, and the amount of external ectomycorrhizal mycelia was correlated to the distribution of the roots. The uptake rate of [⁸⁶Rb⁺] by oak roots declined with soil depth, while that of beech or spruce roots was not influenced by soil depth. In modelling the nutrient sustainability of forest soils, the utilization of nutrient resources in deep soil layers has been found to be a key factor. The present study shows that the more shallow-rooted spruce can have a similar capacity to take up nutrients from deeper soil layers than the more deeply rooted oak. The distribution of roots and mycelia may therefore not be a reliable parameter for describing nutrient uptake capacity by tree roots at different soil depths.     

Seasonal variations in bulk tissue, fatty acid and monosaccharide δC values of leaves from mesotrophic grassland plant communities under different grazing managements

Leaves of 26 grass, herb, shrub and tree species were collected from mesotrophic grasslands to assess natural variability in bulk, fatty acid and monosaccharide δ¹³C values under different grazing management (cattle- or deer-grazed) on three sample dates (May, July and October) such that interspecific and spatiotemporal variations in whole leaf tissues and compound-specific δ¹³C values could be determined. The total mean leaf bulk δ¹³C value for plants was −28.9‰ with a range of values spanning 7.5‰. Significant interspecific variation between bulk leaf δ¹³C values was only determined in October (P = <0.001) when δ¹³C values of the leaf tissues from both sites was on average 1.5‰ depleted compared to during July and May. Samples from May were significantly different between fields (P = 0.03) indicating an effect from deer- or cattle-grazing in young leaves. The average individual monosaccharide δ¹³C value was 0.8‰ higher compared with whole leaf tissues. Monosaccharides were the most abundant components of leaf biomass, i.e. arabinose, xylose, mannose, galactose and glucose, and therefore, fluctuations in their individual δ¹³C values had a major influence on bulk δ¹³C values. An average depletion of ca. 1‰ in the bulk δ¹³C values of leaves from the deer-grazed field compared to the cattle-grazed field could be explained by a general depletion of 1.1‰ in glucose δ¹³C values, as glucose constituted >50% total leaf monosaccharides. In October, δ¹³C values of all monosaccharides varied between species, with significant variation in δ¹³C values of mannose and glucose in July, and mannose in May. This provided an explanation for the noted variability in the tissue bulk δ¹³C values observed in October 1999. The fatty acids C_16:0, C_18:2 and C_18:3 were highly abundant in all plant species. Fatty acid δ¹³C values were lower than those of bulk leaf tissues; average values of −37.4‰ (C_16:0), −37.0‰ (C_18:2) and −36.5‰ (C_18:3) were determined. There was significant interspecific variation in the δ¹³C values of all individual fatty acids during October and July, but only for C_18:2 in May (P = <0.05). This indicated that seasonal trends observed in the δ¹³C values of individual fatty acids were inherited from the isotopic composition of primary photosynthate. However, although wide diversity in δ¹³C values of grassland plants ascribed to grazing management, interspecific and spatiotemporal influences was revealed, significant trends (P = <0.0001) for fatty acid and monosaccharide δ¹³C values: δ¹³C_16:0 < δ¹³C_18:2 < δ¹³C_18:3 and δ¹³C_arabinose > δ¹³C_xylose > δ¹³C_glucose > δ¹³C_galactose, respectively, previously described, appear consistent across a wide range of species at different times of the year in fields under different grazing regimes.     

Tree and grass rooting patterns in an African humid savanna

Abstract. Spatial and temporal soil partitioning between roots of the two savanna plant components, i.e. trees and grasses, were investigated in a West African humid savanna. Vertical root phytomass distribution was described for grass roots, large (> 2 mm) and fine (< 2 mm) tree roots, in open sites and beneath tree canopies. These profiles were established monthly over one year of vegetation growth. Natural ¹³C abundance measurement was used to determine the woody/herbaceous phytomass ratio in root samples. Tree and grass root distributions widely overlapped and both were mostly located in the top 20 cm of the soil. Grass root phytomass decreased with depth whereas woody root phytomass peaked at about 10 cm depth. No time partitioning was detected. These structural results do not support the hypothesis of soil resource partitioning between trees and grasses and are thus consistent with functional results previously reported.     

The vertical distribution of N and K uptake in relation to root distribution and root uptake capacity in mature Quercus robur, Fagus sylvatica and Picea abies stands

We have measured the uptake capacity of nitrogen (N) and potassium (K) from different soil depths by injecting ¹⁵N and caesium (Cs; as an analogue to K) at 5 and 50 cm soil depth and analysing the recovery of these markers in foliage and buds. The study was performed in monocultures of 40-year-old pedunculate oak (Quercus robur), European beech (Fagus sylvatica) and Norway spruce (Picea abies (L.) Karst.) located at an experimental site in Palsgård, Denmark. The markers were injected as a solution through plastic tubes around 20 trees of each species at either 5 or 50 cm soil depth in June 2003. After 65 days foliage and buds were harvested and the concentrations of ¹⁵N and Cs analysed. The recovery of ¹⁵N in the foliage and buds tended to be higher from 5 than 50 cm soil depth in oak whereas they where similar in spruce and beech after compensation for differences in immobilization of ¹⁵N in the soil. In oak more Cs was recovered from 5 than from 50 cm soil depth whereas in beech and spruce no difference could be detected. Out of the three investigated tree species, oak was found to have the lowest capacity to take up Cs at 50 cm soil depth compared to 5 cm soil depth also after compensating for differences in discrimination against Cs by the roots. The uptake capacity from 50 cm soil depth compared with 5 cm was higher than expected from the root distribution except for K in oak, which can probably be explained by a considerable overlap of the uptake zones around the roots and mycorrhizal hyphae in the topsoil. The study also shows that fine roots at different soil depths with different physiological properties can influence the nutrient uptake of trees. Estimates of fine root distribution alone may thus not reflect the nutrient uptake capacity of trees with sufficient accuracy. Our study shows that deep-rooted trees such as oak may have lower nutrient uptake capacity at deeper soil layers than more shallow-rooted trees such as spruce, as we found no evidence that deep-rooted trees obtained proportionally more nutrients from deeper soil layers. This has implications for models of nutrient cycling in forest ecosystems that use the distribution of roots as the sole criterion for predicting uptake of nutrients from different soil depths.     

Roots,soil water and crop yield: tree crop interactions in a semi-arid agroforestry system in Kenya

Variations in soil water, crop yield and fine roots of 3–4 year-old Grevillea robusta Cunn. and Gliricidia sepium (Jacq.) Walp. growing in association with maize (Zea mays L.) were examined in semiarid Kenya during the long rains of 1996 and 1997. Even although tree roots penetrated more deeply than maize roots, maximum root length densities for both tree species and maize occurred in the top 200 mm of the soil profile where soil moisture was frequently recharged by rains. Populations of roots in plots containing trees were dominated by tree roots at the beginning of the growing season but because tree roots died and maize root length increased during the cropping season, amounts of tree and maize roots were similar at the end of the season. Thus, there was evidence of temporal separation of root activity between species, but there was no spatial separation of the rooting zones of the trees and crops within that part of the soil profile occupied by crop roots. Tree root length density declined with increasing distances from rows of trees and with depth in the soil profile. Although Grevillea trees were largest, plots containing G. sepium trees always contained more tree roots than plots containing G. robusta trees and Gliricidia was more competitive with maize than Grevillea. Overall, Gliricidia reduced crop yield by about 50% and Grevillea by about 40% relative to crop yield in control plots lacking trees and reductions of crop yield were greatest close to trees. There was less soil moisture in plots containing trees than in control plots. Such difference between control plots and plots containing trees were maximal at the end of the dry season and there was always less soil moisture close to trees than elsewhere in the plots. Plots containing Gliricidia trees contained less soil water than plots containing Grevillea trees.     

Effects of filamentous algal mats on sulfide invasion in eelgrass (Zostera marina)

The effect of filamentous algae invasion into Zostera marina meadows on water quality, sediment sulfur pools and sulfide invasion into plant tissues was studied experimentally. Sulfide invasion was assessed through analysis of sulfur isotopic composition (δ³⁴S) and total sulfur (TS) concentrations in plant tissues. The algal mats (5 and 10 cm thickness) depleted oxygen in the mats and increased the pools of sulfides in the sediments. Plants exposed to algal mats had δ³⁴S signals closer to the δ³⁴S of sediment sulfide, whereas plants with no mats present had δ³⁴S signals closer to the δ³⁴S of seawater sulfate, indicating a higher sulfide invasion in plants exposed to algal mats. The δ³⁴S varied between the plant tissues with the leaves having more positive δ³⁴S signals than roots and rhizomes, indicating that sulfide was invading into the roots and moved to the other tissues through the lacunae. TS concentrations were higher in plants exposed to algal mats suggesting that sulfur derived from sediment sulfide accumulated in the plants. F_sulfide showed that up to 50% of the sulfides in the plants were derived from sedimentary sulfides. The combined effect of water column anoxia in the lower parts of the meadow and high sulfide invasion into the plants lead to significantly reduced growth rates after 3 weeks and the below-ground tissues showed signs of degradation suggesting that algal mats invasion in to Zostera marina meadows can result in seagrass decline.     

Temporary drought can selectively suppress Schoenoplectus mucronatus in rice

Outdoor pot experiments were conducted in California to quantify differences in rice and Schoenoplectus mucronatus susceptibility to drought and to identify morphological and physiological traits that would favor rice over S. mucronatus under drought. Plants were grown in flooded soil for approximately 5 weeks, and then subjected to different drought periods after which pots were re-flooded. Chlorophyll fluorescence assays revealed that rice and S. mucronatus Fv/Fm first became <0.8 after leaf water potential (Ψ_leaf) had decreased to approximately −4 MPa and −2 MPa, respectively. Thus, by suffering less photosynthetic damage from drought, rice had better recovery after re-flooding than S. mucronatus. When drought reduced Ψ_leaf to −3 MPa, S. mucronatus re-growth was nearly suppressed but that of rice was unaffected. Rice plants depleted soil moisture 1.6 faster than S. mucronatus due to larger and deeper roots and a high water-spending strategy (when Ψ_leaf decreased from approximately −0.5 MPa to −2.5 MPa, ¹³δ increased from −27.8 to −27.4 and from −28.1 to −26.0 for rice and S. mucronatus, respectively). Rice under interspecific competition sustained its Ψ_leaf by extracting more water from greater depths, while causing severe moisture stress and photosynthetic damage to S. mucronatus. Thus temporary drought enhanced rice competitiveness over S. mucronatus, supporting the concept of using brief drought as a tool for S. mucronatus suppression in rice. The Ψ_leaf developed by the end of the drought period predicted rice yields (R² = 0.77, P < 0.0001) and the capacity of S. mucronatus to recover from drought upon irrigation resumption (R² = 0.62, P < 0.001). Brief (8-10 d) drought imposed on 5-week-old rice did not significantly depress late-season rice biomass growth or grain yields, while S. mucronatus never fully recovered from drought. Rice yields were only reduced after Ψ_leaf reached values below approximately −2.5 MPa. Longer drought (∼20 d) delayed maturity and reduced rice yields by approximately 60-80%. The dry-down approach could help suppress weeds similar to S. mucronatus in organic rice where premium prices can compensate for lower grain yield.     

Small differences in root distributions allow resource niche partitioning

1. Deep roots have long been thought to allow trees to coexist with shallow‐rooted grasses. However, data demonstrating how root distributions affect water uptake and niche partitioning are uncommon.
2. We describe tree and grass root distributions using a depth‐specific tracer experiment six times over two years in a subtropical savanna, Kruger National Park, South Africa. These point‐in‐time measurements were then used in a soil water flow model to simulate continuous water uptake by depth and plant growth form (trees and grasses) across two growing seasons. This allowed estimates of the total amount of water a root distribution could absorb as well as the amount of water a root distribution could absorb in excess of the other rooting distribution (i.e., unique hydrological niche).
3. Most active tree and grass roots were in shallow soils: The mean depth of water uptake was 22 cm for trees and 17 cm for grasses. Slightly deeper rooting distributions provided trees with 5% more soil water than the grasses in a drier season, but 13% less water in a wetter season. Small differences also provided each rooting distribution (tree or grass) with unique hydrological niches of 4 to 13 mm water.
4. The effect of rooting distributions has long been inferred. By quantifying the depth and timing of water uptake, we demonstrated how even small differences in rooting distributions can provide plants with resource niches that can contribute to species coexistence. Differences in total water uptake and unique hydrological niche sizes were small in this system, but they indicated that tradeoffs in rooting strategies can be expected to contribute to tree and grass coexistence because 1) competitive advantages change over time and 2) plant growth forms always have access to a soil resource pool that is not available to the other plant growth form.

     

Below-ground competition between trees and grasses may overwhelm the facilitative effects of hydraulic lift

Under large East African Acacia trees, which were known to show hydraulic lift, we experimentally tested whether tree roots facilitate grass production or compete with grasses for below‐ground resources. Prevention of tree–grass interactions through root trenching led to increased soil water content indicating that trees took up more water from the topsoil than they exuded via hydraulic lift. Biomass was higher in trenched plots compared to controls probably because of reduced competition for water. Stable isotope analyses of plant and source water showed that grasses which competed with trees used a greater proportion of deep water compared with grasses in trenched plots. Grasses therefore used hydraulically lifted water provided by trees, or took up deep soil water directly by growing deeper roots when competition with trees occurred. We conclude that any facilitative effect of hydraulic lift for neighbouring species may easily be overwhelmed by water competition in (semi‐) arid regions.     

Assessing nitrogen fluxes from roots to soil associated to rhizodeposition by apple (<Emphasis Type="Italic">Malus domestica</Emphasis>) trees

The mass transfer from root to soil by means of rhizodeposition has been studied in grasses and forest trees, but its role in fruit trees is still unknown. In this study, N fluxes from roots to soil were estimated by applying a ¹⁵N mass balance technique to the soil–tree system. Apple (Malus domestica) trees were pre-labelled with ¹⁵N and then grown outdoors in 40 L pots for one vegetative season in (1) a coarse-textured, low organic matter soil, (2) a coarse-textured, high organic matter soil, and (3) a fine-textured, high organic matter soil. At tree harvest the ¹⁵N abundance of the soils was higher than at transplanting, but the total amount of ¹⁵N present in the tree–soil system was similar at transplanting and tree harvest. The soils had a strong effect on N fluxes from and to the soil. In the fine-textured soil, 11% of the total plant-derived nitrogen was transferred to the soil, compared with 2–5% in the two coarse-textured soils. Rhizodeposition was higher in the fine soil (18% of the primary production) than in the coarse-textured soils, whereas higher soil organic matter depressed rhizodeposition. Nitrogen uptake was almost double in the coarse-textured, high organic matter soil versus the other soils. Our results indicate that belowground primary productivity is significantly underestimated if based on root production data only. Rhizodeposition represents a major process, whose role should not be underestimated in carbon and nitrogen cycles in orchard ecosystems.     

Decomposition and nutrient release of grass and tree fine roots along an environmental gradient in southern Patagonia

Decomposition of fine roots is a fundamental ecosystem process that relates to carbon (C) and nutrient cycling in terrestrial ecosystems. However, this important ecosystem process has been hardly studied in Patagonian ecosystems. The aim of this work was to study root decomposition and nutrient release from fine roots of grasses and trees (Nothofagus antarctica) across a range of Patagonian ecosystems that included steppe, primary forest and silvopastoral forests. After 2.2 years of decomposition in the field all roots retained 70–90% of their original mass, and decomposition rates were 0.09 and 0.15 year^?1 for grass roots in steppe and primary forest, respectively. For N. antarctica roots, no significant differences were found in rates of decay between primary and silvopastoral forests (k = 0.07 year^?1). Possibly low temperatures of these southern sites restricted decomposition by microorganisms. Nutrient release differed between sites and root types. Across all ecosystem categories, nitrogen (N) retention in decomposing biomass followed the order: tree roots > roots of forest grasses > roots of steppe grasses. Phosphorus (P) was retained in grass roots in forest plots but was released during decomposition of tree and steppe grass roots. Calcium (Ca) dynamics also was different between root types, since trees showed retention during the initial phase, whereas grass roots showed a slow and consistent Ca release during decomposition. Potassium (K) was the only nutrient that was rapidly released from both grass and tree roots in both grasslands and woodlands. We found that silvopastoral use of N. antarctica forests does not affect grass or tree root decomposition and/or nutrient release, since no significant differences were found for any nutrient according to ecosystem type. Information about tree and grass root decomposition found in this work could be useful to understand C and nutrient cycling in these southern ecosystems, which are characterized by extreme climatic conditions.     

Soil physical properties and carbon/nitrogen relationships in stable aggregates under legume and grass fallow

Short-season fallow with legumes and/or grasses can restore the soil organic C and nitrogen (N) and improve soil structure. In this study, we accessed the effects of 2-season legume and grass fallow on structural properties and C/N relationships in aggregates of a sandy loam soil. Two legumes (Calopogonium mucunoides and Centrosema pubescens), and two grasses (Guinea grass (Panicum maximum) and goose grass (Eleusine indica) were used. Results showed that Calopogonium and Centrosema increased soil total porosity and reduced soil bulk densities, while goose grass increased bulk density and reduced total porosity of the soils at 0–15 and 15–30?cm depths. Guinea grass significantly increased the saturated hydraulic conductivity (50.4?cm?h^?1) and water holding capacity of the soils. Aggregates, 4.75 to 0.5?mm were greater in Guinea grass and least in goose grass fallowed soils. Calopogonium increased macro-aggregates at 0–15?cm soils by 48%, and mean weight diameter (MWD) by 44%. Organic carbon in 0.5–0.25?mm and <0.25?mm aggregate sizes was higher in Guinea grass soils. Generally, grasses had 4-fold increases of C:N contents in dry aggregates. In conclusion, short-season fallow with Guinea grass, Calopogonium and Centrosema, increased soil C and N and protected them from losses in stable aggregates.     

盐胁迫对2种栎树苗期生长和根系生长发育的影响

以低浓度(50 mmol/L)和高浓度(150 mmol/L)NaCl处理弗吉尼亚栎(Quercus virginiana)和麻栎(Quercus acutissima)1年生幼苗,研究了2种栎树在盐胁迫下的生长、对盐分的敏感性和耐受性及其根系形态学参数变化以及根系对盐离子的吸收与积累。结果表明,高浓度盐胁迫明显抑制了2种栎树地上部生物量的积累(P0.05),而低浓度盐胁迫对弗吉尼亚栎地上部干重的影响不明显,但显著抑制了麻栎地上部干重(P0.05);2种栎树的根冠比在盐胁迫下呈增加趋势,特别是在高浓度盐胁迫下,2种栎树的根冠比明显增加(P0.05),盐胁迫下增加生物量在根部的分配是植物应对盐胁迫的方式之一。2种栎树根部生物量积累在盐胁迫下变化不明显,但2种栎树根系形态学参数在盐胁迫下的响应不同,弗吉尼亚栎根系总长度、总表面积和总体积在盐胁迫下均有不同程度增加,特别是在低浓度盐胁迫下,根系形态学参数明显增加(P0.05),但麻栎根系形态学参数有下降趋势,但与对照相比变化不明显;通过对不同径级根系总长的分析发现,弗吉尼亚栎根系总长度的增加主要是由于直径小于2 mm的细根总长的增加,细根长度的增加对于植物吸收水分和营养物质具有重要意义;通过对Na+和Cl-在根系的含量分析表明,盐胁迫下2种栎树根系盐离子的积累均有明显增加,但弗吉尼亚栎根系盐离子的含量在低浓度和高浓度盐胁迫下的差异不明显,而麻栎在高浓度盐胁迫下根系盐离子的含量明显高于弗吉尼亚栎。综合2种栎树盐胁迫下的生物量分配策略和根系形态学响应以及盐离子的积累规律,证明2种栎树尽管在生物量分配策略方面具有相同的特点,但根系的响应策略截然不同,弗吉尼亚栎在盐胁迫下能够扩大根系吸收范围,维持较高的K+/Na+比值,而麻栎在盐胁迫下根系由于吸收过多的盐离子,导致根系的生长发育受到抑制,影响了根系在逆境中的分布范围,从而在一定程度上避免了进一步的盐害。     

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.     

Deuterium labelling of roots provides evidence of deep water access and hydraulic lift by Pinus nigra in a Mediterranean forest of NE Spain

We studied the tree access to deep water sources and the possibility of hydraulic lift from the deep roots of a Pinus nigra tree to the shallow soil layers in a Mediterranean forest of NE Spain. We also studied the use of hydraulically lifted water by neighboring trees, shrubs, and sprouts. We enriched the roots of a large P. nigra (10 m tall) with deuterium by accessing them from a below ground cave. During the next 3 days we measured stable deuterium isotopic composition of xylem sap, shoot predawn and midday water potentials, and the leaf δ¹³C and δ¹⁵N of the P. nigra tree, neighboring Quercus ilex ballota trees and sprouts, and Juniperus oxycedrus shrubs. The study was conducted both in dry summer and in wet spring. In summer, deuterated water absorbed by deep roots of P. nigra appeared in the stem water of neighboring plants and in surface soil. The most δD-enriched plant xylem sap was found in the enriched P. nigra tree, followed by the Q. ilex sprouts, the small Q. ilex trees and the surface soil (15 cm). All these trends disappeared in the wet spring season, when HDO only slightly appeared in the surface soil. The results show that the studied P. nigra tree accesses deep water source and conducts hydraulic lift in this Mediterranean forest in dry summer but not necessarily in wet spring.     

Paleosalinity in a brackish lake during the Holocene based on stable oxygen and carbon isotopes of shell carbonate in Nakaumi Lagoon, southwest Japan

Based on the relationship between salinity and δ¹⁸O and δ¹³C of modern shells in the Lake Nakaumi-Shinji lagoon system (southwestern Japan), where the salinity changes regularly from ca. 1 PSU to 34 PSU, a paleosalinity record for Nakaumi Lagoon during the Holocene has been derived from bulk mollusk shell δ¹⁸O and δ¹³C data. The robust relationships between the salinity and modern shell δ¹⁸O_ar and δ¹³C_ar (aragonite) were used to calibrate the paleosalinity reconstruction. The salinity relationships are expressed by the regressions:

Salinity (PSU)=3.86 δ¹⁸O_ar(‰ VPDB)+33.9 (n=18, r=0.978)     

Interspecific effects on plant size inequality: evidence from a temperate savanna community

The influences of intraspecific competition on plant size inequality have been well documented, but interspecific effects on this topic remain little understood. Here we examined the effects of canopy shading and fine roots of the trees (Elaeagnus angustifolia) on size inequality of the grasses (Achnatherum splendens) in a temperate savanna community in northwest China. Three study plots of 400 m² were divided into 4-m² quadrats, within each of which (1) canopy shading was quantified by modeling cumulative direct solar radiation (CDSR) and (2) the root effect was quantified using an empirical relationship between tree fine root density (TFRD) and relative distance to tree bases. Morphological traits were measured to represent grass size. Redundancy analysis (RDA) was conducted to examine the relative influences of grass density, CDSR and TFRD on the coefficient of variation of grass size. Results showed that no significant correlation occurred between grass density and grass size inequality. Both CDSR and TFRD had significant negative correlations with grass size inequality, suggesting that canopy shading and the presence of fine roots of trees can, respectively, increase and reduce grass size inequality. Canopy shading and TFRD played competitive roles in determining grass size inequality, where the root effect was a stronger factor than canopy shading. The tree effects can substantially alter the intensity of water stress. In response, size inequality of the grasses could be influenced through size-specific growth/mortality and slowed size divergence. These mechanisms could operate together in the savanna community.     

Unusual Fine Root Distributions of Two Deciduous Tree Species in Southern France: What Consequences for Modelling of Tree Root Dynamics?

The spatial distribution of fine roots of two deciduous tree species was investigated in contrasting growing conditions in southern France. Hybrid walnut trees (Juglans regia×nigra cv. NG23) and hybrid poplars (Populus euramericana cv. I214) were both cultivated with or without annual winter intercrops for 10 years on deep alluvial soils. Soil samples for measuring the fine root distribution of both trees and crops were obtained by soil coring down to 3-m depth at several distances and orientations from the tree trunk. The distribution of live fine roots from walnut and poplar trees was patchy and sometimes unexpected. In the tree-only stands, fine root profiles followed the expected pattern, as fine root density decreased with increasing depth and distance from the tree trunk. However, many fine root profiles under intercropped trees were uniform with depth, and some inverse profiles were observed. These distributions may result from a high degree of plasticity of tree root systems to sense and adapt to fluctuating and heterogeneous soil conditions. The distortion of the tree root system was more pronounced for the walnut trees that only partially explored the soil volume: in the tree-only stand, the walnut rooting pattern was very superficial, but in the intercropped stand walnut trees developed a deep and dense fine root network below the crop rooting zone. The larger poplars explored the whole available soil volume, but the intercrop significantly displaced the root density from the topsoil to layers below 1 m depth. Most tree root growth models assume a decreasing fine root density with depth and distance from the tree stem. These models would not predict correctly tree–tree and tree–understorey competition for water and nutrients in 3D heterogeneous soil conditions that prevail under low-density tree stands. To account for the integrated response of tree root systems to such transient gradients in soils, we need a dynamic model that would allow for both genotypic plasticity and transient environmental local soil conditions.