Variation in nitrogen-15 natural abundance and nitrogen uptake traits among co-occurring alpine species: do species partition by nitrogen form?

In the N-limited alpine tundra, plants may utilize a diversity of N sources (organic and inorganic N) in order to meet their nutritional requirements. To characterize species-level differences in traits related to N acquisition, we analyzed foliar '¹⁵N, nitrate reductase activity (NRA) and mycorrhizal infection in co-occurring alpine species during the first half of the growing season and compared these traits to patterns of N uptake using a ¹⁵N (¹⁵N-NH₄⁺, ¹⁵N-NO₃^-) or ¹³C,¹⁵N ([1]-¹³C-¹⁵N-glycine) tracer addition in the greenhouse. ¹³C enrichment in belowground tissue indicated that all species were capable of taking up labeled glycine, although only one species showed uptake of glycine potentially exceeding that of inorganic N. Species showing the most depleted foliar '¹⁵N and elevated NRA in the field also tended to show relatively high rates of NO₃^- uptake in the greenhouse. Likewise, species showing the most enriched foliar '¹⁵N also showed high rates of NH₄⁺ uptake. The ratio of NO₃^-:NH₄⁺ uptake rates and growth rate explained 64% and 72% of the variance in foliar '¹⁵N, respectively, suggesting that species differ in the ability to take up NO₃^- and NH₄⁺ in the field and that such differences may enable species to partition soil N on the basis of N form.     

Photosynthesis–nitrogen relationships in species at different altitudes on Mount Kinabalu, Malaysia

In situ photosynthetic nitrogen-use efficiency (PNUE, photosynthetic capacity per unit leaf nitrogen) was investigated in species that commonly distributed at different altitudes (600–3700m above sea level) on Mount Kinabalu. Photosynthetic nitrogen-use efficiency was lower in species at higher altitudes, with a mean PNUE at 3700m being one-third as large as that at 600m. This difference in PNUE was larger than that explained by the biochemical response to lower air pressures only. Across altitudes a negative correlation between ¹³C abundance (¹³C) and PNUE was found. Species at higher altitudes tended to have higher ¹³C, suggesting that they had a lower conductance for CO₂ diffusion from the air to chloroplasts. The lower conductance might be responsible for the lower PNUE in species at higher altitudes. Although leaf nitrogen content per unit area tended to be higher at higher altitudes, it did not seem to contribute to increasing photosynthetic rates. Thus, the idea that a higher nitrogen content at higher altitudes is a compensation for a lower PNUE was not supported. In contrast to the large difference in PNUE among altitudes, PNUE tended to converge within a narrow range among species growing at the same altitude.     

Root age distribution: how does it matter in plant processes? A focus on water uptake

Aims and background

Root growth creates a gradient in age at both the scale of the single root, from distal to proximal parts, but also at the root system level when young branch roots emerge from the axis or new nodal roots are emitted that may reach same soil domain as older roots. It is known that a number of root functions will vary with root type and root tissue age (e.g. respiration, exudation, ion uptake, root hydraulic conductance, mucilage release…) and so will the resulting rhizosphere properties. The impact of the distribution of root demography with depth, and related functions, on the overall functioning of the root system is fundamental for an integration of processes at the root system scale.

Scope and conclusion

Starting from methods for measuring root demography, we discuss the availability of data related to root age and its spatial distribution, considering plant types (monocot/dicot, perennial/annuals) which may exhibit different patterns. We then give a detailed review of variation of root/rhizosphere properties related to root age, focusing on root water uptake processes. We examine the type of response of certain properties to changes in age and whether a functional relationship can be derived. Integration of changing root properties with age into modelling approaches is shown from 3D models at the single plant scale to approaches at the field scale based on integrated root system age. Functional structural modelling combined with new development in non-invasive imaging of roots show promises for integrating influence of age on root properties, from the local to whole root system scales. However, experimental quantification of these properties, such as hydraulic conductance variation with root age and root types, or impact of mucilage and its degradation products on rhizosphere hydraulic properties, presently lag behind the theoretical developments and increase in computational power.

     

Effect of tillage and straw return on carbon footprints,soil organic carbon fractions and soil microbial community in different textured soils under rice–wheat rotation: a review

Reviews in Environmental Science and Bio/Technology - Measuring the influence of long-term agricultural tillage practices on soil organic carbon (SOC) is of great importance to farmers and...     

Characteristics and underlying mechanisms of plant deep soil water uptake and utilization: Implication for the cultivation of plantation trees北大核心CSCD

Root water uptake is an essential part of tree water relations and plays a crucial role in tree physiological activities. Water resource in deep soil is relatively abundant and can provide plenty of water to trees to guarantee their survival and healthy growth during dry seasons. Thus, a good comprehension of the characteristics and underlying mechanisms of deep soil water uptake and utilization by trees will deepen the understanding of the interaction between trees and the environment, tree survival and growth strategies, coexistence and competition among different species, etc. This knowledge is important in establishing green cultivation schemes for plantations, which depend less on the external water resources input and avoid the adverse effects on the water ecological environment. From existing studies, the characteristics and underlying mechanisms of deep water uptake and utilization by trees are reviewed. Firstly, the definition of deep roots and deep soil is discussed, and 1 m depth is recommended as the average (reference) definition standard in main forest vegetation types except the boreal forest. The reasons for the formation of deep tree roots around the globe were also determined. Secondly, the observed deep soil water uptake characteristics of trees and their influencing factors are summarized. Then, from the aspects of the adjustment of deep root traits and the coordination of hydraulic traits of different organs, the mechanisms of deep water uptake by trees are discussed. For example, the spatial, temporal and efficiency adjustment strategies of deep roots can be used to facilitate the absorption of deep soil water. Finally, some implications of deep soil water uptake for the cultivation of plantations are proposed, such as “for water management in plantations, trees should be induced to moderately utilize some deep soil water and an appropriate irrigation frequency should be selected”, “appropriate mixed planting of different tree species can facilitate the buffering effect of deep soil water storage”, “developing techniques of selecting trees for thinning based on the water uptake depths of different species”, etc. Deficiencies of existing studies and some future research directions were also pointed out. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All Rights Reserved.     

Nitrogen uptake by crops,soil distribution and recovery of urea-N in a sorghum—wheat rotation in different soils under Mediterranean conditions

The N uptake by crops, soil distribution and recovery of 15N labelled urea-N (100 kg N ha-1) were investigated in a sorghum-wheat rotation in two silty clay soils (Foggia and Rieti Casabianca) and one silt loam soil (Rieti Piedifiume) under different mediterranean conditions. Non-exchangeable labelled NH4-N represented an important pool at both Rieti sites with higher values (p<0.05) under sorghum (14.0 and 24.6% of the urea N in the 0-20 cm layer at the end of the cropping season) than wheat whereas it was much less important in the Foggia soil (10.0% in the surface soil under sorghum). This is probably related to the clay minerals composition of the three soils; because vermiculite was present in both Rieti sites but not in the Foggia soil. At harvest from 4.4 to 5.3% of the urea N initially applied was present as microbial biomass N in the surface soil layer with no generally significant differences due to location and type of crops. Both sorghum and wheat N yields were higher in the driest site (Foggia) probably due to better light conditions, higher temperatures and irrigation during summer of the sorghum cropping period. The recovery of plant fertilizer N (about 21% for sorghum and 27% for wheat) and the percentage of N in the plant derived from the fertilizer (NDFF) were the lowest at Rieti-Casabianca probably as the result of the protection of immobilized fertilizer N against microbial mineralization by the swelling clays. The fertilizer N unaccounted for was nil or very low (10.8% at Rieti-Casabianca under wheat and 11.8 and 4.9% at Rieti-Piedifiume under sorghum and wheat, respectively). Urea-N losses occurred when Rieti Piedifiume and Rieti Casabianca soils were kept bare. In this case the urea N unaccounted for ranged from 12 to 56% of the urea N with higher losses in Rieti-Piedifiume than in Rieti-Casabianca. The higher recoveries in the latter soil were probably confirmed by the stabilizing effect of clays on the immobilized urea N. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of nitrogen species (NH4 + and NO3 −) on the dynamics of P in water–sediment–Salvinia herzogii systems

Water – Salvinia herzogii – sediment systems were exposed to different phosphorus and nitrogen combinations in outdoor experiments. The aim was to estimate the amounts of P immobilized in macrophytes and sediments, as well as to elucidate whether or not the presence of N affects the retention of P. The following components were added: o-P, o-P + NH₄ ⁺, o-P + NO₃ ^– + NH₄ ⁺, o-P + NO₃ ^–. The concentration of nutrients was periodically determined throughout the experiment (28 days). The concentrations of P and N in plant tissues and sediments were determined at the beginning and the end of the experiment. Sequential extractions of P-fractions in sediment were performed using the EDTA method (Golterman, 1996). The removal efficiency of P in water was 95–99%. The removal of NH₄ ⁺ (97–98%) was more effective than that of NO₃ ^– (44–86%). The presence of nitrogen species increased the removal velocity of o-P from water, NH₄ ⁺ was the most effective species. Sediments not only had higher P removal rates than macrophytes but, in the control treatment without macrophytes, they reached the values obtained by macrophytes plus sediments in the other treatments. The adsorption of P takes place at the surface layer of the sediment (1 cm). Most of the P incorporated into the sediment during the experiment was sorbed by the fraction Fe(OOH)P. The addition of nutrients to water modified the leaves/lacinias weight ratio.     

Heterogeneity in soil water and light environments and dispersal limitation: what facilitates tree species coexistence in a temperate forest?

In the present study, we analysed the habitat association of tree species in an old‐growth temperate forest across all life stages to test theories on the coexistence of tree species in forest communities. An inventory for trees was implemented at a 6‐ha plot in Ogawa Forest Reserve for adults, juveniles, saplings and seedlings. Volumetric soil water content (SMC) and light levels were measured in 10‐m grids. Relationships between the actual number of stems and environmental variables were determined for 35 major tree species, and the spatial correlations within and among species were analysed. The light level had no statistically significant effect on distribution of saplings and seedlings of any species. In contrast, most species had specific optimal values along the SMC gradient. The optimal values were almost identical in earlier life stages, but were more variable in later life stages among species. However, no effective niche partitioning among the species was apparent even at the adult stage. Furthermore, results of spatial analyses suggest that dispersal limitation was not sufficient to mitigate competition between species. This might result from well‐scattered seed distribution via wind and bird dispersal, as well as conspecific density‐dependent mortality of seeds and seedlings. Thus, both niche partitioning and dispersal limitation appeared less important for facilitating coexistence of species within this forest than expected in tropical forests. The tree species assembly in this temperate forest might be controlled through a neutral process at the spatial scale tested in this study.     

Tracing Si–N–P ecosystem-pathways: is relative uptake in riparian vegetation influenced by soil waterlogging,mowing management and species diversity?

Despite the growing concern about the importance of silicon (Si) in controlling ecological processes in aquatic ecosystems, little is known about its processing in riparian vegetation, especially compared to nitrogen (N) and phosphorus (P). We present experimental evidence that relative plant uptake of N and P compared to Si in riparian vegetation is dependent on mowing practices, water-logging and species composition. Results are obtained from a controlled and replicated mesocosm experiment, with a full-factorial design of soil water logging and mowing management. In our experiments, the Si excluding species Plantago lanceolata was dominant in the mown and non-waterlogged treatments, while Si accumulating meadow grasses and Phalaris arundinacea dominated the waterlogged treatments. Although species composition, management and soil moisture interacted strongly in their effect on relative Si:N and Si:P uptake ratios, the uptake of N to P remained virtually unchanged over the different treatments. Our study sheds new light on the impact of riparian wetland ecosystems on nutrient transport to rivers. It indicates that it is essential to include Si in future studies of the impact of riparian vegetation on nutrient transport, as these are often implemented as a measure to moderate excessive N and P inputs.     

Quantitative trait loci for plant height in Maresi × CamB barley population and their associations with yield-related traits under different water regimes

High-yielding capacity of the modern barley varieties is mostly dependent on the sources of semi-dwarfness associated with the sdw1/denso locus. The objective of the study was to identify quantitative trait loci (QTLs) associated with the plant height and yield potential of barley recombinant inbred lines (RILs) grown under various soil moisture regimes. The plant material was developed from a hybrid between the Maresi (European cv.) and CamB (Syrian cv.). A total of 103 QTLs affecting analysed traits were detected and 36 of them showed stable effects over environments. In total, ten QTLs were found to be significant only under water shortage conditions. Nine QTLs affecting the length of main stem were detected on 2H-6H chromosomes. In four of the detected QTLs, alleles contributed by Maresi had negative effects on that trait, the most significant being the QLSt-3H.1-1 in the 3H.1 linkage group. The close linkage between QTLs identified around the sdw1/denso locus, with positive alleles contributed by Maresi, indicates that the semi-dwarf cv. Maresi could serve as a donor of favourable traits resulting in grain yield improvement, also under water scarcity. Molecular analyses revealed that the Syrian cv. also contributed alleles which increased the yield potential. Available barley resources of genomic annotations were employed to the biological interpretation of detected QTLs. This approach revealed 26 over-represented Gene Ontology terms. In the projected support intervals of QGWS_l-5H.3-2 and QLSt-5H.3 on the chromosome 5H, four genes annotated to ‘response to stress’ were found. It suggests that these QTL-regions may be involved in a response of plant to a wide range of environmental disturbances.     

Changes in soil organic carbon and total nitrogen after 28 years grassland afforestation: effects of tree species, slope position, and soil order

The effect of conversion of grassland to woodland on organic carbon (OC) and total nitrogen (TN) has significance for global change, land resource use and ecosystem management. However, these effects are always variable. Here, we show results of a study in an arid area in China on profile distribution of OC and TN in soils covered by two different woody tree canopies and outer canopy space (grassland between woody plant canopies). The soils were at various slope positions (upper, middle and lower slopes) for Chinese pine (Pinus tabulaeformis) and Korshrinsk peashrub (Caragana korshinskii) lands, and of different soil orders (Castanozems, Skeletal, Loessial and Aeolian soils). The objectives were to relate the effects of land use change on OC and TN to slope position and soil order. Soil OC and TN were significantly larger at Korshrinsk peashrub slope locations than at Chinese pine slope locations. Soil OC and TN were small at the lower slope position for Korshrinsk peashrub, however, they were largest at the middle slope for Chinese pine. Korshrinsk peashrub always increased soil OC and TN under brush canopy at the three slope positions, while Chinese pine increased them at lower slopes and decreased them at upper slopes. For the soil types, OC and TN in Korshrinsk peashrub land were in the order of Castanozems > Skeletal > Loessial > Aeolian soils. Korshrinsk peashrub also increased OC and TN under brush canopy in the four soils. Our results indicated that soil OC and TN in canopy soils differed greatly from associated values in the outer canopy soils, and the effects of grassland afforestation varied significantly with tree species, slope position, and soil type. Therefore, we suggest that differentiating such factors can be an effective approach for explaining variances in OC and N changes caused by land use conversion.     

Characteristics of soil water distribution and evaluation of recharge rate under different grazing history in the Xilin Gol Steppe北大核心CSCD

Aims In the Xilin Gol Steppe, human-induced grassland degradation and land desertification are becoming increasingly severe. Critical evaluation of its impact on soil water and recharge rate is important for sustainable management of soil health and water resources in the region. Methods In order to determine the effect of different grazing history on dynamics of deep soil moisture contents and precipitation infiltration in the Xilin Gol Steppe, three sites with different grazing history (ungrazed since 1979 or UG79, ungrazed since 1999 or UG99, and continuously grazed or CG) were selected with two sampling spots for each site. The precipitation infiltration was estimated using the chloride mass balance method. Important findings The results showed that: 1) Average soil water content of 0–5 m was 7.1%, 6.9%, and 6.3% for UG79, UG99, and CG, respectively, with no significant difference. In the soil layer of 0–2 m, the soil water content of UG79 was 26.6% and 33.7% higher than that of UG99 and CG, respectively. The soil water content of UG79 was significantly higher than that of UG79 and UG99 (p < 0.05) with no significant difference between UG99 and CG. The soil water storage capacity of UG79 was 87.19 mm higher than UG99 and 82.52 mm higher than CG. In the deep layer of 2–5 m, no significant difference in the soil water content and the water storage among different grazing history. 2) The factors influencing soil water differed among different grazing treatments. The soil water content was mainly affected by the vegetation conditions and soil properties for the 0–2 m soil layer, but by the composition of soil particles for the 2–5 m soil layer. The effect of soil organic matter (SOM) content on soil water increased with time without grazing. Soil water content of the entire soil profile of UG79 was significantly correlated with soil texture and SOM content (p < 0.01). Soil water content of 0–2 m was significantly correlated with SOM content (p < 0.01), soil water content of 2–5 m was significantly correlated with the soil texture (p <0.01), but soil moisture content of UG99 and CG had no significant correlation with SOM content. 3) Annual recharge rate was 5.64, 3.54, and 2.45 mm·a –1 for UG79, UG99 and CG, respectively. The recharge rate increased by 44.5% and 130.2% for the site without grazing for 15 and 35 years, respectively. The recharge rate in the study area ranged from 1.95 to 7.61 mm·a –1 , accounting for only 0.55%–2.13% of the precipitation. In summary, ungrazing treatment can increase soil water retention, total water storage capacity, and recharge. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All Rights Reserved.     

Effects of growth temperature and nitrogen nutrition on expression of C3–C4 intermediate traits in Chenopodium album

Proto-Kranz plants represent an initial phase in the evolution from C₃ to C₃–C₄ intermediate to C₄ plants. The ecological and adaptive aspects of C₃–C₄ plants would provide an important clue to understand the evolution of C₃–C₄ plants. We investigated whether growth temperature and nitrogen (N) nutrition influence the expression of C₃–C₄ traits in Chenopodium album (proto-Kranz) in comparison with Chenopodium quinoa (C₃). Plants were grown during 5 weeks at 20 or 30 °C under standard or low N supply levels (referred to as 20SN, 20LN, 30SN, and 30LN). Net photosynthetic rate and leaf N content were higher in 20SN and 30SN plants than in 20LN and 30LN plants of C. album but did not differ among growth conditions in C. quinoa. The CO₂ compensation point (Γ) of C. album was lowest in 30LN plants (36 µmol mol^–1), highest in 20SN plants (51 µmol mol^–1), and intermediate in 20LN and 30SN plants, whereas Γ of C. quinoa did not differ among the growth conditions (51–52 µmol mol^–1). The anatomical structure of leaves was not considerably affected by growth conditions in either species. However, ultrastructural observations in C. album showed that the number of mitochondria per mesophyll or bundle sheath (BS) cell was lower in 20LN and 30LN plants than in 20SN and 30SN plants. Immunohistochemical observations revealed that lower accumulation level of P-protein of glycine decarboxylase (GDC-P) in mesophyll mitochondria than in BS mitochondria is the major factor causing the decrease in Γ values in C. album plants grown under low N supply and high temperature. These results suggest that high growth temperature and low N supply lead to the expression of C₃–C₄ traits (the reduction of Γ) in the proto-Kranz plants of C. album through the regulation of GDC-P expression.

     

The influence of arbuscular mycorrhizal colonization on soil–root hydraulic conductance in Agrostis stolonifera L. under two water regimes

The hypothesis that mycorrhizal colonization improves the soil–root conductance in plants was experimentally tested in a growth chamber using pot cultures of Agrostis stolonifera L. colonized by Glomus intraradices. Plants were grown in 50-l pots filled with autoclaved sand/silt soil (1:1), with and without the mycorrhizal fungus. Within the mycorrhizal treatment, half of the pots remained well watered, while the other half was subjected to a progressive water deficit. Soil water potential (estimated as plant water potential measured at the end of the dark period), xylem water potential measured at the tiller base, transpiration rate, and soil water content were monitored throughout the experiment. Soil–root hydraulic conductance was estimated as the ratio between the instantaneous transpiration rate and the soil and xylem water potential difference. To obtain cultures with similar nutritional status, the P in the modified Hoagland’s nutrient solution was withheld from the inoculated pots and applied only once a month. Even though there were no differences on growth or nutrient status for the mycorrhizal treatments, water transport was enhanced by the inoculum presence. Transpiration rate was maintained at lower xylem water potential values in the presence of mycorrhizae. The analysis of the relationship between soil–root hydraulic resistance and soil water content showed that mycorrhizal colonization increased soil–root hydraulic conductance as the soil dried. For these growing conditions, this effect was ascribed to the range of 6–10%.     

Influence of co-inoculation of bacteria-cyanobacteria on crop yield and C–N sequestration in soil under rice crop

The performance of three selected bacterial strains—PR3, PR7 and PR10 (Providencia sp., Brevundimonas sp., Ochrobacterium sp.) and three cyanobacterial strains CR1, CR2 and CR3 (Anabaena sp., Calothrix sp., Anabaena sp.), and their combinations was evaluated in a pot experiment with rice variety Pusa-1460, comprising 51 treatments along with recommended fertilizer controls. Highest yield enhancement of 19.02% was recorded in T12 (CR2), over control, while significant enhancement in nitrogen fixing potential was recorded in treatments involving combination of bacterial-cyanobacterial strains—T37 (PR3 + CR1 + CR3) and T21 (PR7 + CR1). Organic carbon was significantly increased in all microbe-inoculated treatments, which could be correlated with microbial biomass carbon values and activities of all the enzymes tested in our study. Also, panicle weight and plant biomass were highly correlated with soil microbial carbon. Comparative evaluation revealed the superior performance of strains CR2, CR1 (both Anabaena sp.) and PR10 (Ochrobacterium sp.) in increasing the growth and grain yield of rice and improving soil health, besides N (nitrogen) savings of 40–80 kg ha⁻¹. The study for the first time illustrated the positive effects of co-inoculation of bacterial and cyanobacterial strains for integrated nutrient management of rice crop.     

Plant–soil feedback effects on the performance and functional traits of Dodonaea viscosa in a dry-hot valley,China

Plant–soil feedback has been widely studied and may be particularly important in resource-poor areas. However, the roles of direct and indirect biotic effect in affecting plant growth and functional traits in this process still remained unclear. The aim of this study was to explore the roles of soil biota in affecting plant performance and traits by conducting a two-phase feedback experiment in a dry-hot valley, with a conditioning phase during which there were Dodonaea viscosa or no D. viscosa growing in the soil and a feedback phase in which the effect of the conditioned soil biota on D. viscosa performance was measured. Soil N was reduced by the presence of D. viscosa during the conditioning phase. However, D. viscosa showed a positive plant–soil feedback. In the feedback phase, the D. viscosa-conditioned soil increased the stem diameter, total biomass, and leaf dry mass content of D. viscosa, while the specific leaf area was significantly lower in D. viscosa-conditioned soil than that in bare soil. In contrast, soil sterilization had a negative effect on the growth of D. viscosa, with a significant reduction in biomass, especially in D. viscosa-conditioned soil, and soil sterilization significantly increased the root:shoot biomass ratio and litter mass. Furthermore, we showed that although the biota-driven changes in enzyme activities correlated with leaf N and especially P amount, the enzyme activity was not the main reason to promote D. viscosa growth in conditioned soil. These results suggest that the positive biotic feedback effect was superior to the negative nutrient-depleted effect on D. viscosa growth, and the direct biotic feedback effect would drive the positive effect of soil biota to a greater extent than the indirect effect. The positive biotic plant–soil feedback in dry-hot valley plays an important role in ecosystem restoration and helps in understanding plant adaptation to the local environment in this area.

     

Seasonal variations in leaf δ13C values: implications for different water-use strategies among species growing on continuous dolomite outcrops in subtropical China

Plant species growing in shallow-soil habitat are likely to experience water deficit especially in seasonally dry or arid regions. However, only scarce studies focused on their water-use strategies. The current study aimed to reveal water-use strategies of different species growing on continuous dolomite outcrops (a typical shallow-soil habitat) in subtropical China that relied on different water sources, and to investigate the differences between narrow endemic and widespread species, based on season variations in leaf δ¹³C values. Leaf samples of six plant species (Radermachera sinica, Sapium rotundifolium, Sterculia euosma, Schefflera octophylla, Alchornea trewioides, and Vitex negundo, in different life-forms and leaf phenologies) were collected for carbon isotope measurements in the wet and dry seasons, respectively. Contrary to the expectation, the evergreen big shrub species, S. octophylla, which always relied on deep water sources, exhibited the most positive δ¹³C values (high water-use efficiency, WUE), indicating more conservative water-use strategies. While the two deciduous small shrubs, A. trewioides and V. negundo, which always relied on shallow water sources, exhibited the most negative δ¹³C values (low WUE). This result was associated with their short life spans, indicating an opportunistic water-use strategy. Leaf δ¹³C values of almost all (except for S. octophylla) the selected species were significantly (P < 0.05) higher in the dry season than in the wet season. This indicated that it was a common strategy for species in rocky karst habitat to improve their WUE in dry season. Despite the similar water sources utilized by the selected three tree species, the widespread one (R. sinica) exhibited greater improvement in leaf δ¹³C values than the narrow endemic ones (S. rotundifolium and S. euosma). This suggested that the widespread tree species had more flexible water-use strategies. It was further speculated that broad spatial distribution of widespread species may contribute to their highly plastic responses to changes in environmental conditions rather than always maintaining high WUE.     

DenNit – Experimental analysis and modelling of soil N2O efflux in response on changes of soil water content, soil temperature, soil pH, nutrient availability and the time after rain event

To quantify the effects of soil temperature (T_soil), and relative soil water content (RSWC) on soil N₂O emission we measured N₂O soil efflux with a closed dynamic chamber in situ in the field and from soil cores in a controlled climate chamber experiment. Additionally we analysed the effect of soil acidity, ammonium, and nitrate concentration in the field. The analysis was performed on three meadows, two bare soils and in one forest. We identified soil water content, soil temperature, soil nitrogen content, and pH as the main parameters influencing soil N₂O emission. The response of N₂O emission to soil temperature and relative soil water content was analysed for the field and climate chamber measurements. A non-linear regression model (DenNit) was developed for the field data to describe soil N₂O efflux as a function of soil temperature, soil moisture, pH value, and ammonium and nitrate concentration. The model could explain 81% of the variability in soil N₂O emission of all individual field measurements, except for data with short-term soil water changes, namely during and up to 2 h after rain stopped. We validated the model with an independent dataset. For this additional meadow site 73% of the flux variation could be explained with the model.     

Aspects of carbon and nitrogen cycling in soils of the Bornhöved Lake district II. Modelling the influence of temperature increase on soil respiration and organic carbon content in arable soils under different managements

Based on field measurements in two agriculturalecosystems, soil respiration and long-term response ofsoil organic carbon content (SOC) was modelled. Themodel predicts the influence of temperature increaseas well as the effects of land-use over a period ofthirty years in a northern German glacial morainelandscape. One of the fields carried a maizemonoculture treated with cattle slurry in addition tomineral fertilizer (maize monoculture), the otherwas managed by crop rotation and recieved organicmanure (crop rotation). The soils of both fieldswere classified as cambic Arenosols. The soilrespiration was measured in the fields by means of theopen dynamic inverted-box method and an infrared gasanalyser. The mean annual soil respiration rates were 268 (maizemonoculture) and 287 mg CO₂ m^-2 h^-1(crop rotation). Factors controlling soil respirationwere soil temperature, soil moisture, root respirationand carbon input into the soil. Q₁₀-valuesof soil respiration were generally higher in winterthan in summer. This trend is interpreted as anadaptive response of the soil microbial communities.In the model a novel mathematical approach withvariable Q₁₀-values as a result oftemperature and moisture adjustment is proposed. Withthe calibrated model soil respiration and SOC werecalculated for both fields and simulations over aperiod of thirty years were established. Simulationswere based on (1) local climatic data, 1961 until1990, and (2) a regional climate scenario for northernGermany with an average temperature increase of 2.1 K.Over the thirty years period with present climateconditions, the SOC pool under crop rotation wasnearly stable due to the higher carbon inputs, whereasabout 16 t C ha^-1 were lost under maizemonoculture. Under global warming the mean annualsoil respiration for both fields increased and SOCdecreased by ca. 10 t C ha^-1 under croprotation and by more than 20 t C ha^-1 undermaize monoculture. It was shown that overestimationof carbon losses in long-term prognoses can be avoidedby including a Q₁₀-adjustment in soilrespiration models.     

Effects of elevated CO₂, warming and drought episodes on plant carbon uptake in a temperate heath ecosystem are controlled by soil water status

The impact of elevated CO₂, periodic drought and warming on photosynthesis and leaf characteristics of the evergreen dwarf shrub Calluna vulgaris in a temperate heath ecosystem was investigated. Photosynthesis was reduced by drought in midsummer and increased by elevated CO₂ throughout the growing season, whereas warming only stimulated photosynthesis early in the year. At the beginning and end of the growing season, a T × CO₂ interaction synergistically stimulated plant carbon uptake in the combination of warming and elevated CO₂. At peak drought, the D × CO₂ interaction antagonistically down‐regulated photosynthesis, suggesting a limited ability of elevated CO₂ to counteract the negative effect of drought. The response of photosynthesis in the full factorial combination (TDCO₂) could be explained by the main effect of experimental treatments (T, D, CO₂) and the two‐factor interactions (D × CO₂, T × CO₂). The interactive responses in the experimental treatments including elevated CO₂ seemed to be linked to the realized range of treatment variability, for example with negative effects following experimental drought or positive effects following the relatively higher impact of night‐time warming during cold periods early and late in the year. Longer‐term experiments are needed to evaluate whether photosynthetic down‐regulation will dampen the stimulation of photosynthesis under prolonged exposure to elevated CO₂.