Description: Background and aims Legumes integrated in crop rotations are intended to improve crop nitrogen (N) supply and yield. In conservation agriculture (CA) systems under low input conditions on highly weathered tropical soils, experimental evidence for these benefits is lacking. To understand the mechanisms and evaluate the impact of the legume N on the subsequent crop, an in-depth study on N dynamics in the soil-plant system was conducted. Methods In Madagascar, a CA based crop rotation with the perennial forage legume Stylosanthes guianensis (stylo) and upland rice (rice/stylo – stylo - rice/stylo) was established under three fertilization regimes. In addition, rice was grown in a non-CA bare fallow rotation without fertilizer. Over the three years N 2 fixed in stylo shoots, the incorporation of stylo shoot (mulch) N into soil N pools and mulch N uptake by rice was quantified using 15 N techniques and mulch and stylo root residue decomposition was investigated in a litterbag study. Results N 2 fixed in stylo shoots ranged from 96 to 122 kg N ha −1 . Between 50 to 70% of stylo mulch and root residues decomposed during the third cropping season. Without fertilizer, grain yield of rice after the fallow with stylo was about 70% greater than after bare fallow, corresponding to 11 kg N ha −1 greater N uptake. Recoveries of stylo mulch N after rice harvest were on average 64% in soil, with about 3% in each of the microbial and mineral N pools, with 39% on the soil surface, and 6% in the rice crop. The N input via stylo seed, leaf litter and belowground N totalled about three times the amount of N contained in stylo mulch, which usually is considered as major rice N source. Conclusions Legumes, like stylo, can improve crop N supply and yield in low input CA cropping systems on highly weathered tropical soils. To explain the impact and mechanisms involved requires a consideration of all legume-N components beyond the mulch N present at the onset of the rice-cropping season.

Description: Aims Belowground legume nitrogen (N) composed of roots and rhizodeposition is an important N input to soils, but published data of belowground N vary broadly, probably due to extrapolation from short-term experiments and dissimilar growing conditions. We quantified belowground N inputs of red clover ( Trifolium pratense L.) during two consecutive years in a clover-grass sward along a soil nutrient availability gradient. Methods We established a red clover-perennial ryegrass ( Lolium perenne L.) model sward in microplots located in field plots of the DOK experiment, which has a 33-year history of organic and conventional cropping, resulting in a soil nutrient availability gradient. Four treatments were examined: the zero fertilisation control, bio-organic with half and full dose manure application, and the conventional system with mineral fertilisation at full dose. We studied the development of clover aboveground and belowground N using multiple pulse 15 N urea leaf labelling. Results Belowground clover N increased over time and with rising nutrient availability and was proportional to aboveground clover N at all times. Belowground clover N amounted to 40% of aboveground clover N during two consecutive years, irrespective of the nutrient availability status. Belowground clover N development was initially dominated by fast root growth, followed by enhanced root turnover during the second year. Potassium availability limited clover growth and total N accumulation in treatments with low nutrient availability. Conclusions Belowground red clover N inputs could be estimated from aboveground N by a constant factor of 0.4, regardless of the nutrient availability and cultivation time. Root turnover led to a distinct absolute increase of N rhizodeposition over time. Hence, N rhizodeposition, with an 80% share of belowground N, was the predominant N pool at the end of the second year.

Description: Aims The effects of shrub encroachment on plant and soil properties have been well studied. However, little is known about how shrub encroachment influences soil bacterial communities. We investigated the effects of shrub encroachment on grassland soil bacterial communities along a soil depth gradient in the Inner Mongolian region of China. Methods The belowground bacterial communities were examined using high-throughput sequencing of the 16S rRNA gene (V4-V5 region, Illumina MiSeq). Results Bacterial alpha-diversity was higher in shrub-encroached soils than in control grassland soils. Bacterial OTU richness was highest at 0–20 cm soil depths, while phylogenetic diversity was greatest at 10–20 cm soil depths. At each soil depth layer, shrub encroachment was associated with a significant shift in bacterial community composition. Change in soil pH was the factor most strongly related to change in bacterial community composition associated with shrub encroachment at all four depth horizons in the soils. Shrub encroachment appears to alter the distribution of bacterial life history strategies in the surface soil (i.e., showing an enrichment in copiotrophs and a depletion in oligotrophs) and shrubs are associated with an increase in nitrification potential in deeper soil horizons. Conclusions Our results indicate that the influence of shrub encroachment on bacterial community composition extends deep into the soil. The intensity of shrub encroachment at this study site suggests that this ecosystem is undergoing dramatic succession towards shrub-dominance, which will likely trigger shifts in ecosystem function.

Description: Aims The detailed Se distribution in plants has been poorly described. This study was performed to determine the optimal dose of selenite for enhanced Medicago sativa growth and comprehensively explore the distribution characteristics of Se in this plant. Methods Alfalfa pot experiments were conducted in a growth chamber. The plants were grown in sterilized quartz sand and treated with different selenite levels for 21 days to determine the effect of Se on growth. Plants exposed to 1 and 10 μM selenite were used to clarify the Se distribution in alfalfa. Results Alfalfa growth was significantly stimulated ( P  〈 0.05) under 〈5 μM Se. Three linear correlations were found between the applied Se doses and Se concentrations in alfalfa roots, stems, and leaves. The following patterns of Se concentrations were observed: root〉leaf〉stem in the organs; rhizodermis and cortex〉stele in the tissues; and younger leaf 〉mature leaf〉older leaf and marginal leaf〉midrib〉internal leaf in the leaves. In addition, Se concentration in the cytoplasm fraction was significantly higher than that in cell wall fraction at 1 μM selenite, and the opposite result was found at 10 μM selenite. Conclusions Appropriate selenite addition (1 μM) benefited alfalfa. Se binding in the rhizodermis and cortex of the root caused relatively low transport efficiency of Se from the root to the shoot. Se redistributions may be a possible important factor affecting the transportation of Se in shoot and Se was transported along with the transpiration stream within a single leaf. Cell wall immobilization might be a major strategy to protect plant organs from potential Se toxicity.

Description: Background and aims Rooting plasticity is critical for plants exploiting patchy soil-water resources, but empirical evidence remains controversial due to complex root/soil interactions in natural and agricultural environments. We compared cultivated and wild Chenopodium populations from distinct agroecological background to assess their rooting plasticity when exposed to contrasting wet-dry soil profiles in a controlled environment. Methods Four treatments of increasing dryness were applied during 6 weeks in plants of Chenopodium hircinum , Chenopodium pallidicaule and two ecotypes (wet- and dry-habitat) of Chenopodium quinoa grown in rhizotrons. Root system architecture and growth were sequentially mapped. At the end of the experiment, plant and root morphological traits and dry biomass were measured. Results Contrary to the other two species, C. quinoa showed accelerated taproot growth in dry soil conditions. The dry-habitat C. quinoa ecotype showed consistently higher plant traits related to longer, coarser, and more numerous root segments which give it a faster taproot growth and sustained root branching at depth in dry soil. Conclusions High rooting plasticity confers the advantage of fast root elongation and deep soil exploration under soil water deficit. Variation in intrinsic root traits and plastic responses among Chenopodium populations controls their root foraging capacity facing patchy soil-water resources.

Description: Background and aims The main difficulty in the use of 3D root architecture models is correct parameterization. We evaluated distributions of the root traits inter-branch distance, branching angle and axial root trajectories from contrasting experimental systems to improve model parameterization. Methods We analyzed 2D root images of different wheat varieties ( Triticum aestivum) from three different sources using automatic root tracking. Model input parameters and common parameter patterns were identified from extracted root system coordinates. Simulation studies were used to (1) link observed axial root trajectories with model input parameters (2) evaluate errors due to the 2D (versus 3D) nature of image sources and (3) investigate the effect of model parameter distributions on root foraging performance. Results Distributions of inter-branch distances were approximated with lognormal functions. Branching angles showed mean values 〈90°. Gravitropism and tortuosity parameters were quantified in relation to downwards reorientation and segment angles of root axes. Root system projection in 2D increased the variance of branching angles. Root foraging performance was very sensitive to parameter distribution and variance. Conclusions 2D image analysis can systematically and efficiently analyze root system architectures and parameterize 3D root architecture models. Effects of root system projection (2D from 3D) and deflection (at rhizotron face) on size and distribution of particular parameters are potentially significant.

Description: Aims Although arbuscular mycorrhizal symbiosis is common in many plants with either C 3 or C 4 photosynthesis, it remains poorly understood whether photosynthesis type has any significant impact on carbon (C) fluxes in mycorrhizal plants. Thus, we compared mycorrhizal and non-mycorrhizal (NM) plants belonging to Panicum bisulcatum (C 3 ) to its congeneric P. maximum (C 4 ). Methods Plants were or were not exposed to arbuscular mycorrhiza (AM) fungal inoculation and/or phosphorus (P) fertilization. Plants’ C budgets were assembled based on 13 CO 2 pulse-chase labelling and sequential harvesting. Results Mycorrhizal plants allocated on average 3.9% more recently fixed C belowground than did their NM counterparts. At low P, mycorrhizal C 3 - Panicum plants allocated less C to aboveground respiration as compared to their respective NM controls. In contrast, mycorrhizal C 4 - Panicum increased the rates of photosynthesis and allocated more C to aboveground respiration than the respective NM controls. At high P, the differences were less prominent. Conclusions We demonstrated consistent differences in aboveground C allocation due to AM symbiosis formation in congeneric C 3 and C 4 grasses. Both grasses benefited from AM symbiosis in terms of improved P uptake (at least at low P). These results advocate a holistic (whole-plant) perspective in studying C fluxes in mycorrhizal plants.

Description: Background and aims Leaf-litter decomposition rate ( k L ) regulates nutrient dynamics and is affected at microsite level by species traits, soil biota and microclimate conditions. Fallen fruits form part of the litter and some, particularly fleshy fruits, contain large quantities of nutrients and sugar. We estimated the amount of fruit fall to litter, and evaluated the effect of its decomposition and sugar content on k L in dominant species of NW Patagonia shrublands. Methods We selected six woody species, four with fleshy and two with dry fruit. We followed 224 decomposition bags with leaf or leaf+fruit throughout 1 year. Fruit-litter and fruit sugar content were also measured. Results and conclusions Fleshy fruit decomposition rate was associated with changes in k L , while no effects of dry fruit on k L were registered. We found that three of the fleshy fruits ( R. cucullatum , R. rubiginosa and S. patagonicus ) had a positive influence on k L due to their sugar content. In contrast, Berberis microphylla fruit had a negative effect on k L , probably due to the presence of antimicrobial substances in the fruit. Considering the abundance of these species and their copious fruit production, the fleshy fruits could play an important role in determining soil fertility.

Description: Aims Nitrogen supply and atmospheric CO 2 concentration ([CO 2 ]) could influence root exudates directly by altering compound concentrations in roots and indirectly by regulating root morphology. This study assessed these direct and indirect effects on cucumber root exudation. Methods Cucumber roots with various morphological traits were obtained in different combinations of nitrogen supplies and [CO 2 ] treatments. Then, the correlations between ten compounds in root exudates and their concentrations in root extracts as well as root morphological traits were evaluated. Results In case of root exudates, the amounts of sugars were more closely correlated to the root surface area, whereas organic acids and amino acids were more closely associated with the number of root tips. Moreover, fructose, glucose, sucrose and oxalic acid in root exudates were correlated to their concentrations in root extracts, whereas there was little correlation between root exudates and extracts for malic acid, citric acid or four amino acids. Conclusions Sugars were probably released from the whole roots by passive or facilitated diffusion, so both the direct and indirect effects were important. Organic acids and amino acids were mainly secreted from the root apices by active transport, thus the indirect effect was more important.

Description: Background and aims Plants grown under elevated CO 2 (eCO 2 ) demand more nitrogen from soil and invest more labile carbon (C) compounds into below-ground. This would potentially affect microbial decomposition of soil organic C (SOC) in the rhizosphere- namely rhizosphere priming effect (RPE). This study aims to reveal how eCO 2 and nitrogen (N) supply affect the RPEs under wheat and white lupin. Methods Wheat ( Triticum aestivum L. cv. Yitpi) and white lupin ( Lupinus albus L. cv. Kiev) were grown at two N addition rates under ambient CO 2 (400 μmol mol −1 ) and eCO 2 (800 μmol mol −1 ) for 32 and 52 days in a C4 soil. Rhizosphere priming of SOC was quantified using the stable  13 C isotopic tracing technique. Results Relative to adequate N supply, low N increased the RPEs under both species at Day 32, but decreased the RPEs under wheat while had no effect on RPE under white lupin at Day 52. Elevated CO 2 increased the RPE except that under wheat at Day 52. Conclusions Low N availability in soil increased the RPE probably via stimulated microbial N mining while eCO 2 and severe N limitation synergistically decreased the RPE under wheat but not under N 2 -fixing white lupin.