Plant and Soil

Assimilate allocation by rice and carbon stabilisation in soil: effect of water management and phosphorus fertilisation Abstract Background and aims Water and nutrient management influences the allocation and stabilisation of newly assimilated carbon (C) in paddy soils. This study aimed to determine the belowground allocation of C assimilated by rice and the subsequent C stabilisation in soil aggregates and as mineral-organic associates depending on combined alternate wetting and drying (AWD) versus continuous flooding (CF) and P fertilisation. Methods We continuously labelled rice plants in 13CO2 atmosphere under AWD versus CF water management, and at two P fertilisation levels (0 or 80 mg P kg−1 soil). The 13C allocation to soil and its incorporation into the wet-sieved aggregate size classes and density fractions of the rhizosphere and bulk soils were analysed 6, 14, and 22 days after the labelling was started (D6, D14, and D22, respectively). Results Under both water regimes and P fertilisation levels, the proportion of photoassimilates was the highest in the silt- and clay-size aggregate classes and in the mineral-associated fraction. On D6 and D14, P fertilization resulted in smaller 13C incorporation into soil, independent of water management. In the rhizosphere soil, at D22, P fertilisation increased 13C incorporation over no P amendment in macroaggregates (>250 μm) by 32% (AWD) and 42% (CF), in microaggregates (250–53 μm) by 97% (CF), and in the silt + clay size class (<53 μm) by 83% (CF). Further, P fertilisation led to larger 13C incorporation into the rhizosphere soil light fraction (75% at AWD and 90% at CF) and dense fraction (38% and 45%, respectively), and into the bulk soil macroaggregates (71% and 78%, respectively). Conclusions Phosphorus fertilisation increased the contents of recent photoassimilates in soil aggregate classes with longer residence time as well as of the particulate organic matter with the continuation of plant growth. This positive response of the stabilisation of recent plant photosynthates in soil to P fertilisation can increase the potential of paddy soil for C sequestration. This potential is not limited by the introduction of alternate wetting and drying water-saving technique.

Localization of typical and atypical Frankia isolates from Casuarina sp. in nodules formed on Casuarina equisetifolia Abstract Aims Members of the nitrogen-fixing actinobacterial genus Frankia are typically isolated from root nodules and generally infective on the same plant species. Several Frankia strains originally isolated from Casuarina species, however, have been found to be non-infective on Casuarina species. The goal of this study was to investigate the potential role of infective isolates from Casuarina species on the potential establishment of these non-infective Frankia strains in root nodule formation on Casuarina equisetifolia. Methods Soil microcosms were established with plants of C. equisetifolia and inoculated with Frankia casuarinae strain CcI3 or cluster 3 strain R43, or combinations of both at different densities. Basic plant growth characteristics, root nodule formation and localization of both Frankia strains in nodule periderm and cortex, as well as population development in soils were monitored. Results The presence of strain R43 did not affect plant growth performance nor root nodule formation, while inoculation with strain CcI3 enhanced plant growth and resulted in root nodule formation. qPCR analyses on selected nodule lobes revealed the presence of strain CcI3 in cortex samples in all treatments, while strain R43 was not detected in any cortex samples but in 40% of the periderm samples from lobes from treatments with highest inoculation values. In situ hybridization detected cells of strain R43 on the outside of the nodules, i.e. on the periderm only. Conclusions These results demonstrate that the cluster 3 Frankia strain R43 is not co-infecting root nodules formed by the F. casuarinae strain CcI3 on C. equisetifolia, but has likely been isolated as a surface contaminant from Casuarina nodules.

The roles of interspecific variability in seed mass and soil resource availability in root system development Abstract Aims Because plant roots serve mainly as organs for the uptake of water and nutrients, we aimed to test whether the development of seedling roots is influenced by the two principal nutrient sources—the substrate and the pool stored in the seed itself. Methods Using seven Fabaceae species that differ in seed mass, we observed their early root system development under four levels of nutrient availability. Transparent-wall rhizoboxes allowed us to track root development and to score root system structure (length and number of roots), size (depth and width of root system) and shape (relative depth and width of root system). Results Seedling root system development depended on both the amount of nutrients contained in the substrate and on the seed mass of the species. Compared to an average seedling, effects of these two nutrient pools were (i) opposite and (ii) did not fully overlap. Small-seeded species developed wider root systems that branched earlier than large-seeded species. Increased availability of nutrients in the substrate led to proliferation of lateral roots, without any substantial impacts on the shape of root system or beginning of branching. Conclusions The source of the nutrients affected the way they were used throughout early root system development, leading to different structures and dynamics. This may be one of the mechanistic links connecting seed mass and the realized niche of the species.

Plant-assisted selection: a promising alternative for in vivo identification of wheat ( Triticum turgidum L. subsp . Durum ) growth promoting bacteria Abstract Background and aims In this work we present the development of an easy and feasible in vivo alternative to identify promising Plant Growth Promoting Bacteria (PGPB), using wheat -as a model plant- growing under variable soil and climate conditions. Methods The identification of promising strains was carried out by Plant-Assistant Selection (PAS) (compared with the conventional PGPB selection, named in this work as Metabolic Traits Selection or MTS). We validated the ability of the obtained strains by PAS to promote wheat growth, by analyzing biometric and nutrimental parameters, as well as the relative expressions of NRT1.4, GluTR, and 6-SFT1 genes. Results Twenty strains were obtained by PAS (170 bacterial strains were originally co-inoculated to plants), of which, twelve strains showed the ability to promote wheat growth mainly by the stem development and the number of leaves. Moreover, thirteen strains up-regulated the 6-SFT1 gene, and three strains up-regulated the GluTR gen. Thus, the strains Enterobacter cloacae TS3, Microbacterium foliorum TS9, Bacillus cereus TS10, Paenibacillus lautus TE8, and Paenibacillus lautus TE10 were identified as promising PGPB, showing strong wheat growth promotion events compared with those strains obtained by MTS. Conclusions PAS is an easy and feasible alternative for identification of PGPB. However, ecological and economic factors need to be investigated to use the obtained strains by PAS for commercial microbial inoculants formulations.

Effect of contaminated soil on multitrophic interactions in a terrestrial system Abstract Background and aims The effect of contaminated soil on multitrophic interactions in a terrestrial system was studied in a mesocosm experiment with European beech, larvae of summer chafer, and entomopathogenic nematodes (EPN). Methods Beech seedlings were grown in non-contaminated forest and garden soil contaminated with potential toxic elements Cd, Pb and Zn, and in the absence/presence of summer chafer larvae. Root morphology and ectomycorrhizal communities were analysed. Volatile organic compounds (VOC) emitted from beech roots were investigated by GC-MS. Humulene, β-caryophyllene, borneol and camphor were selected for chemotaxis assay with EPN Steinernema feltiae, S. carpocapsae and Heterorhabditis bacteriophora. Results Increased root tip density was observed as a response to soil contamination and root herbivory. Soil contamination decreased ectomycorrhizal species richness. Analyses of VOC yielded 14 different compounds. Results showed that the least mobile nematode species towards the VOC tested was S. feltiae. β-caryophyllene was an attractant for S. carpocapsae and humulene a weak attractant for H. bacteriophora. Conclusions Beech roots emitted VOC that affected movement of EPN. β-caryophyllene was detected in beech roots regardless of treatment, indicating that VOC that affect movement of EPN are emitted even in the absence of direct root herbivore attack.

The leaf economic spectrum drives leaf litter decomposition in Mediterranean forests Abstract Background and aims Leaf litter decomposition is an important process controlling nutrient cycling in most terrestrial ecosystems. We evaluated the relationships among traits of green leaves and decomposition rates of leaf litter (k) at different environmental scales and organisational levels (species and community). We also assessed the relationships at community level between k and the Leaf Economic Spectrum (LES) and between k and different soil variables. Methods We measured leaf traits in 38 woody species distributed in nine sampling sites along a topographic gradient in southern Spain. Leaf litter was collected for each species in each sampling site and incubated in a microcosm experiment with soil collected from the top 20 cm of each site. Results We found positive relationships between k and specific leaf area (SLA), leaf N, K and P and negative relationships with leaf dry matter content (LDMC) and leaf C isotopic composition (δ13C), both at species and community levels. Decomposability was positively related with the first PCA axis describing the LES and the relationships were consistent across all sites and within different zones or topographic positions. In addition, community weighted mean values of leaf traits (LESCWM) were stronger predictors of litter decomposition than soil variables. Conclusions A major finding of the present study is the main role that leaf traits, and the covariation among them (LES), play on decomposition process in Mediterranean ecosystems both at the species and community levels. In summary, our results support the idea that the suites of leaf traits have a strong control on the pace of C cycling, being the best drivers of decomposition processes under similar climatic conditions.

Plant nutrition and soil fertility: synergies for acquiring global green growth and sustainable development

Correction to: Diel plant water use and competitive soil cation exchange interact to enhance NH 4 + and K + availability in the rhizosphere In Table 1 of the original publication, values and units for parameters used to simulate root nutrient uptake (Vmax and Km) were incorrect. Here we present the correct values and units for these parameters.

Expansion of rice enzymatic rhizosphere: temporal dynamics in response to phosphorus and cellulose application Abstract Aims The rhizosphere has ecological importance as a microbial hotspot to understand the ‘real’ processing rates of element cycles without being misled by inactive bulk soil. It is thus essential to estimate the rhizosphere size and its response to anthropogenic distribution during crop growth. Methods In situ β-glucosidase, cellobiohydrolase (C-acquiring), and acid and alkaline phosphatase (P-acquiring) activity was examined using soil zymography in rice rhizosphere. Temporal dynamics of enzymatic rhizosphere size under phosphate and cellulose fertilization were calculated based on the expansion of enzyme activity hotspot. Results After 35 days of root development, radial expansion of cellobiohydrolase and acid phosphatase from the root centre to bulk soil was further than that of β-glucosidase and alkaline phosphatase, respectively. Root development expanded β-glucosidase hotspot in rhizosphere, but inhibited rhizosphere expansion of phosphatase activities. P fertilization caused strong N competition between plants and microorganisms, reducing rhizosphere expansion of all tested enzyme activities. Cellulose had no significant effects on C-acquiring enzyme activity expansion, but led to extended acid and alkaline phosphatase hotspots in the rice rhizosphere under P fertilization. Conclusion i) Plant growth stage affects the rice enzymatic rhizosphere size; ii) P fertilization in P-limited soil enhances rhizosphere enzyme activities but reduces the radial expansion; iii) non-labile C application affects enzymatic rhizosphere expansion in an enzyme-specific manner interactively with P fertilization.

Comments on “unravelling community assemblages through multi-element stoichiometry in plant leaves and roots across primary successional stages in a glacier retreat area” by Jiang et al.