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Aims: Visualization of enzymatic activity links microbial functioning to localization in heterogeneous soil habitats. To assess enzymatic reactions in soil thin layer at the microscopic level, we developed a micro-zymography approach and tested it by visualization of the potential activity of phosphomonoesterase for aggregates collected from the rhizosphere of Zea mays L. Methods: We evaluated micro-zymography by applying fluorogenically-labeled substrate i) on individual soil aggregates freshly sampled from the rhizosphere, ii) on thin layers of aggregates (≈ 500 µm) saturated with substrate to assess the dynamics of phosphomonoesterase activity, and iii) on maize roots under laser scanning microscope upon the identified hotspots by membrane-based zymography. Results: We found super transparent silicon as the most appropriate fixative to prevent sample drying. We optimized microscope settings to eliminate the soil auto-fluorescence. The fluorescent signal shifted from the free liquid phase towards the aggregate boundaries within 30 min after substrate addition and was finally detectable at the surface of a few aggregates. This was probably due to higher microbial abundance and enzymatic activity on the soil aggregates compared to the liquid phase. The enzymatic activity appeared patchy at the aggregate and root surfaces indicating heterogeneous distribution of hotspots. Conclusions: The methodology including calibration, sample preparation, fixation, and monitoring was developed. The novel membrane-free micro-zymography approach is a promising tool to identify functional specificity and niche differentiation on roots and soil aggregates. This approach revealed unexplained complexity of competing processes (biochemical, hydrolytic, and physical) due to differently charged reaction products and enzyme-clay complexes. [ABSTRACT FROM AUTHOR]

Comprehensive climate change mitigation necessitates soil carbon (C) storage in cultivated terrestrial ecosystems. Deep-rooted perennial crops may help to turn agricultural soils into efficient C sinks, especially in deeper soil layers. Here, we compared C allocation and potential stabilization to 150 cm depth from two functionally distinct deep-rooted perennials, i.e., lucerne (Medicago sativa L.) and intermediate wheatgrass (kernza; Thinopyrum intermedium), representing legume and non-legume crops, respectively. Belowground C input and stabilization was decoupled from nitrogen (N) fertilizer rate in kernza (100 and 200 kg mineral N ha−1), with no direct link between increasing mineral N fertilization, rhizodeposited C, and microbial C stabilization. Further, both crops displayed a high ability to bring C to deeper soil layers and remarkably, the N2-fixing lucerne showed greater potential to induce microbial C stabilization than the non-legume kernza. Lucerne stimulated greater microbial biomass and abundance of N cycling genes in rhizosphere soil, likely linked to greater amino acid rhizodeposition, hence underlining the importance of coupled C and N for microbial C stabilization efficiency. Inclusion of legumes in perennial cropping systems is not only key for improved productivity at low fertilizer N inputs, but also appears critical for enhancing soil C stabilization, in particular in N limited deep subsoils. [ABSTRACT FROM AUTHOR]

• Legumes interacted with abiotic site-characteristics affecting soil microbial activity. • Abiotic environmental factors indirectly controlled the basal respiration of microorganisms. • Soybean cultivation helped to overcome C limitation in savannah soils with low C stocks. Unsustainable farming systems for a growing population in Sub-Saharan Africa stress natural resources and lead to soil degradation. Legume cultivation, however, promotes soil microbial communities and may help reverse soil degradation. The soil properties (pH, texture, C and nutrient contents) of a set of contrasting sites from two agro-ecological zones of Cameroon were determined. This study characterises the microbial activities and functioning in rhizosphere soils of the four legumes Phaseolus vulgaris L., Glycine max L., Arachis hypogea L. and Vigna subterranea L. in a laboratory incubation experiment. Ecological indicators (total respiration, microbial biomass determined as total phospholipid fatty acids, metabolic quotient) of soil health were quantified after microbial activation by 14C-glucose and related to site-specific parameters as well as to the effects of legume cultivation. Microbial activation after glucose addition was frequently site-specific. Cumulative glucose respiration increased over time, and glucose pulses repeatedly boosted glucose respiration and total respiration at all sites. The microbial biomass was lowest after the experiment in the soil from the High Guinean Savannah except for soils under soybean cultivation. Among the four legumes cultivated, only soybeans strongly increased microbial biomass in the High Guinean savannah reaching even the level of microbial biomass in Western Highland soils. The lowest metabolic quotient in soybean rhizosphere soil and lowest glucose respiration compared to the soils of other legumes suggests a high carbon use efficiency, presumably due to C limitation in the High Guinean Savannah. Abiotic soil properties (pH, clay content) strongly influenced microbial habitat properties and thus had a positive effect on the CO 2 efflux, irrespective of glucose addition. Within the set of tested legumes, soybean cultivation can, therefore, be strongly recommended as a long-term sustainable crop to boost soil microbial functioning and probably also nutrient cycling, especially under low total organic carbon stocks. [ABSTRACT FROM AUTHOR]

Plastics accumulating in the environment, especially microplastics (defined as particles <5 mm), can lead to a range of problems and potential loss of ecosystem services. Polyhydroxyalkanoates (PHAs) are biodegradable plastics used in mulch films, and in packaging material to minimize plastic waste and to reduce soil pollution. Little is known, however, about the effect of microbioplastics on soil-plant interactions, especially soil microbial community structure and functioning in agroecosystems. For the first time, we combined zymography (to localize enzyme activity hotspots) with substrate-induced growth respiration to investigate the effect of PHAs addition on soil microbial community structure, growth, and exoenzyme kinetics in the microplastisphere (i.e. interface between soil and microplastic particles) compared to the rhizosphere and bulk soil. We used a common PHAs biopolymer, poly (3-hydroxybutyrate- co -3-hydroxyvalerate) (PHBV) and showed that PHBV was readily used by the microbial community as a source of carbon (C) resulting in an increased specific microbial growth rate and a more active microbial biomass in the microplastisphere in comparison to the bulk soil. Higher β-glucosidase and leucine aminopeptidase activities (0.6–5.0 times higher V max) and lower enzyme affinities (1.5–2.0 times higher K m) were also detected in the microplastisphere relative to the rhizosphere. Furthermore, the PHBV addition changed the soil bacterial community at different taxonomical levels and increased the alpha diversity, as well as the relative abundance of Acidobacteria and Verrucomicrobia phyla, compared to the untreated soils. Overall, PHBV addition created soil hotspots where C and nutrient turnover is greatly enhanced, mainly driven by the accelerated microbial biomass and activity. In conclusion, microbioplastics have the potential to alter soil ecological functioning and biogeochemical cycling (e.g., SOM decomposition). [Display omitted] • Microplastisphere (soil-MPs interface) is localized and visualized by zymography. • MPs stimulates microbial turnover and nutrient efficiency in microplastisphere. • MPs increases soil enzyme activity and shifts bacterial community to K -strategy. • MPs have the potential to alter soil functioning and biogeochemical cycling. [ABSTRACT FROM AUTHOR]

Root morphology reflects plant adaptations to phosphorus (P) deficiency. We hypothesized that changes in rice root morphology reflect P deficiency decrease after ferric iron (Fe(III))-bound phosphate (Fe–P) dissolution in low-redox paddy soils. We developed a novel in-situ 32P phosphor-imaging approach under flooding to estimate P uptake by rice roots released from Fe–P dissolution. 32P-labeled ferrihydrite (31 mg P kg−1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots but not microorganisms from direct Fe–P mobilization, or (2) directly mixed with soil to enable roots and microorganisms unrestricted access to the Fe–P. The establishment of low redox conditions (E h values between −176 and −224 mV) drove the reductive dissolution of Fe–P. Rice root-derived organic acids alone were unable to control Fe–P dissolution, and Fe(III) reduction is predominately a microbially-mediated process. Direct root access to Fe–P raised both the number and mean diameter of crown roots and root tips, and increased P uptake by 149–231%. Crown root elongation rate, 32P activities along roots and root tips were 5–133% higher when roots directly accessed Fe–P compared to Fe–P excluded from roots in mesh bags. Iron accumulation on roots depended on the rice growth stage, but not on their access to Fe–P. Roots' access to Fe–P increased rice crown roots elongation and branching and increased P accessibility under P deficiency. [Display omitted] • Fe–P dissolution increased rice root growth and induced root morphological changes. • Increased root tip numbers raised P uptake by roots from the soil. • Organic acids exuded by rice roots were unable to control Fe–P dissolution. • Iron accumulation on root surface was independent of root's access to Fe–P. [ABSTRACT FROM AUTHOR]