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Despite its fundamental role for carbon (C) and nutrient cycling, rhizodeposition remains 'the hidden half of the hidden half': it is highly dynamic and rhizodeposits are rapidly incorporated into microorganisms, soil organic matter, and decomposed to CO2. Therefore, rhizodeposition is rarely quantified and remains the most uncertain part of the soil C cycle and of C fluxes in terrestrial ecosystems. This review synthesizes and generalizes the literature on C inputs by rhizodeposition under crops and grasslands (281 data sets). The allocation dynamics of assimilated C (after 13C- CO2 or 14C- CO2 labeling of plants) were quantified within shoots, shoot respiration, roots, net rhizodeposition (i.e., C remaining in soil for longer periods), root-derived CO2, and microorganisms. Partitioning of C pools and fluxes were used to extrapolate belowground C inputs via rhizodeposition to ecosystem level. Allocation from shoots to roots reaches a maximum within the first day after C assimilation. Annual crops retained more C (45% of assimilated 13C or 14C) in shoots than grasses (34%), mainly perennials, and allocated 1.5 times less C belowground. For crops, belowground C allocation was maximal during the first 1-2 months of growth and decreased very fast thereafter. For grasses, it peaked after 2-4 months and remained very high within the second year causing much longer allocation periods. Despite higher belowground C allocation by grasses (33%) than crops (21%), its distribution between various belowground pools remains very similar. Hence, the total C allocated belowground depends on the plant species, but its further fate is species independent. This review demonstrates that C partitioning can be used in various approaches, e.g., root sampling, CO2 flux measurements, to assess rhizodeposits' pools and fluxes at pot, plot, field and ecosystem scale and so, to close the most uncertain gap of the terrestrial C cycle. [ABSTRACT FROM AUTHOR]

Introduction: Root-mediated changes in soil organic matter (SOM) decomposition, termed rhizosphere priming effects (RPE), play crucial roles in the global carbon (C) cycle, but their mechanisms and field relevance remain ambiguous. We hypothesize that nitrogen (N) shortages may intensify SOM decomposition in the rhizosphere because of increase of fine roots and rhizodeposition. Methods: RPE and their dependence on N-fertilization were studied using a C-to-C vegetation change. N-fertilized and unfertilized soil cores, with and without maize, were incubated in the field for 50 days. Soil CO efflux was measured, partitioned for SOM- and root-derived CO, and RPE was calculated. Plant biomass, microbial biomass C (MBC) and N (MBN), and enzyme activities (β-1,4-glucosidase; N-acetylglucosaminidase; L-leucine aminopeptidase) were analyzed. Results: Roots enhanced SOM mineralization by 35 % and 126 % with and without N, respectively. This was accompanied by higher specific root-derived CO in unfertilized soils. MBC, MBN and enzyme activities increased in planted soils, indicating microbial activation, causing positive RPE. N-fertilization had minor effects on MBC and MBN, but it reduced β-1,4-glucosidase and L-leucine aminopeptidase activities under maize through lower root-exudation. In contrast, N-acetylglucosaminidase activity increased with N-fertilization in planted and unplanted soils. Conclusions: This study showed the field relevance of RPE and confirmed that, despite higher root biomass, N availability reduces RPE by lowering root and microbial activity. [ABSTRACT FROM AUTHOR]

Background and aims: Rhizodeposition of plants is the most uncertain component of the carbon (C) cycle. By existing approaches the amount of rhizodeposition can only roughly be estimated since its persistence in soil is very short compared to other organic C pools. We suggest an approach to quantify rhizodeposition at the field scale by assuming a constant ratio between rhizodeposited-C to root-C. Methods: Maize plants were pulse-labeled with CO under controlled conditions and the soil CO efflux was separated into root and rhizomicrobial respiration. The latter and the C activity remaining in the soil corresponded to total rhizodeposition. By relating rhizodeposited-C to root-C a rhizodeposition-to-root ratio of 0.56 was calculated. This ratio was applied to the root biomass C measured in the field to estimate rhizodeposition under field conditions. Results: Maize allocated 298 kg C ha as root-C and 166 kg C ha as rhizodeposited-C belowground, 50 % of which were recovered in the upper 10 cm. The fate of rhizodeposits was estimated based on the C data, which showed that 62 % of total rhizodeposition was mineralized within 16 days, 7 % and 0.3 % was incorporated into microbial biomass and DOC, respectively, and 31 % was recovered in the soil. Conclusions: We conclude that the present approach allows for an improved estimation of total rhizodeposition, since it accounts not only for the fraction of rhizodeposits remaining in soil, but also for that decomposed by microorganisms and released from the soil as CO. [ABSTRACT FROM AUTHOR]

Abstract: The origin and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. The present study aimed to elucidate and quantify the carbon (C) flow from both root and shoot litter residues into soil organic, extractable, microbial and fungal C pools. Using the shift in C stable isotope values associated with replacing C3 by C4 plants we followed root- vs. shoot litter-derived C resources into different soil C pools. We established the following treatments: Corn Maize (CM), Fodder Maize (FM), Wheat+maize Litter (WL) and Wheat (W) as reference. The Corn Maize treatment provided root- as well as shoot litter-derived C (without corn cobs) whereas Fodder Maize (FM) provided only root-derived C (aboveground shoot material was removed). Maize shoot litter was applied on the Wheat+maize Litter (WL) plots to trace the incorporation of C4 litter C into soil microorganisms. Soil samples were taken three times per year (summer, autumn, winter) over two growing seasons. Maize-derived C signal was detectable after three to six months in the following pools: soil organic C (Corg), extractable organic C (EOC), microbial biomass (Cmic) and fungal biomass (ergosterol). In spite of the lower amounts of root- than of shoot litter-derived C inputs, similar amounts were incorporated into each of the C pools in the FM and WL treatments, indicating greater importance of the root- than shoot litter-derived resources for the soil microorganisms as a basis for the belowground food web. In the CM plots twice as much maize-derived C was incorporated into the pools. After two years, maize-derived C in the CM treatment contributed 14.1, 24.7, 46.6 and 76.2% to Corg, EOC, Cmic and ergosterol pools, respectively. Fungi incorporated maize-derived C to a greater extent than did total soil microbial biomass. [Copyright &y& Elsevier]

Translocation of recently photoassimilate plant carbon (C) into soil via root exudates or mycorrhizal fungi is important to understand global carbon cycling. However, it is unclear that how saprotrophic fungi or its interaction with mycorrhizal fungi will affect these process. To address this question we inoculated Picea abies with ectomycorrhizal fungi (ECM), or saprotrophic fungi (SAP), or a mixture of both (ECM- SAP) using rhizobox systems. Based on 14C labelling and phosphor imaging, we visualized the allocation of photoassimilate into roots, and estimated the spatial distribution of hotspots at the root tips. The spatial distribution of assimilate allocation was evaluated based on the 14C specific activity expressed as photostimulated luminescence (PSL) of imaging plates. Areas with high relative 14C activity were classified as hotspots. Besides, the 14C distributions in microbial biomass carbon pool and dissoluble organic pools were also quantified. The result indicated that root biomass in ECM fungi-inoculated plants were 50% lower than that in non-inoculated plants (control plant), while roots of saprotrophic fungi inoculated plants showed the same root biomass as that in non-inoculated plants. Both mycorrhizal or saprotrophic fungi inoculation show 1.7-7.5 times greater carbon allocation to the root system compare to control plant. The amounts of 14C-labeled C in the microbial biomass carbon (14C-MBC) were 40% higher in ECM plants than in control plants, whereas SAP inoculation did not change 14C-MBC. Dual inoculation with mycorrhizal and saprotrophic fungi in ECM- SAP plant reduce 14C-MBC by 28% compared with ECM inoculated plants. SAP inoculated plants have the highest 14C-labeled C in the dissolved carbon (14C-DOC), which was 50% higher than in control plants. Compare to ECM inoculated plants, dual inoculation with saprotrophic fungi indicated a significant increased 14C loss in DOC. Overall, observations in our study suggested that mycorrhizal and saprotrophic fungi both can enhance photoassimilate allocation, which might affiliate the plant to explore nutrients from soil. However, ECM inoculated plants invested photoassimilate allocation into hyphae growth, but SAP inoculated plants might enhance the release of plant root exudates. Dual inoculation with ECM and SAP demonstrated the competition interaction between both fungal, which have change the patterns for photoassimilate distribution. Such changes in rhizosphere carbon inputs might have great impacts on soil nutrients cycles, therefore, further visualization on soil enzyme activities using zymography are under further analyzation. [ABSTRACT FROM AUTHOR]