Although it is commonly accepted that root exudation enhances plant-microbial interactions in the rhizosphere, experimental data on the spatial distribution of exudates are scarce. Our hypothesis was ...that root hairs exude organic substances to enlarge the rhizosphere farther from the root surface.
Barley (Hordeum vulgare 'Pallas' - wild type) and its root-hairless mutant (brb) were grown in rhizoboxes and labelled with 14CO2. A filter paper was placed on the soil surface to capture, image and quantify root exudates.
Plants with root hairs allocated more carbon (C) to roots (wild type: 13 %; brb: 8 % of assimilated 14C) and to rhizosheaths (wild type: 1.2 %; brb: 0.2 %), while hairless plants allocated more C to shoots (wild type: 65 %; brb: 75 %). Root hairs increased the radial rhizosphere extension three-fold, from 0.5 to 1.5 mm. Total exudation on filter paper was three times greater for wild type plants compared to the hairless mutant.
Root hairs increase exudation and spatial rhizosphere extension, which probably enhance rhizosphere interactions and nutrient cycling in larger soil volumes. Root hairs may therefore be beneficial to plants under nutrient-limiting conditions. The greater C allocation below ground in the presence of root hairs may additionally foster C sequestration.
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.
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 C fluxes in terrestrial ecosystems. This review synthesizes and generalizes the literature on C inputs by rhizodeposition under crops and grasslands.
Data for stable C and N isotope natural abundances of arbuscular mycorrhizal (AM) fungi are currently sparse, as fungal material is difficult to access for analysis. So far, isotope analyses have ...been limited to lipid compounds associated with fungal membranes or storage structures (biomarkers), fungal spores and soil hyphae. However, it remains unclear whether any of these components are an ideal substitute for intraradical AM hyphae as the functional nutrient trading organ. Thus, we isolated intraradical hyphae of the AM fungus
Rhizophagus irregularis
from roots of the grass
Festuca ovina
and the legume
Medicago sativa
via an enzymatic and a mechanical approach. In addition, extraradical hyphae were isolated from a sand-soil mix associated with each plant. All three approaches revealed comparable isotope signatures of
R. irregularis
hyphae. The hyphae were
13
C- and
15
N-enriched relative to leaves and roots irrespective of the plant partner, while they were enriched only in
15
N compared with soil. The
13
C enrichment of AM hyphae implies a plant carbohydrate source, whereby the enrichment was likely reduced by an additional plant lipid source. The
15
N enrichment indicates the potential of AM fungi to gain nitrogen from an organic source. Our isotope signatures of the investigated AM fungus support recent findings for mycoheterotrophic plants which are suggested to mirror the associated AM fungi isotope composition. Stable isotope natural abundances of intraradical AM hyphae as the functional trading organ for bi-directional carbon-for-mineral nutrient exchanges complement data on spores and membrane biomarkers.
Drought and the availability of mineable phosphorus minerals used for fertilization are two of the important issues agriculture is facing in the future. High phosphorus availability in soils is ...necessary to maintain high agricultural yields. Drought is one of the major threats for terrestrial ecosystem performance and crop production in future. Among the measures proposed to cope with the upcoming challenges of intensifying drought stress and to decrease the need for phosphorus fertilizer application is the fertilization with silica (Si). Here we tested the importance of soil Si fertilization on wheat phosphorus concentration as well as wheat performance during drought at the field scale. Our data clearly showed a higher soil moisture for the Si fertilized plots. This higher soil moisture contributes to a better plant performance in terms of higher photosynthetic activity and later senescence as well as faster stomata responses ensuring higher productivity during drought periods. The plant phosphorus concentration was also higher in Si fertilized compared to control plots. Overall, Si fertilization or management of the soil Si pools seem to be a promising tool to maintain crop production under predicted longer and more serve droughts in the future and reduces phosphorus fertilizer requirements.
•High microbial activity in topsoil causes apparent PE shortly after substrate addition.•The input of C from fertilizers induces apparent PE at the 10–30 cm depth.•The release and decomposition of ...mineral-bound organic C shortly after substrate addition contributes to positive PE.•Negative PE in topsoil during later incubation stages is linked to reduced microbial biomass and substrate depletion.•In subsoil, positive PE and net C loss was caused by microbial N mining.
Fertilization is crucial for increasing crop productivity and it alters soil microbial biomass and activities. These alterations exert implications for soil carbon (C) stocks, primarily through the priming effect (PE). Here, we investigated the mechanisms underlying PE and their impact on soil C stocks in paddy soils subjected to long-term (31 years) fertilization. Soils from four depths within the upper 40 cm layer, subjected to four fertilization types (no fertilizer, mineral fertilizer, mineral + straw, mineral + chicken manure), were incubated for 60 days with or without 13C-labeled glucose. Following substrate addition, an increase in isotopically unlabeled CO2 efflux may stem from accelerated microbial turnover (apparent PE) or modified soil organic matter (SOM) mineralization (real PE). By analyzing microbial biomass (MBC), CO2 and dissolved organic C (DOC) dynamics, we explored the partitioning of C from glucose into three pools. Following glucose addition (day 2), the SOM-derived MBC decreased by 4.7 % at 0–10 cm in unfertilized and by 6.3 % in mineral-fertilized soils. SOM-derived MBC also decreased in soils fertilized with straw- and chicken manure at 0–20 and 0–30 cm. Hence, the apparent PE closely correlated with fertilization types. At day 2, the SOM-derived DOC increased with the increase in extractable Fe2+ content, indicating high contribution of the released mineral-bound organic C in the DOC pool. Microbial decomposition of SOM-derived DOC and loss of unlabeled C through apparent PE increased the release of unlabeled CO2, resulting in a positive but low PE. After 60 days of incubation, the addition of glucose to the subsoils (30–40 cm) caused a net loss of C (positive PE) of 6.6–19 mg kg−1. In contrast, negative PEs were observed in the topsoil, indicating a decrease in microbial activity and C turnover. These findings suggested that fertilization regimes, whether involving long-term extra C input or not, can alter microbial biomass and turnover rates. Moreover, the C turnover mechanisms in paddy soils are complex, including apparent PE, release of mineral-bound organic C, and negative PE. Collectively, these processes can consequently change soil C stocks.
Background and aims
Plant growth affects soil moisture, mineral N and organic C availability in soil, all of which influence denitrification. With increasing plant growth, root exudation may ...stimulate denitrification, while N uptake restricts nitrate availability.
Methods
We conducted a double labeling pot experiment with either maize (
Zea mays
L.) or cup plant (
Silphium perfoliatum
L.) of the same age but differing in size of their shoot and root systems. The
15
N gas flux method was applied to directly quantify N
2
O and N
2
fluxes in situ. To link denitrification with available C in the rhizosphere,
13
CO
2
pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil.
Results
Plant water and N uptake were the main factors controlling daily N
2
O + N
2
fluxes, cumulative N emissions, and N
2
O production pathways. Accordingly, pool-derived N
2
O + N
2
emissions were 30–40 times higher in the treatment with highest soil NO
3
−
content and highest soil moisture. CO
2
efflux from soil was positively correlated with root dry matter, but we could not detect any relationship between root-derived C and N
2
O + N
2
emissions.
Conclusions
Root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth.
Arbuscular mycorrhizal fungi (AMF) influence plant nitrogen/phosphorus (N/P) by modifying plant N and P uptake, which further affects plant stoichiometric N/P homeostasis. Plant species and community ...stoichiometric N/P homeostasis can impact plant species and community stability, respectively, in response to variations of soil N and P availabilities. We investigated interspecific plant interactions via AMF in regard to plant and soil microbial N/P stoichiometry across different soil N and P availabilities induced by N and P addition (0 mg N kg−1, 25 mg N kg−1, 50 mg N kg−1, 30 mg P kg−1 and 100 mg P kg−1). We selected one dominant (Bothriochloa ischaemum; C4 grass) and one subordinate (Lespedeza davurica; legume) species in a natural grassland climax community. We examined how AMF influences stoichiometric N/P homeostasis in monoculture and mixed culture systems, and the resulting consequences for temporal stability of plant species and community in response to variations in soil N and P availability.
The AMF mitigated the P limitation of soil microbial communities and decreased the degree of stoichiometric N/P homeostasis of host plants in monoculture. Through their resource‐scavenging and soil organic matter mineralisation functions, AMF enhances plant ‘luxury consumption', promoting species stability in monoculture in response to soil N and P availability variations. Compared with plants in monoculture, the interaction between B. ischaemum and L. davurica via AMF increased shoot N/P under soil N‐poor conditions, leading to an enhanced degree of stoichiometric N/P homeostasis in both plant species, especially the legume.
Our results suggest that interspecific plant interaction between C4 grass and legume mediated by AMF confers an advantage in complementarity in plant N acquisition under N‐poor conditions, leading to increased stability of plant communities and better maintenance of subordinate species (legume) in response to soil N deficiency.
As the dynamics and magnitude of rhizodeposition vary considerably among cropping systems, we investigated effects of cropping system on the incorporation of root-derived carbon (C) into Collembola, ...a dominant taxon of soil microarthropods. In the field, we used
13
CO
2
to pulse label a crop monoculture (oilseed rape,
Brassica napus
L.), a mixed-grass community (dominated by
Lolium perenne
L. mixed with clover
Trifolium repens
L.), and a tree plantation (willow,
Salix schwerinii
E.L. Wolf and
Salix viminalis
L.). During 28 days, the incorporation of
13
C was traced in nine species of Collembola including epedaphic (surface-dwelling), hemiedaphic (litter-dwelling), and euedaphic (soil-dwelling) functional groups. Incorporation of
13
C into Collembola reached a plateau before day 3 after the labeling in grass and willow, but increased up to day 14 in rape. While euedaphic Collembola incorporated less root-derived C than epedaphic and hemiedaphic Collembola in rape and willow, the incorporation of
13
C was similar among functional groups in grass. Differential incorporation of
13
C in euedaphic species points to niche differentiation within the same functional group. Our findings highlight that cropping system not only affects the flux of root C into soil mesofauna, being slower in rape than in grass and willow, but also the utilization of root-derived resources by functional groups and species of Collembola. The results indicate that pronounced differences in belowground C inputs between cropping systems affect microbivores as basal species and thereby soil food webs and their functioning and services.
Introduction
Microbial abundance and activities in soils are predominantly determined by soil carbon (C), nitrogen (N) and phosphorus (P) availability. Much research has focused on the effects of ...soil N than P availability on soil microbial biomass and enzyme activities as sensitive proxies of microbial ecophysiology highlighting the need to investigate how microbes will respond to P availability in soil, especially in cropping systems.
Materials and Methods
The effect of P fertilisation on microbial biomass‐C, ‐N and ‐P, and the kinetic parameters (maximal velocity Vmax, Michaelis constant Km and catalytic efficiency Ka) of β‐1,4‐glucosidase (BG; C‐acquiring), leucine‐aminopeptidase (LAP; predominantly N‐acquiring) and acid phosphomonoesterase (PHO; P‐acquiring) were measured in a nutrient‐poor agricultural soil (devoid of fertiliser application since 1942).
Results
This study showed that P fertilisation led to a 65% and 56% increase in microbial biomass‐N and ‐P, respectively, indicating severe P limitation and inefficient N acquisition by microbes without P availability. Increased Ka values of LAP with P fertilisation further hint toward the production of efficient isoenzymes to avoid resource tradeoffs for nutrient acquisition.
Conclusions
Overall, these results decipher microbial metabolic and catalytic adjustments mediated by soil P availability. Increased microbial biomass‐N and ‐P with P fertilisation indicated microbial N and P colimitation that was partly overcome by the production of efficient enzymes for N acquisition with P fertilisation. We argue to incorporate microbial enzyme activities as a response to different management strategies to better inform us about soil biogeochemical cycles in cropping systems.
Numerous 13CO2 labeling studies have traced the flow of carbon from fresh plant exudates into rhizosphere bacterial communities. However, the succession of the uptake of carbon leaving the roots by ...distinct rhizosphere microbiota has rarely been resolved between microbial kingdoms. This can provide valuable insights on the niche partitioning of primary rhizodeposit consumption, as well as on community interactions in plant-derived carbon flows in soil. Here, we have traced the flow of fresh plant assimilates to rhizosphere microbiota of maize (Zea mays L.) by rRNA-stable isotope probing (SIP). Carbon flows involving bacteria, unicellular fungi, as well as protists were observed over 5 and 8 days. Surprisingly, labeling of Paraglomerales and several bacteria including Opitutus, Mucliaginibacter and Massilia spp. was especially apparent in soil surrounding the strict rhizosphere after 5 d. This highlights the central role of arbuscular mycorrhizal fungi (AMF) as a shunt for fresh plant assimilates to soil microbes not directly influenced by root exudation. Distinct trophic webs involving different flagellates, amoeba and ciliates were also observed in rhizosphere and surrounding soil, while labeling of filamentous saprotrophic Ascomycota or Basidiomycota was not apparent. This challenges the proposed “sapro-rhizosphere” concept and demonstrates the utility of rRNA-SIP to disentangle inter-kingdom microbial relationships in the rhizosphere.
Display omitted
•rRNA-SIP was used to trace plant-derived carbon flows in the rhizosphere of maize.•13C-labeling of mycorrhiza was stronger in surrounding soil than in the rhizosphere.•Labelling of bacteria incl. Opitutus spp. was also stronger in surrounding soil.•AMF are a shunt of plant assimilates to microbes outside the strict rhizosphere.
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