7 Summary / Zusammenfassung
Major-Findings
As hypothesized, mycorrhizal receiver plants gained higher quantities of fungal mediated N from donor plants that had been colonised. This increase may have been due to a higher hyphae length density in the soil adjacent to the colonised roots. The results indicate a possible re-mobilisation of N reserves from fungal structures located within the root and a subsequent export to the receiver plant through hyphal networks. The amount of N transferred was low compared to the direct uptake by the plant root. The results confirm the suggestion made in earlier studies that in soils with a low N availability AM fungal N transfer alone cannot meet the N demands of a fast growing plant being N deficient. However, a considerable portion of the N contained in the dead root of the donor plant was transported by the fungus to the receiver plant. If, and under which circumstances AM fungi by absorbing nutrients may reduce N losses through leakage has to be investigated in further studies.
Hypothesis 2
When soil containing an established mycorrhizal network is mechanically disturbed, AM fungal N transfer to a colonised host plant will be significantly reduced. (Chapter 3)
Major-Findings
In contrast to the hypothesis the N transfer to a receiver plant was increased as a consequence of soil disturbance. The fragmentation process probably lead to elevated N losses from root fragments and thereby increased the nutrient availability in the soil readily taken up and transferred by the fungus. As long as the mycelium was symbiotically associated with a host plant the fungus might have a high capacity to re-establish the network after fragmentation and again function as a nutrient transport vessel. A soil fragmenting technique in combination with the incorporation of plant residues may aid a fast assimilation of mobile, inorganic N into the mycelium. This result is contrary to earlier studies where intensive forms of soil disturbance showed negative impacts on the functioning of the AM symbiosis.
Hypothesis 3
When fungal colonisation of plants is established exclusively by the ERM, AM fungal isolates with a higher extent of ERM proliferation in the soil volume prior to mycelium excision will have a higher inoculum potential and growth promoting effect on the subsequent crop.
(Chapter 4)
Major-Findings
After excision from a host plant, AM fungal isolates with a higher extent of ERM spread in soil also showed a higher root colonisation rate and contributed to growth of a subsequent plant.
Also, AM fungi were able to partly compensate for a slow development of their ERM at the beginning of the symbiosis, possibly by means of a high fungal capacity for nutrient uptake and transfer, as also suggested in earlier reports. Under conditions of low P availability, mycorrhiza-responsive plants may benefit enormously from the colonisation in early phases of growth. AM fungal isolate-specific patterns of vertical root colonisation and horizontal spread of the ERM in soil were consistent in association with both, the initial host (maize) as well as with the successor host plant (sweet potato). This indicates that AM fungi maintain specific growth patterns, irrespective of the host plant species.
Hypothesis 4
The mechanical fragmentation of detached ERM, induced by soil disturbance, reduces AM fungal inoculum potential and consequently reduces fungal contribution to P uptake and growth of the next plant. (Chapter 4)
Major-Findings
There was no evidence that soil disturbance affected the fungal contribution to P uptake or growth of mycorrhizal host plants, when the soil profile was maintained. The tested AM fungal inocula had a high inoculum potential, irrespective of the spatial distribution of the ERM before the disruption. The results suggest that established and fast growing mycelia are not negatively affected by management practices that are loosening and fragmenting the soil without turning it upside down.
Hypothesis 5
Spore development within dead roots will not depend on whether the root originated from a host or a non-host plant species, but rather will increase with root diameters. (Chapter 5) Major-Findings
The presence of dead roots in soil clearly stimulated hyphal proliferation as well as sporulation on the surface and inside root fragments, irrespective of whether roots originated from a host or a non-host species. In accordance with the outlined hypothesis, the average number of spores per unit root length was higher in root fragments with larger diameter. In comparison with the surrounding soil or air gaps contained within, AM fungal sporulation occurred preferably in dead roots. This observation may be justified by the fact that AM fungi usually prefer to
proliferate into soil partition with elevated nutrient availability. The organic matter from decaying roots release nutrients available to the fungus (directly or indirectly after microbial mineralisation). Since decaying plant roots are widespread in vegetated soils they may serve the fungus as important physical shelter to protect fungal propagules before the establishment of a new symbiosis. Spore aggregation within root fragments may represent a potential technique to obtain AM fungal spores in a low-weight carrier material. This would meet the present demand for the development of AM inoculum products, easy to transport and allowing effective application.
When cultivated in C-loess already in an early developmental stage mycorrhizal sweet potato plants took up more than double the amount of P than non-mycorrhizal plants. The speed with which some of the AM fungal isolates contributed to growth of the next plant after the infection which can partly be explained by the fast spread of mycelium into the soil, known for some AM fungi. Despite a low degree of colonisation in the bulk soil and in roots, the AM fungi showed a high potential to contribute to P uptake of the next crop in an early time of the symbiosis. In condition of low P availability the AM symbiosis may significantly improve the growth of plants which have a relatively low capability to forage for P.
The advantages of the AM symbiosis in plant production can be maximised by implementing suitable culture systems. One suitable method may be to apply conservation tillage combined with reduced fertilisation. Therewith, agricultural practices that do not change the vertical soil profile but are rather fragmenting the soil structure obviously do not have a relevant impact on AM fungal growth or infection potential. Nutrients that derive from plant residues in soil likely play a significant role in low input culture systems. The results showed that AM fungi have a targeted growth towards decaying roots to deposit nutrient reserves therein in the form of spore material. The fungi may absorb nutrients released from the present organic material (such as from decaying roots) and distribute the resources within the hyphal network. Where N is mainly in an immobile form, for example as organic N or ammonium it can become scarcely available to plants. Via the profusely branched mycelium the AM fungus may reach such N sources that are less accessible even for fine plant roots. The fungus can compete with other soil microorganisms for the recently mineralised nutrients (such as N). Even if only part of the assimilated N is transferred to the host plant via the mycelium, the most might be kept in fungal tissues located in top soil layers and may become available for the pants in a long term.
A complete extraction of the ERM from the soil for AM fungal studies bears a considerable operating expense and might explain the predominant use of soil-less media for studies on AM fungal physiology. With the development of techniques that allow the extraction of almost intact ERM it became possible to directly examine effects on morphology and physiology of AM fungi in soils. For the better understanding of cultivation processes and their consequences for the AM symbiosis in plant production future studies should focus on benefits of AM fungal colonisation in early stages of host plant growth in different soil types. More information should be obtained on targeted acquisition of nutrients from organic material by the external mycelium and its closely associated microorganisms.