AM fungi as determinants of plant resource capture from organic patches followed by isotopic and molecular techniques

Project: Research project (funded)Research

Project Details

Description

Traditionally the arbuscular mycorrhizal (AM) symbiosis is viewed as a classic mutualism, an interaction in which both partners benefit. The fungi appear to acquire their entire carbon supply from the plant, and although colonisation of roots by AM fungi (AMF) can confer a wide range of benefits to the plant (Newsham et al. 1995), the most widely cited benefit is that of enhanced phosphorus (P) acquisition. In contrast, as this type of mycorrhizal association tends to dominate in systems where bacterial decomposition and nitrification processes are favoured, thus the main form of N in these systems is nitrate, it was previously thought as nitrate is highly mobile in soil the AM association would not be important for plant N acquisition (Read, 1991). However, in our previous studies on root proliferation in plants grown in interspecific plant competition we found the species which proliferated the most roots in the organic patch subsequently captured the most N from the patch, even when the N was released as inorganic N (including nitrate) by microbial decomposition (Hodge et al., 1999). Thus, it was important to investigate the question of the role AM fungi in N capture from organic patches anew. Because of their smaller size, AM hyphae should be:
i) less expensive to construct than fine roots in terms of both carbon and nitrogen
ii) able to respond to short-lived nutrient patches that the roots cannot detect
iii) able to penetrate to the sites of patch decomposition and therefore be able to compete directly with other soil micro-organisms for the nutrients released.
Also as AM fungi are obligate biotrophs and obtain all their carbon from their host plant, this should give the AM fungi a competitive advantage over other soil micro-organisms should carbon become limiting in the patch zone. Thus, proliferation of AMF hyphae instead of roots, should in theory, be more beneficial to the host plant.

Thus the objectives of this fellowship are:
OBJECTIVE 1: Determine if AM colonisation enhances patch exploitation.
OBJECTIVE 2: Examine if root proliferation and turnover is modified by i) AM colonisation and ii) the extent of AM colonisation.
OBJECTIVE 3: Manipulate carbon (C) flux into the rhizosphere and mycorrhizosphere in the presence of an organic patch.

Layman's description

Alterations in patterns of land use worldwide including an increased emphasis on extensification and a reduction in fertiliser inputs demand that we understand how recycling of nutrients occurs both in natural and agro-ecosystems. Sustainable agricultural systems therefore depend upon efficient recycling of nutrients from organic sources, which are patchily distributed in soils. The cycling of nitrogen (N) in the soil-plant system is particularly important as it is a key nutrient in plant growth and also causes environmental problems when lost from the soil-plant system. Although there is some evidence that plants can take up simple organic N compounds, such as amino acids, intact, in practice in most ecosystems microbial decomposition is the first step. Through a previous BBSRC grant (87/DO5140) we demonstrated that plant roots are effective competitors with micro-organisms for N released from complex organic patches but only in the longer term, due to the slower turnover times of roots compared to micro-organisms. However, many plants also have an additional nutrient capture mechanism, namely mycorrhizal symbiosis. Thus, the overall objective of this fellowship programme was to determine if arbuscular mycorrhizal (AM) associations between AM fungi and plant roots modified the host roots response to organic patches and to follow the subsequent impact for nitrogen capture from the patch by the plant. I focused upon arbuscular mycorrhizal associations because this type of mycorrhizal symbiosis is the most common of the different types of mycorrhizal associations and can form on two-thirds of all land plant species, including most crop species. Furthermore, this type of mycorrhizal association tends to dominate in systems where bacterial decomposition and nitrification processes are favoured, thus the main form of N in these systems is nitrate. As nitrate is highly mobile in soils it has previously been assumed that the AM association is not important for N capture. However, in our previous studies on root proliferation in plants grown in interspecific plant competition we found the species which proliferated the most roots in the organic patch subsequently captured the most N from the patch, even when the N was released as inorganic N (including nitrate) by microbial decomposition (Hodge et al., 1999). Thus, it was important to investigate the question of the role AM fungi in N capture from organic patches anew.

Key findings

The key findings are:
1. When both AMF and roots are present in an organic patch, the response by the root system is more important. However, AMF colonisation may indirectly modify this response.
2. When AMF only have direct access to an organic patch they can proliferate extensively within that patch and use the decomposition products from the patch to support their own growth.
3. When C supply from their associated host plant is restricted the ability to proliferate hyphae in the patch does not appear to be adversely affected although internal colonisation by the AMF declines.
4. Access to an organic patch makes both G. hoi and G. mosseae more aggressive colonisers both of un-inoculated plants and of their original host plant.
5. Low N availability in the bulk soil increases AMF hyphal proliferation.
6. AMF do not appear to take up organic N forms directly.
StatusFinished
Effective start/end date1/02/0030/09/07

Funding

  • BBSRC: £242,768.00