Interactions Of Arbuscular Mycorrhizal (AM) Fungi And Several Crop Species Under Disturburbance And Competition

Arbuscular mycorrhizal (AM) fungi widely exist in agricultural ecosystems and can form symbiosis with approximately 80% of vascular plants. The symbiosis usually enhances plant mineral nutrient uptake, improves water stress tolerance and disease resistance. It plays an important role in agricultural production, vegetation components and succession. A two-year field survey was undertaken in 2004 and 2005 to investigate the effects of various tillage systems on AM fungi infection in the Loess Plateau of Gansu China. These were conventional tillage (CT), no tillage (NT), conventional tillage with stubble retention (TS) and no tillage with stubble retention (NTS). The rotation crops were maize (Zea mays), winter wheat (Triticum aestivum) and soybean(Glycine max). To further understand the effect of soil disturbance and residue application on the interactions of plant and AM fungi, a greenhouse experiment was conducted using wheat and a wild variety (A17) of medic (Medicago truncatula var. Jemalong) to simulate a single rotation of wheat and medic. Soils were disturbed or non-disturbed to simulate tillage or no tillage, with or without application of residue to simulate stubble retention. The two AM fungi used in the trial were Glomus intraradices and Gigaspora margarita. The role of AM fungi in medic growth competition was also studied by using the wild (A17) and mutant (dmil) variety of medic as competition combinations.32P labelled soil was employed to measure the direct P uptake by AM fungi. The results were as the follows:1. AM colonisation of the crops was well established at seedling stage and varied with agricultural practices and growth stage. All crops had higher AM colonisation at the flowering/podding stage than that of the seedling stage. Percentage of root colonisation was 11%-30% for maize,8%-31% for winter wheat and 3%-30% for soybean. In general, NT and TS increased AM colonisation of the three crops by 10%-15% compared with CT and NTS. There was no effect of residue application or tillage treatment on grain yield, biomass and P uptake of the three crops in either year, however the trends were similar for grain yield, biomass and P uptake under different agricultural practices.2. In the greenhouse, the two stage experiment was successful in simulating the wheat/medic rotation. Soil disturbance and residue application did not affect AM colonisation in medic nor did it affect hyphal length density in hyphal compartments (HCs) and root hyphal compartments (RHCs). Four weeks after seedling emergence, AM colonies in the medic were well established from the inoculation. The mean colonisation rate was 70%,30% and 60%, by G intraradices, Gi. margarita and mixtures of both species respectively. Percentages of AM colonisation from mixed inoculation was in between inoculation of a single species, however the infection rate by single inoculation of G. intraradices and Gi. margarita was 60%, whereas the rate by mixed inoculation was 25% 7 weeks after emergence. Plants with AM fungi showed an advantage in competition to infection site and growth space in roots. Non mycorrhizal medic did not show any response to soil disturbance, but medic with AM treatment was significantly affected by soil disturbance, which suggests that the effect of soil disturbance on medic was via its effects on AM fungi. Soil disturbance significantly decreased medic P uptake and biomass. Compared with the undisturbed treatment, soil disturbance decreased P uptake of medic inoculated with the mix of the two AM fungi by 37.64% or with Gi. margarita by 69.67% while it had no effect on the G intraradices inoculation. Soil disturbance only decrease P uptake of medic colonized by G intraracides by 30.29% compared to the undisturbed treatment at 7 weeks after emergence. Residue application had little effect on the interactions of medic and AM fungi, it only increased hyphal length density in HCs or RHCs in disturbed and undisturbed treatment. The effects of treatments on size distribution of water stable aggregates was an increased number of particles>2 mm in all inoculated treatments compared with NM treatments. Soil disturbance decreased the number of particles at size of 1-2 mm and 0.25-5 mm where as residue application increased numbers of particles of 1-2 mm and 0.25-0.5 mm.3. Medic plants of wild type A17 and mutant dimil were chosen as a positive and non response plant to AM fungi in the growth competition experiment. The results showed that A17 had an advantage compared to dmil in P absorbance and that the P extracted via AM fungi was transferred into biomass of A 17 at both A17+A17 and A17+dmil combinations. The biomass of A17 was 1-2 times greater than dim1 and the 32P uptake by A17 was about 100-200 times to dmi1. There is function diversity among plant-AM fungi combination in competition situations. The symbiosis of A17 and AM fungi was well established in the range of 40%-80%. The colonisation of A17 was lower at high P than that in the low P treatment. A 17 also had higher AM colonisation in A17+A17 than in A 17+dmil. The dmi1 showed different competition strategy to AM colonisation, it had higher AM colonisation at A17+dmi1 (15%) than in dmi1+dmi1(2%).