Soil Nitrogen Form and Availability affect the role of Arbuscular Mycorrhizal Fungi on Nitrogen Uptake and Nitrogen Recovery in Durum Wheat

The term Arbuscular Mycorrhizal (AM) is referred to the symbiosis between arbuscular mycorrhizal fungi (AMF) and plant roots. Such symbiosis is the most widespread among plants (two out of three of the all plant taxa) including the majority of crops. AMF belong to the monophyletic subphylum Glomeromycotina which include obligate biotrophs that entirely relay on the host plant for the carbon source. In exchange, AMF provide their host several benefits which have been recognise in mitigation of biotic and abiotic stress, even though the main benefit ascribe to AMF is improving P uptake. However, although the AM symbiosis is considered as a classic example of a mutualistic symbiosis, many factors influence the outcome of this interaction (e.g. fungal and plant species involved in the symbiosis, nutrient availability, etc.), and under certain condition the presence of AMF have been shown to do not influence or depress the plant growth. Soil N availability is one of the main factors affecting the plant production, and AMF have been shown to be able to transfer N to the host plant. However, the conditions that ruling the uptake and the following transfer of N via AMF to the host plant are still unclear. Here, durum wheat has been grown with or without AMF inoculum in presence of different N form and availability to study if the amount and the form of soil N affect the outcome of the AM symbiosis. The experiment was conducted in pot under natural climatic condition. The soil moisture was monitored twice a week with gravimetric methods and additional water was added when the soil moisture reached the 70% threshold of the holding capacity. Three treatments have been compared: 1) native substrate N content (soil Navailability= 0.45 ‰; control); 2) mineral N in form of (NH4)2SO4 applied in equivalent quantity to 75 kg of N ha-1 (N-min); 3) organic N in from of organic matter of Lolium multiflorum applied in equivalent quantity to 6.5 Mg ha-1(N-org). The organic matter was applied before the sowing phase, distributing the fertilizer homogenously in the substrate 5 to 7 cm below the sowing bed. The mineral treatment was applied in two steps: the first application was done 10 days after the emergence (DAE) distributing 2/3 of the total amount, the second was done 50 DAE applying the rest. In each case the fertilizer had a known amount of 15N to monitoring the N fate. Each treatment was replicated 5 times and the experiment was conducted in a completely randomize design. The experiment lasted until the crop reach the flowering phase (75 DAE). At the end of the experiment the biomass was harvested and split in the two parts below and above; the fresh and dry weights were recorded and the total N and 15N content were assayed. The below biomass was used to assay the total root length and the AMF infection. The data were used to determine the plant biomass production, N uptake and N recovery in the plant biomass. The data obtained were analysed in R using a two-way ANOVA. When statistical significance occurred, means were compared using the Tukey’s test. The results, as expected, have shown a marked effect of the fertilizer treatments. In fact, the treatment N-min determined an average increment respect to the control for both above and below ground biomass; while the treatment N-org determined a decrement for both the biomass portion compare to the control. Such trend was consistent for the others variables investigated. As concern the AMF inoculum, on the biomass, the effect was confined to a decrement of the above ground biomass in the N-org treatment, while no significant effect was detected for the same parameter on the others treatments and on the below ground biomass. A strong interaction has been shown between Fertilization and AMF inoculum in N uptake and N recovery. In particular, the presence of AMF determined a decrement of both parameter in the control and N-org, while a marked increase has been detected for both the parameters in the N-min. In conclusion, the effect of AMF inoculum on total plant biomass was very contained. By contrast, on the others parameters a competitive behaviour for the N source has been shown in a poor-N-environment, while a cooperative behaviour was observed in a reach-N-environment.