Effect of tree age and agronomic treatments on mycorrhizal colonization in apple orchards

Background/Question/Methods

Arbuscular mycorrhizal fungi (AMF) colonize plant roots and spread in the surrounding bulk soil enhancing the nutrient uptake from the soil to the plant and stimulating the transfer of photosynthates from the plant to the soil. The importance of this symbiotic association in agro-ecosystems is related to an increasing attention to management sustainability and to the role of these systems as sinks of carbon dioxide (CO₂). The aim of our research was to understand if AMF colonization differs with plant age and with soil treatments. We studied three apple orchards of 1, 2 and 5 year old, grafted on M9 rootstock. In each orchard four plots of 20 trees each are treated with chemical fumigation, mycorrhizal inoculation and no treatment (control). We measured soil respiration in 60 positions per orchard (tot = 180 measurements) in September 2012 with a survey chamber coupled with a gas analyser (LI-8100, Li-Cor). At each position we sampled the soil for the analysis of AMF colonization on apple roots, enzymatic activity, total C and microbial C concentrations.  

Results/Conclusions

AMF colonization increases with the age of plants. After one year from planting 50% of roots were colonized, after 2 years 80% and up to 90% after 5 years. We did not notice a difference in AMF colonization due to soil treatments. After one year from the treatment soil respiration was higher in fumigated than in inoculated and control plots (5.9 ± 0.8, 5.1 ± 0.9 and 5.0 ± 0.8 μmol CO₂ m² s^-1, respectively, p<0.001). However, the effect vanished over time and no differences were noted after 2 and 5 years from the treatment. Microbial carbon was higher in control than in fumigated plots after one year from treatment (151 ± 22, 101 ± 11 mg kg^-1, respectively, p<0.001). The addition of AMF inoculum does not significantly increase the mycorrhizal colonization in the root system, probably due to the natural abundance of AMF under field condition. Although this research is preliminary it supports the hypothesis that AMF have an important role in carbon cycle of apple orchards. This might have positive consequences for field management and provide a support for the carbon sink potentiality of permanent crops.