The effect of mycorrhizal fungi associated with willows growing on marginal agricultural land

This thesis involved a farm-scale field experiment that ran for three growing seasons from 2010 to 2012, and covered almost 1.4 hectares in total. The experiment used two cultivars of shrub willows grown for biomass, Salix miyabeana Seeman SX61 and SX64, on three very different marginal fields, with and without a nitrogen fertilization treatment of 75 kg/ha, and with and without a mixed symbiotic fungal inoculation treatment of the arbuscular mycorrhizal (AM) species Rhizoglomus irregulare and the ectomycorrhizal (EM) species Hebeloma longicaudum. This experiment was unique from most mycorrhizal inoculation experiments with trees or even most vegetables, because willow cuttings are planted directly into farm soil (instead of started as seedlings in a nursery or greenhouse). Both the control willows and treated willows interacted from the beginning with intact, unsterilized soil. The inoculum had to compete throughout with the native microorganisms already present, providing a robust proxy for agricultural systems planting with seed and increasing the practical relevance and applicability of these results. The results will also not contain a hidden bias that we believe could affect inoculation experiments whose control plants grow in completely sterile soil during their initial growth period. The fact that our experiment includes the imperfectly understood wild organisms that exist in unsterile farm soil, explains why a large part of this thesis is also an exploration of the native mycorrhizal community. PCR-cloning-sequencing targeting the universal fungal Internal transcribed spacer (ITS) as well as an AM specific 18S fragment, was used on DNA extracted from 36 root and rhizospheric soil samples obtained at the end of the first growing season in 2010. Then more comprehensive Illumina MiSeq sequencing targeting the fungal ITS was used on DNA extracted from 96 rhizospheric soil samples obtained at the end of the second growing season in 2011. We did observe a marked increase in growth with nitrogen fertilization on marginal land, as well as a substantial increase in initial growth correlated with a higher-moisture, higher-sand-percentage-soil field, though these findings are limited to qualitative observation as the experiment was not designed to test them specifically. Notably, this experiment conducted in unsterilized farm soil showed no growth benefit from inoculating with mycorrhizal fungi, in direct contrast with lab and greenhouse results obtained in the past. Furthermore, the inoculum did not appear to compete successfully with numerous indigenous fungi. Interestingly, the majority of the indigenous fungi present have never been cultured or studied. Also, even though many of the fungi are likely mycorrhizal and all associated with cuttings of the same two willow cultivars, the fungal community in one of the test fields is very different from those fungi in the other two fields. This difference could correlate to a marked difference in soil type.

Keyphrases: mycorrhizal fungi, willow, inoculation, agricultural field experiment, rhizospheric soil, indigenous fungi, champignons mycorhiziens, saules, inoculation