Effects of Arbuscular Mycorrhizal Symbiotic Interaction with Roots on Plant Growth and Greenhouse Gas Emissions Under Changing Moisture Regimes.

Increased nutrient and/or water uptake by arbuscular mycorrhizal (AM) symbiosis may affect soil biochemical properties and greenhouse gas (GHG) emissions. A greenhouse experiment was carried out to compare mycorrhizal tomato (76R MYC) and its non-mycorrhizal mutant (rmc) on the CO₂ and N₂O emissions from an organically-managed soil. Plants were grown for 10 weeks in pots with compost amended soil and subjected to two consecutive dry down cycles to simulate changing moisture regimes in the field. Dry downs were applied gradually through controlled watering treatments. The effects of AM and soil moisture in GHG emissions were assessed in root in-growth PVC cylinders installed in the pots. Gas samples were taken from the cylinders using static chambers 4 hours after each watering event. Photosynthetic rates and stomatal conductance of the plants were assessed after watering using a field portable open flow infra-red gas analyzer. Soil moisture was monitored throughout the experiment. Plant biomass and total shoot N, P and K as well as soil content of DON, DOC, NH₄⁺-N, NO₃^- -N and microbial biomass C, were assessed at harvest. For the same shoot growth and nutrient content, rmc plants allocated more resources to root biomass than mycorrhizal plants. AM symbiosis improved the capacity of the plants to adapt to changing soil moisture, increasing photosynthetic rates and stomatal conductance at high soil moisture but decreasing them when soil moisture was lower. In addition AM symbiosis helped to regulate N₂O emissions at high soil moisture. Control over N₂O emissions by AM plants seemed to be driven by a higher use of soil water and not by increased N uptake. Soil management that enhances the colonization of crop roots by arbuscular mycorrhizae may contribute to a reduction in N₂O emissions and more efficient use of water under changing environmental conditions.