Arbuscular Mycorrhizae and Greenhouse Gas Emissions
Arbuscular mycorrhizae fungi (AM) mediate in the uptake of several forms of soil N. Higher N interception could reduce its availability for microbial processes such as nitrification and denitrification, thereby reducing the risk of N loss through gas emissions. AM can also affect soil CO2 fluxes either due to direct respiration of the fungi or due to indirect impacts of AM on heterotrophic microorganisms. AM could indirectly influence the release of greenhouse gas emissions (GHG), through the change in soil physical conditions such as moisture, aggregation and aeration that largely influence the production and transport of greenhouse gases in soil. The role of AM fungi on plant nutrition and soil processes has shown to be particularly relevant in those environments where resources such as water and nutrients are scarce or less available. This is the case of low-input agricultural production systems where large part of the plant nutrients is found in organic form.
A greenhouse experiment was carried out in which we compared the impacts of a wild genotype of tomato and its non-mycorrhizal mutant on the CO2 and N2O emissions from an organically-managed soil amended with compost. Plants were grown in 12L pots and subjected to two dry downs. The impacts of AM and moisture regimes in GHG emissions were assessed in root in-growth PVC cylinders installed in the pots. First dry down was carried in 8 week-old plants, and consisted of a 3% daily reduction in soil moisture over 4 days, starting at 20% gravimetric moisture and ending at 11%. Pot moisture was then taken again to 20% in order to allow plants to recover briefly from the water stress. Six days later, 9 week-old plants were subjected to a second experimental dry down over 9 days, in this case two different moisture regimes were applied: (i) a drought treatment where the entire soil volume in the pots gradually decreased from 22% to 10% gravimetric moisture over the nine days, and (ii) a patchy watering moisture regime, where the pots were also subjected to the same decrease in moisture but where PVC cylinders were kept at 22% moisture by regular watering. Gas samples were taken 4 hours after watering events, daily in the first dry down and every other day in the second, using the static chamber method. Further, photosynthetic rates, stomatal conductance and water use efficiency of the tomato plants was assessed using a field portable open flow infra-red gas analyzer (IRGA) (Model 6400, LI-COR Inc., Nebraska, USA). Plants were harvested after the second dry down and their biomass and total N content assessed. Soil moisture was monitored throughout the experiment, and samples were collected from the pots at harvest in order to determine DON, DOC, NH4+, NO3-, as well as microbial biomass C and N.
Preliminary data showed a role of AM fungi on the release of N2O depending on the soil moisture. A further analysis of the growth and physiology of AM plants under different soil moisture regimes indicated that the role of AM fungi on soil emissions could be indirect through the different use of the available water in the soil. The present results provide evidence of the key role of AM fungi for the sustainability of cropping systems.
Lazcano, C., F.H. Barrios-Masias, and L.E. Jackson. 2014. Arbuscular mycorrhizal effects on plant growth and soil greenhouse gas emissions under changing moisture regimes. Soil Biology and Biochemistry 74:184-192. Lazcano et al. 2014