Dynamics of mycorrhizal respiration in relation to GPP in a Central-Hungarian dry grassland

Soil respiration is a highly complex process including a wide range of soil biota (autotrophic and heterotrophic functioning) and different pathways of carbon cycling (decomposition, arbon allocation), all being under the control of environmental and biotic drivers. The most important biotic driver is the photosynthetic activity of the vegetation providing supply mainly for the autotrophic component of soil respiration: plant roots and their symbiotic partners - such as arbuscular mycorrhizal fungi (AMF). By acting as a source of CO₂ and a pathway of carbon to the SOM, the role of AMF in carbon balance is unquestionable, not to mention that mycorrhizal C allocation could determine the long-term C storage potential of an ecosystem.

The objective of this study was to describe the time-lagged relationship between gross primary production (GPP) and the mycorrhizal soil respiration component, so to determine the amount of carbon derived from GPP appearing as mycorrhizal mycelial respiration. Measurements of CO₂ efflux were conducted in three different treatments – i) undisturbed, root and AMF-included (R_s), ii) root-excluded (R_{basal + myc}) and iii) root- and AMF-excluded (R_basal) plots – for three consecutive years in a Central-Hungarian dry sandy grassland between July 2011 and May 2014. GPP data were derived from eddy-covariance (EC) measurements, while an automated soil respiration system (SRS) consisting of ten chambers was used for continuous and long-term measurement of soil CO₂ efflux. We analysed the relationship between mycorrhizal mycelial respiration and GPP by using cross-correlation and GAMs (generalized additive models). Besides, we used sine wave models to describe the diel pattern of basal and mycorrhizal fungi respiration in the soil together with the diel patterns of soil temperature and GPP.

Considering the whole dataset correlation between GPP and mycorrhizal fungi respiration was highest at 13.5 hours time lag, while the average difference between peak timing of mycorrhizal fungi respiration and peak timing of GPP was 15 hours. However, the time lag and the peak timing difference varied from 10-24 hours. According to the results, carbon allocation to mycorrhizal fungi is a fast process in dry grasslands and the main driver of this respiration component is the GPP.