drivers of soil respiration of root and microbial ... - Unitus DSpace

6.4. Discussion
6.4.1. Mesh-exclusion
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Mesh-exclusion technique is a type of widely used root exclusion methods. Such type of
partitioning methods potentially changes soil nutrient and water status and in combination with a
lack of competition with roots and mycorrhiza could alter microbial respiration of SOM and litter in
the root free soil. Because of the absence of root exudates in the soil core it is not possible also to
account for any priming effect, which is usually appears to be under the natural soil conditions
(Kuzyakov, 2002; Kuzyakov and Bol, 2005; Kuzyakov, 2006). The modification of the method and
introduction of the nylon mesh bags with various apertures seems promising, allowing the flow of
water and dissolved substances through the mesh, thus minimizing the disturbance of natural soil
environment.
However, there are some particular shortcoming and limitation associated with the utilized
methodology, which result in a possible overestimation of microbial-derived respiration and
underestimation of root-derived one.
Namely, the shortcomings of the method are: disturbance of the soil structure by sieving
and lateral diffusion of CO2 (Jassal and Black, 2006; Moyano et al., 2008) to the mesh bags from
the surrounding soil and inability to separate root respiration from rhizomicrobial one.
We tried to minimise the effect of the soil disturbance on microbial activity and respiration
fluxes by installing an additional mesh of 1cm, and obtaining the value of root-derived respiration
as a difference between two meshes with the equally disturbed soil. However, here we make an
assumption that there is no difference in the in-growth patterns of roots between non disturbed soil
and soil which was previously sieved. Microbial-derived respiration was calculated further from the
difference between control (non disturbed soil) plots and root respiration.
Presence of the CO2 from the surrounding soil in the mesh bags by its lateral diffusion
through the soil pores was confirmed by the pulse labeling of the adjacent soil in 13 CO2 atmosphere
and its tracing in the 13 CO2 respired from the mesh bags. The magnitude of the influence of the
lateral CO2 on the respiration rate is however difficult to estimate. Moyano et al. (2008) reported a
value of ca. 10%, obtained by inserting a PVC tubes over the mesh soil cores. As the procedure is
associated with additional disturbance of the measurement plots we preferred to avoid it. Presence
of laterally diffused CO2 results in a systematic overestimation of the microbial-derived
respiration. Here we make a second assumption: the influence of the lateral flow of CO2 is constant
during the growing season, and doesn’ t modify the seasonal trend in contribution of soil
respiration components to the total one. We also argue that an introduction of the additional 1 cm
mesh core with the same influence of the incoming CO2 from the surrounding soil could potentially
minimize the error in estimation of root-derived respiration.