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

2.4. Discussion
2.4.1. Partitioning of soil respiration
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In our study the relative contribution of Ra to total CO2 efflux over the 3 years amounted to
29%. During the growing seasons the relative contribution of Ra varied from 2 to 70%, however an
increase in the relative input of root respiration to the total one was associated mainly with a
considerable decrease in microbial respiration under the water stress conditions (Fig. 3). The
observed average value of root contribution to soil respiration is less than 38-40% and 43-56%
reported respectively by Craine et al. (1999) and Wang et al. (2007) for temperate grasslands, and
39-41% reported by Tang et al. (2005) for oak-savannah. The lower value could be attributed to the
fact that Amplero is subjected to the annual mowing and grazing, which influence the
photosynthetic C supply to roots and thus root respiration itself (see below). In fact, our estimate is
consistent with the results obtained in semi-arid grazed grassland of China (15%-37% by Li et al.,
2002). The methods used for partitioning also vary between various studies, each with its own
shortcomings and limitations, which could also influence the final result, making the comparison
between studies especially difficult.
As all soil respiration partitioning methods, the calculated here fluxes are an approximation
of real field fluxes. The mesh-exclusion technique we used to partition soil respiration is associated
with various limitations and assumptions, 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, lateral diffusion of CO2 (Jassal and Black,
2006) to the mesh bags from the surrounding soil and inability to separate root respiration from
rhizomicrobial one. The advantage of the nylon mesh bag technique over other trenching and root-
exclusion methods is that it permits to exclude and control the roots in-growth inside the plot and in
the same time allows the exchange of water and other substances with an exterior soil.
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. The rate of the influence
of the lateral diffusion of CO2 from the surrounding soil is difficult to estimate. Moyano et al.
(2008) reported a value of ca. 10%. It 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.