Relative degradability of exogenous organic carbon in function of its concentration and the interaction with native soil organic matter content

The soil environment impacts carbon (C) turnover in a multitude of ways. It has been shown that C emissions are higher when exogenous carbon is locally concentrated than when it is spread in the soil matrix. Hence, the spatial separation between decomposers and their substrate may cause ineffective dispersion of the produced enzymes within the soil matrix. It is not clear, however, if and how lower doses of exogenous C also impact its degradability and if an interaction exists with native soil organic matter (SOM) level. Therefore, a seventy-day incubation experiment with six varying exogenous OM doses (0-6 g C kg-1) in soil mesocosms of loamy sand subsoil (WFPS 50%) with three different native SOM contents (i.e. 0.1, 0.5, and 1.0 %) was carried out. CO2 emissions and isotopic signature measurements of 13CO2 by means of a CRDS-analyser were used to track the degradation of added maize shoots with a δ13C=-12.7 ‰ that contrasts to native SOM (δ13C=-25 ‰). Mineral N was added, and the redox potential (Eh) monitored. Eh suggested that O2 may have not become limiting for C mineralization, even at higher doses, but it did not clearly relate to the added C dose. Cumulative CO2 emissions increased proportionally with C dose and so our hypothesis that a lower dose dispersion limits microbial activity was disproved. However, a higher native SOM level led to higher relative degradability of added C for the highest doses (i.e. 3 and 6 g kg-1 soil). Furthermore, a negative impact of OM doses above 1 g kg-1 soil on native SOM degradation was found. Growth of the microbial biomass was only stimulated proportionally above 1 g C kg-1, and this extra biomass was unexpectedly not involved in degradation of native SOM but instead exclusively in the decomposition of the added maize residues. Hence, we can conclude that positive priming of native SOM mineralisation did not occur, but rather negative priming effect was found for higher doses (> 1 g C kg-1). The resulting disproportional response of microbial biomass carbon (MBC) and soil C mineralisation at lower doses suggests that dose does impact the apparent microbial metabolic quotient (qCO2). Ongoing PLFA analyses will further reveal the soil microbial community structure along the different treatments and help us interpret these observations