Peter Maenhout

Quantifying the Contribution of Entire Free-Living Nematode Communities to Carbon Mineralization under Contrasting C and N Availability

To understand the roles of nematodes in organic matter (OM) decomposition, experimental setups should include the entire nematode community, the native soil microflora, and their food sources. Yet, published studies are often based on either simplified experimental setups, using only a few selected species of nematode and their respective prey, despite the multitude of species present in natural soil, or on indirect estimation of the mineralization process using O2 consumption and the fresh weight of nematodes. We set up a six-month incubation experiment to quantify the contribution of the entire free living nematode community to carbon (C) mineralization under realistic conditions. The following treatments were compared with and without grass-clover amendment: defaunated soil reinoculated with the entire free living nematode communities (+Nem) and defaunated soil that was not reinoculated (-Nem). We also included untreated fresh soil as a control (CTR). Nematode abundances and diversity in +Nem was comparable to the CTR showing the success of the reinoculation. No significant differences in C mineralization were found between +Nem and -Nem treatments of the amended and unamended samples at the end of incubation. Other related parameters such as microbial biomass C and enzymatic activities did not show significant differences between +Nem and -Nem treatments in both amended and unamended samples. These findings show that the collective contribution of the entire nematode community to C mineralization is small. Previous reports in literature based on simplified experimental setups and indirect estimations are contrasting with the findings of the current study and further investigations are needed to elucidate the extent and the mechanisms of nematode involvement in C mineralization.

Nitrogen Limitations on Microbial Degradation of Plant Substrates Are Controlled by Soil Structure and Moisture Content

Mineral nitrogen (N) availability to heterotrophic micro-organisms is known to impact organic matter (OM) decomposition. Different pathways determining the N accessibility depend to a large extent on soil structure. Contact between soil mineral and OM substrate particles can facilitate N transport toward decomposition hot spots. However, the impact of soil structure on N availability to microbes and thus heterotrophic microbial activity and community structure is not yet fully understood. We hypothesized that carbon mineralization (Cmin) from low-N substrate would be stimulated by increased N availability caused by closer contact with soil particles or by a higher moisture level, enhancing potential for N-diffusion. Under opposite conditions retarded heterotrophic activity and a dominance of fungi were expected. A 128-days incubation experiment with CO2 emission monitoring from artificially reconstructed miniature soil cores with contrasting soil structures, viz. high or low degree of contact between soil particles, was conducted to study impacts on heterotrophic activity. The soil cores were subjected to different water filled pore space percentages (25 or 50% WFPS) and amended with either easily degradable OM high in N (grass) or more resistant OM low in N (sawdust). X-ray μCT image processing allowed to quantify the pore space in 350 μm around OM substrates, i.e., the microbial habitat of involved decomposers. A lower local porosity surrounding sawdust particles in soils with stonger contact was confirmed, at least at 25% WFPS. Mineral N addition to sawdust amended soils with small particle contact at 25% WFPS resulted in a stimulated respiration. Cmin in the latter soils was lower than in case of high particle contact. This was not observed for grass substrate particles or at 50% WFPS. The interactive effect of substrate type and soil structure suggests that the latter controls Cmin through mediation of N diffusion and in turn N availability. Phospholipid fatty acid did not reveal promotion of fungal over bacterial biomarkers in treatments with N-limited substrate decomposition. Combining X-ray μCT with tailoring soil structure allows for more reliable investigation of effects on the soil microbial community, because as also found here, the established soil pore network structure can strongly deviate from the intended one.