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

Abstract

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.

Fig 1. Schematic representation of the procedure followed to prepare the composite soil sample for gamma irradiation and incubation.

Large sized PVC tubes were used for gamma irradiation and small sized cores for incubation.

Fig 2. Nematode abundances (individual nematodes per gram of dry soil) in amended and unamended soil at the beginning (t0) and end (tf) of the incubation period.

The error bars indicate standard error of the mean (n = 3) and different letters indicate significant statistical differences.

Fig 3. Cumulative C mineralization (μg CO₂-C g⁻¹ soil) as a function of the incubation time (a) and cumulative C mineralized from the grass-clover amendment (μg CO₂-C g⁻¹soil) over time calculated as a simple difference between each treatment of the grass-clover amended and unamended samples at each sampling date.

Vertical bars indicate standard error of the mean (n = 3).

Fig 4. PLFA mean scores (n = 3) with standard error of the mean for the CTR,-Nem and +Nem in grass-clover amended (Am) and unamended (U) soil at the end of the incubation as analyzed by the principal component analysis.