Assistant Professor McGill University, Quebec, Canada
Abstract: Models of soil foodwebs gauge carbon (C) and nitrogen (N) flux, particularly when including microbivorous nematodes -a ubiquitous microbivore. Bacterivorous nematodes have a higher C:N (~8) than do bacteria (~4), therefore mineralize N at a higher rate than fungivorous nematodes that have a C:N similar to that of fungi. Respiration and production rates are known for many nematode taxa. Thus, by including nematodes in the foodweb, we can determine the rate of C and N mineralization.
Organic soils are ideal for growing vegetables, but rapid C mineralization as well as wind erosion threaten the future of these soils. Amending organic soils with plant litter could mitigate wind erosion, but could increase C mineralization as well as N immobilisation.
The objective of this study was to assess the impact of four plant litter types of C:N ranging from 65 to 340 on the micro-foodweb and the consequences to C and N flux as lettuce increases in biomass over time. We hypothesised that 1) the detritusphere will exhibit more C flux than the rhizosphere because microbial biomass and nematode population size will be larger in the detritusphere than the rhizosphere, 2) N flux will be highest in the rhizosphere where root exudates are a constant source of labile C for bacteria, resulting in more bacterivores than fungivores and thus faster N mineralization.
We conducted a field study with two lettuce crops over two years. We measured microbial biomass, soil NH4+ and NO3-, absolute nematode abundance and determined the structure of bacterial, fungal and nematode communities from a metabarcoding analysis that targeted the 16S/ITS/18S rRNA genes. These measurements were taken at six times points per season. We also sequenced N-cycling genes using RNA at the end of the second season.
Preliminary results show smaller lettuce in miscanthus treatments than all other treatments (ANOVA, P< 0.001). Contrary to our hypothesis, we found less soil NO3- in the rhizosphere than the detritusphere, but only with miscanthus, larch, and willow, which differed significantly from the control (P=0.002, 0.009, 0.012, respectively). Nematodes in the miscanthus treatment increased in abundance from mid-season to harvest, although not significant. These results raise caution when using miscanthus to mitigate wind erosion and C loss on organic soils. We will also present microbial biomass C and N, microbial and nematode community composition and abundance as well as N-cycling RNA genes.
