Agriculture and Agri-Food Canada Saint-Jean-sur-Richelieu, Quebec, Canada
Organic soils are ideal for growing vegetable crops, but are threatened by erosion and soil subsidence. Adding ligneous plant litter is expected to mitigate these losses. However, their relatively high amounts of carbon (C) and low nitrogen (N) could affect microbial populations and C and N cycling.
Our objective was to assess the change in nematode and microbial communities in a cultivated organic soil after applying ligneous plant litter to the soil. We hypothesized that nematode Fungivore:Bacterivore positively correlates with lignin:N of the added litter because of increased fungal prey density. We also hypothesized that incorporating litter into the soil enriches nematodes because litter nutrients are more available for microbial growth. We tested the hypotheses in a greenhouse experiment with five litter treatments × two placements and used lettuce (Lactuca sativa cv. Estival) as a model crop. We tested four litters: tamarack ((Larix lyallii, lignin:N = 338.9), white ash (Fraxinus americana, lignin:N =131.7), willow (Salix sp., lignin:N = 64.7), miscanthus grass (Miscanthus sinensis, lignin:N=126.1) and a 50:50 mix of willow and ash (lignin:N =104.08). We deduced the structure of bacteria, fungi and nematode communities from a metabarcoding analysis that targeted the 16S/ITS/18S rRNA genes.
Nematode communities were distinct when litter was on top of the soil versus incorporated (p < 0.001). The ratio of the relative abundance of Cephalobidae (cp-2) / Rhabditidae (cp-1) was 5 times higher when plant litter was incorporated compared to when it was place on top (5.24 vs. 0.99). Neither litter type or placement influenced nematode alpha diversity except for incorporated Larch that decreased nematode diversity. Litter placement explained 20% of the variation of nematode beta-diversity (PERMANOVA, p < 0.001) and litter type explained 19% of nematode community composition (p < 0.001). Amongst the fungivores: Aphelenchoididae were favored over Aphelenchidae. The fungivore/bacterivore nematode ratio increased with willow plant litter. Fungi communities significantly differed between plant litter types and placement (p< 0.001). Soil ammonium and nitrate did not differ significantly between treatments and neither did lettuce biomass.
While these results suggest that the shifts in the micro-food web did not influence N flux, the trends in soil nitrate are complementary to those found in the nematode communities. Nematode enumeration, lettuce C/N as well as soil dissolved organic carbon are being analyzed to further understand the patterns of C and N flux in relation to the micro-food web.