Brassicaceous and rye cover crops altered free-living soil nematode community composition

Nematode community analysis was utilized to evaluate the biofumigant or allelopathic effects of brassicaceous and rye winter cover crops on non-target nematodes in three experiments (two sites) in Maryland. The cover crop treatments included mustard blend (Sinapis alba and Brassica juncea) 'Caliente', rapeseed (B. napus) 'Essex'/'Humus', forage radish (Raphanus sativus) 'Dichon', oilseed radish (R. sativus) 'Adagio'/'Colonel', rye (Secale cereale) 'Wheeler' and a no cover crop (winter weeds) control. Soil samples (0-15 cm) were collected two or three times per year and extracted nematodes were identified to genus or family. Nematode response parameters were genus, family, trophic group population density, and percent distribution of trophic groups in the entire nematode community divided into colonizer-persister ranks. The parameters refer only to free-living nematodes, however facultative hyphal-root hair feeding Tylenchidae were included. Cover crops had unique impacts on nematode communities, but these impacts appeared to be associated more with quality of organic matter inputs rather than biofumigation or allelopathy. Across all dates and seasons, and four to nine months after winter-kill, dormant bacterivore (dauer larvae) nematode populations in the forage radish (C/N shoots ∼10) plots ranged from 3.5 to 15.7 times higher (P < 0.10) than in the control plots. Plant-associate (Tylenchidae) nematodes were 4-6.5 times higher (P < 0.10) in rapeseed or rye (C/N shoots ∼25) plots compared to the control in June of two experiments. Across experiments fungivore nematode abundance was increased in either rapeseed 'Essex' or rye compared to radishes or the control. Correlations of nematode community groups with cover crop and soil parameters suggested that dauer larvae abundance was associated with soil moisture in radish plots, and tissue quality and quantity at the time of cover crop termination was associated with nematode community response. Canonical discriminant analysis suggested that rapeseed and rye had similar effects on the nematode community composition, as did the two radish cultivars, though distinct from the effect of rye and rapeseed. Overall, results suggest that radishes stimulated a bacterial decomposition pathway, while rapeseed and rye stimulated a proportionally greater fungal-based food web.