Contrasting effects of cover crops on ‘hot spot’ arbuscular mycorrhizal fungal communities in organic tomato

Soil: Grow cover crops in arable fields

A replicated, controlled study in 2011–2012 in an irrigated tomato field near Pisa, Italy, found that similar numbers of tomato roots were colonized by mycorrhizae (beneficial fungi), but found more mycorrhizae spores, and more mycorrhizae species, in soils with planted cover crops, compared to resident (unplanted) vegetation. Soil organisms: Similar numbers of tomato roots were colonized by symbiotic fungi in plots with cover crops, compared to resident vegetation (28–42% vs 30–37% of roots were colonized). More mycorrhizae spores were found in soils with cover crops, in three of six comparisons (10.3–18.5 vs 7–8.5 spores/g soil). More mycorrhizae species were found in soils with cover crops, in two of six comparisons (when tomatoes were harvested, in plots that were cover cropped with Brassica juncea or a mixture of species: 29–30 vs 24 species). Implementation options: More tomato roots were colonized by mycorrhizae in plots that were cover cropped with Vicia villosa (42% of roots were colonized) or a mixture of species (35% of roots were colonized), compared to B. juncea (28%), in one of two comparisons (when tomatoes were flowering). More mycorrhizae spores were found in plots that were cover cropped with V. villosa (14.2–18.5 spores/g soil), compared to the other two cover crops (species mixture: 10.3–10.8; B. juncea: 7.8–9.1), and more spores were also found in plots with the species mixture, compared to B. juncea, in one of two comparisons (when tomatoes were flowering: 10.3 vs 7.8). More mycorrhizae species were found in plots that were cover cropped with B. juncea or the species mixture, compared to V. villosa (29–30 vs 25 species). Methods: There were three plots (plot size not reported) for each of three winter cover crops (B. juncea, V. villosa, or a mixture of seven species) and three control plots (without cover crops, but with resident vegetation). Cover crops were sown on 19 October 2011, and then mown and incorporated into the soil in spring 2012. Tomato seedlings were transplanted into the plots (into raised beds) on 30 May 2012. Tomatoes were drip irrigated. Soil samples were collected when the tomatoes were flowering (10 April 2012) and when they were harvested (20 September 2012) (four soil cores/plot, 0–20 cm depth). Half of the seedlings were inoculated with two species of mycorrhizae.