Defoliation‐induced changes in carbon allocation and root soluble carbon concentration in field‐grown Lolium perenne plants: do they affect carbon availability, microbes and animal trophic groups in soil?

Abstract

Summary It is hypothesized that defoliation‐induced changes in plant carbon allocation and root soluble C concentration modify rhizosphere C availability and, further, the abundance and activity of soil microbes and their grazers. To test this hypothesis, field‐grown Lolium perenne swards were defoliated twice during their second growing season at two nitrogen availabilities (added N or no added N). Plant, soil and microbial attributes were measured 2 and 4 days after the last defoliation, and nematode abundance was measured 6 days after the last defoliation. Defoliation decreased shoot production in plots where N was added, but had no significant effect in plots where N was not added. Root biomass and the ratio of root mass to shoot production were not affected. Defoliation increased root soluble C concentration by 26% at the first harvest (2 days after defoliation) and by 18% at the second harvest (4 days after defoliation). Leaf N concentration was 27% lower in defoliated than in non‐defoliated swards at the first harvest, while that of stems was 14% higher in defoliated swards at both harvests, and that of roots was not affected. Defoliation increased root C : N ratio, decreased stem C : N ratio, and did not have a statistically significant effect on leaf C : N ratio. Soil attributes (soil soluble C concentration and soil C and N concentrations) were not affected by defoliation. Similarly, microbial attributes such as microbial C and N content, bacterial abundance in rhizosphere soil, and diversity of C sources utilized by the rhizosphere microbial community, did not differ between defoliated and non‐defoliated swards. Among nematode trophic groups, defoliation reduced the abundance of fungivorous and herbivorous nematodes by 70 and 47%, respectively, but did not affect the abundance of bacterivorous, omnivorous and predatory nematodes. Although defoliation altered plant C allocation and root soluble C concentration, these changes did not influence C availability, soil microbial growth or the abundance of bacteria‐feeding nematodes in the plant rhizosphere. Instead, the effects on root‐ and fungus‐feeding nematodes suggest that the effects of defoliation on soil communities were propagated not through the effects of root‐released C on bacteria and bacterial grazers, but through effects of root quality on root‐feeders and possibly through effects of mycorrhizal fungi on fungus‐feeders.