Soil biota responses to long-term atmospheric CO2 enrichment in two California annual grasslands

Root, arbuscular-mycorrhizal (AM), soil faunal (protozoa and microarthropod s), and microbial responses to field exposure to CO2 for six growing season s were measured in spring 1997 in two adjacent grassland communities. The g rasslands showed contrasting root responses to CO2, enrichment: whereas roo t length was not affected in the sandstone grassland, it was greater in the serpentine grassland, as was specific root length. AM fungal hyphal length s were greater in the sandstone, but were unaffected in the serpentine comm unity. This lent support to the hypothesis that there may be a tradeoff in resource allocation to more fine roots or greater mycorrhizal extraradical hyphal length. AM root infection was greater in both communities at elevate d CO2, as was the proportion of roots containing arbuscules. Our data on to tal hyphal lengths, culturable and active fungi, bacteria, and protozoa sup ported the hypothesis that the fungal food chain was more strongly stimulat ed than the bacterial chain. This study is one of the first to test these h ypotheses in natural multi-species communities in the field.