Comparative ecology of arbuscular mycorrhizal fungi in lowland tropical forest and pasture.

Abstract

Throughout the Neotropics forests are being converted to agricultural systems, especially pastures. Little is known about the impacts of such habitat conversion on the soil microbes that are associated with the diverse, native forests. In this study I assess the impacts on one group of soil microbes, arbuscular mycorrhizal fungi (AMF). These critical, ubiquitous fungi form mutualistic associations with most plant species, and they are especially important in nutrient-poor, tropical soils. A better understanding of their ecology could improve restoration and reforestation in old tropical pastures. I compared the following attributes between forests and pastures in eastern Nicaragua: (1) AMF species diversity, abundance, and community composition; (2) effect of successional status of hosts and fungi regarding their ability to associate and sporulate; (3) soil infectivity and effects of soil disturbance; and (4) soil aggregate stability. First, according to field-collected spores as well as trap cultures, AMF diversity is surprisingly high in tropical pastures. Most species are equally or more abundant in pasture than in their native forest. Species richness (via accumulation curves) and evenness are at least as high in pasture as in forest. Although relative abundances in pasture shift towards some species in the Acaulosporaceae and Glomaceae, community composition is similar to that in forest. Second, the change from forest trees to pasture plants may account for some of the shift in AMF species composition. Some AMF species that are especially abundant in pasture preferentially sporulate with pasture plants, in contrast to other AMF and in contrast to other more shade-tolerant hosts. Third, soil infectivity is higher in forest than in pasture. AMF apparently rely more on hyphal networks for infection in forest soil than in pasture soil, because soil disruption reduces infectivity significantly more in forest than in pasture. Fourth, however, in neither habitat did hyphal networks play a measurable role in soil aggregate stability. Rather, iron oxides seem to be the primary agent binding macroaggregates in the Ultisols studied here. In conclusion, despite the subtle changes in AMF ecology following conversion of tropical forest to pasture, AMF are not likely to be an important factor limiting tree growth in many lowland, wet tropical pastures.