Resilient populations of root fungi occur within five tomato production systems in southeast Florida

Abstract

Farming practices are known to impact arbuscular mycorrhizal (AM) fungi and other soil microbial communities in agroecosystems. The effects of divergent land management strategies on the incidence and infectivity of AM and other fungal root endophytes were evaluated in a 5-year tomato ( Lycopersicon esculentum Mill.) cropping systems study. Two of the five treatments utilized farming practices considered detrimental to AM fungal populations, including the tillage-mediated elimination of vegetation and soil fumigation. The remaining three treatments used practices thought to be more conducive to the presence of AM fungi, including organic production methods, bahiagrass pasture and undisturbed weed fallow. In years four and five of the study tomato roots and rhizosphere soil were collected. Roots were examined for colonization by AM and other fungal root endophytes, and rhizosphere soil was assayed to measure the amount of infective inoculum present based on maize ( Zea mays L.) seedling infection. Tomato roots and rhizosphere soil were also analyzed for the AM fungal fatty acid biomarker 16:1ω5c. Sudangrass ( Sorghum sudanense (Piper) Stapf) trap cultures were initiated using field soil to assess the diversity of AM fungal spore morphotypes. Soil disturbance and phosphorus (P) levels had the greatest influences on AM fungal infectivity and abundance. All plots had high levels of available soil P, resulting in low levels of colonization across treatments. Bahiagrass ( Paspalum notatum Flugge) pasture was the only treatment without repeated, intensive soil mixing, and had the highest level of field root colonization by AM fungi. Field roots were more heavily colonized by other fungal endophytes than by AM fungi in all treatments. Tomato roots from organic plots were apparently unique in encouraging colonization by fungi that appeared to be Microdochium bolleyi (R. Sprague) de Hoog & Herm.-Nijh. Infection by AM and other fungal root endophytes were positively correlated in all studies. Flooding and a shortened growing season likely contributed to reduced infection potentials in all treatments except for organic plots in year five compared to year four. Areas of high disturbance from frequent tillage had the lowest levels of primary inoculum, but recovery to levels comparable to less disturbed treatments occurred after a single season of host root growth. Diversity of AM fungal morphotypes was typical of agricultural fields, with at least 10 spore morphotypes present across treatments; Glomus spp. were the dominant spore type recovered in all treatments.