The effect of slash/mulch and alleycropping bean production systems on soil microbiota in the tropics

Mulch applied to tropical soil may increase soil health both through stimulation of microbiota beneficial to plant nutrient uptake and the suppression of plant disease. To test this hypothesis, we compared beans in three cropping systems: (1) mulched with secondary vegetation (slash/mulch), (2) mulched with foliage from alleycropped nitrogen-fixing trees, Calliandra calothyrsus, Gliricidia sepium and Inga edulis (alleycrop/mulch), and (3) unmulched, for their effect on positive microbial interactions (nodulation with Rhizobium and arbuscular mycorrhiza (AM)), and pathogenic associations (foliar and root diseases) in long-term field experiments in Costa Rica. In some dry years slash/mulched beans nodulated significantly more than the unmulched, though in wet years the nodule biomass was not significantly different between treatments. Beans grown in alleycropping systems mulched with foliage from Calliandra and Inga had lower nodule biomass than beans in slash/mulched and Gliricidia mulched plots at 3 and 5 weeks, probably due to high nitrogen levels from the decomposing vegetation of the tree mulch. Roots of bean plants were 95–98% colonized by AM fungi and there were no differences between the slash/mulched beans and the unmulched treatments. Slash/mulch had no effect on the foliar diseases angular leaf spot and web blight, due to their naturally low incidence in that year; slash/mulched decreased the severity of anthracnose (caused by Colletotrichum lindemuthianum). Slash/mulch decreased severity of a Fusarium-type root rot, but increased a Rhizoctonia-type. In the laboratory, microbial activity as determined by measurements of CO2 respiration was greater in the slash/mulched than the unmulched system due to the high respiration of the mulch material. Our hypothesis that the use of mulch would favor mutualistic symbionts was not supported, which may be due to weather and nutrient interference. However, the hypothesis that mulch would reduce disease incidence was supported for certain diseases. A comparison of the microbial community with those of nearby natural systems can help provide a standard for the assessment of microbial health in agroecosystems and allow us to avoid the circularity of defining microbial communities as ‘healthy’ because they are found in cropping systems whose plants appear without disease. © 2000 Published by Elsevier Science B.V.