Long-term effect of tillage systems on soil microbiological, chemical and physical parameters and the incidence of charcoal rot by Macrophomina phaseolina (Tassi) Goid in soybean

Abstract

A 20-year field experiment was employed with the aim of evaluating the effect of tillage systems onbiological, chemical and physical aspects of the soil, and to establish whether there was a correlation ofthese parameters with the incidence of charcoal rot (Macrophomina phaseolina) of soybean and cropyield. The tillage systems evaluated were direct seeding (DS), DS þ scarifier (DS þ S), minimum tillage(MT) and conventional tillage (CT). DS presented higher values than CT in culturable total fungi(26.33 105 vs. 2.33 105 CFU g1 dry soil), total bacteria (182 107 vs. 64 107 CFU g1 dry soil),microbial respiration (0.77 mg CO2 g1 week1 vs. 0.45 mg CO2 g1 week1) and fluorescein diacetate(FDA) hydrolysis (4.17 ug fluorescein g1 h1 vs. 1.70 ug fluorescein g1 h1 in CT. Fungal and bacterialcommunity fingerprints, by terminal restriction fragment length polymorphism (T-RFLP) analysis, ofIntergenic spacer regions of rRNA and 16S rRNA genes, respectively, were influenced by the tillagesystem. Also FAME (fatty acid methyl ester) profiles showed that microbial community structure in DSand CT was clearly different. DS samples contained significantly higher total microbial biomass than theother tillage treatments, but there were no significant differences in fungal biomass or any consistenttrend with respect to stress index. Our results showed that microbial communities were more abundantand active in DS than in CT in response to high nutrient content in soil. Indeed, DS systems presentedhigher soil OM, total N, K and Ca than CT. Electrical conductivity and aggregate stability (AS) were alsoimproved by DS. Soybean grown in high-quality soil was not affected by charcoal rot, however, under CT,disease incidence in soybean was 54%. These differences were correlated to the higher microbialabundance and activity under DS, the biological component being a key factor determining soil capacityto suppress the soilborne pathogen.