Soil fungal and bacterial biomass determined by epifluorescence microscopy and mycorrhizal spore density in different sugarcane managements

Abstract

Crop productivity and sustainability have often been related to soil organic matter and soil microbial biomass, especially because of their role in soil nutrient cycling. This study aimed at measuring fungal and bacterial biomass by epifluorescence microscopy and arbuscular mycorrhizal fungal (AMF) spore density in sugarcane (Saccharum officinarum L.) fields under different managements. We collected soil samples of sugarcane fields managed with or without burning, with or without mechanized harvest, with or without application of vinasse and from nearby riparian native forest. The soil samples were collected at 10cm depth and storage at 4°C until analysis. Fungal biomass varied from 25 to 37µg C g-1 dry soil and bacterial from 178 to 263µg C g-1 dry soil. The average fungal/bacterial ratio of fields was 0.14. The AMF spore density varied from 9 to 13 spores g-1 dry soil. The different sugarcane managements did not affect AMF spore density. In general, there were no significant changes of microbial biomass with crop management and riparian forest. However, the sum of fungal and bacterial biomass measured by epifluorescence microscopy (i.e. 208-301µg C g-1 dry soil) was very close to values of total soil microbial biomass observed in other studies with traditional techniques (e.g. fumigation-extraction). Therefore, determination of fungal/bacterial ratios by epifluorescence microscopy, associated with other parameters, appears to be a promising methodology to understand microbial functionality and nutrient cycling under different soil and crop managements.