Relationships between mycelial and bacterial populations in stored, air-dried and glucose-amended arable and grassland soils

Abstract

Microbial morphology and volume (estimated by microscopy), and eukaryote and prokaryote substrate-induced respiration rates (estimated by a selective respiratory inhibition method) were measured on one arable (Greytown) and two grassland (Pomare and Waikanae) soils that had been stored (−50 kPa, 25°C), or amended with glucose (1% w/w) or air-dried. Mycelial and bacterial volumes, surface areas and rates of respiration were lowest in the arable soil. Bacteria had the greatest surface area whilst mycelia had the greatest volume, mass and rates of respiration in all soils. Mean bacterium volume (0.076 μm 3) and morphology (92% coccal) were similar between soils. Mean mycelial diameters and length-to-mass ratios were similar in Greytown and Waikanae soils (averaging 3.7 μm and 0.85 m μg −1 mycelial C), but different in Pomare soil (2.7 μm, 1.53m μg −1 C). In general, storage and air-drying decreased mycelial lengths, bacterial numbers, volumes and surface areas, but substrate amendment increased these measurements. The frequency of low diameter mycelia increased in dried soil whilst bacterium volume and the frequency of rod-shaped bacteria increased in substrate-amended and decreased in stored and air-dried soil. In Greytown and Waikanae soils, the total microbial volume ( T v ) was related to the ratio of mycelial: bacterial volume ( R v ), whilst in each soil total respiration ( T r ) was related to the ratio of eukaryote: prokaryote respiration ( R t ), by hyperbolic functions. The ratios R v , and R r of mycelia (eukaryotic and hence fungal) to bacteria (prokaryotic) at high values of T v and T r both tended to 1.2 (fungal dominant). Greytown and Waikanae soils contained a constant amount of senescent myeelia (0.81 mm 3 g −1 soil respiring at a substrate-induced rate of 1.7μ1 CO 2 h −1), whilst in all three soils all the bacteria appeared viable. Thus disappearance of dead bacteria was probably more rapid than that of dead mycelia.