Temporal variations of the distribution of water-stable aggregates, microbial biomass and ergosterol in temperate grassland soils with different cultivation histories

In grassland soils, the temporal dynamics in different fractions of soil organic carbon (SOC) is not completely understood. We investigated the temporal variations of water-stable aggregates, microbial biomass C (C-mic) and ergosterol concentrations (as a proxy for fungal biomass) in loamy sandy soils under grassland (north of Kiel, Germany). Samples were taken six times between October 2010 and October 2011 from i) a permanent grassland, ii) an arable plot converted to grassland and iii) a grassland with a one-time tillage operation (both in September 2010). Water-stable aggregate concentrations were determined by a wet-sieving procedure, C-mic concentrations by chloroform-fumigation-extraction and ergosterol concentrations by ethanol extraction. Temporal variation in aggregate distribution of soil samples from the permanent grassland was considerable: the large macroaggregate (>2000 mu m) concentrations (mean +/- standard error, n = 3) in the surface soil (0-10 cm) were highest in October 2011 (666 +/- 12 g kg(-1)) and lowest in May (206 +/- 49 g kg(-1)). The tillage operation in grassland led to lower concentrations of large macroaggregates, ergosterol and C-mic in the surface soil than in the permanent grassland. The conversion of arable land into grassland did not affect macroaggregate concentrations, despite an increase in C-mic (1.4-fold) and ergosterol concentration (3.3-fold) in the surface soil. Multiple linear regression analysis described the variations in the large macroaggregate concentration satisfactorily (R-2 = 0.60) and indicated that the gravimetric moisture content and the C-mic concentration had positive effects and the pH a negative effect. While ergosterol concentrations (mean +/- standard error) showed some temporal variations (values for the surface soil in the grassland ranged from 0.5 +/- 0.1 to 1.5 +/- 0.1 mg kg(-1)), C-mic concentrations varied much less, indicating a higher sensitivity of ergosterol to soil cultivation or changing environmental conditions. (C) 2014 Elsevier B.V. All rights reserved.