Applied Soil Ecology 46 (2010) 390–397 Contents lists available at ScienceDirect Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil No-till soil management increases microbial biomass and alters community profiles in soil aggregates B.L. Helgason a,∗ , F.L. Walley b , J.J. Germida b a Agriculture and Agri-Food Canada, Saskatoon Research Centre, 107 Science Place, Saskatoon, SK, S7N 0X2, Canada b Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8, Canada article info Article history: Received 19 February 2010 Received in revised form 30 September 2010 Accepted 5 October 2010 Keywords: No-till Tillage Aggregates Microbial biomass Microbial community abstract Aggregation is important for soil functioning, providing physical protection of organic matter and micro- bial inhabitants. Tillage disrupts aggregates, increases wind and water erosion of soils and exposes formerly protected organic matter to decomposition and losses. Microbial biomass and community dynamics in dry-sieved aggregate-size classes from long-term no-till (NT) and conventionally tilled (CT) soils were examined using phospholipid fatty acid analysis (PLFA). Bacterial, fungal, and total biomass were up to 32% greater in NT compared to CT aggregates. Aggregate size also affected microbial biomass, which was highest in the 1–2 mm size class. Arbuscular mycorrhizal fungi (AMF) were particularly affected by tillage disturbance with increases of 40–60% among aggregate-size classes in NT vs. CT, but glomalin related soil protein concentration was not different between tillage treatments or among aggregate-size classes. Bacterial stress biomarkers were higher in CT than NT aggregates but were not significantly correlated with total C, total N or C:N ratio, indicating that the physiological status of bacteria within aggregates was not simply governed by the quantity of available resources. Ordination analysis of PLFA profiles demonstrated a shift in microbial community structure between NT and CT aggregates, cor- related with AMF abundance in NT aggregates and increased bacterial stress biomarkers in CT aggregates. Our results demonstrated greater microbial biomass and altered microbial community structure in NT vs. CT aggregates. This work demonstrates that tillage management influences microbial community struc- ture within aggregates and may provide a potential explanation for differences in process rates observed in NT vs. CT soils. Further research into the processes that govern community structure in aggregates from NT and tilled soils is needed to better understand how the interaction of microorganisms with their physical environment affects nutrient turnover and availability. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. 1. Introduction Aggregation is an important facet of soil structure, providing resistance to wind and water erosion, physical protection of organic matter and microsites for microbial activity. Agroecosystem pro- ductivity and sustainability are dependent on the maintenance of key microbial processes. Aggregates provide habitat for microbial activity and understanding how management affects the distribu- tion of microbial functional groups among aggregates will lead to a better understanding of the regulation of microbial processes including soil C storage, nutrient turnover and trace gas emissions. The primary mechanisms to influence soil microbial communi- ties are through the alteration of soil structure or substrate inputs (i.e., crop residues) (Elliott and Coleman, 1988). In this way, soil biological activity and the physical soil environment are related ∗ Corresponding author. Tel.: +1 306 975 6510; fax: +1 306 966 4226. E-mail address: helgasonb@agr.gc.ca (B.L. Helgason). through dynamic feedback mechanisms which inextricably link these two primary foundations of soil functioning (Young and Ritz, 2000). Tillage affects soil aggregation directly through physical disruption, and indirectly through influences on the broader bio- logical and chemical soil environment (Young and Ritz, 2000). Macroaggregate stability is especially susceptible to agronomic practices (Six et al., 2004). Macroaggregates are stabilized by plant roots and fungal hyphae, as well as by the byproducts of microbial metabolism (Tisdall and Oades, 1982) and they pro- vide habitat for soil bacteria while spatially protecting organic matter. Tillage affects both the level of aggregation and the rate of aggregate turnover (Six et al., 1998). Disruption during tillage events releases particulate organic matter from macroaggregates and increases organic matter turnover (Six et al., 2000). As a result, slower macroaggregate turnover under NT has been suggested as a potential mechanism for increased C storage in NT soils (Six et al., 1999). The formation of microaggregates within macroag- 0929-1393/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2010.10.002