Influence of dry-wet cycles on the interrelationship between aggregate, particulate organic matter, and microbial community dynamics

Aggregate dynamics and their relationship to the microbial community have b een suggested as key factors controlling SOM dynamics. Dry-wet (DW) cycles are thought to enhance aggregate turnover and decomposition of soil organic matter (SOM), particularly in tilled soils. The objective of this study wa s to evaluate the effects of DW cycles on aggregate stability, SOM dynamics , and fungal and bacterial populations in a Weld silt loam soil (Aridic Pal eustoll). Samples, taken from 250 mum sieved air-dried soil (i.e. free of m acro aggregates > 250 mum), were incubated with C-13-labeled wheat residue. In one set of soil samples, fungal growth was suppressed using a fungicide (Captan) in order to discern the effect of dry-wet cycles on fungal and ba cterial populations. Aggregate formation was followed during the first 14 d of incubation, After this period, one set of soil samples was subjected to four DW cycles, whereas another set, as a control, was kept at field capac ity (FC). Over 74 d, total and wheat-derived respiration, size distribution of water stable aggregates and fungal and bacterial biomass were measured. We determined native and labeled C dynamics of three particulate organic m atter (POM) fractions related to soil structure: the free light fraction (L F), and the coarse (250-2000 mum) and fine (53-250 mum) intra-aggregate POM fraction (iPOM). In the fungicide treated soil samples, fungal growth was significantly reduced and no large macroaggregates (>2 mm) were formed, whe reas without addition of fungicide, fungi represented the largest part of t he microbial biomass (66%) and 30% of the soil dry weight was composed of l arge macroaggregates. During macroaggregate formation, labeled free LF-C si gnificantly decreased whereas labeled coarse iPOM-C increased, indicating t hat macroaggregates are formed around fresh wheat residue (free LF), which is consequently incorporated and becomes coarse iPOM. The first drying and wetting event reduced the amount of large macroaggregates from 30 to 21% of the total soil weight. However, macroaggregates became slake-resistant aft er two dry-wet cycles. Fine iPOM-C was significantly lower in soil after tw o dry-wet cycles compared to soil kept at FC. We conclude that more coarse iPOM is decomposed into fine iPOM in macroaggregates not exposed to DW cycl es due to a slower macroaggregate turnover. In addition, when macroaggregat es, subjected to dry-wet cycles, became slake-resistant (d 44) and conseque ntly macroaggregate turnover decreased, fine iPOM accumulated. In conclusio n, differences in fine iPOM accumulation in DW vs. control macroaggregates are attributed to differences in macroaggregate turnover. (C) 2001 Elsevier Science Ltd. All rights reserved.