Aggregate hierarchy in soils

Abstract

An Alfisol, a Mollisol and an Oxisol were fractionated into different particle sizes after a range of disaggregating treatments from gentle to vigorous. The Alfisol and the Mollisol appeared to break down in steps; macroaggregates >250 �m diameter breaking down to microaggregates 20-250 �m diameter before particles <20 �m were released. Vigorous disruption led to particle size distributions similar to those obtained by classical methods used to determine particle size distributions. The Oxisol was stable to rapid wetting treatments but when aggregate disruption was initiated by vigorous treatments particles <20 �m diameter were released and there was no evidence of aggregate hierarchy. Scanning electron microscopy of particles of different sizes showed distinctly single grain particles and aggregates. The microscopic studies indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates. Plant debris was not visible in aggregates <20 �m but clay microstructure was evident. It is suggested that aggregate hierarchy occurs in Alfisols and Mollisols because organic materials are the dominant stabilizing agents in larger aggregates but in the Oxisol oxides are dominant stabilizing agents and prevent the expression of aggregate hierarchy caused by organic materials.

Carbon cycling by arbuscular mycorrhizal fungi in soil–plant systems

“…In doing so, they help to create a mechanism for increasing the residence time of organic debris within soil macroaggregates. Mechanistically, the ERH contribution to soil aggregation can be viewed as a 'sticky-string-bag' mechanism, in which the hyphae help to entangle and enmesh soil particles to form macroaggregate structures [16,17]. The physical dimensions of the ERH allow them to grow and to ramify through soil pores the size of those between macroaggregates.…”

Storage and stability of organic matter and fossil carbon in a Luvisol and Phaeozem with continuous maize cropping: A synthesis

“…Several physical procedures have been proposed to isolate physically stabilized SOM fractions with different composition and stability and to characterize and quantify mechanisms of SOM stabilization in soils. These methods include aggregate-size fractionations (e.g., Oades and Waters, 1991;Jastrow et al, 1996;Puget et al, 2000), particle-size fractionations (e.g., Tiessen et al, 1981;Christensen, 2001;Jolivet et al, 2003), density fractionations (e.g., Balesdent et al, 1998;Baisden et al, 2002;Yamashita et al, 2006), and their combinations (e.g., Cambardella and Elliott, 1993;Rodionov et al, 2000;Six et al, 2002;John et al, 2005). To account for chemical stabilization processes, different extraction procedures (Balesdent, 1996;Ludwig et al, 2003), wet oxidation (Eusterhus et al, 2005;Mikutta et al, 2005), acid hydrolyses (Paul et al, 2001;Poirier et al, 2003;Plante et al, 2006), and various combinations of these procedures (Helfrich et al, 2007) have been used, frequently on physically isolated soil fractions (cf., e.g., von Lützow et al, 2007 for a recent review).…”

Mineralogia, morfologia e análise microscópica de solos do bioma cerrado

“…Os microagregados de solos tropicais intemperizados são muito resistentes e tiveram papel de destaque na formulação da hipótese da hierarquia da agregação dos solos (Oades & Waters, 1991). Mais recentemente, outros pesquisadores levantaram dados e novas hipóteses surgiram para explicar a maior resistência e durabilidade desses microagregados, quando comparados aos de solos menos intemperizados (Schulze & Stott, 1997;Vrdoljak, 1998;Marques, 2000;Schaefer, 2001).…”