Formation of aggregates by plant roots in homogenised soils

Abstract

The influence of root growth and water regime on the formation of aggregates was studied in modified minirhizotrons under controlled conditions. Two soils, a black earth (67% clay) and a red-brown earth (19% clay) were ground and forced through a 0.5 mm sieve. Ryegrass, pea and wheat were grown for fifteen wetting and drying (wd) cycles for 5 months. Another set of minirhizotrons was not planted and served as a control. Measurements of aggregate size distribution (ASD), aggregate tensile strength (ATS), aggregate stability (AS), aggregate bulk density (ABD) and organic carbon (OC) were made on single aggregates of the 2-4 mm fraction.The results showed that aggregates of the black earth which has a high clay content and shrink/swell properties had more smaller aggregates with higher ATS, AS and ABD than those from the red-brown earth. It was also found that for both soils: (1) w/d cycles and higher root length density (RLD) increased the proportions of smaller aggregates and aggregate strength; (2) differences in the ability of the plant species to influence aggregation was evident and seemed to be related to the RLD. The RLD was in the order ryegrass > wheat > pea. Mechanisms likely to be involved in processes of aggregate formation and stabilization are discussed. They include cracking of soil due to tensile stresses generated during drying of a shrinking soil; changes in pore water pressure within the soil mass caused by water uptake by plant roots generating effective stresses; and biological processes associated with plant roots and root exudates.

Quantificação da resistência tênsil e da friabilidade de um latossolo vermelho distroférrico sob plantio direto

“…Neste sentido, os resultados de Bartoli et al (1992), para Latossolos (Oxissolos) provenientes do Brasil e da Índia, indicaram maiores valores de RT nos solos que apresentaram maior percentagem de argila em associação com o maior teor de C orgânico, atribuindo este resultado ao efeito agregador da matéria orgânica na estrutura do solo. Outros autores também constataram que solos com elevados teores de argila tendem a apresentar maiores valores de RT (Kemper et al, 1987;Guérif, 1990;Materechera et al, 1992;Imhoff et al, 2002).…”

“…Os sistemas de manejo do solo influenciam diferentes fatores associados à resistência tênsil dos agregados e a friabilidade do solo, tais como ciclos de umedecimento e secamento (Utomo & Dexter, 1981;Kay & Dexter, 1992), conteúdo de água (Chan, 1989;Watts et al, 1996), dispersão e floculação das argilas (Shanmuganathan & Oades, 1982;Kay & Dexter, 1992;Barzegar et al, 1994), concentração e composição da solução do solo (Rahimi et al, 2000), formação e estabilidade dos agregados (Materechera et al, 1992) e teor de matéria orgânica do solo (Perfect et al, 1995;Imhoff et al, 2002). Por essa razão, a resistência tênsil dos agregados constitui uma medida sensível aos efeitos de sistemas de uso e manejo na estrutura do solo (Chan, 1995;Munkholm & Schjønning, 2004;Blanco-Canqui et al, 2005).…”

Plant exudates improve the mechanical conditions for root penetration through compacted soils

“…Field based evidence of the capacity of plant roots to enhance mechanical resilience of soil was provided by Gregory et al (2007), who found penetration resistance of a compacted soil to decrease far more in the presence of roots than in fallow soil in a sandy loam soil. The capacity of plant roots to restructure compacted soils is well reported (Uteau et al 2013;Bodner et al 2014), driven by a combination of roots fracturing soil, enhancing cycles of wetting and drying, producing biopores and secreting exudates (Gregory et al 2013;Materechera et al 1992).…”

Reclamation of a Degraded Coal-Mining Area with Perennial Cover Crops

“…Materechera et al (1992) also observed a lower proportion of large aggregates in agricultural clay soils cultivated with Lolium multiflorum compared to Triticum spp and Pisum sativum, attributing this result to the greater root length of Lolium multiflorum. The root system of grasses can lead to the change of more compact aggregates into less compact aggregates (Portella et al, 2012) by breaking the aggregates and then reshaping them (Terpstra, 1990) or by stresses generated in the soil-root interface during water extraction, which cause cracks in the soil (Ball et al, 2005).…”