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Soil structure has a huge impact on plant root growth, but it is difficult to isolate from other soil properties in field studies, and generally overlooked in laboratory studies that use sieved and homogenised repacked soil. This study aimed to compare root and shoot growth under controlled soil conditions where only soil structure varied. Soil treatments used soil sieved to < 2 mm, packed in uniform layers to create a homogenous structure. A heterogeneous structure was packed from artificially formed aggregates created by breaking apart the homogeneous soil after intense compaction. Barley, peas and Arabidopsis, selected for contrasting root sizes, were grown under three levels of compaction (1.25 g cm−3, 1.40 g cm−3, 1.55 g cm−3) in both homogeneous and heterogeneous structured soils for 10 days. Penetration resistance increased from about 0.4 MPa at 1.25 g cm−3 to 1.3 MPa at 1.55 g cm−3 for either soil structure. Soil structure was quantified from water retention characteristics and X-ray Computed Tomography (CT) as complementary methods to assess the soil's pore size distribution and properties. Heterogenous soil had 50% more macropores at 1.55 g cm−3 when compared to homogenous soils. Pore structure complexity in the heterogeneous structure was found to be beneficial for root growth of peas and barley but not Arabidopsis. Shoot biomass of peas grown in heterogeneous soil at 1.55 g cm−3 increased by 65% when compared to homogenous soil, whereas barley and Arabidopsis shoot biomass did not differ significantly between any treatments. Chlorophyll, flavonoid, and nitrogen content could only be measured on barley or peas due to shoot size, but only minor differences were observed between soil structures. Soil structural heterogeneity influenced many root properties and above-ground biomass, with impacts found to be species-dependent and likely caused by the interaction between root size and preferential growth in macropores.
Giuliani et al. (Fri,) studied this question.