A physiological mechanism for the formation of root casts

“Root casts” are calcified structures in the shape of roots often found exposed on aeolian sand-dunes. A central channel of the cast may be void or filled with soft carbonate. Transpiration drives water flux towards the roots resulting in ‘mass-flow’ of dissolved nutrients towards the root. It was hypothesised that root casts form in cation-rich substrates as a consequence of transpiration-driven mass-flow of cations to roots in excess of that required by the plant, with subsequent cementation of sediments around the root mould through formation of carbonates. The elemental compositions of both ancient root casts (6870 ± 60 BP) of an unknown plant from Still bay (Western Cape, South Africa) and concreted sediments associated with cluster roots of a presently occurring dune plant, Morella cordifolia (L.) Killick (Myricaceae), were found to have similar elemental compositions and were positively correlated. Both root casts and cluster root sediments had higher [Ca] and other cations and also higher inorganic [C] than that of dune sand. From isotope analysis, the most likely sources of C for formation of carbonates in the cluster root sediment (δ13C = − 0.8 ± 0.29‰) and in the root casts (δ13C = − 5.3 ± 0.6 to − 7.6 ± 0.3‰) was a varying combination of the bulk atmosphere (predicted δ13C of concretion = 1.3‰) and plant respiration derived CO2 (predicted δ13C of concretion = − 11.5‰). Despite extremely low [P] (0.05 μmol g− 1 sand) in dune sand, the leaves of M. cordifolia accumulated amounts of P comparable to other cluster-rooted plants in the adjacent, somewhat nutrient-richer Fynbos. It was concluded that in the generally nutrient-poor but cation-rich sand, transpiration-driven water flux would deliver scarce nutrients to the cluster root rhizosphere of M. cordifolia. In the process of this mass-flow, Ca is delivered in excess of plant demand and accumulates in both the root tissue and the rhizosphere. This Ca is chemically converted to CaCO3 over time, resulting in the formation of persistent root casts.