Effect of Root Exudates From Corn and Bromegrass on Soil Structural Stability

The effect of root exudates on structural stability and the hypothesis that the growing roots of corn (Zea mays L.) can destabilize soil structure by chelating Fe and Al involved in mineral-metal-organic matter linkages were investigated. Exudates were removed from 14 C-labelled corn and bromegrass (Bromus inermis Leyss.) plants grown in sand using sequential leaching with water and acetone. The exudates were concentrated by flash evaporation and incubated with 1–2 mm aggregates of a calcareous silt loam soil for up to 240 h. Aggregates to which exudates from bromegrass were added had a higher wet aggregate stability (WAS) and lower dispersible clay content (DC) than aggregates to which exudates from corn were added. The greatest increase in stability occurred on addition of the water-extracted exudates (9.6% for WAS, 27.1% for DC). The increase in stability correlated positively with a release into solution of Ca and Mg from the soil solids. Addition of CaCl 2 to the aggregates, to give corresponding amounts of Ca and Mg in solution, had similar effects on stability suggesting that the ions released by the exudates were not in a chelated form. The quantity of carbon added in the exudates and the mineralization of this carbon were positively and negatively correlated with stability, respectively. There was little evidence that corn exudates caused a destabilization of structure.