Influence of ryegrass on physico-chemical properties of a fragipan soil

Fragipan horizons are naturally occurring impermeable layers found in certain soil types which constrain crop yields. While abundant literature exists regarding the processes of fragipan genesis, there is little recognition given to potential alterations in the pan with surface management, particularly important in light of the fact that these horizons can occur near the surface. We collected soil cores from a Zanesville silt loam (Typic Fragiudalf) and seeded them to annual ryegrass in the greenhouse and determined relevant soil properties after one growing cycle. The presence of ryegrass lowered bulk densities in the Btx1 and Btx2 horizons by 8% and 6% when compared with the counterpart horizons in the control, suggesting an increase in porosity. These findings were paralleled by a 62% and 52% reduction in tensile strength of ryegrass-grown aggregates in these same horizons when compared with control aggregates. These trends are likely due in part to solubilization of cementing agents by ryegrass root exudates. Glycolate, azelaic acid, and diphenolics appeared as components of the water soluble organic fraction (WSOC) pool and the diphenolics were in greater concentrations in ryegrass soil cores. Diphenolic concentrations were associated with elevated water-soluble aluminum and iron levels, suggesting that ryegrass might be solubilizing these cementing agents and affecting bulk density. Biological drilling by ryegrass roots might also be involved to increase porosity in fragipan horizons. Our findings raise questions about the ability of ryegrass to ameliorate impermeable subsurface fragipan horizons and how the improvement in porosity might impact subsequent row crop production.