Cover crop residue moisture content controls diurnal variations in surface residue decomposition.

• Cover crop residue water retention properties changes during decomposition. • Cover crop residue lignin concentrations can be used as a proxy for water retention properties. • Cover crop surface residues experience extreme diurnal fluctuations in moisture and temperature compared to underlying soils. • Diurnal variations in decomposition tracked variations in cover crop residue moisture content. • Cover crop decomposition in no-till systems is strongly moisture-limited. The effect of cover crop (CC) surface residues on water, carbon, and nitrogen cycling in no-till systems depends in part on the water retention properties of decomposing residues and the extent of decomposition. This study (1) examined the effect of decomposition on residue water retention properties; (2) characterized diurnal variations in residue decomposition rates in response to changes in soil-residue-air environmental conditions; and (3) examined the diurnal relationships between cover crop surface residue decomposition and residue environment (moisture and temperature). Maximum gravimetric water content (θ g) and characteristic water release curves were determined for red clover (Trifolium pratense L.) and cereal rye (Secale cereale L.) residues collected at 0, 4, 10, and 16 weeks after termination for red clover, and at 2, 5, and 18 weeks for cereal rye. In addition, residue carbon dioxide (CO 2 -C) flux, along with soil-residue-air environmental conditions, were measured diurnally for red clover at 4, 10, and 16 weeks after termination, and for cereal rye at 5 weeks after termination. Maximum residue θ g decreased as decomposition progressed. Cover crop residue decomposition also influenced water release curves such that the water retained at any given water potential (ψ r e s i d u e) declined with increasing decomposition. These decomposition-associated changes in residue water retention properties were strongly related to residue lignin concentrations. Cover crop surface residue CO 2 -C flux showed distinct diurnal patterns that were strongly related to ψ r e s i d u e or residue θ g. At a diurnal scale, residue CO 2 -C flux increased during the nighttime from 18:00 to 06:00 h when residues gain moisture from the atmosphere and soil, and decreased during the daytime from 06:00 to 18:00 h when residues lost moisture via evaporation. Increase in temperature decreased residue CO 2 -C flux due to moisture limitations. Therefore, CC surface residue decomposition models must address both diurnal changes in ψ r e s i d u e and the changes in water retention properties as residues decompose. [ABSTRACT FROM AUTHOR]