Water retention and field soil water states in a vertisol under Long-Term direct drill and conventional tillage

Understanding differences in the agro-hydrologic performance of Vertisols under conventional tillage (CT) and direct drill (DD) requires a thorough knowledge of the soil hydraulic properties. We measured water retention on 54 undisturbed topsoil (0-0.05 m) samples collected at the CT and DD plots from a long-term experiment. Water retention was significantly larger in DD (p < .05) for absolute pressure heads (|h|) ranging from 63 to 3.2 x 10(3) cm, and at 1.8 x 10(4) and 3.3 x 10(4) cm. A comparison of the equivalent pore-size distributions showed combined effects of tillage in the CT topsoil and compaction as a result of machinery traffic and natural consolidation in the DD topsoil, increasing and decreasing the amount of the largest pores in CT and DD, respectively, in favour of a larger abundance of smaller equivalent pore-sizes in DD than in CT. Significant differences in water retention and abundance of equivalent pore sizes near the dry end of the soil water retention curve (|h| approximate to 4 x 10(4) cm) appear to be associated with the larger organic matter content in DD. These results were corroborated by field-measured soil water content data (0-0.10 and 0.25-0.35 m), showing a persistently larger soil water content in DD than the spatial average of both tillage systems. Differences in the observed trimodal soil water content probability density functions between CT and DD were related to equivalent pore-sizes for which significantly different water retentions were measured. This work elucidates direct and indirect effects of soil management on water retention and the equivalent pore-size distribution, with important consequences for the soil ' s agro-hydrologic performance.

Automated summary

Understanding the differences in agro-hydrologic performance of Vertisols under conventional tillage and direct drill requires a thorough knowledge of soil hydraulic properties. Water retention in direct drill was significantly larger for absolute pressure heads ranging from 63 to 3.2 x 10(3) cm, attributed to higher organic matter content. The study also found differences in equivalent pore-size distributions, with direct drill showing a larger abundance of smaller equivalent pore-sizes compared to conventional tillage, impacting soil water content and agro-hydrologic performance.