Effects of perennial and cover crops on hydrology in Iowa
Since European settlement, and beginning in the 1940's, two dramatic shifts in land use have occurred in Iowa - the first from prairie and forest to tile-drained farmland, and the second from diverse rotations to a heavier concentration of corn-soybean rotations and continuous corn. These shifts in land use and management have altered hydrological and biogeochemical cycles in the Upper Midwest, but perennial and cover crops have the potential to assist in mediating changes in these cycles.
The first study in this thesis examines how the perennial forage (PF) crop orchardgrass (Dactylis glomerata) affects subsurface drainage as compared to a corn-soybean rotation or continuous corn (row crops, or RC). Over the entire drainage season (March -November) over 22 years, PF did not reduce subsurface drainage, but during May, PF reduced subsurface drainage by 32% (p < 0.05). May is a critical period for drainage in Iowa, as wet field conditions and a lack of vegetative cover contribute to a majority of drainage and leaching of NO3-N from row crop fields during this period.
The second study investigates how cereal rye (Secale cereale L. ssp. cereal) cover crop influences soil water dynamics in two fields in Iowa. During the spring growth period of rye, at a site in central Iowa, rye plots to be planted to soybeans significantly increased the rise in magnitude of soil moisture following rainfall events in the top 0-20 cm of soil as compared to fallow plots. Different types of rainfall events caused differing responses in soil water redistribution.
In the third study, the effect of a rye cover crop on soil water content and soil water storage during the spring and early summer in a drought year is examined. In one field in central Iowa, rye was able to conserve water in the top soil layers (0-20 cm) and increase soil water storage in a corn-soybean rotation.
Because of public health and ecological concerns, and in light of economic and ecological uncertainties posed by climate change, more research should be directed toward perennial and cover crops because of their beneficial contributions to hydrological processes and biogeochemical cycling.