Investigating Winter Cover Crop Effects II: Soil water Conservation and Pore Characteristics

Investigating winter cover crop effects II: Soil water conservation and pore characteristics Ole Wendroth, Hanna Poffenbarger, and Montse Salmeron This project is submitted being complementary to the project “Investigating winter cover crop effects on soil health in sloping cropland” by Poffenbarger et al. There exist several reasons for growing winter cover crops: Other than mere crop residue that can be easily translocated by wind and surface water runoff in slopy terrain, cover crop plants remain in their position and protect the soil surface because their roots anchor them in the soil at their position of growth. Winter cover crops take up nitrogen, lower the nitrate concentration in the soil profile, and protect nitrate against leaching into groundwater and surface waters. Intuitively, we might assume that cover crops through their water uptake increase the consumption of water and reduce the available water storage for the following crop. Recent research from Dr. Poffenbarger’s group shows that this is not necessarily the case. While the effect of landscape position – shoulder, backslope, footslope – on water stress of corn differed further studies will need to improve understanding of the occurrence of water stress and its effect on crop yield. While backslope positions revealed water stress, soil water content measurements in plots grown with cover crops before showed higher soil water contents than in those plots not grown with cover crops. This phenomenon was observed, however, the processes leading to this unexpected positive effect of cover crops are not well understood. In the main project “Investigating cover crops …” Poffenbarger et al. propose the quantification of water-stable aggregates to identify whether special binding effects increase water infiltration and prevent crusting. Accordingly, we hypothesize that cover crops increase aggregate stability and favor the flow of water through soil pores. In addition, the roots of cover crops create and conserve a macropore system. The resulting higher infiltration and redistribution of water in the soil profile coincide with a larger storage capacity in the soil. As a consequence, when the following main crop is planted, the higher soil water storage capacity after cover crops prevents water stress. The objective of this study is to identify soil pore processes that trigger higher water availability in soils previously grown with cover crops, and whether water-stable aggregates are an indicator for the favorable effect of cover crops with regards to soil structure, soil water conservation and prevention of water stress. For this purpose, soil water dynamics over time in different soil layers (4-inch-increments down to 40 inches depth or to bedrock) will be monitored once a week in plots managed with and without cover crops. Undisturbed soil cores will be used for measuring the soil hydraulic properties (soil water retention curve and soil hydraulic conductivity function) in the laboratory. In addition, the pore geometry at the soil surface will be quantified with special types if infiltrometers that allow measuring permeability of different pore classes. These measured characteristics will be used in a simulation model to capture the dynamics of the water balance, i.e., evapotranspiration, infiltration, redistribution, and seepage. We expect that our measurements and results will complement the findings of the project “Investigating winter cover crop effects on soil health in sloping cropland”. We ask for support for a student worker ($6,000) who will perform field and lab measurements, for field and lab supplies ($3,000), and for replacing and repairing water content access tubes that have become dysfunctional due to field operations ($600).