Impacts of Crop Sequence and Minimum- and No-till Cropping Systems on Soil Water in South-Central North Dakota, USA.

As part of a long-term study near Mandan, ND, to evaluate effects of crop sequence and tillage on crop yield, water use, and soil properties, we measured soil water at various depths to 4 feet (121.9 cm) from 1994 to 2011. We used changes in profile soil water over the growing season, together with precipitation data, to estimate water use by spring wheat grown in different crop sequences. The experiment was a split-plot, with crop sequences as whole plots and tillage (minimum tillage or no-till) as subplots. Crop sequences included continuous spring wheat (Triticum aestivum L.) with crop residue left on the soil surface, continuous spring wheat with crop residue removed, spring wheat–millet (Setaria italica (L.) Beauv.), spring wheat–safflower (Carthamus tinctorius L.)–fallow, spring wheat–safflower–rye (Secale cereale L.), and spring wheat–fallow. Profile soil water, measured at planting and at harvest, with a neutron probe, was unaffected by crop sequence but higher under no-till management than minimum tillage (P≤0.1). Conversely, the difference between planting and harvest measurements was smaller for continuous spring wheat plots than for sequences with a fallow phase, and decreased with depth (P≤0.0001). However, an interaction (P≤0.1) revealed differences between sequences only at depths between 30.5 and 91.4 cm. Average seasonal depletion of soil water varied with crop sequence (P≤0.0005), from 8.5 cm for spring wheat-fallow to 6.5 cm for continuous spring wheat. Grain produced per unit water varied with crop sequence (P≤0.01), highest for spring wheat–safflower–fallow and lowest for continuous spring wheat, and was also higher under no-till management (P≤0.08). Crop sequence and tillage management can affect average seasonal crop water use efficiency but the response to atypical annual or even intraseasonal patterns of precipitation and temperature may be of greater importance in the northern Great Plains.