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Wet Season Aeration Problems Beneath Surface Mulches in Dryland Winter Wheat Production¹

Denitrification was a suspected cause of localized poor growth in winter wheat (Triticum oestivum L.) fields in the northwest. Experimental objectives included monitoring soil aeration, soil nitrogen supply and plant growth in field soils with poor and normal plant growth to establish the probability of localized denitrification. Measured anaerobic conditions occurred above and in restricting layers of slowly draining soils sup orting poor growth but not in adjacent, unlayered s o d supporting good growth. The layered soils were Natrixerolls occuring in a Calcic Haploxeroll which is the major soil of the landscape in experimental area. Laboratory measurements of soil texture and soil and plant N combined with neutron meter field measurements of soil water, platinum microelectrode measurements of soil 0₂ diffusion rate, and plant dry matter accumulation explained growth and yield differences.

Surface mulch of burlap (simulating a heavy straw cover) increased water infiltration in both soils but by doing so, prolonged low 0₂ levels in slowly draining soils. Although both soils had been fertilized at the same rate and were no more than 3 m apart, plants growing on the slowly draining soil had lower N contents than those on the normal (well drained) soil. Beneath the mulch essentially all of the fertilizer N was lost on the slowly draining soil and yield was 35% lower than that on mulched normal soil. Without the mulch, the slowly draining soil reduced yield by 15%. Loss of N through denitrification was the most probable cause of reduced plant growth and yield. Localized application of N late in the spring or trial of nitrification inhibiting compounds were suggested as remedies to counteract the problem.

Key Words: Denitrification • Triticum aestivum L. • Straw mulch • Slow drainage • Oxygen diffusion rate • Nitrogen

² Respectively, soil scientist and plant physiologist, USDASEA-AR, Pendleton, OR.

Received for publication November 26, 1979.