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Fallow Tillage Influence on Spring Populations of Soil Nitrifiers, Denitrifiers, and Available Nitrogen¹

ABSTRACT

The influence of plow, subtillage, and no-tillage fallow management on nitrifier and denitrifier populations and available N in surface soil (300 mm) was evaluated at two sites in western Nebraska during spring growth of winter wheat (Triticum aestivum L.). Previous management differed considerably and tillage comparisons at one site were established 11 years earlier in land previously cultivated and at the other 9 years earlier in native sod. At both sites, nitrifier populations in the surface 150 mm of no-tillage and subtillage soils were 22 to 56% and 16 to 35% lower, respectively, than those in plowed soil. Soil denitrifier populations tended to increase with less tillage. Differences in soil water content and temperature were major factors influencing differences in microbial populations between tillage practices. Higher water content in reduced tillage soils during early spring paralleled lower nitrifier and higher denitrifier populations as compared with plowed soils. These differences apparently resulted from greater aeration in plowed soils. Soil temperatures during the spring were well below optimum for mineralization and nitrification, particularly on subtillage and no-tillage soils which averaged 1 and 2°C cooler, respectively, than plowed soils. Surface applications of 44 kg N/ha as NH₄NO₃ increased soil nitrifier populations six to nine fold in the surface 0- to 150-mm layer; this effect was not influenced by tillage practice. Lower populations of nitrifiers and higher populations of denitrifiers were paralleled by lower NO^-₃-N levels in no-tilled compared with plowed soils. Levels of potentially mineralizable N (PMN) in the surface 0- to 150-mm layer of no-tillage and subtillage soils at the previously cultivated site averaged 20 and 12% higher, respectively, than plowed soils. At the native sod site, differences in soil PMN between tillage practices were minimal. Changes in the soil environment and availability of N as a result of tillage can be modified by previous management history.

¹ Contribution from the Agricultural Research Service, USDA, in cooperation with the Nebraska Agric. Exp. Stn., Lincoln. Published with the approval of the director as Paper no. 7216, Journal Series, Nebr. Agric. Exp. Stn.

² Graduate Research Assistant, Univ. of Missouri, Columbia (formerly Univ. Nebraska-Lincoln); Soil Scientist, ARS, USDA, Lincoln, NE; Professor, Univ. of Nebraska-Lincoln; and Professor Emeritus, Univ. of Nebraska, Panhandle Station.

Received for publication June 30, 1983. Accepted for publication March 24, 1984.