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Nitrification in Grassland Soils¹

ABSTRACT

Nitrification studies were made on seven paired soil samples representing closely adjacent grassland and intertilled sites in the following soils: Codorus silt loam, Bladen clay loam, Cecil sandy loam, Hixton fine sandy loam, Fayette silt loam, Honeoye silt loam, and Pachappa sandy loam. All samples were incubated unamended and with 0.1% urea for periods of 2, 4, 6, and 9 weeks. Determinations were made for ammonia, nitrite, and nitrate nitrogen, and for urea nitrogen when indicated.

Grassland samples of the Codorus and Bladen soils, pH 4.7 and 4.9, respectively, showed accumulations of ammonia when incubated unamended. Such accumulations were not apparent in the five remaining grassland samples ranging in pH from 5.7 to 7.9. All grassland samples, irrespective of pH, when amended with 0.1% urea showed higher retention of ammonia and lesser nitrate production than did their intertilled counterparts. Urea-amended soils initially alkaline or becoming alkaline following addition of urea showed nitrite accumulation, irrespective of whether the samples came from grassland or intertilled sites. Soils showing nitrite accumulation showed greater nitrogen deficits than did the remaining soils. Greater nitrogen deficits were commonly encountered in the grassland than in the intertilled members of paired, urea-treated samples. There appeared a direct relationship between nitrite accumulation and magnitude of nitrogen deficit.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA.

² Bacteriologist, Eastern Soil and Water Management Research Branch, Beltsville, Md.; and Microbiologist, Western Soil and Water Management Research Branch, Ft. Collins, Colo., respectively.

Received for publication February 17, 1958. Accepted for publication March 17, 1958.