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Heterogeneity of Soil Mineral Nitrogen in Pasture Grazed by Cattle

Dep. of Soil Sciences North West Frontier Province Agricultural Univ., Peshawar, Pakistan

W.A. Adams^*

Soil Science Unit, Dep. of Biochemistry, Univ. College of Wales, Aberystwyth SY 23 3DE, Wales, UK

*Corresponding author.

ABSTRACT

Herbage growth on grazed pasture is commonly limited by N availability. The availability of N in soils is affected by inputs of fertilizer N through N mineralization from soil organic matter and by the return of N on local areas in animal excreta. Our objective was to describe the spatial and vertical heterogeneity in soil mineral N caused by the deposition of excreta on pasture grazed by dairy cattle (Bos taurus). The persistence and change in pattern of heterogeneity was examined both during and between grazing seasons. The soil is a Dystric Eutrochrept and perennial ryegrass (Lolium perenne L.) was the dominant species in the sward. Intact soil cores were taken at 10-cm intervals on randomly selected 5-m transects during the grazing season in 1989 and immediately prior to grazing in 1990. Fresh cattle dung and simulated cattle urine were applied both during and at the end of the grazing season. Spatial variation in soil mineral N developed during the grazing season with differences in NO₃-N up to 66 mg N L^–1 and total mineral N (TMN) up to 250 mg N L^–1 occurring between sampling points at 10-cm centers. Sites unaffected by excreta had TMN concentrations of 11 mg L^–1 with less than one-quarter of this as NO₃-N. Dung patches applied in summer increased TMN in the 0- to 2-cm depth range only. Left over winter, they increased the soil TMN content significantly but did not increase mineralizable N. Urine patches caused a much greater increase in TMN than dung and the pattern of heterogeneity with depth changed with time. Soil NO₃-N remaining in spring beneath simulated urine patches applied in autumn was distributed throughout a depth of at least 1 m and vertical heterogeneity persisted into the summer.

Received for publication March 23, 1991.

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