Nitrogen Fertilization of Semiarid Grasslands: Plant Growth and Soil Mineral N Levels
1971
Power, J. F. | Alessi, J.
To evaluate the fate of mineral N in a grassland ecosystem, fertifizer N was applied to native grassland in NorthD akotaa t rates upt o 540 kg N/ha either (a) all in year, (b) one-third in each of 3 years, or (c) one-sixth each of 6 years. Cumulativep roductiono f grass tops for the 6 years increased with increased amounts of total N applied, but was not affected by timing of fertilizer application. With continued annual fertilization, responses were measureda t progressively lower rates. At 270-kg N or more, residual effects were still significant 6 years later. Dry weights of three oats crops grown on these soils in the growth chamber indicated that greater N availability in fertilized soils contributed appreciably to these residual effects. Soil NH₄⁺- and NO₃⁻-N levels for many treatments exhibiting residual effects were no higher than for unfertilized plots, indicating residual N was present in other than mh~eral forms. Fertilizer N is probablyi mmobilizedb y all componentos f the soil-grass ecosystem (especially grass roots) and the mineralization of this immobilizedN mayc ontribute to residual effects. Fertilizer N applied in excess of the immobilizing capacity of the ecosystem remains in mineral form until required by the grass. Consequently, the presence of a mineral N pool indicates that N has been eliminated as a growthlimiting factor and maximumpr oduction from the available water is being achieved.
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