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The Effect of Moisture on Fertilizer Nitrogen Immoblization in Grasslands

ABSTRACT

The fate of 500 kg of fertilizer N per ha applied to Bromus inermis L. was followed for 2 years for various moisture levels and times of application. Analysis of the plots at the end of 2 years of irrigation (with no leaching) showed that about 40% of the fertilizer N was contained in grass tops and 25 to 35% in roots, while 5 to 10% was in the soil in mineral form and 20% was not accounted for. For dryland plots this distribution was 25%, 10 to 25%, 25 to 65% and 0 to 20%, respectively. The primary effect of increased water was to hasten plant absorption of fertilizer N. Fertilizer N losses were not affected by moisture supply, thus with proper fertilization it may be possible to maintain a constant pool of available N in grassland soils of semiarid regions without experiencing undue losses. Maximum growth could then be achieved whenever water is available by eliminating N as a growth-limiting factor.

Weekly applications of 17 kg of fertilizer N in fallow plots resulted in about 175% of the fertilizer N being accounted for, but when the fallow plots received all the fertilizer N in the spring only 50 to 60% was accounted for. These differences may have been caused by differences in rates of mineralization and denitrification. Soil NO₃-N levels on fertilized dryland often rose during periods of drought. Soil NO₃-N levels in fertilized irrigated plots declined with time. Unfertilized plots were conistently very low in soil NO₃-N, regardless of time or water supply.

² Soil Scientist, Northern Great Plains Research Center, Mandan North Dakota.