Range Fertilization: Nitrogen and Phosphorus Uptake and Recovery over Time

Little information has been published concerning the long-term effects of N and P fertilization on nutrient cycling and availability of N and P as related to quantity and quality of native grassland herbage. Factorial combinations of ammonium nitrate at rates of 0, 112, 336, and 1,008 kg N/ha and concentrated superphosphate at rates of 0, 112, and 224 kg P/ha were broadcast once in the spring of 1969 on a native range site (Bouteloua-Carex [Stipa] faciation of a mixed prairie association). During the next 8-years, plant N and P content of grasses and nongrasses increased for periods of time proportionated to the rate of N and P applied. Plant N content tended to be low in "wet" years and relatively high in "dry" years. Conversely, plant P content ended to be high in "wet" years and relatively low in "dry" years. After the first 2 years, the increase in plant N and P uptake, resulting from a given level of N-P fertilization, continued at a rather stable rate as compared with the unfertilized check. In 1973, the unfertilized check had 20,700 kg/ha of root material in the upper 30 cm of soil. The fertilized (336 kg N/ha plus 224 kg P/ha) grassland had 24,310 kg/ha of root material which contained 116 kg/ha more N and 8 kg/ha more P than did the check. Therefore, the below-ground root system is a nutrient-deficient sink which has a high potential to immobilize relatively large quantities of applied N and P fertilizer materials. This study revealed the long-term benefits of N and P fertilization on forage quality which may persist for several years after yield responses are no longer apparent.