Response of Semiarid Grassland Sites to Nitrogen Fertilization: I. Plant Growth and Water Use

Although it has been repeatedly demonstrated that most perennial grassland ecosystems respond to N fertilization, knowledge of factors controlling N transformations involved in such responses is fragmentary. Eight rate-of-N-fertilizer experiments were conducted for 5 or 6 years on established perennial grass sites near Mandan, North Dakota. Experiments were conducted within essentially the same precipitation and temperature regimes on sites differing in soil type, exposure, and grass species to determine their influence on N response. Above-ground vegetation response to N fertilization depended primarily upon species — smooth bromegrass (Bromus inermis) was generally more responsive than crested wheatgrass (Agropyron desertorum) or native mixed prairie. No consistent patterns of root biomass response to N fertilization were obvious. At some sites or in certain years root biomass responses were fairly consistent, whereas at other sites or in other years, little response to N occurred. Water use efficiency (WUE) was greatest for bromegrass and least for native grasses, and was generally increased by N fertilization. WUE increased as aboveground growth increased, whether as a result of N fertilization or because of less evaporation at more protected sites. Cumulative dry matter response to N fertilization was highly correlated with cumulative fertilizer N added, with a different curvilinear relationships for each site. These results indicated that the magnitude of fertilizer N responses by perennial grasslands is site specific. Although the same general mechanisms seem to operate at the different sites, magnitude of response was controlled by the plant species present and by some soil type × location factors that are presently ill-defined. These latter factors appear to influence the capacity of the soil to immobilize fertilizer N, capability of above-ground plant growth to take up fertilizer N, and many other N transformations, and may be related to relative availability of water. Consequently interpretations of fertilizer experiments conducted on only one soil type or with only one species must be interpreted with caution.