Primary Shoot Production of Blue Grama Grassland in South-Central New Mexico under Two Soil Nitrogen Levels

Net primary productivity was determined on a blue grama (Bouteloua gracilis) grassland under two levels of soil nitrogen. Ammonium sulfate was added by hand in the amount of 67 kg of nitrogen per ha. The added nitrogen increased standing crop of all herbage on most sampling dates for both years. The standing crop curves showed a rapid increase during the growing season followed by a sharp decline early in the dormant season and then a gradual leveling off in the late dormant period. The added nitrogen stimulated mostly blue grama and other grasses to some extent but not forbs. Net primary productivity calculated from only peak community standing crop underestimated net primary productivity compared to that calculated from the sum of peaks of species groups and that calculated from the sum of positive increments of species groups. Leaf blades and sheaths contributed most of the herbage weight of blue grama plants early in the growing season, but culms contributed most of the weight later in the growing season and contributed the most of any part to annual net primary productivity during a favorable growing season under added nitrogen. Considering changes in development of plant parts gives a higher estimate of net primary productivity than considering only the entire plant weight of each species