Mowing and Fertilization Effect on Productivity and Spectral Reflectance in Bromus Inermis Plots

Experiments were conducted to examine the potential role of grazing on ecosystem-level parameters as part of the NASA-sponsored First Isabela (International Satellite Land Surface Climatology Programmed) Field Experiment (FIFE) conducted at Konza Prairie Research Natural Area in 1987. Here we report results of one experiment conducted in a field consisting primarily of Bromus inermis, a cool season C₃ grass. The experiment involved four simulated grazing components (unmowed control, 20-, 10-, and 5-cm mowing heights) and fertilization (untreated control and ammonium nitrate application). The plots were mowed to ground level and raked in April, following which they were mowed seven times during the growing season from May to October. Biomass production, N production, and spectral reflectance data were collected with a hand-held radiometer throughout the growing season, with standing crop estimates taken at two periods (7 August [day 219] and 27 October [day 300]) to correlate with the remote sensing information base. Standing crop values of mowed plots were as much as 67-70% lower than controls, but they produced significantly larger amounts of both biomass and total N. Maximum seasonlong production values in the mowed plots were °43% above controls, with major differences developing as a result of fertilization. Fertilized plots produced 67% more foliage than unfertilized plots. Our data show over-compensatory growth as a result of the simulated grazing treatments. Indexes (NDVI [normalized difference vegetation index] and greenness) derived from the reflectance data were poorly correlated with biomass. The correlation of NDVI with N content of the canopy foliage was somewhat stronger, particularly if stratified by mowing class. NDVI was a better predictor of vegetation status than the greenness indexes, but in plots stimulating heavily grazed areas where leafy vegetation was sparse and soil became more visible from above the canopy its utility decreased significantly.