Above-Ground Biomass and Nitrogen Quantities in a Big Sacaton [Sporobolus wrightii] Grassland

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Live and standing dead biomass, standing crop, and total nitrogen, within each component, were measured in a big sacaton (Sporobolus wrightii Monro) grassland in southeastern Arizona for 3 years to determine annual fluctuations in above-ground biomass and nitrogen. Mean live biomass varied from 150 kg/ha in February to 2,000 kg/ha in August. Standing dead biomass accumulated after the summer growing season and rapidly disappeared following either fall, winter, or summer moisture, but was the predominant vegetative component for about 49 weeks of each year. Standing crop (live plus standing dead) was greatest in August and averaged 4,450 kg/ha. Total nitrogen varied from 2 to 31 kg/ha in live biomass, from 5 to 15 kg/ha in standing dead biomass, and from 9 to 40 kg/ha in standing crop. The rapid disappearance of standing dead suggests that stocking rates should be based on standing crop just prior to the grazing period rather than peak standing crop after the summer growing season. Big sacaton (Sporobolus wrightii Monro) grasslands produce abundant forage in summer (Gavin 1982) and pure stands once existed along the channels and tributaries associated with the San Pedro and Santa Cruz Rivers in southeastern Arizona (Griffiths 1901). These grasslands, which are located on low alluvial flats or flood plains (Wooten and Standley 1912), naturally spread flood waters, trapped sediments (Hubbell and Gardner 1950), limited soil erosion (Humphrey 1958), and provided a forage resource for one million cattle before 1890 (Cox et al. 1983). Range managers and researchers have assumed for more than 100 years that the standing dead biomass of big sacaton disappears slowly, and this assumption has been used to justify the need for burning and mowing. Data to support this slow disappearance hypothesis is not available in the literature. Some work has been reported on the response of big sacaton following burning (Bock and Bock 1978) and mowing (Haferkamp 1982). However, information on the cycling of above-ground biomass and nitrogen in undisturbed big sacaton grasslands is currently unavailable. The objectives of this study were to (1) investigate the seasonal dynamics of live and standing dead biomass and nitrogen of big sacaton grasslands under natural conditions, and (2) to interpret biomass and nitrogen dynamics in relation to annual climate.