Nitrogen and biomass dynamics following brush control in the cross timbers

Converting marginal hardwood forest to grassland has the potential of increasing economic output with livestock grazing. Nitrogen (N) management during conversion needs to be evaluated to minimize possible adverse effects on the environment. This study was conducted to determine temporal changes in quantities of N and biomass within ecosystem compartments after herbicide application in a mature post oak (Quercus stellata Wangenh.)-blackjack oak (Quercus marilandica Muenchh.) forest. The 4 treatments evaluated included; 1) no brush kill with no grass overseeding, 2) brush kill with no grass overseeding, 3) brush kill with cool-season grass overseeding, 'K-31' tall fescue (Festuca arundinacea Schreb.), and 4) brush kill with warm-season grass overseeding, 'Plains' Old World bluestem [Bothriochloa ischaemum var. ischaemum (L.) Keng.]. Excellent brush kill was accomplished with 2.2 kg a.i. ha(-l) tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2 yl]-N,N'-dimethylurea) applied in spring 1993. Total N was measured during a 21-month period (June 1993 to February 1995), in 15- X 25-m plots, within 8 compartments; soil (0 to 60 cm), roots (0 to 60 cm), litter, top growth of herbaceous plants, woody plants <2.5 cm in diameter, and leaves, branches, and boles from trees >2.5 cm in diameter. Stored N in tall fescue forage was 6 to 7 times greater than in Old World bluestem or native vegetation by June 1994 and was 2 times greater in October 1994 and February 1995. Above-ground biomass of tall fescue and Old World bluestem was not different in October 1994 or February 1995, but both were greater than native vegetation. Litter biomass and total N in litter decreased, especially in the tall fescue overseeding treatment. Total N in the soil for all treatments averaged 5,100 kg ha(-1) and fluctuations were not detectable among treatments. Total N changes in other compartments were not observable or were minimal. Brush kill and overseeding with grass had little influence on total N stored within the ecosystem for 21 months after treatment. N was redistributed to the herbaceous biomass compartment and away from the litter compartment after herbicide application, regardless of the overseeding treatment applied.