NITRATE DYNAMICS FOLLOWING BRUSH CONTROL IN A POST OAK-BLACKJACK OAK FOREST

Converting marginal hardwood forests to grass may increase economic ou tput from livestock production. Nitrogen dynamics during conversion ne ed to be evaluated to determine if conversion releases excessive quant ities of NO3-N, with potential adverse effects on the environment. Thi s study was conducted to determine the amount of NO3-N present during conversion of a mature post oak-blackjack oak (Quercus stellata Wangen h.-Q. marilandica Muenchh.) forest and to identify optimum time period s for grass uptake of available mineralized N after herbicide treatmen t. Four treatments were evaluated: (i) no brush kill with no grass ove rseeding, (ii) brush kill with no grass overseeding, (iii) brush kill with cool-season grass overseeding, and (iv) brush kill with warm-seas on grass overseeding. The cool-season (Go) grass was 'K-31' tall fescu e (Festuca arundinacea Schreb.); 'Plains' Old World bluestem [Bothrioc hloa ischaemum var. ischaemum (L.) Keng] was the warm-season (C-4) gra ss. Soil profile NO3-N to a depth of 60 cm and NO3-N concentrations in soil leachate at 60 cm were measured in 15- by 25-m plots during a 22 -mo period (June 1993 to March 1995). Soil NO3-N increased from <5 kg ha(-1) initially to >50 kg ha(-1) when brush was killed with tebuthiur on hylethyl)-1,3,4-thiadiazol-2-yl]-N,N'-dimethlurea] application. Soi l and leachate NO3-N in tall fescue overseeded plots returned to near pretreatment levels by the end of the first rapid growth phase of tall fescue in June 1994. Soil NO3-N in old World bluestem overseeded and unseeded brush kill plots remained elevated throughout the experiment, and leachate NO3-N concentrations rose from 0 to more than 70 mg L(-1 ). The risk of NO3-N leaching after brush control is reduced by overse eding with tall fescue.