NITRATE REDUCTASE ACTIVITY AND N FRACTIONS IN TIMOTHY AND SWITCH GRASS AS INFLUENCED BY N AND S FERTILIZATION

The effects of N and S fertilization on nitrate reductase activity (NRA) and N fractions were studied in timothy (Phleum pratense L.) and switch grass (Panicum virgatum L.). Soil cores of timothy and switch grass plants were obtained from a marginally S-deficient area at Madison, Wisconsin. The cores were placed in growth chambers maintained near the optimal temperature for each species. Three rates of N (0, 224, 448 kg N/ha as Ca(NO3)2) and three rates of S (0, 11.2, and 22.4 kg S/ha as CaSO4) were applied in all possible combinations. Plants were harvested at anthesis and NRA in the leaves (blades) was determined. Total N, reduced N, nitrate-N (NO−3-N), and free α-amino N concentrations (concns.) were measured in leaves and stems (culms, leaf sheaths, and inflorescences). Fertilization with S had little effect on the concn. of any N fraction or on NRA in either species. Switch grass and timothy were not S-deficient as indicated by tissue S concn. Total N, NO−3-N, and free α-amino N concns. increased significantly in all plant fractions of both species with each increment of N fertilization. "Toxic" concns. of NO−3-N accumulated in timothy leaves when N was applied. Switch grass leaf NO−3-N concn. was less than one-half that found in timothy. NRA and concn. of reduced N in timothy leaves increased significantly only with the first increment of N fertilization. NRA and concn. of reduced N in switch grass leaves increased significantly with each increment of N fertilization. NRA was significantly correlated (0.01 level) with the concn. of every N fraction in the leaves of both species.