RESPONSES OF LOBLOLLY-PINE, SWEETGUM AND CRAB GRASS-ROOTS TO LOCALIZED INCREASES IN NITROGEN IN 2 WATERING REGIMES

Root responses to differences in availability of nitrogen and soil wat er were studied in loblolly pine (Pinus taeda L.) seedlings grown in m onoculture and in competition with sweetgum (Liquidambar styraciflua L .) or crab grass (Digitaria spp.). Rhizotron cells were maintained at high soil water availability (approximately -0.1 MPa) or subjected to three dry-down cycles to low soil water availability (approximately -1 .0 MPa), over two growing seasons. Localized increases in nitrogen ava ilability were created by adding nitrogen in solution to root ingress cores placed in each rhizotron cell. Presence of competitors reduced l oblolly pine root growth regardless of the nitrogen or soil water trea tment. On average, both total root length density and root surface are a were reduced 60% when loblolly pine seedlings were grown with crab g rass and 31% when grown with sweetgum. Low water availability reduced loblolly pine root length density and root surface area by 25 and 28%, respectively, compared with well-watered seedlings. Sweetgum root sur face area was reduced 18% by the low water availability treatment, whe reas crab grass root surface area was unaffected by this treatment. At all soil depths, loblolly pine root surface area and root length dens ity were increased in localized areas of increased nitrogen availabili ty. Sweetgum and crab grass root surface areas were also greater in ar eas of increased nitrogen availability. In the high soil water availab ility treatment, loblolly pine root surface area increased 128% in loc alized areas of increased nitrogen in all competition treatments. In t he low soil water availability treatment, loblolly pine roots responde d to increased nitrogen only in the absence of competitors. In general , loblolly pine and sweetgum roots responded to increases in resource availability similarly, whereas crab grass roots were relatively less affected.