Responses of loblolly pine, sweetgum and crab grass roots to localized increases in nitrogen in two watering regimes.

Root responses to differences in availability of nitrogen and soil water were studied in loblolly pine (Pinus taeda L.) seedlings grown in monoculture 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 availability were created by adding nitrogen in solution to root ingress cores placed in each rhizotron cell. Presence of competitors reduced loblolly pine root growth regardless of the nitrogen or soil water treatment. On average, both total root length density and root surface area were reduced 60% when loblolly pine seedlings were grown with crab grass 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 surface area was reduced 18% by the low water availability treatment, whereas crab grass root surface area was unaffected by this treatment. At all soil depths, loblolly pine root surface area and root length density were increased in localized areas of increased nitrogen availability. Sweetgum and crab grass root surface areas were also greater in areas of increased nitrogen availability. In the high soil water availability treatment, loblolly pine root surface area increased 128% in localized areas of increased nitrogen in all competition treatments. In the low soil water availability treatment, loblolly pine roots responded 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.