Nitrogen source influences on forage and soil in young southern-pine silvopasture

Southern-pine silvopasture can be established by thinning an existing forest plantation then adding or improving a forage component, or by adding low densities of trees to existing pasture. Studies in mature loblolly (Pinus taeda L.) pine silvopasture (26 years) developed from a thinned plantation on the Western Coastal Plain of Louisiana, USA estimated higher forage biomass production for open-pasture versus silvopasture. However, little is known about temporal and spatial dynamics of forage productivity and soil quality in permanent pastures being converted to silvopasture on the Southern Coastal Plain of the Southeastern USA. The objectives of this research were to determine the impact of nitrogen (N) source (legume-N versus fertilizer-N), pasture type (silvopasture versus open-pasture), and alley position relative to trees in young Southern Coastal Plain silvopasture on (1) forage productivity and quality; (2) soil aggregate stability, density of fungal hyphae, and soil penetration resistance. This research was conducted in a randomized complete block design with three replications from 2005 to 2007 at Americus, Georgia, USA in a young longleaf pine (Pinus palustris Mill.)-bahiagrass (Paspalum notatum Flugge) silvopasture and adjoining bahiagrass pasture without trees (open-pasture). Treatments included either fertilizer-N or overseeded crimson clover (Trifolium incarnatum L. ‘Dixie’). Silvopasture forage parameters were monitored at two (2005: 1.0 and 6.1 m) or three (2006–2007: 1.0, 3.5, and 6.1 m) alley positions relative to the tree base; soil parameters were monitored at two alley positions (1.0 and 6.1 m). Cool-season (April or May) forage biomass was 40% higher and forage N concentration 27% higher for the legume-N versus fertilizer-N treatment. When compared to the 3.5- or 6.1-m alley positions, forage productivity at the 1.0-m alley position decreased when the pines were 6 years old. Lower N and higher acid detergent fiber (ADF) levels were found in forage from silvopasture versus open-pasture in August 2006, and July and September 2007, the result of pine straw accumulation in the forage alleys. Water stable aggregates were 5% lower in silvopasture versus open-pasture. Soil penetration resistance was lower in silvopasture versus open-pasture at 10–15- and 15–20-cm in 2005, and at 15–20-cm in 2007. This research found that forage productivity and forage and soil quality can be improved, and N fertilizer additions replaced by introduction and maintenance of crimson clover into young longleaf pine-bahiagrass silvopasture during the hay-production period of pasture to silvopasture conversion on Southern Coastal Plain soil in the Southeastern USA.