Streamside Management Zones Affect Movement of Silvicultural Nitrogen and Phosphorus Fertilizers to Piedmont Streams

Abstract

Forestry best management practices (BMP) recommendations for streamside management zones (SMZs) are based on limited data regarding SMZ width, partial harvests, and nutrient movements after forest fertilization. Agricultural fertilization is commonly linked to increased stream nutrients. However, less is known about effectiveness of SMZ options for controlling nutrient movements after silvicultural fertilization. Diammonium phosphate and urea were applied to 12 subwatersheds in 3-year-old loblolly pine (Pinus taeda L.) plantations in the Virginia Piedmont. Three replicates of four SMZ treatments were superimposed on 12 subwatersheds in a previous SMZ harvest sediment study (7.6-m SMZ, 15.2-m SMZ thin, 15.2-m SMZ, and 30.5-m SMZ). Surface, near-surface, subsurface, and stream water samples were collected monthly for 1 year and analyzed for nitrate (NO3 ), ammonium (NH4 ), and orthophosphate (ortho-P). Transected measurements from streamside to fertilized plantations allowed interpretations of spatial nutrient measurements across SMZs. When compared with wider SMZs, 7.6-m SMZs had 3–10 surface water NO3 ,3 – 6 near-surface water NO3 , and 1–2 more stream water NO3 . No significant differences were detected for NH 4 for any SMZ treatment. The 15.2-m SMZ thin had small but significant increases (2– 8) in surface runoff for ortho-P relative to other SMZ treatments, perhaps because of increased surface water movement along thinning corridors. Across all SMZ treatments, comparisons of stream edges with fertilized stands indicated NO3 reductions of 33–98%, NH4 reductions of 68 –97%, and ortho-P reductions of 70 –98%. A 39% rainfall deficit during the study influenced results, but conventional SMZs 15.2 m protected streams from fertilization nutrient increases.