Water Quality Effects of Clearcut Harvesting and Forest Fertilization with Best Management Practices
Nine small (2.5 ha) and four large (70-135 ha) watersheds were instrumented in 1999 to evaluate the effects of silvicultural practices with application of best management practices (BMPs) on stream water quality in East Texas, USA. Two management regimes were implemented in 2002: (1) conventional with clearcutting, herbicide site preparation, and BMPs and (2) intensive, that added subsoiling, aerial broadcast fertilization, and an additional herbicide application. Watershed effects were compared with results from a study on the same small watersheds in 1981, in which two combinations of harvesting and mechanical site preparation without BMPs or fertilization were evaluated. Clearcutting with conventional site preparation resulted in increased nitrogen losses on the small watersheds by about 1 additional kg ha-1 each of total Kjeldahl nitrogen (TKN) and nitrate nitrogen (NO3-N) in 2003. First-year losses were not significantly increased on the large watershed with a conventional site preparation with BMPs. Fertilization resulted in increased runoff losses in 2003 on the intensive small watersheds by an additional 0.77, 2.33, and 0.36 kg ha-1 for NO3-N, TKN, and total phosphorus (TP), respectively. However, total loss rates of ammonia nitrogen (NH4-N) and NO3-N were overall low and only accounted for a small percentage (~7%) of the applied N. Mean loss rates from treated watersheds were much lower than rainfall inputs of about 5 kg ha-1 TKN and NO3-Nin 2003. Aerial fertilization of the 5 yr old stand on another large watershed did not increase nutrient losses. Intensive silvicultural practices with BMPs did not significantly impair surface water quality with N and P.