Impacts of Fertilization on Soil Properties in Loblolly PINE Plantations in the Southeastern United States.

Plant growth is partially dependent on the quantity of available nutrients in the soil, primarily nitrogen (N) and phosphorous (P).  Significant improvements on tree growth and volume have been achieved after N and P fertilizer additions.  Long-term impacts of P have been discovered in sandy soils following fertilization while little effect on mineral soil N has been found.  Nevertheless, there is still uncertainty on the primary transformations of these nutrients in the soil. Our research focused on determining whether fertilization has long-term impacts of N and P on a variety of soil types in pine plantation systems across the southeastern United States.  To examine the effects of N and P applications, forest floor and mineral soil samples were collected from 13 sites of a regional fertilization study that was initiated during the early 1990s. The experimental sites are located in the Piedmont and the Atlantic Coastal Plain with a range of soil texture and drainage classes. Application rates of N were 0, 67, 135, 202, and 269 lbs/acre at 2 and 4-application frequency and P fertilizer was added at 10 percent of N.

Results from our research show that fertilization impacts positively both N and P on the forest floor. Nevertheless, we found little response on N in mineral soil so that we concluded that there are not long-term impacts on nutrient availability. Part of the N added is only being recovered on the forest floor, but it is unlikely that these recoveries will improve site quality over time since after harvesting litter decomposes rapidly. Fertilization with P, on the other hand, accumulates through time in the mineral soil corroborating former researches conducted on sandy sites.