Trichloroacetic Acid Extractable N and P as Indicators of Site Quality Changes in Pine Plantations in the South.

In southeastern U.S. plantation forestry, fertilization is widely implemented to improve growth rates on infertile, sandy soils. Our overall objective is to determine if fertilization has improved site quality for successive rotations. Specifically, we are examining the impact of fertilization on pools and fluxes of N and P in the forest floor and mineral soil.  The Southeast Tree Research and Education Site (SETRES), a 24-year old loblolly pine (Pinus taeda L.) plantation, has received annual fertilization and irrigation after age seven. Fertilization applied N, P, K, and other macro- and micro-nutrients based on the results of annual foliar analysis. Irrigation maintains a soil water content >40% field capacity during the summer. Fertilization increased both N and P in the forest floor with increases of 74 kg ha^-1 and 10 kg ha^-1, respectively. Fertilization increased mineral soil P in the 0-15 cm depth by approximately 40 kg ha^-1 while no effect was observed for N. This suggests that the forest floor is a sink for N but releases P over time. The main question to be addressed is why the forest floor is a sink for N and not for P. To address this question, we quantified N and P fractions released from fresh litterfall in order to identify fluxes during the initial stages of decomposition. Sequential cold (0.30 M) and hot (0.15 M) trichloroacetic acid (TCA) extractions were performed to assess organic and inorganic N and P flux from fresh litterfall. In addition, extracts were analyzed for sugar and polyphenol C fractions which are known to enhance and inhibit, respectively, microbial and fungal activity. Identifying these N, P, and C fractions will enable inferences to be made on processes affecting retention or release of N and P from pine litter.