Effect of Soil Moisture and Fertilization on Nitrogen Availability in a Loblolly Pine (Pinus Taeda) Plantation

Abstract

The Southeast is considered to be one of the more vulnerable regions in the United States to climate change. This will have an impact on future southern pine productivity, specifically loblolly pine (Pinus taeda L.), one of the most intensively managed forest ecosystems in the region. Because of this, it is important to understand how shifts in the soil nitrogen (N) supply under varying degrees of soil moisture and fertilization will impact loblolly pine productivity. Doing so would allow forest landowners to adapt their management practices in order to more efficiently utilize fertilizer inputs while maintaining productivity. The primary objective of this study was to examine the influences of soil moisture and fertilization on the belowground processes affecting loblolly pine productivity by assessing how fertilization with projected changes to the precipitation regimes in the southeastern United States will influence soil N availability. This study was located on a seven-year-old loblolly pine plantation in McCurtain County near Broken Bow, OK. A randomized complete block 2x2 factorial design was utilized, incorporating two levels of fertilization (none or fertilized) and rainfall manipulation (none or 30% throughfall reduction). Ion exchange resin bags and intact soil cores were used to index N availability and determine N mineralization rates over the course of 486 days. Previous literature estimates net mineralization rates in loblolly pine plantations around 22.4 to 96.4 kg N ha-1 soil yr-1. The mineralization rates in this study ranged from 37.84 (wet), 51.71 (dry), 25.59 (wet+fert), and 30.22 (dry+fert) kg N ha-1 soil yr-1. The major finding in this study was that fertilization increased N mineralization rates as hypothesized, particularly for nitrate. While not significant, there were also indications supporting this for NO3 accumulation on the core resins as well. The second major finding of this study was the fluctuations in N mineralization rates did not follow the patterns observed in previous studies, which is attributed to unexplored fluctuations in the microbial community.