Soil Carbon After Midrotation Phosphorus Fertilization of Loblolly Pine (Pinus Taeda L.) Stands in East Texas

Pine plantation management impacts forests' role as a carbon (C) sink. We evaluated the status of forest floor mass and soil C content 8 years after phosphorus (P) fertilization of midrotation loblolly pine (Pinus taeda L.) plantations at seven sites of different drainage characteristics (excessively, well, and poorly drained) in East Texas. The impacts of stand age, site drainage class, and P fertilization were evaluated. Mean C accumulation in the forest floor near the end of a 30-year rotation was 10 Mg C ha−1. Phosphorus fertilization resulted in a mean increase of 3 Mg C ha−1 in the forest floor across drainage conditions. However, the direction and magnitude were markedly influenced by site drainage characteristics. Forest floor mass was inversely related to surface soil water content, with the highest C accumulation recorded on the excessively drained site (19 Mg ha−1 vs. mean of 8 ± 2 Mg ha−1 in all other sites). Yet, the poorly drained sites showed the highest forest floor C accumulation after fertilization (up to 77% increase). No significant differences in forest floor C and N composition or in soil C content were observed among sites or treatments. Carbon sequestration in the mineral soil was not affected by P fertilization and seems to be limited by the mineralogy and the coarse texture of the forest soils. Our results suggest that a site's inherent moisture regime dominated soil C accumulation processes. Because the relative P fertilization effect on forest floor mass accumulation followed the order poorly drained sites > well drained > excessively well drained, moisture limitations likely suppressed decomposition rates more than it did stand growth, resulting in overall decreases in forest floor C accumulation near the end of a 30-year rotation in the order excessively well-drained > well-drained > poorly drained sites.