Soil nitrogen dynamics as an indicator for longleaf pine restoration

Assessing the status of soil nutrients with their corresponding microbial communities provides important information about degraded soils during the restoration of coastal wet pine forests. Net nitrogen mineralization, nitrogen-oxidizing bacteria (NOB), and soil microbial biomass were compared with patch-derived volume along a 110-year longleaf pine (Pinus palustris Mill.) chronosequence for identifying a trajectory and ecological benchmark during forest restoration. Net nitrogen mineralization rates decreased significantly in the maturing-aged, pine patches, driven by a larger drop in net nitrification. Net nitrification and abundance of NOB were higher in young pine patches compared to soils from the maturing (86-110 years) pine patches. Gross nitrate fluxes followed the nonfungal portion of the soil microbial biomass along the chronosequence, declining in 64-year-old pine patches. Microbial biomass peaked in patches 17-34 years of age, but significantly declined in the older patches. Fungal biomass leveled off without decline. Ammonium was the major source of nitrogen within the maturing pine patches as well as the wetland patches, indicating that ammonium maintains longleaf pine during growth-limiting conditions. Nitrate dominated during rapid tree growth, optimally in mesic conditions. The relative amounts of available ammonium to nitrate can be used to model nitrogen cycling in facultative-wetland pine forests of the coastal United States as soils alternate between wet and mesic conditions. A key restoration benchmark occurred after 86 years of pine development when pine patch growth rates slowed, with lower numbers of NOB, when the nonfungal biomass leveled off, and net nitrification rates are at a minimum, during pine maturation.