Soil nitrogen dynamics as an indicator for longleaf pine restoration

Abstract

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.

Low‐dose Herbicide Effects on Tree Establishment and Soil Nitrogen Biogeochemistry within Pine Savannas

“…Soil samples (90) were taken from the top 10 cm monthly within experimental Blocks 1, 4, and 6 during 2002 and 2003; the time period covering the two chemical applications. Samples were also collected from all six experimental blocks during spring and summer of 2005 and 2006 for microbial and fungal biomass analysis (McCaskill et al, 2018). All of the samples were immediately stored at 4°C until analysis.…”

“…A total of 180 samples were analyzed for ammonium (NH 4 + ) and nitrate (NO 3 − ) using a continuous auto‐flow analyzer. Net N mineralization was calculated as the difference between incubated‐ and initial‐N mineralization, corrected for soil moisture (Keeney and Nelson, 1982; McCaskill et al, 2018; McCaskill and Jose, 2012).…”

“…Total organic carbon (TOC) was determined on a Shimadzu TOC‐VCSH analyzer according to Vance and Entry (2000). Microbial biomass carbon was calculated as: mg C MB kg soil −1 = [(TOC fumigated – TOC control )/0.51]/(soil dry mass) (Joergensen, 1996; McCaskill et al, 2018).…”

“…Second, Gulf coastal wet flatwoods are unique habitats because they are located on low, rain‐fed coastal terraces where weather patterns maintain high soil moisture conditions for extended periods during the growing season (McCaskill and Jose, 2012; Messina and Conner, 1998). These two factors along with the application of herbicides make it imperative to research the soil biogeochemical properties of these ecosystems during restoration (Harris, 2003; Heneghan et al, 2008; Johnston and Crossley, 2002; McCaskill et al, 2018).…”

“…Only imazapyr and hexazinone were applied to some of the control plots for establishing second year‐only treatments. The project was also expanded to include the addition of soil biochemical assays for determining the impacts of chemical treatments on nitrogen (N) cycling (McCaskill et al, 2018). The main objectives of the expanded study were to determine the effects of applying the original herbicide treatments in the second year after planting compared to first‐year treatments, consecutive herbicide treatments (first and second years), and whether a March or April application changes the herbicide effects on longleaf pine development during the second growing season.…”