Joseph J. Hendricks

Fine root branch orders respond differentially to carbon source-sink manipulations in a longleaf pine forest.

Fine roots are a key component of carbon (C) flow and nitrogen (N) cycling in forest ecosystems. However, the complexity and heterogeneity of the fine root branching system have hampered the assessment and prediction of C and N dynamics at ecosystem scales. We examined how root morphology, biomass, and chemistry differed with root branch orders (1–5 with root tips classified as first order roots) and how different root orders responded to increased C sink strength (via N fertilization) and reduced carbon source strength (via canopy scorching) in a longleaf pine (Pinus palustris L.) ecosystem. With increasing root order, the diameter and length of individual roots increased, whereas the specific root length decreased. Total root biomass on an areal basis was similar among the first four orders but increased for the fifth order roots. Consequently, total root length and total root surface area decreased systematically with increasing root order. Fine root N and lignin concentrations decreased, while total non-structural carbohydrate (TNC) and cellulose concentrations increased with increasing root order. N addition and canopy disturbance did not alter root morphology, but they did influence root chemistry. N fertilization increased fine root N concentration and content per unit area in all five orders, while canopy scorching decreased root N concentration. Moreover, TNC concentration and content in fifth order roots were also reduced by canopy scorching. Our results indicate that the small, fragile, and more easily overlooked first and second order roots may be disproportionately important in ecosystem scale C and N fluxes due to their large proportions of fine root biomass, high N concentrations, relatively short lifespans, and potentially high decomposition rates.

Season of burn and nutrient losses in a longleaf pine ecosystem

Fire regulates the structure and function of longleaf pine ecosystems, including potential nutrient controls on productivity, forest floor and groundcover nutrient pools, and nutrient availability. Little is known about comparative influences of seasonality of fire, litter types, and mass on N and P balance and soil processes in longleaf pine ecosystems. This study primarily addresses the hypothesis that nutrient volatilization during growing season burning, due to combustion of live biomass, exceeds losses from winter burning of standing dead plant litter. Summer and winter burns were conducted experimentally in different groundcover types with ambient, double-ambient and no litter loadings to contrast 2–3 years of litter accumulation with very low and high fuels. As a comparison, the seasonal burns were repeated with fuel and temperature measurements on sites that had actual fuel accumulations ranging from 1 to 3 years following the last fire. Peak fire temperatures and duration of burning were similar, but with high variation across groundcover types and seasons due to variation in fuel moisture content. The highest pine litter loadings produced maximum mineral soil/litter interface temperatures that never exceeded 700°C. Groundcovers without pine litter burned incompletely and with low temperatures. Biomass and N content were greater in summer groundcover than winter groundcover, and were greater in wiregrass than old-field groundcover. More N was lost from growing season burning as biomass had higher N in green foliage at that time. With ambient litter loadings, mass losses were 88–94% of total litter and groundcover. Percentages of N lost were comparable (80–90% across all groundcovers and seasons), but amounts of N lost were below that estimated to be replenished by legume N fixation and regional atmospheric deposition over a dormant season prescribed fire cycle. Net N balances with growing season fire were generally negative only if growing season burning was projected exclusively over the long-term. P content was not significantly different among groundcovers, but summer standing stocks were higher than winter. No P losses were detected with any experimental treatments and, following burning, all P was returned to soil pools, attributable to soil surface temperatures remaining largely below 700°C. We conclude that frequent, dormant season, or even variable season burning should not seriously deplete long-term nitrogen balance of longleaf pine ecosystems.

Assessing the Nitrogen-15 Concentration of Plant-Available Soil Nitrogen

Abstract Three techniques (buried bag, resin membrane, and cellulose disk approaches) for assessing the nitrogen-15 (15N) concentration of the soil N pool available to plants were evaluated by initiating buried bag N-mineralization, resin membrane, and cellulose disk incubations in coordination with the germination of two non-N2-fixing reference plants in four types of 15N-enriched soil. In turn, the incubation samples and reference plants were collected after 45 and 90 days for 15N analysis. The deviation from the reference plant 15N concentration averaged across all four soil types was least for the cellulose disk technique (3.8 ± 2.3 and 3.5 ± 3.8‰) followed by the buried bag (9.8 ± 5.5 and 4.5 ± 2.5‰) and resin membrane (38.4 ± 11.9 and 21.7 ± 1.6‰) techniques on days 45 and 90, respectively. Also, the linear relationships between soil 15N concentrations and corresponding cellulose disk and reference plant 15N concentrations had high coefficients of determination (average R 2 = 0.96 ± 0.03) and comparable slope and intercept parameters, whereas the linear relationships between soil 15N concentrations and corresponding buried bag soil and resin membrane 15N concentrations were not significant. Overall, the relatively new cellulose disk technique was considered to be the most reliable and practical (i.e., inexpensive and easy to employ) approach for assessing plant-available 15N concentrations in the relatively coarse textured and infertile soils used in this assessment.