Ectomycorrhizal fungal mycelia turnover in a longleaf pine forest.

Elucidation of the patterns and controls of carbon (C) flow and nitrogen (N) cycling in forests has been hindered by a poor understanding of ectomycorrhizal fungal mycelia (EFM) dynamics. In this study, EFM standing biomass (based on soil ergosterol concentrations), production (based on ergosterol accrual in ingrowth cores), and turnover rate (the quotient of annual production and average standing biomass estimates) were assessed in a 25-yr-old longleaf pine (Pinus palustris) plantation where C flow was manipulated by foliar scorching and N fertilization for 5 yr before study initiation. In the controls, EFM standing biomass was 30 ± 7 g m(-2) , production was 279 ± 63 g m(-2)  yr(-1) , and turnover rate was 10 ± 3 times yr(-1) . The scorched × fertilized treatment had significantly higher EFM standing biomass (38 ± 8 g m(-2) ), significantly lower production (205 ± 28 g m(-2)  yr(-1) ), and a trend of decreased turnover rate (6 ± 1 times yr(-1) ). The EFM turnover estimates, which are among the first reported for natural systems, indicate that EFM are a dynamic component of ecosystems, and that conventional assessments have probably underestimated the role of EFM in C flow and nutrient cycling.

A 15N tracer technique for assessing fine root production and mortality.

We tested a 15N tracer technique to assess fine root production and mortality based on temporal measurements of the 15N mass in fine root structural tissues and the 15N concentration of the plant-available soil N pool. The results of a pilot study indicated that this technique is based on sound methods and reasonable assumptions. The 15N tracer technique avoids most of the major limitations which hinder existing methods and may provide valuable insight into the rates and controls of fine root production and mortality in terrestrial ecosystems.

Fine root heterogeneity by branch order: exploring the discrepancy in root turnover estimates between minirhizotron and carbon isotopic methods.

Fine roots constitute a large and dynamic component of the carbon cycles of terrestrial ecosystems. The reported fivefold discrepancy in turnover estimates between median longevity (ML) from minirhizotrons and mean residence time (MRT) using carbon isotopes may have global consequences. Here, a root branch order-based model and a simulated factorial experiment were used to examine four sources of error. Inherent differences between ML, a number-based measure, and MRT, a mass-based measure, and the inability of the MRT method to account for multiple replacements of rapidly cycling roots were the two sources of error that contributed more to the disparity than did the improper choice of root age distribution models and sampling bias. Sensitivity analysis showed that the rate at which root longevity increases as order increases was the most important factor influencing the disparity between ML and MRT. Assessing root populations for each branch order may substantially reduce the errors in longevity estimates of the fine root guild. Our results point to the need to acquire longevity estimates of different orders, particularly those of higher orders.

Nitrogen decreases and precipitation increases ectomycorrhizal extramatrical mycelia production in a longleaf pine forest.

The rates and controls of ectomycorrhizal fungal production were assessed in a 22-year-old longleaf pine (Pinus palustris Mill.) plantation using a complete factorial design that included two foliar scorching (control and 95% plus needle scorch) and two nitrogen (N) fertilization (control and 5 g N m(-2) year(-1)) treatments during an annual assessment. Ectomycorrhizal fungi production comprised of extramatrical mycelia, Hartig nets and mantles on fine root tips, and sporocarps was estimated to be 49 g m(-2) year(-1) in the control treatment plots. Extramatrical mycelia accounted for approximately 95% of the total mycorrhizal production estimate. Mycorrhizal production rates did not vary significantly among sample periods throughout the annual assessment (p = 0.1366). In addition, reduction in foliar leaf area via experimental scorching treatments did not influence mycorrhizal production (p = 0.9374), suggesting that stored carbon (C) may decouple the linkage between current photosynthate production and ectomycorrhizal fungi dynamics in this forest type. Nitrogen fertilization had a negative effect, whereas precipitation had a positive effect on mycorrhizal fungi production (p = 0.0292; r (2) = 0.42). These results support the widely speculated but poorly documented supposition that mycorrhizal fungi are a large and dynamic component of C flow and nutrient cycling dynamics in forest ecosystems.

Measuring external mycelia production of ectomycorrhizal fungi in the field: the soil matrix matters.

Assessing mycorrhizal fungi production in field settings has been hindered by the inability to measure external mycelia. Recently, external mycelia production was measured in the field using a novel in-growth core technique with acid-washed sand as the in-growth matrix. Here, we tested the assumption that external mycelia production in acid-washed sand is representative of that in native soil. External mycelia production was estimated as the difference in fungal growth between closed (allowing only saprotrophic fungal production) and open (allowing mycorrhizal and saprotrophic fungal production) cores using a factorial design of soil matrices (acid-washed sand vs native) and fertilization treatments (control vs nitrogen (N)) in a longleaf pine (Pinus palustris) plantation. In native soils, the ectomycorrhizal to saprotrophic fungal biomass signal was strong and consistent facilitating the assessment of external mycelia production, which was 300% higher than corresponding rates in acid-washed sand and inversely correlated with soil N. These results demonstrate the efficacy and importance of using native soil as the in-growth matrix to measure ectomycorrhizal fungi external mycelia production in field settings.

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.