BG1-3: Root respiration and root-derived heterotrophic respiration account for an increasing proportion of soil respiration as NPP declines because of increased root allocation and more rapid tissue turnover and decomposition of below- than aboveground tissues in unproductive stands

Fine Root Survival and Decomposition

Fine root survival and decomposition rates are obtained from minirhizotron data using a parameter estimation program developed for the analysis of marked animal populations, Program MARK (White and Burnham 1999). Because the condition and fate of individual roots are followed, known-fate survival of individuals can be estimated in much the same manner as is done for marked animal populations (Lebreton et al. 1992), without disturbing the root/mycorrhizal association. Following an earlier protocol (Ruess et al. 1998, 2003), encounter history files are created that include a listing for each individual root, coded for every time period as either “1” (alive), or “0” (dead or missing). MARK is then used to generate fine root survival estimates (noted as phi, f) after constructing models incorporating direct effects of time of year, age, cohort (when the root first appeared), and site, along with their interactions, on fine root longevity. The most parsimonious models are identified using the information-theoretic Akaike’s Information Criterion (AIC), and likelihood ratio tests (LRT). The identification of models follows the notation of Lebreton et al. (1992). For example, a model including both site and time effects (abbreviated as f_time*site) can be compared statistically with a reduced model including only time effects (f_time), which can be compared to the most reduced model (f_o). In this manner, the effects of time, cohort, age of the root, site, and their interactions are evaluated. We also test for differences in survival between growing season and over-winter periods (abbreviated as f_season), and for site differences for growing season (abbreviated as f_{site*growing season}) and over-winter periods (abbreviated as f_site*winter) separately. Mean life span (MLS) of roots imaged during the growing seasons are computed from survival estimates outputted from models adjusted for interval lengths (MLS = - 1/ln(f_t)), which typically varies from 20 to 35 days during the growing season. The standard error of MLS is calculated using the “delta method” as described by Seber (1982).

Data sets are modeled as “live recaptures” with all recapture parameters fixed at 1, since we are assured of “recapturing” a root unless it has decomposed. We are confident in this known-fate approach, because it is unlikely that many roots would appear, die and completely decompose between sampling periods. The exception to this is perhaps the over-winter period. In addition to modeling fine root longevity, we also estimate the probability that dead fine roots will decompose between successive time periods by modeling the “survival” of dead roots. This is accomplished by recoding the data to create encounter history files which identified dead roots as “1”, and roots at time of first disappearance as “0”. Fine root decomposition estimates are then simply taken as 1 - dead root survival. A similar model selection approach to that used for fine root longevity is then used to determine factors affecting fine root decomposition (Ruess et al. 1998). To evaluate the effects of root longevity on decomposition rate, we write a separate dBASE program that estimated the life span of every root. Roots are assumed to have been born half way through the period prior to first appearance. Similarly, the death or disappearance of a root is assumed to have occurred half way through the period prior to final appearance as live. Age at death is then entered as a continuous individual covariate in MARK to examine both linear and non-linear constraints on dead to missing survival analysis (Schmutz and Ely 1999). Individual covariates are standardized in MARK to have a mean between 0 and 1, to ensure that the numerical optimization algorithm identified the correct parameter estimates. We then test the hypothesis that age at death, in combination with other factors, influenced decomposition rate using LRTs with more reduced models.

Fine Root Respiration

Fine root respiration is measured in the field as CO₂ production (Burton et al. 2002; Ruess et al. 2003). Fine roots (< 1mm diameter, approximately 2 g fresh weight) are excised from the near-surface fibric horizon, brushed free of soil and organic material, and placed immediately in an aluminum cuvette (5.4 cm diameter X 15 cm in length) coupled to a LICOR-6400 photosynthesis system (LICOR, Lincoln, NE). The cuvette contains a solid aluminum plug (263 cm³), and is cooled on ice prior to measurements to facilitate equilibration of the cuvette to ambient soil temperatures. Both soil and cuvette temperatures are monitored and cuvette temperature is adjusted to ambient soil temperature prior to measurements by burying the cuvette closer to or away from the cold soil. This enables root respiration to be measured at ambient soil temperatures, so that thermal acclimation responses throughout the growing season can be incorporated into our seasonal temperature response relationship (Tjoelker et al. 1999). Cuvette CO₂ concentration is adjusted to 1000 ppm and respiration rates are measured after stabilized readings were observed, which typically took 10 minutes. Root respiration is measured on 4 to 5 root samples collected at locations selected randomly at each site during each sampling period. After measurements, roots are placed on ice and transported to the laboratory, where samples are washed free of adhering soil and organic matter, dried at 60 °C for 48 hr, and analyzed for total N on a LECO 2000 CNS Autoanlyzer (St. Joseph, MI).

Respiration rates (nmol CO₂ g_DWT fine root^-1 s^-1) collected from all sites throughout the season are related to soil temperatures using non-linear regression (PROC NLIN) (SAS, 1999). The slope, k, of the non-linear regression (R_FR = a * e^-k*TSOIL) is used to estimate Q₁₀, (Q₁₀ = e^10*k), and the standard error of Q₁₀, SE(Q₁₀), is derived as SE(Q₁₀) = SE(k) * 10(Q₁₀). An estimate of total growing season (15 May to 15 September) fine root respiration (g C m^-2 growing season^-1) is obtained by combining this equation with daily soil temperature means, and live fine root biomass values obtained from soil cores. Soil cores (5 cm diameter * 20 cm length) are taken from each site and hand-sorted into live and dead roots.