Estimating root respiration, microbial respiration in the rhizosphere, and root-free soil respiration in forest soils

Abstract

We hypothesized that respiration measurements made using both the basal and excised-root respiration methods would allow us to quantify three important components of soil respiration: root respiration (R_root), microbial respiration in the rhizosphere (R_rhizo), and root-free soil respiration (R_rfs). Root respiration determined by the basal method was approximately one-third greater than root respiration determined by the excised-root method (52 versus 32% of total soil respiration, respectively). Results from a decomposition model constructed for the root C fractions (easily decomposable, slowly decomposable, and recalcitrant) showed that the easily and slowly decomposable C fractions disappeared approximately 3 months after the basal respiration measurements began. Since these C fractions contained the majority of the C source for rhizosphere microorganisms (i.e., rhizodeposition), microbial respiration in the rhizosphere must have been severely reduced, indicating that respiration measured after 3 months must be dominated by microorganisms in the root-free soil (R_rfs); this indicated that the basal method actually measured root-free soil respiration. This allowed us to fractionate the components of soil respiration based on the relationship: total soil respiration (R_total)=R_root+R_rhizo+R_rfs; subtracting R_{root(excised-root method)} and R_{rfs(basal method)} from R_total gave us an estimate of microbial respiration in the rhizosphere; and the contribution of these three components to R_total: R_root=32%, R_rhizo=20%, and R_rfs=48%. These results are important because they suggest a way that soil respiration can be separated into at least three functionally different components, and they show that microbial respiration in the rhizosphere is a significant sink for photosynthetically-fixed C in forests.

Introduction

Researchers have been interested in measuring soil respiration in forested ecosystems for many years. Soil respiration measurements are used for developing forest carbon budgets (e.g., Edwards, 1982; Tate et al., 1993), determining mineralization rates (e.g., Kelly and Strickland, 1984) and as an index of root respiration (e.g., Raich et al., 1990; Vose et al., 1995). Interpretation of soil respiration data for the previously stated and other objectives is confounded by an inability to differentiate biologically-relevant soil respiration compartments (e.g., root and microbial) based solely on a measurement of total soil respiration (Bowden et al., 1993).

At a minimum, we can recognize three biologically relevant compartments among which C is transferred in soils, and thus should be accounted for in most soil respiration studies in forests. Bazin et al. (1990)defines these compartments as root tissue, rhizosphere, and root-free soil. The root tissue compartment is the most easily distinguished, consisting of living roots bounded by the soil matrix. The rhizosphere compartment is more diffuse, being roughly defined as the region of soil under the influence of the root (Paul and Clark, 1989, pp. 81–82) inhabited by a relatively large microbial community which utilizes root-derived organic matter as the primary energy substrate. This root-derived organic matter, defined as rhizodeposition, consists of water-soluble root exudates, secretions, lysates (e.g., cell walls and root turnover), and gases (Whipps, 1990). The root-free soil compartment consists of a much smaller microbial community relative to the rhizosphere; this microbial community obtains its energy from secondary products which diffuse into the root-free soil from the rhizosphere and organic matter from above ground litter.

When scientists separate root respiration from microbial respiration in forest soils they usually do not attempt to distinguish rhizosphere respiration from root-free soil respiration; microbial respiration is considered as one compartment in a two-compartment model consisting of root respiration and microbial respiration (Raich and Nadelhoffer, 1989) (Fig. 1A). We know through studies of root production and turnover in forest ecosystems that rhizodeposition via fine-root turnover accounts for approximately 50% of the annual C allocated belowground (Edwards and Harris, 1977; Persson, 1978; Fogel and Hunt, 1983). What is not known is how much additional C is transferred from the root to the rhizosphere via other rhizodeposition materials. This lack of accounting is mainly due to inherent difficulties in differentiating between root, rhizosphere, and root-free soil respiration (Helal and Sauerbeck, 1991).

Carbon 14 pulse-labeling experiments with agricultural crops suggest that other rhizodeposition materials such as root exudates may account for a significant amount of plant C lost via rhizodeposition (Cheng et al., 1993); however, Rygiewicz and Andersen (1994)suggested that a negligible amount of C was lost from rhizodeposition via root exudation based on the results of a ¹⁴C pulse-labeling study of carbon allocation in greenhouse-grown ponderosa pine seedlings (Pinus ponderosa Laws). Such discrepancies between results illustrate the need for continued research on C losses through rhizodeposition, but techniques such as ¹⁴C pulse-labeling cannot be practically applied in forest systems.

We suggest that the root tissue, rhizosphere, and root-free soil compartments as defined by Bazin et al. (1990)can be separated using a combination of easily applied techniques to arrive at a three-compartment model for soil respiration (Fig. 1C). This methodology is based on direct measurements of live root respiration and root-free soil respiration, with microbial respiration in the rhizosphere being determined by difference.

Three general methods exist for measuring root respiration: in situ, excised-root, and the basal method. In situ measurements involve excavating live roots without severing them, placing them in a cuvette, and measuring the respiration rate with an infrared gas analyzer (IRGA) (Sisson, 1983; Cropper and Gholz, 1991). In the excised-root method, soil core samples are taken and the roots are removed; root respiration is then measured either using the Warburg method (Steinbeck and McAlpine, 1966) or with an IRGA. Microbial respiration can then be calculated by subtracting root respiration from total soil respiration (Fig. 1A). The basal method (Marshall and Perry, 1987) uses the two-compartment soil respiration model directly. A volume of soil is isolated with a trenched plot and all aboveground biomass is removed. Total soil respiration is measured periodically. As the freshly-severed root material decomposes, total soil respiration decreases, eventually reaching a horizontal asymptote. The respiration rate at this asymptote represents soil microbial respiration or “basal” respiration. Root respiration is then obtained by subtracting the basal respiration rate from total soil respiration (Fig. 1B).

Each of these methods have potential artifacts associated with them: the in situ method assumes that the cuvette does not alter environmental conditions that in part regulate respiration; the excised-root method assumes that severing the roots prior to inserting them into the IRGA does not change the respiration rate; and the basal method assumes that killing the roots will not affect microbial respiration. Considering the potential artifacts associated with each method, and the relative ease of measuring multiple samples, we believe that the excised-root method is the most reliable and practical procedure for measuring root respiration. This contention is based on a study which showed that respiration rates of excised and unsevered roots remained similar for approximately 2 h before the rates began to diverge (Bloom and Caldwell, 1988).

We have compared the excised-root and basal respiration methods. It was hypothesized that the basal method would overestimate root respiration through elimination of the rhizosphere respiration compartment. It was further hypothesized that the basal method could be used in conjunction with the excised-root method to partition respiration among three compartments (Fig. 1C): root respiration (R_root) determined by the excised-root method, root-free soil respiration (R_rfs) determined by the basal respiration method and rhizosphere respiration (R_rhizo) determined by subtracting the sum of R_root and R_rfs from total soil respiration (R_total).