Abstract
Background and aims
Trees allocate a high proportion of assimilated carbon belowground, but the partitioning of that C among ecosystem components is poorly understood thereby limiting our ability to predict responses of forest C dynamics to global change drivers.
Methods
We labeled sugar maple saplings in natural forest with a pulse of photosynthetic 13C in late summer and traced the pulse over the following 3 years. We quantified the fate of belowground carbon by measuring 13C enrichment of roots, rhizosphere soil, soil respiration, soil aggregates and microbial biomass.
Results
The pulse of 13C contributed strongly to root and rhizosphere respiration for over a year, and respiration comprised about 75 % of total belowground C allocation (TBCA) in the first year. We estimate that rhizosphere carbon flux (RCF) during the dormant season comprises at least 6 % of TBCA. After 3 years, 3.8 % of the C allocated belowground was recovered in soil organic matter, mostly in water-stable aggregates.
Conclusions
A pulse of carbon allocated belowground in temperate forest supplies root respiration, root growth and RCF throughout the following year and a small proportion becomes stabilized in soil aggregates.
Similar content being viewed by others
Explore related subjects
Discover the latest articles, news and stories from top researchers in related subjects.Avoid common mistakes on your manuscript.
Introduction
Global changes in atmospheric CO2 concentration and climate have stimulated increased interest in understanding the carbon dynamics of ecosystems. Forests play a particularly prominent role in the global C cycle acting as a significant C sink (Houghton 2005) and responding to changing atmospheric CO2 concentration (Korner et al. 2005) and climate change (Boisvenue and Running 2006). Uncertainty about the short-and long-term fate of photosynthetically-fixed C in forests, and especially soils, constrains our ability to predict the interactions and feedbacks among forests, atmospheric CO2 and climate. In particular, predictions of the response of ecosystem respiration and soil C sequestration to environmental changes are limited by incomplete understanding of how plant carbon is allocated to and utilized by different ecosystem components (Carbone et al. 2007). Photosynthetic C is allocated within trees to supply cellular growth and respiration, and a large proportion of the assimilated C is allocated belowground where it enters a variety of pools with varying turnover rates including, fine and coarse roots, microbial biomass, soil organic matter (Keel et al. 2006). Although the magnitude of total belowground carbon allocation (TBCA) is well known from ecosystem C flux measurements (Raich and Nadelhoffer 1989, Davidson et al. 2002), the partitioning of TBCA among soil components and fluxes is not well understood. Because the magnitude and timing of belowground C allocation and its subsequent transformations are likely to be sensitive to changes in environment and disturbance regimes, better understanding of the partitioning of belowground C in forests is needed.
Total soil respiration (TSR) consists of respiration by roots and mycorrhizae and by soil heterotrophs and typically comprises over half of total ecosystem respiration in temperate and boreal forests (Granier et al. 2000, Hogberg et al. 2001). Soil heterotrophic respiration is fueled by root turnover and aboveground litterfall as well as rhizosphere carbon flux (RCF). Recent studies indicate that TSR in temperate forests is supplied in roughly equal proportions by root/mycorrhizal and heterotrophic respiration (Hanson et al. 2000, Hogberg et al. 2002). Although the rapid link between root and rhizosphere respiration and photosynthetic C assimilation has been demonstrated (Ekblad and Hogberg 2001, Steinmann et al. 2004), a significant proportion of root processes is supplied by carbon reserves temporarily stored in the trees (Gaudinski et al. 2009). Distinct seasonal allocation patterns in temperate zone trees allow the use of excess assimilates to fuel maintenance respiration in the dormant season, new growth in the spring and subsequent growth flushes in summer (Dickson 1991). This role of tree C reserves may be particularly sensitive to stressful conditions associated with environmental change and natural disturbances (Gaudinski et al. 2009).
The transport of photo-assimilated C to roots and the long-term fate of TBCA in forests has received limited study (Endrulat et al. 2010) in part because of practical problems of measurement. Recent syntheses (Jones et al. 2009) have argued that isotope pulse-labeling approaches are most effective for this purpose. The advantage of using 13C for this study is that the pulse of 13C could be detected even in the large plant and soil pools with high precision for a relatively long time. An analogous approach based on an inadvertant release of radioactive 14C provided parallel results but with attendant environmental hazards (Gaudinski et al. 2009). Based upon the observations from tree girdling experiments of the rapid link between photosynthetic C fixation and soil respiration (Hogberg et al. 2001), we hypothesized that the 13C label would be detected in a large pulse of root and rhizosphere respiration immediately after labeling in early fall. Based on principles of tree assimilate distribution and storage (Dickson 1991), we hypothesized that root growth and respiration would be supplied throughout the following winter and summer from stored labeled C. We also hypothesized that the labeled, plant labile pools would be largely depleted by the end of the first growing season (Gaudinski et al. 2009), resulting in constant, low enrichment of TSR supplied mostly by heterotrophic activity. We also expected RCF (Grayston et al. 1997) to result in C accumulation in the rhizosphere soil, and together with turnover of roots and microbes, RCF would supply relatively stable soil carbon stored in soil aggregates (Six et al. 2000).
Materials and methods
Site and 13C pulse labeling
Sugar maple trees growing in a natural forest stand in the Arnot Forest in central New York were labeled with 13C. The Arnot Forest is a hardwood forest on the northern Allegheny Plateau (Fain et al. 1994). Annual precipitation averages 90 cm and average June and December temperatures are 22° and −4 °C, respectively. Soils are acidic Dystrochrepts with pH 4.5–5.0. The site chosen for study was in a ca. 90-yr-old stand that had been selectively harvested in 2000, leaving 50 overstory trees per ha; this thinning released a dense understory of young sugar maple trees. At the time of labeling in September 2006, this sapling layer averaged 2.5 m in height.
The labeling procedure has been described in detail by Horowitz et al. (2009). Four aluminum-frame chambers (2.5 m tall x 3 m diameter) were positioned over pure sugar maple sapling groves (about 10 stems/m2) in June 2006. The maximum distance between the chambers was 50 m, and all were on moderately-well drained soils on a gentle NE-facing slope. Although sugar maple saplings were ubiquitous across the stand, these groves were more uniformly pure sugar maple than average. The soil and root systems in each chamber were isolated by trenching around the chambers to 0.5 m depth, lining the trenches with 6 mil polyethylene and backfilling with soil. After enclosing the chambers with 6 mil polyethylene, the saplings were labeled with 13C by injecting 40 atom % enriched 13CO2 into each chamber on 13 sunny days between 1 and 20 September 2006 (hereafter, “year 0”); this procedure was designed to maximize the incorporation of the label into plant tissues. The procedure involved sealing the tops of the chambers at dawn on each day, scrubbing ambient CO2 in chamber atmosphere to ca. 60 ppm and then adding 13CO2 until total CO2 concentration reached about 500 ppm. Chamber tops were removed each day after about 0.5 to 1 h when chamber CO2 was reduced by photosynthesis to nearly constant concentration.
Sample collection and processing
All freshly-fallen leaf litter was removed from the chambers in fall of year 0 (2006) and year 1 (2007) for use in a separate experiment (Fahey et al. 2011). The total dry mass of leaf litter in each year was measured and subsamples were obtained for isotope analysis. Unlabeled sugar maple litter from an adjacent, 90-yr-old sugar maple stand was added in each year to replace the removals.
Roots were collected from the 0–5 cm soil depth on three dates after leaf senescence in fall of year 0 (10 October, 24 October, 10 November), on two dates prior to leaf out in spring (30 April, 15 May) and in late summer (15 August) of year 1. These samples were obtained by loosening soil and excavating small root fans with attached rhizosphere soil. The aim was to obtain roots and rhizosphere soil while minimizing soil disruption which was a recurring challenge within the chambers of limited size. Adhering rhizosphere soil was manually removed (Phillips and Fahey 2006), roots were sorted visually into fine (<1 mm) and coarse (1–5 mm) classes, and samples were dried (55 °C) and stored for isotope analysis. Roots and soil were also sampled on 1 June and 15 October of year 2 (2008) and 15 October of year 3 (2009). For these samples four soil cores (5 cm diameter) were collected to 10 cm depth, roots were manually separated from soil and sorted by size class, and roots and soil were dried, pooled within chambers and stored for isotope analysis. Roots and soil for natural abundance measurements were collected in fall of year 0 in adjacent areas outside the chambers. Four cores were collected by the same method and processed as above.
Root in-growth cores were used to obtain newly-grown fine roots for isotope analysis over four intervals: 15 April to 13 July and 13 July to 2 October of year 1, and 15 November to 15 May and 1 June to 15 October of year 2. On each date two soil cores (5 cm diameter) were excavated to 10 cm depth in each chamber, soil was sieved to remove roots, the core hole was lined with coarse mesh and sieved soil was backfilled. Root ingrowth samples were sieved from soil, washed with d H2O, dried and stored for isotope analysis. Reference samples were also obtained from four ingrowth cores in adjacent areas outside the chambers.
We collected CO2 emitted from soil in the chambers and from adjacent reference sites using a NaOH trap method (Phillips and Fahey 2005). Sampling was conducted on two dates in fall of year 0 (26 September, 7 November), seven dates from April to October of year 1 and seven dates from June to November of year 2. The CO2 emitted from the soil was trapped in 10 mL of 0.1 mol/L NaOH inside two sealed 5-cm diameter cylinders in each chamber over an 8 h daytime incubation period. Samples were pooled within chambers, sealed in vials and stored for isotope analysis. Following the addition of 1 M BaCl2, the base trap solution was titrated with 0.1 M HCl. To collect the BaCO3 precipitate for 13C analysis, the precipitate was washed with degassed dH2O, centrifuged at 2,200 rpm for 5 min., and the supernatant decanted. This washing procedure was repeated three times and any remaining solution was evaporated at 65 °C for 24 h. 13C was measured on a Finnegan isotope ratio mass spectrometer (IRMS) at the University of Georgia. Vanadium pentoxide was added as a catalyst to ensure complete recovery of 13C during combustion of BaCO3 precipitate (Harris et al. 1997).
Soil CO2 emission was measured using a LI-COR soil respiration chamber (model LI-6000-09; LI-COR BioSciences, Lincoln, Nebraska) and LI-6200 infra-red gas analyzer (Norman et al. 1992). Four soil collars were placed in each chamber and measurements were conducted on seven dates from early June to early November of year 2 following the procedure detailed by Fahey et al. (2005a).
Microbial biomass C and 13C were measured using the chloroform-fumigation method (Jenkinson and Powlson 1976) on 0–5 cm soil cores collected from each chamber in October of year 2. Soils were fumigated to kill and lyse microbial cells and fresh soil was used to inoculate samples with microbes that used lysed cells as substrate. Flushes of CO2 over a 10 day incubation at field moisture content were assumed to be proportional to C in microbial biomass after applying a standard (0.45) proportionality constant. Gas chromatography was used to measure CO2 and a Europa Integra IRMS to measure 13C at the University of California-Davis.
After drying to constant mass at 70o C, tissue and soil samples were finely ground and homogenized for isotope analysis. Leaf litter was ground with a mortar and pestle; fine roots and soil with a ball mill. The isotopic composition (13C) of these samples was measured on a Finnegan IRMS at the Cornell Stable Isotope Laboratory. At each of the IRMS laboratories appropriate NIST standards were used for normalization correction (CBT, HCRN), instrument linearity (methionine) and precision purposes (NIST calibrated rice, poplar, BCBG). Samples from the October year 2 soil collections were processed for analysis of isotope enrichment of soil aggregate fractions following a procedure modified from Fonte et al. (2007). Subsamples of air-dried soil were wet sieved to generate three aggregate size classes: macroaggregates (>250 μm), microaggregates (53–250 μm) and silt plus clay (<53 μm) as well as low-density organic matter that floated in water. The macroaggregate fraction was further separated by wet sieving following the procedure of Six et al. (2000) to yield three size classes: coarse particulate organic matter (>250 um), microaggregates, and silt plus clay. Subsamples of each of these fractions were finely ground and analyzed for total C, and C isotope ratios by IRMS at Cornell, as described above.
Data analysis
Recovery of added 13C in soil pools and fluxes was calculated after correcting for atom % 13C concentration of reference pools and fluxes measured in areas adjacent to the chambers (termed excess 13C). The excess 13C pool in soil was estimated for 0–10 cm depth based on bulk density measured in similar soils at Arnot Forest (0.56 g/cm3; Fahey et al. 2011). For the three dates when rhizosphere soil δ13C was measured (see above), we assumed no enrichment in bulk soil (Keel et al. 2006) and that rhizosphere represented 26 % of soil mass, the figured obtained by Phillips and Fahey (2006) using the same method for nearby sugar maple stands on the same soil type. To account for roots to 30 cm depth, we assumed that roots collected from 0 to 10 cm soil had the same isotope signature as those from 10 to 30 cm. We estimated excess 13CO2 flux from soil respiration on the basis of short-term measurements of δ13CO2 of soil emissions and LI-COR chamber measurements of soil CO2 flux. In general, these calculations were performed using linear interpolations to obtain estimates during the intervals between measurements. For the winter period (December-March) we assumed soil CO2 emission was the same as estimated for nearby sugar maple stands on the same soil type in the Arnot Forest by Fisk et al. (2004), i.e. 16 g C m−2 month−1, and we applied the average of δ13CO2 values from the last autumn and first spring measurement in each year (Fig. 1). Also, spring (April-May) soil CO2 emissions were assumed to mirror those in fall (October-November), as previously indicated for Arnot Forest (Fisk et al. 2004). All the assumptions above result in uncertainty for our budgetary calculations, as detailed in the Discussion.
Statistical significance of differences across dates within years in δ13C of TSR was evaluated using ANOVA. In our ANOVA, year (0, 1, 2, 3) was fixed, month of sampling was random and nested within season (spring, summer, or fall) with chambers as replicates for the dependent variable, δ13CO2. This model was used because of large between-year differences and because the day of the year for sampling differed between years. Statistical differences in δ13C and in the excess 13C pool (relative to natural abundance) in fine roots and rhizosphere soil were evaluated with one-way ANOVA; values of the dependent variables (δ13C and excess 13C) were log transformed to fit normal distribution. Post-hoc comparisons for these ANOVAs were made using Tukey’s test. Differences in δ13C across soil aggregate fractions were evaluated using repeated measures ANOVA as all the values of each replicate were obtained from the same soil sample.
Results
We were able to detect the partitioning of a large pulse of photo-assimilated 13C through the tree-soil system across 3 years. During leaf senescence soon after labeling, 13C in fine and coarse roots rapidly increased (see Horowitz et al. 2009) reaching peak values by November of year 0, but remaining highly enriched in year 1 and in spring of year 2, before declining significantly thereafter (Table 1; ANOVA for fine roots: F6,21 = 11.34, p < 0.001; coarse roots: F6,21 = .90; p < 0.05). By comparison, enrichment of 13CO2 in soil emissions (TSR) declined rapidly from a peak value of 848 ± 151 ‰ 6 days after completion of labeling to −20 ± 1 ‰ by spring of year 2 (Fig. 1). Moreover, during year 1 significant seasonal fluctuations in δ13C of TSR were observed (month * year; F9,71 = 1.95, p < 0.05), with a seasonal minimum in mid-May at the time of leaf emergence, followed by a pronounced peak in early summer. The large rhizosphere soil pool showed the slowest enrichment of 13C; only slight enrichment was observed in fall of year 0 (−23 ± 1 ‰), but it increased markedly over winter (−13 ± 5 ‰) and reached a peak value in late summer of year 1 (−2 ± 6 ‰). During the growing season of year 2 excess 13C in soil declined significantly by over half (F5,18 = 7.62, p < 0.001), with a smaller, non-significant decline in year 3. Two years after labeling the highest δ13C enrichment among soil fractions was in low density particulate organic matter (presumably mostly dead fine roots), and significantly higher enrichment was observed in both macroaggregates and microaggregates than in adsorbed pools on silt plus clay (Fig. 2).
We estimated the fluxes and pool sizes of excess 13C in the plant-soil system through time after labeling correcting for natural abundance in roots, soil and TSR (Table 2). The daily excess 13C flux from soil was calculated as the product of TSR and excess 13C in TSR using the interpolations described in Methods. The largest 13C fluxes in TSR occurred in year 0 immediately after labeling, but high fluxes also were observed in year 1. These fluxes can be compared with the soil and root (<5 mm diameter) pool sizes and temporal changes in their magnitude (Table 2). For example, excess 13C in TSR during year 0 and the growing season of year 1 greatly exceeded accumulation in roots and rhizosphere soil; thereafter the significant decline of excess 13C in roots (F5,18 = 4.44, p < 0.01) and soil was comparable to TSR.
A whole-ecosystem budget of the 13C pulse enrichment also was estimated. Much higher 13C enrichment was observed for year 1 than year 0 leaf litter and the total flux of excess 13C for the 2 years (885 mg 13C/m2) was larger than the maximum pool size in small roots (336 ± 104 mg 13C/m2) or rhizosphere soil (531 ± 111 mg 13C/m2) but smaller than TSR flux during the first 2 years after labeling (2,872 ± 207 mg/m2; Table 1). By comparison the pool of excess 13C in microbial biomass in 0–10 cm soil in fall of year 2 was very small (3.9 mg 13C/m2), although it was highly enriched in 13C (δ13 C – 1 ± 3 ‰). An estimate of TBCA of the 13C pulse is the sum of TSR in year 0/1 (2,340 mg 13C/m2; Table 1) and the maximum pool size in roots and soil (867 ± 215 mg 13C/m2), i.e. 3,207 mg 13C/m2. The immediate allocation of this TBCA to root and rhizosphere respiration is about 40 % based on fall/winter TSR. Allocation to RCF through the dormant season was about 7.5 % of TBCA and allocation to new roots a minimum of 9 % (i.e., based on root pool size in June of year 2 and assuming no turnover of new roots). An additional 9 % of TBCA supplied some of RCF during the following growing season.
Newly-grown fine roots collected using in-growth cores exhibited much lower 13C enrichment than observed for in situ roots. In-growth cores roots collected in July and October of year 1 averaged δ13C −17.8 ± 1.1 ‰ and −16.9 ± 0.9 ‰, respectively. In-growth roots collected in mid-May of year 2 had slightly but not significantly higher δ13C (−13.6 ± 4.6 ‰), but in-growth roots produced during the growing season of year 2 were not enriched compared with natural abundance values (−27.8 ± 0.2 ‰ vs. −27.6 ± 0.2 ‰).
Discussion
We followed a large pulse of photo-assimilated 13C into the belowground components of a young sugar maple forest and quantified the partitioning of this pulse among soil pools and fluxes over 3 years. In particular, we evaluated the temporal sequence of C partitioning to total soil respiration (TSR), root growth, rhizosphere carbon flux (RCF) and relatively stable soil organic matter (SOM). Our observations demonstrated that the pulse of late-season belowground C was allocated to immediate root and rhizosphere respiration during the dormant season (about 40 % of TBCA), but that a large proportion of the pulse was stored in roots and supplied subsequent root respiration, root growth and RCF for an entire growing season. Thereafter, the pulse apparently was utilized in heterotrophic respiration and after 3 years about 3 % of the pulse was stabilized in SOM, mostly in aggregates and particulate organic matter derived from root turnover. This is one of the first reports of the mid-term partitioning of belowground C among forest ecosystem components and will help inform understanding of the likely responses of belowground C dynamics to changes in CO2, climate and disturbance regimes.
Our first hypothesis, based on observations from a boreal evergreen forest (Ekblad and Hogberg 2001), was that photo-assimilated 13C would be rapidly transported belowground to fuel root and rhizosphere respiration. This hypothesis was supported by very high 13C enrichment observed on our first measurement date for TSR 6 days after the last labeling event (20 September; Fig. 1). Although abiotic soil 13CO2 efflux might contribute slightly to this enrichment, observations of Subke et al. (2009) indicated that abiotic efflux from soil pores could be detected for only 2 days under labeling conditions similar to ours.
Enrichment of 13C in TSR remained high during the fall (δ13C = 684 ‰ on 7 November; Fig. 1), illustrating that translocation of photosynthate to root systems occurred both during and after leaf senescence and resulted in a gradual increase of δ13C in fine and coarse roots during the fall (Horowitz et al. 2009). The high metabolic demand of roots contributes to the sink strength driving carbohydrate transport to roots during autumn senescence (Farrar and Jones 2000), but much of this C probably goes into temporary storage pools because minimal root growth of sugar maple occurs after leaf senescence (Tierney et al. 2003). We also observed relatively large accumulation of 13C in rhizosphere soil between October and May (Table 2), indicating that substantial RCF occurs during the dormant season. Cold soil temperatures through most of this interval would suppress heterotrophic utilization of this RCF, promoting its accumulation.
Our second hypothesis was that the fall pulse of TBCA would contribute significantly to root respiration, growth and RCF during some or all of the following growing season, conforming with general principles of tree assimilate distribution and storage (Dickson 1991). Our observations supported this hypothesis, as the 13C enrichment of TSR, fine roots and rhizosphere soil remained very high throughout year 1 (Fig. 1, Table 1, 2). These results corroborate recent observations of Gaudinski et al. (2009) that over half of annual new root growth in a temperate deciduous oak forest was supplied by stored C (vs. fresh assimilate). Similarly, Endrulat et al. (2010) demonstrated the remobilization of starch from a late fall 13C pulse into new fine roots the next spring in a temperate conifer forest. However, the seasonal patterns in 13C enrichment of TSR that we observed during year 1 provided new evidence about the timing of partitioning of stored C in temperate deciduous forest trees. A pronounced (though non-significant) decline in δ13CO2 of TSR during leaf out in May was followed by a large and significant (p < 0.05) increase during early summer. The early decline coincided with a decline in coarse root δ13C (Table 1) presumably reflecting the mobilization of stored C to the strong aboveground C sink (Cannell and Dewar 1994); perhaps the coincident decline in δ13C of TSR reflected consequent diversion of stored C from roots. The early summer peak in δ13C of TSR coincides with the peak of root growth in sugar maple and other temperate deciduous trees (Joslin et al. 2001, Tierney et al. 2003), strongly suggesting that new root growth and additional growth respiration (Veen 1981) are substantially dependent on stored carbon. Thus, any factors reducing the availability of stored C (e.g., canopy disturbance, late-summer drought) might be likely to disrupt peak root production during the growing season, with possible consequences for soil resource acquisition. Conversely, extension of the growing season in fall owing to climate warming would be likely to increase stored C and its allocation to RCF and root production.
The sustained high 13C enrichment of fine root biomass from year 0 through year 1 and spring of year 2 presumably reflects a complex balance between remobilization of root storage pools (Endrulat et al. 2010) and coincident growth and mortality of fine roots. Although our measurements cannot distinguish among these processes, it is notable that in fall of year 1, δ13C of structural fiber (mostly lignocellulose) in very fine roots (root order 1–2; Fahey et al. 2012) was 93.32 ±11.18 ‰, suggesting that stored 13C was allocated to the growth of these short-lived roots (Hendrick and Pregitzer 1993, Tierney and Fahey 2002), and supporting our earlier assertion that the early summer, year 1 peak in δ13C of TSR was associated with stored 13C allocated to root growth respiration.
The strikingly lower 13C enrichment of newly-grown fine roots collected from in-growth cores (range = −13.56 to −17.80 ‰ during year 1 and spring of year 2) compared with in situ roots (59.62 to 88.13 ‰; Table 1) supports recent observations of the complexity and heterogeneity of C supply to new root growth (Guo et al. 2004, Joslin et al. 2006, Vargas et al. 2011). Clearly, roots in the in-growth cores were grown almost entirely with new photoassimilate. The 13C in the in situ roots consisted of both stored 13C (i.e. reflected in their high initial enrichment; Table 1), as well as growth of new daughter roots derived from this stored 13C. One possible explanation of this striking difference is based on the observation of Eissenstat and Achor (1999) that most of the roots colonizing in-growth cores are “pioneer” roots derived from the cut ends of a few lateral roots. If these pioneer roots and their daughter laterals are supplied mostly by new photoassimilate whereas new fibrous, lateral roots arising directly from other mother laterals receive lots of C stored in those laterals, then such a large contrast in 13C enrichment could occur. This observation emphasizes the need to use caution when interpreting root growth and production measured with in-growth cores (Tierney and Fahey 2007).
Between fall of year 0 and spring of year 1 δ13C of TSR declined markedly and remained relatively constant supporting our third hypothesis that nearly all of the labile 13C storage in the trees would be exhausted during year 1. Leaf and fine root tissues produced during the growing season in year 2 were not significantly higher than natural abundance values; hence, 13C enrichment of TSR in year 2 and 3 probably represented almost entirely heterotrophic metabolism of RCF and decaying dead roots. The pool of 13C in rhizosphere soil more than doubled during the growing season of year 1, suggesting that RCF (including C allocation to extra-matrical mycorrhizal hyphae) greatly exceeded its utilization by soil heterotrophs. This observation clearly indicates that a substantial portion of RCF is derived from stored photosynthate from the previous growing season. Undoubtedly some turnover of labeled fine roots occurred during this interval, but based on the method we employed to collect rhizosphere soil (i.e., brushing adhering soil from roots), root turnover was probably not a major source of 13C in soil samples from August of year 1. However, during years 2 and 3 bulk soil samples were used to quantify the soil 13C pool; these samples would have included considerable root detritus. Continued heterotrophic processing of root-derived carbon was indicated by the parallel decline of 13C enrichment of both soil and fine root pools during years 2 and 3, as well as the continued elevated 13CO2 flux in TSR (Table 2). Notably, 13C enrichment of microbial biomass (−0.967‰ ± 2.705) was much higher than for TSR in fall of year 2.
Rhizosphere carbon flux is recognized as a major soil carbon flux in forest ecosystems (Grayston et al. 1997), but accurate measurement is notoriously difficult. Phillips and Fahey (2005) used a 13C pulse-labeling method and estimated that RCF during the growing season comprised 7 % of net photosynthesis in sugar maple saplings. Our conservative estimate of RCF during fall and winter was about 7.5 % of TBCA. In addition, the observation that 13C continued to accumulate in the rhizosphere during the growing season in year 1 (representing 9 % of TBCA) suggests that RCF is considerably greater than estimated by Phillips and Fahey (2005). For example, assuming that TBCA represents about 55 % of net photosynthesis in sugar maple dominated northern hardwood forest (Fahey et al. 2005b), we would estimate that total annual RCF is at least 16 % of net photosynthesis in young sugar maple forest. A substantial proportion of RCF enters non-labile pools including, for example, glycoproteins produced by arbuscular mycorrhizal trees like sugar maple (Wright and Upadhyaya 1996).
In support of our fourth hypothesis that a substantial proportion of the TBCA pulse would contribute to relatively stable soil C, 3 years after pulse labeling about 3.8 % of TBCA was recovered in SOM. Separation of SOM into aggregate fractions (Fig. 2) illustrated the processes resulting in this mid-term stabilization of belowground C. The highest 13C enrichment was observed in low density particulate organic matter (presumably mostly dead roots) and in macroaggregates and free microaggregates. Lower enrichment was noted for silt plus clay fractions (Fig. 2). Both RCF and the rapid turnover of microbial biomass and mycorrhizal hyphae may contribute to the enrichment of aggregate fractions and the possible role of glycoproteins associated with the arbuscular mycorrhizae of sugar maple in formation and stabilization of aggregates is noteworthy (Rillig 2004).
We emphasize that our budgetary estimates are subject to several sources of uncertainty and error associated with the assumptions described under Methods. First, to estimate 13C flux of TSR we used linear interpolations between measurement dates both for TSR and δ13CO2 of TSR. Perhaps most important for flux estimates was the overwinter interpolation (November-April); although winter TSR is undoubtedly low in soils near 0° (Groffman et al. 2006), δ13CO2 was high (Fig. 1). If the change in δ13CO2 departed from the linear decline that we assumed, significant error in the TSR 13C flux would result. Second, we assumed that rhizosphere soil comprised 26 % of soil mass (Phillips and Fahey 2006); departures from this assumption would result in proportional errors in RCF estimates. Moreover, our approach would overestimate RCF to the extent that root tissues may have been included in rhizosphere soil, and it would underestimate RCF because of heterotrophic utilization of rhizosphere C. The challenges of quantifying RCF are well known (Jones et al. 2009). Third, to calculate the root 13C pool we assumed that 10–30 cm roots had the same δ13C as 0–10 cm roots and we ignored deeper roots. Because roots of different orders form differently (Guo et al. 2008), some error from this assumption is likely. However, if deeper roots had lower δ13C this source of error would be counterbalanced by the additional mass of roots at depths >30 cm. Finally, variation in the δ13C signature of both rhizosphere soil and roots was highly variable (Table 1, 2) both within chambers and across chambers, contributing to uncertainty in pool sizes and fluxes.
Our budgetary estimates can be compared to some literature values obtained with different methods to help evaluate confidence in pool sizes and fluxes. In a summary of TBCA for global forests based on measurements of TSR and aboveground litterfall, Davidson et al. (2002) reported a mean value of 3.8 (range = 1.5–6.8) for the ratio of TBCA/aboveground litterfall C flux. Our estimate of this ratio for excess 13 flux is 3.6 (i.e., 3,207/885 mg 13C/m2; Table 2). This value is somewhat higher than for a complete C budget for a sugar maple forest in NH, USA (2.23; Fahey et al. 2005a); our value may overestimate proportional TBCA because aboveground C demand at the time of labeling in late summer would be lower than the growing season average. Hogberg et al. (2002) estimated that about 75 % of TBCA goes to root and soil respiration in a boreal pine forest. Our comparable figure is 73 % based on year 0 + 1 TSR and maximum soil and root pool sizes (i.e., 2,340/3,207 mg 13C/m2). Estimation of this proportion is complicated in our approach by coincident turnover occurring in the root and rhizosphere pools; that is, our peak 13C pool size estimate of RCF and root production does not account for 13C supply to these pools that counterbalances losses to root mortality or heterotrophic respiration. Nevertheless, these comparisons suggest that our C budget estimates are reasonably accurate.
In conclusion, ours is the first study to trace a large pulse of photo-assimilated 13C into belowground pools and fluxes over the long term in a natural forest, thereby providing useful insights for predicting responses of forest C dynamics to environmental change. We show that stored C from late summer photosynthesis is utilized for root growth and RCF during the following growing season. Thus, these processes could be particularly sensitive either to disruption or enhancement of late-season photosynthetic activity associated with changing climate or canopy disturbance. Our results also indicate substantial RCF during the dormant season (over 6 % of TBCA). Thus, accounting for growing season RCF of stored TBCA plus immediate allocation of fresh photosynthate to RCF (Phillips and Fahey 2005), we suggest that total RCF in young sugar maple forest exceeds 16 % of net photosynthesis. Together with fine root turnover this RCF supplies C sequestered in stabilized SOM. After 3 years nearly 4 % of the photosynthetic 13C pulse was retained in mineral soil mostly in microaggregates and macroaggregates. Longer term tracing of this pulse in SOM would be useful. Moreover, studies of how the partitioning of TBCA among soil ecosystem components varies across the growing season would provide a more complete picture of belowground carbon dynamics in temperate forests.
References
Boisvenue C, Running SW (2006) Impacts of climate change on natural forest productivity—evidence since the middle of the 20th century. Global Change Biol 12:862–882
Cannell MGR, Dewar RC (1994) Carbon allocation in trees: A review of concepts for modeling. Adv Ecol Res 25:60–103
Carbone MS, Czimczik CI, McDuffee KE, Trumbore SE (2007) Allocation and residence time of photosynthetic products in a boreal forest using a low-level 14C pulse-chase labeling technique. Global Change Biol 13:466–477
Davidson EA, Savage K, Bolstad P, Clark DA, Curtis PS, Ellsworth DS, Hanson PJ, Law BE, Luo Y, Pregitzer KS, Randolph JC, Zak D (2002) Belowground carbon allocation in forests estimated from litterfall and IRGA-based soil respiration measurements. Agr For Met 113:39–51
Dickson RE (1991) Assimilate distribution and storage. In: Raghavendra AS (ed) Physiology of Trees. John Wileys and Sons, New York, pp 51–85
Eissenstat DM, Achor DS (1999) Anatomical characteristics of citrus rootstocks that vary in specific root length. New Phytol 141:309–321
Ekblad A, Hogberg P (2001) Natural abundance of 13C in CO2 respired from forest soils reveals speed of link between tree photosynthesis and root respiration. Oecologia 127:305–308
Endrulat T, Saurer M, Buchmann N, Brunner I (2010) Incorporation and remobilization of 13C within the fine root systems of individual Abies alba trees in a temperate coniferous stand. Tree Physiol 30:1515–1527
Fahey TJ, Jacobs KR, Sherman RE (2012) Fine root turnover in sugar maple estimated by 13C isotope dilution. Can J For Res doi:10.1139/x2012-128
Fahey TJ, Yavitt JB, Sherman RE, Groffman PM, Fisk MC, Maerz JC (2011) Transport of carbon and nitrogen between litter and soil organic matter in a northern hardwood forest. Ecosystems 14:326–340. doi:10.1007/s10021-011-9414-1
Fahey TJ, Tierney GL, Fitzhugh RD, Wilson GF (2005a) Soil respiration and soil carbon balance in a northern hardwood forest ecosystem. Can J For Res 35:244–253
Fahey TJ, Siccama TG, Driscoll CT, Likens GE, Campbell J, Johnson CE, Aber JD, Cole JJ, Fisk MC, Groffman PM, Hamburg SP, Holmes RT, Schwarz PA, Yanai RD (2005b) The biogeochemistry of carbon at Hubbard Brook. Biogeochemistry 75:109–176
Fain JJ, Volk TA, Fahey TJ (1994) Fifty years of change in an upland forest in south-central New York: General patterns. Bull Torrey Bot Club 121(2):130–139
Farrar JF, Jones DL (2000) The control of carbon acquisition by roots. New Phytol 147:43–53. doi:10.1046/j.1469-8137.2000.00688.x
Fisk MC, Fahey TJ, Groffman PM, Bohlen PJ (2004) Earthworm invasion, fine- root distributions, and soil respiration in north temperate forests. Ecosystems 7:55–62
Fonte SJ, Kong AYY, vanKessel C, Hendrix PF, Six J (2007) Influence of earthworm activity on aggregate-associated carbon and nitrogen dynamics differs with agroecosystem management. Soil Biol Biochem 39:1014–1022
Gaudinski JB, Torn MS, Riley WJ, Swanston C, Trumbore SE, Joslin JD, Majdi H, Dawson TE, Hanson PJ (2009) Use of stored carbon reserves in growth of temperate tree roots and leaf buds: Analyses using radiocarbon measurements and modeling. Global Change Biol 15(4):992–1014
Granier A, Ceschia E, Damesin C, Dufrene E, Epron D et al (2000) The carbon balance of a young Beech forest. Funct Ecol 14:312–325
Grayston SJ, Vaughn D, Jones D (1997) Rhizosphere carbon flow in trees, in comparison with annual plants: The importance of root exudation and its impact on microbial activity and nutrient availability. Appl Soil Ecol 5:29–56
Groffman PM, Hardy JP, Drisoll CT, Fahey TJ (2006) Snow depth, soil freezing and trace gas fluxes in a northern hardwood forest. Global Change Biol 12:1748–1760
Guo D, Li H, Mitchell RJ, Wenxuan H, Hendricks JJ, Fahey TJ, Hendrick RL (2008) Fine root heterogeneity by branch order: Exploring the discrepancy in root turnover estimates between minirhizotron and carbon isotopic methods. New Phytol 177:443–456
Guo D, Mitchell R, Hendricks J (2004) Fine root branch orders respond differentially to carbon source-sink manipulations in a longleaf pine forest. Oecologia 140:450–457. doi:10.1007/s00442-004-1596-1
Hanson PJ, Edwards NT, Garten CT, Andrews JA (2000) Separating root and soil microbial contributions to soil respiration: A review of methods and observations. Biogeochemistry 48:115–146
Harris D, Porter LK, Paul EA (1997) Continuous flow isotope ratio mass spectrometry of carbon dioxide trapped as strontium carbonate. Comm Soil Sci Plant 28:747–757
Hendrick RL, Pregitzer KS (1993) The dynamics of fine root length, biomass, and nitrogen content in two northern hardwood ecosystems. Can J For Res 23:2507–2520. doi:10.1139/x93-312
Hogberg P, Nordgren A, Agren GI (2002) Carbon allocation between tree root growth and root respiration in boreal pine forest. Oecologia 132:579–581
Hogberg P, Nordgren A, Buchmann N, Taylor AFS, Ekblad A, Hogberg MN, Nyberg G, Ottosson-Lofrenius M, Read DJ (2001) Large-scale forest girdling shows that current photosynthate drives soil respiration. Nature 411:789–791
Horowitz ME, Fahey TJ, Yavitt JB, Feldpausch T, Sherman RE (2009) Patterns of late-season photosynthate movement in sugar maple (Acer saccharum Marsh) saplings. Can J For Res 39:2294–2298
Houghton RA (2005) Aboveground forest biomass and the global carbon balance. Global Change Biol 11:945–958
Jenkinson DS, Powlson DS (1976) The effects of biocidal treatments on metabolism in soil. V. A method for measuring soil biomass. Soil Biol Biochem 8:209–213
Jones DL, Nguyen C, Finlay RD (2009) Carbon flow in the rhizosphere: Carbon trading at the soil-root interface. Plant Soil 321:5–33
Joslin JD, Gaudinski JB, Torn MS, Riley WJ, Hanson PJ (2006) Fine-root turnover patterns and their relationship to root diameter and soil depth in a 14C-labeled hardwood forest. New Phytol 172:523–535. doi:10.1111/j.1469-8137.2006.01847.x
Joslin JD, Wolfe MH, Hanson PJ (2001) Factors controlling the time of root elongation intensity in a mature upland oak stand. Plant Soil 228:201–212
Keel SG, Siegwolf RTW, Korner C (2006) Canopy CO2 enrichment permits tracing the fate of recently assimilated carbon in a mature deciduous forest. New Phytol 172:319–329
Korner C, Ashoff R, Bignucolo O, Hatten-Schwiler S, Keel SG, Palaez-Ridel S, Pepin S, Siegwolf RTW, Zotz G (2005) Carbon flux and growth in mature deciduous forest trees exposed to elevated CO2. Science 309:1360–1362
Norman JM, Garcia R, Verma SB (1992) Soil surface CO2 fluxes and the carbon budget of a grassland. J Geophys Res 97:18845–18853
Phillips RP, Fahey TJ (2006) Tree species and mycorrhizal associations influence the magnitude of rhizosphere effects. Ecology 87(5):1302–1313
Phillips RP, Fahey TJ (2005) Patterns of rhizosphere carbon flux in sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) saplings. Global Change Biol 11:983–995
Raich JW, Nadelhoffer KJ (1989) Belowground carbon allocation in forest ecosystems: Global trends. Ecology 70:1346–1354
Rillig MC (2004) Arbuscular mycorrhizae, glomalin, and soil aggregation. Can J Soil Sci 84:355–363
Six J, Elliott ET, Paustian K (2000) Soil macroaggregate turnover and microaggregate formation: A mechanism for C sequestration under no-tillage agriculture. Soil Biol Biochem 32:2099–2103
Steinmann K, Siegwolf RTW, Saurer M, Korner C (2004) Carbon fluxes to the soil in a mature temperate forest assessed by 13C isotope tracing. Oecologia 141:489–501
Subke J-A, Vallack HW, Magnuson T, Keel SG, Metcalfe DB, Hogberg P, Ineson P (2009) Short-term dynamics of abiotic and biotic soil 13CO2 effluxes after in situ 13CO2 pulse-labeling of a boreal pine forest. New Phytol 183:349–357
Tierney GT, Fahey TJ (2007) Estimating belowground primary productivity. In: Fahey TJ, Knapp AK (eds) Principles and standards for measuring primary production. Oxford University Press, Inc, New York, Ch 7
Tierney GL, Fahey TJ (2002) Fine root turnover in a northern hardwood forest: A direct comparison of the radiocarbon and minirhizotron methods. Can J For Res 32(9):1692–1697. doi:10.1139/x02-123
Tierney GL, Fahey TJ, Groffman PM, Hardy JP, Fitzhugh RD, Driscoll CC, Yavitt JB (2003) Environmental control of fine root dynamics in a northern hardwood forest. Global Change Biol 9:670–679
Vargas R, Baldocchi DD, Bahn M, Hanson PJ, Hosman KP, Kulmala L, Pumpanen J, Yang B (2011) On the multi-temporal correlation between photosynthesis and soil CO2 efflux: Reconciling lags and observations. New Phytol 191:1006–1017
Veen BW (1981) Relation between root respiration and root activity. Plant Soil 63:73–76
Wright SF, Upadhyaya A (1996) Extraction of an abundant and unusual protein from soil and a comparison with hyphal protein of arbuscular mycorrhizal fungi. Soil Sci 161:575–586
Acknowledgements
For their assistance with field and laboratory work on this study, the authors thank P. Bohlen, M. Fisk, J. Maerz, J. Beem-Miller, F. Chen, M. Dempsey, T. Feldpausch, A. Heinz, L. Martel, and L. Stoschek. We appreciate the insightful comments of two anonymous reviewers of an earlier draft of this Ms. This research was supported by a grant from the Ecosystem Studies Program, National Science Foundation.
Author information
Authors and Affiliations
Corresponding author
Additional information
Responsible Editor: Eric Paterson.
Rights and permissions
About this article
Cite this article
Fahey, T.J., Yavitt, J.B., Sherman, R.E. et al. Partitioning of belowground C in young sugar maple forest. Plant Soil 367, 379–389 (2013). https://doi.org/10.1007/s11104-012-1459-1
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s11104-012-1459-1