Summary We estimated carbon allocation to belowground processes in unfertilized and fertilized red pine (Pinus resinosa Ait.) plantations in northern Wisconsin to determine how soil fertility affects belowground allocation patterns. We used soil CO2 efflux and litterfall measurements to estimate total below-ground carbon allocation (root production and root respiration) by the carbon balance method, established root-free trenched plots to examine treatment effects on microbial respiration, estimated fine root production by sequential coring, and devel-oped allometric equations to estimate coarse root production. Fine root production ranged from 150 to 284 gm−2 year−1 and was significantly lower for fertilized plots than for unfertilized plots. Coarse root production ranged from 60 to 90 gm−2 year−1 and was significantly lower for fertilized plots than for unfer-tilized plots. Annual soil CO2 fluxes ranged from 331 to 541 g Cm−2 year−1 and were significantly lower for fertilized plots than for unfertilized plots. Annual foliage litterfall ranged from 110 to 187 g C m−2 year−1 and was significantly greater for fertilized plots than for unfertilized plots. Total belowground carbon allocation ranged from 188 to 395 g Cm−2 year−1 and was significantly lower for fertilized plots than for unfertilized plots. Annual soil CO2 flux was lower for trenched plots than for untrenched plots but did not differ between fertilized and unfertilized trenched plots. Collectively, these independent estimates suggest that fertilization decreased the relative allo-cation of carbon belowground.
Keywords: carbon balance, fertilization, litterfall, Pinus resi-nosa, root production, soil respiration, trenched plots.
Introduction
Despite the importance of root production in forest carbon budgets, it is still unclear how soil fertility affects the alloca-tion of carbon to roots. In many western conifers, the absolute amount of carbon allocated to fine root production is inversely related to site quality (Keyes and Grier 1981, Vogt et al. 1983, Santantonio and Hermann 1985, Vogt et al. 1987, Comeau and Kimmins 1989, Kurz 1989). A fertilization study has also shown that increased nutrient availability decreased fine root production of a Douglas-fir forest in New Mexico (Gower et al. 1992). However, Nadelhoffer et al. (1985) reported that, in forests in Wisconsin, belowground carbon allocation is
posi-tively correlated to nitrogen availability and aboveground pro-ductivity.
It is difficult to reconcile the differing effects of nutrient availability on carbon allocation patterns because different methods of estimating fine root production have been used to study forests in different climates. Furthermore, all of the methods used to estimate fine root production require assump-tions that are difficult to test, and belowground spatial hetero-geneity is large. Most estimates of fine root production are based on changes in fine root biomass in sequential soil cores. However, width and depth of soil cores, frequency of sam-pling, root separation methods and classification of roots as live or dead can all significantly affect estimates of fine root productivity (Vogt et al. 1986, Vogt et al. 1989, Publicover and Vogt 1993). In-growth cores, where root production is meas-ured as colonization of root-free soil (Persson 1983, Ahlström et al. 1988), generally give low estimates relative to sequential coring and carbon budgets (Nadelhoffer and Raich 1992, but see Neill 1992). The N budget technique uses a mass balance approach to estimate root production and requires accurate measurement of several N fluxes (Nadelhoffer et al. 1985).
An alternative explanation for the different patterns of be-lowground carbon allocation in response to nutrient availabil-ity is that there is an interaction of climate and nutrition on carbon allocation. For example, several researchers have re-ported a negative relationship between belowground carbon allocation and latitude (Schlesinger 1977, Vogt et al. 1986) or mean annual temperature (Gower et al. 1994), implying a strong influence of climate.
Fine and mycorrhizal root primary production is only one of the major processes determining the total amount of carbon allocated to roots. Total belowground carbon allocation can be separated into two primary components: root and mycorrhizal respiration, and root and mycorrhizae production. Because direct measurements of root respiration are difficult to obtain, total soil CO2 efflux is often used as an index of root and mycorrhizal respiration (Raich and Nadelhoffer 1989). Various factors influence soil respiration, but soil temperature and water content are the main abiotic factors (Singh and Gupta 1977). Soil respiration for a Pinus radiata D. Don forest in Australia has been successfully modeled with these two vari-ables alone (Carlyle and Than 1988).
The usefulness of soil respiration as an index of
below-Belowground carbon allocation in unfertilized and fertilized red pine
plantations in northern Wisconsin
BRENT E. HAYNES and STITH T. GOWER
Department of Forestry, University of Wisconsin-Madison, 120 Russell Laboratories, 1630 Linden Drive, Madison, WI 53706, USA
Received February 22, 1994
ground carbon allocation is greatly improved if the proportions of respiration attributable to roots and mycorrhizae and free-living microbes are determined. In situ measurements of root respiration are difficult to obtain (Vogt et al. 1989); however, comparison of soil CO2 flux between root-free trenched plots and untrenched plots in forests may provide a simple approxi-mation of the contribution of root and mycorrhizal turnover and respiration to total soil CO2 efflux. For example, Bowden et al. (1993) and Ewel et al. (1987b), using this method, have estimated that roots account for 33--62% of soil CO2 efflux.
Raich and Nadelhoffer (1989) have proposed that soil respi-ration, together with aboveground litterfall, can be used to estimate total belowground carbon allocation (root production + root respiration) in forest ecosystems by the equation:
Rs−Pa≈Pb+Rr,
where Rs is the annual soil respiration, Pa is the aboveground detritus production, Pb is the belowground detritus production, and Rr is the root respiration. For this case, soil carbon content is assumed to be at steady state, and leaching losses are as-sumed to be negligible. Raich and Nadelhoffer (1989) used this approach to show that belowground carbon allocation is posi-tively correlated to aboveground litterfall, and thus above-ground productivity, for forests of the world.
The objective of our study was to use two independent methods to estimate total belowground carbon allocation (by the carbon balance approach) and fine and coarse root produc-tivity in unfertilized and fertilized red pine plantations. The study was conducted in Wisconsin because Nadelhoffer et al. (1985), using the N budget technique to estimate fine root production, found that belowground carbon allocation to fine roots is positively correlated to nitrogen availability in Wiscon-sin forests. We reasoned that if several independent methods to estimate belowground allocation showed a similar effect of fertilization, the results would clarify the effects of nutrient availability on carbon allocation patterns in forests.
Methods Study site
The study was conducted in a 31-year-old (age in 1990) red pine (Pinus resinosa Ait.) plantation located approximately 10 km northwest of Boulder Junction, WI (46°10′ N, 89°40′ W) (Table 1). Seedlings were planted at a 2 × 2 m spacing, and the stand was never thinned. Soil parent material was glacial outwash, and the soil was classified as a sandy, mixed, frigid, entic Haplorthod (USDA 1988). The surface soil texture was a loamy fine sand. During the winter (December--March), the soil was usually frozen to a depth of 50 cm (Haynes, unpub-lished data).
The experiment was based on a completely randomized block design with a 25 × 25 m fertilized plot and an unfertilized control plot in each of three blocks. A treated 5-m buffer was maintained around each plot for the duration of the study, although all the data reported in this study are based on the 25 × 25 m plots. Within each block, the distance between the outer
edges of the treated buffers of the fertilized and unfertilized plots was > 15 m. Fertilized plots received 150 kg N ha−1 in the spring and fall of 1990, 1991 and 1992, and spring 1993. Nitrogen was applied in equal proportions of NH4+ and NO3−, and other essential macro- and micronutrients were applied in the ratio 24/6/12/1/1/1.2 (N/P/K/Ca/Mg/S). In October 1990, one 2.5 × 2.5 m trenched plot was established in the buffer strip of each plot. To prevent including a tree in the trenched plot, we selected locations where seedlings had died during early stand development. The trenches were dug to a depth of least 1.5 m, lined with two layers of 4.5 mil black polyethylene sheeting, and backfilled. The trenched plots were maintained free of vegetation for the duration of the project by monthly weeding.
Field measurements
Soil respiration
We measured soil respiration monthly from July 1990 to Oct-ober 1993, except for months when the site was under continu-ous snowpack, by the soda-lime chamber method (Edwards 1982). To avoid a disturbance effect during the measurement period, we inserted 28-cm diameter plastic collars into the ground at least 2 weeks before soil CO2 flux was measured. At the time of measurement, each collar was replaced with a 28-cm diameter plastic chamber (12.6 l) under which was placed an uncovered tin containing soda lime. All soil CO2 flux measurements were made for 24 h. The ratio of the tin surface area to chamber surface area exceeded the 6% minimum sug-gested by Raich and Nadelhoffer (1989). Soil CO2 flux was measured at 10 random locations on each untrenched plot and at two locations on each trenched plot. Three covered tins were left on the soil surface of each plot during this same period as blanks. We calculated actual CO2 absorbance as the difference
Table 1. Physiographic, climatic and structural characteristics of un-fertilized and un-fertilized red pine plantations in northern Wisconsin (as of 1990). Climate characteristics are from long-term regional weather station data.
(A) Physical and climate characteristics
Elevation (m) 500
Slope (%) 0--5
Average January air temperature (°C) −10.4 Average July air temperature (°C) 19.5 Growing season precipitation (mm) 586 Average annual snowfall (mm) 2660
(B) Stand characteristics (± 1 SE) Unfertilized Fertilized (from Gower et al. 1993)
Trees.ha−1 2106 ± 229 2016 ± 180 Basal area (m2 ha−1) 42.4 ± 2.7 38.8 ± 3.1 Average stem diameter (cm) by canopy class
between the weight gain for each tin and the mean weight gain of the three corresponding blanks on the plot, corrected for water loss on CO2 absorption (Edwards 1982). The soda lime was replaced after each field season.
Monthly soil respiration rates were calculated by assuming the measured daily respiration rate was the mean for the month. Soil respiration rates for April 28, 1993, when soil temperatures were ~ 0 °C, were multiplied by the time of continuous snowpack (approximately 5 months each year) to obtain estimates of winter respiration, and these were added to the monthly totals to obtain annual estimates of soil respira-tion.
From June to October 1993, we made direct comparisons of measurements of soil respiration by the soda-lime and IRGA methods. At each sampling period (n = 8), we selected four soda-lime chambers per treatment plot for the comparison. We placed three 10.15-cm diameter PVC collars 2 cm into the ground around each selected soda-lime chamber (external col-lars), and placed one collar inside each chamber (internal collar). Soil respiration from these collars was measured with an LI-6200 infrared gas analyzer (Li-Cor Inc., Lincoln, NE) with a prototype of the LI-6000-09 soil respiration cuvette following the measurement protocol described by Norman et al. (1992).
Measurements on the internal collars were taken immedi-ately before and after each 24-h soda-lime absorption period. Two sets of measurements were taken on the external collars during each absorption period to account for spatial and tem-poral variability. To compare the two techniques, we developed a regression equation relating soda-lime soil respiration meas-urements to their corresponding IRGA measmeas-urements for all measurement periods.
Soil temperature
A datalogger was used to measure soil temperature at a depth of 10 cm on Plots 1 (fertilized) and 2 (unfertilized) from September 1992 to October 1993. During each soil respiration measurement period, we also measured the soil temperature at a depth of 10 cm adjacent to each chamber with digital soil thermometers (Fisher Scientific).
Litterfall
Ten 0.25-m2 litter screens (8.7 cm deep, raised on 10 cm legs) were randomly placed on each plot in May 1990. Litterfall was collected monthly from July 1990 to October 1993, except for periods of continuous snowpack. Litter samples were taken to the laboratory, sorted into pine foliage and miscellaneous tis-sue, dried at 70 °C and weighed. Annual litterfall mass was calculated by summing all monthly collections plus the first collection of the following year, which consisted of litter shed the previous year that remained lodged in the canopy. Foliage litterfall was assumed to be 48% carbon (Raich and Nadelhof-fer 1989).
Fine root biomass
Fine root biomass was estimated from cores taken in April and July 1991 (n = 20), and May, July and October 1992 (n = 10).
The sampling scheme was based on the fine root phenology of red pine reported by Aber et al. (1985) and from root periscope data from this study (Haynes, unpublished data). The corer was 41.5 mm in diameter and 57 cm in length. Cores were kept at 2 °C until processed.
Intact roots (> 1 cm in length) were removed from the cores, sorted into live and dead roots by visual inspection under a dissecting microscope, and further sorted into diameter classes (< 1 mm and 1--5 mm). For the first four sampling periods, root tips < 1 cm in length were separated from each soil core with a hydropneumatic root elutriator (Smucker et al. 1982). Or-ganic matter from the elutriator screens was placed in a shal-low round water-filled pan (40-cm diameter) and homogenized. Root tips were removed from one quarter of the pan area. Visual inspection of the collected tips showed that nearly all were dead. The five categories of fine roots (> 1 mm live, < 1 mm live, > 1 mm dead, < 1 mm dead, and dead tips) from each core were dried at 70 °C and weighed. Root catego-ries were compiled by plot, ground in a Wiley mill, and sub-samples were ashed at 425 °C in a muffle furnace to determine the percent ash content of samples by weight.
Fine root productivity was determined on a plot basis by the max--min and decision matrix methods (McClaugherty et al. 1982, Publicover and Vogt 1993). Root tip biomass did not change substantially between any of the first four sampling periods and was therefore excluded from calculations. Annual fine root production for 1991 was based on changes in live fine root biomass between April 1991 and May 1992, and produc-tion for 1992 was based on changes in root biomass between May and October 1992. We assumed that no root growth occurred during the winter months (November to April or May).
Coarse root biomass
In October 1992, we excavated the coarse root systems of seven unfertilized red pine trees, located less than 100 m from the study plots, using water pressure from a fire truck. The harvested trees represented a range of stem diameters (7.5 to 21.7 cm) similar to those found on the plots. Each root system was taken to the laboratory and separated into five diameter classes (1.0--1.5, 1.5--2.5, 2.5--5.0, 5.0--10.0 and > 10 cm, or stump). All root samples were washed and dried at 70 °C to constant weight. A subsample of each root sample was ground in a Wiley mill and ashed at 425 °C to determine the ash content by weight. Regression equations relating coarse root diameter class biomass and total coarse root biomass to diame-ter at breast height were developed for the seven excavated trees, using the REG procedure of SAS (SAS Institute Inc., Cary, NC).
Statistical analyses
Main effects of fertilization on soil respiration and root biomass for each sampling period, as well as annual soil respiration, root production and litterfall totals, were deter-mined by an analysis of variance on treatment means. Block-ing effects were rarely significant at α = 0.05 and rarely affected the significance of treatment effects. Therefore, all analyses of treatment effects were made assuming a com-pletely randomized design. Determinations of trenching main effects on respiration were made using a conservative F-test with reduced degrees of freedom to account for unequal sub-sample numbers. Interaction between fertilization treatment and trenching was determined by analysis of variance on differences between main and trenched plot means. All analy-ses were performed with the GLM procedure of SAS. An α = 0.10 level was used to detect statistically significant differ-ences in all analyses.
Results
Root biomass and production
Total live fine (< 5 mm diameter) root biomass ranged from 59 to 431 gm−2, whereas total intact dead root biomass ranged from 74 to 141 g m−2 (Figure 1). Fertilized plots had signifi-cantly lower < 1 mm and total live fine root biomass and
significantly greater 1--5 mm dead root biomass in July 1991 and May 1992 than unfertilized plots. Fertilized plots also had significantly greater (P < 0.05) < 1 mm dead root biomass in May 1992, as well as greater total dead fine root biomass in April 1991 (P < 0.09) and October 1992 than unfertilized plots. Fine root net primary production was significantly lower in fertilized than in unfertilized plots in 1991 by both methods of calculation (P < 0.03 max--min, P < 0.09 decision matrix), but the fertilizer effect was not significant in 1992 (Table 2).
Coarse root biomass was positively correlated with stem diameter for all diameter classes (Table 3). Total coarse root biomass averaged 2260 g m−2 for fertilized plots and 2460 g m−2 for unfertilized plots in March 1993 (Table 4). Total coarse root productivity averaged 60 and 90 g m−2 year−1 for fertilized and unfertilized plots, respectively, and the treatment effect was significant.
Soil respiration
Excluding the trenched plots, soil respiration rates ranged from 0.097 (April 1993, fertilized treatment) to 0.462 (August 1993, unfertilized treatment) g CO2m−2 h−1. Soil respiration was significantly lower for fertilized plots than for unfertilized plots for most measurement periods in each growing season (Figure 2). Soil respiration rates for the trenched plots ranged from 0.079 (April 1993, unfertilized treatment) to 0.426 (May 1991, fertilized treatment) g CO2m−2 h−1. Soil respiration rates did not differ significantly between unfertilized and fertilized trenched plots (Figure 3); however, soil respiration was signifi-cantly lower in trenched plots than in untrenched plots for most of the 1991 and 1992 growing seasons for both fertilized
Table 2. Mean (± 1 SE) annual fine root production (gm−2 year−1). An asterisk denotes a significant difference between treatments (P < 0.10).
Max--min Decision matrix
1991
Unfertilized 251 ± 39* 284 ± 39* Fertilized 94 ± 26 150 ± 44
1992
Unfertilized 180 ± 32 182 ± 31 Fertilized 194 ± 32 208 ± 39
Table 3. Allometric equations for ash-free coarse root component biomass (n = 7) (log10(biomass) = a + b[log10(dbh)], biomass in grams, dbh in centimeters).
Component a b SEE r2
Stump 0.454 2.654 0.165 0.981
10.0--5.0 cm −3.992 5.795 1.065 0.732 5.0--2.5 cm 0.054 2.706 0.241 0.962 2.5--1.5 cm 0.895 1.775 0.127 0.975 1.5--1.0 cm 0.415 1.970 0.157 0.969
Total 0.821 2.636 0.139 0.986
(Figure 4) and unfertilized (Figure 5) treatments.
Annual soil respiration rates based on the soda-lime method ranged from 374 to 541 g C m−2 year−1 (Table 5). Fertilization significantly (P < 0.05) decreased annual soil respiration in the untrenched plots in 1992 and 1993, but not 1991. Annual soil respiration did not differ significantly between trenched
unfer-tilized and trenched ferunfer-tilized plots in any year. Soil respiration in the fertilized trenched plots was significantly lower than in the fertilized untrenched plots in 1992 (P < 0.10), whereas soil respiration in the unfertilized trenched plots was significantly lower than in the unfertilized untrenched plots in 1991 and 1992 (P < 0.10).
The relationship between the soda-lime and IRGA measure-Table 4. Mean (± 1 SE) coarse (> 1.0 cm diameter) root biomass and annual production estimates (n = 3). An asterisk denotes a significant difference between treatments (P < 0.10).
Treatment April 1990 coarse root biomass March 1993 coarse root biomass Annual coarse root productivity
(g m−2) (gm−2) (g m−2 year−1)
Unfertilized 2190 ± 100 2460 ± 90 90 ± 7*
Fertilized 2070 ± 130 2260 ± 130 60 ± 6
Figure 2. Mean untrenched plot soil respiration rates (± 1 SE) for individual sampling periods, 1990--1993 (n = 3). An asterisk denotes significantly (P < 0.10) different means.
Figure 4. Mean fertilized plot soil respiration rates (± 1 SE) for individual sampling periods, 1991--1993 (n = 3). An asterisk denotes significantly (P < 0.10) different means.
Figure 3. Mean trenched plot soil respiration rates (± 1 SE) for individual sampling periods, 1991--1993 (n = 3). An asterisk denotes significantly (P < 0.10) different means.
ments of soil respiration rates was similar to that reported by Ewel et al. (1987a) (Figure 6). Using this relationship, we calculated annual soil respiration corrected for soda-lime bias (Table 5). The correction factor increased annual soil respira-tion by 11 to 78%, and made the trenching effect significant for fertilized (P < 0.06) and unfertilized (P < 0.03) plots in 1993.
Relationship between soil temperature and soil respiration
We found a significant relationship between soil temperature and soil respiration. The best model was an exponential rela-tionship (respiration rate = 0.158 × 100.02266(temperature) , P < 0.0001, Q10 = 1.685) that explained 54% of the variation in soil respiration. Excluding data for October 1990 and November
1992, when we hypothesize that advective forces may control CO2 flux (soil warmer than air), the model is (respiration rate) = 0.129 × 100.02870(temperature) (r2 = 0.80, P < 0.0001, Q10 = 1.94).
Litterfall
Mean annual foliage litterfall mass ranged from 150 to 387 g m−2 (Figure 7). Annual foliage litterfall was significantly higher in the unfertilized plots in 1990, the first year of fertili-zation treatment, but foliage litterfall was significantly greater in the fertilized plots in 1991, 1992 and 1993. Annual miscel-laneous litterfall ranged from 68 to 154 gm−2 and was signifi-cantly (P < 0.052) greater for fertilized plots than for unfertilized plots in 1992.
Carbon balance analysis
When we used the carbon balance approach to estimate maxi-mum belowground carbon allocation, we found that total be-lowground carbon allocation was significantly lower for fertilized plots than for unfertilized plots for 1991, 1992 and 1993 (Table 6). Total belowground carbon allocation ranged from 188 to 395 gm−2 year−1. However, when we based our calculations on the IRGA-corrected respiration rates, the range was 253 to 791 g Cm−2 year−1, and the significant treatment effect for 1991 was lost.
Discussion
Our maximum estimate of live fine root biomass (431 g m−2) is comparable to values reported for other red pine forests in Wisconsin (441 g m−2, Aber et al. 1985) and Massachusetts (510 g m−2, McClaugherty et al. 1982), and is typical for cold-temperate conifers. Our estimates of annual fine root production are also similar to those derived by Nadelhoffer et al. (1985) (120 g m−2 year−1) and Aber et al. (1985) (200 g m−2 year−1) for red pine plantations in Wisconsin. The estimates based on the decision matrix are higher than those from the max--min method (cf. Publicover and Vogt 1993). Although the Table 5. Mean ± 1 SE (n = 3) annual soil respiration (g Cm−2 year−1)
by treatment. Different lower case letters denote significant differ-ences (P < 0.10) among treatment means for each individual year and method.
Treatment Soda lime IRGA corrected
1991
Unfertilized untrenched 541 ± 30 a 904 ± 85 a Unfertilized trenched 430 ± 40 b 563 ± 75 b Fertilized untrenched 478 ± 17 ab 696 ± 97 a Fertilized trenched 461 ± 26 ab 783 ± 203 ab
1992
Unfertilized untrenched 483 ± 20 a 769 ± 18 a Unfertilized trenched 336 ± 39 bc 391 ± 31 bc Fertilized untrenched 374 ± 7 b 451 ± 17 b Fertilized trenched 331 ± 17 c 369 ± 15 c
1993
Unfertilized untrenched 505 ± 20 a 901 ± 18 a Unfertilized trenched 395 ± 56 ab 534 ± 100 bc Fertilized untrenched 404 ± 6 b 505 ± 17 b Fertilized trenched 379 ± 17 b 442 ± 16 c
Figure 6. Soda-lime soil respiration rates versus the natural logarithm of simultaneous IRGA measurements. The solid line (ln(IRGA) = 3.94 + 0.00718(soda lime), r2 = 0.585) represents data from this study. The dashed line (ln(IRGA) = 3.98 + 0.00653(soda lime)) from Ewel et al. (1987a).
treatment difference was significant only in 1991, the esti-mates corroborated our soil respiration results.
Our coarse root biomass estimate is also similar to those from conifer stands of similar age and density summarized by Santantonio et al. (1977); however, few estimates of coarse root production are available for comparison. The coarse root net primary production rate (90 gm−2 year−1) of our unfertil-ized plots is similar to that reported for roots > 5 mm in diameter of Pinus contorta var. latifolia Engelm. on a mesic site in British Columbia (Comeau and Kimmins 1989), but their estimate for a xeric site (40 gm−2 year−1) is significantly lower. Gower et al. (1992) reported high coarse root production values (136 to 187 gm−2 year−1) in a New Mexico Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest, but they also reported a negative effect of fertilization and irrigation on coarse root production.
The soda-lime soil respiration rates on our plots (0.097 to 0.462 g CO2m−2 h−1) are similar to those reported by Bowden et al. (1993) for a temperate hardwood stand in Massachusetts and Ewel et al. (1987a) for a warm-temperate Pinus elliottii Engelm. forest in Florida. Our annual estimates are also well within the reported range for temperate conifer forests (Schlesinger 1977, Raich and Nadelhoffer 1989), but annual estimates for the unfertilized red pine plantation are low com-pared to the prediction (604 g Cm−2 year−1) for our site based on latitude (Schlesinger 1977). The low annual soil CO2 flux for our site may be due to the small amount of organic matter in the sandy soil.
We feel that our correction of annual respiration totals for winter respiration is valid. Although CO2 diffusion may be restricted by frozen soil and snowpack, soil microbes and deep roots may have a basal respiration rate that we did not measure. Sommerfeld et al. (1993) measured mean respiration rates of 0.031 to 0.114 g CO2m−2 h−1 from snow in the Rocky Moun-tains. Our soil respiration measurements for April 1993, when most of the soil was still frozen < 10 cm deep, are probably reasonable estimates of soil respiration rates for winter condi-tions. Because soil respiration during the winter months can comprise up to 25% of the annual totals, it is unwise to assume winter soil respiration is negligible.
The relationship between soda-lime and IRGA measure-ments is similar to that derived by Ewel et al. (1987a), although the r2 value is lower (0.59 versus 0.80). These results suggest that the relationship between the soda-lime and IRGA meas-urements may be applicable under a range of conditions.
Fertilization significantly decreased soil respiration in the red pine plantation. This response may be explained by one of two mechanisms. First, several scientists have reported that fertilization reduces microbial activity (Foster et al. 1980, Fog 1988). However, this explanation does not appear to be valid for this study, because annual soil surface CO2 flux did not differ significantly between trenched fertilized and trenched unfertilized plots. The alternative explanation is that fertiliza-tion decreased root and mycorrhizal growth or respirafertiliza-tion, or both. This explanation is consistent with the fine root biomass data for the unfertilized and fertilized forests in this study.
Comparison of annual respiration from trenched and un-trenched plots may provide a first approximation of the propor-tion of soil respirapropor-tion that can be attributed to roots and mycorrhizae. Our soda-lime data suggest that roots and mycor-rhizae contributed 21 to 30% of total soil respiration for unfer-tilized red pine plantations, and 4 to 12% for ferunfer-tilized red pine plantations, depending on the year. When corrected for soda-lime bias, root contribution to total soil respiration was 38 to 49% for unfertilized plots. The IRGA-corrected values showed fertilized trenched plots had 13% greater respiration in 1991, and 17 and 13% lower respiration in 1992 and 1993, respec-tively. We speculate that the decay of severed roots in the fertilized trenched plots increased soil respiration in 1991.
Our estimate of root contribution to soil respiration in unfer-tilized plots for 1992 (30%) is similar to the values of 33% for a mixed-hardwood stand in Massachusetts (Bowden et al. 1993) also derived using alkali absorption, and 35% for a mixed-hardwood stand in Tennessee (Edwards and Sollins 1973), derived using a respirometer. However, our IRGA-cor-rected estimates of the contribution of root respiration in un-fertilized plots are closer to the 58--62% determined by Ewel et al. (1987b) for a slash pine stand in Florida (also IRGA corrected) and 50% estimated for Japanese red pine (Nakane et al. 1983). If the IRGA values of soil respiration rates are Table 6. Maximum belowground carbon allocation (± 1 SE) (sensu Raich and Nadelhoffer 1989) for unfertilized and fertilized red pine plantations for 1991, 1992 and 1993. An asterisk denotes a significant difference (P < 0.10) between treatments.
Treatment Soil CO2 flux Foliage litterfall Belowground carbon allocation
Soda lime IRGA corrected (g C m−2 year−1) Soda lime IRGA corrected (g Cm−2 year−1) (g C m−2 year−1) (g C m−2 year−1) (g Cm−2 year−1)
1991
Unfertilized 541 ± 30 904 ± 85 146 ± 3* 395 ± 33* 758 ± 89
Fertilized 478 ± 17 696 ± 97 162 ± 3 317 ± 14 535 ± 95
1992
Unfertilized 483 ± 20* 769 ± 18* 151 ± 3* 332 ± 19* 618 ± 21*
Fertilized 374 ± 7 440 ± 15 187 ± 4 188 ± 12 253 ± 18
1993
Unfertilized 505 ± 20* 901 ± 18* 110 ± 21* 395 ± 19* 791 ± 18* Fertilized 404 ± 6 505 ± 17 164 ± 121 240 ± 13 342 ± 5
correct, then we can state that roots generally contribute 40 to 65% of total soil respiration for pine forests.
The slightly higher soil temperature of the unfertilized plots compared with the fertilized plots may have affected soil respiration rates (Haynes, unpublished data). To test this, we applied the derived 10-cm Q10 value (1.685) to observed mean differences in soil temperature between treatments for each sampling period in 1993. We found that temperature accounted for only a 4.9% difference in respiration between the fertilized and unfertilized plots.
In conclusion, based on two independent methods to esti-mate belowground carbon allocation in unfertilized and fertil-ized red pine forests, we found that belowground carbon allocation was inversely related to nutrient availability (Kurz 1989, Gower et al. 1992). Our estimates of total belowground carbon allocation by the carbon balance method showed sig-nificantly lower belowground carbon allocation in fertilized red pine forests than in unfertilized red pine forests in 1991, 1992 and 1993. Compared with the unfertilized plots, total soil respiration was significantly lower in 1992 and 1993, and foliage litterfall was significantly higher in fertilized plots each year. Similar results have also been reported for unfertilized and fertilized P. radiata plantations in Australia (Pongracic 1993). These results conflict with the hypotheses that below-ground carbon allocation is positively correlated to nutrient availability (Nadelhoffer et al. 1985) and aboveground litter-fall (Raich and Nadelhoffer 1989). Although these hypotheses may hold true on a global scale, it appears that within a given ecosystem, belowground carbon allocation is negatively re-lated to nutrient availability and aboveground productivity. Although our fine root production estimates did not always show a significant treatment effect, we always observed a shift in the proportion of carbon allocated belowground. Above-ground net primary production estimates for the site showed significantly greater aboveground production for fertilized plots than for unfertilized plots (568 versus 431 g C m−2 year−1 in 1991) (Gower et al., unpublished observations). Therefore, even if we assume fine root net primary production did not differ (as in 1992), a lower proportion of carbon was allocated belowground.
Acknowledgments
The authors thank Mr. Ralph Hewett and Mr. Phillip Theiler, Wiscon-sin Department of Natural Resources, Trout Lake, WI, for their help in locating the study site and for the use of their fire truck. Special thanks are also extended to Tom Steele, Director of Kemp Natural Resources Station, Woodruff, WI, for his invaluable assistance with all phases of this research. We also thank Dr. Erik Nordheim for his valuable statistics advice. Finally, we thank Daniel Olson, Karin Fass-nacht, Charles Brooks, Joseph House and the many other student workers who made this study possible. This research was supported by an NSF grant (BSR-8918022) to S.T. Gower, S.W. Running and H.L. Gholz, and an NSF doctoral dissertation improvement grant (DEB-9212873) to B.E. Haynes and S.T. Gower.
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![Table 3. Allometric equations for ash-free coarse root componentbiomass (n = 7) (log10(biomass) = a + b[log10(dbh)], biomass ingrams, dbh in centimeters).](https://thumb-ap.123doks.com/thumbv2/123dok/1014581.923579/4.612.57.260.404.703/table-allometric-equations-componentbiomass-biomass-biomass-ingrams-centimeters.webp)
