Xuejun Dong, Ph.D. Ecophysiologist, CGREC
Soil carbon dynamics and sequestration have significant impacts on ecosystem productivity and environmental quality worldwide, and thus have been studied extensively. While many achievements have been made in different ecosystems, scientists still do not fully understand the control and regulation mechanisms of carbon flux. Finding a solution to this complex problem, which involves the interplay of soil chemical, physical, biological, and ecological mechanisms, requires a multidisciplinary approach that focuses the research effort on a well-chosen field site. At the CGREC, we are collaborating with other scientists from NDSU, USDA Agricultural Research Service in Mandan, ND, and Philadelphia, PA, to study the problem. This report highlights the results obtained on root biomass, respiration, and decomposition, which are useful as input to an ecosystem modeling study of carbon dynamics at the CGREC.
Plant roots and soil carbon sequestration
Carbon sequestered in plant roots is the central focus of this report. Although plant roots (live and dead) in grasslands do not represent a large carbon storage pool (Kumar, et al. 2006), they are an important first step for the accumulation of the soil organic matter (SOM). A rangeland manager is aware of the appearance of vegetation, because the appearance changes rapidly with the season and with grazing pressure. The status of the vegetation is highly coordinated with the growth of the unseen roots. In contrast, an appreciable change in SOM usually occurs over a longer time period. The vegetation and roots are sensitive indicators of the need to adjust the management strategy for sustained productivity of the rangelands. The basic considerations of this study are as follows: (a) live roots are added annually to the soil ecosystem as a function of grazing intensity and weather; (b) an estimate of the amount of total root mass present in soils should consider the different age classes of the root biomass.
Impact of grazing management on total root biomass
|Figure 1. Dr. Xuejun Dong and international student Jinhui Wang at the root biomass and decomposition study at CGREC, summer 2007.|
A root study was conducted in September 2006 (Figure 1) to measure root biomass and respiration. Roots were collected from 48 soil samples from three moderately grazed and three heavily grazed pastures of the Grazing Intensity Study. Each sample was collected from a plot 10 in. X 10 in. and 6 in. deep. Roots were separated from dead material and into three functionally different groups based on texture and visual appearance: fine roots (<2mm in diameter resulting from the current year’s growth), coarse roots (>2mm in diameter), and rhizomes. Fine roots were cut from coarse roots, and for coarse roots and rhizomes, we collected the part that looked new. Some portion of these two groups may come from the previous year’s growth. The roots were washed and tested for respiration rates (next section). Roots (with the three groups separated) were then oven-dried and weighed, but were considered as representing the first 6-inch soil depth (without further subdividing for soil depth). Our results indicate that grazing intensity had a significant influence on total root biomass in 2006. Assuming root density decreases exponentially in the soil profile (Dahlman and Kucera, 1965) and most roots are located within the 6-foot soil depth, pastures under moderate grazing had an average total root biomass of 11,109 lbs/acre, while pastures under heavy grazing had an average of 8,787 lbs/acre. Grazing pressure had a negative effect on coarse roots and rhizomes. The amounts of coarse roots in moderate and heavy grazing pastures were 2,888 and 1,142 lbs/acre, respectively. The amounts of rhizomes in moderate and heavy gazing pastures were 1,333 and 703 lbs/acre, respectively. In contrast, fine roots showed a very similar density in pastures of the two grazing treatments with an average of 6,921 lbs/acre. This result for fine roots was unexpected. It is possible that the dry summer of 2006 discouraged fine root growth in both moderate and heavily grazed pastures. Because of contributions from previous years, coarse roots and rhizomes still showed a higher value under moderate grazing, which was not surprising to us, because the long-term Grazing Intensity Study has shown that moderate grazing promotes a healthier rangeland condition in terms of sustained use for forage production and cattle grazing (Patton, et al. 2007).
Root respiration and its response to grazing pressure and temperature
After the roots were washed, we put the moistened roots under an incubator (Figure 2) to equilibrate for at least 30 minutes at a target air temperature (50-100°F). Then the LI-COR 6400 gas exchange system (Figure 3) was used to measure root respiration. The relationship between respiration and temperature was used to estimate an important parameter called Q10, indicating the temperature sensitivity of respiration. While the majority of roots from both grazing treatments had a similar Q10 of 2.16 (unitless), about 20% of samples from moderately grazed sites had a significantly higher Q10 of nearly 5. This suggests that despite the 2006 summer drought, a portion of roots under moderate grazing still showed physiological vigor.
|Figure 2. A small refrigerator was converted into an incubator for temperature control in root respiration measurement.||Figure 3. Li-Cor 6400 and root respiration measurement cup (ready to use).|
Six dominant plant species were grown from seed both in and out of a greenhouse (Figure 4), and the results indicate that there were no significant differences among species in temperature sensitivity of root respiration. Roots of plants grown outside did not differ in temperature sensitivity when compared to those of plants grown under warmer conditions inside the greenhouse (with an average temperature about 7°F higher than the outside temperature).
| Figure 4. The
barrels used for growing native plant species for root
Root decomposition: results for 2006-2007
In our 2006 study, after roots were dried, samples were put into 192 fine-nylon bags and buried in the field to measure decomposition rate. The root bags were retrieved and weighed twice, first in April 2007, then in September 2007. For this report, we used only data of the initial (September 2006) and final (September 2007) weights to calculate the decomposition rate constant (k-value) according to Lee et al. (2003). Our results indicate that for fine roots and rhizomes, the decomposition rates between treatments were similar, with k-values of 0.80 for fine roots and 0.64 for rhizomes. However, the decay constant for coarse roots was lower under heavy grazing (k=0.45) than it was under moderate grazing (k=0.67). These measured k-values are similar to the values reported by Lee et al. (2003) from a study in a forest ecosystem. In our 2006 root survey, for fine roots, only those roots that were believed to be current year’s growth were measured; for coarse roots and rhizomes, the measured biomass probably include contributions from both the current year and previous years. To estimate the total root biomass, we should consider the fact that at any given time the total root biomass buried in the soil is composed of contributions from several (possibly 4-5) different years (Dahlman and Kuera, 1965). So, the measured k-values are useful for estimating how fast each year’s production (of fine roots, coarse roots, or rhizomes) decomposed and how much it contributed to the total below-ground plant biomass. The result is shown in the following sections.
Estimating changes in total root mass
We believe that the fine roots obtained in our 2006 study represent the current year’s growth. However, we are not sure about the age structures of the coarse roots and rhizomes we collected, because it is extremely difficult to judge age just by visually inspecting the washed roots. While we did not quantify what proportion of rhizomes was from previous years, we believe that this perenniality plays an important role in the plant’s eco-physiology. Also, for coarse roots, although some forbs may have only annual growth, shrubs such as buckbrush (Symphocarpos occidentalis, shown in Figure 5) must produce roots that are functional over several years, just like the above-ground shoots. Although accurate quantification of the below-ground biomass has to be based on a series of well-developed studies, for now we will use our previously gathered data to provide a rough estimate of long-term below-ground biomass. The estimate is important for system-level studies at CGREC. We first define a term called duration of viability, which is the duration (in years) that a plant organ (a coarse root or rhizome in this report) remains potentially active. We used a simple MINITAB macro to estimate total root carbon in our moderately grazed and heavy grazed pastures under average precipitation condition (18.71 in.) as recorded in the past 19 years. We used the following three assumptions:
For the second assumption, above-ground biomass production and average annual precipitation data (Patton, 2007) were used to find an estimate for the average-year fine root production. For the average-precipitation-year, the adjusted value for fine root production on the moderately grazed pastures is 7,760 lbs/acre, and on the heavily grazed pastures is 7,111 lbs/acre. For coarse roots and rhizomes, we will not provide adjusted values because of their perenniality.
|Figure 5. Roots of buckbrush washed clean and ready for respiration measurement.|
Total below-ground plant biomass and carbon sequestration
How much more carbon is added into the soils through proper grazing management? The simulated results give the total below-ground plant biomass (including both the current year’s growth and previous years’ growth) for the two differently grazed pastures considering a possible duration of viability of 1-10 years for coarse roots and rhizomes.
If we assume the average duration of viability is three years for both coarse roots and rhizomes, then we would have 28,380 lbs/acre of below-ground root biomass under moderately grazed pastures, and 20,100 lbs/acre under heavily grazed pastures. According to a root biomass survey at CGREC in 2004 (Guojie Wang, unpublished), the estimated total root biomass for the top 20-inch soil depth is about 20,000 lbs/acre, which is very close to our simulated value for root biomass under the heavily grazed pastures. The simulated numbers are different from the ones that were based on what we had measured in 2006 (that is, 11,109 lbs/acre for moderate grazing and 8787 lbs/acre for heavy grazing pastures, respectively), because these are the predicted results of total below-ground plant biomass for (a) an average precipitation condition, (b) with adjusted fine roots production, and (c) a duration of viability of three years. If 40% of this biomass is carbon (Kuzyakov, et al. 2001), then we will have 11,352 and 8,040 lbs carbon/acre for pastures of moderate and heavy grazing, respectively (further details, as well as the MINITAB macro for reproducing these results, will be posted on the web-version of this report).
From our data, the annual production values of below-ground biomass under moderate and heavily grazed pastures are estimated to be 9,291 and 7,916 lbs/acre, respectively, for the dry year of 2006, which are about three times the current year’s above-ground production (Patton, et al. 2007). For the year with average precipitation, the estimates of annual production are 10,130 and 8,107 lbs/acre of roots for the moderate grazing and heavy grazing pastures, respectively. These estimates for the Missouri Coteau prairie are about 50% higher than the estimates in grasslands of India (Kumar, et al. 2006) and Germany (Kuzyakov, et al. 2001). From our study, a pasture with a moderate grazing intensity, in a year with average precipitation (18.71 in.), could provide a root carbon sequestration offset of 1.35 metric tonne of CO2 per acre per year, and with a dry year of 2006, the sequestration offset estimated as 0.91 metric tonne per acre (the unit of “metric tonne” is used by the Chicago Carbon Exchange, or CCX). Based on a study by Rogers (2002), we estimate that for the 14 years from 1989 to 2002, about 0.6 metric tonne of CO2 per acre per year was incorporated into the SOM in the rangelands near the CGREC due to moderate grazing in comparison with the less-sustainable heavy grazing. Based on our simulation results, about 11% of the below-ground total plant biomass was incorporated into SOM in order to produce this amount of carbon sequestration offset (i.e., 0.6 metric tonne CO2 per acre per year). This would amount to an income of $1.2 per acre per year for Missouri Coteau ranchers considering the current carbon price of $2 per metric tonne of CO2. For a major part of North Dakota rangelands, however, the CCX uses a rather conservative estimate of the carbon offsets: 0.12 metric tonne CO2 per acre per year for normal rangelands and 0.27 metric tonne CO2 per acre per year for degraded rangelands. These offsets are associated with the assumption that about 2% to 5% of the below-ground total plant biomass is considered as permanent sequestration of carbon. These percentages are too low if the data from our field measurements and simulations, as well as from several other cited studies conducted at CGREC, are to be trusted. For an accurate accounting of the relationship of carbon sequestration and good rangeland management options, it is necessary to further study the eco-physiology (such as longevity, turn-over, and decomposition) of below-ground plant organs. With the facts from basic studies such as this one, it is possible that the CCX may be convinced to adopt a higher estimate of the soil carbon sequestration potential for North Dakota, as well as other regions. That would make the carbon market a better investment for everyone, as well as good for the environment.
Dahlman, R. C. and Kucera, C. L.: 1965, Root productivity and turnover in native prairie, Ecology 46, 84–89.
Kumar, R., Pandey, S. and Pandey, A.: 2006, Plant roots and carbon sequestration, Curr Sci 91, 885–890.
Kuzyakov, Y., Ehrensberger, H. and Stahr, K.: 2001, Carbon partitioning and belowground translocation by Lolium perenne, Soil Biol Biochem 33, 61–74.
Lee, M., Nakane, K., Nakatsubo, T. and Koizumi, H.: 2003, Seasonal changes in the contribution of root respiration to total soil respiration in a cool-temperate deciduous forest, Plant Soil 255, 311–318.
Patton, B. D., Dong, X., Nyren, P. E. N. and Nyren, A.: 2007, Effects of grazing intensity, precipitation, and temperature on forage production, Rangelands Ecol Manage 60, 656–665.
Rogers, W. 2002. North Dakota State University Master Thesis. (Advisor: Kirby, D.)
I take this opportunity to thank Mr. Paul Nyren, Director and Range Scientist at CGREC, for advice and input, and Mr. Bob Patton, Range Scientist at CGREC, for collaboration. I thank Anne Nyren at CGREC for encouragement and help. The following NDSU faculty members in the NDSU Soil Science Department brought their expertise and knowledge to this project: Dr. Larry Cihacek, Dr. Tom DeSutter, and Dr. Lyle Prunty. I thank Dr. Mark Liebig at USDA ARS-Mandan for his collaboration. I appreciate Dr. Shiping Wang at the Institute of Plateau Biology, Chinese Academy of Sciences, for discussions. Dr. Wang’s students, Guojie Wang (now a Ph.D. candidate at NDSU), Jinzhi Wang, Danjun Wang, Xueyan Zhao and Jinhui Wang, provided tremendous help both in fieldwork and lab measurements. I appreciate Mr. Tom Maresca of Philadelphia, PA, for his advice and input on computer programming. Finally, I appreciate Joan Dolence and Janet Patton for their patience and skills to improve the English usage as well as scientific clarity of the report.