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Carbon sequestration on rangelands:
The role of plants

Xuejun Dong, Central Grasslands Research Extension Center, Streeter, North Dakota


 

Introduction

In terrestrial ecosystems, carbon sequestration is defined as the uptake of carbon through photosynthesis followed by storage in plants and soil. This type of carbon storage plays an important role in reducing the rise of atmospheric CO2. In past decades, more ecological studies on carbon sequestration were carried out in forests and croplands than in rangelands. Because rangelands occupy about one-half of the world’s land area, more scientists are now studying the potential of rangeland carbon sequestration. However, the high variability in floristics, soil types, and topography pose many challenges to this research. Current studies focus on soil organic carbon dynamics or on the overall balance of carbon exchange (carbon sink vs. source) on rangelands over time and under different management regimes. However, a scarcity of data on key eco-physiological mechanisms is hindering further understanding of this topic, which is important to both rangeland managers and the general public.

At the CGREC, we have developed a field-plot-scale study linking plant photosynthesis to rangeland carbon sequestration. After two years of field-plot preparation and pilot measurements, full-scale measurements started in 2008 and will continue for five years. The main objective of the study is to document plant photosynthetic activity and the components of soil respiration on pastures subjected to both grazing and drought treatments. The study will provide site-specific data that accounts for the vegetation’s contribution to rangeland carbon sequestration. In particular, this study considers the following ideas:

This study provides opportunities for observing the “behavior of the rangelands in terms of water use and carbon flux as a function of grazing intensity and drought. In the study, three types of manipulations are used:

 

Xuejun Dong placing root-filled nylon net bags in the soil to study carbon sequestration.

 

The results from 2008 include:


The outcome of this study will:


Detailed results as of 2008

Photosynthesis of leaves of green plants is a remarkable process capable of efficiently absorbing and fixing CO2 from the atmosphere to produce carbohydrates. The accumulated or sequestered carbohydrates become the basis of plant biomass and will reside in the ecosystem (including in animals, insects, and microorganisms) before the “reduced” form of carbon is recycled back to the atmosphere as the “oxidized” form of carbon (free CO2). While this cycling of carbon occurs naturally on earth’s surface, human management of natural resources may strongly influence this natural cycling of carbon by shifting the cycle toward the oxidized end (excessive carbon release to the atmosphere leading to global warming) or toward the reduced end (increased carbon retention in the ecosystem leading to carbon sequestration). The major goals of eco-physiological studies on agricultural carbon balance are to determine the nature and directions of human impacts on the major ecosystem processes of carbon cycling. In rangelands, animal grazing as an important type of land management tool may alter eco-physiological traits of plants and eventually lead to changes in species composition. The changed physiology, biochemistry, and species composition may not only alter the photosynthetic capacity of the whole plant community, but also may impact litter decomposition rate, which determines how long the primarily “reduced” form of carbon is retained in the ecosystem. In this report, we will outline our recent research results on two aspects of the above-mentioned processes. First, we will look at field data regarding specific leaf area (SLA) of two dominant grass species to see how they are related to the overall capacity of grassland photosynthetic water use and carbon sequestration capacity in this mixed-grass prairie. Second, we will discuss the impact of grazing and season on the decomposition rate of plant roots.

Ecosystems containing naturally diverse plant species are better able to absorb carbon dioxide than commercial plantations (Steinbeiss, et al. 2008). One reason is that in naturally diverse ecosystems, many different plant species provide opportunities for efficient use of resources such as water and nutrients. In the mixed-grass prairie, where drought is a major factor limiting forage production, the coexistence of species adapted to relatively dry soil conditions and species adapted to relatively wet soil conditions is important for maintaining the long-term stability of forage production. Photosynthetic production relies heavily on effective stomatal regulation for efficient water use. For a given plant species, this happens under proper leaf water status. As shown in Figure 1, western wheatgrass is an example of a dry-adapted species (with a stomatal conductance most responsive to changes in water potential when it is drier), while Kentucky bluegrass is an example of a wet-adapted species (with a stomatal conductance most responsive to changes in water potential when it is wetter). When leaf water status is poorer than the optimal range (where stomatal conductance responds to water potential rapidly), plants might be partially wilted and become photosynthetically inactive. However, one of the major problems in the management of rangelands in this region is that Kentucky bluegrass seems to over-dominate the plant community to such an extent that the diversity of the species composition for optimal forage production has been affected. To understand the eco-physiological consequences of this impact from various environmental scenarios, we need a good mathematical model to relate leaf stomatal conductance to photosynthesis, plant production, and carbon sequestration (see report by Jinzhi Wang). In this report, we will discuss the use of an easily obtained index of leaf photosynthetic capacity, specific leaf area (SLA), to make predictions regarding the management of Kentucky bluegrass-dominated rangelands through animal grazing in both dry and wet years.

In eco-physiological studies, a higher SLA suggests a higher photosynthetic potential, but it may also suggest a lower drought resistant potential. In this section, we will discuss SLA and its components (leaf density and thickness) in western wheatgrass and Kentucky bluegrass and point out some management implications. In western wheatgrass, grazing intensity did not have an effect on the relationship between SLA and its two components, because under both heavily grazed and ungrazed situations, SLA strongly correlated with leaf thickness but not with leaf density Figure 2. However, in Kentucky bluegrass, grazing had an impact on SLA, with SLA significantly correlated with leaf density under ungrazed exclosures, but correlated with both density and thickness under heavy grazing, leading to a higher SLA Figure 3. The following are some management implications:

  1. Grazing management has minimum impact on SLA of western wheatgrass but has some impact on SLA of Kentucky bluegrass.
  2. One difficulty in grazing management is that usually in wetter years grazing pressure tends to be higher in order to fully utilize the available forage for maximum animal production. However, with high water and high grazing pressure, Kentucky bluegrass becomes more photosynthetically active and potentially grows more vigorously than some native species such as western wheatgrass. This may lead to the overdominance of Kentucky bluegrass in the rangeland plant community in wetter years.
  3. A good time to reduce Kentucky bluegrass’s dominance is in a dry year, or in several consecutive dry years, during which the leaves of Kentucky bluegrass can become more prone to drought stress with a higher grazing pressure than with a lower grazing pressure. Increasing grazing pressure during dry years may help to weaken the vigor of Kentucky bluegrass.

According to Weaver (1926), both Kentucky bluegrass and western wheatgrass produce numerous underground rhizomes for vegetative reproduction. However, our experience in a greenhouse study indicates that Kentucky bluegrass is more capable of producing rhizomes than is western wheatgrass. If this trend also applies to typical field conditions, the control of Kentucky bluegrass can be more difficult. In sum, in terms of optimal use of rangeland soil water and carbon sequestration, the coexistence of Kentucky bluegrass and other native species seems to be excellent. However, the over-dominance of Kentucky bluegrass reduces the stability of long-term forage production in rangeland due partly to the reduction in plant species diversity. Some easily measurable eco-physiological indices, such as SLA, can be used to manage Kentucky bluegrass-dominated rangelands through animal grazing.

Finally, we discuss the effects of grazing intensity (moderate vs. heavy grazing) and season (winter vs. summer months) on the decomposition rates of fine roots, coarse roots, and rhizomes in the mixed-grass prairie. According to our measurement in the dry year of 2006 (see Xuejun Dong 2007 report) and the measurement from a report by Bob Patton (2008 report), about 75% of total plant biomass is below-ground. Thus, the dynamics of below-ground biomass would have a dominant effect on carbon sequestration in rangelands of this region. The following is an incomplete report from a multi-year decomposition study. The results are summarized in Figure 4. A two-way ANOVA indicates that (1) decomposition rate of different sub-types of the below-ground plant biomass tended to be higher in the heavily grazed pastures. However, only the results in coarse roots are significant (p=0.042); (2) decomposition rates of different sub-types of the below-ground plant biomass tended to be lower in the winter months (October through April) than in the summer months (May through September), with significant effects on fine roots (p=0.003) and rhizomes (p=0.002), but not on coarse roots; (3) there are no statistically significant interactions in grazing and season on decomposition rate of any sub-type of biomass.

In the moderately and heavily grazed pastures, the average percentage (% total below-ground biomass) of fine roots, coarse roots, and rhizomes are 82%, 14%, and 4%, respectively. So, the decomposition of total below-ground biomass is determined primarily by fine roots, for which the decomposition rate in the first winter (Fall 2006 to Spring 2007) is 51% of the first summer (Spring 2007 to Fall 2007), and the decomposition rate of the second winter (Fall 2007 to Spring 2008) is 55% that of the second summer (Spring 2008 to Fall 2008). Temperature is probably the leading factor contributing to the observed between-season contrast in decomposition rate.

Despite the relatively smaller fraction, the fact that coarse roots decomposed more rapidly in the heavily grazed than in the moderately grazed pastures is interesting. We noted in field survey that the coarse roots in the heavily grazed pastures on average had smaller diameter than those in the moderately grazed pastures. A likely cause of this difference is that the density of the major shrub (western snowberry) is much lower in the heavily grazed pastures than in the moderately grazed pastures after nearly 20 years of cattle grazing (see report by Bob Patton. The overall tendency of decomposition rate being higher in the heavily grazed pastures might be caused by differences in (1) biomass chemical quality (due to differences in physiological traits or in species composition), (2) soil temperature and moisture condition, and (3) soil microbial species composition between the moderately and heavily grazed pastures. Continued measurement, more in-depth data analysis, and further studies are needed to answer these questions.


Acknowledgments

I thank Janet Patton, Joan Dolence and Anne Nyren for valuable editorial comments that have improved the presentation.

 

References

Steinbeiss, S., H. Bessler, C. Engels, V. M. Temperton, N. Buchmann, C. Roscher, Y. Kreutziger, J. Baade, M. Habekost and G. Gleixner. 2008. Plant diversity positively affects short-term soil carbon storage in experimental grasslands. Global Change Biology. 14: 2937-2949.

Weaver, J. E.. 1926. Root Development of Field Crops. McGrow-Hill Book Company, Inc. New York. Available at http://www.soilandhealth.org/01aglibrary/010139fieldcroproots/010139toc.html.


NDSU Central Grasslands Research Extension Center
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