Soil carbon sequestration in managed grasslands: An ecological perspective

1. % of global soil C t C ha-1 CENTURY model: Carbon flows Net Primary Production Structural C Metabolic C CO2 Active soil C CO2 CO2 Slow soil C CO2 Passive soil C Soil carbon sequestration in managed grasslands: An ecological perspective Richard T. Conant and Keith Paustian Can changes in grassland management in the Southeastern US sequester soil C? How much? How do different types of grassland management affect soil C? Why is soil C in grasslands important? Grasslands cover more than one third of the terrestrial surface of the earth. Much of the earth’s grassland area is over utilized and poorly managed. Grassland overuse and land conversion into grasslands are driven by the demand for forage production We compiled data from the literature on the influence of grassland management and land use conversion to grassland on soil C. In order for data to be useful for this analysis, studies examining land management must have been designed so that management was the primary factor influencing soil C. Pastureland is an important land resource in the southeastern United States. In 1992 pastureland covered more than 13 MHA, or 12% of the total land area in the nine states comprising the southeastern region (AL, FL, GA, KY, MS, NC, SC, TN, and VA). Pastureland in the Southeast supports 6.5 million beef cattle and more than 990,000 dairy cows. Sales of both dairy ($1.8 billion) and beef ($2.7 billion) products produced in the Southeast are substantial. Scant data exist on the current state of pasture management in the Southeast. Since no published source of information for the extent of adoption of improved management practices exists, we contacted state coordinators for Studies from 17 countries were included in this review, but 87% of the studies were from Australia, the United Kingdom, New Zealand, Canada, Brazil, and the United States. Soil samples were collected at depths ranging from 2 to 800 cm, with an average of 32.5 cm. The shortest treatment duration was one year and the longest treatment lasted 200 years. While the average duration was 23.7 years, the median was 18.0 years, because treatment duration was less than 40 years for 81% of the studies. The most common types of improvement were fertilization, improved grazing management, and conversion to pasture from native and cultivated lands, accounting for over 90% of all of studies. Soil C increased following management improvements for 76% of studies reporting changes in C. The average change for content data was an increase of 20% and the average for concentration data was a 29% increase. For studies that showed an increase in soil C, soil C concentration increase averaged 51% and soil C content increased by an average of 31%. The 20% of the studies with the smallest increases averaged 3.8% for soil C concentration data and 4.0% for soil C content data, while the average increase for the 20% with the since significant portions of world milk (27%) and beef (23%) production occur on grasslands managed solely for those purposes. Grasslands have high inherent soil organic matter (SOM) content that supplies plant nutrients, increases soil aggregation, limits soil erosion, and also increases cation exchange and water holding capacities. Thus, maintenance of SOM is a key factor in the sustainability of grassland ecosystems. Soil organic matter in temperate grasslands averages 331 Mg ha-1, and grasslands contain 12% of the earth’s SOM. Grassland SOM can be strongly influenced by management. Historically, intensive cultivation has resulted in the transfer of 993 Tg of SOM to the atmosphere in the form of CO2 in the US alone, much of which was lost from native grasslands. Soil organic matter losses due to conversion of native grasslands to cultivation are both extensive and well documented and losses due to the Grazing Lands Conservation Initiative (GLCI) program within each state to obtain state-by-state assessments. Each coordinator was asked to estimate the portion of pastureland in the state that is regularly fertilized (to soil test recommendations at least once every two years) and the area currently sown with legumes. We assumed that answers to the questions as phrased would indicate the portion of land area realizing maximum potential production. These data were then weighted by the amount of grazing land within each state and summarized for the southeastern United States. We used these data and C sequestration rates from the literature (112 data points for fertilization; 9 for sowing legumes) to estimate current regional C sequestration rates due to improved pasture management. GLCI coordinators for states in the Southeast reported a largest increases was 154% and 76% for soil C concentration and content data, respectively. For those studies with a net loss of C with improved management, content and concentration data decreases both averaged very close to 12.0%. Soil C increased for nearly two-thirds of the studies examining native land range of adoption rates for both fertilization and sowing legumes. Between 25 and 85 percent of the pastureland is fertilized, with an area weighted average of 45 percent, but a median value of 33 percent. Legumes are sown on between zero and fifty percent of the pastureland in the Southeastern states, with an area-weighted average and mean near 16 percent. conversions and for all but one (98%) of the studies evaluating cultivation to pasture conversions. Slightly more than half of the studies evaluated reported values in soil C per unit surface area or contained enough information to calculate soil C per unit surface area. No attempt was made to correct for bulk density since studies occurred on a wide variety of soils under various types of management. Introduction of earthworms, sowing of improved grass species, and sowing legumes had very high rates of C storage, though the number of observations was limited and results were influenced by unique conditions including allophanic soils and some controversial results. Conversion from cultivated land to grassland also had high sequestration rates, likely due to prior soil C depletion following cultivation. Carbon sequestration rates overgrazing and poor pasture management have also been observed. However, historical SOM losses can potentially be reversed, and atmospheric C sequestered, with good agricultural management. In the United States, agricultural conservation practices such as Carbon sequestration rates derived from the literature ranged from 0.13 to 0.38 MgC ha-1 yr-1 for fertilization and sowing legumes. Soil C increased more when legumes were sown than when fertilizer was added, regardless of the PPT/PET ratio. For areas sown with legumes, C sequestration was greater in wetter areas, while the opposite was true for fertilized pastures. Average changes in soil C were not directly related to depth. More data points were used for the estimate of the influence of fertilization on soil C sequestration than for the effect of sowing legumes. All states but Tennessee had greater C sequestration due to fertilization than due to sowing legumes (Fig. 4). Sequestration rates for fertilization ranged from 0.05 to 0.27 TgC yr-1 with the most C sequestered in Florida and Georgia and the least in Mississippi and North Carolina. Sequestration rates due to sowing legumes ranged from zero for Kentucky (due to very low adoption reduced tillage, improved fertilizer management, elimination of bare fallowing, the use of perennials in rotations, and the use of cover crops can potentially sequester large amounts of atmospheric C. Similarly, areas converted from cultivation and maintained under well managed permanent grassland, as pastures or rangelands, constitute potential C sinks. Within established pastures, soil C can be increased by eliminating disturbances to the soil and by increasing primary production. The purpose of this poster is to explore different aspects of grassland management and their impact on soil C. Dynamics of soil organic matter in grasslands have been studied extensively. Indeed, the Century model was developed, in part, based on data collected in grassland ecosystems. Grasslands typically contain large amounts of organic matter which is influenced by many different types of management. Further, a large portion of the earth’s grasslands are poorly managed. Therefore, grassland soils have the potential to sequester substantial amounts of atmospheric C. Mg C ha-1 yr-1 Improved soil C (Mg C ha-1 yr-1) rates) to 0.32 TgC yr-1 in Tennessee, which has more area sown with legumes than average and substantial area in pasture. Total C sequestration for the Southeast was 1.00 TgC yr-1 due to fertilization and 0.75 TgC yr-1 due to sowing legumes. Unimproved soil C (Mg C ha-1 yr-1) Substantial amounts of C can be sequestered with increased adoption of improved management practices (Fig. 6). With moderate adoption of between two and five percent, improved management practices store a total of less than 0.5 TgC yr-1 in the Southeast. However, with wider adoption, improved fertilization and sowing of legumes may result in C sequestration rates of between almost one TgC yr-1 (with 10% adoption) and more than four TgC yr-1 (with 50% increase in adoption). Current management of pastureland is storing as much as 1.75 TgC yr-1 due to fertilization (1 TgC yr-1) and sowing legumes (0.75MMtC yr-1). Our results indicate that land use change will continue at decreasing rates and will sequester 9.0 TgC between now and 2010. Modest increases in adoption of improved pasture management could sequester between 0.44 and 4.5 additional TgC by 2010. for other management types were substantially lower, resulting in an overall average sequestration rate of 0.54 Mg C ha-1 yr-1. When exceptional results from Columbia were excluded, sowing legumes and grasses sequestered only 0.10 and -0.12 Mg C ha-1 yr-1 and the overall average dropped to 0.43 Mg C ha-1 yr-1. Tg C ha-1 yr-1 Can Century be used to predict changes in soil C following changes in grassland management? The Century model contains multiple soil C compartments that turn over at different rates. Rates of transfer between pools are influenced by soil temperature, moisture, soil physical characteristics, and management. Carbon fluxes to the soil are directly related to net primary production (NPP) and are divided into structural and metabolic components based on decomposability. Since NPP is a major driving variable within Century, many different types of grassland management can be accurately predicted. Century has been widely used to evaluate various effects of different types of grassland management. For example, Century can be used to evaluate the influence of different management practices such as fertilization, grazing, and sowing different species on production and soil C. Century, like other soil C models, generally predicts increased soil C with increased forage production (C inputs). However, in some cases, poor grassland management can lead to increased production without a corresponding increase in forage production. For example, warm season grass cover has increased in many regions in response to heavy grazing, resulting in decreased forage production but increased soil C. Also, sowing grass species with substantially greater belowground productivity increases soil C at the expense though aboveground productivity could decrease. Similarly, sowing forage species with different litter qualities or changing the quality of inputs by increasing manure inputs and decreasing grass inputs (through increased grazing) could alter soil C without corresponding changes in aboveground production. Century does not predict species changes with changes in management (such as those brought about by overgrazing), but Century can reflect input and soil C dynamics given the pertinent information. We used the Century model to estimate changes in soil C following changes in grassland management for three counties in the SE United States. Rates of change were similar to those estimated based on literature values. Mg C ha-1 yr-1 Tg C ha-1 yr-1