Agroecosystems /
Carbon Sequestration
Research Focus
Iowa Final Report
Phase II: Methodology
Phase II: Methodology
Initial Contacts And Expectations
Representatives from the Iowa Department of Natural Resources, Iowa Conservation Districts, National Association of Conservation Districts (NACD), Colorado State University, Natural Resource Ecology Laboratory (NREL), and NRCS, met in 1998 to discuss GHGs, soil C modeling, and the potential for Iowa agriculture to sequester C. The Iowa C Storage Project provided education on C sequestration and collection of land use data not available in the literature. All 99 counties were involved in the project, and the 100 conservation districts provided local data through the use of the CSRA. NACD coordinated with the conservation districts and the state partners to assist in data collection and to establish an information and education program.
A database that provides each county with estimated amounts of C sequestered under various management practices will be available to the Iowa Conservation Partners. The database has been tailored to address the specific climate, soils, and current cropland management systems, and allows the user to project changes in soil C due to changes in crop and tillage practices. Each conservation district agreed to report their 1998 C sequestration amounts to DOE using the 1605(b) reporting procedure. Figure 14 is a flow diagram of the conservation partners involved in Phase II of this project and details how the involved parties communicated and the process of how data is transferred between groups.
Figure 14
The CSRA consists of a series of data sheets detailing historical land-use, dominant management practices (drainage, irrigation, crop rotations, tillage, fertilization) over time, and installation of conservation practices (e.g., CRP, grassed waterways, buffers) compiled by local experts in each county. A five-county pilot study was conducted to determine the availability of local data and the willingness of conservation districts to provide data and to refine the process of collecting local data at a very large scale for all 99 counties. Adair, Clay, Fayette, Hardin and Wapello Counties participated in the 1998 pilot study. These counties are diverse in their agricultural production, varying from row crops grown on drained prairie soils in Hardin County to the row crop-grass rotations on sloping lands in Wapello County. Their respective locations in Iowa provide a good mix of North-to-South and East-to-West landscape positions. Information and ideas provided by the conservation districts and NRCS people in the pilot counties provided the bases of the CSRA. Table 4 details the types of data provided by the Conservation Districts through the use of the CSRA.
Table 4: Types of data provided by the CSRA
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The CSRA sheets (Appendix A) are supported by background information from published databases including current land use/land cover obtained from the Geographic Analysis Program for Biodiversity, 1991-1992 (GAP) records (Scott et al., 1997) and historical land use/land cover obtained from the General Land Office (GLO) surveys of pre-settlement vegetation (Iowa Department of Natural Resources, 1996). STATSGO soils information, CTIC tillage information and NASS crop information provided additional background information to support the CSRA. For each county, maps of historical (pre-settlement) and current land cover were developed (Figure 15).
Figure 15
Figure 16
Figure 17
Databases
Data on climate, soils, land use, and management practices used in the analysis were assembled from a variety of sources. Individual counties are the spatial unit for representing climate factors. In other words, counties were assumed to be homogeneous with respect to the temperature and precipitation driving variables.
Temperature (mean monthly maximum and minimum) and precipitation (monthly total) were obtained from the PRISM monthly climate data set (Daly et al., 1994). PRISM uses point data from the U.S. network of weather stations and a digital elevation model (DEM) to orographically adjust climate variables for 4 km grid cells across the coterminous U.S. The data used in our analysis consisted of long-term (1961-1990) monthly averages (Figure 18). Area-weighted mean values of monthly temperature and precipitation variables were calculated for each county.
Figure 18
Figure 19
Figure 20
CSRA Relational Database
A relational database was developed to manage the data provided by the various Iowa counties for the county level assessments. This database was necessary to define the relationships between the various crops, tillage operations, rotations, and cropping histories. The data were then fed directly from the database to a series of PERL computer scripts that built the schedule files necessary to run the Century Model for the various combinations of crop histories, soils types, and hydric conditions. Developing the database and moving the data between the data entry spreadsheets to the database took a substantial amount of effort. Quality control of the CSRA data was necessary to provide consistent terminology, definitions, and units between counties. The various tillage events then had to be organized into tillage sequences that the Century Model could interpret appropriately. Finally, the data had to be organized from the spreadsheets into a set of standard query language (SQL) strings in order to insert the data into the relational database. The result was a straightforward and highly adaptable relational database structure that improved the efficiency of the model runs. The final data set was the end result of almost two dozen sets of modeling runs. Each of the interim model runs that was done prior to the final result led to new discoveries about the data set, requiring minor modifications and corrections to the input data. Having the input data in a relational database substantially eased the process of doing the model reruns. Century Modeling and Analysis
Initial model parameters were set according to the procedure outlined in the Century Model Description section of this report. The equilibrium Century runs provide the initial soil organic matter levels in the different pools. The model then simulated changes in soil C as a function of past agricultural practices based on dominant crop rotations and management practices reported in the CSRA. The average date reported for the onset of cultivation occurred during the 1860s-1880s for most counties, and cropping histories were divided into periods between 1860-1920, 1920-1950 and 1950-1974. Crop production potentials were also varied over time to mimic long-term changes in crop yields as reported by NASS, with yields increasing by 1-2% per year since the 1950s. For each time period, the local experts completing the CSRA specified the crop rotations and management practices (i.e., tillage, fertilization, manuring) that were representative for their area. Each county reported a single representative history prior to 1974, which varied between counties. However, most counties reported similar trends in the dominant cropping practices with corn, oats, wheat, and hay as the dominant crops prior to 1950, followed by a rapid shift towards feed-grain dominated rotations (i.e., corn and soybean) and a substantial reduction of hay in rotation. Drainage of hydric soils occurred in two phases; the first phase being a partial drainage in the early 20th century, and more complete drainage by 1950-1970. Using the relational database, the CSRA provided the crop rotation, tillage practices and fertilizer used in the individual counties. Each individual county has its own history beginning at the time the soils were broken out for cultivation and extending until 1974. Appendix B details the actual crop system and drainage dates for each county from 1861 to 1974. Starting in 1974, four crop rotations (continuous corn, corn-soybean, corn-soybean-oat-alfalfa-alfalfa, and oat-alfalfa), for each of three tillage regimes (intensive tillage, moderate tillage and no tillage), were simulated for a 20-year period (1974-1994) in each county. Intensive tillage was defined as multiple tillage operations every year, including significant soil inversion (i.e., plowing, deep disking) and low surface residue coverage. This definition corresponds to the intensive tillage and ‘reduced’ tillage systems as defined by Conservation Technology Information Center (CTIC, 1998). No tillage was defined as not disturbing the soil except through the use of fertilizer and seed drills and where no-till is applied to all crops in the rotation. Moderate tillage made up the remainder of the cultivated area, including mulch tillage and ridge tillage as defined by CTIC (CTIC, 1998) and intermittent no-till (see below).
Estimates of the percent of total cropland in each of the four crop rotations were provided as part of the information collected in the CSRA. The CTIC reports the area in various tillage systems by individual crops on an annual basis; however, it does not differentiate between long-term no tillage practices versus intermittent or ‘rotational no tillage' (i.e., tilled corn – no-tilled soybean rotations). For agronomic reasons, (i.e., low residue amounts under soybean and use of herbicide-resistant soybeans), the percent area of soybeans managed under no-till was generally higher than for corn. Thus, to estimate the area of continuous no tillage as opposed to rotational no tillage, we based the percent area of continuous no tillage on the acreage of corn under no tillage, assuming that if corn were no-tilled it was likely that other crops in the rotation (e.g., soybean or oats) would also be no-tilled. The remaining area reported as no tillage by CTIC was assumed to represent rotational no tillage and was included as part of the moderate tillage category. The moderate tillage category also included areas reported as mulch-till and ridge-till by CTIC. The area under intensive tillage was then calculated by difference.
Areas under the different tillage systems were estimated from the CTIC database, which has reported area by tillage system and county on an annual basis since 1989. To represent changes over time in tillage practices within time blocks simulated in the model we assumed: 1) that all annual row crop area had been managed with intensive tillage prior to 1974; and 2) 75% of the moderate till area reported in 1998 represented area converted to moderate till during the period 1974-1994 with the remaining area converted from intensively tilled systems since 1994. Since the area under continuous no tillage prior to 1989 was minimal, we used the soil C change rates calculated for the adoption of no tillage beginning in 1994 for the estimates of current C sequestration rates under no tillage. Since no information was available on the relative distribution of tillage systems by crop rotations, we applied the relative distribution estimated for total cropland to each of the four crop rotations simulated. The 'Iowa Carbon Potentials ' database (enclosed CD-ROM) provides the rate of soil C change for each type of conservation treatment on cropland.
To simulate changes due to the Conservation Reserve Program (CRP), all four crop rotations, under intensive tillage, were modeled with a change to CRP grass plantings for a ten-year period, starting in 1985. Table 5 shows the combinations of crop rotations, CRP, and tillage regimes modeled from 1974 to 1994.
From 1994, all of these options were continued for an additional 20 years, along with all combinations of changes between crop rotations, CRP, and tillage regimes. This provided 280 simulations for each soil texture/hydric combination in each county or approximately 203,000 simulations for the entire state. Again, the 'Iowa Carbon Potentials' database provides the rate of soil C change for each of these management combinations and conservation practices.
Additional information was compiled from the literature to estimate net soil carbon changes for minor land use practices that were not modeled by Century, including changes associated with tree conversion and wetland restoration on former cropped land and cultivation of organic soils. Mean rates of carbon change (on a per hectare basis) for cropland conversion to trees were taken from Lal et al. (1998). The rates for cropland conversions to wetland and cultivation of organic soils were taken from Armentano and Menges (1986). CSRA provides the area associated with the tree conversion and wetland reversion conversion practices. Phase I of this study provides the impact of cultivation of organic soils.
Table 5: Crop rotations and tillage interactions: 1974-1994
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