Agroecosystems /
Carbon Sequestration
Research Focus
|
|
Iowa Final Report
Phase II: Results
State Summaries
The principle management trends affecting simulated soil C stock changes for the state of Iowa were the increase in the adoption of moderate tillage and no tillage systems for row crop production, and the introduction of the CRP. In addition, there is a general long-term trend of increasing crop residue inputs, associated with productivity gains (on the order of 1-1.5% per year) since the 1950s (Reilly and Fuglie, 1998), which contributes to increasing soil C stocks in the annual crop systems, even for some intensively tilled soils.
No tillage and moderate tillage systems have increased in Iowa over the past several years from < 3% and 20% of annual cropland in 1989 to 11% and 28% in 1998, respectively. Simulated increases in C stocks of soils under continuous no tillage averaged about 0.6 tonnes ha-1 (0.27 tons ac-1) for the state as a whole. The gain of soil C on reduced (i.e. ‘moderate’) tillage soils averaged about 0.25 tonnes ha-1 (0.11 tons ac-1) across the state. Examples of mean rates of soil C change for different tillage systems and selected soil types under corn-soybean rotations are shown in Figure 21.
Figure 21
Rates are state averages over the period 1994-2004. Bars within columns show the range of values across counties within the state. Rates for moderate tillage and no tillage are averages for a ten-year period (1994-2004) following conversion from intensive tillage. Also shown are projected rates of change with continuation of intensive tillage practices. In non-hydric (well-drained), intensively tilled soils (Figure 21 A), a low rate of increase in soil C is predicted, driven by increasing crop residue additions. In contrast, hydric (poorly drained) soils under conventional, intensive cultivation (Figure 21 B) are predicted to be losing C in most locations, due to the stimulus of soil drainage on decomposition rates that overrides the positive effects of increasing residue inputs. In summary, estimates of the current rates of C change under the predominant crop (corn-soybean) rotation in Iowa are due largely to changes in tillage practices, but with an underlying influence of increasing crop residue inputs for all systems.
Carbon sequestration rates predicted for Iowa soils with adoption of no tillage are in line with results from several long-term studies in the Corn Belt Region (Paustian et al., 2001a). Recent regression based estimates of C accumulation under no-till from 15 long-term sites in the Midwest show average annual rates of 0.72 tonnes ha-1 (M. Eve, pers. comm.). Numerous other studies of tillage impacts illustrate the general trend of increased C sequestration from reducing or eliminating tillage, although rates vary considerably according to soil, climate, and management variables (Paustian et al., 1997c; West and Marland, 2001). In a few cases, negligible effects of tillage reduction on soil C have also been reported (e.g., Wander et al., 1998). The variability of the modeled response of soil C to adoption of no tillage is less than might be expected based on comparisons across different field studies with simulated rates of increase with no tillage adoption varying between about 0.5 and 0.7 tonnes ha-1 yr-1 (0.22 and 0.31 tons ac-1 yr-1) across all soils. Additional sources of variability in response to tillage changes that can occur at a site-specific level, such as reduced productivity with unsuccessful no-till management, are not captured in the model application at county and state scales.
Conversion of annual cropland to CRP grasslands was estimated to yield C sequestration rates of about 1.3 tonnes ha-1 yr-1 (0.58 tons ac-1 yr-1), average across the state. Simulated rates varied across counties and soil types, ranging from about 1.2 to 1.7 tonnes ha-1 yr-1 (0.54 and 0.76 tons ac-1 yr-1) (Figure 22)
Figure 22
In comparison, Follett et al. (2001) estimated rates of C sequestration for 14 sites in the Central US, based on field sampling of paired CRP sites, and
averaging 0.9 tonnes ha-1 yr-1 (0.40 tons ac-1 yr-1). Sites in Iowa had the highest rates of C increase, ranging up to 4 tonnes ha-1 yr-1 (1.78 tons ac-1 yr-1). Paustian et al. (2001b) document several field studies of attributing increases in soil carbon with prairie restoration and application of CRP on former annual cropland, with values of around 1 tonnes ha-1 yr-1 (0.40 tons ac-1 yr-1) for conditions similar to those in Iowa. As for reduced tillage effects, the model does not reflect the full range of variability in C change under CRP that would be expected through site-specific effects (e.g., poor stand establishment, high residual nutrient levels, pest effects), which cannot be captured in a regional assessment. It should also be noted that assumptions regarding nitrogen availability have a significant impact on the predicted response of CRP. For the present simulations, we assume that CRP planting included a legume component to help meet demands for nitrogen by the perennial vegetation. The same assumption was used for other grass conversions (e.g. grassed waterways, filter strips) given that these areas would likely receive significant N inputs from runoff and/or through the presence of legumes. Simulations with pure grass, with no fertilization and minimal pre-CRP residual nitrogen, were predicted to yield only about half of the rates reported here (unpublished data). Thus our estimates for CRP could be somewhat high if there are areas of CRP with significant nutrient limitations on productivity.
The simulation results were mapped back to soil polygons to show how predicted changes in soil carbon storage vary geographically across the state (Figure 23).
Figure 23
For the conversion of corn-soybean under intensive tillage to no tillage, average rates of increase of C stocks during the first 10 years varied from less than 0.5 tonnes ha-1 yr-1 to greater than 0.65 tonnes ha-1 yr-1 (0.22 and 0.29 tons ac-1 yr-1). The spatial patterns that emerge are a function of a number of interacting factors such as soil texture and hydric characteristics, climate influences on productivity and decomposition, and regional differences in management practices such as the relative dominance of grain versus hay crops and the extent and timing of soil drainage. Thus, interpretation of the regional patterns is complex. Rates of increase under no tillage are somewhat lower along the western side of the state where productivity tends to be lower due to less precipitation. Locally, the greatest response to no tillage is predicted to occur on the lighter textured (sandy) soils in the eastern half of the state (shown in red in Figure 23). Although C stocks on these soils are considerably lower than on surrounding silt and clay-loam soils, the predicted increase in soil C following adoption of no tillage is predicted to be higher. The next highest response is shown on scattered soil polygons having a low incidence of hydric soils (< 10%) dominated by loam textures in the northeastern part of the state. As shown in Figure 19, C increases under no tillage are predicted to be higher on non-hydric compared to hydric soils. In the south-central part of the state, greater increases due to no tillage are shown to occur on loam compared to silt loam soils. Most of the rest of the state is predicted to show an intermediate response to no tillage adoption on the order of a 0.5-0.6 tonnes ha-1 yr-1 (0.22-0.27 tons ac-1 yr-1) increase.
To estimate current changes in soil C storage under present management systems, we used the mean annual rates of C change for the simulated period 1994-2004 for each management sequence X soil X county combination, multiplied by the area represented by that combination. Compiling all of the model-based estimates for managed cropland and grass with separate calculations for tree conversion, wetland restoration and cultivated organic soils, we estimate that Iowa soils are currently (i.e., based on 1998 data) a net sink for CO2, accumulating soil C at a rate of about 3.1 MMT per year (Table 6).
Table 6: 1998 summary of C sequestered by tillage system in Iowa
|
Management System
|
Metric Units
|
English Units
|
|
Hectare
|
Tonne C
|
Tonne CO2
|
Acres
|
Ton C
|
Ton CO2
|
| Cropland |
9,339,452
|
1,902,551
|
6,982,362
|
23,078,197
|
2,097,203
|
7,696,735
|
| CRP Land |
785,712
|
840,862
|
3,085,964
|
1,941,529
|
926,891
|
3,401,690
|
| Grass Conversion |
311,437
|
494,939
|
1,816,426
|
769,575
|
545,577
|
2,002,268
|
| Tree Conversion |
22,617
|
11,308
|
41,500
|
55,888
|
12,465
|
45,747
|
| Wetland Reversion |
37,646
|
18,070
|
66,317
|
93,025
|
19,919
|
73,103
|
| Cult Organic Soils |
22,000
|
-170,000
|
-623,900
|
54,363
|
-187,393
|
-687,732
|
| |
| State Total |
10,518,864
|
3,097,730
|
11,368,669
|
25,980,360
|
3,414,662
|
12,531,811
|
|
The largest contributions to this C sequestration is attributed to the reduction in area under intensive tillage over the past 10-20 years, and the conversion of formerly annually cropped area to perennial grasses through the CRP, as well as the increased installation of grass waterways, field buffers, filter strips, terrace walls and other conversion to grassed conservation practices. More than one-half of Iowa’s 10.6 million hectare (26 million acre) of cropland are still managed using conventional tillage practices, predominately under corn-soybean rotations. While many conventionally managed soils may be net sources of CO2 (particularly artificially drained hydric soils), our analysis predicts an overall slow rate of increase of soil C for the conventionally managed cropland in the state due to increasing amounts of crop residues added to soil over the past three to four decades. Others (Cole et al., 1993; Allmaras et al., 2000) have also suggested that the general trends in crop productivity since WWII have changed agricultural soils from being a net C source to a net sink in the US. Wetland restoration is projected to represent a net carbon sink, but the overall effects on the C balance for the state are minor due to the relatively small 37,000 hectare (91,000 acre) of associated area. Phase I details the extent of the cultivation of 22,000 hectare (54,000 acre) of organic soils, and the resulting source of CO2 emissions, particularly considering the limited extent of this practice in the state. These areas should be identified to document what land use and management decisions are presently occurring on them. Assuming these areas are being cropped, the application of C conserving conservation practices can have a large impact on this potential source of C from the soil.
County Summaries
The 1998 calculated effects of management, tillage and conservation practices for each county are summarized in several tables in Appendix C. Table C-1 details the area affected by each tillage system, the C sequestered by tillage system, and the CO2 removed from the atmosphere for each system. All values were converted to units of CO2 removal from the atmosphere using the conversion factor of 3.67. Figures 24-26 show the distribution of the C being sequestered in 1998 throughout the state for the three types of tillage practices (intensive, moderate and no tillage). Table C-2 is the summary of the C sequestered on cropland for each county in SI and English units.
Figure 24
Figure 25
Figure 26
Table 7: 1998 total C sequestered by tillage system in Iowa
|
Tillage System
|
Metric Units
|
English Units
|
|
Hectare
|
Tonne C
|
Tonne CO2
|
Acres
|
Ton C
|
Ton CO2
|
| Intensive Tillage |
5,241,467
|
427,394
|
1,568,536
|
12,951,896
|
471,121
|
1,729,014
|
| Moderate Tillage |
2,997,209
|
816,811
|
2,997,696
|
7,406,235
|
900,380
|
3,304,395
|
| No Tillage |
1,100,776
|
658,346
|
2,416,130
|
2,720,066
|
725,702
|
2,663,326
|
| |
| State Total |
9,339,452
|
1,902,551
|
6,982,362
|
23,078,197
|
2,097,203
|
7,696,735
|
|
Table C-3 summarizes the effects of CRP in each county, the C sequestered, and the CO2 removed from the atmosphere for CRP lands. Our analysis assumes a 50% legume-50% non-legume plant community, which provides a source of nitrogen due to the fixing capacity of legume plants. Figure 27 shows the distribution of C sequestered in 1998 due to CRP throughout the state. Since these lands have been in grass for over 10 years, the rates of C sequestration are declining. The CRP lands provide valuable cover for wildlife, reduce erosion and provide water quality benefits. Should the land manager decide to return these lands to crop production, the 'Iowa Carbon Potentials' database can provide the effects of different management options to assess the changes in soil organic matter. The database also provides land managers the ability to calculate the projected C sequestration potential of new lands that are enrolled in CRP.
Figure 27
Figure 28
Figure 29
Figure 30
Figure 31
Results from the 203,000 model runs for each combination of climate (i.e. county average), soil type, and management sequence were compiled and entered into a distributed database that can be used to estimate current soil carbon changes, as well as potential C sequestration rates for the whole state. To provide a planning and assessment tool for land managers, model simulation results were organized into an Access (Microsoft Corp.) database with facilities to query and graph the results. The 'Iowa Carbon Potentials' database provides this interface with supporting user manual documentation (Appendix D) and illustrative presentation (enclosed CD-ROM). The user selects the desired county, major soil types within the county, and then selects from the menu crop rotations and tillage management sequences for each of two time periods (1974-1994 and 1994-2014). Two contrasting scenarios can be specified and displayed at the same time, allowing comparison of management alternatives. In addition, a table is produced showing the difference in C stock change (for both soil organic matter and crop residues) between scenarios. The data are configured to display the relative changes since the base year of 1974, but actual simulated C stocks are given in the accompanying data sheets.
|
|