Agroecosystems / Carbon Sequestration Research Focus |
Nebraska Phase I Progress Report
Phase I: Results Century Modeling and Analysis Initial model parameters were set according to the procedure outlined in the Modeling Soil Organic Matter section of this report. The equilibrium Century runs provide the initial soil organic matter levels in the different pools. The model then simulates the change in soil C as a function of past agricultural practices as described in the Cropping Histories and Land Management section of this report. Modern agricultural practices are simulated as described in the Modern Crop Histories and Land Management section of this report. To simulate changes due to the Conservation Reserve Program (CRP), all non irrigated crop rotations, under intensive tillage, were modeled with a change to CRP grass plantings starting in 1986. Drainage of hydric soils occurred in two phases; the first phase being a partial drainage in the early 20th century, and more complete drainage in 1970. It was assumed that irrigation systems are not used on hydric soils. Each model run is associated with a specific soil texture and mean climate for each county. These are weighted based on the area each represents in a county and provides the mean value for each crop management system. The weighted mean soil C values is then associated with the are of each specific tillage system as described below. 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 in each county 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. This procedure was followed to account for the other crop rotations modeled in each county. Estimated C Sequestration It is estimated that current conservation practices are sequestrating 1.7 million tonnes (1.9 million tons) of C annually. This state wide analysis only addresses cropland and is based on our ability to estimate past landuse histories, current cropping and tillage systems. This estimate includes the impact of CRP and reflects lower C sequestration rates occurring on these lands due to the length of time they have been managed as a grass system. We estimate that slightly over 3 million hectares (7.5 million acres) of Nebraska is being currently irrigated, 4.7 million hectares (11.6 million acres) is currently in non-irrigated cropland and 0.5 million hectares (1.2 million acres) are in CRP. These areas were distributed to the three tillage systems (intensive tillage, moderate tillage and no tillage). The modeled C changes for each system in each county were joined and provide the C sequestration rates for the different systems. We also estimate that by intensifying cropping systems in the wheat-fallow rotations and going to a no tillage farming systems for all Nebraska cropland will sequester a total of 2.3 million tonnes (2.5 million tons) of C annually over 10 years. It should be noted that this increase of 0.6 million tonnes (0.7 million tons) of C is offsetting lower C sequestration rates from CRP and continuing no tillage systems. Large increases can be expected through the conversion for cropland to no tillage systems as shown in Table 5. We are continuing to refine the model for Nebraska agriculture systems. In collaboration with University of Nebraska, we are collecting data on irrigated crop systems, which will provide better estimates of irrigated biomass production, high yielding crops and effects of additional moisture on decomposition of organic matter. The phase II study is on going with each county collecting additional data using the CSRA. These activities will continue over the summer of 2001 and will provide additional refinements to the model. Upon completion of the phase II study, local land managers will have a tool to estimate C sequestration rates for their system and will allow individuals to estimate soil C changes based on land management decisions. Table 5: Phase I: Annual C Sequestration Rates.
The existing databases on land-use history, drainage, fertilizer amendments, crop rotations and tillage systems provided a good basis for the Phase I study, but they were limited in their ability to characterize local conditions. Thus a Phase II study was designed to help in bringing C sequestration information to the local land manager. The Phase II study utilized the information gained from the Phase I study, and expanded the scope and complexity of the analysis to include the local land managers’ decision-making and expert knowledge to assist in providing inputs into the Century model. This additional information was gathered for each county in the state, using a new survey instrument -- the Carbon Sequestration Rural Appraisal (CSRA) -- (Brenner et al. 2001). |