Thomson, Allison (Joint Global Change Research Institute, 8400 Baltimore Ave., Suite 201, College Park, MD, 20740; Phone: 301-314-6750; Email: allison.thomson@pnl.gov)

 

Simulating Historical Soil Carbon Dynamics in Semi-Arid Rangelands

 

A.M.Thomson *, R.C. Izaurralde, M.P. McClaran, S.P. McLaughlin, N.J. Rosenberg

 

Semi-arid rangelands cover a large area in the US southwest and have changed extensively over the past century due to human influence. Although soil carbon content is low, a large area of land (3.8 million ha) in Arizona is managed by the State Lands Department. Therefore, if a management change was found to increase soil C and implemented across the region, it could result in significant amounts of C sequestration. First we must understand the physical potential for soil carbon sequestration in these rangeland ecosystems, how it is influenced by management practices, and how it has responded to the changes of the past century. Research on range management and ecosystem composition has been conducted for over 100 years, however analysis of soil properties has not been included in this research. By reconstructing the management and vegetation history in a process-based ecosystem model we can examine the impact on soil properties. We can then use that knowledge to project the potential of these semi-arid rangelands to act as a source or sink of soil C in response to management practices and climate variability and change. Like many semi-arid grasslands around the world, southeastern Arizona has experienced woody encroachment by velvet mesquite (Prosopis velutina). Mesquite are a physical impediment to grazing and also reduce the grass productivity of a pasture, therefore mesquite removal treatments have been attempted by clear-cutting and herbicide application. For this study, control and treatment pastures on the Santa Rita Experimental Range (SRER) were selected for biomass measurements and soil sampling. Livestock grazing has also evolved over the past century as unsustainable year-round grazing has been replaced by rotational grazing systems and intensive grazing systems in order to maintain productivity of the grasslands. The intensive grazing system referred to as holistic resource management (HRM) has been applied for the past 20 years to a private ranch adjacent to conservation land the Audubon Appleton-Whittell Research Ranch (AWRR). Biomass measurements and soil sampling were also conducted at these sites to assess the influence of intensive grazing and rest from grazing on lands which have not experienced mesquite encroachment. The patterns of mesquite encroachment and control and the details of grazing history are reconstructed in the simulation model EPIC (Environmental Policy Integrated Climate), a process based model which operates on a daily time step and simulates biomass production and soil C dynamics. Soils are initialized using field values for sites approximating the history of land use from 1900 and the model simulation of grass and mesquite biomass is calibrated using field data. The different management practices are simulated for the next 100 years. The impact of nitrogen-fixing mesquite encroachment on soils is positive and significant. While the management treatments (clear cut in 1930s and herbicide in 1960s) reduce mesquite influence, the impact on biomass does not last more than a decade. The model illustrates that grazing systems do have an impact on soil C, with variation in response due primarily to interannual climate variations. By reconstructing the past century of biomass and soil dynamics in the simulation model, we establish a basis for evaluating soil C sequestration potential and considering soil C in the development of land management plans for the region.