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)
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evolved over the past century as unsustainable year-round grazing has been
replaced by rotational grazing systems and intensive grazing systems in order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 1930’s and
herbicide in 1960’s) 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.