Estimating the contribution of agricultural land use to terrestrial carbon fluxes in the continental US Keith Paustian 1,2, Steven Ogle 2, Scott Denning 3 and Erandi Lokupitiya 3 1 Dept. of Soil and Crop Sciences, Colorado State University 2 Natural Resource Ecology Lab, Colorado State University 3 Dept. of Atmospheric Sciences, Colorado State University

Outline Agriculture’s role in the US C balance Bottom-up ‘inventory’ modeling of C dynamics in cropland and grassland Agricultural influences on process-based CO 2 flux and transport modeling

The (familiar) overarching ?s What is the current C balance and magnitudes of sources and sinks in the US? Will sinks decrease in the future with LU changes and/or with CC? Can sinks be increased by management? How much? How fast?

What is agriculture’s role in all of the above? Responsible for ~ 7% of US GHG emissions (mostly from non-CO 2 ) C balance on agricultural land dominated by soil carbon stock changes (and commodity exports) Policy needs –National inventory reporting (UNFCCC) –National (and market-based) GHG mitigation efforts ( Tg C/yr potential) –Resource conservation policies (national, state, local) –Biofuel development the new ‘wildcard’

Database Management Run Control Simulation Model: Century Structural Uncertainty Estimator Management Activity Environmental Conditions Point Scale Data (NRI Survey) PDF Model Inputs Database Results Database Bottom-up modeling framework

Data sources National Resource Inventory (NRI) –Statically-based sample of ca. 800,000 points since 1979 –LU, soils, crop rotations/vegetation –Most land management practices NOT collected County-, state- and regional survey data of management practices –E.g. tillage, fertilization, manuring, irrigation Regional-level land use practices (pre-1980)

Totals for US Croplands (1990s) : ± 22% Tg CO 2 eq. yr : ± 16% Tg CO 2 eq. yr -1

Current status – bottom-up efforts At national level, estimates of average soil C stock changes are relatively well constrained. However, at subregional & local levels uncertainties are high Current efforts focus on: –Reducing finer scale uncertainty –Improving estimates of NPP and C inputs –Obtaining bench-mark data on soil C stocks under field conditions

Approach Adapting MODIS/EVI-based production estimates from NASA-CASA into Century Testing NPP and yield estimates at field-scale experiments Field-scale observation of soil C stocks and establishment of a pilot monitoring system Applying the new model to NRI inventory points

MODIS Enhanced Vegetation Index (EVI) Frequent (10 d) return interval Low cost (primary data free) Better resolution (250m) than previous imagery (e.g. AVHRR) often used for regional crop modeling But still challenges dealing with mixed pixels and using traditional LTEs for ground-truthing.

CASA CQUEST MODIS Enhanced Vegetation Index (EVI) - May 2004 Spatial Resolution: 250 meter Farm boundaries and production patterns begin to emerge in details of landscape at 250-m resolution Courtesy Chris Potter – NASA-Ames

Agricultural influences on process- based CO 2 flux and transport modeling What’s the interannual variability in C fluxes from croplands and how does it influence interannual variability in cropland soil C stocks? Capturing within-season dynamics of cropland C fluxes.

Cropland NPP the mid-continent region in a dry year (1988) and a wet year (1997)

Lokupitiya et al Tg C yr -1 Variability in residue C inputs Permanent area for major crops (90 Mha)

Annual anomalies in residue C and SOC stocks Lokupitiya et al. in prep (all land use and management changes are excluded)

Measured and modeled (SIB3-RAMS) CO 2 concentrations at WLEF tower (400 m) in Wisconsin Courtesy Scott Denning

Revised phenology and leaf area development algorithms Cropland modifications to SIB3 Lokupitiya et al. unpubl.

a. before b. now Results from testing SIB3 for maize-soybean flux site – Bonneville, IL Lokupitiya et al. unpubl.

Midcontinent Intensive (MCI) Focus for initial inter- comparisons and synthesis of bottom-up approaches and bottom-up/top-down estimates within NACP Field-campaign

Conclusions Soil C on US ag. land are currently a small sink Estimates relatively well constrained at national level thanks to abundant activity data and LTEs But, uncertainties high at local scale – where management decisions are implemented. Broad-based soil monitoring network and better fine- scale estimates of C additions to soil needed to reduce uncertainties at local scales. Large short-term C fluxes and large interannual variability associated with agricultural crops pose challenges for flux/transport based estimates of long- term C balance in agriculturally dominated ecosystems