1863 Ecology, 86(7), 2005, pp. 1863–1872  2005 by the Ecological Society of America THE IMPACT OF CROPPING ON PRIMARY PRODUCTION IN THE U.S. GREAT PLAINS JOHN B. BRADFORD, 1,2,3 WILLIAM K. LAUENROTH, 2 AND INGRID C. BURKE 2 1 Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80521-1472 USA 2 Department of Forest, Rangeland and Watershed Stewardship, Colorado State University, Fort Collins, Colorado 80521-1472 USA Abstract. Land use and altered carbon dynamics are two of the primary components of global change, and the effect of land use on carbon cycling is a crucial issue in regional scale biogeochemistry. Previous studies have shown that climate and soil conditions control net primary production (NPP) at regional scales, and that agricultural land use can influence NPP at local scales through altered water availability and carbon allocation patterns. How- ever, few studies have attempted to quantify the effect of cultivation on NPP at regional scales, and no studies have examined this relationship for the most heavily cultivated region of the United States, the Great Plains. We quantified current regional aboveground and belowground productivity (including cultivation) for nine years on a county basis from (1) USDA agricultural census data, and (2) STATSGO range site production values. By com- paring these data with values of native vegetation NPP (precultivation) derived from STATSGO, we estimated that cultivation is increasing regional NPP by 10%, or 0.046 Pg C/yr. In addition, we examined the relationship between cultivation of particular crops and NPP change and characterized the influence of individual crops on primary productivity. Key words: agriculture; carbon; cropping; grassland; land use; primary production; regional scale. INTRODUCTION Land use and altered carbon cycling are two major components of global change (Vitousek 1994). Land use practices often include dramatic modification of vegetation and are therefore some of the most direct and common ways that humans impact ecosystems (Houghton 1995, Houghton et al. 1999). Cropping is a widespread component of land use, covering over 1.5  10 9 ha worldwide (Richards 1990). Although crop- ping has obvious effects on vegetation structure, it also influences ecosystem processes, including the magni- tude and direction of the carbon flux between the at- mosphere and the soil–vegetation system (Houghton et al. 1999). Despite the potential impact of land use on ecosys- tem processes, few studies have attempted to quantify changes in long-term primary productivity of cultivat- ed areas over large scales. Many regional scale studies have focused on patterns in native plant production in relation to climatic and/or edaphic conditions (e.g., Lieth 1975, Sala et al. 1988, Epstein et al. 1997). Of the studies that have examined the relationship between land use and carbon cycling, most have either (1) ex- Manuscript received 12 March 2004; revised 27 September 2004; accepted 9 November 2004. Corresponding Editor: S. D. Smith. 3 Present address: USDA Forest Service, Rocky Mountain Research Station, 240 West Prospect, Fort Collins, Colorado 80526 USA. E-mail: jbbradford@fs.fed.us amined carbon release from initial land conversion (ei- ther into or out of cultivation) at large scales, (2) quan- tified changes in carbon cycling processes at small scales, or (3) used ecological simulation models, re- mote sensing, and/or atmospheric models to estimate large-scale carbon cycling processes. The first type of studies combines information about the amount of land converted with results about gain or loss of carbon (soil carbon or standing biomass) as a result of conversion (e.g., Dale et al. 1991, Houghton 1995, Dale 1997, Paustian et al. 1999). For example, land conversion from native vegetation into cropping in the United States prior to 1945 is estimated to have released 27 Pg of carbon, whereas subsequent aban- donment, forest regrowth, and fire suppression have sequestered 2 Pg C (Houghton et al. 1999). Although this work provides insight into the initial impact of land conversion and subsequent carbon exchange, it does not help us to understand how land use practices influence primary productivity in areas already con- verted. The second type of studies relies on site-level mea- surements of productivity in cropped and uncropped comparisons to quantify the long-term changes in pro- ductivity. These results indicate that cropping in tem- perate grasslands typically increases aboveground pro- ductivity (Buyanovsky et al. 1987, Kucharik et al. 2001), can decrease belowground productivity (Smith 2003), and almost always decreases soil carbon (Burke et al. 1997). Although these studies provide valuable