Estimating Source Carbon from Crop Residues, Roots and Rhizodeposits Using the National Grain-Yield Database

Crop residue management received little attention until about 1970. Records of crop residue production are limited, but crop yield databases have been available since 1865. Carbon sequestration and other conservation benefits require a detailed knowledge of crop residue production and management. Our objectives are to: (i) review grain and biomass yield, harvest index (HI), and root C/shoot C ratios (k) of major grain crops in the USA; (ii) discuss historical agriculturalpractice impacts on soil organic C (SOC); and (iii) compare estimates of total (aboveand belowground) source C production (ESC) relative to minimum source C inputs required to maintain SOC (MSC). Aboveground MSC input averaged 2.5 6 1.0 Mg C ha yr (n 5 13) based on moldboard plow sites and 1.8 6 0.44 Mg C ha yr (n 5 5) based on no-till and chisel plow sites. These MSC values included only aboveground source C, thus underestimate the total MSC. When ESC is estimated from k, including rhizodeposition (krec), the true magnitude of the C cycle is at least twice that when ESC is estimated using k excluding rhizodeposition (khis). Neglecting rhizodeposition C underestimates the net production of C in cropland. Current yields and measured MSC predict continued SOC loss associated with soybean [Glycine max (L.) Merr.] and some wheat (Triticum aestivum L.) production management unless conservation tillage is used and ESC is increased. The adequacies of ESC to maintain SOC has direct implications for estimating the amount of crop residue that can be harvested and yet maintain SOC. SOIL organic C is the net result of simultaneous processes, the addition of floral and faunal biomass and the C losses due to biological respiration and physical losses related to tillage, erosion, and leaching and runoff of dissolved organic C as part of the C cycle. Terrestrial C stocks are sensitive to changes in land management (e.g., conversion to cropland, or changes in crop rotation or tillage system), climate, and soil. Inventories of soil C stocks are necessary for the development of strategic policies regarding U.S. agriculture and its associated environment (Eve et al., 2001). Inventories (e.g., Eve et al., 2001, 2002) are helpful; however, an improved inventory of current and potential source C inputs is needed. Source C, as used in this review, refers to the organic C inputs derived from plant biomass (Allmaras et al., 2004). A net loss of SOC (20–60%) was measured in most agricultural land in the first 50 yr after conversion from its native prairie or forest state (Huggins et al., 1998a; Janzen et al., 1998; Lal et al., 1998; Follett et al., 1997). In prairie soils, C losses were 17% in the upper landscape position and.70% in the footslope position (Slobodian et al., 2002). Greater biomass production and greater retention of crop residues through adoption of a diverse crop rotation and conservation tillage (especially no tillage) that retains crop residue on or near the soil surface can often reverse this loss and increase SOC (Lal et al., 1998; Allmaras et al., 2000; Reicosky and Allmaras, 2003). An increase in organic C inputs relative to the CO2 efflux or other losses (e.g., tillage, erosion, or leaching) is necessary to increase SOC. Before we continue our discussion of the C cycle, we would like to define related terms. We use the term soil organic matter (SOM) as defined by Stevenson (1994) to include the whole of organic matter in soils including litter, light fraction, microbial biomass, water-soluble organics, and stabilized organic matter. Soil organic C is the C fraction of these pools. Carbon from all these various pools is included when SOC is determined by combustion methods, except surface litter, if the samples are sieved to remove surface litter. According to the Stevenson (1994) definition, most SOM is comprised of stable organic matter, which are protected pools having a mean residence time of .5 yr, to the very recalcitrant C, which may have mean residence times on the order of centuries or longer. Stable organic matter or humus corresponds to the Soil Science Society of America (1998) definition of SOM. Generally, SOM is calculated by assuming SOM is 40% C by mass, therefore values reported in the literature usually fit the Stevenson (1994) definition of SOC and SOM. Therefore, in this review, SOM reflects the broader definition as defined by Stevenson (1994). The simple C cycle showing the flow of C from photosynthesis to the roots, through the soil, and back to the atmosphere is presented in Fig. 1 to exemplify complexities of understanding the rapid C cycling within an agricultural system. In this schematic, the C found in various pools (stable SOM, decomposing tissue, soil organisms, free organic C, exudates, etc.) all contribute to SOC. Photosynthate provides the energy for plant J.M.-F. Johnson, USDA-ARS, North Central Soil Conservation Research Lab., 803 IowaAve., Morris, MN 56267; R.R. Allmaras, USDAARS and Univ. of Minnesota, St. Paul, MN (retired); and D.C. Reicosky, USDA-ARS, Morris, MN. The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the USDA or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable. Received 15 June 2005. *Corresponding author ( [email protected].