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 (RI), and root C/shoot C ratios (k) of major grain crops in the USA; (ii) discuss historical agricultural-practice impacts on soil organic C (SOC); and (iii) compare estimates of total (above- and belowground) source C production (ESC) relative to minimum source C inputs required to maintain SOC (MSC). Aboveground MSC input iveraged 2.5 +/- 1.0 Mg C ha(-1)yr(-1) (n = 13) based on moldboard plow sites and 1.8 +/- 0.44 Mg C ha(-1)yr(-1) (n = 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 (k(rec)), the true magnitude of the C cycle is at least twice that when ESC is estimated using k excluding rhizodeposition (k(his)). Neglecting rhizodeposition C underestimates the net production of C in cropland. Current yields and measured MSC predict continued SOC loss associated with soybean [Glycine mar (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.