Submitted to: Carbon Sequestration In Soil An International Symposium
Publication Type: Book / chapter
Publication Acceptance Date: 8/3/2001
Publication Date: 8/3/2002
Citation: OLNESS, A.E., LOPEZ, D., CORDES, J., SWEENEY, C., MATTSON, N., VOORHEES, W.B. APPLICATION OF A MANAGEMENT DECISION AID FOR SEQUESTRATION OF CARBON AND NITROGEN IN SOIL. KIMBLE, J.M. ET AL., EDITORS. LEWIS PUBLISHERS, BOCA RATON, FL. AGRICULTURAL PRACTICES AND POLICIES FOR CARBON SEQUESTRATION IN SOIL. 2002. P. 245-253. Interpretive Summary: Agricultural production has relied on production of Nitrogen (N) for crops from microbial decay of soil organic matter. However, storage of carbon (C) in soil is now viewed as a means of reducing the global greenhouse gas emission of carbon dioxide (CO2). Storage of C in soil has a natural limit that is determined by the rate of microbial decay. Five factors largely control whether C is emitted from soil or stored in soil. These factors interact to control soil microbial decay of dead plant matter. These factors are soil pH, soil water content, soil bulk density, soil temperature and soil clay content. Soil bulk density determines the total amount of void space in soil. Void space in soil is occupied by water and air. Soil clay content determines the amount of water that is retained or held in the soil. Microbial decay is maximized when about two-thirds of the void space is filled with water. A new energy model is used to describe the erelationship of soil clay content and water-filled pore space. Soil temperature seems the most likely and inexpensive means of slowing soil microbial decay. Nitrogen is a critical component for storage of C in soil. About 10% of the soil organic matter is N. The opportunity cost for the U.S. Corn Belt of stored N is about $31 to $62 billion for each 1% of C stored in the soil. Impact: The theories contained in this paper serve as a guide to policy makers, scientists and crop producers as 1) a means of increasing storage of C in soil, 2) determining the value of stored organic matter, and 3) the most likely cost efficient means of increasing soil C.
Technical Abstract: The US Corn Belt has more than 53 million hectares under cultivation annually. These soils are continually being mined of their carbon (C) and nitrogen (N) for crop production. Carbon fixation depends largely on N. The export of N from farms has exacerbated loss of C from soil. Mineralization of N from soil organic matter is accompanied by a loss of organic C from soil. A recently developed N fertilizer decision aid uses five factors to estimate the amount of N mineralized from soil organic matter. These factors are clay content, soil pH, soil bulk density, temperature, and rainfall (water balance). Clay content plus organic matter determines the range of soil water content between -33 and -1500 kPa suction; this, plus the net difference between rainfall and evaporational loss, is the water available for crop production. Soil bulk density determines the range of air filled pore space for a given soil texture. Soil pH (hydrogen ion activity) controls microbial enzymatic efficiency; N is mineralized and nitrified most rapidly at a pH optimum of about 6.7. Differences in rates of mineralization of N vary by > 100% in adjacent soils. The N decision-aid suggests that organic C sequestered in the soil increases when microbial activity decreases. This is effected by a) cooling the soil through increased residue cover, b) adjusting soil bulk density to achieve sub-optimal aeration for aerobic microbial activity and c) effecting a soil pH which is sub-optimal for microbial activity. Within the Corn Belt, the opportunity cost of increasing soil organic C by 1% in the surface 15-cm of the soil is estimated at $27 to $66 billion dollars in N alone. The most likely source of this amount of N seems to be symbiotic fixation with legumes.