REDUCED TILLAGE AND INCREASING CROPPING INTENSITY IN THE GREAT-PLAINSCONSERVES SOIL C

Concern about soil organic matter losses as a result of cultivation ha s been voiced consistently since the early part of the 20th century. S cientists working in the US. Great Plains recognized that organic matt er losses from an already small pool could have major negative consequ ences on soil physical properties and N supplying capacity. The advent of reduced- and no-till systems has greatly improved our ability to c apture and retain precipitation in the soil during the non-crop period s of the cropping cycle, and has made it possible to reduce fallow fre quency and intensify cropping systems, The purpose of this paper is to summarize the effects of reduced tillage and cropping system intensif ication on C storage in soils using data from experiments in North Dak ota, Nebraska, Kansas, Colorado, and Texas. Decades of farming with th e wheat (Triticum aestivum L.)-fallow system, the dominant fanning sys tem in the Great Plains, have accentuated soil C losses. More intensiv e cropping systems, made possible by the greater water conservation as sociated with no-till practices, have produced more grain, produced mo re crop residue and allowed more of it to remain on the soil surface. Combined with less soil disturbance in reduced- and no-till systems, i ntensive cropping has increased C storage in the soil, We also conclud e that the effects of cropping system intensification on soil C should not be investigated independent of residue C still on the surface. Th ere are many unknowns regarding how rapidly changes in soil C will occ ur when tillage and cropping systems are changed, but the data summari zed in this paper indicate that in the surface 2.5 cm of soil, changes can be detected within 10 years, It is imperative that we continue lo ng-term experiments to evaluate rates of change over an extended perio d, It is also apparent that we should include residue C, both on the s urface of the soil and within the surface 2.5 cm, in our system C budg ets if we are to accurately depict residue-soil C system status. The a ccounting of soil C must be done on a mass basis rather than on a conc entration basis. (C) 1998 Elsevier Science B.V. All rights reserved.