SOIL C AND N CHANGES UNDER TILLAGE AND CROPPING SYSTEMS IN SEMIARID PACIFIC-NORTHWEST AGRICULTURE

Soils in semi-arid regions are highly susceptible to soil organic matt er (SOM) loss when cultivated because of erratic yield, removal of cro p residue for feed or fuel, uncontrolled soil erosion, and frequent fa llowing to increase water storage. It is important to quantify the eff ect of each factor to be able to identify agoecosystems that are susta inable and recognize the management practices that best sequester C in soil. We identified changes in SOM in long-term experiments, some dat ing from the early 1900s, by evaluating tillage and crop rotation effe cts at several locations in semi-arid regions of the US Pacific Northw est. The major factors influencing changes in organic C and N were the frequency of summer-fallow and the amount of C input by crop residue. Soil erosion was low in long-term studies, but even limited soil loss can have a substantial impact on C and N levels if allowed over many years. Yearly crop production is recommended because any cropping syst em that included summer-fallow lost SOM over time without large applic ations of manure. We conclude that most of the SOM loss was due to hig h biological oxidation and absence of C input during the fallow year r ather than resulting from erosion. Decreasing tillage intensity reduce d SOM loss, but the effect was not as dramatic as eliminating summer-f allow. Crop management practices such as N fertilization increased res idue production and improved C and N levels in soil. SOM can be mainta ined or increased in most semi-arid soils if they are cropped every ye ar, crop residues are returned to soil, and erosion is kept to a minim um. SOM loss may be more intense in the Pacific Northwest because fall owing keeps the soil moist during the summer months when it would norm ally be dry. Our experiments identify two primary deficiencies of long -term studies to measure C sequestering capability: (1) soil C loss ca n be partitioned between erosion and biological oxidation only by esti mation, and (2) C changes occurring below 30 cm in grassland soils can not be quantified in many instances because samples were not collected . (C) 1998 Elsevier Science B,V. All rights reserved.