Biol Fertil Soils (1996) 22:184-190 9 Springer-Verlag 1996 Y.K. Soon 9M.A. Arshad Effects of croppingsystemson nitrogen, phosphorus and potassiumforms and soil organic carbon in a Gray Luvisol Received: 23 June 1994 Abstract The effects of up to 23 years of agricultural cropping of a boreal forest soil on soil organic carbon (SOC) and N, R and K pools were studied. The cropping systems studied were: (a) continuous barley, (b) continu- ous forage bromegrass, (c) continuous forage legume, and (d) barley/grass-legume forage rotation. Continuous bro- megrass increased while other cropping systems decreased SOC in the surface soil. Kjeldahl N in soil approximately followed the trend in SOC. The net gain in N under con- tinuous grass was attributed mostly to nonsymbiotic N fixation. Changes in SOC content appeared to be also in- fluenced by cropping and tillage frequencies. Changes in fixed (intercalary) ammonium were small. There was no measurable change in total R in part, because input was only slightly higher than crop offtake. Organic P increased under continuous bromegrass, and tended to decrease un- der continuous legume. The C/N and C/P ratios of soil organic matter decreased slightly with cropping. Ex- changeable K (Kex) was decreased by cropping systems containing a legume crop to a greater extent than those without a legume crop. Most of the decrease occurred in the 0-15 cm depth. Nitric acid extractable K was not af- fected by cropping. Since net loss of Kex to 30 cm depth was substantially less than crop offtake, it is suggested that subsoil K reserves and matrix K were supplying a ma- jor portion of the crops' K requirement. It is concluded that the effects of cropping systems on SOC, N, P and K are influenced by crop type, and cropping and tillage fre- quencies. Key words Cropping systems 9 Soil organic matter 9 Nitrogen 9 Phosphorus 9 Potassium Y.K. Soon ( ~ ) - M.A. Arshad Northern Agriculture Research Cenlxe, Agriculture and Agri-Food Canada, PO Box 29, Beaverlodge, Alberta, TOH OCO, Canada Fax (+1) 403 3548171 Introduction The effects of cultivation on changes in soil nitrogen (N), phosphorus (P) and potassium (K) have been reported in several studies. Nonexchangeable ammonium was virtually unchanged by cultivation, although a decrease in other pools of N was apparent (Keeney and Bremner 1964; Hinman 1964). Smith and Young (1975) reported an 11% increase in nonexchangeable ammonium due to cultivation on soils that had received between 12 and 336 kg fertilizer N ha-1 year 1 prior to sampling. Tiessen et al. (1982) found that cultivation decreased P in three surface soils, primarily in the organic fi'action (Po). Inorganic P (Pi) in- creased in one soil and decreased in another. Sharpley and Smith (1983) reported similar trends for eight pairs (culti- vated vs virgin) of soils representative of different United States cropping areas. They attributed observed increases in Pi to fertilizer input and mineralization of Po- Sharpley and Smith (1988) also found increased exchangeable K (K~x) and acid-extractable K in four cultivated soils that had received a total of 200-5000 kg K ha -1 through fertili- zation prior to sampling. Of the remaining four cultivated soils that had decreased Kex and acid-extractable K, two had not received any potash fertilizer. Most of the above studies, in particular the Canadian ones (Hinman 1964; Tiessen et al. 1982), were conducted on soils that were originally under grasslands or range- lands. Also, these studies did not examine the effects of different cropping systems. We set out to determine the ef- fects of long-term cropping on soil organic matter, and N, P and K following clearing of a boreal forest, and to com- pare the effects of four low-input cropping systems. This study is based on 18-23 years of data from replicated plots. Cultivation has been shown to change the quality of soil organic matter, aggregate stability and organic matter distribution among different aggregate sizes (Whitehead et al. 1975; Elliot 1986; Gupta and Germida 1988). Some of these aspects will be reported in a companion paper.