Fertilization Effects on Soil Organic Matter Turnover and Corn Residue C Storage E. G. Gregorich,* B. H. Ellert, C. F. Drury, and B. C. Liang ABSTRACT Soil organic matter turnover is influenced by N; thus long-term fertilization of corn (Zea mays L.) may significantly affect soil organic matter levels. Effects of fertilization on soil organic matter turnover and storage of residue C under continuous corn were evaluated using soils from a long-term field experiment in Ontario. Total organic C and natural I3 C abundance measurements indicated that fertilized soils had more organic C than unfertilized soils, the difference accounted for by more C 4 -derived C in the fertilized soils. About 22 to 30% of the soil C in the plow layer had turned over and was derived from corn in the fertilized soils; in unfertilized soils only 15 to 20% was derived from corn. Assuming that organic matter turnover follows first-order kinetics, the half-life of Cj-derived C in the surface 10 cm of both soils was the same, about 19 yr. Natural I3 C abundance measurements and estimates from a soil organic matter model indicate that 10 to 20% of the added residue C was retained in the soil. Fertilized soils had more light fraction (LF) C than unfertilized soils. More than 70% of the C in the LF of fertilized soils was derived from corn; in unfertilized soils only 41% was derived from corn. The half-life of C.i-derived C in the LF was shorter than 10 yr. These results indicate that adequate fertilization increases crop yields, in turn leading to greater C storage, and that fertilization does not significantly alter the rate of turnover of native soil organic matter. T HE AMOUNT OF ORGANIC MATTERin soil is a function of the amount of plant residues returned to the soil and the rate at which those residues decompose. It is often reported that organic residue addition is one of the most important factors influencing organic matter levels. Larson et al. (1972) found that changes in soil organic C were linearly related to the amount of residue applied to soil under continuous corn. Rasmussen et al. (1980) made similar observations and also noted that the changes were independent of the type of residue applied. Many soils have received applications of inorganic amendments for several decades, and it is recognized that the addition of fertilizer on a regular basis leads to an increase in soil organic matter (Campbell and Zentner, 1993; GlendiningandPowlson, 1991). The rate of change in soil organic matter is dependent on a number of factors, including the initial level of organic matter (Campbell et al., 1976) and texture (Bauer and Black, 1981). Liang and Mackenzie (1992) observed that soil C content in- creased by 18% after 6 yr of continuous corn fertilized at relatively high rates. The LF is a transitory pool of organic matter between fresh plant residues and humified soil organic matter. The LF concentration in soil is highly variable and depends on the amount and characteristics of C inputs and soil environmental factors that affect rates of decomposition E.G. Gregorich and B.C. Liang, Agriculture and Agri-Food Canada, Centre for Land and Biological Resources Research, Ottawa, ON, K1A OC6 Canada; B.H. Ellert, Agriculture and Agri-Food Canada, Research Centre, Lethbridge, AB, T1J 4B1 Canada; and C.F. Drury, Agriculture and Agri-Food Canada, Research Centre, Harrow, ON, NOR 1GO Canada. Received 15 Feb. 1995. "Corresponding author. Published in Soil Sci. Soc. Am. J. 60:472-476 (1996). (Gregorich and Janzen, 1995). The LF has been sug- gested as a sensitive indicator of changes of soil organic matter because of its responsiveness to management prac- tices (Gregorich et al., 1994). Field studies have sup- ported this concept and showed that soil organic matter accumulation is linked to accumulation of LF (Wander et al., 1994). Studies in western Canada have shown that application of N fertilizer significantly increases LF C in continuous cropping systems (Janzen et al., 1992; Biederbeck et al., 1994). Data from field experiments with 14 C-labeled plant material have been used to develop models that describe the decomposition and turnover of soil organic matter (Jenkinson, 1977; Voroney et al., 1989). These models usually partition the incoming residue into two compart- ments, each decomposing by a first-order process, but one much more quickly than the other. The introduction of C 4 plants to soil previously devel- oped under Cs vegetation results in the soil organic matter containing two isotopically different sources of C and provides a means of partitioning soil organic matter as to origin. The natural 13 C abundance method has been used to estimate soil organic matter turnover both in tropical (Martin et al., 1990) and temperate soils (Bales- dent et al., 1987; Gregorich et al., 1995). The objective of this study was to use the I3 C isotopic method to determine the long-term effects of fertilization on the turnover of soil organic matter and storage of C derived from corn residue in a medium-textured soil in southwest Ontario. METHODS AND MATERIALS The soil used in this experiment is a Brookston clay loam, a poorly drained soil (clayey, mixed, mesic Typic Haplaquoll) located at the Eugene F. Whelan experimental farm (Agricul- ture and Agri-Food Canada, Woodslee, Ontario). The average annual temperature at the experimental site is 8.7°C; the average maximum growing season (May-September) tempera- ture is 24°C and the average minimum temperature is 13°C. The average annual total precipitation is 876 mm, with rainfall accounting for 769 mm. The average maximum evapotranspira- tion rate is 654 mm. The average textural analysis for this soil is 280 g kg-' sand, 350 g kg~' silt, and 370 g kg" 1 clay. Although complete records of agricultural management of the experimental site prior to 1954 are not available, it is known that alfalfa (Medicago saliva L.) and red clover (Trifolium pratense L.) were grown for several years between 1940 and 1954. The land was summer fallowed in 1954; tile drains (100-mm diam.) were installed in 1955 at a depth of 71 cm and a spacing of 12.2 m. The plots, 76.2 m long by 12.2 m wide, were centered longitudinally above the tile drains (Bolton et al., 1970). Corn was grown on all plots from 1956 to 1958 to reduce residual effects and obtain uniform data. In 1959, 12 plots, consisting of six cropping treatment plots with fertil- izer and six without, were implemented. A continuous corn treatment with fertilizer and one without fertilizer as well as the fertilized continuous bluegrass (Poa pratensis L.) sod treatments were used in this study. The fertilized treatments received 16.8 kg N ha~', 67.2 kg P ha" 1 , and 33.0 kg K 472