ratio mass spectrometer (Isotech, Middlewich, England). The natural abundance of heavy isotopes is expressed as parts per thousand (%o), relative to the ...
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
(Gregorich and Janzen, 1995). The LF has been suggested as a sensitive indicator of changes of soil organic matter because of its responsiveness to management practices (Gregorich et al., 1994). Field studies have supported 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 14C-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 compartments, each decomposing by a first-order process, but one much more quickly than the other. The introduction of C4 plants to soil previously developed 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 13C abundance method has been used to estimate soil organic matter turnover both in tropical (Martin et al., 1990) and temperate soils (Balesdent et al., 1987; Gregorich et al., 1995). The objective of this study was to use the I3C 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.
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 I3C abundance measurements indicated that fertilized soils had more organic C than unfertilized soils, the difference accounted for by more C4-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 I3C 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 MATTER in 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 increased 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
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 (Agriculture 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) temperature 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 evapotranspiration 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 fertilizer 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
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).
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ha~' at planting and 112 kg N ha~' of NH4NO3 sidedressed (incorporated). Corn was planted in 1.0-m rows at a density of 37 000 to 50 000 seeds ha~' from 1956 to 1982 and at 55 000 seeds ha~' from 1982 to 1990. Herbicides were applied as required to control weeds. Ten rows of corn (33 m in length) were harvested from each treatment. Corn residues were incorporated into the soil by moldboard plow each autumn whereas the bluegrass was left on the soil surface. Prior to seeding in the spring of 1991, four replicate soil samples were obtained from the plow layer (0-26 cm) of each treatment. One sample was also taken from the 15-cm layer below the plow layer in each treatment for a total sampling depth of approximately 40 cm. Soil samples were mixed, air dried, and sieved through a 2-mm sieve to remove visible plant fragments. The LF organic matter from the surface 10 cm of each soil was separated by flotation on a solution of Nal with the density adjusted to 1.8 g cm~ 3 (Gregorich and Ellert, 1993). The organic C content of samples was determined using a Carlo Erba T1500 elemental analyzer (Carlo Erba Strumentazione, Milan, Italy). Because the soils were free of carbonates to a depth of 60 cm, total soil C is equivalent to soil organic C. Bulk density of the plow layer was determined using soil cores; bulk density values for depths below the plow layer were obtained from published data measured at the same site (McKeague et al., 1987). The 8I3C values were determined by combustion of 3 mg C mixed with CuO (1:50) in the vacuum-combustion system described by Swerhone et al. (1991). Carbon dioxide generated in the combustion tubes was separated by cryogenic distillation, collected in breakseals, and analyzed on a VG-SIRA 12 isotope ratio mass spectrometer (Isotech, Middlewich, England). The natural abundance of heavy isotopes is expressed as parts per thousand (%o), relative to the international PDB standard using delta'units (8). The 813C value is calculated from the measured C isotope ratios of the sample and standard gases as 513C(%0) = [(^sample ~ /?standard)//?standanl]103
[1]
where R is the I3C/12C ratio of the sample or standard gas. The soil under the sod treatment was taken as the reference, and the proportion of soil C derived from corn residues since the experiment was initiated (X) was calculated as X=(6-5 s )/(6 c r -6s)100
for the fertilized corn and 30% for the unfertilized corn (Geisler and Krutzfeldt, 1984). A minimum value of 3 Mg ha"1 yr~' of corn residue was assumed for the unfertilized corn when the crop failed (no grain yield), as occurred in 1983 and 1985, and when the grain yield was 12% of the C4-C in the surface 10 cm of soil under fertilized corn, but only about 5% of the C4-C under unfertilized corn. The larger amounts of corn residue returned to the soil (discussed below) apparently have a direct effect on the amount of LF C. The C3-C content of the LF of both fertilized and unfertilized soils was the same, 0.3 g kg"1 soil. The labile nature of the LF is shown by the fact that 94% of the original €3 -C had turned over since the start of the experiment. The estimated half-life of the LF Cs-C in both corn soils was about 8 yr, the same as that estimated by Gregorich et al. (1995) in a soil that was under continuous corn for 25 yr. This half-life is similar to the half-lives reported in studies using 14C-labeled plant residues (Jenkinson, 1977; Voroney et al., 1989). Angers et al. (1995), who used natural 13C abundance to estimate the half-life for macroorganic matter in Quebec soils that had been under continuous corn for 11 yr, obtained a value of 10 yr.
1965
1985
1990
Year Fig. 2. Soil organic C derived from corn residue under fertilized and unfertilized treatments, assuming that the decomposition of the corn residue in soil follows a double exponential model (Woodrow, 1949). The C content of the corn residue was assumed to be 45%.
fertilized soils and 7.8 Mg ha"1 for the unfertilized soils (Fig. 2). Fertilized soil had larger amounts of C derived from corn (22-30% of the total C) than unfertilized soil (1520% of the total C). In the fertilized soil, the estimate by the above-cited model of the amount of C4-C remaining after 32 yr (14.1 Mg ha"1) was slightly lower than the amount measured by the natural abundance of 13 C (19.5 Mg ha"1). In the unfertilized soil, the amount of C4-C measured by the natural abundance of 13C (12.4 Mg ha"1) was larger than that estimated by annual inputs of corn residue. These data indicate that after 32 yr of cropping to corn, between 10 and 20% of corn residue-C inputs was retained as total soil C, similar to the findings of Balesdent et al. (1990). The smaller amount of corn-derived C estimated for the unfertilized soil by the decomposition model may have been due to the underestimation of the root/shoot ratio. Geisler and Kriitzfeldt (1984) reported that the ratio of shoot to root for dry matter increased with increasing N concentration in the soil. In addition, according to Jenkinson (1977), a two-compartment model is an oversimplification of the real processes of decay in soil. It takes no account of the formation and decay of biomass, nor of the formation and decay of the inert material that radiocarbon dating has shown is present.
CONCLUSIONS Estimates of Storage from Yield Data The amount of C remaining in the fertilized and unfertilized soils was estimated from assumed crop residue input using a double exponential model (Woodruff, 1949; Bartholomew and Kirkham, 1960). The model assumes that soil organic matter is heterogeneous and composed of two components that decompose at different rates (Jenkinson, 1977). The estimated amount of C stored in soil, derived from the annual additions of corn residue, including stover and roots, was 14.1 Mg ha~' for the
Soils under continuous corn, fertilized for >30 yr, had greater amounts of soil C than systems that were unfertilized. The difference between the soils was accounted for by estimating the C4-derived C using the natural 13C abundance method. We estimated that in fertilized soils, from 22 to 30% of soil C in the plow layer had turned over and was derived from corn residues, whereas in unfertilized soils, only 15 to 20% was derived from corn residue. Only a small portion (1020%) of the corn residue C inputs remained in these soils after 32 yr. Estimated half-lives of the surface soil
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C indicated that fertilization did not significantly alter the decomposition rate of C3-derived C. The LF organic material in the surface 10 cm of both soils was labile; between 40 and 70% of the LF C was derived from corn residues. However, the LF C content of fertilized soils was more than two times that of the unfertilized soils, and the difference between the soils was related to the amount of C4-derived C. The estimated half-life of C3 -C in the LF was shorter than 10 yr, which is consistent with findings of other studies conducted on temperate soils. ACKNOWLEDGMENTS B.C. Liang acknowledges the Natural Sciences and Engineering Research Council of Canada for providing a postdoctoral scholarship. This research was conducted as part of the Evaluating Changes in Soil Organic Matter study of the National Soil Conservation Program. Funding for this research was provided by Research Branch, Agriculture and Agri-Food Canada, and the Program of Energy Research and Development (Natural Resources Canada). The "C determinations were performed at the Stable Isotope Lab., Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, Saskatchewan.
