Non-Peer ReviewedNatural 13C abundance was measured in two Gleysols, one from a forest and one which had been under continuous corn for more than 25 yr. Delta 13C values increased slightly with depth in the forest soil. In the corn soil, delta 13C values were similar throughout the plow layer but lowest in the deepest layer sampled, 30-50 cm. Between 29 and 3 5% of the carbon in the plow layer of the cultivated soil was derived from corn. The 0-5 cm layer of each soil was separated into particle size fractions; delta 13c values indicated that the sand fraction had the largest proportion of carbon derived from corn whereas the coarse silt had the smallest proportion

Ellert, B.H.

Gregorich, E.G.

Monreal, C.M.

English

University of Saskatchewan Research Archive

NATURAL CARBON-13 DISTRIBUTION IN FORESTED AND CULTIVATED GLEYSOLS E.G. Gregorich1 , C.M. Monreal2 and B.H .. Ellert1 Centre for Land and Biolo~ical Resources . Research, Agriculture Canada, Ottawa, 2saskatoon. ABSTRACT Natural 13c abundance was measured in two Gleysols, one from a forest and one which had been under continuous corn for more than 25 yr. c5 13C values increased slightly with depth in the forest soil. In the corn soil, c5 13c values were similar throughout the plow layer but lowest in the deepest layer sampled, 30-50 em. Between 29 and 3 5% of the carbon in the plow layer of the cultivated soil was derived from corn. The 0-5 em layer of each soil was separated into particle size fractions; c5 13c values indicated that the sand fraction had the largest proportion of carbon derived from corn whereas the coarse silt had the smallest proportion. INTRODUCTION The heavy carbon isotope, 13c, accounts for about 1% of all carbon atoms in nature. The proportion of 13c in natural material varies as a result of isotopic fractionation during physical, chemical and biological processes. Plants with a c3 photosynthetic pathway reduce C02 to 3 -c compounds using an enzyme (RuBP carboxlyase) which discriminates against 13c, resulting in low cS 13c values in the plant material. The more e'fficient C4 plants reduce C02 to 4-C compounds using an enzyme (PEP carboxylase) which discriminates less against 13c; thus the plant material in c4 plants has relatively higher o13c values. There is little 13c enrichment during the transformation of plant residues into soil organic matter (Stout et al., 1981) so the 13c content of soil organic matter corresponds closely to the 13c content of the plant material from which it is derived. Consequently, the introduction of c4 plants to soil previously developed under c) vegetation results in the soil organic matter containing two ~sotopically different sources of carbon and provides a means of following changes in soil organic matter turnover with changes in vegetation cover. Several investigators have used this isotopic method to estimate organic matter turnover in tropical (Martinet al., 1990), subtropical and temperate soils (Balesdent et al., 1987; Balesdent et al., 1988). The objective of this study was to assess the effects of cul ti vat ion on organic matter in a Gleysol and use the natural 13c distribution to partition the soil organic matter as to origin. 158 MATERIALS AND METHODS The soils used in this study are classified as Orthic Gleysols (Canada Soil Survey Committee, 1~78). The cultivated soil was tile drained and had been under continuous corn for more than 25 yr. Tillage operations include fall moldboard plowing with secondary tillage with an offset disk in the spring. Three pits were excavated in both the corn soil and the soil of a mixed hardwood forest within 3 km. Soil samples were taken from the o-5, 5-10, 10-15, 15-30, and 30-50 em depth of each pit. Bulk density was estimated using soil cores in the forest soil;· a nuclear probe was used at the cultivated site. In the latter technique gamma ray attenuation is used to measure wet bulk density and volumetric water content (Culley and McGovern, 1990) and dry bulk density is calculated from these values. Soil samples were mixed, air-dried and sieved through a 2 mm sieve which removed plant fragments coarser than 2 mm. Soil samples were separated into particle size fractions by sieving and sedimentation. The organic carbon content of samples were determined using a combustion method with direct titration of the evolved co2 as carbonic acid in a NaOH trap (Tiessen et al., 1981). The mass of organic carbon per hectare was calculated as follows: Mg C ha -1 = OC * BD * L * 0 • 1 where oc = or~anic carbon concentration mg C g· 1 soil, BD = bulk density (Mg m· ), L =layer thickness (em). Samples of leaves, surface roots, twigs and bark were collected from the forest site and corn leaves, roots, cobs with grain and stalks were collected from the cultivated field prior to harvest. Plant and litter samples were oven dried at 60° c and ground in a laboratory mill. The o13c values were determined by combustion of 3 mg c mixed with cuo (1: 50) in the vacuum-combustion system described by Schoenau and Bettany (1988). Purified C02 was collected in breakseals and run through a VG Isogas SIRA 12 spectrometer. The natural abundance of heavy isotopes is expressed (as parts per thousand(~)] relative to the international standard PDB using delta units (6). The o13c value is calculated from the measured carbon isotope ratios of the sample and standard gases as: o13c (~) = ( (R sample - R standard) /R standard] X 103 where R is the mass 45/44 ratio of the sample or standard gas. The proportion of carbon derived from corn residues was calculated as: '159 where 0 = o13c value of sample from cultivated soil, ocr = -13.191», the o13c value of corn residues (excluding cobs and grain), and of = o13c value of sample from forest soil. RESULTS AND DISCUSSION 13c values of plant material and· soil The o13c values of forest plant parts ranged from a m1.n1.mum of -28.391» for surface roots to a maximum value of -25.591» for bark. corn plant parts had higher o13c than forest plant parts and ranged from -12.8 to -13. 491» (Table 1) • Thus, the range of o13c values for plant parts from ~ (forest) and c 4 (corn) species did not overlap and the average values for · all plant parts differed by approximately 14~, which is consistent with the survey by Smith and Epstein (1971). Variation among corn plant parts was 0.591» and was lower than that in the forest plant parts. The o13c values show a consistent increase- (291» total) with depth in the forest soil profile (Table . 2) • This phenomenon has been noted in other studies (O'Brien and Stout, 1978; Vitorello et al., 1989; Martinet al., 1990) and may be accounted for by any or all of· the following: i) l.'l.) iii) Migration and redeposition of clay which has a higher 13c content than that of the whole soil. In the case of Alfisol "lessive" soils, Becker-Heidmann and Scharpenseel (1986) suggested that protected clay-humic complexes ( 13c enriched) migrated a shorter distance than young soluble carbon and became insoluble in the B horizons. Change in isotopic composition of the forest vegetation with time. Atmospheric 13c-co2 has . decreased by approximately 1.291» over the past 130 years (Freidli et al., 1986) o The dilution of 13c content of the atmosphere has been attributed to fossil fuel combustion and deforestation. Certain biochemical fractions of ·plant litter are slightly heterogeneous isotopically and decompose at different rates. Cellulose and hemicellulose are 1-291» more enriched in 13c, whereas lignin is 2-6~ lowe.r in 13c than whole plant tissue (Benner et al., 1987). Lignin decomposes more slowly than cellulose and the isotopic composition of litter may change during decomposition. The o13c values of the corn soil profile do not show a consistent chan¥,e with depth. The layers comprising the plow layer have similar o1 c values, indicating that mixing of corn residues with soil occurred with plowing. 160 Soil organic carbon in forest and cultivated Gleysols The concentration of organic carbon in the forest soil showed a sharp decline with depth whereas in the corn soil it was similar in the layers down to 30 em (Table 3) • The concentration of organic carbon in the surface 5 em was an order of magnitude higher in the forest soil than in the corn soil. These data reflect the large amount of organic matter in the forest which is concentrated on the surface of the soil. The mass of organic carbon in the surface 10 em of the forest soil was 3-4 times greater than that of the corn soil (Table 3). Below 10 em the quantity of organic carbon in the forest and corn soil was similar. The amount of organic carbon to 50 em depth was 132 Mg haG 1 in the forest soil and 74 Mg ha-1 corn soil. Thus, cultivation resulted in a loss of 44% of the organic carbon content of the soil. Detailed records of the start of cultivation were not found, but the period of cultivation was estimated at 80-100 yr, based on the history of settlement in the area. The mass of organic carbon in the corn soil increased slightly in each depth increment to 30 em. A thin band of residues between 20-25 em which was observed during excavation of the corn soil may account for this incr.ease in carbon with depth. These buried residues are the result of the inversion of crop residues during moldboard plowing. carbon derived from corn The proportion of carbon derived from corn was greatest in the top 5 em, was similar for depth increments to 15 em and ranged from 29-35% (Table 3). The lowest proportion of carbon derived from corn was found deepest in the profile, at 30-50 em. This depth is below the plow layer and the corn carbon is probably from corn roots and soluble corn organic material. The slightly lower proportion of carbon derived from corn in the 15-30 em layer was probably due to the incorporation of soil low in corn carbon from below the plow layer. More than 16 Mg c ha-1 was derived from corn and· contained in the top 50 em of the soil profile. More than 60% of this carbon was contained within the top 15 em. The o13c values of the particle size fractions from the top 5 em of the forest soil were similar to the average o13c values of the forest whole soil and litter. In the forest soil, the variation between particle size fractions was 1.19fxl. In contrast, the o13C values of particle size fractions in the corn soil varied by 49fxl. The o13c value of the corn soil sand fraction was about 29fxl higher than that of the whole soil indicating it was relatively enriched in 13c and in carbon derived from corn. This enrichment would be expected for the fraction in which non-decomposed residues predominate and indicates that the turnover of organic matter is the most rapid in the sand fraction. The readily decomposable 161 nature of organic material in the sand fraction has been observed in mineralization studies (Christensen, 1987; Gregorich et al., 1989). The fractions smaller than sand size had lower o13c values than did whole soil, indicating less enrichment of carbon derived from corn and slower turnover of carbon. The proportion of carbon derived from corn decreased sharply to 16% in the coarse silt fraction and increased to about 26% in the fine silt and fine clay. The slow turnover of carbon in the coarse silt indicates the organic matter is older and more stable. Anderson and Paul (1984) used radiocarbon dating to show that the organic carbon associated with the coarse clay/fine silt was the oldest fraction. Tiessen and Stewart (1983) found that the stability of organic matter increased from sand to fine silt and decreased from fine silt to fine clay. The differences in observations of the stability and age of organic matter associated with particle size fractions by our study and these studies may be due to genetic differences in the soi1s, as well as differences in mineralogy, management and crop history. CONCLUSIONS Cultivation of c4 plants on a soil previously under c3 vegetation allowed the partitioning of soil organic matter as to origin. Measurement of natural 13c abundance was used to estimate the quantity of carbon derived from corn on the whole soil and partic'le size · fractions of a Gleysolic soil in Ontario. About one-third of the organic matter in the top 15 em had turned over since the start of continuous cropping to corn more than 25 yr ago. The turnover of organic matter was most rapid in the sand fraction and slowest in the coarse silt fraction. 162 REFERENCES Anderson, D.W. and Paul, E.A. 1984. Organa-mineral complexes and their study by radiocarbon dating. Soil ·sci. Soc. Am. J. 48, 298-301. Balesdent, J., Mariotti A. and Guillet, B. 1987. Natural 13c abundance as a tracer for soil organic matter dynamics studies. Soil Biol. Biochem. 19, 25-30. Balesdent, J., Wagner, G.H. and Mariotti, A. 1988. Soil organic matter turnover in long-term field experiments as revealed by the carbon-13 abundance. Soil Sc. Soc. Am. J. 52, 118-124. Becker-Heidmann, P. and Scharpenseel, H.W. 1986. Thin-layer c5 13c and 14c monitoring of 1 lessive 1 soil profiles. Radiocarbon 28, 383-390. Benner, R., Fogel, M., Sprague, E. and Hodson, R. 1987. Depletion of 13c in lignin-and its implications for stable carbon isotope studies. Nature 329, 708-710. Canada Soil Survey Committee. 1978. The Canadian System of Soil Classification. Publication 1646. Research· Branch, Canada Department of Agriculture. Christensen, B.T. 1987. Decomposability of organic matter in particle size fractions from field soils with straw incorporation. Soil Biol. & Biochem. 19, 429-435. Culley, J.L.B. and McGovern, M.A. 1990. Single and dual probe nuclear instruments for determining water contents and bulk densities of a clay loam soil. Soil & Tillage Res. 16, 245-256. Friedli, H., Lotscher, H., Oeschger, H., Siegenthaler, U. and Stauffer, B. 198 6. Ice core record of the 13c;12c ratio of atmospheric C02 in the past two centuries. Nature 324, 237-238. Gregorich, E.G., Kachanoski, R.G. and Voroney, R.P. 1989. Carbon mineralization in soil size fractions after various amounts of aggregate disruption. J. Soil Sci. 40, 649-659. Martin, A., Mariotti, A., Balesdent, J., Lavelle, P. and Vuattoux, R. 1990. Estimate of organic matter turnover rate in a savanna soil by 13c natural abundance measurements. Soil Biol. Biochem. 22, 517-523. O'Brien, B.J. and Stout, J.D. 1978. Movement and turnover of soil organic matter as indicated by carbon isotope measurements. Soil Biol. & Biochem. 10, 309-317.· 163 Schoenau, J.J. and Bettany, J.R. 1988. A method for determining sulfur-34 abundance in soil and plant sulfur fractions. Soil Sci. Soc. Am. J. 52, 297-300. Smith, B.N. and Epstein, s. 1971. Two categories of ·13c;12c ratios for higher plants. Plant Physiol. 47,380-384. Stout, J.D., Goh, K.M. and Rafter, T.A. 1981. Chemistry and turnover of naturally occurring resistant organic compounds in soil. pp. 19-24 In E.A. Paul and J.N. Ladd (ed.) Soil Biochemistry. Vol. 5. Marcel Dekker, New York. Tiessen, H. Bettany, J.R. and Stewart, J.W.B. 1981. An improved method for the determination of carbon in soils and soil extracts by dry combustion. Comm. Soil Sci. Plant Ana1. 12, 211-218. Tiessen, H. and Stewart, J.W.B. 1983. Particle size fractions and their use in studies of soil organic matter. It. cultivation effects on organic matter composition in size fractions. Soil Sci. Soc. Am. J. 47, 509-514. Vitorello, V.A., Cerri, c.c., Andreux, F., Feller, c. and Victoria, R.L. 1989. Organic matter and natural carbon 13c distribution in forested and cultivated Oxisols. Soil Sci. Soc. Am. J. 53, 773-778. 164 Table 1. Variations in the c5 13c values (%o) of litter material from a forest and corn Gleysol (Mean± Std. Dev.). Plant parts Forest litter Leaves -27.6 ± 0.4 Surface roots -28.3 ± 1.6 Twigs -27.8 + 0.5 Bark -25.5 + 0.4 Average of all plant parts -27.3 ± 1.35 Corn litter Leaves -13.4 ± 0.6 Surface roots -12.8 ± 0.5 Cobs & grain -13.0 ± 0.6 Stalks -13.1 ± 0.1 Average of all plant parts -13.1 ± 0.5 Table 2. Variations in the c5 13c values (%o) with depth in Gleysols under a forest and corn crop (Mean± Std. Dev.). Depth (em) Forest soil Corn soil 0-5 -28.3 ± 0.3 -22.9 + 0.5 5-10 -27:3 ± 0.2 -23.2 ± 0.5 10-15 -26.7 ± 0.5 -22.5 ± 0.6 15-30 -26.7 ± 0.1 -23.9 ± 0.2 30-50 -26.2 ± 0.3 -25.3 + 0.7 165 Table 3. Concentration and mass of organic carbon in forested and cultivated Gleysols and the proportion and quantity of carbon derived from corn. Depth em 0-5 5-10 10-15 15-30 30-50 0-5 5-10 10-15 15-30 30-50 Or9janic carbon mg g· Mg ha·1 Forest soil 140e7 87.2 51.3 12.8 4.3 45.7 28.3 16.7 27.8 13.2 corn soil 17.6 14.7 17.2 12.9 4.9 10.2 10.4 11.4 26.5 15.2 Carbon derived from corn % Mg ha-1 35 29 30 20 7 3.6 3.0 3. 4-5.3 1.1 Table 4. &13c values (96o) of particle size fractions in forested and cultivated Gleysols and the proportion of carbon derived from corn. carbon soil fraction Forest soil corn soil Fl:all corn (%) Whole soil -28.3 -22.9 45 Size fraction: Sand -27.0 -20.8 45 Coarse silt -27.0 -24.8 16 Fine silt -27.8 -24.0 26 Coarse clay -27.0 -23.6 24 Fine clay -26.7 -23.1 27 166 

Natural carbon-13 distribution in forested and cultivated gleysols

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Natural carbon-13 distribution in forested and cultivated gleysols

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