Scientific registration no: 1949 Symposium no: 4 Presentation: oral Concentric Gradients within Stable Soil Aggregates Gradients concentriques à l intérieur d agrégats stables de sols SMUCKER Alvin J M (1), SANTOS Djail (1), KAVDIR Yasemin (1), PAUL Eldor A (1) (1) Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI Soil structure research has been concerned with the organization of primary particles into aggregates, with little attention to the micro-scale heterogeneity or gradients of soil physical, biological, and chemical properties within aggregates. The hierarchical model of particle organization into structures of increasing size appears to be most applicable to soils having a long history of continuous contributions by plant roots (Oades 1993). Elliott (1986) and Miller and Jastrow (1990) support the hierarchical model of aggregate development, however, these similar interpretations may be biased by potential redistribution of SOM during the rigorous physical and chemical treatments used to separate fractions of soil aggregates (Cambardella and Elliott 1992). Consequently, Oades (1993) concludes that a great deal of confusion and contradiction exists in the literature dealing with soil aggregate development and stabilization. More recently, Wander and Yang (1995) reported contrasting concentrations of C associated with different sizes of sieved soil aggregates. Greater quantities of C were reported for the largest soil aggregates. We also reported that the highest percentages of total soil C, nearly 57 %, was located in larger aggregates, ranging from 0.004 to 0.0063 m across, with 34 % of C originating from the current maize crop (Paul et al 1998). Recent developments in the mechanical separation of layers within soil aggregates, by the soil aggregate erosion (SAE) apparatus (Santos et al 1997), provide a method for the quantitative separation of concentric surface layers from both air-dry and moist aggregates from most soil types. Analytical evaluations of these separates provide valuable knowledge essential for understanding those mechanisms which control stability responses by soil aggregates to a myriad of plant and soil management systems. Furthermore, greater sequestration of contaminants, by weathered clay minerals, provide opportunities for more extensive bioremediation of many environmental pollutants by roots, C, and associated soil organisms. Delta (*) 13 C techniques for identifying sources of C, produced by plants having different photosynthetic pathways (Balesdent et al 1993) and expanded by Paul et al (1998), provide excellent analytical approaches for identifying the sources of C energy and associated cementing agents which contribute to soil aggregation processes. Plants with C 3 pathways generate average * 13 C values of -27 by discriminating against 13 CO 2 during photosynthesis. In contrast, C 4 plants generate an average * 13 C signature of -12. Data reported in this paper identify the sources and distributions of C in aggregates of soils subjected to different management regimes. Quantification of these chemical gradients and 1
associated mechanisms, which control soil aggregate stabilization, are essential before new management programs can be designed to promote the sustainability of agricultural systems. Soil type, sampling, and concentric erosion Soil sampled for this report was a Kalamazoo loam (fine-loamy, mixed, mesic Typic Hapludalf) taken from the conventional (CT) and no tillage (NT) fertilized treatments of the Agroecosystem Interactions experimental site at the W.K. Kellogg Biological Station in southwestern Michigan. The experimental design is a randomized complete block of four treatments: Two tillage (CT and NT) by two nitrogen rates (0 and 125 kg N per ha) with four replicates. All treatments were continuously cropped, from 1990-97: corn ( 90), bromegrass ( 91), corn ( 92), soybeans ( 93), corn ( 94), alfalfa ( 94-96), corn ( 96 and 97). Four samples were obtained from 0-0.05 and 0.05-0.15 m depths on May and November 1996. Samples were taken in the row and in areas between the row without wheel tracks. Samples were manually broken along the lines of least resistance, air-dried, and gently sieved through a 0.020 m screen to remove stones and plant residues and to define the maximum dimensions of soil aggregates. Dry soils were further sieved and surface concentric layers were removed from six aggregates, 0.012-0.016 m, from each plot, by a modified air-tight SAE (Agriculex, Inc., Guelph, Ont. CA). Soil C and N analyses The ratio of 13 C/ 12 C (* 13 C signature) of soil C in concentric layers of soil aggregates were determined by mass spectroscopy. Soil samples were finely ground in a mortar and pestle and precisely weighed. Samples were introduced to the ratio mass spectrometer, via a CHN hightemperature oxidation furnace analyzer, using an auto sampler. Carbon was converted to CO 2 and carried to the triple collector isotope ratio mass spectrometer. All 13 C analyses automatically provided total C. Total N was determined by the CHN analyzer and C/N ratios calculated for each aggregate layer. Carbon sources which accumulated in each soil layer were calculated by the equation: % from C 3 plant = * 13 C from each layer - * 13 C from original soil x 100 (1) * 13 C from C 3 plant - * 13 C from original soil Since * 13 C values of the original Kalamazoo loam soil were -21.8 (CT) and -21.5 (NT) there was an effective working range of 5.4, for C 3 plants, with a precision of + 0.3. Clay mineralization and accelerated weathering Quantification of the chemical and physical changes associated with accelerated clay mineral weathering in concentric layers of soil aggregates were evaluated by x-ray diffraction (XRD) spectroscopy. Clay minerals were removed from soil materials in concentric layers by particlesize distribution analyses, according to Stoke s law of sedimentation. A micro-syringe apparatus, shown to be in excellent agreement with the standard pipette method of Burt et al (1993), was used to separate the clay-sized fraction from very small samples, 2-6 g, of soil removed as concentric layers from aggregates. Vermiculitization of the chlorite and illite clays was semiquantified by a Philips APD 3720 x-ray diffractometer (Philips Electronic Instruments Co., Mahwah, NJ) with Cu-K" radiation, n=1.54. Clay samples were saturated by 2
1 M KCl or 0.5 M MgCl 2. Following K-saturation, samples were heated to 300 o C then 500 o C for four hours. Mg-saturated samples were further saturated by vaporization with pure ethylene glycol, under vacuum. Ratios of diffractogram peaks for illite and chlorite were closely examined to semiquantitatively determine the clay mineralization ratio (CMR) for estimating weathering of clays analyzed (Cremeens and Mokma 1987). More details of these procedures were published earlier (Smucker et al 1996). The equation used to calculate the CMR for soil layers removed from aggregates is: CMR L = 1.0 nm (K) _ 1.0 nm (Mg) 1.4 nm (K) 1.4 nm (Mg) (2) which gives the difference between XRD peak ratios of K- and Mg-saturated illite (1.0 nm) and K- and Mg-saturated chlorite (1.4 nm) Results and Discussions Accumulations of C 3 -C materials in surface layers of soil aggregates 0.012-0.016 m across were greater for aggregates sampled near the soil surface, Table 1. * 13 C values for the centers of aggregates were unaffected by depth of sampling. However, the external and transitional layers of aggregates were 2.5 to 3.0 more negative and they approached * 13 C values similar to those of alfalfa, when sampled in the top 0.05 m of soil. Similar * 13 C values in whole aggregates and the interior regions of aggregates, Table 1, suggest that all of these C gradients have been concealed by current C analyses of finely ground bulk soil samples. Twice as much C was identified in surface layers of soil aggregates than the interior regions of aggregates, Table 2. This highly labile soil C appears to be greatest when plants are growing and subsides later in the season. It contributes to microbial respiration and several organic constituents associated with increased aggregation stability. The more negative * 13 C signature, -26.5, in external layers of aggregates from CT soils strongly suggests that much of this labile C, nearly 80 % of the C located within the surface layers, originates from the current alfalfa crop. In contrast, 24-29 % of C sequestered within aggregates originates from current or previous C 3 crops, Table 2. Although more total C was identified in external layers of aggregates from NT soils, * 13 C values were two units less than those of the external soil layer for CT aggregates, indicating that 45 % of the C in external soil layers of NT aggregates originated from the current alfalfa crop. Clay contents in surface layers of aggregates averaged 20% less than the interior regions of soil aggregates and appeared to be unaffected by tillage treatments nor sampling depths, Table 3. Interior regions of aggregates sampled at greater depths appeared to have higher clay contents then aggregates sampled from shallower depths. Clay weathering, as demonstrated by increasing CMR values, was 3-fold greater for clay-sized minerals of illite and chlorite, located in surface layers of CT aggregates, Table 3 and equation (2). No-tillage also appeared to weather clays in surface layers of aggregates less than CT. Accelerated vermiculitization of these minerals is the first microsite evidence that more clay is weathered at surfaces of aggregates. Weathering and subsequent illuviation of clays to greater depths appears to occur along surfaces of aggregates and may be protected from weathering only when sequestered in centers of larger soil aggregates. 3
Carbon and nitrogen (C:N) ratios were approximately two-fold greater in surface soil layers of aggregates from conventional (CT) and no tillage (NT) soils, Table 4. Greater C:N ratios on the CT aggregates resulted from the relatively lower (60%) nitrogen in external soils from CT aggregates (data not presented). No tillage slightly reduced C:N ratios of their external layers. Although no differences in C:N ratios were observed for the central regions of aggregates, C:N ratios were approximately 10, regardless of tillage or depth of sampling. The relatively high C contents in external soil layers of aggregates suggest large population differences of microbiological activities in different regions of each aggregate. These greater quantities of bacteria, fungi, and mesofauna greatly contribute to the stability of soil aggregates. Significantly lower concentrations of Ca, Mg, and K within external layers of a Udollic Ochraqualf loam soil aggregate suggest there is significant leaching of these cations from surface soils of these aggregates, Table 5. There were clear gradients of these three cations from the lowest exterior to the highest concentrations in the central regions of aggregates for this soil type from Michigan. In contrast, opposite cation gradients were observed, from the highest concentrations in the external to the lowest concentrations within aggregate interiors, for the Typic Hapludalf soil from Germany. Variations in the degree of aggregation, internal porosities, and combinations of different textural separates identified in the layers of these soil aggregates may have caused the higher concentrations of exchangeable cations in the internal regions of the Typic Hapludalf (Horn 1987). These results confirm that the formation and stabilization of aggregates in NT soils is an important mechanism for protecting and maintaining soil carbon which would be lost under CT practices (Beare et al 1994). Although NT soil aggregates retain less labile carbon, they retain larger quantities of more recalcitrant C resulting in greater total C. Textural components, particularly the weathering of the clay fraction, appeared to be enhanced by CT. Lower vermiculitization of illite and chlorite associated with surface regions of CT aggregates suggest that 10 years of NT may have begun to decrease the weathering of clays at surfaces of NT aggregates. Mechanical fractionation of larger aggregates into smaller and more biophysically meaningful components provides additional opportunities for understanding the mechanisms which explain how C, N, clay, and cations gradients, within soil aggregate layers, contribute to the physical-chemical bonding of stable soil structural units. Table 1. Sampling depth modifications of the * 13 C signatures in layers of aggregates from a Kalamazoo loam following 20 months of a C 3 -C alfalfa (-27 ), n=4, +Sd. Concentric soil aggregate layers Whole Soil depth External Transitional Internal aggregate m -------------------------------* 13 C ------------------------------ 0-0.05-26.5 +0.6-26.7 +0.6-23.2 +0.6-23.1 +0.4 0.05-0.15-24.0 +0.9-23.7 +0.5-23.4 +0.8-23.6 + 0.5 4
Table 2. Changes in the * 13 C, percent from C 3 -C plant source, and total C in the external and internal layers removed from soil aggregates sampled at 0-0.05 m from a Kalamazoo loam subjected to CT and NT following 20 months of alfalfa, n=4 + Sd. Soil layer * 13 C C 3 -C source Total C % g kg -1 CT External -26.5 +0.6 78 +9 22.6 +5.0 CT Internal -23.2 +0.6 24 +10 10.8 +1.6 NT External -24.5 +1.8 45 +11 29.9 +3.2 NT Internal -22.5 +1.2 29 +0 9.7 + 0.7 Table 3. Clay content and affiliated clay mineralization ratios (CMR) in concentric layers in Kalamazoo soil aggregates from two soil depths in CT and NT treatments, n=4, +Sd. Tillage Sample depth CT NT Concentric layer Clay CMR Clay CMR _ m g kg -1 g kg -1 0-0.05 External 118 +16 0.83 116 +18 0.28 Transitional 119 +7.0 0.13 96 +19 0.01 Internal 141 +16 0.07 140 +6.2 0.05 0.05-0.15 External 126 +8.3 0.15 128 +13 0.23 Transitional 129 +15 0.05 123 +21 0.04 Internal 162 +11 0.04 163 +11 0.04 CMR is the difference between spectra of 1.0 0m to 1.4 0m ratios of K- and Mg-saturated clay. 5
Table 4. Soil management and sampling depth modifications of C:N ratios for C and N extracted, by CHN analyzer, from aggregate layers of a Kalamazoo loam soil, n=4, +Sd. Concentric soil aggregate layer Management Depth External Transitional Internal m ------------------------- C:N ratio -------------------------- CT 0-0.05 23.6 +4.1 22.4 +2.8 10.0 +0.4 CT 0.05-0.15 12.0 +0.9 12.1 +2.1 10.7 +1.7 NT 0-0.05 19.9 +2.6 27.5 +4.4 10.0 +0.3 NT 0.05-0.15 12.7 +1.6 11.0 +1.2 10.4 +1.2 Table 5. Exchangeable cation concentrations from within aggregate layers for two soils from Michigan (Conover) and Germany (Parabraunerde), sampled at 0-0.15 m, n=10. _ Soil type Concentric Ca Mg K soil layer ------------- cmol c kg -1 ------------- Udollic External 1.00 0.14 0.05 Ochraqualf Transitional 1.02 0.15 0.05 (Michigan) Internal 1.49 0.21 0.07 LSD (0.05) 0.25 0.05 0.02 Typic External 7.09 3.26 0.44 Hapludalf Transitional 7.20 3.27 0.46 (Germany) Internal 7.00 3.00 0.35 LSD (0.05) NS 0.25 0.04 6
References Balesdent, J., C. Girardin, and A. Mariotti. 1993. Site-related 13C of tree leaves and soil organic matter in temperate forest. Ecology 74:1713-1721. Beare, M.H., M.L. Cabrera, P.F. Hendrix and D.C. Coleman. 1994. Aggregate-protected and unprotected organic matter pools in conventional- and no-tillage soils. Soil Sci. Soc. Am. J. 58:787-795. Burt, R., T.G. Reinsch and W.P. Miller. 1993. a micro-pipette method for water dispersable clay. Commun. in Soil Sci. Plant Anal. 24:2531-2544. Cambardella, C.A. and E.T. Elliott. 1992. Particulate soil organic matter changes across a grassland cultivation sequence. Soil Sci. Soc. Am. J. 56:777-783. Cremeens, D.L. and D.L. Mokma. 1987. Fine clay mineralogy of soil matrices and clay films in two Michigan hydrosequences. Soil Sci. Soc. Am. J. 51:1378-1381. Elliott, E.T. 1986. Aggregate structure and carbon, nitrogen and phosphorus in native and cultivated soils. Soil. Sci. Soc. Am. J. 50:627-633. Horn, R. 1987. Die Bedeutung der Aggregierung für die Nähstoffsorption in Böden. Z. Pflanzenernähr. Godenkd. 150:13-16. Miller, R.M. and J.D. Jastrow. 1990. Hierarchy of root and mycorrhizal fungal interactions with soil aggregation. Soil Biol. Biochem. 22:579-584. Oades, J.M. 1993. The role of biology in the formation, stabilization, and degradation of soil structure. Geoderma 56:377-400. Paul. E.A., H.P. Collins and S. Haile-Miriam. 1998. Analytical determination of soil C dynamics. ISSS Proceedings (This volume). Santos, D., S.L.S. Murphy, H. Taubner, A.J.M. Smucker and R. Horn. 1997. Uniform separation of concentric surface layers from soil aggregates. Soil Sci. Soc. Am. J. 61:720-724. Smucker, A.J.M., D. Santos and Y. Kavdir. 1996. Concentric layering of carbon, nitrogen, and clay within soil aggregates from tilled and non-tilled agroecosystems. D. Angers (ed.), Third Eastern Canada Soil Structure Symposium Proceedings pp. 129-140. Wander, M.M. and X.M. Yang. 1995. Crop and tillage effects on the features of organic matter associated with dry-sieved aggregates. Agron. Abst. 87:241. Keywords : soil structure, soil carbon, clay mineralization, tillage Mots clés : structure du sol, carbone du sol, minéralisation, pratiques culturales 7