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Yasemin Kavdir 1 , Alvin J. M. Smucker 2 1 Assistant Professor at Canakkale 18 Mart University, Lapseki Agricultural Collage Canakkale-Turkey 2 Professor at Michigan State University, Crop and Soil Sciences Department East Lansing, MI- USA Abstract Rye (Secale cereale L.) roots and shoots release C and N compounds in situ during their decomposition. Plant deposition of N by rye roots and shoots into soil aggregates was determined by labelling rye shoots with stable N isotope during rye cover cropping of corn agroecosystems. Rye plants were labelled with foliar applications of solutions containing 99% atom (15NH4)2SO4. Isotopic enrichment of soil aggregates ranging from 2.0 - 4.0, 4.0 - 6.3 and 6.3 - 9.5 mm across was determined following plant residue applications. Concentric layers of aggregates were removed from each aggregate by newly designed meso soil aggregate erosion (SAE) chambers. Significant correlation observed between change in ratios of N in external layer/N in internal regions of aggregates and corn biomass for 6.3-9.5 mm aggregates in 1999 (r2=0.88 for no cover crop and r2=0.71 for rye cover crop treatments). Non-uniform distributions of total N and recently derived rye N in soil macroaggregates, across time suggested that the formations and functions of macroaggregates are very dynamics processes. Rye roots contributed as much N as rye shoots to the soil N pool. Therefore maintaining active plant root in the soil and keeping N on the surfaces of macroaggregates are the best management systems for maximizing soil N availability and reducing N leaching. Introduction Cover crops used to reduce leaching of NO3 (Ditsch et al., 1993) also contribute to the improvement of soil organic matter through the addition of residues in the early spring and throughout the summer. Microbes rapidly deplete decomposing cover crop residues of most sugars, amino sugars, organic acids, starches, and simple proteins (Paul and Clark, 1996). Kladivko (1994) reported that microbial decomposition of fresh organic material produced polysaccharides and other compounds that became the main contributors to the soil aggregate stabilization. Therefore, living rye roots, decomposition and by-products associated with the rye root and shoot residues are effective contributors to soil nutrient cycling and aggregate formation and stabilization. Numerous studies have been reported on the formation, stabilization, and effect of different soil and crop management systems on soil aggregation (Wood et al., 1991, Roberson et al., 1995). However, there is little information on the location of recently decomposed plant residues within soil aggregates (Angers et al., 1997). Clay illuviation, preferential movement of water, weathering of clay and preferential growth of roots can change the compositions of aggregate surfaces (Smucker et al., 1997, Whiteley and Dexter, 1983). Living roots may create rapid wetting-drying cycles that enhance SOM degradation (Bottner, 1985). They may induce microbial activity and increase SOM decomposition (Cheng and Coleman, 1990). Roots control the concentrations and fluxes of soil N by absorbing soil water and soluble N compounds (Harper, 1995 and Frensch, 1996). Released N in situ, from decomposing plant roots and shoots contribute to stabilizing soil aggregation processes (Oades, 1993). Nitrogen is deposited in the rhizosphere as NH4, NO3, and root debris. It is assumed that, some of the extracted plant available N forms and mineralized N from rhizodeposits are reabsorbed by the plant. Recent studies showed that soil aggregates develop by adding concentric layers of cations, carbon (Santos et al., 1997, Horn 1990, Smucker et al. 1997) and heavy metals (Wilcke and Amelung,1996). Short term effects of cropping on soil organic matter and associated rhizodeposition can be determined more quickly when concentric layers are removed from soil aggregates. Six weeks after planting ryegrass in a greenhouse potted study (Santos, 1998) showed exterior layers of soil aggregates contained 20% newly deposited C while interior regions contained only 8% new C3-C. Therefore, under field cover crop conditions, it is suggested that recently derived rye cover crop shoot and root nitrogen could be deposited at greater concentrations on the surfaces of soil aggregates than interiors. To understand cover crop contributions of N to successive plant uptake and soil aggregation processes, sources and specific locations of N must be identified within soil aggregates. In this study, we determined whether plant-derived organic N, sequestered at different locations in soil aggregates, affected N absorption by subsequent corn crops. The objective of this study was to identify the contributions of rye root and shoot N to different regions within aggregates ranging from 2.0 to 9.5 mm across in the Ap horizon of a Kalamazoo loam soil. Materials and Methods A two- year field experiment (1997-1999) was conducted on a Kalamazoo loam soil (coarse-loamy, mixed, mesic Typic Hapludalf) at the KBS/LTER (long term ecological research) site in southwestern Michigan. There were four treatments; 1)Bare soil control 2)Bare soil where rye shoots were applied before corn planting (shoots) 3) Rye where shoots were cut and removed and roots in the soil remained in situ (roots) 4) Rye cover crop roots and shoots (roots+shoots) where rye shoots were cut and placed on soil surface. Each treatment was replicated four times in a randomized complete block design. Two open-ended PVC cylinders, 30 cm in diameter and 60 cm in depth, were driven through the Ap horizon and into the center of the Bt2 horizon in each plot by a front fork loader of a tractor in each year. Approximately 45 rye seeds were planted into each PVC cylinder of the rye treatment plots. In an effort to avoid soil contamination by 15N, the soil surface in each cylinder was covered with plastic sealed around the walls of the PVC cylinder and each row of rye using nontoxic clay sealant. Pine wood shavings were placed on the plastic to absorb any mist or droplets of the 15N labeled spray materials preventing them from contacting the to soil surface. Rye plants were labeled with 15N by foliar applications of solutions containing 6.39 g (15NH4)2SO4 containing 99 atom% 15N dissolved in 9 L of distilled water in May. This solution was applied in 3 to 4 separate applications to prevent run off. Following a two-week translocation period, the rye plants were spray-killed with Roundup Ultra without ammonium sulphate that (4.5 L ha-1) mixed with 186 L ha-1 water in early May of 1998 and 1999. Above ground plant parts of rye were manually cut, weighed and subsamples were taken for analyses. Rye shoots were removed or placed on the soil surface inside PVC cylinders with appropriate treatments. Corn seeds (6-8) were hand planted into each PVC cylinder. Following germination PVC 15N lysimeters were thinned to two corn plants, 2 days after emergence. Thinned corn plants were left on the soil surface of the chambers to retain 100% of 15N within the PVC lysimeters. Background soil samples (0-5 and 5-15 cm) were taken from each 15N lysimeters using a small (2.5 cm in diameter) PVC pipe before 15N application. Soil samples were air-dried and manually sieved from the 9.5 mm sieve. Rye root and shoot sub samples were taken before and after the 15N labeling to determine initial and final 15N contents of plant shoots and roots. Rye root samples were extracted from the top 15 cm depth of soil surface by pressing PVC cores (117 cm3) into the soil to sample rye roots before and after 15N application. Roots were removed from this sample by developing a slurry of distilled water which was poured through a 53 um screen and the retained roots were washed under water. Fine and white roots were picked from the sand and residue remaining on the screen by tweezers. Both roots and shoots were oven dried at 70oC for 24 h. Concentric layers of aggregates were removed from each aggregate by the meso soil aggregate erosion (SAE) chamber technique reported by Smucker et al. (1999). Following the separation of aggregates into 3 equal concentric layers, samples were further processed by grinding in mortar and pestle. Sand was removed by sieving a sample through a 53 um screen to increase the concentration of the 15N and N in the small sample size associated with each concentric layer of each aggregate. Resultant clay and silt samples were weighed into small tin capsules, approximately to 10 mg and placed into an autosampler. Total C and N of plant and soil materials were determined by the dry combustion method (Kirsten, 1983) using a C/N/S analyzer NA 1500 series 2 (Carlo Erba Stumentazione, Milano, Italy) and %15N by using Isotope Ratio Mass Spectrometer Model 2020 (Europa Scientific, Crewe, UK). Total C content of a Kalamazoo loam soil from 0-5 cm depth was assumed to be equal to soil organic carbon as reported by Santos (1998). 
Treatment effects on measured parameters were estimated by a PROC-GLM procedure using Statistical Analysis System (SAS Institute, 1999). Duncan's multiple range test was used to separate means of measurements. Carbon, nitrogen and 15N contents of exterior and interior layers of soil aggregates were compared by paired t-test using Statistical Analysis System (SAS). Correlation analysis was used to determine relationship between plant and soil parameters. All significant tests were set at the 0.05 level. Results and Discussion Nitrogen from rye roots and shoots could be detected on the exterior layers of soil aggregates of 4.0 - 6.3 and 6.3 - 9.5 mm as early as 17 days after rye shoot applications to the soil surface in 1998 (Figure 1). Rye root contributions of N were greater than that of rye shoot N, presumably due to the more rapid decomposition and direct contact of rye roots to soil aggregates. Both content and contrasting gradients of 15N, derived from rye increased with increasing aggregate size (Figure 1). Similar increases of 15N gradients with aggregate sizes were also observed in July 1999 (Figure 2). These results support that most of roots grow preferentially around surfaces of soil aggregates rather than through aggregates. Although concentrations of 15N on surface layers and interior regions of aggregates 2 - 4 mm across were the same as the surface layers of larger aggregates, no gradients of 15N from rye cover crops were observed for aggregates 2 - 4 mm across (Figures 1 - 4). Organic materials derived from rye roots and shoots appeared to be uniformly distributed throughout aggregates 2.0 - 4.0 mm across with minimum 15N gradients at the beginning of the corn growing season (Figures 1 and 2). Contents of 15N within aggregates 2.0 - 4.0 mm across decreased with no gradients were observed at harvest (Figure 3). The 15N gradients developed within larger soil aggregates, 6.3-9.5 mm across, decreased in October 1998, 116 days after rye shoot application (Figure 3). Nitrogen isotope gradients between external layers and internal regions of soil aggregates 4.0 - 6.3mm across developed early in the summer (Figure 1) diminished as the season progressed (Figure 3). Exterior layer of soil aggregates contained similar concentrations of 15N as interior regions at corn harvest (Figures 3 and 4). In summary, there seemed to be a migration of 15N materials from rye roots and shoots into soil aggregates at a constant rate. Early in the season, more 15N migrated to the interior regions of the smallest aggregates, 2 - 4mm across, but was limited to only surfaces and transitional regions of the larger aggregates, 6.3 - 9.3 mm across. At harvest, more of the 15N located within interior regions of the smallest sized aggregates was withdrawn by corn growth while more 15N remained within the interior regions of the medium sized aggregates, 4 - 6.3 mm across (Figs. 1-3 and 2-4). Living roots provide large quantities of C compounds to the surfaces of soil aggregates (Santos, 1998) promoting microbial biomass activities and greater N mineralization (Texier and Billes, 1990). Therefore, corn roots appear to be important C sources for stimulating microorganisms in the soil. Their specific locations on soil aggregates of different sized fractions need further investigation. When roots preferentially grow on the surfaces of soil aggregates, as discussed above, these roots should increase N mineralization in the external regions of aggregates. Frequent wetting-drying cycles will diffuse more N into interior regions of soil aggregates of all sizes. Mean-free pathways, however, limit the diffusive distance or depths within aggregates of different size fractions. However, when roots are present or when soil water contents are high, highly mobile mineral N, located on surface layers of larger aggregates and throughout smaller aggregates can either be absorbed or leached from these respective areas of multiple sized soil aggregates with subsequent diffusion from their interior regions towards their exterior regions. The good correlation (r2=0.68) between 15N ratio of exterior layers to interior regions of soil aggregates and 15N uptake by corn plant support these conclusions. Similar correlations were found between changes in the ratios of total N of external layers and internal regions of soil aggregates at the beginning and end of the corn growing season and corn biomass in 1999. Increases in the ratios of Ne/Ni (in July) - Ne/Ni (in September) demonstrate root uptake of N during the corn growing season. As these ratios increased, greater corn biomass was observed Thus, it is clear that uptake of N is more efficient from the surface of aggregates from a Kalamazoo loam soils larger than 4 mm across. Most of the 15N presented in the interior regions of soil aggregates greater than 4 mm across was preserved at the time of corn harvest. Especially since many of the roots appear to grow around exterior regions of soil aggregates (Allison, 1973, Whiteley and Dexter, 1983). It was also observed that approximately 20% of the aggregates contained some of the finer roots which had penetrated and passed through soil aggregates. These soil aggregates containing roots were not selected for analyses. Any fine rye root fragment located within aggregates of any size would result in the deposition of mineralized 15N which could be sequestered within larger aggregates and become unavailable to corn roots unless they penetrated the same larger soil aggregate (Rasse and Smucker, 1999). More rye root-derived N accumulated on exterior layers of soil aggregates 6.3 - 9.5 mm across than rye shoot-derived N (Figure 1). In the first year of experiment soil aggregate samples were taken 17 days after application of labeled rye shoots on the PVC chambers. During the labeling period some 15N was transferred from rye shoots to roots and was released to soil by rye roots as root exudates. During applications of Round Up and cutting rye shoots, dead roots continued to release N compounds to the soil. Therefore, more rye root-derived N was deposited on the exterior layers of aggregates in 1998 (Figure 1). In the second year of the experiment, first samples were taken 51 days after application of labeled rye shoots. During that time root derived N was already utilized by corn and shoot derived N concentration was greater than root derived N (Figure 2). Separation of external layers of individual aggregates demonstrated the contributions of short-term rye shoot, root and shoot plus root to soil N pools. Nitrogen derived from root (Ndfr) and shoot (Ndfs) located in the exterior layers diminished from planting to harvest. The percentage of total N derived from rye shoot and rhizodeposition from rye roots was calculated using equation [1] (Kavdir, Y. 2000) Exterior layers of aggregates 6.0 - 9.5 mm across retained 1.6% of the Ndfr in July 1999, three times more than their interior regions. This was slightly greater than the %Ndfs. One month later, during the corn growing season %Ndfr and %Ndfs were nearly equal in exterior layers and interior regions of soil aggregates, possibly due to diffusion within larger aggregates and uptake by corn. At harvest, there were greater or equal quantities of rye-N located in interior regions compared to exterior layers of aggregates. In the case of N fertilization, diffusion rate of N from exterior layers to interior regions of aggregates and even leaching could be faster limiting availability of N to the plant. Kinyangi (2000), reported that if there is more P in the exterior layers of soil aggregates, 4.0-8.0 mm across, than interior regions at the beginning of corn season, corn roots can easily utilize this P resulting in increased corn yield. Aggregate sizes used in this study covered only 34% of the total soil by weight. Additional research on the best management practices for maintaining more N on surfaces of larger soil aggregates during crop growth as well as sequestering mobile soil N within larger soil aggregates during wet soil periods between cash crops needs to be completed. Aggregates greater than 2 mm contain many longer roots and hyphae than microaggregates (Jastrow and Miller, 1998 and Degens et al., 1994). Aggregates greater than 2 mm had 150 times longer hyphal lengths per aggregate than aggregates smaller than 0.5 mm. Aggregates greater than 2 mm also had 7 times longer root lengths within compared to soil aggregates 1.0-2.0 mm across (Degens et al., 1994). Therefore, main stabilizing factors for macroaggregates are roots, root derived materials and hyphae. While rye and corn roots develop between the planes of weakness and along surfaces of aggregates, root derived materials also help to stabilize macroaggregate surfaces. Continuous addition of SOM through dead and leaving roots and uptake of nutrients and waters by roots contributes development and stabilization of soil aggregates. If we assume the aggregate hierarchy model, proposed by Oades and Waters (1991) is the only model for the soil structure formation of the Kalamazoo loam soil, then macroaggregates should consist of mostly microaggregates and properties of the macroaggregates should be similar all the way across the aggregate. The formation and function of soil macroaggregates are very dynamic processes utilizing many biogeochemical factors. These factors include: continuous additions of C and N compounds by roots (Santos et al, 1998: Kavdir, 2000); additions of N and P by fertilizers (Kinyangi, 2000); weathering of clay minerals by water, microbes and roots (Santos et al., 1997); dessication of aggregates by the uptake of water by plant roots (Sissoko, 1997); nutrient extraction by plant roots and leaching; frequent wetting and drying cycles and countless microbial activities (Guggenberger et al., 1999) all appear to develop concentric layers of various properties into the interior regions of soil aggregates. In a summary, it was found that concentric gradients of rye residue-derived N increased with aggregate size. The location of the N within soil aggregate played an important role on corn N uptake. Rye root and shoot derived N in exterior layers of larger aggregates decreased by time. Therefore these studies suggest increasing soil aggregate size and maintaining active plant root systems are the best strategies for maximizing soil N availability to cash cropping systems and reducing N leaching.  References . Allison, F.E. 1973. A factor in soil aggregation and root development. In. Soil organic matter and its role in crop production. p:314-345. . Angers, D. A., S. Recous & C. Aita. 1997. Fate of carbon and nitrogen in water-stable aggregates during decomposition of 13C15N labeled wheat straw in situ. European Journal of Soil Sci. 48:295-300. . Bottner, P. 1985. Response of microbial biomass to alternate moist and dry conditions in a soil incubated with 14C and 15N labeled plant material. Soil Biol. Biochem. 17: 329-337. . Cheng, W. & D. C. Coleman. 1990. Effect of living roots on soil organic matter decomposition. Soil Biol. Biochem. 22: 781-788. . Degens, B. P., G. P. Sparling & L. K. Abbott. 1994. The contribution from hyphae, roots and organic C constituents to the aggregation of a sandy loam under long-term clover based and grass pastures. European J. of Soil Sci. 45:459-468. . Ditsch, D. C., M. A. Alley, K.R. Kelley &Y.Z. Lei.1993. Effectiveness of winter rye for accumulating residual fertilizer N following corn. J. Soil Water Conserv. 48:125-132. . Guggenberger, G., E.T. Elliott, S. D. Frey, J. Six & K. Paustian. 1999. Microbial contributions to the aggregation of a cultivated grassland soil amended with starch. Soil Biol. Biochem. 31:407-419. . Harper, L. A., P. F. Hendrix, G.W. Langdale & D. C. Coleman. 1995. Clover management to provide optimum nitrogen and soil water conservation. Crop Science. 35:176-182. . Horn,R. 1990. Aggregate characterization as compared to bulk soil properties. Soil Till. Res. 17:265-289. . Jastrow, J. D. & R. M Miller. 1998. Soil aggregate stabilization and carbon sequestration: feedbacks through organomineral associations. In:Soil processes and the carbon cycle. R. Lal., J. M. Kimble., R. F. Follet and B. A. Stewart (Ed.). CRC Press, p: 207-223. . Kavdir, Y. 2000. Distribution of cover crop N retained by soil aggregates in rye-corn agroecosystem. Thesis (Ph.D) Michigan State University. Dept. of Crop and Soil Sciences, p:157. . Kinyangi J. M. 2000. Carbon, nitrogen and phosphorus sequestration within soil aggregates of maize and tree based agroforestry systems. M.S. Thesis. Michigan State University. Dept. of Crop and Soil Sciences. . Kladivko, E. J. 1994. Residue effects on soil physical properties. In: Managing Agricultural Residues. Unger P.W. ed., Lewis Publishers, Boca Raton, FL. p:123-141. . Kirsten, W.J. 1983. Organic elemental analysis. Academic Press, New York, NY. . Oades J. M. & A. G. Waters. 1991. Aggregate hierarchy in soils. Aust. J. Soil Res. 29:815-828. . Paul, E.A. & F.E. Clark. 1996. Soil microbiology and biochemistry, 2nd ed., Academic Press, San Diego, California. 273 p. . 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Concentric layering of carbon, nitrogen, and clay within soil aggregates from tilled and nontilled ecosystems. D. Anger (ed.)., Third Eastern Canada Soil Structure Symposium Proceedings. p. 129-141. . Smucker A. J. M., C. J. Dell. J. M. Kinyangi, Y. Kavdir & J. D. Jastrow. 1999. Quantitative removal of concentric layers from soil aggregates by soil aggregate erosion chambers. Abstracts for ASA-CSSA-SSSA, 91st Annual Meeting, Salt Lake City, Utah, October 1999. . Texier, M. & G. Billes. 1990. The role of rhizosphere on C and N cycles in a plant-soil-system. Symbiosis. 9:117-123. . Whiteley, G. M. & A. R. Dexter.1983. Behavior of roots in cracks between soil peds. Plant and Soil. 74:153-162. . Wood, C.W., D.G. Westfall, & G.A. Peterson. 1991. Soil carbon and nitrogen changes on initiation of no-till cropping systems. Soil Sci. Soc. Am. J. 55:470-476. . Yoneyama, T., T. Muraoka, N. Boonkerd, P. Wadisirisuk, S. Siripin & K. Kouno. 1990. Natural 15N abundance in shrub and tree legumes, Casuarina, and non-fixing plants in Thailand. Plant Soil. 128: 287-294. Acknowledgements The authors wish to thank the Corn Marketing Board of Michigan and LTER graduate student research grant for their financial support during this research at Michigan State University. |