 |  |
| Bildiri Özetleri | |
| Ana Sayfaya Dönüş |
| Back To The Main List |
|
A.J.M. Smucker 1 , Y. Kavdir 2 1 Department of Crop and Soil Sciences, Michigan State Univ., E. Lansing, MI 48824 USA 2 Department of Soil Science, Canakkale University, Canakkale, Turkey Abstract Increasing soil carbon (C) generally increases the stability of soil aggregates. Physical and chemical analyses of extracted soil organic matter (SOM) have produced considerable quantities of information. Yet, little is known of the in situ biophysical locations and interactions between the SOM and the microbial biomass within soil aggregates. Using small soil aggregate erosion (SAE) chambers for separating individual soil aggregates into concentric layers, we have been able to identify gradients of carbon, nitrogen (N), several ions, clay minerals, and microbial communities within soil aggregates. Recent developments of computer microtomography (CMT) evaluations of whole aggregates also provide opportunities to view internal porosities of 3D images of these small aggregates. Soil aggregates extracted from no-till (NT) soil management of continuous maize production for nearly 40 years, contained nearly twice the total C concentration of Wooster silt loam aggregates sampled at 0-5 cm depths. Short-term cover crop management systems have demonstrated clear accumulations of both C and N within concentric layers of soil aggregates. Introduction Soil structure includes a vast array of heterogeneous and separable individual soil aggregates that are repositories of C, water, microbial communities, plant nutrients, and pollutants within the soil profile. These biophysical polymorph structures control the absorption, storage and losses of most soil constituents. Nitrogen is highly mobile, especially in soils containing low levels of soil organic matter. Increasing concentrations of soil nitrate (NO3) in fertilized agroecosystems are leaching into groundwaters creating potential health risks. Timing of N fertilizers combined with proper tillage and cover cropping are excellent management approaches for minimizing the leaching of soil nitrates. Living biomass is the best possible approach for reducing N leaching through coarse and medium textured soils. Microbes in the rhizospheres of plants develop into large reservoirs of N metabolites retained by the living microbial biomass. Therefore, it is essential that living plants and associated rhizospheres remain active for the duration of each year. Cover crops are excellent bridge crops for retaining an active microbial biomass as well as additional absorptive surfaces for retaining soluble soil N from leaching beneath the root zone. Soil aggregates develop and function through a complex biological, chemical and physical interactions with climate, water, ion and many anthropogenic actions upon the soil matrix (Dexter and Horn, 1998). Formation processes include the cementation of adjacent smaller soil aggregates into larger aggregates and/or the accumulation, sorption, and cementation of soil minerals, ions, and particulate organic matter (POM) (Dexter and Horn, 1998; Smucker, et al., 1998). Repeated wetting and drying cycles develop stronger soil aggregates than surrounding bulk soil and create smaller internal pores than the surrounding macropores of bulk soil. Root growth, exudation, fungal associations and death contribute substantial quantities of C into soil aggregates (Jastrow, et al., 1996). Continuous rhizodeposition of C into soil aggregates can be considered one of the major contributions of crop management to soil structure formation and stability. There seems to be very little information on biogeochemical mechanisms associated with the improvement of C fixed by plants and retained for prolonged periods of time. Many of the mechanisms controlling soil aggregation processes remain unknown. These and other dynamic feed-forward and feed-back root and soil activities are being investigated. Some are reported in this paper. Approach Measurement of most soil properties has been limited to evaluations of composite samples of bulk soils or individual aggregate fractions. Although this approach has led to the discovery of many soil properties, the bulk soil approach provides little information on the microsite sequestration or protection of soil carbon (C), nitrogen (N) and other ions that influence the root soil interface and microbial communities. We mechanically removed concentric layers from soil aggregates by rotating individual aggregates in soil aggregate erosion (SAE) chambers, Fig. 1, until pre-designated quantities of soil material are removed by erosion. Using more than 130 SAE chambers enables one individual to remove multiple soil layers from literally 100s of aggregates daily. Most frequent rotational speeds range from 250 to 450 revolutions per minute (rpm) and are dependent upon the tensile strength and erosion resistances of layers within each aggregate. Nondestructive CMT characterization of pore networks, within aggregates, is essential to our understanding of the intra-aggregate porosity. Knowledge of pore connectivities will greatly assist our knowledge of fluid transport and residence times of soil solutions among connected pores within aggregates. More than a century of conventional tillage (CT) of virgin forest soils in Ohio has reduced total porosities of soil aggregates, 2-9.5 mm across, by as much as 17%. Approximately half of this lost porosity was recovered by 40 years of NT. Measurements of the ratios between hydraulic conductivity (Ks) and aggregate porosity for aggregates sampled from these expanding and contracting Wooster silt loam soils demonstrated fewer connected pore networks within CT and NT aggregates than similar sized aggregates from nearby forest soils (Park et al., 2001). These significant interactions among C and N substrates, SMB, and the number of wetting and drying cycles for different management treatments of the same soil type as well as the discontinuous porosities of tilled agricultural soils suggest specific intra-aggregate process-level biogeochemical mechanisms, eg., changes in the internal pore connectivities during frequent W/D cycles of soil aggregates having different microbial populations, SOM levels, ion concentrations, textures, and clay minerals. Results and Conclusions Additions of simulated root exudates to the Kalamazoo loam soils of our 20-year Long Term Ecological Research programs in Michigan, increased the soil microbial biomass (SMB) of soils by 15% and 70% for agricultural and grassland ecosystems. When root exudates were combined with nine wetting and drying cycles, the mean weight diameter indices of aggregate stability increased nearly 4-fold (Sissoko, 1997). Differences among the changes in aggregate stabilities could not be contributed exclusively to changes in C and N substrate and associated SMB. In separate studies, C contents of thin soil layers were 2.4-fold greater on exterior regions of NT than CT aggregates (24 and 10 g C kg-1) and 3-fold greater on interior regions of NT than CT aggregates (21 and 7 g C kg-1) (Dell et al., 2002). Significantly greater quantities of labile C respired from surface layers of soil aggregates than from soil layers extracted from internal regions of aggregates during the first 88 d of incubation. Therefore, most recently deposited root C appears to be more rapidly respired by microbial communities located on surfaces of soil aggregates than their interiors. Nearly 75% of the catch crop N was released by the decomposing roots of an irridicated rye cover crop resulting in the absorbance of 40 kg N ha-1 per year by two successive maize crops (Kavdir, 2000). Decomposing rye roots deposited organic and inorganic N to surface layers of soil aggregates forming gradients of N within soil aggregates in the rhizosphere. When ratios (Ne/Ni) of external N (Ne) to internal N (Ni) concentrations within soil aggregates exceeded 1.2, both corn plant biomass and grain yield increased. There were significant correlations (r2=0.88) between Ne/Ni and corn biomass, suggesting root deposits of N on surfaces of soil aggregates became more available to successive corn crops. These results demonstrate, from the plant root's perspective, that the time between N deposition by catch or inter crops and the primary crop are critical and can be better evaluated when soil aggregates are separated into layers and analyzed before the best management practices for controlling plant N can be determined for either high or low input crop production systems of a sustainable agroecosystem. Image processed information of digitally scanned washed roots (Smucker and Aiken, 1992) enabled the calculations of soluble nitrogen (N) uptake by roots of rye catch crop experiments in the Ap and Bt horizons of a Kalamazoo loam soil (coarse-loamy, mixed, mesic Typic Hapludalf) during a 39-day period in April and May 1998. Measuring 15N and total nitrate uptake by rye roots, Kavdir (2000) reported 27 (+ 5) mg N were absorbed per m2 of rye root surfaces per day in the Ap horizon and 9 (+ 6) ) mg N were absorbed per m2 of rye root surfaces per day in the Bt horizon. These types of data can be used to estimate nitrogen uptake by the root systems of different cover crop species when minirhizotron observations of root demographics are combined with washed roots and frequent soil N evaluations within the soil profile. These combined approaches are most useful for comparing nitrogen uptake efficiencies for different plant species, especially when identifying the best cover crops for the phytoremediation of nitrate, pesticide, or heavy metal contaminations of groundwater beneath high-risk soils.
References . Dell, C.J., Smucker, A.J.M. and Paul, E.A., 2002. Carbon mineralization from concentric layers of soil aggregates. Soil Sci. Soc. Am. J. (submitted). . Dexter, A.R. and Horn, R., 1998. Effects of land use and clay content on soil structure as measured by fracture surface analysis. Z. Pflanzenernahrung Bodenk. 151, 325-330. . Jastrow, J.D.; Boutton, W.; Miller, R.M., 1996. Carbon dynamics of aggregate-associated organic matter estimated by carbon-13 natural abundance. Soil Sci. Soc. Am. J. 60, 801-807. . Kavdir, Y., 2000. Distribution of cover crop nitrogen retained by soil aggregates within a rye-corn agroecosystem. Ph.D. Dissertation. Michigan State University. 157 p. . Park, E.J., Aditjandra, K.L. and Smucker, A.J.M., 2001. Spatial variability of porosity, tensile strength, and mineralogy within soil aggregates. ASA Abstracts, Charlotte, NC. . Sissoko F., 1997. Enhancement of soil aggregation by the combined influences of soil wetting and drying and root-microbial associations. MS thesis, Michigan State University, East Lansing. 109p. . Smucker, A.J.M. and Aiken, R.M., 1992. Dynamic root responses to water deficits. Soil Science 154,281-289. . Smucker, A.J.M., Santos, D., Kavdir, Y., Paul, E.A. and Snider, R., 1998. Concentric gradients within stable soil aggregates in Proceedings of the 16th World Congress of Soil Science, France, August 1998. . Van Gestle, M., Ladd, J.N. and Amato, M. 1991. Carbon and nitrogen mineralization from two soils of contrasting texture and microaggregate stability: Influence of sequential fumigation, drying and storage. Soil Biol. Biochem. 23, 313-322. |
Figure 1. Soil aggregate erosion (SAE) chambers are constructed of machined stainless steel tubes with a 350 micron sized screen welded on to the base of the erosion chamber (left). Inside walls of the erosion chamber are finely knurled to create an abrasive surface above the screen, retaining the unpeeled portion of the soil aggregate (left). Bases of the SAE (right) are machined from a stainless steel rod, with the bottom drilled into a concave surface that can be easily cleaned.