AN ORGANIC MATTER TRAIL: POLYSACCHARIDES TO WASTE MANAGEMENT TO NITROGEN/CARBON TO HUMIC SUBSTANCES

C. Edward Clapp

USDA-ARS, Department of Soil, Water & Climate, University of Minnesota, St. Paul, MN 55108

1 INTRODUCTION

A farm boy from Princeton, MA went to the University of Massachusetts to study chemistry and learn how to make dynamite. After 4 years with a chemistry major and agronomy minor he proceeded to Cornell University to become a soil biochemist. This paper represents a summary of the research and experiences initiated in 1952 under the guidance and direction of Professor Jeffrey Earl Dawson, who started me on the way along the Organic Matter Trail. Jeff was an outstanding teacher and researcher who thought thermodynamically and later became a true friend and colleague. His life and career were cut short due to health complications, and he passed away at age 49. This paper is dedicated to his memory. Along with the highlights of research on organic matter, many references will be made to the people, colleagues and friends who influenced the Trail throughout the years.

2 AN ORGANIC MATTER TRAIL

The Organic Matter Trail started for the author in the Organic Soils Laboratory of the Department of Agronomy at Cornell University in Ithaca, NY in June 1952. After M.S. and Ph.D. degrees were completed, the trail led to Beltsville, MD in October 1956 with the Soil and Water Conservation Research Division (SWCRD) of USDA-ARS. Work on soil and rhizobial polysaccharides was carried out to study aggregate stabilization. After 5 years, a transfer of the organic matter/soil structure project to the Cornbelt Branch of the SWCRD in the Department of Soil Science at the University of Minnesota was completed in May 1961. In August, a visiting scientist arrived from Australia to spend a year investigating organic matter and soil crumbs. This work continued with students and other support for 10 years. When funding for basic research became difficult to justify, a call was issued for more practical and field-oriented projects. This led to waste management experiments involving application of municipal sewage sludges and wastewaters to agricultural land.

After ten more years, in 1980, a series of field and laboratory experiments was started involving nitrogen-tillage-residue management (NTRM) including 15N and later 13C isotope studies. Many graduate students, ARS Soil and Water Research Unit colleagues and Soils Department cooperators were involved in research and publication activities. During the late 1980s, the Organic Matter Trail was side-tracked but later developed into a super highway! Humic substances were re-introduced during an educational leave in 1988-89 at the University of Birmingham in England, and continued in 1989 at the Hebrew University of Jerusalem in Rehovot, Israel. Many facets of humic substances research have continued through the 1990s and into the 21st century.

Involvements with the International Humic Substances Society (IHSS), including many former associates and friends, led to an IHSS Board position and a relationship with the IHSS Standard Sample Collection. Together, we have set out on a combination of research projects, Society programs, publication endeavors (some controversial) and other exciting activities. With cooperating humic scientists we are looking at new isolation and characterization techniques, the sizes and shapes of humic macromolecules, at organic contaminant studies, at metal-humic interactions, at water treatment and at humic-plant stimulation, among other topics. The research areas in this version of the Organic Matter Trail will serve to summarize the career of one soil biochemist who has chosen to spend his time in the laboratory and field.

2.1 Polysaccharides

An agricultural muck soil from Orange County, NY was extracted with hot water in a Soxhlet system to produce several kilograms of lyophilized organic matter. After concentration, electrodialysis and ultrafiltration, a low-charged polysaccharide was isolated using convection as well as continuous flow paper curtain electrophoresis. Electrophoretic mobilities in phosphate and borate buffers were determined for the isolated polysaccharide using a column electrophoresis apparatus. The polysaccharide was hydrolyzed and component sugars identified by comparison of their relative electrophoretic mobilities with the relative mobilities of pure sugars (Figure 1). Earlier experiments by Dawson (in cooperation with Arne Tiselius) laid the groundwork for the electrophoresis studies.

The first real job started in October 1956 to study the effects of organic matter on soil structure at the USDA-ARS Soils Laboratory at Beltsville, MD under Hub Allaway. Administrative guidance was provided by Frank Allison and cooperative projects on rhizobial and soil polysaccharide effects on aggregate stability were carried out with Bob Davis.

A transfer in 1961 to the USDA-ARS Cornbelt Branch under C.A. Van Doren in the Soil Science Department (Bill Martin, Head) of the University of Minnesota, St. Paul, led to a continuation of the organic matter/soil structure work. Administratively, the St. Paul Unit was directed by Bob Holt of the USDA Morris, Minnesota Lab. The one-year sabbatical of Bill Emerson, CSIRO soil physicist from Adelaide looking at soil crumb stability' and clay-polysaccharide complexes provided a cooperative and highly rewarding venture. Tables 1 and 2 show the effects of organic matter extraction coupled with periodate oxidation of polysaccharide components. A1 Olness joined the group as a graduate student and we carried on the polysaccharide-clay interaction studies. X-ray diffractograms of clay-Polytran complexes (Figure 2) show two first-order (001) peaks and shoulders of the 3.6, 7.3 and 16.6% dextran complexes, indicating more than one basal spacing. No evidence of spacings greater than 15 Å was found in the diffractograms of the Polytran complexes dried at 105°C.

2.2 Waste Management

The Organic Matter Trail continued when Bill Larson came to St. Paul in 1968 as Research Leader of the ARS Soils Unit. With the advent of the waste management studies in 1972, field and laboratory experiments were initiated at St. Paul, Rosemount and Elk River, MN. Other chief players were Soil Scientists Bob Dowdy and Dennis Linden, along with graduate students Steve Stark and Scott Harding. A 40-acre terraced sewage sludge watershed was constructed at Rosemount where crop yields, nutrient uptake and soil and water quality were investigated for over 20 years, with corn (lea mays L.) and reed canarygrass (Phalaris arundinacea L.) as crops. Nitrogen mineralization and availability from application of different types of sewage sludge was also a featured topic (Figure 3). Results indicated that the sludge application rate was more important than sludge type. An 15N-labeled fertilizer experiment was superimposed on sewage sludge field plots to further study N availability.

A field experiment applying treated municipal wastewater (secondary effluent) to corn and 8 forage grasses was co-sponsored by the US Army Corps of Engineers (CRREL) and carried out at Apple Valley, MN. Main investigators, in addition to the ARS-Soils group, were Gordon Martin (ARS-Crops), Tony Palazzo (CRREL), Jim Nylund (Soils Dept.) and Karen Chopp (a graduate student). Results of the 6-year study showed that municipal wastewater could be applied effectively to agricultural crops with a reduction of nitrogen and phosphorus to acceptable safe levels for surface and ground water quality. A summary of nitrogen uptake by reed canarygrass (Figure 4) shows the large amount of nitrogen removed from wastewater compared with a fertilizer control. Additional studies involving microbial activity and nitrate risk assessment were carried out with Ed Schmidt (Soils Dept.).

2.3 Nitrogen/Carbon

A series of field and laboratory experiments started in 1980 involved nitrogen-tillage-residue management (NTRM), including 15N and 13C isotope studies. An earlier experiment initiated by Bill Larson at Ames, IA was completed and the effects of residue additions on gain and loss of soil carbon were compared (Figure 5). It would take approximately 6 Mg ha-1 yr-1 of corn stalks to prevent loss of organic matter in a soil containing about 1.8% C.

The NTRM experiment involved 8 large tractor-size blocks with three nitrogen levels (0-100-200 kg ha-1), three tillage management systems (no-till, moldboard-plow, and chisel-plow) and two residue treatments (returned or harvested). A comparison 15N-labeled fertilizer study on smaller plots (the most expensive 100 m2 area in Dakota County!) was carried out during the same period under the initial inspiration and guidance of Art Edwards. Treatments included two fertilizer levels (2 and 20 g N m-2 with 40 and 4 atom-% 15N, respectively) applied for 15 years, two tillage practices (no-till or rototill) and two residue choices (returned or harvested). Reports of the early years are given by Dave Clay (graduate student), including predictions based on models NCSOIL and NCSWAP (Figure 6) by Jean Molina (Soils Dept.) and Randy Deans (graduate student). A recent modeling paper based on data from the NTRM experiment reported the soil conditions necessary to reproduce the measured kinetics of seven variables over a 13-year period.

Continuing the organic matter theme resulted in a review on interactions of organic macromolecules with soil colloids at a NATO conference A summary review of 13C abundance in world-wide soil and water sources was prepared for a Dublin conference. Access to other long-term field experiments came about with the addition to the team of Ray Allmaras (ARS, St. Paul) and Dave Huggins (Soils Dept., Lamberton). A 10-year corn-soybean cropping sequence study provided a unique opportunity to investigate C dynamics using natural 13C abundance. Our results have shown that long-term field experiments are among the best means to predict soil management impacts on soil C storage. Soil organic C and natural abundance 13C (δ13C) were sensitive to tillage, stover harvest and N management during 13 years of continuous corn in the NTRM study. Figure 7 shows results where δ13C values generally increased in both the 0-15 and 15-30 cm layers. All slopes except those in the NTh treatment in the 0-15 cm layer were greater than 0, irrespective of N rate.

2.4 Humic Substances

The re-introduction of humic substances research actually occurred with the IHSS Conferences in 1983 at Estes Park, CO and in 1984 at Birmingham, UK. A chapter on viscosity measurements of humics was presented and written for an IHSS book. Another book co-edited with Pat MacCarthy, Ron Malcolm and Paul Bloom on humic substances in soil and crop sciences resulted from a symposium at the SSSA meeting in 1985. The educational leave at Birmingham in 1988-89 brought together two old Cornell lab-mates and friends. Mike Hayes completed his M.S. with Jeff Dawson in 1955 and we relived many old memories. We carried out many experiments involving isolation and characterization of humic substances using techniques that Mike had been developing at Birmingham. Some of this work was reported at a conference in Lancaster, UK and published in 1993. Another research paper introducing a new extraction method was presented and published as part of the proceedings of the IHSS Conference in Trinidad in 1994.

A research project started during the extended educational leave in Israel (with Uri Mingelgrin and Yona Chen) was completed and published in 1997. Plots in Figure 8 show significant differences in complexation coefficients (Kc) between the two herbicides napropamide and atrazine. The dialysis experiments carried out demonstrated a procedure by which the extent of complexation of small organic molecules with stable humic acids could be quantitatively estimated.