Research on Soil Quality under Different Tillage Systems in Northeast Oregon

 

S.L. Albrecht, Microbiologist, C.L. Douglas, Jr., Soil Scientist, R.W. Rickman, Soil Physicist, and P.E. Rasmussen, Soil Scientist, USDA-ARS, CPCRC, Pendleton, OR 97801-0370

INTRODUCTION

Soil quality and soil health are terms that are generating increasing discussion in the agricultural community. Both terms encompass a complex, dynamic concept, difficult to define and hard to measure. The Soil Science Society of America has recently published two volumes offering definitions of and methods for assessing soil quality (Doran et al, 1994; Doran and Jones, 1996).

Many changes in the soils of agroecosystems can require 10-20 years to identify and quantify. Long-term experiments can benefit agricultural productivity by demonstrating the effects of management, including crop rotations, fertilization, erosion, variety development, and tillage on soil quality. Deterioration of soils can be reduced or prevented by an improved understanding of biological, chemical and physical reactions in soil that may be accelerated or interrupted by management practices.

LONG-TERM EXPERIMENTs

The Pendleton Agricultural Research Center has several ongoing long-term experiments (Table 1). The Residue Management, Continuous Cereal and Grass Pasture experiments were started in 1931 and are among the oldest replicated research experiments west of the Mississippi River. The crop variety, tillage, date of seeding, and grain yield have been documented for all experiments. The experiments are representative of cropping systems in the Pacific Northwest that receive less than 18 inches of rainfall.

 

Table 1. Long-term research experiments at Pendleton, OR

Experiment Name

Year Initiated

Treatment Variables

Grass Pasture

1931

None

Continuous Cereal

1931

Tillage

Residue Management

1931

Nitrogen, Manure, Burning

Tillage-Fertility

1940

Tillage, Fertility

Wheat-Pea

1963

Tillage

No-till Wheat

1982

Nitrogen

The Continuous Cereal and the No-till experiments were modified in 1997 to include no-till treatments in the former and expanded area in the latter, allowing cropped and fallow treatments during the same year.

 

EVALUATING SOIL QUALITY

Improving soil quality has excellent short- and long-term benefits for agricultural production and for the environment and should be an important part of any management plan. However, data must be available to determine soil quality. It has been proposed that a minimum data set, consisting of 16 physical, chemical and biological parameters, be used to assess soil quality (Pierce and Larson, 1993; Doran et al, 1994). This data set includes soil organic carbon. The loss of soil organic carbon is a major, if not the major, problem for maintaining soil quality in the Pacific Northwest. Soil organic carbon can be an indicator of soil organic matter, which is very important in water holding capacity, tilth, the retention of macro and micronutrients, and the promotion of microbiological activity. Maintaining or increasing soil organic carbon or soil organic matter will maintain long-term productivity and reduce runoff and erosion which will, in turn, offer significant benefit to stream water quality through reductions in sediment and nutrient loads.

SOIL ORGANIC MATTER

When cultivated, soils in the Pacific northwest are susceptible to the loss of soil organic matter. Irregular yields, summer fallowing, and burning are all contributors to long-term reduction in soil organic matter and erosion. Changes in organic carbon and nitrogen have been documented in the long-term experiments. The two major factors influencing changes in organic carbon and nitrogen are the frequency of summer fallow and the amount of carbon input by crop residue. Most of the organic carbon is lost during the fallow year of a crop cycle; the loss is both from continuing biological activity and the absence of carbon input. In a fallow system, carbon input occurs every other year, which exacerbates the oxidation of soil organic carbon by soil organisms. Biological activity is greater in fallow soils because the soil organisms are not competing with plants for soil moisture, hence the soil microorganisms have adequate moisture for growth and activity during the warm summer months. While erosion is minimal in the long-term experiments and less important than biological oxidation in the loss of soil carbon, it may have a long-term impact on carbon and nitrogen levels. Although decreasing tillage intensity reduces soil carbon and nitrogen loss, it is not as effective as eliminating summer fallow.

ORGANIC MATTER LIGHT FRACTION

It has been suggested that the amount of light fraction in soil organic matter can provide a good indication of the status of soil management (Christensen, 1992). A light fraction can be separated from soil organic matter by density fractionation. Light fraction was isolated from several of the long-term experiments to evaluate its relationship to long-term management practices. The light fraction differed distinctly among management systems. It ranged from 0.4 g per kg in soils from a conventional management regime (moldboard plow, summer fallow, residue burned) to 6.1 g per kg in soils from a continuous grass pasture. Light fraction increased with increased nitrogen fertilizer (Table 2), and decreased with soil depth. Soils from annual

 

Table 2. Soil organic matter light fraction (0-8 in.) from the Crop Residue long-term research experiment at Pendleton, OR

Fertilizer

 

Light Fraction

Nitrogen

 

Free

Intra-aggregate

Total

lbs/ac

- - - - - - - - - - - - - - - - - g/kg - - - - - - - - - - - - - - - -

0

1.24 0.24

0.45 0.15

1.69 0.12

80

2.52 0.11

0.57 0.09

3.09 0.02

Manure

3.96 0.07

0.34 0.01

4.30 0.07

Manure addition = 10 tons acre-1 crop-1 wet wt (47.5% dry matter; 1557 lb. C and 130 lb. N acre-1 crop-1)

cropping management systems, where the residue is retained on the surface, showed greater organic matter light fraction accumulation than soils from systems with summer fallow and residue burning. The light fraction can be separated into material that is not closely associated with any soil aggregates (free) and a portion that is intimately bound to soil aggregates (intra-aggregate). Preliminary results show that only a small portion of the light fraction is intra-aggregate, which suggests that these soils have little aggregate stability.

CARBON DIOXIDE LOSS

Climatic conditions in the Pacific northwest are much different than other parts of the USA. Most summer precipitation evaporates before it can be utilized by crops or soil microorganisms. Under annual cropping, when soils are dry, there is little microbial activity in the summer; the soil organisms are most active during the spring and fall when soils are moist and temperatures are relatively warm. The carbon dioxide flux following tillage from soils at the Pendleton Experiment Station have been measured. It was found that short-term, i. e. in the first few hours following tillage, carbon dioxide release resulted from soil fracturing during tillage with the resultant loss of trapped gas. This is in agreement with the report of Reicosky and Lindstrom (1993), who also reported that during short-term release, depth of soil disturbance was more important than the amount of residue incorporated, and that tillage methods limiting soil disturbance have the least impact on carbon dioxide loss. After the initial release, the carbon dioxide flux from the tilled soil remained low, until approximately three days following tillage when it began a steady increase, due probably to the initiation of decomposition of freshly incorporated residue. Carbon dioxide production reached a maximum after 15 days, then slowly declined. The carbon dioxide flux was greatest following moldboard plowing and reduced following chisel plowing. Carbon dioxide flux from no-till plots remained relatively stable. It is proposed that the carbon dioxide from the decomposition of incorporated residue is the difference between the amount of carbon dioxide released from the tilled treatments and the carbon dioxide lost from the no-till treatment. Table 3 shows the comparison of plow and chisel tillage on the loss of carbon dioxide from soil.

 

Table 3. Carbon loss from Walla Walla soil, Pendleton Experiment Station.

   

Carbon

 

Residue

Treatment

 

Trapped

Decomposing

 

Equivalent

 

- - - - - - - - - - - - lbs/acre - - - - - - - - - - - -

Chisel  

8.5

52.7

 

132.1

Plow  

8.8

303.6

 

758.9

 

SUMMARY

The loss of soil carbon, and soil organic matter, has a substantial effect on soil quality in the Pacific northwest. Soil organic matter can be maintained or increased in most semi-arid soils if they are cropped every year, the residues are returned to the soil, and erosion reduced or eliminated. Summer fallow, while improving soil moisture for the crop, is very detrimental to soil organic matter retention and soil quality. Crop management practices, such an nitrogen fertilization, increase residue production and improve carbon and nitrogen levels in the soil. We conclude that most of the loss of organic carbon and nitrogen is due to biological oxidation and the lack of carbon input into soils. Yearly crop production with a reduction in tillage is recommended for maintaining soil quality in the soils of the Pacific northwest.

Acknowledgments: We thank Amy Baker, Roger Goller, Tami Johlke, Chris Roager, Katherine Skirvin, and Claude Sterling, USDA-ARS, Pendleton, for technical support, and Saran Sohi, IACR Rothamsted (UK), for discussion. Part of this study was supported by grant SCPUK-27 from the USDA, Foreign Agricultural Service.

REFERENCES

Christensen, B.T. 1992. Physical fractionation of soil organic matter in primary particle size and density separates. Advances in Soil Science 20:2-90.

Doran, J.W., D.C. Coleman, D.F. Bezdicek and B.A. Stewart (Eds.). 1994. Defining Soil Quality for a Sustainable Environment. Special Publication No. 35, Soil Science Society of America, American Society of Agronomy, Inc., Madison, WI. 244 pp..

Doran, J.W. and A.J. Jones (Eds.). 1996. Methods for Assessing Soil Quality. Special Publication No. 49. Soil Science Society of America, American Society of Agronomy, Inc. Madison, WI. 410 pp..

Pierce, F.J. and W.E. Larson. 1993. Developing criteria to evaluate sustainable land management. pp. 7-14. In J.M. Kimble (Ed.) Proc. 8th Int. Soil Management Workshop. May 1993. USDA-SCS. Lincoln, NE.

Reicosky, D.C. and M.J. Lindstrom. 1993. Effect of fall tillage method on short-term carbon dioxide flux from soil. Agronomy Journal 85:1237-1243.