Soil organic matter assessments in a long-term cropping system study1

Gary E Varvel,Mark A Liebig,John W Doran

doi:10.1081/css-120005752

Abstract

Assessing effects of cropping systems on soil organic matter (SOM) and soil carbon (C) changes are necessary to make accurate projections regarding sequestration and emission of CO2 by agricultural soils. This process requires substantial annual management inputs and large outlays for soil sampling and analyses. Our objectives were (1) to evaluate and test an alternative method for soil organic matter determination, (2) to determine if crop rotation and N fertilizer management significantly affected soil organic matter at the beginning (1986) or after 12 yrs (1998), and (3) to determine if total soil organic matter levels have changed after 12 years in a long-term cropping system study. Soil samples were taken in 1986 and 1998 to a depth of 150 cm in 30 cm increments. Total soil organic C and organic matter by weight loss-on-ignition concentrations were determined for the soil samples taken in 1998. Results from both methods of analyses for the 1998 samples were highly correlated. No significant differences in soil organic matter by weight loss-on-ignition or total soil organic C concentrations between crop rotations or nitrogen (N) fertilizer rates were obtained for either sampling date, in the change in soil organic matter concentrations between dates, or total soil C amounts in the profile (0 to 150 cm) after 12 yrs (1998). Although no differences in soil organic matter (soil C) were obtained in the study, the excellent correlation between results of the two methods of organic matter analyses demonstrates that the less expensive and easier to use weight loss-on-ignition method can be used to make these types of assessments. *Joint contribution of USDA-ARS and the Nebraska Agricultural Research Division, Lincoln, NE, as paper no. 13512.

Highlights

Measurement of the long-term effects of crop and soil management practices on soil organic matter content provides necessary information to evaluate their effect on the environment
Trends toward reduced tillage and more diverse cropping systems should affect the magnitude of these emissions, but assessment of these affects on soil organic matter content must be continually monitored
All soils have various levels of quality, which are basically defined by stable natural or inherent features related to soil forming factors, and the loss of soil organic matter generally results in a reduction in soil quality

Summary

Introduction

Measurement of the long-term effects of crop and soil management practices on soil organic matter content provides necessary information to evaluate their effect on the environment. Soil and crop management can affect global balance of CO2 as soils serve as both a source and sink for this greenhouse gas.[1] The mix of tillage practices used in 1990 for U.S agriculture produced an emission rate of 8 Mg CO2 yr from soil organic matter.[2] Trends toward reduced tillage and more diverse cropping systems should affect the magnitude of these emissions, but assessment of these affects on soil organic matter content must be continually monitored This information can be used to predict the effects of cropping and tillage systems on degradation, maintenance, or improvement of soil organic matter and the resulting sequestration or emission of CO2 by agricultural soils. This process is both difficult and costly because of the amount of organic matter present in soils and the time required for management practices to change soil organic matter levels

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Conclusion