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Soil Carbon Levels Highest in Upper Soil Layers

Author

Steve L. Watson,
Colorado State University

Executive Summary 
Three K-State long-term tillage and fertility studies have recently been analyzed for changes in soil organic carbon (SOC) levels.



Three K-State long-term tillage and fertility studies have recently been analyzed for changes in soil organic carbon (SOC) levels. This analysis was done by Chuck Rice, K-State professor of agronomy, and his graduate student Karina Fabrizzi.  

     
Rice and Fabrizzi analyzed soil samples from ongoing studies at K-State experimental sites in Tribune, Hays, and Parsons. At all three locations, soils were sampled at three depths: 0-5 cm, 5-15 cm, and 15-30 cm. Results were a little different in each region of Kansas.  

     
Tribune: This 15-year study involves a wheat-sorghum-fallow rotation under three tillage treatments (no-till, reduced-till, and conventional-till) and a native sod area for comparison. The soil in a Richfield silt loam. The soils were sampled in November 2003, both in fallow ground and in wheat-stubble ground.  

     
At the 0-5 cm depth, total SOC was highest in the no-till plots, followed by reduced-till then conventional-till. Averaged over the entire 30 cm depth, however, tillage had very little effect on total SOC when the soil was sampled in the fallow phase of the rotation. When the soil was sampled in the wheat phase of the rotation, the no-till plots had 2.9 tons/acre more SOC than the conventional-till plots at the 0-30 cm depth. The sod plots had consistently higher total SOC levels than any of the plots in the wheat-sorghum-fallow rotation.  

     
Hays: This 38-year study also involves a wheat-sorghum-fallow rotation under three tillage treatments (no-till, reduced-till, and conventional-till). An additional factor at this site was four nitrogen rates, ranging from 0 to 60 lbs per acre. The soil was a Harney silt loam. Soils were sampled in March 2003, following sorghum harvest.  

     
As with the Tribune study, total SOC was highest in the no-till plots at the 0-5 cm depth at Hays. Conventional-till plots had higher SOC levels at the other two depths. And over the entire 30-cm depth, conventional-till plots had the highest total SOC levels.  

     
No-till plots had the highest level of soil aggregation at the 0-5 cm depth, which means improved soil structure.  

     
Parsons: This 20-year study involves a sorghum-soybean rotation under three tillage treatments (no-till, reduced-till, and conventional-till) and two nitrogen rates (0 and 100 lbs per acre). The plots were sampled in December 2003.  

     
Total SOC was highest in the no-till plots at the 0-5 cm depth. No-till also had the highest total SOC when averaged over the entire 30 cm depth.  

     
SOC levels were slightly higher overall in the plots receiving 100 lbs per acre of N than in the unfertilized plots.  

     
Summary: At all three locations, no-till increased total SOC in the upper layer of the soil (0-5 cm). At Parsons, the no-till plots had about 1.3 tons/acre more C than the conventional-till plots after 20 years. At Hays, the no-till plots had about 0.4 tons/acre more C than the conventional-till plots after 38 years. At Tribune, the no-till plots had about 1.5 tons/acre more C than the conventional-till plots after 15 years.  

     
When averaged over the 0-30 cm depth, the results are not consistent.  

     
In eastern Kansas (Parsons), where the plots were in continuous cropping, no-till increased total SOC at the 0-30 cm depth by about 1.05 tons per acre after 20 years.  

     
In western Kansas (Hays) under a wheat-sorghum-fallow rotation, total SOC levels were lower under no-till. One possible explanation for this is that under no-till, the soil remained wetter than under conventional-till, even at the lower depths of soil. This resulted in an increase in microbial decomposition of organic matter in no-till systems. At the same time, the fallow period did not add any additional organic material from crop production. So the net result was a slight decrease in SOC in no-till at the 0-30 cm depth.  

     
On the other hand, no-till consistently improved soil structure and resulted in larger soil aggregates at the 0-5 cm depth at all locations, and for all rotations.  

     

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