A Comparison of Microbial Respiration in Soils After a 20-Year Period of Subtilling or Plowing

Respiration of organisms was determined on soil samples taken at 0- to 1-, 1- to 6-, and 6- to 12-inch depths from subtilled or plowed Sharpsburg silty clay loam soil in a corn-oats-wheat rotation at the Agronomy farm at Lincoln, Nebraska. Plots had been subtilled or plowed continuously over a 20-year period. Respiration was greater in samples from subtilled plots at 0- to 1-inch depth than in similar samples from plowed plots. This difference was probably due to a higher organic matter and microbial content of the stubble-mulch soil. There was less difference in respiration due to tillage at the 1- to 6- and 6- to 12-inch depths. Statistically significant differences in respiration of soil samples from the corn, oats, and wheat plots were found at the 0- to 1-inch level. Oxygen uptake by the incubated soil samples decreased with depth of sampling. Drying and remoistening of samples was found to increase respiration rates. Respiration was measured after various organic materials (starch, mannitol, dextrose, wheat straw, lignin), herbicides (2, 4-D, dalapon, carbamate), inhibitor (KCN), and mineral nutrient solution were added at several concentrations to soil from both subtilled and plowed plots. Soil samples from subtilled plots generally gave greater respiration than samples of plowed soil, regardless of additive. Increasing concentrations of test materials added to soil generally increased respiration, except KCN which with increasing concentrations decreased respiration. The effects of the various additives were similar regardless of the tillage practice to which the sample had been subjected. This would indicate that the nature of the microbial respiration in both tillage systems was similar qualitatively, although quantitatively subtilled surface soil gave greater respiration.