Effect of crop management on C and N in long-term crop rotations after adopting no-tillage management: Comparison of soil sampling strategies

Abstract

Society is interested in increasing C storage in soil to reduce CO2 concentration in the atmosphere, because the latter may contribute to global warming. Further, there is considerable interest in the use of straw for industrial purposes. Using soil samples taken from the 0- to 7.5-cm and 7.5-to 15-cm depths in May 1987 and September 1996, we determined organic C and total N in five crop rotations (nine treatments) using automated Carlo Erba combustion analyzer. The experiment was managed using conventional mechanical tillage from 1957 to 1989; it was changed to no-tillage management in 1990. Our objective was to determine: (a) if change to no-tillage management had changed soil C and N storage, and (b) if method of calculating organic C and N change would influence interpretation of the results. All three methods of calculation confirmed the efficacy of employing best management practices (e.g., fertilization based on soil tests, reducing summerfallow, including legumes in rotations) for increasing or maintaining soil organic matter, and showed that the latter was directly associated with the amount of crop residues returned to the soil. Where bulk density was significantly different between sampling times, the often used mass per fixed depth (MFD) (i.e., volume basis) calculation can lead to erroneous conclusions. When the recently recommended mass per equal depth (MED) method of calculation was used, it showed that 6 yr of no-tillage did not increase soil organic C or total N. However, in unfertilized systems, where crop yields are gradually decreasing since the change, there is an accompanying decrease in organic matter, while fertilized, or high-fertility systems that include legume hay crops, in which wheat yields have been maintained have tended to maintain the organic matter level over time. When the MFD calculation was used, there was no change in C over time when straw was harvested in the F-W-W system; however, the MED calculation and concentrations tend to show a decrease in soil C and N. This suggests that in time, industrial use of straw may have negative consequences for soil conservation. We concluded that concentrations may be as effective as MED for assessing changes in organic matter, provided 'amounts' are not required.