Effect of Management On Storage and Turnover of C and N in Upper Midwest Cropping Systems.

The objectives of this research were to determine the effect of long-term management differences (tillage intensity and crop rotation) on soil C and N storage and dynamics, using ¹³C and ¹⁵N labeled shoot, root and nutrient inputs at the Variable Input Crop Management Study research trial in Lamberton, MN.  Management systems established in 1989 were: low and high purchased inputs (LPI and HPI), and organic management with (ORG+CC) and without (ORG-CC) cover crops, under 2-yr (corn-soybean) and 4-yr (corn-soybean-oat/alfalfa-alfalfa) rotations. Microplots were labeled in 2004 and 2005 and followed through Fall 2006.  The ORG, LPI and 4-yr HPI treatments had higher total C and N than 2-yr HPI. There were significantly more macroaggregates (> 250 µm) in the ORG and LPI than in the HPI management systems.  Neither the fraction of root-derived C and N or shoot-derived C and N in the total soil differed by management, but ORG systems had less root-derived C and N in the occluded fraction than HPI because of the relative sizes of the occluded C pools (larger in ORG and smaller in HPI).  The amount of shoot-derived C and N in the occluded fraction was highest in the LPI 4-yr system. Roots contributed approximately 3 times as much as shoots to soil C.  Roots had lower soluble sugars and starch content but greater amounts of hemicellulose, cellulose, and lignin than shoots, favoring slower decomposition of root material. After 2 years, 39% of the ¹³C added in roots remained, compared to 17% of that added to the soil in labeled shoots. As expected, the mass of labeled C disappeared more quickly than that of labeled N.  Fractions of shoot-derived C and N were more highly correlated over time than root-derived C and N, indicating that turnover was more closely coupled in the shoot residue.