Willson, T. 1998. Managing nitrogen mineralization and biologically active organic matter fractions in agricultural soils. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

Integrated cropping systems use biological resources such as organic fertilizers, nitrogen fixing plants, and crop and cover crop residues to reduce the need for chemical inputs and improve sustainability. This research explores the effect of integrated management on nitrogen mineralization in a variety of corn-based agricultural rotations and early successional treatments at the Kellogg Biological Station (KBS) in Southwestern MI. Part 1 defines the effect of management on the seasonal dynamics of nitrogen mineralization potential (NMP), the intrinsic ability of a soil to mineralize N over time (ideally a growing season). Measurements of NMP, based on the accumulation of inorganic N in soils incubated under aerobic conditions for 10, 30, 70, 150 and 310 days at 25 °C, are evaluated using 1st order mineralization kinetics and repeated measures analysis of variance.

Nitrogen mineralization potential was greatest in the spring and lowest in the fall in all treatments. It increased in response to compost additions, was greater in low-input rotations with cover crops than in conventional rotations, and was greater in successional treatments without tillage than in tilled succession or agronomic treatments. During a corn-corn-soybean-wheat rotation, NMP was lowest under 2nd year corn and highest under wheat and 1st year corn, reflecting both legume inputs and the lack of spring tillage in wheat. Part 2 of the research found that macroorganic matter was correlated with NMP in most treatments, but that microbial biomass was not. The strongest correlation (r2=.41) was between macroorganic C in the 53-2000 m m size class and inorganic N accumulation in 150 day incubations. Macroorganic matter increased more rapidly after compost additions than NMP, but was less affected by residue inputs. Part 3 of the research found that conventional fertilization produced higher corn and wheat yields and greater leaching loss than additions of leaf and dairy compost. Net annual N mineralization (calculated from plant uptake and leaching loss) was greater in the compost treatments, but N availability was low because of lower than expected compost mineralization (9% yr-1). Nitrogen mineralization was greatest in years with corn production and lowest in years with soybean and wheat production, reflecting (in part) differences in residue input from the previous crop.

Much of the variation in N mineralization could be predicted by laboratory measurements of NMP and macroorganic matter, particularly total macroorganic N (53 – 2000 mm), potentially mineralizable organic matter (No), and N mineralization predicted in 70 and 150 day incubations based on the 1st order kinetics. Leaching loss (October through September) was best predicted by initial inorganic N and total inorganic N (initial plus mineralized) in incubations performed after sidedress fertilization in the preceding season. This research shows that N mineralization varies appreciably within and between growing seasons, and responds predictably to integrated agricultural management.

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