Monday, November 5, 2007
96-25
Nitrogen Cycle among Soil Organic Matter Fractions in Agricultural Systems.
Ana Wingeyer, University of Nebraska-Lincoln, 245 Keim Hall, Lincoln, NE 68503-0915 and Daniel Walters, 261 Plant Science Bldg, PO Box 830915, University of Nebraska-Lincoln, University of Nebraska, Dept. of Agronomy & Horticulture, Lincoln, NE 68583-0915.
Soil nitrogen (N) storage and release is intimately related to energy and nutrient residue inputs and their transformations to soil organic matter (SOM) fractions. We monitored the fate of N and C added to two distinct soils (Sharpsburg silt, and Kennebec silt), previously labeled with 15N in a field experiment, during a 300-d incubation (25ºC and 60%WFPS). Prior to incubation, crop residues as either “corn” stover (C:N=75) or “soybean” leaf (C:N=12) were added at a rate of 23.7 mg N.kg soil-1, and an unammended “control” was also monitored. The N and C cycling was studied in the following SOM fractions: light fraction (LF), a density (1.6 g.cm-3) fraction of fast turnover-time; mobile humic acid (MHA), a humified young age fraction with ~5%N, Calcium humate (CaHA) an older chemically-protected fraction of ~2%N; and Humin, the insoluble soil residue left after humic acid extraction. Prior to incubation, all SOM fractions were found to contain detectable 15N enrichment. Pre- and post-incubation soils were analyzed for C and N content, 15N atom% enrichment, and d13C signature in whole soil and all designated SOM fractions. Cumulative mineralized N and 15N were calculated in periodic leaching of soil incubation vessels over the 300 d period. Application of “soybean” resulted in net N mineralization and “corn” in net N immobilization compared to control. Comparison of pre and post-incubation 15N atom% excess in SOM fractions indicated most dynamic N cycling occurred form LF > MHA > CaHA >> Humin over the course of the 300-d incubation. Both humic acid fractions showed a significant %C increase and %N decrease post-incubation suggesting a N donor function of these under conditions that promote aerobic microbial activity. The addition of residues changed the d13C signature of post-incubation LF (p=0.03) with a preferential C flux from residues to LF.