Agronomy and Horticulture Department


Date of this Version



A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Agronomy, Under the Supervision of Professor Daniel T. Walters. Lincoln, Nebraska: September, 2007

Copyright (c) 2007 Ana B. Wingeyer


Identifying soil organic matter (SOM) fractions that contribute to soil indigenous nitrogen (N) supply and understanding their turnover under different management constitute necessary tools toward an efficient N use. The objectives of this study were: i) trace the endpoint of carbon (C) flux from residue inputs into SOM; and ii) asses the role of the light fraction (LF), mobile humic acid (MHA) fraction and calcium humate (CaHA) fraction as N sources for heterotrophic decomposition of fresh plant residues with contrasting C:N ratio. A long-term aerobic soil incubation was carried out on 15N-labeled soil samples from Lincoln and Mead, NE. Pre-incubation three residue treatments were assigned: MAIZE stover; SOYBEAN leaves, and NO-RESIDUE added. Pre- and post-incubation LF, MHA and CaHA were extracted. The soil was periodically leached, and the leachate was analyzed for N and 15N atom%. SOM fractions were analyzed for %C, %N, δ13C‰ and 15Natom%. Cumulative mineralized N was ~60% higher at Mead. MAIZE addition resulted in N immobilization at Mead (until t=90d) and Lincoln (until t=300d), while SOYBEAN increased N mineralization by 42% at Lincoln and 23% at Mead. Post-incubation, CaHA mass was reduced by 16 and 11% at Lincoln and Mead, respectively, and MHA and LF mass varied among treatments , with a significant increase of both fractions at Lincoln and no differences at Mead. All SOM fractions had a significant loss of 15Natom% and 15N mass across treatments. The relationship between 15N mass loss and change in N mass indicated CaHA as a N donor fraction with a preferentiall loss of recently added materials. The turnover of MHA reflected a wider range of situations. A N donor under N mineralization at Mead, and N and C storage under N immobilization at Lincoln. The LF %C4-C increased under MAIZE, supporting LF as the primary pool for residues into SOM. This study indicates that the C and N flux from Residue > LF > MHA > CaHA, can be modified to CaHA > MB > MHA > LF under high N demand for decomposition of C, where both humic fractions constitute N sources to LF and residue decomposition.