Quantitative Analysis of the Change in Humic Acid N Storage Under Conditions of Intense N Immobilization.

Soil nitrogen (N) storage in humic acids  constitutes 20 to 35% of total soil N. The chemical extraction of humic acid fractions before and after an acid wash isolates two pools of different composition and chemical stabilization: the mobile humic acid (MHA) fraction and the polyvalent cation bound humic acid (CaHA) fraction. MHA constitutes a N rich (4.0 to 5.5% N) fraction with mean residence time of carbon (MRT) on the order of 10 - 100 years, while CaHA constitutes an older fraction (MRT ~250-1000 years) with 2.4 to 3.0 %N.  Our objectives were: i) determine the role of MHA and CaHA fractions as N sources for heterotrophic decomposition under contrasting N mineralization conditions. A long-term (300 d) 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 MHA and CaHA were extracted. The soil was periodically leached to remove net N  mineralization.  To assess the relative contribution of sources to the soil’s labile indigenous N pool, an N turnover statistic (TS) relating the net change in ¹⁵N mass to the net change in total N mass for each fraction was computed. At both sites pre-incubation mass of humic acids was ~10900 mg kg soil^-1. Post-incubation MHA mass significantly increased by 6% (SOYBEAN), 10% (MAIZE), and 17% (NO-RESIDUE) at Lincoln, while at Mead site the addition of residues resulted in 12% smaller MHA mass than under no residue addition. The post-incubation CaHA fraction mass was reduced by -16 and -11% at Lincoln and Mead, respectively.. At Mead there was a higher CaHA-¹⁵N mass loss under NO-RESIDUE (-35%) compared to SOYBEAN (-27%) or MAIZE (-17%).   The %N of CaHA mass lost during incubation was higher than the original CaHA material (3.7% vs 2.9%), indicating a differential degree of N condensation within the CaHA fraction .   There was also a higher proportion of ¹⁵N lost from CaHA than total N indicating more recently incorporated CaHA N was preferentially mineralized over older CaHA-N.  The loss in ¹⁵N mass observed in the MHA fraction confirmed that the degree of ¹⁵N atom% reduction was due to both ¹⁵N dilution from massive N deposition to this pool as well as flow of ¹⁵N mass out of the fraction and that MHA was far more active in N and C turnover than CaHA.  The results suggest that the CaHA pool is composed of materials with different degree of stabilization, and that the new added materials are N-rich compounds that are relatively labile and not completely chemically stabilized against loss.