Use of a Modified Chemical Fractionation Scheme to Characterize Organic Nitrogen in Wetland Soils

Abstract

Soil organic nitrogen (SON) is the main source of available N to microbes through mineralization. Although the importance of SON is recognized, the chemical nature of SON is not well described mainly because of methodological limitations. The objective of this study was to modify the operationally defined SON fractionation scheme to improve sample processing time. The operationally defined SON pools are amino sugar N, amino acid N, and hydrolyzable unknown N. Modifications to the method included: SON hydrolysis using an electric griddle, use of 0.05 mol L−1 H2SO4 NH3 traps, and use of a forced-air oven during amino acid N deamination. The modified method improved sample throughput by (i) increasing soil extractions from one to 12 samples at a time, (ii) decreasing sample analysis time by using colorimetric methods, and (iii) reducing temperature variability during amino acid N deamination. We estimate that the method modifications save on average 5 h per 12 samples. The modified method was then applied to 10 flooded or drained wetland soils that ranged from 3 to 34 g kg−1 total N (TN). The labile SON pools ranged from 0.06 to 1.29 g kg−1 amino sugar N and 0.74 to 10.5 g kg−1 amino acid N. Amino sugar N linearly increased with soil total C (TC; R2 = 0.60). Amino acid N exponentially increased with TC (R2 = 0.80), suggesting that conditions in wetlands preferentially conserve amino acid N. A decline in amino acid N with drained conditions highlighted the potential loss of TN stored in wetlands, estimated at 50 to 75 Pg.