Estimation of Mineralizable Nitrogen from 15N-Labelled Crop Residues using Alkaline-Hydrolyzable Nitrogen Methods

Abstract

Crop residues play a significant role in soil N cycling. The type of residue, C/N ratio, tillage, and soil moisture influence potentially mineralizable soil-N. This study was established to estimate the N mineralization potential of various crop residues using Direct Steam Distillation (DSD) and the Illinois Soil Nitrogen Test (ISNT). Corn (Zea mays L.), soybean (Glycine max L.), wheat (Triticum aestivum L.), rice (Oryza sativa L.), and grain sorghum (Sorghum bicolor L.) residues were grown in a greenhouse and labeled with 15N using 10 atom% 15N labeled-urea. To assess the N mineralization potential of various crop residues, 0.2 g of residue was subjected to both the DSD and ISNT. Hydrolyzed-N was captured and analyzed for atom% 15N to compare fertilizer atom% 15N to that of the original residue. Total N was quantified to establish recovery percentage. For percentage of N recovery there was a significant residue by method interaction (p < 0.0001) indicating that the two methods recovered varying amounts of N based on the type of residue. Atom% 15N recovered from the soybean residue as alkaline hydrolyzable-N (AH-N) was significantly lower than what was quantified in the plant tissue using TN. Conversely, atom% 15N recovered from the rice residue as AH-N was significantly greater than that which was quantified in the original plant tissue using TN. Comparison of atom% 15N in the residue and recovered AH-N suggested that certain crop species partition fertilizer N differently. Specific estimation of N mineralization potential of crop residues could aid producers in determining fertilizer N needs and encourage the development and implementation of soil-based N tests.