Quantifying Nitrogen Availability From Crop Residue Using N-Star.

Crop residues play a significant role in soil N cycling. The type of residue, C:N ratio, tillage, and soil moisture can dictate potentially mineralizable soil-N. Previous studies indicate that for crops possessing wide C:N ratios, a net N immobilization will occur, whereas crops with low C:N ratios can increase N mineralization in the soil. This study was to 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), soybean (Glycine max), wheat (Triticum aestivum L), rice (Oryza sativa), and grain sorghum (Sorghum bicolor) residues were labeled with 15N using 10 atom% urea. To assess the N mineralization potential of various crop residues, 0.2g of residue was subjected to both the DSD and ISNT. Hydrolyzed-N was captured and analyzed for 15N content to compare to the atom % 15N of the residue to the hydrolyzed N and total N was quantified to establish percent recovery. For percent 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 hydrolyzable-N was significantly lower than what was quantified in the plant tissue. Comparison of atom % 15N in the residue and recovered hydrolyzable-N suggested that certain crop species may partition N fertilizer differently and thus influence the quantification of hydrolyzable-N using the DSD and ISNT. Additional studies are needed to compare N mineralization potential to actual N mineralization in controlled incubations. Specific estimation of N mineralization potential of crop residues could aid producers in determining N fertilizer needs and help encourage the development and incorporation of soil-based N tests.