Nitrogen and Carbon Mineralization From Canola, Pea, and Wheat Residues With Differing N Content and Carbohydrate Composition.

Understanding the decomposition of crop residues and the release of nitrogen (N) is critical when implementing diversified crop rotations in no-till systems.  Nitrogen content, proximate fiber analysis [Neutral Detergent Fiber (NDF) and Acid Detergent Fiber (ADF)], and spectroscopic techniques are often proposed as suitable predictors of carbon (C) and N mineralization from crop residues.  This study compared the C and N dynamics in soil over a period of 112 days following the additions of 3 types of canola and wheat, and 2 types of pea residues, with differing C:N ratios and contrasting biochemical composition.  After 7 days, soil amended with the pea and canola residue types sustained greater rates of CO₂ evolution than the wheat residues. The lower C mineralization rate corresponded with a smaller NDF fraction in the pea and canola residues than wheat, thus comprising of a lesser proportion of structural carbohydrate compounds (e.g. hemicellulose, cellulose, and lignin).  However, these differences were reduced after 14 days; and by 42 days, the CO₂ emissions did not differ from the unamended soil.  The addition of all residue types resulted in initial net N immobilization as compared to the soil without residue.  By 28 days, soil amended with the high N pea residue (C:N ratio of 25:1) had a positive N balance, while all other residues maintained negative N balances relative to the control soil possibly due to their higher ADF fraction (between 0.45 and 0.50). The overall net N immobilization was strongly correlated with N content (R² = 0.90), but poorly related to C:N ratios (R² = 0.58).  Despite differences in the NDF fraction among crops, structural carbohydrate content did not greatly influence N mineralization dynamics (R² = 0.53).  ¹³C NMR CP/MAS analysis is currently being conducted to examine differences in C-chemistry among the residue types.