Abstract

The plant available nitrogen (PAN) content of dairy manure is commonly calculated using concentration and availability coefficients for organic nitrogen (N) and ammonium N (NH4), but the carbon (C) fraction of the manure also influences the availability of N over time. We evaluated the interactive effect of manure C and N from nine dairy manures during a 176 days aerobic incubation. All of the manures had appreciable NH4 content, and varied widely in fibrous C. The incubation was conducted using sandy loam (coarse-loamy, mixed, frigid, Typic Haplorthod) and silt loam (fine, illitic, non-acid, frigid, Aeric Epiaquepts) soils at 25 °C and 60% water-filled pore space. There were clear differences in nitrate (NO3) accumulation over time, including manures that resulted in net nitrification and net immobilization. For both soils, the rate of nitrification at 7 and 56 days after application, and the amount of NO3 accumulated at the end of the incubation (176 days) were strongly correlated (r = −0.88) with C: NH4 and also to the ratio of neutral detergent fiber (NDF):NH4 (r = −0.90). The addition of manure C also resulted in significant net immobilization, compared to addition of mineral N fertilizer alone. These studies demonstrate that increased understanding of manure C and N interactions may lead to improved prediction of manure PAN.

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