Liquid swine manure application timing and Instinct™ impacts on net soil nitrogen mineralization and corn yield

Nitrogen (N) is one of the most expensive variable input costs for corn production. Optimized manure management is essential to ensure maximum crop N utilization and to reduce the risk of N loss. A field study was conducted in central Indiana from 2011-2013 to assess liquid swine ( Sus scrofa L.) manure N availability when applied at different times or co-applied with InstinctTM, a microencapsulated form of nitrapyrin. Soil inorganic N contents, soil N mineralization, corn yield, grain N content, whole plant N concentrations of corn at the V6 growth stage, and ear leaf N concentrations of corn at the R1 growth stage differed from year to year and were greatly influenced by manure application timing and climatic conditions. A static cup incubation was used to assess N mineralization from soil samples collected at two depths (0-30 cm and 30-60 cm) at various times after manure application (early winter, planting and corn V6 growth stage). The cumulative net N mineralization was fitted to a first-order exponential model to determine potentially mineralizable N (N0) and the N mineralization rate constant (k). Cumulative mineralizable N up to 16 weeks was not affected by liquid swine manure application timing or the use of InstinctTM at both soil depths. Differences in total soil inorganic N recovered after incubation were mainly due to initial inorganic N differences. Negligible amounts of NH4+-N were recovered from soil extracts in all treatment plots during the incubation study, indicating that nitrification occurred and that there was little efficacy from InstinctTM at controlling nitrification at the time of soil sample collection. Spring application of liquid swine manure (165 kg manure total N/ha) showed greater corn grain yield and grain N content (6.9 Mg/ha and 61.2 kg N/ha) compared to early fall applications (138 kg manure total N/ha, 5.1 Mg/ha and 44.6 kg N/ha) at Location 3 in the 2013 growing season, while in the 2012 growing season corn yield was unaffected by manure application timing. Distinct climatic conditions, especially precipitation, between these two years were considered a main source of yearly inconsistence. Corn growth and yield was unaffected by the addition of InstinctTM at any location. Primary factors that could have affected the efficacy of this nitrification inhibitor include: i) InstinctTM runoff or leaching from the soil before nitrapyrin is released from the polymer microcapsules; ii) inadequate release rate and concentration of nitrapyrin from InstinctTM with time in soil; iii) sorption of released nitrapyrin by organic matter and clay minerals in soils; iv) efficacy of released nitrapyrin on nitrifiers under the varying soil moisture and temperatures encountered during this field study; and v) insufficient rainfall to cause significant N loss in the field