Nitrogen dynamics and biological response to dairy manure application
Bierer, Andrew M.
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Animal manures are land applied in agronomic systems to supply essential crop nutrients and decrease dependency on chemical fertilizers. Liquid manures are traditionally surface broadcast to fields and sometimes incorporated to reduce odor and nutrient losses; however, incorporation is incompatible with no-till agriculture. Subsurface manure injection is a no-till compatible alternative application method which addresses these concerns, but likely changes the dynamics of nutrient cycling. Comparison of the two application methods has yielded mixed results and warrants further research. Therefore, the objectives of this research were to contrast the surface broadcast and subsurface injection of dairy slurry on nitrogen and carbon cycling, crop yield, and biologic responses to proxy soil health. In a forced air-flow laboratory incubation, manure injection reduced ammonia volatilization by 87% and 98% in a sandy loam and clay loam soil, respectively. The increased ammoniacal nitrogen recovery resulted in increases of soil nitrate of 13% for the sandy loam and 26% for the clay loam after 40 days of incubation. Microbial measurements were inconclusive in the laboratory. In 7 site-years of field study, soil nitrate was greater in 7 of 25 measurements under manure injection and 30% higher under injection on average during the corn pre side-dress nitrate test (PSNT) time. Soil nitrate sampling methods were assessed for fields injected with manure; a standard random sampling method had a coefficient of variation (C.V.) of 28% and was as equally repeatable as utilizing an equi-spaced distribution of cores taken across an injection band, C.V. of 30%. Both biological responses, carbon mineralization (C-min) and substrate induced respiration (SIR), were not different between application methods; both were highly variable and C-min was especially intensive logistically. Corn yield showed no consistent response to application method, but probably was not nitrogen limited. In 2 years of field study conducted on a university research farm injection resulted in greater 0-15cm soil nitrate levels than surface broadcast 1 week after application and persisted for 9 additional weeks. In injected plots, nitrate was concentrated in the injection band; nitrate movement was significant only 10cm lateral to the injection band but overall distribution fit well to a second degree polynomial, especially 2 and 4 weeks after application, R2>0.80. Evidence of leaching was observed in one year after receiving considerable rainfall in weeks 1 and 2 after application. When corn grain yield was averaged year over year, injection was 26% greater than the no- manure control, and 15% greater than surface application. Both biological metrics, C-min and microbial biomass, were stratified by depth; C-min was concentrated within the manure band leading to greater mineralization under injected applications. Microbial biomass was significantly higher under injection at the 15-30cm depth. Overall biological response to manure application method was inconclusive, however manure injection is superior to surface application in terms of nitrogen recovery.
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