Manure injection alters the spatial distribution of soil nitrate, mineralizable carbon, and microbial biomass

Abstract

Subsurface injection of liquid manure is a no-till compatible application method that may fundamentally change nutrient cycling dynamics. This study investigated subsurface injection and surface broadcast application methods on soil nitrate-nitrogen (NO₃^–-N) at 0, 1, 2, 4, 6, 8, 10, 14, 18, and 22 weeks after first-year applications of liquid dairy slurry in 0 to 15 cm and 15 to 30 cm sampling depths. The effect on corn (Zea mays L.) grain yield and quality parameters was considered. Soil health was assessed by proxy on weeks 0, 2, 8, 14, and 22 by two microbial indicators sensitive to change: carbon (C) mineralization and microbial biomass C. Soil NO₃^–-N and the microbial indicators were also measured at distances from an injected band of manure: In-band, 10 cm, 20 cm, and 36 cm. The injection application resulted in significantly (p < 0.05) higher soil NO₃^–-N levels relative to the surface application and no-manure control from week 2 to week 10 after manure application. Soil NO₃^–-N concentrations in the 15 to 30 cm sampling depth suggested NO₃^–-N leaching was exacerbated by the injection application method in 1 of 2 years. Soil NO₃^–-N decreased with increasing sampling distance from an injected manure band following quadradic plateau models at weeks 1, 2, and 4 each year. Crop yield was ~14.5% higher under injection application relative to surface application; however, grain quality parameters were not significantly (p > 0.05) different between treatments. Carbon mineralization was significantly (p < 0.05) increased under only the injection application relative to the control. The increase in C mineralization was entirely dependent on the In-band sampling distance as the 10, 20, and 36 cm sampling distances were not significantly (p > 0.05) different from the no-manure control. Microbial biomass C was significantly (p < 0.05) higher in the In-band samples in either the 15 to 30 or 0 to 15 cm sampling depths each year relative to the control, but was not appreciably affected by surface application. Findings from this study refine the understanding of nutrient concentrations around an injection band and accentuate the potential for manure N retention under surface and injected manure applications. However, as treatments were not reliably differentiated by C mineralization nor microbial biomass C, the capability of these tests to identify first-year management changes is disputable.