Soil nitrogen, active carbon, corn, and small grain response to manure injection

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Virginia Tech


Manure injection is an alternative manure application method that places manure in subsurface bands rather than spreading it evenly across the soil surface as done with the typical broadcasting method. The reduced exposure of manure to air under injection can lead to greater N retention when compared to broadcasting, but also alters the spatial distribution of manure. This altered spatial distribution of manure could alter soil nutrient dynamics and crop growth; however, literature exploring this subject is limited. Therefore, this dissertation aimed to compare soil nitrate and active carbon levels between manure injection and broadcasting, assess the spatial distribution of soil N under injection, and determine if the subsurface bands under injection cause differential crop growth. An 8-site on-farm study was conducted comparing spring manure applications under corn silage. This study found that soil NO3-N was the same under injection and broadcasting but did alter the spatial distribution of soil NO3-N as it was consistently elevated in the injection band compared to between bands. No differences in active carbon were observed, even when measuring the injection band directly. This finding calls into question the usefulness of measuring active carbon in manured systems. Corn silage yields were only significantly increased at 1 of 8 sites, and this occurred at the one site that did not receive a sidedress N application, which suggests that N was not limiting at the other seven sites. A small-scale research plot study examining fall manure applications under small grains found similar results to the previous study. No consistent differences in soil NO3-N were observed between injected, broadcast, and control plots; however, soil NO3-N was greater in the injection band compared to between, a difference that persisted for two months after manure application. Evidence of soil NO3-N leaching was observed in one study year, suggesting soil NO3-N leaching under fall manure applications should be examined. No consistent differences in soil active carbon were observed, either between manure application methods or injection bands. Furthermore, the alteration in soil NO3-N under injection did not lead to differential small grain growth. A 24 site on-farm study was conducted to assess potential differential growth of small grains following manure injection. This study found that soil NO3-N in the manure injection band compared to between bands was significantly increased in 13 of 24 sites and was on average 137% greater in-band at the 0-15 cm depth. This difference did not persist through small grain silage harvest as only 1 of 24 sites showed a significant difference in-band. Small grain maturity did not show any difference in 2021 due to late planting dates, but some differences were observed in the injection band compared to between bands one month after planting. As with soil NO3-N, these differences did not persist through silage harvest. Small grain forage quality parameters were not different in-band compared to between-band at harvest, while DM yield only differed in 3 of 24 sites, with 2 of those 3 sites being under wheat. The data presented in this dissertation indicates that manure injection causes differential soil NO3-N levels from banding. Accurately measuring soil NO3-N levels under injection was difficult due to the injection band being difficult to fully sample and suggests injected soil NO3-N levels were underestimated. No meaningful changes in crop growth were observed due to banding or different manure application methods.



Manure injection, soil nitrate, soil carbon, forage quality, small grains