Date of this Version
Research Report. Environment. NPB # 15-045
Swine manure has been used as a soil amendment for crop production because it can provide nutrients, increase soil productivity, improve water infiltration, and reduce the potential for soil erosion. It is important to understand how different land application strategies may affect the fate and transport of various manure constituents in the environment. The objective of this project was to determine how the method and timing of swine manure application may impact the levels of multiple manure constituents in soil. The manure constituents included nutrients, antimicrobials, and antimicrobial resistance genes (AMR genes).
A series of plot-scale field experiments were performed. In these experiments, swine manure slurry from a commercial farm was either broadcast or injected into test plots by a commercial manure applicator. A set of three 30-min simulated rainfall events, 24 hour apart, were initiated on the manure amended plots 1 day (and referred as 0 week thereafter), 1 week, 2 weeks, or 3 weeks after the manure application. Soil cores were collected before and after the rainfall simulation tests and analyzed for nutrients using standard methods, for antimicrobials using liquid chromatography tandem mass spectroscopy, and for AMR genes using quantitative polymerase chain reactions (qPCR).
Broadcast resulted in higher nitrate concentrations in soil than did injection. In terms of application timing, three of the four nutrient compounds tested (i.e., nitrate, water soluble phosphorus, and Bray 1 phosphorus) did not show significant decrease in broadcast plots during the three weeks following manure application. Ammonium concentration dropped significantly in the third week. Simulated rainfall events lowered the concentrations of the nitrogen species in top soils significantly, and showed minor impacts on the phosphorus species in the top soils.
For top soils the antimicrobial concentrations in broadcast plots were higher than those in injection plots. For broadcast plots, chlortetracycline was detected in both the top and bottom of the soil cores before and after rainfall events. Lincomycin and tiamulin were only detected in the top of the soil cores. The antimicrobial concentrations in top soils decreased with time after manure application, although the trend is significant only for lincomycin.
AMR gene levels in top soil were not affected by land application methods. For broadcast plots, the length of the time between manure application and rainfall events had no significant effects on the abundance of three of the four AMR genes tested. The exception was tet(Q), which increased in Week 3. The abundance of the AMR genes in soil were not affected by the simulated rainfall events. The only treatment factor that showed significant effect was soil position: the AMR genes were more abundant in the top portion than the bottom portion of the soil cores collected.