Summer fertigation of dairy slurry reduces soil nitrate concentrations and subsurface drainage nitrate losses compared to fall injection

Authors: Gamble, Joshua; Feyereisen, Gary; Schneider, Sharon; Wente, Christopher - Chris; Baker, John

Submitted to: Trade Journal Publication
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/23/2018
Publication Date: 5/15/2018
Citation: Gamble, J.D., Feyereisen, G.W., Papiernik, S.K., Wente, C.D., Baker, J.M. 2018. Summer fertigation of dairy slurry reduces soil nitrate concentrations and subsurface drainage nitrate losses compared to fall injection. Frontiers in Sustainable Food Systems. https://doi.org/10.3389/fsufs.2018.00015.
DOI: https://doi.org/10.3389/fsufs.2018.00015

Interpretive Summary: Leaching of nitrate nitrogen from manure-applied cropping systems represents a substantial N-loss to the environment for dairy farms, particularly in fields with artificial subsurface drainage. This study evaluated the potential for reducing nitrate losses in tile drianage by converting from fall injection to summer fertigation of manure through center-pivot irrigators. The study was conducted in a 65-ha silage corn field in west-central Minnesota over 9 growing seasons. We found that switching from fall injection to summer fertigation allowed the producer to reduce manure N rates, thereby reducing nitrate losses, without impacting corn silage yield. The treatment effect of fertigation was the result of a combination of factors. Manure N rates were reduced during fertigation, which undoubtedly helped reduce losses, but the timing and placement of manure N was also important. Crop N uptake and manure N-use efficiency were greater during fertigation than injection, which suggests that summer fertigation is a more efficient means of delivering N to the crop in both space and time. This greater utilization, in turn, enables lower manure application rates and results in reduced N losses in drainage. Results from the soil analysis support these findings. Spring soil nitrate concentrations were lower at most depths during fertigation than injection, and fall residual nitrate concentrations were also reduced below 30 cm depth. Overall, results suggest that summer fertigation is a viable method for reducing drainage N losses without impacting yield of irrigated silage corn in the U.S. Midwest.

Technical Abstract: Leaching of nitrate (NO3-N) from manure-applied cropping systems represents a substantial N-loss to the environment for dairy farms, particularly in fields with artificial subsurface drainage. In this on-farm study, we used a Before/After analysis to assess the effectiveness of summer fertigation (years 2010 – 2015) versus fall injection (2007 – 2009) of dairy slurry in terms of subsequent corn silage yield, corn N uptake, soil NO3-N accumulation, and NO3-N losses in subsurface tile drainage effluent from a 65-ha field in west-central Minnesota, U.S.A. Corn silage yield was similar between periods (average of 18.8 Mg ha-1), but crop %N, total N removal, and manurial N-use efficiency were 15, 12, and 126% greater during the fertigation than injection period. Fertigation reduced spring soil NO3-N concentrations to 60-cm depth by an average of 53% relative to injection, except in the 15 to 30 cm increment, where no difference was found. Similarly, fall soil nitrate-N concentrations from 30 to 90 cm were 48% lower, on average, under fertigation than injection. Weekly flow-weighted mean NO3-N concentration in tile drainage was lower during fertigation (47.7 mg L-1) than injection (56.8 mg L-1), though weekly drainage depth was greater during fertigation (2.3 versus 1.1 mm). This resulted in similar weekly loads between periods (mean of 0.96 kg NO3-N ha-1), but average annual loads were lower during fertigation (47.1 versus 55.5 kg NO3-N ha-1 yr-1). Relationships between drainage and NO3-N load showed log–log slopes of near 1.0 for injection and 0.97 for fertigation, indicating that concentrations were diluted with increased flows during fertigation, but not during injection. Differing intercepts indicated a treatment effect of fertigation, independent of flow effects. The intercepts corresponded to loads of 16.0 kg NO3-N ha-1 for injection and 12.15 kg NO3-N ha-1 for fertigation, a reduction of 24% at 10 mm flow depth. Given the difference in log-log slopes, the magnitude of the reduction in NO3-N load increased with drainage depth. Results suggest that summer fertigation is a viable method for reducing drainage N losses without impacting yield of irrigated silage corn in the U.S. Midwest.