Carbon quality of four-year-old woodchips in a denitrification bed treating agricultural drainage water

Authors: GHANE, EHSAN - University Of Minnesota; Feyereisen, Gary; ROSEN, CARL - University Of Minnesota; TSCHIRNER, ULRIKE - University Of Minnesota

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2018
Publication Date: 7/1/2018
Citation: Ghane, E., Feyereisen, G.W., Rosen, C.J., Tschirner, U.W. 2018. Carbon quality of four-year-old woodchips in a denitrification bed treating agricultural drainage water. Transactions of the ASABE. 61(3):995-1000. doi: 10.13031/trans.12642.
DOI: https://doi.org/10.13031/trans.12642

Interpretive Summary: It is well-known that tile-drainage water from croplands transports nitrogen in the form of nitrate to receiving waters. One edge-of-field method of reducing the nitrate loss is to route the water through a denitrifying bioreactor bed consisting of wood chips. Not much information is known about the condition of these chips in the field over time. Also, field bioreactor research has been unreplicated in real-world settings. This paper reports on physical properties of four-year old wood chips that were excavated along the length of a 350-foot long bioreactor bed near Willmar, MN, USA, during segmentation of the bed into eight smaller beds for replicated study. Wood chips were sampled at the inlet end of the bioreactor and at ˜44-foot intervals. Testing included sieve analysis for wood chip particle size, carbon (C) and nitrogen (N) composition to determine C:N ratio, and carbon makeup, i.e., percentages of cellulose, hemicellulose, and lignin. The particle size and C:N ratio were smaller and the relative % of lignin was higher at the inlet position, where most biological activity had occurred. The lower C:N ratio and higher % lignin at the inlet end means that less carbon was available to support denitrification. From 85 to 350 feet along the bed, the wood chips had similar properties. Thus, the conclusion was that the six beds toward the outlet can be used for a replicated experiment without biasing the results. The findings of this research will be of interest to researchers, agency personnel, producers/producer groups with interest in improving tile drainage water quality.

Technical Abstract: A denitrification bed is a system that can reduce nitrate concentration in subsurface drainage water. There is a need to investigate the carbon quality of old woodchips to gain a better understanding of the effect of age on the woodchip properties. The objectives of this study were to characterize the carbon quality and carbon to nitrogen (C/N) ratio of old woodchips, and to examine the suitability of a denitrification bed for a replicated experiment. To achieve these goals, we excavated four-year-old woodchips along the length of a 106.4 m long denitrification bed near Willmar, Minnesota, USA, and analyzed them for particle size, C/N ratio and carbon quality. Particle size analysis showed similarities from 12.5 to 106.4 m along the bed. We found a mean C/N ratio ranging from 58.4 ± 3.17 to 153.4 ± 9.57 (smallest at the inlet). The mean lignocellulose index (LCI) of four-year-old woodchips ranged from 0.47 to 0.57 (highest at the inlet). The woodchip particle sizes, C/N ratios, and LCI from 25.9 to 106.4 m along the bed length were similar, and thus, they will not bias an upcoming replicated experiment. In conclusion, C/N ratio and LCI of four-year-old woodchips showed greater decomposition and more recalcitrant woodchip carbon, respectively, than fresh woodchips reported in the literature.