Analysis of plant-growth promoting compounds in the biogas digestate liquid

Abstract

Biogas digestate liquid (BDL) is a byproduct derived from the anaerobic (co)-digestion of organic wastes, which can be applied as an organic fertilizer since it usually contains plant nutrients. In this work, we aim to analyze plant nutrients and plant hormones/plant-growth promoting compounds in unheated and heated BDL sample sets (fresh, 1-month-, 2-month-, and 3-month-storage sets), obtained from the anaerobic co-digestion of animal manure inoculum and food waste, by analytical techniques, CNS analyzer, AAS, and LC-MS/MS. The results obtained from the CNS and AAS analysis showed that all of the BDL sample sets contained high concentrations of essential plant macronutrients (N, P, K, Ca, and Mg) and micronutrients (Fe, Cu, Zn, Mn, and Mo), and very low concentrations of toxic elements (As, Pb, Cd, and Cr). The LC-MS/MS analysis using ampicillin as an internal standard demonstrated that only five plant hormones including, indole 3-acetic acid (IAA), abscisic acid (ABA), salicylic acid (SA), kinetin, and zeatin were detected in unheated BDL samples of the fresh set with an approximate concentration of 1.5 µM, 1.7 nM, 0.18 µM, 1 nM, and 0.2 nM, respectively. However, it was found that these plant hormones, except zeatin, were trivially decreased after storage for 1 month and could not be detected after storage for more two months. It was interesting to note that all of the plant hormones detected, except zeatin, were likely stable over heat treatment, which was agreed well with the finding that these plant hormones were stable at 40 and 100 oC. To evaluate all of BDL sample sets with different treatments could promote plant growth, we hence employed a hydroponic assay using green oak lettuce as a model. The bioassay results showed that only the 1HO+ formula (heated BDL sample of 1-month-storage set mixed with 170X-diluted hydroponic fertilizer) can promote the growth of green oak lettuce. The results suggested that heated BDL sample of the 1-month-storage set was suitable for plant growth in the presence of hydroponic fertilizer. Taken together, the data indicated that BDL obtained from the anaerobic co-digestion of animal manure inoculum and food waste can serve as an environmentally friendly organic supplement for plant growth.

-