Nutrient dynamics in pak choi cultivation fertilized with biogas digestate : effects of non-nutrient digestate elements and amendment with mineral nutrients

Abstract: Anaerobic digestion in biogas plants produces renewable energy and a residue which is rich in plant nutrients. This residue is called digestate. Today, these digestates are mostly spread directly onto fields as manure. However, due to their high content of plant-available macroand micronutrients, digestates also have the potential to replace synthetic fertilizers in protected horticulture in soilless systems, contributing to the completion of global energy and nutrient cycles. However, this places more demands on their nutrient composition because, unlike fertilizers in soil systems, fertilizers in soilless systems have to provide the crop with all essential macro- and micronutrients at sufficient levels during the whole cropping cycle. Most reports from trials in soilless systems emphasize that the high ammonium nitrogen (NH4-N) to nitrate nitrogen (NO3-N) ratio in digestates constitutes a problem. Furthermore, low concentrations and recovery efficiency of phosphorus (P) and sulphur (S) have been highlighted as limiting factors for growth. However, as all digestates differ in composition, other nutrients might also be present at insufficient levels. Accordingly, it has been recommended that a share of the NH4-N in the digestate is converted to NO3-N before application, and that the digestate is supplemented with the missing macro- and micronutrients. However, to date, trials with digestate fertilizers in protected horticulture are limited, and the results are conflicting. In addition to plant nutrients, digestates contain a complex mixture of partially degraded organic matter and inorganic compounds, including substances that, when derived from other organic source materials, have been reported to have biostimulatory properties. Digestates derived from protein-rich feedstocks have been reported to contain the auxin indole-3-acetic acid (IAA), as well as other plant hormones, at concentrations sufficient to regulate plant development. This has been related to improved growth and nutrient stress tolerance in digestate growth trials. As part of this thesis, a greenhouse pot trial with pak choi (Brassica rapa, ssp. chinensis, ‘Joy Choi’) grown in peat was set up to evaluate the plant-nutrient dynamics and biostimulatory effects of a digestate collected at the municipal Karpalund biogas plant in Kristianstad, southern Sweden. The digestate was nitrified in a moving bed biofilm reactor prior to the experiment in order to lower the NH4-N:NO3-N ratio. The study was designed with three objectives: (i) to assess the plant availability of macro- and micronutrients in the digestate with particular focus on P and S recovery; (ii) to assess the plant availability and effect of added mineral P, S, magnesium (Mg), manganese (Mn), boron (B), and molybdenum (Mo) to the slightly alkaline digestate; and (iii) to assess the possible biostimulatory properties of the Karpalund digestate (i.e., the effects unrelated to the nutrient content) on plant yield and stress tolerance. The result showed that the recovery of P and S was significantly lower in the digestate treatment than the mineral control with the same total P and S content (65% for P and 67% for S was recovered in the above-ground parts of the plant in the digestate treatment compared to 83% for P and 95% for S in the mineral control). The shoot tissue concentrations of S (1.6 g kg-1) and B (10 mg kg-1) in the digestate treatment were below the threshold recommended for optimal growth. The value for P (2.8 g kg-1) was within the recommended limits but on the verge of a possible shortage of P. Supplementing the digestate with mineral P, S, Mg, Mn, Mo, and B resulted in sufficient plant tissue concentrations of all nutrients with the exception of S, and in higher fresh matter yields. The supplemented digestate performed as well as the synthetic control with respect to fresh matter yield, and outperformed it with respect to dry matter yield. It might be speculated that the higher dry matter yield was a result of biostimulatory compounds contained in the digestate. However, it cannot be excluded that it was caused by higher concentrations of potassium (K) and chlorine (Cl). Finally, the digestate was not found to alleviate plant response to nutrient stress. To summarize, the results are promising and show that, after some modifications, the Karpalund digestate can be used successfully as a fertilizer in soilless production of leafy vegetables.