Plant and soil responses to hydrothermally converted sewage sludge (sewchar)

Abstract

This study compared the effects of sewchar and mineral fertilizer on plant responses in beans (Phaseolus vulgaris, var. "Jalo precoce") and soil properties in a pot experiment in a completely randomized design with two harvests. The initial treatments consisted of a control, sewchar doses of 4, 8, 16 and 32 Mg ha^-1 and mineral fertilizer (30 mg N, 90 mg P₂O₅ and 60 mg K₂O kg^-1). The treatments (4 replications each) were fertilized with 135 mg P₂O₅ kg^-1 at the second harvest. The sewchar application rates correlated positively with the CEC, the water holding capacity, the availability of Zn, Ca, Fe, Cu, and P, and the concentrations of nitrate, ammonium, total N, total organic carbon and hot water extractable carbon. They correlated negatively with the Mg availability and the soil C: N ratio. Additionally, they correlated positively with the P, Zn and Ca uptake from the soil. For both harvests, the 16 Mg ha^-1 sewchar treatment had a total dry matter equivalent to that of the mineral fertilizer. After the second harvest, the 16 Mg ha^-1 sewchar treatment revealed 96% higher plant biomass than the control and 79% higher biomass than it did during the first period. The positive effect of sewchar in addition to phosphorous on the plant response and soil properties suggests that the residual effect of sewchar could be a promising alternative as a soil amendment for partly replacing mineral fertilizers. In future, further studies are necessary to evaluate long-term residual effects of sewchar in soil.

Keywords: Biosolids; Hydrochar; Plant growth; Sludge; Trace elements.