Impact of pyrochar and hydrochar on soybean (Glycine max L.) root nodulation and biological nitrogen fixation

Summary

The aim of this study was to identify effects of carbonized organic material (‘‘biochar’’) on soybean growth, root nodulation and biological nitrogen fixation, and to elucidate possible underlyingmechanisms.Soybean (Glycine max L.) was grown in four arable soils amended with carbonized organic material produced from wood or maize as feedstocks, by pyrolysis (‘‘pyrochar’’) or hydrothermal carbonization (‘‘hydrochar’’). Nodulation by Bradyrhizobium, biological nitrogen fixation (BNF) assessed by15N techniques, plant growth, nutrient uptake and changes in chemical soil propertiesafter soil amendment were determined. Data were analyzed by means of a three way ANOVA onthe factors soil, carbonization technique and feedstock. It turned out that soybean root nodulationand BNF was influenced by the carbonization technique used to prepare the soil amendment.Hydrochar, in average and across all soils, increased nodule dry matter and BNF by factors of3.4 and 2.3, respectively, considerably more than pyrochar, which led to 1.8 and 1.2 fold increases, respectively. Nodule dry matter and BNF correlated positively with available soil sulfurand negatively with available soil nitrogen. Hydrochars provided more available sulfur than pyrochars, and hydrochars caused a decrease in nitrogen availability in the soil solution, thereby exerting a positive influence on nodulation and BNF. Pyrochar amendment increased soil pH buthad no effect on nodulation and BNF. Plant growth was affected by the soil and by the feedstockused for the ‘‘biochar’’, and increased slightly more in treatments with pyrochar and hydrocharmade from maize, which was richer in nitrogen and potassium.The results show that carbonized organic materials, and specifically hydrochar, have the capacity to increase BNF in soils. We suggest that this enhancement in BNF in response to soilamendments with carbonized organic materials is due to an increase in available sulfur and a reduction of available soil nitrogen.