Investigating the release of nitrogen, phosphorus and potassium from biochars of grape waste, straw and wheat stubble and walnut shell

To investigate the release of nitrogen. Phosphorus and potassium elements from biochar, an experiment was conducted in a completely randomized design in three replications. Biochars were used by 2% in weight (W/W). The experimental treatments included control soil (CS), Soil+ Grape waste biochar (GSB), Soil + wheat straw and stubble (WSB) + brown walnut shell biochar (NSB) and Soil. The samples were incubated for about four months and sampling was done at days of 0, 7, 14, 28, 42, 56, 88 and 120. The results showed that the biochar addition significantly increased the concentrations of nitrogen, phosphorus and potassium in all biochar-amended soils compared to the control soil. Nitrogen concentration ranged from 766.6 mg kg-1 in control (average of sampling times) to 1272.2, 1366.7, and 1488.9 mg kg-1 in GSB, WSB and NSB treatments, respectively. Phosphorus concentration enhanced from 44.5 mg kg-1 in control (time-average of sampling) to 79.1, 67.1, and 70.8 mg kg-1 in GSB, WSB and NSB, respectively, and the concentration of potassium in the control increased from 103 mg kg-1 to 656.5, 293.8, and 125.6 mg kg-1 in the GSB, WSB, and NSB treatments, respectively. The WSB and NSB treatments released an average of 52% and 50.5% of their nitrogen content, respectively during 120 days, increased while GSB released only 28.6% of its total nitrogen on average. The order of nitrogen release rates in biochar were as follows: WSB≥ NSB> GSB. Regarding the release of phosphorus, NSB was considered to be the most efficient biochar, which was able to release 5% of its phosphorus during 4 months, while, GSB was the lowest effective biochar releasing only 0.5% of its phosphorus during the corresponding time. The order of phosphorus release from biochar treatments was as follows: NSB > WSB > GSB. The amount of available potassium in GSB was 2.3 to 6.7 times higher than the other treatments. The order of potassium release from biochar treatments was as follows: GSB > WSB > NSB. The results of infrared spectroscopy showed that the dissolution of amides containing nitrogen is the main mechanism of N release. The decrease in pH caused by nitrification, probably facilitated Ca-P dissolution from biochar and led to a higher phosphorus release.