Introduction

Every year, a large amount of agricultural waste is produced in the world, which can make an important contribution to the supply of soil organic matter. One of the effective ways to provide soil organic matter is to convert these wastes into compounds that, in addition to providing energy, also reduce the volume and weight of waste materials. These compounds, while increasing the efficiency of water consumption, do not endanger the health of the soil and the environment. One of these compounds is biochar. Biochar is a product of thermal chemical combustion and carbonization of organic biomass, which is usually produced through pyrolysis of biomass at different temperatures under low oxygen conditions. Biochar decomposition rate is much lower than other organic materials and it can remain in the soil for hundreds to thousands of years. Biochar has a high capacity in reducing greenhouse gases and can store carbon in the soil for a long time. Biochars have attracted the attention of researchers in order to increase soil carbon (C) storage, increase crop yield, improve physical, chemical and biological properties of soil and accelerate the cycle of soil nutrients. Therefore, the purpose of this study was to investigate the release of some essential nutrients from different biochars and carbon sequestration.

Materials and methods
This research was conducted in order to investigate the release of some nutrients from different biochars and carbon sequestration. For this purpose, a composite soil sample was prepared from Malayer city located in Hamadan province. The soil sample was transferred to the laboratory and after being air-dried, it was passed through a 2 mm sieve. Some physical and chemical characteristics were measured in the soil sample with usual laboratory methods. The biomass used to prepare biochar was grape waste, brown walnut shell and poultry manure, which grape waste was obtained from the vineyards of Melayer city, brown walnut shell from Tuysarkan city and poultry manure from Bijar city, Kurdistan province. The biomass was sent to a knowledge-based company in Ahvaz, Khuzestan province to be converted into biochar. In this center, the biomass was converted into biochar at a temperature of 400 degrees Celsius in an environment with low oxygen for two hours. Some chemical characteristics of biochars were measured. To carry out this research, the ratio of 10 percent by weight of biochars was used. For this purpose, 10 grams of each biochar was added to 90 grams of soil. Also, the control soil was prepared in three replicates. The experimental treatments included soil + grape waste biochar (GSB), soil + brown walnut shell biochar (NSB), soil + poultry manure biochar (PMB) and control soil (CS). The samples were incubated for two months at a temperature of 25 ± 3. During this period, the humidity was maintained at 70% of the field capacity and soil samples were taken at 5, 10, 30 and 60 days to determine the release of phosphorus, potassium, iron and zinc elements and to calculate carbon sequestration.

Results and discussion
The results showed that the addition of biochar caused an increase in exchangeable potassium compared to the control. Potassium concentration from 243.6 mg kg-1 in control soil (average of all times), to 1679.3 mg kg-1 in GSB treatment, 1445.1 mg kg-1 in PMB treatment and 379.7 mg kg-1 reached in NSB treatment. The amount of exchangeable potassium in GSB treatment (on average 1.2-6.9 times) was higher than other treatments. As the incubation time increased, the exchangeable potassium concentration increased. NSB treatment released an average of 10% of its potassium during 60 days of incubation. The order of potassium release from the biochar treatment was as follows: NSB> PMB> GSB, regarding potassium release, NSB was the most efficient treatment and GSB was the least efficient. This study showed that the release of potassium from biochars was higher compared to other elements during the incubation time. By adding biochar to the soil, the absorbable phosphorus concentration increased compared to the control. So that the concentration of absorbable phosphorus from 157.4 mg kg-1 in control soil (time average) to 202.2, 179.0 and 1.329 mg kg-1 respectively in GSB treatment. NSB and PMB arrived. Phosphorus release percentage in PMB treatment was more than other treatments. However, compared to other elements, the release of phosphorus continued with a slower process. This indicates that biochars can release phosphorus to the soil for more than one cropping season. The order of phosphorus release from different treatments was as follows: PMB > NSB > GSB. By adding biochars, the absorbable iron concentration increased. But this increase in GSB treatment compared to other treatments was significant (average 2.6 times during incubation time). Iron concentration increased from 12.0 mg kg-1 (average of all times) to 31.6, 12.8 and 14.4 mg kg-1 in GSB, NSB and PMB treatments, respectively. The order of iron release from the treatments was as follows: GSB > NSB > PMB. By adding biochar, the concentration of zinc in the soil increased compared to the control. Zinc concentration increased from 8.1 mg kg-1 in control soil (average of all times) to 9.8, 11.1 and 12.7 mg kg-1 in GSB, NSB and PMB treatments, respectively. Regarding zinc release, NSB was the most effective treatment and PMB was the least effective. The release order was: NSB > GSB > PMB. The highest carbon pool index (CPI) during incubation was seen in NSB treatment which was about 30% more than GSB treatment and 18% more than PMB treatment. The order of CPI index in different treatments was as follows: NSB > PMB > GSB. In the cluster analysis, it was found that NSB treatment had the greatest effect on the measured factors.

Conclusion
Biochar is an environmentally friendly material and its use in the soil can lead to an increase in organic matter and the content of nutrients. The results of this research showed that the addition of biochar to the soil increased the macronutrient and micronutrient studied. It also increased carbon sequestration in the soil. But it is difficult for a specific biochar to act as a complete nutrient. Therefore, it can be stated that the biochars used have long-term agricultural value, and considering Iran's climate and the severe lack of organic matter, biochar can be recommended as a stable organic matter in the soil.