Acid-modified Biochar Effect on Some Physiological Indicators and Micronutrient Availability of Quinoa (cv. Gizavan) in a Calcareous Soil

Organic matter and alkaline pH are the main causes of nutrient deficiencies in calcareous soils of arid and semi-arid regions. The availability of some nutritional elements, including the micronutrients such as iron, zinc, copper, and manganese is very low in calcareous soils, although the total concentration of these elements may be relatively high.  Burning crop residues results in substantial loss of nutrients, and may lead to air pollution and human health problems. An alternative approach is to apply crop residues to soil in the form of biochar. The biochar modification with acid may increase the solubility of nutrients (P, Fe, Zn, Cu, Mn) present in biochar, thereby significant improvement in mineral nutrition of plants grown in calcareous soils. Therefore, the object of this study is to investigate the effect of acid-modified biochar from rice residues on the amount of chlorophyll and the micronutrient concentration of quinoa plant (Chenopodium quinoa) in a calcareous soil.

Methods and Materials:

The soil was air-dried and ground to pass through a 2-mm sieve then was analyzed to determine various soil physico-chemical properties using standard methods. To achieve the aim of this study the factorial experiment was carried out based on a completely randomized design in 4 replications. Factors include 3 types of biochar (unmodified, modified by pre-acidic method and modified by post-acidic method) and different levels of biochar (0, 2, and 5% by weight). Then 10 quinoa seeds were planted in each pot at 2-cm depth which after emergence, declined to 3 plants in each pot. The pots were randomly moved twice a week during the growth period to eliminate environmental effects. Irrigation and weeding operations were performed by hand. Determination of chlorophyll content (a, b, and ab) and carotenoids were measured precisely before harvesting in fresh plants using Arnon method.  Plants were harvested at 187 days after planting, washed with distilled water and dry with tissue paper. The samples were air-dried and then oven dried at 65˚C to a constant weight in a forced air-driven oven. Then the total micronutrient content of the plant was determined after dry ashing. The statistical results of the data were analyzed using SAS software (9.4) and LSD test (at 5% level) was used for comparing the mean values.

Based on the variance analysis, all attributes responded positively to different types and levels of biochar and modified biochar (p<0.01). The comparison of the average effect of the studied treatments showed that with the increase in the levels of all three types of biochar, the amount of chlorophyll a, b, total, and carotenoid increased so the highest amount of chlorophyll a, b, total, and carotenoid respectively, with an average of 2.58 and 1.54, 4.13 and 1.36 mg g-1 were obtained from the treatment of 5% post-acidic biochar. The results showed that the highest amount of Fe concentration in shoots with an average of 229.48 mg kg-1 was obtained from the treatment of 5% post-acidic biochar, although there was no statistically significant difference with the treatment of 5% pre-acidic biochar with an average of 220.48 mg kg-1 and its lowest value with an average of 95.95 mg kg-1 was related to unmodified biochar. The highest amount of Zn concentration in shoots with an average of 13.42 mg kg-1 was related to the treatment of 5% post-acidic biochar which showed an increase of 13.24 and 33.26% compared to the treatment of 5% pre-acidic and unmodified biochar, respectively. Also, the highest concentrations of Cu and Mn in shoots were obtained with an average of 3.85 and 23.37 mg kg-1 respectively, from the treatment of 5% post-acidic biochar.

Post-acidic biochar had better results in terms of physiological indices and the concentration of micronutrients (Fe, Zn, Cu, and Mn) than unmodified biochar in quinoa. The increase of nutrients in quinoa can be attributed to the dissolution of biochar nutrients after being modified with acid and the reduction of pH and the availability of these elements in the soil. Therefore, biochar modified with acid or biochar produced from sources that have acidic properties can be recommended as a suitable method for improving fertility and increasing micronutrients in calcareous soils affected by salt.