Effects of Biochar on Soil Fertility and Water Use Efficiency of Black Seed (Nigella sativa L.) under Water Stress Conditions

Application of chemical fertilizers since green revolution, make the crop production doubled or even tripled in some crops. However, long-term overuse of these fertilizers decreased soil quality. Thus to obtain the same yield, the rate of inorganic fertilizer application steadily increases from year to year. Recent studies revealed that in order to increase the chemical fertilizer efficiency, it needed to integrate organic resources with chemical fertilizers to soil. However, using organic manure in the soil will be decomposed very rapidly. Biochar is a source of high organic carbon which highly resistant to decomposition. It has a porous quality and large surface area which reduces leaching of nutrients. Biochar addition to soils can change microbial biomass, adsorb toxic compounds and improved soil water and pH status. The internal porosity of biochars may help soil microorganisms avoid grazers. Globally demands for plant- based medicines are growing. Black seed (Nigella sativa L.) is annual plant to Ranunculaceae family, is grown in arid and semi-arid regions of the world. Black seed considered as a spice, medicinal plant and used as seasoning in cooking and foodstuffs. Recently, there has been an increasing interest in integrated approaches in improving crop production to resist conditions of nutrient-poor soil, drought, and salinization. Since, a few attempt has been made to investigate the effects of biochar amendment on medicinal plants in semi-arid agricultural systems, the present study was initiated in agro ecological condition of Mashhad. A field experiment was conducted as split plots factorial layout based on a randomized complete block design with three factors and three replications at the Research Station, Faculty of Agriculture, Ferdowsi University of Mashhad, Iran during growing season of 2016. Three levels of irrigation (100, 70, and 40 percent of water irrigation requirement) were assigned as main plots and the combination of three levels of biochar (0, 10, and 20 ton ha-1) and two levels of chemical fertilizers (without and with chemical fertilizer) were allocated as sub plots. Recommended dose for nitrogen, phosphorus, and potassium was 150 kg ha-1, 75 kg ha-1 and 100 kg ha-1, respectively. The size of each experimental unit was 3×2 m2. Black seed thinning was performed after emergence to get a plant population density of 200 plants m-2. Post experimental soil which was amended with biochar and fertilizer were analyzed for total nitrogen, available phosphorus, potassium, soil organic matter, and microbial biomass carbon. Plant criteria which studied were such as grain yield (kg ha-1), straw yield (kg.ha-1) and water use efficiency (base of grain yield). Analysis of Variance (ANOVA) and Duncan multiple range test were used to compare the recoded data. Soil N, K, SOM, and Microbial biomass carbon were significantly affected by biochar application. Phosphorus was not affected by biochar. The highest biochar effect on soil characteristics was found under 20 ton ha-1 biochar application treatment. The results showed that application of 10 and 20 ton ha-1 of biochar improved soil nitrogen compared with control treatment. Soil organic matter was increased by 1.56% compared with 1.33% in control when 20 ton ha-1 biochar was used. It has been reported that the biochar may increase the organic matter of the poor soils. The soil microbial biomass carbon was increased by biochar and fertilizer application. It seems that the reason for increasing soil microbial biomass by adding biochar to the soil is enhancing available soil nutrients, adsorption of toxic compounds and improved soil water and pH status. The internal porosity of biochars may help soil microorganisms avoid grazers. Combine use of fertilizer and biochar were more affected on plants growth than biochar or fertilizer alone. The interaction between biochar, chemical fertilizer and water requirements significantly affected the seed and straw yield. The significant increase of grain yield, straw yield observed in I2B2F2 (70% water requirement+10 ton ha-1 biochar+ with chemical fertilizer) and I2B3F2 (70% water requirement+20 t.ha-1 biochar+ with chemical fertilizer) treatments. Biochar significantly increased water use efficiency which shows that under water deficiency, biochar can increase the uptake of water and nutrients. As a result, plants tolerance against water deficiency increase and more yields will be produced for a specific value of water. This means the water use efficiency can increase and the use of water will decrease.   This study confirmed that biochar application can help in increasing of black seed production through the improving some physical and chemical properties of the soil such as soil pH, soil nutrient availability, soil nutrient holding capacity, and soil microorganism conditions. Although the application of 20 ton ha-1 biochar improved soil properties compared with 10 ton ha-1, the difference was not significant.  Application of biochar with chemical fertilizers had better effects on seed and straw yield, compared with solely application of biochar and chemical fertilizers.  It may belong to the biochar ability to decrease nitrate and other elements leaching due to its high porosity and high water retention capacity which increased water use efficiency in the plant. This is important in particular, in arid and semi-arid areas, where reduces the amount of water consumed and increases water yields for plants that are under water stress. The seed production of black seed was not affected by reducing water requirements up to 70 % of its total water requirements when biochar was applied. This means that, by using the biochar in soil, only 70% water irrigation required. These results may use for water saving in agriculture in arid and semi-arid conditions.