Standardization of cold plasma treatment for nitrogen fixation in major soil types of Kerala and its effect on seedling growth of cowpea

Abstract

Plasma, often referred to as the fourth state of matter is an ionized gas that can be generated in various gases or gas mixtures across a wide range of pressures and temperatures. Cold plasma assisted nitrogen fixation is a green technology to fix atmospheric nitrogen (N₂ ) through the generation of highly energetic electrons and reactive radicals and this also involves synthesis of ammonia and nitrogen oxides (NOx) or nitrates/nitrites (NO₃ ⁻ /NO₂ ⁻ ). A study titled “Standardization of cold plasma treatment for nitrogen fixation in major soil types of Kerala and its effect on seedling growth of cowpea” was conducted to explore the potential of plasma treatment for nitrogen fixation at the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellanikkara during 2023-24. Surface soil samples were collected from seven major soil types of Kerala viz laterite soils, red loam soils, Onattukara sandy soils, coastal sandy soil, Kari soils, Pokkali soils and black soils. The soils were initially characterized for physico-chemical and biological properties and were subjected to cold plasma and Plasma Activated Water (PAW) treatments at low (5 kV), medium (10 kV) and high (15 kV) voltages for 10 minutes. Among the studied soil types, coastal sandy soil (7.34) was neutral; black soil (7.50) was of alkaline reaction and the other soils were acidic in reaction with pH varying from 2.95 (ultra-acidic) in kari soil to 6.32 (slightly acidic) in red loam soil. Pokkali soil recorded higher status with respect to EC (6.57 dS m-1), available N (570.75 kg ha-1), P (168.84 kg ha-1), K (442.55 kg ha-1) and B (2.00 mg kg-1). High values were recorded in Kari soil for parameters like organic carbon (99.60 g kg-1), Fe (1004.50 mg kg-1), S (651.30 mg kg-1) and Zn (4.18 mg kg-1). Soil pH and organic carbon content has significantly increased after cold plasma treatment in all the soil types under study except Kari soil, which showed reduction in both parameters. Electrical conductivity (EC) has decreased in all soils except Pokkali soil. Cold plasma treatment could enhance nitrogen status in all the selected soil types. Available nitrogen level was highest at 15 kV treatment for all the soils except Kari and coastal sandy soil, which have shown the peak value at 10 kV. The highest per cent increase in available nitrogen content has shown by coastal sandy soil at 5 kV (214.30 per cent) and 10 kV (457.20 per cent) and by black soil at 15 kV (269.56 per cent). Highest ammoniacal nitrogen content was observed at 5 kV in Kari (198.80 mg kg-1), coastal sandy (142.80 mg kg-1) and Pokkali soils (205.80 mg kg-1), at 10 kV in red loam soil (159.60 mg kg-1) and at 10 kV (161.00 mg kg-1) and 15 kV (155.40 mg kg-1) in laterite soil. Microbial activity has reduced in all soils after cold plasma treatment. Plasma Activated Water (PAW) has significantly reduced soil pH in all soils except Onattukara sandy soil. Organic carbon content has shown significant reduction in all soils, except red loam soil. Available nitrogen content was highest at 15 kV for all soils except red loam soil, which has shown peak value at 10 kV. Coastal sandy soil has shown highest per cent increase at 5 kV (214.31 per cent), 10 kV (500.05 per cent) and 15 kV (557.20 per cent). Ammoniacal and nitrate nitrogen content was highest at 15 kV for all the selected soils except red loam soil, for which nitrate nitrogen was highest at 10 kV. Microbial activity and dehydrogenase activity has significantly reduced after PAW treatment, while urease activity has increased. A pot culture study was conducted to study the effects of plasma treatment on seedling growth of cowpea. Treatments included control (T1), plasma treatment (T2), plasma activated water treatment (T3) and cold plasma and plasma activated water treatment (T4). Total chlorophyll and protein content were significantly higher under treated conditions. Total nitrogen content was significantly higher in seedlings grown under plasma (T2) and plasma and PAW treated (T4) laterite (50.05 g kg-1) and black soil (50.05 g kg-1) and in plasma treated (T2) Kari soil (80.50 g kg-1). Seedlings grown under PAW treated black soil has shown significant increase in phosphorus (0.29 g kg- 1), zinc (94.44 mg kg-1) and copper (32.22 mg kg-1) content. Results of the study indicate that, cold plasma and plasma activated water treatment could assist nitrogen fixation in all the selected soils and treatment at a voltage of 5 kV can be considered as optimum for sufficient nitrogen fixing in the soils with low energy consumption and minimum impact on soil microbes. The feasibility and ease of application make plasma activated water more effective approach for enriching soil nitrogen.