Impact of Natural Rubber Tire Waste Charcoal on Selected Soil Physical Characteristics of Tea Growing Soils

The present study assessed the impact of application of natural rubber tire waste charcoal (NRTWC) on selected physical characteristics of tea cultivated soils which belongs to Red Yellow Podzolic great soil group. Natural rubber tire waste charcoal was applied as a soil amendment at different rates (0%, 1%, 1.6%, 2.2% and 2.8% w/w). Physical attributes of soil; bulk density, particle density, porosity, texture and aggregate stability were assessed at 10 and 20 weeks after application of NRTWC. Natural rubber tire waste charcoal significantly improved physical characteristics of soil by reducing soil bulk density, and increasing particle density, porosity and aggregate stability. Natural rubber tire waste charcoal treated soils showed significantly ( p ≤ 0.05) lower bulk densities for the application rates of 2.2% and 2.8% (1.21 and 1.17 gcm -3 respectively) compared to the control (1.39 gcm -3 ) at 10 weeks after application. Significantly ( p ≤ 0.05) lower particle density was observed for the application rate of 2.8% (2.52 gcm -3 ) than the control at 20 weeks after application. The increased porosity with time might be attributed to the increased of soil organic matter content with the application of NRTWC. The highest porosity change was observed at 1 % treatment (from 44.9% to 48.8%) and the lowest was at 2.8% (from 49.4% to 56.3%). Despite no significant ( p ≥ 0.05) difference was reported in particle size distribution in soil treated with NRTWC, there was a noticeable decreased in sand percentage and increased in either silt or clay percentages at 20 weeks after application. Soil aggregate stability was found to be significantly increased ( p ≤ 0.05) in all treatments at 10 and 20 weeks after NRTWC application compared to the control. However, further investigations are needed before recommending the addition of NRTWC as an amendment to tea growing soils.


Introduction
An enormous amount of tire wastes has been discarded throughout the world every year causing negative impacts on the environment (Thomas et al. 2016;Czajczynska et al. 2017). A substantial fraction of these tires is used in landfilling or discarded as garbage without proper treatment. The number of waste tires is estimated to be increased to five billion by the year 2030, (Thomas et al. 2016) leading a significant disposal and storage problems in landfill and stockpiles (Dhir et al. 2001). Pyrolysis, incineration, retreading, gasification and landfill are found to be the common method of tire disposal (Undri et al. 2013;Duan et al. 2015;Li et al. 2016). Due to their nonbiodegradable nature, and discarded rubber tires may cause "black pollution" (Nehdi and Khan 2001). Landfilling on the other hand is also not recommended due to possible floating on the top over time (Juma et al. 2006). Therefore, it is of prime importance to discover safe and useful method to discard waste tires without harming the environment.
Pyrolysis oil, pyrolysis gas and solid coke etc. are outcomes of pyrolysis, thus it is considered as an economically viable and environmentally friendly method (Williams 2013).
The solid coke is composed of industrial carbon black (CB), ash, inorganic filler, etc., which can be further recycled (Wang et al. 2014;Zhang et al. 2018). According to Junqing et al. (2020), pyrolytic CB is very much similar in nature to the biochar. Therefore, uses of carbonaceous materials produced through the thermal decomposition are capable of increasing soil organic carbon content. Pyrolytic CB is recalcitrant to decomposition and maintains soil organic carbon levels for a longer period of time.
Tea is a perennial crop with a commercial life span of about nearly 30-50 years (Gunathilaka 2018). Sri Lankan tea industry is considered as one of the largest agriculture based industries in Sri Lanka and it contributes 2% to the Gross Domestic Production (Cental Bank Report 2020). Out of the total land area used for agriculture, 28% is used for tea cultivation (Mapa et al. 2002
Exchangeable K, Na and Ca were determined using a flame photometer (Blackmore et al., 1987) and the wet oxidation method (Tiessen and Moir, 1993) was applied to determine soil organic matter content.  cm 3 ) to determine soil bulk density. Then, the plastic bags containing soil samples were sealed and transported to the laboratory. Soils were air-dried at room temperature for a week and crushed with a rubber tipped pestle. They were sieved using a 2 mm mesh before being analyzed.

Soil Analyses
All the analyses were done at the Soil Pycnometer method (Heiskanen 1992) was employed to determine particle density and soil porosity was calculated based on soil densities. Bouyoucos hydrometer method (Gee and Or 2002) was employed to measure particle-size distribution (soil texture).
Dry sieving method (Elliott 1986) was used to determine the distribution and stability of soil aggregates (oscillatory sieving analyzer, JH-200, Beijing, China).
In order to isolate six aggregate size fractions, soil sample (500 g) was passed through a series of five sieves (3, 2, 1.4, 1 and 0.5 mm). Then, they were shaken at 280 rpm gently for 05 minutes and impurities such as stones and roots etc., were eliminated. Finally, the weights of the mass of different particle sizes were taken and the mean weight diameter (MWD) was calculated using following equation: Where, ̅̅̅ represents the mean diameter of each size fraction (mm) and Wi stands for the proportion of the total sample mass of relevant size fraction.

Soil Texture
No significant (P ≥ 0.05) variations in particle size distribution of the soil treated with NRTWC were observed at 10 and 20 weeks after application (Table 3).
However, as the time progressed, there was a noticeable decreased in sand percentage while increasing silt or clay percentages (Figure 1). The highest clay content (16.9%) was recorded at the 1.6% application rate followed by 2.2% (16.8%) application rate at 20 weeks after NRTWC application. However, the soil texture remained unchanged (sandy loam) even 20 weeks after application of NRTWC.   There is a strong negative correlation between organic matter content and particle density of a soil (Schjønning et al. 2017). Therefore, the decreased trend of soil particle density with the addition of NRTWC is attributed to the high organic matter content and low particle density of charcoal particles.

Conclusions
According to the present findings, the addition of NRTWC had a significant impact on major physical characteristics of tea growing soil.