Mineralization of organic materials and bioavailability of nutrients to partially replace the inorganic fertilization for tea

Soil fertility depletion is one of the main factors for yield stagnation in tea cultivation in Sri Lanka for last few decades. Organic material (OM) incorporation has been identified as the sustainable technique to restore soil fertility in degraded tea fields even though research information is scanty. A series of experiments were carried out to investigate the carbon (C) and nitrogen (N) mineralization dynamics and the effects of OMs, such as Gliricidia (GLI), compost (CMP), tea waste (TW), biochar charged with nutrients (CBC) and raw biochar (RBC) without nutrient charging, on bio-availability of nutrients. Experiments were conducted in completely randomized design at the Universiti Putra Malaysia. The potential for partial substitution of inorganic N fertilizers by aforesaid OMs was tested in split plot design under field condition in Sri Lanka using mineralization and bio-availability indicators. The parallel first-order kinetic model fitted well (R2>0.99) with the C mineralization dynamics demonstrating different C pools in added OMs. All the materials enhanced the soil microbial biomass carbon (MBC) and nitrogen (MBN) over 79 and 70%, respectively. Catalase and dehydrogenase activities were also improved by more than 76%. The carbon mineralization rate was highest in GLI (k=0.7 day-1) and it was lowest in RBC (k=0.01 day-1). Gliricidia showed the highest mineralizable C content of 15,350 mg kg-1, which rapidly promoted the soil's biological activities. The CBC behaved similarly to compost in improving soil health. Gliricidia had faster released of N amounting to 94% of added N in N, P and S mineralization study. Even though CBC increased urease, protease, dehydrogenase and phosphatase activity by 157, 376, 500 and 75%, respectively, CBC probably immobilized/fixed N in the soil making unavailable for plants. It was confirmed by the bioavailability of mineralized products trial evaluated by implementing the exhaustive cropping technique with Napier grass. Approximately, 47% of the mineralized N from GLI was taken up by the plant, and the rest may have been lost or immobilized by microbes. Consequently, GLI showed the highest apparent nutrient recovery (NAR) of 47%. It should be mandatory to fortify biochar with a nutrient solution in order to maximize the utilization of biochar for soil health restoration. The study of different ratios of OMs to inorganic fertilizers based on N requirements showed that all the tested OMs had the potential to replace the inorganic N fertilizer to an extent of 30%. In conclusion, GLI and CBC can be applied to restore fertility and health status respectively in degraded tea lands. Charged biochar showed the maximum benefits for the restoration of degraded soil with the ability to replace inorganic fertilizer by at least 20% for mature tea cultivation, invariably subsequent to supplementary future investigations.

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