Integrated cow urine and rice-husk biochar regimes modulate reproductive phenology, fruit set efficiency and biochemical quality in red-fleshed dragon fruit (Hylocereus polyrhizus)

Abstract:

Can a livestock waste stream and a carbonised crop residue, combined at agronomically rational rates, substitute for a substantial fraction of mineral fertiliser in a high-value cactus fruit system without penalising reproductive output or biochemical quality? This research addressed that question in red-fleshed dragon fruit (Hylocereus polyrhizus) grown on a weathered sandy loam in the lower Shire Valley of southern Malawi. A field experiment was established at the Chikwawa Institute of Tropical Agriculture germplasm block between October 2022 and September 2023, arranged in a randomised complete block design with five treatments and four replications of six posts each. Treatments comprised an unamended control (T0); full mineral nutrition at 100 g urea, 84 g triple superphosphate and 84 g muriate of potash per post (T1); rice-husk biochar at 5 kg per post combined with fermented cow urine at 3 L per post (T2); biochar at 5 kg per post with half-rate mineral fertiliser (T3); and a full integration of biochar, cow urine and half-rate mineral fertiliser (T4). Reproductive phenology was recorded at seven-day intervals from first bud emergence, and fruit were sampled at commercial maturity for physical and biochemical characterisation. Integration in T4 advanced first bud emergence by 11.4 days relative to the control and raised cumulative flower bud number per post to 34.8 against 19.2 in T0, a gain of 81.3 percent. Fruit set efficiency, expressed as the ratio of mature fruit to anthesised flowers, rose from 0.61 in T0 to 0.83 in T4. Mean individual fruit weight increased from 214.6 g to 301.2 g, while total soluble solids advanced from 10.4 to 13.6 degrees Brix and betacyanin concentration from 27.3 to 41.9 mg per 100 g fresh weight. Notably, T2, which received no mineral nitrogen whatsoever, achieved 89.4 percent of the fruit yield recorded in the conventional T1 treatment and exceeded it in betacyanin accumulation. Soil analysis at termination showed cation exchange capacity in biochar-amended plots elevated from 6.8 to 11.2 cmol per kg, with mineral nitrogen retention in the upper 30 cm improved by 43.7 percent. Correlation analysis identified cation exchange capacity as the variable most strongly associated with fruit set efficiency (r = 0.87, p<0.01), suggesting that the co-benefit of the two amendments operates principally through improved retention of the labile nitrogen supplied by urine rather than through direct nutrient addition by the char itself. The research indicates that a biochar-urine platform permits a fifty percent reduction in mineral input while improving both yield and pigment quality, a proposition of direct relevance to smallholder pitaya growers operating on low-fertility tropical soils where fertiliser access is intermittent and costly.