Maize and pigeon pea sole crop and intercrop nutrient response functions for Tanzania

The development of ecological circular agriculture has been highly encouraged by the Chinese government to recycle agricultural wastes, reduce mineral fertilizer input, and protect the environment. Biogas slurry, a byproduct of biogas engineering developed in rural areas of China, could be used as N fertilizer for crop growth. The field experiments were conducted in 2014 and 2015 to study the plant growth responses and environmental impacts of applying biogas slurry to two-season Zizania aquatica Turcz. growth. The potential factors that restrict the rational use of biogas slurry were also clarified. Mineral N fertilizer can be completely or partly substituted by N fertilizer from biogas slurry to satisfy Z. aquatica plant growth. It was not at the cost of sacrificing yield, dry matter accumulation, N accumulation and physiological N use efficiency in the above-ground parts. However, the growth inhibition occurs when the N quantity in biogas slurry was 2 or 2.7 times higher than that of mineral N fertilizer. Vitamin C in non-shell swollen culms (as edible part) of Z. aquatica significantly increased after biogas slurry application. Biogas slurry application substantially increased the N concentrations, i.e., total N, NH₄ ⁺–N, and NO₃ ^?–N in floodwater and delayed the time to reach national discharge standards. However, biogas slurry application did not affect the N concentrations in percolating water compared with the treatment with mineral N fertilizer only. Applying biogass slurry did not generate potential pollution risks by trace elements (Cu, Zn, Pb, Cr, Cd, As, and Hg) in the non-shell swollen culm and soil, and did not increase the nitrate content in non-shell swollen culm. We found the NH₄ ⁺–N concentration in biogas slurry can account for 77–93% of total N and reflects the N level in biogas slurry to a great degree. Semi-quantitative color-based colorimetric methods possessing simple and fast characteristics should be developed to determine the NH₄ ⁺–N concentration with the purpose of promoting reasonable use of biogas slurry in area of crop cultivation. Otherwise, excessive use of biogas slurry can adversely affect crops and increase environmental risks.