The use of organic substrates with contrasting C/N ratio in the regulation of nitrogen use efficiency and losses in a potato agroecosystem

The effect of the application of organic amendments of contrasting C/N ratio, combined with mineral N fertilizer, on the N budget of a potato crop was evaluated. The hypothesis was that the combination of substrates of contrasting chemical composition can improve the synchronization between N crop requirements and N availability in the soil, increasing yields and reducing losses to the environment. Potato plants were cultivated in microplots (0.225 m²) in the tropical uplands of Venezuela, using mineral N (labeled with ¹⁵N), poultry manure (C/N = 12), and rice hulls (C/N = 90) as N sources. Four treatments with the same total dose of N (25.0 g N m⁻²) were applied: (1) MF: mineral fertilizer; (2) P + F: poultry manure and mineral fertilizer; (3) R + F: rice hulls and mineral fertilizer and (4) P + R + F: poultry manure, rice hulls and mineral fertilizer. Labeled and non-labeled N was measured in drained water and plant and soils compartments, and a N budget was established for each treatment. The ratio of N crop uptake to N losses was proposed as an ecological indicator of N use efficiency. The highest value (3.0) of this ratio was obtained in the treatment combining the two organic substrates of contrasting quality, P + R + F, followed by the R + F (2.0) and P + F (1.8) and the lowest ratio was obtained in the MF treatment (0.9). The P + R + F combination may represent a good soil amendment to obtain a high yield with a lower environmental impact, at least in the short-term. The generalization of these results to other soils and climates is discussed.