Objectives We investigated the influence of adding acid-hydrolyzed amino acids (AA) on composting process, N loss prevention, composting quality and product effects of different plant residue wastes.

Methods Nine different composts were made from herb, cassava and mushroom residues, by adding 5%, 10% and 15% of AA (v/w) to the three kinds of composting piles, respectively. After composting, nine bio-organic fertilizers were made by addition of growth-promoting bacteria (Bacillus amyloliquefaciens SQR9) to conduct pepper and eggplant pot experiments, taking the aforementioned composts as treatments as well. The bio-fertilizer and compost of each kind with the best yield effect were chosen for the following field experiment. The pepper and eggplant field experiments contained 14 treatments, no fertilizer and chemical fertilizer (NPK) controls, three composts, three bio-organic fertilizers, and the combination of compost and bio-organic fertilizers with NPK. The physical and chemical properties of the compost, fresh weight, dry weight, height, stem diameter and SPAD of plant, and the quality and effect of the compost products were evaluated.

Results AA addition prolonged thermophilic stage of herb and cassava residue composting, while it shortened that of mushroom residue composting. AA addition decreased the pH of composting piles, and generally the more AA was added, the lower the pH. AA addition significantly promoted the decomposition of cellulose, hemicellulose and total carbon of the three raw materials, and the higher the AA addition, the stronger the promoting effects and NPK nutrient accumulation in the composts. Moreover, the electrical conductivity of the piles slightly increased due to the H⁺ supplementation introduced by AA. In the pot experiment, all the three kinds of composts produced by adding 10% of AA significantly increased the pepper plant biomass and biological indicators, and their combined use with bacteria strain (bio-organic fertilizer) further increased the promoting effects. Combined application of herb residue compost and bio-organic fertilizer had the best growth-promotion effects on pepper, while bio-organic fertilizer plus chemical fertilizer had the best effect on eggplant. With cassava compost, bio-organic fertilizer was the best in both pepper and eggplant. For mushroom residue compost, bio-organic fertilizer plus chemical fertilizer had the best effect on both pepper and eggplant.

Conclusions Addition of acid-hydrolyzed amino acids (10%) decreased compost pile pH, reduced N loss, prolonged the thermophilic stage, consequently improved the carbon degradation rate and increased the relative content of N, P and K content in composts. The combined use of the composts with beneficial bacteria strains is more efficient in increasing crop biomass and yield.