The effect of reducing particle size on physical properties, the methane yield and energy flow were investigated through the biochemical methane potential (BMP) experiment of aerobic-anaerobic digestion (AAD) of rice straw (RS). The whole straw was crushed through four sieves of different aperture sizes (1, 3, 5, and 7 mm) to obtain the actual and non-uniform particle size distribution (PSD). The results indicated that the actual particle sizes were normally or logarithmic normally distributed. Reducing particle size could significantly promote the aerobic hydrolysis and acidification process, increase the content of volatile fatty acids (VFAs) from 4408.78 to 6225.15 mg/L and the degradation of volatile solids (VS) from 40.56% to 50.49%. The results of path analysis suggested that particle size reduction played an important role in improving lignocellulosic degradability, which was the main factor affecting methane production with the comprehensive decision of 0.4616. The maximum methane production obtained at 1 mm sieve size was 176.47 mLCH₄g⁻¹ VS. The phyla of Firmicutes (61.5%), Proteobacteria (9.3%), Chloroflexi (8.3%), Bacteroidetes (4.1%), Cyanobacteria/Chloroplast (4.6%) were mainly responsible for VFAs production and lignocellulose degradation. However, the net negative energy balance was observed at the 1 mm sieve size due to the increased energy input. Therefore, the optimum sieve size for AAD was 3 mm.

Recycling and reusing the nutrient resources from anaerobic digested slurry is very promising for environmental pollution control and agriculture sustainable development. We focus here on nitrogen and phosphorus recycling in treating cattle manure anaerobic digested slurry by a magnesium ammonium phosphate (struvite-MAP) crystallization process and examine the impact of MAP precipitation on plant growth. The MAP crystallization process was studied by a combination of Design-Expert 8.0.6 software, mathematical modeling, and experiments. The influence of Mg/P, N/P and pH on nitrogen (N) and phosphorus (P) recovery was investigated. Then, the fertilizing efficiency of the MAP precipitate on the growth of three vegetables (water spinach (Swamp cabbage), amaranth and Brassica parachinensis) was also evaluated. The results showed that more than 89% of N and 99% of P could be recovered at pH = 10 with molar ratios of Mg/P = 1.6 and N/P = 1.2. Compared with the control pots and potassium chloridepots, the fresh weight, dry weight and average height of swamp cabbage in the MAP pots were obviously enhanced without burning effects. The results showed that MAP precipitation can promote the development of plants, which is promising for its use as a slow-release fertilizer for agricultural production.