Impact of struvite crystallization on nitrogen losses during composting of pig manure and cornstalk.

An absorbent mixture of magnesium hydroxide (Mg(OH)(2)) and phosphoric acid (H(3)PO(4)) was added to compost mixtures of pig manure with cornstalk in different molar ratios (T1, 1:1; T2, 1:2; T3, 1:3) in order to examine its effect on controlling ammonia losses during composting. Based on the principle of struvite precipitation, and with an unamended trial as control (CK), an in-vessel composting experiment was conducted in fermenters (60L with forced aeration) in which the absorbent mixture was added with proportions of 3.8%, 7.3% and 8.9% of dry weight for T1, T2 and T3, respectively. The results showed that the total nitrogen loss was reduced from 35% to 12%, 5% and 1% of initial N mass, respectively. In the final compost, the total nitrogen content in T1, T2 and T3 was improved by 10, 14, 12gkg(-1), and NH(4)(+)-N in T1, T2 and T3 was improved by 8, 9, and 10gkg(-1), respectively, compared with the unamended trial. The results of the germination index test showed that the maturity of treatment T2 was best among the four treatments in the final compost, followed by T1, CK and T3. The results of X-ray diffraction (XRD) confirmed the formation of magnesium ammonium phosphate hexahydrate (MgNH(4)PO(4).6H(2)O:MAP) in the T1, T2 and T3 compost. Based on these results, the adsorbent mixture of Mg(OH)(2)+H(3)PO(4) could control nitrogen loss effectively during composting via struvite crystallization. However, an excess of phosphoric acid (1:3) had a negative influence on composting properties. The pH value decreased which led to reduced microorganism activity, and which finally resulted in reduced biodegradation of the organic matter.