Objectives Abuse of additives containing micro-elements are often happened in the feeds of intensive animal farming, which results in the accumulation of heavy metals in animal manure and the commercial organic fertilizers based on these feces. The dynamic changes of nutrient and heavy metal contents during composting were studied for the safe production of organic fertilizer.
Methods Swine manure was mixed with rice chaff in ratio of 6∶1 (fresh weight) and then aerobically composted for 55 days. The compost piles were turned over every week for aeration. Temperatures inside the composting piles were determined every day, and compost samples were collected at PM 10:00−11:00 since 1, 13, 23, 28, 41 and 55 days, respectively. The moisture content, pH, the total concentrations of N, organic carbon (TOC), P, K, Cr, Cd, Cu, Mn, Ni, Zn, Pb, Hg and As, and the available contents of Cu, Mn and Zn were measured.
Results 1) Total N, total P and total K contents were generally increased during the composting. Total N was firstly increased and reached maximum from 23rd day to 28th day, then started decreasing. Total P and total K contents were kept increased during composting, and reached their maximum values at the 41st day. The TOC content was gradually decreased and kept stable after 28th day. The C/N ratio was deceased firstly, and then kept stable after the 23rd day, and the final C/N ratio was 15.12. 2) The total concentrations of 7 heavy metals were as flows: Cr 6.99−10.43 mg/kg, Cu 106.01−120.81 mg/kg, Mn 663.51−899.48 mg/kg, Ni 11.32−20.67 mg/kg, Zn 1245.18−1552.13 mg/kg, Pb 0.09−0.56 mg/kg and As 0.58−1.25 mg/kg. Cd and Hg were not detectable in the composts. Cr and Ni exhibited a way of decrease, increase, then decrease, till got to a stable level. The maximum contents of Cr and Ni were observed on the 23rd day and then did not significantly change after 28th day. Cu, Mn and Zn contents gradually increased, and reached maximum values on the 41st day, though a slight decrease on 55th day. As content was decreased at the beginning then increased and got to a stable level after 28th day. Pb content was decreased by 61.22% and 81.63% at the 13th and 23rd day, respectively, and was not detectable after 41st day. The available Cu, Mn and Zn contents were in the ranges of 2.35−5.79 mg/kg, 17.82−20.28 mg/kg and 47.39−70.29 mg/kg, respectively, far below their total contents. The available Cu and Zn contents exhibited the maximum from 13rd day to 23rd day, and the afterward decreases were significant after the 28th day and kept stable till the end of composting. The available Mn content was generally stable during the whole composting, except an apparent increase at 41st day. 3) Strong positive correlations were found in available contents between Cu and Zn, and Zn and Mn (P<0.01). Strong negative correlations were found between total P, total K, Cu, Mn, Zn and TOC (P<0.01), which suggested the increases in these fractions due to mineralization of organic matter during composting. However, there existed no correlations between available Cu, Zn, Mn and TOC.
Conclusions From the 23rd to 28th day, the compost temperatures is high enough to meet the demand for non-hazardous livestock. The C/N ratio is reached ideal level and the total nitrogen content is the highest on the 28th day, The metal contents become stable after the 23rd day, except that for Mn content. Therefore, after the 28th day, the compost is matured completely and in the highest safety and nutrition level.
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