Effects of biogas manure replacing chemical fertilizer on accumulation of nutrient and heavy metal in greenhouse vegetable soil

Abstract: Unreasonal application of biogas manure discharged from the biogas plants in livestock farms increased the environmental risk. In this study, 5-year with 8 seasons of tomato-pepper rotating protected-field experiment was conducted to investigate the effects of different applied proportions of biogas manures and chemical fertilizer (100% biogas manures, 3/5 biogas manures plus 2/5 chemical fertilizer, 2/5 biogas manures plus 3/5 chemical fertilizer, 3/5 biogas manures and unfertilized treatments) on vegetable yield and soil accumulation of nitrogen (N), phosphorus (P) and heavy metals, in order to support the possibility of biogas manure to replace chemical fertilizer. The results showed that there were no significant effects on vegetable yields among the treatments with 100% biogas manures, 3/5 biogas manures plus 2/5 chemical fertilizer, 2/5 biogas manures plus 3/5 chemical fertilizer treatments with the same N and potassium (K) input. No significant differences were investigated in the N surplus and soil total N in 0-180 cm soil depth after 5 years experiment. However P surplus significantly increased with the applied proportion of biogas manures. The contents of Olsen-P and CaCl2-P in the surface soil layer (0-30 cm) reached to 151 and 8.0 mg P/kg in treatment with 100% biogas manures application, respectively, much higher than other treatments, which significantly exceeded environmental threshold. Compared with the treatment with 100% biogas manures application, reduced biogas wastes application (3/5 biogas wastes) significantly decreased the nitrate leaching risk and P surplus. Compared with unfertilized treatment, the contents of total Hg in surface soil layer (0-30 cm) and totoal As content in the soil layer (0-180 cm) slightly increased, whereas the contents of total Cr, Cd and Pb in soil decreased, especially Pb, in the treatments with biogas manures application. No heavy metal pollution risk was observed in all treatments. Biogas manure replacing chemical fertilizer can effectively achieve the waste recycling utilization, but increase the risk of soil P accumulation and leaching.