Safety dosage of pig manure based on zinc accumulation in rice and wheat rotated farmland

Objectives In order to solve the outstanding problem of waste ultra-discharge by scale aquaculture in Jiangsu province, which caused serious environmental pollution and massive waste of nutrient resources, the characteristic changes of heavy metal zinc of soil were studied after chemical fertilizer replaced by pig manure, in addition, pig manure application amount of security was discussed based on tilth soil zinc accumulation rate. The results will provide theoretical basis for popularization and application of pig manure security technique in the scale pig farms.

Methods A field experiment was conducted with different dosages of pig manure on rice and wheat rotated farmland. Eight treatments were designed:control (CK), chemical fertilizer (CF), 25% pig manure and 75% chemical fertilizer (25% PM), 50% pig manure and 50% chemical fertilizer (50% PM), 75% pig manure and 25% chemical fertilizer (75% PM), 100% pig manure (100% PM), 125% pig manure (125% PM), 200% pig manure (200% PM), and each treatment was repeated three times. Soil samples were taken in the 0-10 cm and 10-20 cm soil layers in the rice harvest period in 2013 and 2014 respectively. The contents of total and available zinc were determined by atomic absorption spectrometry after HF-HNO₃-HClO₄heating digestion method and DTPA extraction method respectively to analyze the characteristic changes and accumulation rate of total and available zinc under different dosage of pig manure. The analysis of the safety application age limit of pig manure application was according to the secondary standard of agricultural safety production combined with the total zinc annual accumulation rate of different dosages of pig manure in the 0-10 cm soil layer.

Results The content and accumulation rate of total and effective zinc showed a linearly increase with the increased application ratio of pig manure in the 0-10 cm and 10-20 cm soil layers on rice harvest during the first 3-4 years. The contents of total and effective zinc were 91.1-236.5 and 8.9-49.4 mg/kg, and the accumulation rate of total and effective zinc were 6.6-42.9 and 1.9-12.0 mg/(kg·a) in the 0-10 cm soil layer under different pig manure treatments in 2014, respectively. The percentage of accumulation content of effective zinc varied from 28.0% to 31.3% of accumulation content of total zinc in the arable soil layer. By accumulation rate of total zinc, the safety application dosage limit of pig manure showed a decreasing trend of power equation with the increased application ratio of pig manure in the 0-10 cm soil layer. The replacement ratios of pig manure nitrogen were 11.1%-12.5% in 50 years scale, and 33.6%-35.5% in 20 years scale according to the secondary standard of agricultural production.

Conclusions The cumulative effect of zinc existed due to use of pig manure, and a linearly increasing trend is observed with the increased ratios of pig manure in the arable soil layer on rice and wheat rotated farmland. Based on accumulation rate of soil zinc, it is suggested that the replacement ratios of pig manure nitrogen are less than 35% for short-term use, and within 12.5% for medium-and long-term use in the practical use.