Nitrogen losses and Cu, Pb changes of paddy field affected by adding duckweed under biogas slurry application

Nutrient loss and heavy metal accumulation are major problems in the utilization of biogas slurry as a resource. Duckweed, which can absorb nitrogen and phosphorus and take up heavy metals, can be used to control environmental pollution in biogas slurry irrigation. To study the effects of duckweed on nitrogen loss and reduction in Cu and Pb heavy metals in a paddy field fertilized with biogas slurry, a field experiment was conducted in Jinshan District, Shanghai, China. The experiment comprised four treatments: (1) chemical fertilization (CF); (2) chemical fertilization + duckweed (CF + D); (3) biogas slurry (BS); (4) biogas slurry + duckweed (BS + D). The changes in nitrogen concentration in paddy field surface water, nitrogen-loss load in runoff water, and the levels of Cu and Pb in soil, rice grain and straw under different treatments were measured and compared. The results showed that the trends in the variation of total nitrogen (TN) and ammonia nitrogen (NH₄⁺-N) concentrations in the surface water of different treatments were the same. The concentrations reached peak values on the first day after each fertilization, and then decreased day by day. On the fifth day after fertilization, they dropped below 30% of the peak. The peak value of nitrate nitrogen (NO₃^--N) concentration lagged by 3-7 days. The addition of duckweed when biogas slurry application significantly reduced TN content in surface water, and the total TN loss load of BS+D treatment was 3.67 kg·hm^-2. Compared with the CF treatment, the TN loss load with BS+D treatment was lower by 37.2%. The Cu and Pb contents of the soil treated with BS+D was 22.65 mg·kg^-1 and 49.05 mg·kg^-1, which was not significantly different to levels in the other treatments, but the available Cu and Pb levels in the soil treated with BS+D treatment were significantly higher than those treated with CF, by 18.6% and 17.5%, respectively. There was no significant difference in the contents of Cu and Pb in rice straw and grain, but the grain content of Cu in BS+D treatment decreased by 41.1% compared with BS treatment. In summary, adding duckweed to field applied with biogas slurry can reduce nitrogen loss in paddy fields, and can control the increase of Cu and Pb in soil and rice grain and straw to a certain extent. It can be an effective means to control water and soil pollution under biogas slurry irrigation mode.