Abstract: Agricultural use of urban sludge is a main way for processing municipal sludge. However, such applications have the risks of heavy metal pollution. This study is aimed to explore the reasonable application of sludge by a field experiment in Hexi oasis irrigated desert soil. The experiment included five treatments: conventional fertilization (NP) as control, conventional fertilization + urban sludge compost 9000 kg·hm^-2 (NPS₉₀₀₀), conventional fertilization + municipal sludge compost 18000 kg·hm^-2 (NPS₁₈₀₀₀), conventional fertilization + municipal sludge compost 18000 kg·hm^-2 + green manure returning planted after wheat harvest (NPG_rS₁₈₀₀₀), and conventional fertilization + municipal sludge compost 18000 kg·hm^-2 + replanting green manure and removal (NPG_tS₁₈₀₀₀). We examined the effects of sludge and green manure application on soil physical and chemical properties, organic carbon and total nitrogen sequestration rate, nitrogen, phosphorus and potassium activation coefficient, crop yield, and heavy metal contents in a wheatcorn cropping system. The results showed that the effects of four treatments of sludge application on corn yield were equivalent to that of NP, while wheat yield was significantly increased. The difference of wheat yield in the four treatments with sludge application gradually changed from significant to insignificant with increasing duration of sludge application from 2015 to 2019. Sludge application increased the contents of soil organic matter, total nitrogen, total phosphorus, alkali hydrolyzable nitrogen and available phosphorus, activated soil nitrogen, phosphorus and potassium, increased organic carbon and total nitrogen sequestration rate, and reduced soil pH and bulk density. The application effect was positively correlated with application rate. Although sludge application or sludge + green manure returning slightly increased the accumulation of heavy metals in the soil, the contents of all heavy metals were lower than the national safety standards of China. The zinc and lead contents of corn grains in NPS₁₈₀₀₀ treatment and the zinc content of corn grains in NPG_tS₁₈₀₀₀ treatment were significantly higher than those of NP treatment. The input amount of heavy metals by the sludge application was much greater than the output amount by harvesting crops. In summary, the application of sludge could significantlyincrease soil fertility and wheat yield. High-level sludge application combined with removal of green manure could inhibit the absorption of zinc and lead by corn grains and increase the output of heavy metals from the cropping system. To ensure that the zinc and lead contents in corn grains are below safe limits, annual input amount of zinc and lead carried by the sludge application should be lower than 348 g·hm^-2 and 9.5 g·hm^-2.

Key words: urban sludge, soil fertility, crop yield, soil heavy metal.