Effects of long-term chemical fertilizer application with different organic fertilizers on accumulation of heavy metals in soil and maize

Objectives The effects of long-term organic manure application on the accumulation of heavy metal elements in soil, crop grains and straws and their environmental effect were clarified to provide scientific basis for safe and efficient fertilization.

Methods A 10-year field experiment was conducted with winter wheat-summer maize rotation in the new fertilizer long-term experiment station of Beijing Academy of Agriculture and Forestry Sciences. N 180 kg/hm², P₂O₅ 90 kg/hm² and K₂O 90 kg/hm² were applied to crops every season in all fertilization treatments. The experimental treatments were: no fertilizer (CK), single chemical fertilizer (NPK), NPK + organic manure from chicken manure compost (NPKJF), NPK + sludge compost organic fertilizer (NPKWN), NPK + living garbage compost (NPKLJ), NPK + straw return (NPKJG). After maize harvest in the 10th year of the experiment, the total contents of heavy metals in maize stalk, grains and 0–20 cm topsoil were determined.

Results The contents of Cd, Hg, As, Cu and Zn in soil were significantly increased by adding sludge treatment, and the single-factor pollution index increased by 45.10%, 150.00%, 104.00%, 44.60% and 65.80%, respectively, compared with CK. Different fertilization treatments had the effect of enriching Cd, Cr, Hg, Cu and Zn in soil. The single pollution index was 0.02–0.46, far lower than 1, and the comprehensive pollution index was 0.23–0.36, all lower than 0.7. The soil heavy metal in the test area was pollution-free. The contents of Cd, Cu and Zn in maize grain treated with organic waste compost were increased significantly compared with CK, but the contents of Hg, As, Cr, Pb and Ni in different treatments were not significantly different, and the contents of heavy metals in soil and maize grain in the experiment station did not exceed the critical levels after 10 years of fertilization. The enrichment capacity of heavy metals in different parts of the same crop was completely different, and the enrichment capacity of heavy metals in maize stalk was greater than that in grain. The contents of Pb, Cu and Zn in maize grain were positively correlated with the corresponding heavy metal contents in soil, while the contents of Cd, Hg, Cr and Ni in maize stalk were positively or significantly negatively correlated with the corresponding heavy metal contents in soil.

Conclusions Under the constant nitrogen application rate, after 10 years of continuous application of organic compost, the contents of heavy metals in the soil did not exceed the critical levels. The single pollution index of heavy metals in soil was 0.02–0.46, and the comprehensive pollution index was 0.23–0.36, indicating no pollution risk. Only sludge compost needs to strengthen soil Cd monitoring. The enrichment ability of heavy metals in maize stalks is greater than that in grains. The contents of Pb, Cu and Zn in maize grains are positively correlated with the corresponding heavy metal contents in soil, the contents of Cd, Pb, Hg, Zn in straw are positively correlated with the corresponding heavy metal contents in soil, and the contents of Cr and Ni in straw are significantly negatively correlated with the corresponding heavy metal contents in soil.