Effects of tillage measures and organic manure on soil heavy metal pollution in dryland wheat fields

Heavy metal pollution in agricultural soils is a crucial environmental problem. In this study, we investigated the effects of different tillage measures and organic fertilizers on the accumulation of heavy metals in dryland wheat fields and their ecological efficiency based on a long-term tillage and fertilizer experiment (since 2007) with wheat cultivation. Four treatments were applied: deep tillage + chemical fertilizer (T), deep tillage + chicken manure + chemical fertilizer (TM), no tillage + chemical fertilizer (NT), and no tillage + chicken manure + chemical fertilizer (NTM). The effects of these treatments on the physicochemical properties of soil at 0–20 cm depth (pH, electric conductivity, organic matter content, and total nitrogen content) and heavy metals (Pb, Cd, As, Hg, Cr, Cu, Zn, and Ni) contents were investigated. The heavy metal geoaccumulation index and potential ecological risk indice were used to evaluate heavy metal pollution and ecological risk. The results showed that TM, NT, and NTM treatments significantly decreased soil pH by 1.10% to 2.56% and increased soil electric conductivity by 6.19% to 57.08% compared to T. Soil organic matter content increased by 33.22% in NTM treatment compared to that in T treatment. The application of chicken manure significantly affected soil heavy metals contents. The total Hg content of soil was significantly increased by 123.60% and 150.56% in TM and NTM treatments compared with that in T treatment, and the available Cu, Zn, and Cd contents increased significantly by 16.89% to 23.48%, 219.04% to 520.99%, and 2.90% to 20.29%, respectively. The tillage measure also had a significant effect on soil heavy metals contents. Compared to those under TM treatment, the contents of total Hg, total Zn, available Cu, available Zn, and available Cd showed significant increases of 12.06%, 8.11%, 5.64%, 94.65%, and 16.90% under NTM treatment, respectively; whereas the contents of total Pb, available Pb, and total Cr contents significantly reduced by 63.74%, 70.00%, and 3.14%, respectively. The Hg content showed the highest potential ecological risk index of all treatments, expressed as T (32.36) < NT (41.45) < TM (72.36) < NTM (81.09). Other heavy metals were at a low level, with the comprehensive potential ecological risk pollution index expressed as T (79.05) < NT (82.33) < TM (115.27) < NTM (120.00). In summary, long-term no-tillage and chicken manure application significantly decreased pH; increased electric conductivity, contents of organic matter and total nitrogen; and increased Cu, Zn, and Cd availability. The individual and potential ecological risk indices for Hg pollution were at the moderate to strong level when treated with chicken manure, whereas the other heavy metals were at a light level. Moreover, the composite and potential ecological risk indices differed slightly, indicating the requirement to strengthen the safe application of organic fertilizers.