| The application of organic fertilizer has long been considered as an important agricultural non-point source pollutant.Whether the safe utilization of organic fertilizer can be realized is worthy of further study and discussion.Based on this,this paper studied the effects of different fertilization measures on the morphology and distribution of typical heavy metals(Cd,Pb,Cr)in farmland soil and environmental factors under different irrigation conditions.In the experiment,lettuce was used as the test crop,pig manure,chicken manure,sheep manure,cow manure and biogas residue were used as the test organic fertilizers.The soil texture in the study area was silty loam.Two irrigation levels(W1:upper and lower limits of irrigation 65%-75%,W2:upper and lower limits of irrigation 85%-95%)and6 fertilization treatments(CK-no fertilization,MOF-pig manure fertilizer,CMF-chicken manure fertilizer,SOF-sheep manure fertilizer,CMC-cow manure fertilizer,BRF-biogas residue fertilizer:40t/hm~2)were set up.The changes of heavy metals and environmental factors in soil under different sources of organic fertilizers under two irrigation conditions were discussed in depth.The structural equation model was used to explore the response mechanism between environmental factors and heavy metals.In order to provide a theoretical basis for the safe use of organic fertilizer in agriculture.(1)Organic fertilizer can significantly reduce the bioavailability of heavy metals.Under low irrigation amount,the effective form of heavy metals decreased rapidly in 0～10cm soil layer.Compared with CK treatment,organic fertilizer treatment could reduce the effective form concentration of heavy metals in soil by 23%～73%.Compared with high irrigation amount,the effective forms of heavy metals decreased greatly in 10～20 and 20～30cm soil layers,with an average decrease of 4%～59%.(2)Organic fertilizer can significantly increase the relative content of soil DOM,and high irrigation can significantly increase the relative content of DOM in deep soil.It also improves the structural characteristics of DOM.Compared with CK,the fluorescence proportion of C4 changed significantly(P<0.05)under the two irrigation levels,and moved to the deep soil with the increase of irrigation amount.(3)Under different irrigation conditions,the application of organic fertilizer to the soil will affect the mutual transformation of DOM components in the soil.Under the condition of W1 irrigation,when MOF,SOF and BRF were applied,with the increase of soil layer,some humus-like substances were degraded from macromolecular substances with complex structure to protein-like substances and finally transformed into small molecular substances with relatively simple structure.When CMC was applied under W2 irrigation conditions,with the increase of soil depth,some humic-like small molecular substances in DOM were first converted into protein-like substances and then converted into macromolecular humic-like substances.Humus-like components(C1+C2)are the key components controlling the bioavailability of heavy metals.The transformation of C1+C2 in soil corresponds to the change of effective form of heavy metals.Under the condition of W1 irrigation,the humic-like macromolecules in the surface soil first began to transform,and the effective speciation rate of heavy metals in the 0-10 cm soil layer was the largest.Under the condition of W1 irrigation,the effective speciation rate of heavy metals in 10～30 cm soil layer was the highest when the surface soil was transformed from humus-like small molecules to humus-like macromolecules.(4)Organic fertilizer had a very significant positive effect on soil mineral nitrogen,DOM components and characteristics,and ETC(P<0.001).Organic fertilizer had a significant negative effect on p H and Cd(P<0.05),and organic fertilizer had a very significant negative effect on Cr(P<0.001).Soil mineral nitrogen,C1+C2,ETC and p H had negative effects on heavy metals Cr,Pb and Cd,while DOM eigenvalue had significant positive effects on Cr,Pb and Cd(P<0.05). |
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