Effects and Comprehensive Evaluation of Different Organic Fertilizer Substitution Ratios on Maize Yield, Soil Organic Carbon Fractions, Nutrients and Heavy Metals

15 Organic fertilizer substitution technology is an effective measure to solve the excessive application of chemical 16 fertilizers in agricultural production. A pot experiment was set up with 5 treatments: no fertilizer (CK) and 17 organic fertilizer substituting 0% (CF), 8% (OF8), 16% (OF16), and 24% (OF24) of chemical N fertilizer to 18 analyze their effects on maize yield, soil organic carbon (SOC) and its fractions, carbon pool management index 19 (CPMI), nutrients and heavy metals to provide a scientific basis for safe fertilizer application to maize. This 20 study found that OF8, OF16, and OF24 all increased the content and proportion of SOC and labile organic C 21 (LOC) fractions, CPMI, most of the middle and trace elements, and heavy metals content and their pollution 22 indices in soil and grain compared to CF. Grain was more vulnerable to pollution compared to soil. There was a 23 strong positive correlation between the content of middle and trace elements, and heavy metal, SOC and its 24 fractions (except LLOC), and organic fertilizer substitution ratio, all with no significant correlation with yield. 25 OF8 and OF16 promoted maize growth with a significant increase yield of 35.65% and 30.28% ( P <0.05), 26 respectively. A comprehensive analysis determined the optimum substitution ratio of 8% (OF8), which can 27 reduce chemical fertilizer and increase yield, improve soil fertility, low heavy metal pollution risk, is beneficial 28 to promote sustainable agricultural development. effects on and yield; effects of and its effects of theoretical basis for reducing fertilizer and yield,


Introduction
4 more scientific method for using LOC and SOC to jointly evaluate the quality of the organic C pool (Benbi et al.

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In recent years, with the development of China's "Farmland Soil Quality Improvement Project" and the 63 implementation of "Zero Growth Strategy for Chemical Fertilizer", the demand and application amount of 64 organic fertilizer showed an increasing trend. Organic fertilizer not only contains rich nutrient elements for plant 5 of heavy metals in agricultural products, and the degree of influence depended on the amount of commercial 76 organic fertilizer applied. Zhang et al. (2021a) found that organic fertilizer substitution increased the content of 77 Cu and Zn in wheat for two consecutive years, which was lower than the food standard limit in China. At present, 78 research of organic fertilizer substitution on heavy metals mainly focuses on farm manure, and there is little 79 research on commercial organic fertilizer and its different substitution ratios. Most of the above studies focused 80 on the effects of organic fertilizer substitution on SOC and CPMI or heavy metals, while few attentions were 81 paid to the effects of commercial organic fertilizer substituting part of chemical fertilizer on them.

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(i) the effects on maize growth and yield; (ii) the effects of SOC and its fractions, and CPMI; (iii) the effects of 85 nutrients and heavy metals in maize grain and soil; (iv) and through heavy metals risk assessment of soil and 86 grain and comprehensive analysis, and determine the optimum organic fertilizer substitution ratio. It can provide 87 a theoretical basis for reducing chemical fertilizer and increasing yield, improving soil fertility, low risk, and 6 Chemical fertilizer uses urea (46.0% N), diammonium phosphate (18.0% N, 46.0% P2O5), potassium sulfate 98 (51.0% K2O), the commercial organic fertilizer (26.10% C, 1.77% N, 1.95% P2O5, and 0.53% K2O).

Experimental design 100
Before the start of the experiment (early April 2019), the pot experimental soil was selected from the 0-20 101 cm soil of the experimental station, and the fresh soil was air-dried and screened with a sieve of 5 mm to remove 102 stones, rhizomes, and other sundries. The compactness of the soil was the same as that of the field soil when the

Sample collections and chemical determination 118
Before the start of the experiment, the mixed soil samples were air-dried and sieved to determine the basic 119 physical and chemical properties of the soil. A destructive sampling at the jointing (V6), tasseling (VT), and 120 maturity (R6) stages, collecting soil samples and different organs of maize. The dry matter of maize was 121 determined by the drying method, and the content of total N, P, and K in the plant and the content of SOC, total 122 N, P, and K in soil were determined as reference (Bao, 2000). The total amount of the middle elements (Ca, Mg, 123 S, g/kg), trace elements (Mo, B, Fe, Mn, mg/kg), and heavy metals (Cu, Zn, Cd, Cr, Pb, As, and Ni, mg/kg) in  148 Where Pi is the heavy metals pollution index, Ci is heavy metals content; Si is the limit standard of heavy 150 metals content in soil and grain, referring to China's latest limit standard for soil pollution (GB 15618-2018) and 151 food safety (GB 2762-2017) ( Table 2). Pi ≤ 1 is non-pollution level, 1 < Pi ≤ 2 is light pollution level, 2 < Pi ≤ 3    (Fig. 1). Therefore, a suitable organic fertilizer substitution ratio (OF8 and OF16) can promote 174 maize dry matter accumulation and nutrient absorption, thereby increasing yield.

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Maize yield was positively correlated (P <0.05) with grain N, P content, and soil N content, while there was 233 no significant correlation (P >0.05) with the content of middle and trace elements, and heavy metals. This

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indicates that the effect of N, P on yield was greater than that of middle and trace elements under the condition 16 and OF8, (iii) OF16 and OF24; and grain divided into (i) CK and CF, (ii) OF8, and (iii) OF16 and OF24 (Fig. 7).

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In short, although the heavy metals pollution index of grain and soil after organic fertilizer substitution treatment 254 did not reach the pollution level, the pollution risk increased with the increase of the organic fertilizer 255 substitution ratio, and the grain was more vulnerable to pollution than soil.

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In this study, the organic fertilizer substitution treatments (OF8, OF16, OF24) obviously increased the content of 261 SOC and its fractions during the growth stage (V6, VT, and R6), and its content was significantly (P <0.05) 262 positively correlated with the organic fertilizer substitution ratios. This is because the application of organic 263 fertilizer directly inputs organic C into the soil, increasing microbial activity, accelerating the turnover of organic

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Organic fertilizer is not only rich in nutrient elements, but also contains a certain amount of heavy metals.

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Organic fertilizer application and its amount are key factors affecting the content of heavy metals such as Cr, Cu,

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Zn, Cd, Pb and As in soil and agricultural products (Wu, et al. 2020), therefore, clarifying the effect of different 298 organic fertilizer substitution ratios of heavy metals in soil and grain can provide a safe fertilizer application for 299 maize provides a scientific basis. This study found that organic fertilizer substitution treatments obviously 300 increased the content of middle and trace elements, and heavy metals in maize grain and soil, and were 301 positively correlated with the organic fertilizer substitution ratios (Fig. 6). This indicates that their contents were 302 affected by the amount of organic fertilizer applied, which is consistent with the results of some studies (Ning et