To investigate the effects of biogas slurry substitution for fertilizer on rice yield, fertilizer utilization efficiency, and soil fertility, a field experiment was conducted on rice–wheat rotation soil in the Chaohu Lake Basin for two consecutive years, with the following six treatments: no fertilization (CK), conventional fertilization (CF), optimized fertilization (OF), biogas slurry replacing 15% of fertilizer (15% OFB), biogas slurry replacing 30% of fertilizer (30% OFB), and biogas slurry replacing 50% of fertilizer (50% OFB). The field experiment results showed that, compared with CF treatment, OF treatment in 2022 and 2023 significantly increased (p < 0.05) rice yield, promoted the uptake of nitrogen (N), phosphorus (P), and potassium (K) by grains and straws, improved fertilizer utilization efficiency, and increased the contents of soil organic C (SOC), NH₄⁺-N, NO₃⁻-N, hydrolysable N, and available P. The 15% OFB and 30% OFB treatments significantly increased (p < 0.05) rice grain and straw yields compared with CF treatment, and rice grain and straw yields were the highest in the 30% OFB treatment. Compared with CF and OF treatments, 30% OFB treatment significantly increased (p < 0.05) the N, P, and K uptake of grains and straws and increased the fertilizer utilization efficiency. Compared with CF treatment, the grain yield of 50% OFB treatment was significantly decreased (p < 0.05) in 2022, and there was no significant difference in 2023, which may be because the biogas slurry was applied before planting in 2023 to provide more nutrients for early rice growth. Compared with CF treatment, 30% OFB treatment significantly increased (p < 0.05) the contents of SOC, NH₄⁺-N, available K, and hydrolysable N. In summary, optimizing N and K topdressing methods can increase rice yield and improve the fertilizer utilization efficiency and soil fertility. The 30% OFB treatment resulted in the highest rice yield, fertilizer utilization efficiency, and improved soil fertility, indicating that biogas slurry replacing 30% of fertilizer was the best application mode for rice in this region.

Screening for low-accumulation varieties is an effective way to reduce the cadmium (Cd) content in crops and decrease human Cd intake. In this study, a field experiment was conducted to analyze the characteristics of Cd absorption and accumulation in 24 maize varieties at maturity in farmland with different levels of Cd pollution. Cluster analysis and Pareto analysis methods were used to screen for maize varieties with low Cd absorption. In addition, the environmental threshold of Cd in farmland in the study area was estimated based on the Species Sensitivity Distribution (SSD) characteristics of different maize Cd enrichment coefficients (1/BCF). The results showed that maize in the study area was subject to varying degrees of Cd pollution, with total soil Cd content ranging from 1.81 to 2.71 mg·kg⁻¹ at test site 1 and from 0.95 to 1.76 mg·kg⁻¹ at test site 2. Only the Cd content of heavy metals in maize kernels at test site 2 did not exceed China’s national food safety standard (GB 2762-2022, 0.1 mg·kg⁻¹), with the Cd content of different maize varieties at test site 2 ranging from 0.008 to 0.073 mg·kg⁻¹ and the bioconcentration factor (BCF) of maize kernels for Cd ranging from 0.004 to 0.054. According to cluster analysis and Pareto analysis, the low-Cd accumulation maize varieties suitable for local planting were identified as Yufeng 303, Nongda 372, Jingnongke 728, MC121 (Fengda Seed Industry), and Jinyu 1233. In addition, the soil Cd environmental threshold for ensuring that 95% of maize would not be polluted by Cd in soil was derived as 1.39 mg·kg⁻¹.

To identify the optimal substitution ratio of biogas slurry to chemical fertilizer, this study determined the cumulative nitrogen use efficiency (CNUE) of wheat and carbon pool in Lime concretion black soil. The following treatments were applied: control (CK), conventional chemical nitrogen fertilizer application (CN), optimized chemical fertilizer application (ON), and biogas slurry replacing 15% (ONL15%), 30% (ONL30%), and 50% (ONL50%) of fertilizer. The results indicated that CNUE was the highest in the ONL30% treatment and 67.26–80.26% higher in the ONL15%, ONL30%, and ONL50% treatments than it was the CN treatment. The soil dissolved organic carbon of 2023–2024 increased by 11.93–22.93% compared to that in the CN treatment, and the highest particulate organic carbon content was observed in ONL30% treatment. In 2024, the carbon pool management index was 22.20, 42.42, and 29.34% higher in ONL15%, ONL30%, and ONL50% treatments than it was in CN treatment, respectively. In summary, biogas slurry replacing 30% of fertilizer regulated the carbon pool in Lime concretion black soil and improved the yield, quality, and cumulative nitrogen use efficiency of wheat, which was the optimal substitution ratio of biogas slurry to chemical fertilizer in the Northern Anhui Plain of China.