Pig-biogas residue is widely used as organic fertilizer for rice (Oryza sativa L.) in China. To evaluate the risk of heavy metal contamination in paddy soil and rice grains caused by long-term pig-biogas residue fertilization, this study was conducted in paddy fields which had been fertilized continuously with pig-biogas residue for 8 years. We found that pig-biogas residues contained high concentrations of arsenic (As), cadmium (Cd), copper (Cu) and zinc (Zn). As a result, the concentrations of these four metals in paddy soils and rice grains sampled from biogas residue fertilized fields were significantly higher than those from control fields. In addition, in biogas residue fertilized fields, the concentrations of these four metals rapidly increased in the tillage layer compared with those in deep soil layers, and biological availability was significantly higher than in control fields. Moreover, we found that the accumulation of these four metals in paddy soils and rice grains increased in the second year compared with the first year. To reduce heavy metal accumulation in rice grains, we screened for rice genotype among 20 local rice genotypes. We found obvious genotypic differences in grains’ accumulation of heavy metals, and identified some rice genotypes which had low accumulation of multiple heavy metals. These low-accumulating genotypes predicate the possibility to reduce heavy metal accumulation in rice grains grown in pig-biogas residue fertilized fields.

This research was to examine the impacts of long-term integrated rice–crayfish farming on soil nutrients, carbon pools, and rice yields in paddy fields. The aim was to establish a scientific basis for the sustainable development of RS in the northern region of Zhejiang. The results showed that the change from rice monoculture (CK) to rice–crayfish coculture systems (RS) led to a 24.99% increase in the 5-year average of soil ammonium nitrogen (AN), while the soil nitrate nitrogen (NN), available potassium (AK), and available phosphorus content (AP) decreased by 28.02%, 16.05%, and 28.76%, respectively. Moreover, the total organic carbon (TOC), easily oxidizable organic carbon (EOC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) exhibited a reduction of 2.45%, 8.82%, 35.31%, and 65.84%, respectively. Correlation analysis revealed a significant positive correlation between NN, EOC, and MBC in the RS mode. In terms of rice yield, the 5-year average of rice yield in RS decreased by 8.40% compared to CK. The mean yield of early-maturing rice varieties was reduced by 13.16%, while that of late-maturing rice varieties was reduced by 6.00%. These results shed light on the annual variation in soil nutrients, carbon pools, and rice yield in the RS mode, providing insights for the sustainable development of RS in northern Zhejiang.

This research was to examine the impacts of long-term rice-crayfish coculture systems(RS) on soil nutrients, carbon pools, and rice yields in paddy fields , aiming to establish a scientific basis for the sustainable development of RS in the northern region of Zhejiang. The results showed that the change from rice monoculture (CK) to RS resulted in a 24.99% increase in the 5-year average of soil ammonium nitrogen(AN), while the soil nitrate nitrogen(NN), available potassium(AK), and available phosphorus content(AP) decreased by 28.02%, 16.05%, and 28.76% respectively. Moreover, the total organic carbon(TOC), easily oxidizable organic carbon(EOC), dissolved organic carbon(DOC), and microbial biomass carbon(MBC) decreased by 2.45%, 8.82%, 35.31%, and 65.84% respectively. Correlation analysis showed that there was a significant positive correlation between NN, EOC and MBC in the RS. In terms of rice yield, 5-year average of rice yield in RS has decreased by 8.4% compared to CK.The average yield of planting early-maturing varieties of rice was reduced by 13.16%, while the average yield of planting late-maturing varieties of rice was reduced by 6.00%. In conclusion, enhancing water and nutrient management and cultivating late maturing rice varieties are essential strategies for achieving the sustainable development of RS in northern Zhejiang.