Effects of integrated rice-crayfish co-culture on soil fertility and metal element content

The integrated rice-field aquaculture (IRFA) has been practiced successfully for 2000 years in China. The integrated rice-crayfish (Procambarus clarkii) co-culture (IRCC) has contributed more than 50% of the yield of IRFC, currently. Thus it is necessary to conduct researches on the IRCC, including its’ environmental impacts. As commercial feed feeding is very popular in IRCC practice, one of the potential environmental problems is the effects of feeding on the water and soil quality. Some previous studies have indicated that most nitrogen and phosphorus in the feed deposit in soil of crustacean aquaculture system. While it is not clear whether IRCC can increase nitrogen and phosphorus in soil, even some researches have been done. And it’s also not clear the effects of IRCC on the metal element contents in soil. The purpose of this study was to evaluate the impacts of IRCC in soil fertility (nitrogen, phosphorus and organic matter contents), and metal element contents on soil. Six physicochemical indicators and 11 metal elements contents of three layers of soil (0-10 cm, 10-20 cm and 20-30 cm) of the trench (TRC) and paddy (PRC) of IRCC, and rice monoculture fields (RM, as control) in Qianjiang City, Hubei Province, China, were measured, and the culture ages of the two modes were both 5 years. Contents of soil organic matter (SOM) and available phosphorus (AP) were measured by potassium dichromate heating oxidation-volumetrics, and NaHCO₃ extraction, ammonium molybdate-tartaric emetic-ascrbic acid colorimetry method, respectively. Contents of total nitrogen (TN), ammonia nitrogen ( \rmNH_4^ + -N), nitrate ( \rmNO_3^ - -N) and total phosphorus (TP) were measured by flow injection analysis, and contents of Cr, Cu, Cd, Pb, Zn, Ni, As, Fe, Mg, Ca and Mn were measured by microwave digestion-inductively coupled plasma-mass spectrometry. The contents of Cr, Cd, Pb, Zn, Ni, As and Cu were then evaluated according to the risk screening values for soil contamination of agricultural land in <GB15618—2018>. The results showed that the TN, AP and TP content of all layers exhibited a trend of PRC > RM > TRC, while SOM (excepting 20-30 cm layer) and NH₄⁺-N of RM > PRC > TRC and \rmNO_3^ - -N of TRC > PRC > RM. The content of Cu, As, Mg and Mn of all soil layers showed a trend of TRC > PRC > RM, while Cd, Pb, Fe and Ca on the contrary, and less significant differences were noted among TRC, PRC and RM for Cr, Zn or Ni, excepting for 0-10 cm longer. According to the risk screening values for soil contamination of agricultural land in <GB15618—2018>, Cr, Cd, Pb, Zn, Ni and As contents were up to standard, while Cu content was generally out of the limits, with over-limit ratios of 14%-46%. The results indicate that integrated rice-crayfish co-culture could increase nitrogen and phosphorus contents in the paddy soil, while decrease those in the trench soil, and SOM content.