Study On The Effect Of Water Quality In Rice-crab Culture Model

The level changes of water environment factors were studied in rice-craband rice monoculture system. A ring ditch was dug around the paddy inside, werandom selected thee points in ditch, edge and field respectively to collect watersample. We investigated water environment factors include dissolved oxygen （DO）,temperature （T）, pH, nitrate-nitrogen （NO₃-N）, nitrite-nitrogen （NO₂-N）,ammonium-nitrogen （NH₃-N） and orthophosphate （PO4-P） at four rice growth stages（tillering, jointing, flowering and filling stage）. Results indicate that, T, pH and DO ofrice-crab culture have no significant difference at four rice growth stages （p>0.05）,but DO in rice monoculture significantly different （p <0.05） at jointing and fillingstage, at flowering and filling stage, DO of ditch, edge and field in rice-crab havesignificant difference （p <0.05） with DO of ditch, edge and field in rice monoculturerespectively, pH and T of ditch, edge and field in rice-crab have no significantdifference （p>0.05） with that in rice monoculture respectively at whole rice growingstages. At filling stage NO₂-N, NH₃-N and PO4-P show significant difference （p <0.05）; at flowering and filling stage, nitrate-nitrogen （NO₃-N） of ditch, edge and fieldin rice-crab have significant difference （p <0.05） with nitrate-nitro gen （NO₃-N） ofditch, edge and field in rice monoculture respectively. Overall, compared with ricemonoculture, the dissolved oxygen of rice-crab is lower and the NO₂-N, NH₃-N,PO4-P in ditch are higher than field.Concurrent rice-crab system as a new ecosystem that has only been practiced atexperimental stage, which is an agriculture cropping system that integrate crab intorice culture, and has only been trailed in several areas, especially in northeast of China.Rice-crab culture experiment was conducted to determine the diel fluctuation of waterquality, i.e. dissolved oxygen （DO）, temperature （T） and pH under different crab larvae stocking density at four rice growth stages （tillering, jointing, flowering andfilling stage） with three treatments having three replicates for each:（1） ricemonoculture （RM）,（2） rice-crab with low stocking density （RCL）,（3） rice-crab withhigh stocking density （RCH）; low and high stocking density with500and1500crablarvae, respectively. The concentration of DO not differ significantly from each other（p>0.05） expect filling stage （p <0.05）, pH levels did not have any difference amongtreatments at June19, Jul17, Aug14（p>0.05）, but values at Sep16showedsignificant difference （p <0.05） between rice only and rice-crab culture. It seemedthat the presence of crab decreased DO and pH levels. Temperature did not changesignificantly through rice growing seasons. We also investigated the variations ofnitrate-nitrogen （NO₃-N）, nitrite-nitrogen （NO₂-N）, ammonium-nitrogen （NH₃-N） andorthophosphate （PO4-P） through four rice growth stages. The results showed theconcentration of nitrogen was high after fertilizing and then descended rapidly, forrice utilizes nitrogen most at tillering stage. Concentrations of NO₂-N had significantdifference among RM and RCH at Jul17, and had significant difference between riceonly and rice crab culture at Sep16. Concentrations of NO₃-N had significantdifference （p <0.05） among RM and RCH at Jul17.Concentrations of NH₃-N andPO4-P did not differ significantly （p <0.05） among treatments at June19, Jul17, Aug14and Sep16. NH₃-N showed an increasing trend as the days of culture increased,probably due to higher organic decomposition and organic load. Concentrations ofnitrogen and phosphorus in High stocking density were higher than two othertreatments. There was no significant effect of presence of crab on yield of rice paddy,but probably due to the mechanism of “Uninterruptible fertilization”（UPF）, yield ofrice-crab had a little improvement compare with rice monoculture. In general, lowstocking density was considered reasonable in rice-crab culture system.This study is a survey of agriculture practices and pest management strategiesamong rice monoculture and rice-crab farmers in Liaoning province. Based on andeconomic and ecological economic comparison between different farmer categories, itis argued that rice-crab farming provides a sustainable alternative to rice monoculture.The aim should be to reduce the resource use, avoid overuse of agrochemicals andimprove production efficiency through increased recycling of nutrients and matter.The results showed that rice-crab farmers had higher net income than ricemonoculture farmers, and had significant difference with rice monoculture, andrice-crab culture reduced agrochemicals and fertilizer use, improved income of farmers effectively.