Migration And Transformation Of Nitrogen In Rice-crab Culture Systems

Rice is the most widely crop cultivated in the world,and about 40%of the population obtain carbohydrates from rice.China is the world’s largest producer of rice,while it has a large demand for rice.At present,fertilization is the most effective method to increase rice yield,but the efficiency is often less than 50%,causing the problems of wasting resources and environment pollution.Crab is one of the most important crustaceans in China,which has extremely high economic value because of its delicious taste.Therefore,the crab farming is rolling full steam ahead.The formula feed is relied on for higher output and benefits,which is consumed by crab less than 30%.Therefore,the N,P and organic particulates in crab aquaculture water will overload the water,increasing the risk of crab diseases and causing environmental pollution.Rice-crab culture system combines rice planting and crab breeding well.Rice provides a good living environment and diversified bait for crab,reducing the input of artificial compound feed.Meanwhile crab accelerates the turnover of organic matters by feeding on the aquatic species and further supplies nutrition to rice continuously.Moreover,the disturbance of crab in paddy fields promotes the absorption of nutrients by rice roots.Thus,the combination of rice and crab has the characteristics of improving the efficiency of resource,reducing the loss of nutrients,increasing rice yield,and improving overall benefits.At present,the rice-crab culture is in full swing,sharing is in full swing throughout the country.However,there is little knowledge about the migration and transformation characteristics of Nitrogen（N）in soil-water-air systems,its impact on the environment and the relationship between rice growth and N,and so on.Therefore,a field experiment was carried out in Panjin,Liaoning Province on a split-plot design with two factors,no crab and crab as the main factors,with and without fertilizer as sub-factors.The treatments included:（1）rice monoculture without fertilizer（R0M）,（2）rice-crab culture without fertilizer（R0C）,（3）rice monoculture with fertilization（R1M）,（4）rice-crab culture with fertilizer（R1C）,with three replications each.The main contents of this study were:（1）the inorganic N,microbial nitrogen（MBN）and enzyme activity of the soil;（2）N dynamics in flooding water;（3）N losses including NH₃ volatilization,N₂O emission and N leaching;（4）rice growth,yield and fertilizer use efficiency;（5）crab growth and yield attributes;（6）N balance.The results would provide theory basis for increasing N efficiency and reducing N input and N losses in rice-crab culture system.The results were outlined below:1.Fertilization could significantly increase pH and the concentrations of NH₄⁺-N and NO₃^--N in soil（P<0.05）,but had little effect on TN concentrations in soil.The concentration of NH₄⁺-N in the soil in R1C treatment was 13.71 mg·kg^-1,8.8%more than R1M treatment（P<0.05）,but pH as well as the concentrations of NO₃^--N and TN in soil did not differ between the two treatments（P>0.05）.There was no significant difference in pH as well as the concentrations of NH₄⁺-N,NO₃^--N and TN in soil between R0M and R0C treatments.2.The soil MBN increased firstly until peaked at the tillering stage,and decreased subsequently.Urease and catalase activities decreased firstly,then increased later,and declined at last.Proteinase and hydrogenase activities increased at first,then decreased.The concentration of MBN in the 0²0 cm soil layers,the activities of urease,protease and dehydrogenase in the 0²0 cm soil layers as well as the activity of catalase in 0¹0 cm soil layer were improved by use of fertilizer.Compared with R1M treatment,R1C treatment significantly increased the MBN content in the 0²0 cm soil layers,whereas there was little difference between R0M and R0C treatments.The activities of urease,protease in the 0¹0 cm soil layer and dehydrogenase activity in the 0²0 cm soil layers were significantly higher in rice-crab culture system when compared with the rice monoculture system,but there were no difference in the activities of urease,protease in the 10²0 cm soil layer and catalase activity in the 0²0 cm soil layers between the two culture systems.There were significant positive correlations among the four enzyme activities,except for the uncorrelated relation between catalase and dehydrogenase activities.The four enzyme activities were significantly positively correlated with soil MBN content.The activities of urease and protease were significantly positively correlated with the concentrations of NH₄⁺-N and NO₃^--N which were not relate to dehydrogenase activity.The dehydrogenase activity had negative correlation with NO₃^--N content,but had no correlation with NH₄⁺-N content.The results suggest that rice-crab culture system could improve the soil MBN and enzyme activities to a certain extent and stimulate the activity of the enzyme in soil nitrogen transformation,which could promote the effectiveness of nitrogen during the rice growth.3.NH₄⁺-N was the primary form of N in flooding water,accounting for 50.8%of the TN,while NO₃^--N contributed the most to TN leaching,accounting for 58.5%of TN.Fertilization could significantly increase N concentrations in flooding and leaching water（P<0.05）.Rearing crab could significantly increase the concentration of MBN in soil and reduce leaching losses of NO₃^--N（P<0.05）,but had little effect on N concentrations in flooding water and leaching losses of NH₄⁺-N as well as dissolved organic nitrogen（DON）.The concentration of NO₃^--N in the leaching water showed positive correlation with NO₃^--N in flooding water（P<0.01）,while the concentration of DON in the leaching water was negatively correlated with the concentration of soil MBN（P<0.01）.The cumulative TN leaching of R1M and R1C was 7.6%and 6.3%,respectively,which indicated that the N leaching was not the predominant fertilizer N losses.Rearing crab in rice fields could reduced the cumulative TN leaching 15.0%at fertilized plots（P<0.05）,and 7.2%at no fertilized plots（P>0.05）.4.Seasonal ammonia volatilization（AV）losses from R0M,R0C,R1M and R1C fields were 8.56,7.37,45.64 and 41.34 kg N·hm^-2,respectively.N fertilization was the dominant factor which significantly affected AV losses from the treatments.AV losses from R1M and R1C treatments were 4.33 and 4.65 times than those of R0M and R0C,respectively.In fertilized rice fields,more than 67.6%of the AV losses occurred during10 days after flooding.The rates of AV losses from no fertilized paddy field were small and stable.Raising crab could significantly decreased AV losses from rice fields.AV loss from R1C was 28.4%lower than that from R1M treatment after the crab rearing in paddies,but there was little difference between them during the whole rice growth period.The difference of AV was not significant between them under no fertilization.The rates of AV losses were significantly correlated with the concentrations of pH and NH₄⁺-N in flooding water.On the contrary,there was a significantly negative correlation between the amount of AV losses and the N accumulation in rice plants.AV losses from R1M and R1C accounted for 28.5%and 26.0%of the seasonal N inputs,respectively.5.No obvious seasonal trends were observed in N₂O emission,the peak of which appeared at the returning green stage and harvesting stage of rice.Fertilization significantly reduced the emission rates and the cumulative emissions of N₂O.Under fertilization practices in paddy fields,cumulative emission of N₂O in R1C treatment was significantly lower than R1M treatment（P<0.05）,while cumulative emission of N₂O in R0C treatment was slightly higher than R0M（P>0.05）.The GWP-N₂O of R0M,R0C,R1M and R1C treatments were 29.7,33.8,211.2,151.0 kg CO₂ eq·hm^-2,respectively.N₂O flux was showed positive correlation with water DO and soil NO₃^--N,but negative correlation with soil temperature.6.Compared to rice monoculture,rice-crab culture significantly increased spike number,grain number/spike and yield of rice（P<0.05）.There was a significant positive correlation between rice yield and N accumulation.The survival rate and yield of crab in R1C treatment were significantly higher than that in R0C,while the feed conversion rate was significantly lower than that of R0C treatment.The size was slightly lower than those of R0C treatment（P>0.05）.The N fertilizer agronomic efficiency（NAE）,N fertilizer recovery efficiency（NRE）,N fertilizer use efficiency（NPE）,and N fertilizer partial productivity（PFP）of rice-crab culture system were 9.8%（P>0.05）,7.8%（P>0.05）,2.2%（P>0.05）and 12.5%（P<0.05）higher than those of the rice monoculture system,respectively.In addition,the soil N dependence rate（SNDR）and contribution rate of basic soil productivity to yield（BSPCR）in rice-crab culture system were 5.3%（P>0.05）and 2.8%（P>0.05）higher than those of rice monoculture system,respectively.7.Compared to R1M treatment,cumulative emission of N₂O and N leaching losses in R1C treatment were lower in the whole rice growth period,and the AV was not significantly different between the two treatments.There was no significant difference in AV,N₂O emission and N leaching losses between R0M and R0C treatments.However,the total N loss was lower in rice-crab culture system,compared to the rice monoculture system.Rice-crab culture significantly improved N accumulations of rice and crab,as well the capacity of soil N supply.In addition,rice-crab culture weakened the dependence of rice on fertilizer N,thus reduced the input of fertilizer N potentially.Therefore,in the increasingly serious situation of thin resources and environmental pollution,the combination of planting and breeding,with their respective advantages,will fully exploit the functions of different agricultural systems to improve the circulation rate and efficiency of N in the system,and achieve sustainable development of agriculture.Therefore,rice-crab culture system has played a positive role in rice production,N fertilizer efficiency,feed utilization,N losses and environmental pollution,and is suitable for promotion and application.