Study On Reduction In Nitrogen And Phosphorus In Arbuscular Mycorrhiza-paddy Field Ecosystem

As non-point source pollution, nitrogen(N) and phosphorus(P)loss from paddy fields in Northeast China is very serious due to excess fertilization and low nutrient use efficiency. Supported with the theories in Agricultural Ecology, Plant Physiology and Environmental Engineering, our study systematically analyzed the feasibility of reduction in N and P loss from paddy fields by inoculation with arbuscular mycorrhizal fungi(AMF)and quantified the effect of AMF inoculation on N and P input and loss at paddy fields in Northeast China.The abundance of native AMF was investigated and the biotic and abiotic factors limiting AMF growth were analyzed in the experimental site. This led to the comparison between two strains of AMF, Glomus Mosseaeand Glomus Intraradices in promoting the growth of rice seedlings, indicating the dependency of rice on AMF-rice symbiosis. Thetolerance for limiting factors of AMF growth at paddy fields were estimated for the two strains of AMF in order to assess the stability of AMF-rice symbiosis at paddy fields. After that, a field experiment was carried outat paddy fields to quantify the effect of AMF-rice symbiosis on reduction in N and P input and loss at paddy fields.We found that the percentage of root length colonization(RLC) of native AMF in rice roots was low, in a range between 2% and 3%. We found that soil with extremely high N and P availability and long-term floodingall contributed to the low abundance of native AMF. Additionally, analysis on the strategy for increasing the abundance of AMF indicated that AMF inoculation and proper N and P management would lead to successful construction of AMF-rice symbiosis. Better improvements in seedling shoots and roots were found for rice inoculated with Glomus Mosseaethan that inoculated with Glomus Intraradices. Furthermore, it was found that inoculation with Glomus Mosseae led to higher percentage of RLC under high N and P availability and long-term floodingand that higher percentage of RLC was observed for rice inoculated with Glomus Mosseae than that with Glomus Intraradicesafter the recovery from long-term flooding..At maturity, it was found that AMF inoculation increased the biomass allocated to shoots at whole-plant level and then to panicles at shoot level. In addition, we observed that rice inoculatedwith AMF absorbed and allocated more N and P to panicles than their counterparts did, especially under lower N and P supply. The observed economic yield of rice was the greatest without AMF inoculation when N and P input reduced to 80% of the local normof fertilizationwhereas the maximum yield was predictedto occur when N and P input reduced to 85%. Additionally, theeconomicyield of rice without inoculationdecreased with declining N and P input when fertilization was below 85% of the local norm of fertilization. Moreover, we found that there were more seeds with better quality produced in AMF inoculated rice. Importantly, the positive effect of AMF inoculation on rice yield was greater under low nutrient availability than that under high fertilization. N and P input could further cut down 23.5% due to AMF inoculation while high rice yield maintained; while with the same amount of N and P input, AMF inoculation resulted in more economic benefits.Under the local norm of fertilization, the concentrations of total nitrogen(TN) and total phosphorus(TP) were higher than the threshold of Grade V in Quality Standard of Surface Water Environment of China, which increased the risk of pollution of Lalin River and then Songhua River. During rice growing season, the total N and Ploss were 18.9 kg/haand 3.7 kg/ha, respectively, which accounted for 7.9% and 3.5% of annual fertilization. Specifically, dissolved N(DN) was the main form of Nloss while most P in drainage was particle P(PP). We also found under the local water management rainfall and artificial drainage were the main causes of drainage. A small reduction in N and P input, less than 20%, only led to a decrease of N and Ploss by 8.2% and 8.9%, respectively, while further reduction in fertilization(more than 40%) resulted in a decrease in rice yield. Consequently, fertilizer management could not cut down nutrient runoff largely without scarifyingthe economic yield.In Lalin River basin, it was found that the mean concentrations of TN and TP in drainage from paddy fields with non-inoculated rice were higher than that with AMF inoculated rice. In addition, AMF inoculation decreased the concentrations of particle N, dissolved N, particle P and dissolved P. The role of AMF inoculationwas observed in the rice-growing season and it was season-dependent. Specifically, the effect of AMF inoculation was not observed at the initial period of transplantation and during the long period of continuous flooding; while the effect was most significant after the fertilization was finished. The most remarkable decrease in nutrient loss was observed in paddy fields with 80% of fertilization, with N and P cut down by 5.13 kg/haand 1.79 kg/ha, accounting for 27.2% and 44% of the cumulative loss under the local norm of fertilization. The input of N and P from Lalin River to Songhuajiang River would be reduced 143.6 t and 50.1 t, respectively when AMF-rice symbiosis was constructed at all the paddy fields in Lalin River basin.Engineering experiences and theoretical achievements presented inour study systematically elaborated the theory and method of construction of AMF-rice symbiosis to control N and P loss from paddy fields, which provided powerful guidance and reference for spreading the application of AMF-rice symbiosis. The application of AMF-rice symbiosis successfully reduced the N and Pinput and loss at paddy fields, leading to both economic and environmental benefits.