Effects Of Liming And Straw Return On Biological Nitrogen Fixation In An Acidic Paddy Soil

Rice is the most important food crop in China and plays a vital role in ensuring food security.The southern rice region is one of the major rice producing areas in China.However,environmental and human factors have led to increasingly severe soil acidification in southern rice fields,limiting the sustainable improvement of rice production.Our monitoring of a localization experiment on typical acidic rice fields(the localization experiment started in 2015 with one application of cooked lime(Ca（OH）₂at2.1 t hm^-2and 2.0 t hm^-2before early rice planting in 2015 and after late rice harvest in2018,respectively)showed that lime application and straw return had a synergistic contribution to double-season rice yield improvement,mainly because lime improved the nitrogen uptake of rice.Although lime application significantly improved rice nitrogen uptake and promoted ammonia volatilization in paddy fields,lime application did not have a significant effect on soil nitrogen content.We hypothesize that lime may have promoted nitrogen input to the paddy field.Biological nitrogen fixation is an important source of nitrogen in paddy ecosystems.However,current studies have mainly focused on the effects of lime application and straw return on soil fertility and rice yield,while the effects of lime application and straw return on biological nitrogen fixation in acidic paddy soils have been less studied.Therefore,in this study,Xiangzaoxian 45 was used as the early rice test variety and Meixiangzhan 2 was used as the late rice test variety to continued on the locational trial in 2021.There are four treatment groups in this experiment,which are:（1）control,no lime and no straw returned to the field.（2）Lime treatment is applied,and the straw was not returned to the field.（3）Straw return treatment,full amount of straw in situ per season.（4）Straw return treatment with lime application.The main results were as follows:Both lime application and straw return significantly increased the yield of rice.Among them,lime application increased the yield of early and late rice by 3.42%and2.72%,respectively,while straw return to the field increased the yield of early and late rice by 6.67%and 4.73%,respectively.Lime application significantly increased the nitrogen uptake of early rice by 10.55%,and straw return significantly increased the nitrogen uptake of late rice by 14.70%.There was no significant reciprocal effect of lime application and straw return on nitrogen uptake in rice.Lime application significantly increased the pH of the soil.Both lime application and straw return can significantly reduced the concentration of soil exchangeable H⁺,exchangeable Al³⁺and total exchangeable acid.Lime application had no significant effect on nifH gene copy number.Straw return significantly increased the nifH gene copy number by 56.3%.Lime application significantly increased the rate of nitrogen fixation by soil organisms by 60.8%.There was no significant effect of straw return on soil biological nitrogen fixation rate.Lime application significantly changed the community structure composition of nitrogen-fixing microorganisms in acidic rice fields,mainly increasing the relative abundance of Halochromatium,Sideroxydans,Geobacter and Thiothrix.Straw repatriation did not significantly affect the community structure composition of nitrogen-fixing microorganisms in acidic rice fields,but significantly increased the relative abundance of iron-oxidizing spp.In addition,in order to verify the results of the soil culture of the localization test,we conducted a soil culture test with the addition of lime and straw,and also set up four treatments,namely:（1）control,no lime and no rice straw;（2）Treatment with lime;（3）Treatment with rice straw;（4）Adding rice straw with lime treatment.The main study results were as follows:the results showed that both lime addition and rice straw addition can significantly reduced the concentration of soil exchangeable H⁺,exchangeable Al³⁺and total exchangeable acid.The addition of rice straw significantly increased the nifH gene copy number.Both the addition of lime and the addition of rice straw significantly increased the rate of biological nitrogen fixation in the soil.The addition of rice straw mainly promoted the growth of heterotrophic nitrogen-fixing microorganisms.In conclusion,lime application and straw return significantly increased the yield of rice.Lime application significantly increased the rate of soil biological nitrogen fixation and significantly changed the community structure of nitrogen-fixing microorganisms in acidic paddy soils.Lime application promoted biological nitrogen fixation in acidic paddy soils,and the effect of lime on biological nitrogen fixation effect should be considered in the research of nitrogen management and soil improvement in acidic paddy fields.