Effects Of Green Fertilizer Planting And Returning On Coastal Saline-Alkali Soil Characteristics And Greenhouse Gas Emission

Soil salinization is one of the main problems causing soil degradation,which has a negative impact on the sustainable development of agriculture and poses a serious threat to the ecological environment and economic development.Saline-alkali soil is also an important reserve cultivated land resource in China.It is of great significance for promoting agricultural economic development and utilize coastal saline-alkali land rationally.Biological measure is one of the most economical,effective and sustainable methods for the improvement of saline-alkali soil at present.Green fertilizer crops have become an important means for the improvement of saline-alkali soil biological fertilizer due to their strong adaptability,lush branches and leaves,developed roots and other characteristics.In present study,the Yellow River Delta coastal saline-alkali soil was employed as the experimental material,10 kinds of green fertilizer crops were selected for pot experiment,and the influence of green fertilizer planting on soil physical and chemical properties were analyzed,combined with crop biomass analysis,the effect of green fertilizer planting on the coastal saline-alkali soil saline content reduction and soil cultivation were explored.At the same time,green fertilizer crops with bearded root system（sweet sorghum）and straight root system（alfalfa,sesbania,alkalium,vetpea）planted for many years（5 years）in coastal saline soil were selected to explore the structural improvement,fertility improvement and salinity reduction effects of green fertilizer crops with different root types on mild and moderate coastal saline soil.Laboratory simulation culture experiment was used to study the influence of green fertilizer crop returning to the field on greenhouse gas emissions,so as to clarify the influence of green fertilizer planting and returning to the field on the characteristics of coastal saline-alkali soil and greenhouse gas emissions,enriched the theory of bioimproved fertilizer cultivation in coastal saline-alkali land,and established bioimproved technology in coastal saline-alkali land.The main research results are as follows:（1）Through cluster analysis,10 green fertilizer treatments were comprehensively grouped into four categories,and 13 indicators such as soil properties and green fertilizer biomass were grouped into three categories.It was found that the first（sesbania）and second type（arrow pea,ryegrass）green fertilizer treatment were negatively correlated to the classification of indicators representing soil physical properties.The results showed that the two green fertilizer treatments significantly reduced pH,EC,TDS and other indexes.Compared with CK treatment,the soil salt content in 0-20 cm and 20-40cm soil layer was significantly reduced by 47.27%and 57.96%in ryegrass treatment,and the salt reduction effect was the best.Compared with CK treatment,soil EC was significantly reduced by73.36%and 83.90%.Sesbania treatment has the best effect on reducing soil bulk density.The soil bulk density of 0-20 and 20-40cm soil layers were significantly reduced by 24.68%and24.04%,and the nitrate nitrogen content of soil treated by cyanine was significantly reduced thus reducing the risk of nitrate nitrogen loss.The second type of green fertilizer was positively correlated with the index cluster of soil salt and nutrient content.This kind of treatment had a better improvement effect on soil nutrient content.Ryegrass and arrow tongue pea treatments had the best effect on the content of soil available phosphorus and ammonia nitrogen.Compared with CK treatment,soil available phosphorus increased significantly by392.35%and 467.37%,and soil ammonium nitrogen content increased significantly by 21.53and 22.02 times.Compared with CK,the soil microbial biomass carbon content of arrow tongue pea treatment increased significantly by 142.15%,and the carburization effect was significant.There two green fertilizer treatments were positively correlated to the first index of biomass,indicating that two green fertilizer treatments were more salt-tolerant and had better growth.The green fertilizer varieties with better improvement effect on coastal saline-alkali soil are sesbania,arrowtongue pea and ryegrass green fertilizer.（2）The experiment of planting green fertilizer with different root types to improve mild and medium saline-alkali soil showed that the salt reduction effect of green fertilizer with straight root system was significantly better than that with beard root system,and the salt reduction effect on mild and medium saline-alkali soil layers（0-20cm,20-40cm）were21.75%-38.23%.The salt reduction effect of bearded root（TGL）green fertilizer on 0-20cm soil layer was reduced by 23.11%.Green fertilizer with straight roots improved the bulk density and porosity of medium saline-alkali soil better than that of mild saline-alkali soil.Green fertilizer treatment of straight roots had better effects on the contents of total nitrogen and soil organic matter,soil urease and alkaline phosphatase activities in 20-40cm soil layer than that of bearded roots.Green fertilizer of bearded roots had significantly better effects on the contents of available phosphorus,soil organic matter and nitrate nitrogen in 0-20cm soil layer than that of straight roots.Green fertilizer of bearded roots reduced soil CEC and promoted the conversion of ammonium nitrogen into nitrate nitrogen.In summary,straight root green fertilizer was superior to bearded root green fertilizer in salt reduction,bulk density reduction and total nitrogen increment,and its improvement effect on 20-40cm soil layer was higher than that on surface layer.The clustering analysis showed that the green fertilizer with straight roots（alfalfa）had the best salt-reducing effect on coastal saline-alkali soil,and the improvement effect on mild saline-alkali soil was better than that on medium saline-alkali soil.（3）The mild and medium coastal saline-alkali soil were modified and fertilized by planting green fertilizer of different root types.The comparison of soil microbial diversity under different green fertilizer treatments showed that planting green fertilizer reduced soil microbial diversity and increased the dominant fungal community as Ascomycota,the abundance of the dominant bacterial communities as Proteobacteria and Bacteroidota.Among them,Proteobacteria,Bacteroides bacteria and ascomycetes fungi were significantly correlated with soil alkaline phosphatase activity,total nitrogen content and soil organic matter content of environmental factors,and green fertilizer planting treatment was also positively correlated with nitrogen,phosphorus,potassium,soil organic matter and other nutrient indexes,indicating that green fertilizer planting could increase the dominant flora and affect the increase of nutrient content.Aphelidiomycota communities were found in medium saline alkaline soil treated with green fertilizer.It is indicated that planting green fertilizer can reduce soil salinity and improve microbial environment.The treatment of mild and medium saline-alkali soil by alfalfa significantly increased eight bacterial communities,such as Deferrisomatota,WS4 and Halanaerobiaeota,and promoted the microbial diversity of medium saline-alkali soil.The improvement effect on microbial diversity of medium saline-alkali soil was better than that of mild saline-alkali soil.It was further proved that green fertilizer with straight roots had the best improvement effect on coastal saline-alkali soil.（4）In the process of green fertilizer returning to the field,the release fluxes of CH₄and N₂O were low,while the release fluxes of CO₂were high,which showed a trend of increasing first and then decreasing gently with time.The greenhouse gas release rate of mild saline-alkali soil was significantly lower than that of medium saline-alkali soil,and the CO₂release rate of soil under alfalfa returning treatment was significantly higher than that under other green fertilizer treatments,which may be related to the low C/N of alfalfa green fertilizer and it’s easy decomposition and utilization by microorganisms.Soil enzyme activities and nitrate and ammonium nitrogen were significantly lower in the medium saline-alkali soil than in the mild saline-alkali soil.Soil enzyme activities and nitrate and ammonium nitrogen content were significantly correlated with microbial activities,indicating that salt could inhibit soil microbial activities.The soil enzyme activity of green fertilizer returning treatment was significantly increased compared with that of CK1 treatment,but there was no significant difference among all treatments.The soil nitrate nitrogen content of alfalfa returning treatment was significantly higher than that of other green fertilizer treatments.In the green fertilizer returning treatment,soil nitrate nitrogen reached a valley value on the 21st day of culture,at which time soil ammonium nitrogen content reached a peak value and catalase activity reached a valley value,indicating that the decrease of catalase activity was significantly correlated with the conversion of ammonium nitrogen to nitrate nitrogen.In conclusion,the CO₂release rate and nitrate nitrogen content of alfalfa returning treatment were significantly higher than those of other returning treatments,and there was no significant difference in soil enzyme activity and ammonia nitrogen content.