| Camellia oleifera oil is known as "Oriental olive oil" because it is rich in unsaturated fatty acids,and vigorously planting C.oleifera is one of the important national policies to ease the tension of edible oil and ensure the safety of edible oil in China.However,the growth cycle of C.oleifera is long,and long-term planting is prone to a series of problems such as decreasing soil fertility and yield of forest land.Relevant studies have found that the above problems may be related to the inter-root nutrient cycling process of C.oleifera,where microorganisms are key drivers involved in the nutrient cycling and transformation process in the soil,and the life activities of microorganisms are influenced by differences in the size and stability of soil aggregates and nutrient storage capacity.So,how does the nitrogen content of soil aggregates evolve in long-term planted C.oleifera stands?Are there differences in nitrogen effectiveness and bacterial community structure of soil aggregates in C.oleifera stands of different ages?What are the reasons affecting soil nitrogen conversion rate and its regulatory pathways?To address the above questions,a differential study on the nitrogen conversion process and bacterial community characteristics of soil aggregates in C.oleifera stands of different ages is proposed.In this study,the soil agglomerates were classified into three classes:>2 mm,2-0.25 mm and<0.25 mm,and the differences in physicochemical properties,enzyme activities and nitrogen conversion capacity of soil agglomerates in long-term C.oleifera plantations were investigated.The microbiological mechanisms underlying the differences in nitrogen effectiveness of soil aggregates in C.oleifera forests at different growth stages were analyzed by regression analysis,redundancy analysis and structural equation modeling.The main research findings are as follows:(1)The stability of soil aggregates tended to increase and then decrease with increasing stand age,and the content of soil organic matter(SOM)gradually increased;the content of total phosphorus(TP),fast-acting phosphorus(AP),total potassium(TK),and fast-acting potassium(AK)tended to gradually decrease.Compared with the young forest soil,the quick-acting phosphorus content decreased 73.7%and the quick-acting potassium content decreased 66.5%in the declining soil,both reaching a significant level(p<0.05).The soil aggregates of C.oleifera forest in fruiting stage were stable,and the organic matter content of soil increased the most at this growth stage,with an increase of 47.5%,and the change in the stability of the aggregates might be related to the fluctuation of soil organic matter content.The significant decrease in soil fast-acting phosphorus and fast-acting potassium content from the growth to the decline stage indicates that the long-term cultivation of C.oleifera has a large consumption of fast-acting nutrients in the soil,and the significant decrease in soil AP and AK nutrients may be an important reason for the decrease in soil fertility of C.oleifera forest.(2)With the increase of forest age,soil total nitrogen(TN),ammonium nitrogen(NH4+-N),and nitrate nitrogen(NO3--N)content showed a trend of first decreasing and then slowly increasing,and the effect of forest age change on soil N content reached a highly significant level(p<0.01);further analysis showed that particle size affected the distribution of soil total nitrogen and NO3--N in the agglomerates(p<0.05).Microagglomerates adsorbed more NH4+-N due to their larger specific surface area,while NO3--N was mainly distributed in large agglomerates.The variation of NH4+-N content in soil macroaggregates was greatest with increasing stand age,with a decrease of 22.57 mg·kg-1 in NH4+-N content in fruiting season and an increase of 21.56 mg ·kg-1 in later growth to decline,while the variation of NO3--N content with stand age was mainly in microaggregates,with a decrease of 29.11 mg ·kg-1 in fruiting season and an increase of 16.11 mg·kg-1 in later growth to decline.Correlation analysis revealed that N content was significantly correlated with soil pH and geometric mean diameter(GMD),suggesting that the increase in effective soil N content during decline may be related to soil acidification and cluster fragmentation caused by long-term planting.(3)Long-term planting significantly affected the rate of nitrogen turnover,and the rates of net soil mineralization,net nitrification,and denitrification decreased,while the rate of net ammonification showed an increasing trend,indicating that the efficiency of organic nitrogen utilization was increased in C.oleifera stand soils after long-term planting,and soil nitrogen conversion was dominated by ammonification.The net ammonification and mineralization rates were higher in small and microaggregates<2 mm in different stands,and the peak denitrification rates were more likely to occur in large aggregates>0.25 mm.The correlation between environmental factors and enzymatic activities of soil N transformation processes revealed that the fluctuation of SOM and AK content in long-term planting caused an increase in the activity of urease,which is involved in ammonification,and a decrease in the activity of ammonia monooxygenase(AMO),nitrate reductase(NR),nitrite reductase(NiR),and nitric oxide reductase(NOR),which are involved in nitrification and denitrification,thus leading to changes in the effective N content in soils of different forest ages.(4)The richness and diversity of soil bacterial communities were significantly higher after long-term planting,and the structure of soil bacterial communities at young ages was significantly different from that of other forest ages in soil clusters,indicating that long-term planting significantly changed the structure of soil bacterial communities.The functional prediction results showed that the young-age soils were enriched in carbon and nitrogen metabolic pathway processes such as glycolysis,valine,leucine and isoleucine degradation pathways,while the expression for the degradation of chloroalkanes and chloroolefins,toluene degradation and sulfur metabolism were found in the old-age soils,indicating that the old-age soils may contain exogenous substances that are unfavorable to microbial growth.Linear discriminant analysis showed that the phylum Helicobacter nitrification and Proteus phylum,which are involved in soil nitrification process,were enriched in the young-age soil;while the phylum Acidobacter phylum,Phylum Wartybacter phylum,and Phylum Bacillus phylum,which are involved in nitrification process,were found to be enriched in the old-age soil,with stronger nitrogen fixation capacity,higher utilization efficiency of soil organic nitrogen,decreased nitrification rate,and the soil nitrogen transformation was mainly in the ammonium state.(5)We found that the change in forest age affected the soil N transformation process through two pathways.Pathway 1:Long-term planting caused changes in soil environmental factors pH,SOM,AK,AP,TK,and TP,which affected the activities of soil enzymes Ure,ACP,AMO,NR,and Inv,and then caused changes in the abundance and diversity of bacterial communities,which eventually led to differences in soil N transformation rates;Pathway 2:Long-term planting disturbed soil structure and caused changes in the content and distribution of different forms of N in soil aggregates.The second pathway:long-term cultivation disturbs the soil structure and causes changes in the content and distribution of different forms of N in soil aggregates,thus regulating the amount of reactants and products at each stage of N transformation and directly influencing the efficiency of soil N metabolism.Long-term C.oleifera cultivation causes a decrease in the stability of the aggregates and a decrease in the effectiveness of nutrients in the soil,which in turn affects the changes in the abundance of bacterial communities with carbon and nitrogen functions in the soil,decreasing the rate of carbon metabolism,weakening the nitrogen nitrification capacity and increasing the nitrogen fixation capacity. |
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