| Understory vegetation is an important part of the plantation.Previous studies generally considered that the main role of understory vegetation in plantations is to increase the number of species in plantation system.Recently,more and more studies have shown that understory vegetation is beneficial to the nutrient cycling and long-term productivity maintenance of plantation forest systems.Under natural conditions,the understory vegetation in a certain area contains many kinds of plants,and the nutrient function of the understory vegetation species richness in the plantation system is still unclear.Taking the poplar(Populus spp.)plantation as the research object,this study explored the nutrient effects of the diverse understory vegetation and the nitrogen-fixing vegetation.The field experiment was designed with a randomized block,and four understory treatments were designed including the understory vegetation removal,the retention of single main understory vegetation species,the retention of natural diverse understory vegetation and planting nitrogen-fixing plants.The understory vegetation in this experimental forest was dominated by barnyard grass(Echinochloa crusgalli),and it accompanied with Setcreasea purpurea,Rhizoma cyperi and Ammannia coccinea.Therefore,the barnyard grass was retained in the retention of single main understory vegetation species treatment.The Sesbania cannabina was planted in the treatment of planting nitrogen-fixing plants.After one year of treatment,the growth and nitrogen metabolism of poplars were investigated.The nutrient supply,microbial community structure and extracellular enzyme activity of soil were regularly monitored at the same time.We also analyzed the influence of the understory vegetation litter on litter decomposition and soil nutrient supply with an incubation experiment based on field experiment.Finally,the natural isotope abundance technology was used to evaluate the nitrogen utilization and nutrient interaction between poplars and the understory vegetation.Results showed that:1.During the period of rapid growth of poplars(June to August),increments of the height and diameter at breast height(DBH)of poplars were significantly lower under the retention of single species vegetation treatment than under the treatment of vegetation removal.In addition,the species richness of understory vegetation did not significantly affect the growth of poplars during this period.Moreover,increments of the height and DBH of the poplar were significantly higher under the treatment of planting nitrogen-fixing plants than under the retention of single species vegetation treatment.Results also showed that,treatments changed the nitrogen metabolism of poplars.Compared with the treatment of vegetation removal,the retention of single species vegetation treatment resulted in significant decreases in chlorophyll contents and activities of nitrate reductase by 12.96% and 18.71%,respectively,in July 2019.In addition,compared with the retention of single species vegetation treatment,the treatment of planting nitrogen-fixing plants improved the use of nitrogen by poplars.2.Treatments of understory vegetation significantly affected the moisture content,compaction,nutrient content,and microbial flora of the soil,and treatments had strong effects on 0-5 cm soil layer.Compared with the treatment of vegetation removal,the retention of single species vegetation treatment significantly reduced soil compactness,in addition,compared with the retention of single species vegetation treatment,the retention of diverse understory vegetation treatment further reduced soil compactness.Moreover,the treatment of planting nitrogen-fixing plants greatly reduced the soil compaction.The results also showed that,compared with the retention of single species vegetation treatment,the retention of diverse vegetation treatment reduced the content of soil inorganic nitrogen,but it accelerated the conversion process of soil dissolved organic nitrogen(DON)to inorganic nitrogen,it also increased the accumulation of soil microbial biomass and the activity of soil extracellular enzymes involved in the transformation of carbon,nitrogen,and phosphorus.What's more,compared with the retention of single species vegetation treatment,the retention of diverse vegetation treatment resulted in significant increases in soil net nitrogen mineralization rates,net nitrification rates and net change rates of DON by 182.61%,30.77% and 200%,respectively,during June to July,and it also increased the soil microbial biomass and the activity of extracellular enzymes.In addition,compared with the retention of single species vegetation treatment,the treatment of planting nitrogen-fixing plants increased the content of soil inorganic nitrogen,and it improved the activity of enzymes involved in soil nitrogen conversion.Understory vegetation treatments also changed soil microbial communities' metabolic profiles.The main carbon sources utilized by soil microorganism with vegetation removal treatment were a few kinds of carbohydrates,amino acids and esters.By contrast,soil microorganisms under the retention of single species vegetation treatment improved the utilization capacity of the above-mentioned carbon sources and expanded the available carbon sources to some organic acids.At the same time,the retention of natural diverse understory vegetation enabled soil microorganisms to effectively utilize all of the 31 carbon source types.In addition,compared with the retention of single species vegetation treatment,the treatment of planting nitrogen-fixing plants mainly increased the utilization of amino acids and organic acids by soil microorganisms.3.The litterfall of Echinochloa crusgalli,Setcreasea purpurea,Rhizoma cyperi,Ammannia coccinea,and Sesbania cannabina had lower carbon: nitrogen ratios and higher carbon: lignin ratios than those of litterfall of poplars.Compared with the decomposition of poplar leaves,poplar leaves mixed with Echinochloa crusgalli increased the decomposition rate of litterfall,and it accelerated the release of the carbon,nitrogen and phosphorus in litterfall.What's more,compared with the decomposition of poplar leaves mixed with Echinochloa crusgalli,poplar leaves mixed with diverse understory vegetation litters resulted in significant increases in remaining rates of mass and carbon by 6.37% and 12.18%,respectively,when litterfall decomposed to 540 days.In addition,the decomposition rate and nitrogen release rate of poplar leaves mixed with the Sesbania cannabina were much faster than those of poplar leaves mixed with the Echinochloa crusgalli.Results also showed that,compared with the cultivation of pure soil,the turnover of soil available nutrients was improved after adding poplar leaves.In addition,compared with the decomposition of poplar leaves,the addition of Echinochloa crusgalli increased the accumulation of soil microbial biomass,besides,it also increased soil enzyme activities and contents of available nutrients in the soil.More importantly,compared with the decomposition of poplar leaves mixed with Echinochloa crusgalli,poplar leaves mixed with diverse understory vegetation further improved the soil microbial biomass and extracellular enzyme activity,and it also made the soil microbial community structure more complex than before.In addition,compared with poplar leaves mixed with Echinochloa crusgalli,poplar leaves mixed with Sesbania cannabina significantly accelerated the soil nitrogen transformation and the inorganic nitrogen content.4.No biomass and nitrogen accumulation existed in understory vegetation under the retention of single species vegetation treatment in April,while appreciable amount was found under the retention of diverse vegetation treatment at this timing.In addition,the nitrate nitrogen in 10-20 cm soil layer had the most impact on nitrogen utilization of poplars in the treatment of understory removal.Poplars alleviated nitrogen competition with the understory vegetation by increasing the use of ammonium nitrogen.In summary,in the short term,comparing with the understory vegetation removal,the retention of non-nitrogen-fixing understory vegetation may compete with poplars for soil inorganic nitrogen.However,in the long term,comparing with the understory vegetation removal,the retention of understory vegetation,especially diverse understory vegetation,can accelerate the decomposition of the litter,improve the reproduction of soil microorganisms,and significantly promote the supply of soil available nutrients.Planting nitrogen-fixing plants is the most conducive way to increase the nitrogen supply of the poplar plantation,but it also increases costs and management difficulties.The retention of diverse understory vegetation is beneficial to the maintenance of long-term productivity of poplar plantations,so it is a preferable management method for understory vegetation. |
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