| Litter decomposition in the grassland is one of the important ecological processes for the circulation and balance of important elements such as carbon?C?,nitrogen?N?and phosphorus?P?from“grass”to“ground”,which plays an important role in nutrient cycling on the infertile Loess Plateau of China.However,in artificial mixed grassland,the litter has many types.Especially in the litter mixture,there are litter species interactions during the decomposition process such as nutrient transfer,chemical inhibition,alteration in detritivore behavior,and so on.Coupled with the diverse effectors in the environment,the litter decomposition is more complex in the mixed grassland.Therefore,it is very important to reveal what happens during litter decomposition and how it is regulated in the mixed grassland,which will provide practical basis for the establishment and associate field management of mixed grassland.In this study,in mixed grasslands of alfalfa?Medicago sativa L.?and smooth bromegrass?Bromus inermis Layss.?,we intended to analyze the temporal dynamics of the decomposition of 5 types of litter?alfalfa MS,smooth bromegrass BI,80%MS+20%BI,50%MS+50%BI,20%MS+80%BI?,in order to reveal litter decomposition at different decomposition sites,and explore how litter type,soil microorganism and micro-environmental factor affected litter decomposition.The main results were as follows:1.Among 5 decomposition sites,the decomposition of 5 types of litter varied.In general,the decomposition rate k varied from 0.43 to 1.14 year-1.During the first seven months of decomposition,the mass loss of the litter accounted for 23%of the initial mass,and during the following six months,it was 30%of the initial mass.The litter N was directly released or first released,then enriched and finally released,while the litter P was first released,then enriched and finally released.The variation of litter C concentration was mainly affected by decomposition time.Litter C concentration tended to increase during 0-63 d,and then barely changed from 63 to213 d,but decreased afterwards.The variation of litter N and P concentrations were mainly affected by litter type.For the single species litter,the N and P loss of MS treatment was significantly higher than that of BI treatment.For the litter mixture,the N loss was the most at 80%MS+20%BI treatment,accounted for 42.60%of N concentration at initial decomposition,and was the least at 20%MS+80%BI treatment,accounted for 20.24%,while the P loss was the most at 20%MS+80%BI treatment,accounted for 29.42%of P concentration at initial decomposition,and the least at50%MS+50%BI treatment,accounted for 19.84%.In short,for the decomposition of single species litter,the higher litter C/N ratio and C/P ratio limited the loss of litter mass,N and P,while for the decomposition of litter mixture,the loss of litter mass,N and P were much more complex and didn't change consistently with litter C/N ratio and C/P ratio increased or not.2.There was a litter-mixing effect?LME?during the decomposition of litter mixture and the relative individual performance?RIP?of each litter component was significantly different.The 33.33%of litter mixtures showed significant non-additive effect,of which synergistic and antagonistic effects accounted for 58.14%and 41.86%,respectively.The non-additive effect became more significant not only with decomposition time,but also increased first and then decreased with increasing proportion of smooth bromegrass litter?or decreasing proportion of alfalfa litter?in the litter mixture.There were the most synergistic effect and the least antagonistic effect at 50%MS+50%BI treatment.The averaged LME changed with decomposition time.Synergistic effect on litter decomposition could occur on the 63-213th d of decomposition,and antagonistic effect could occur on the 305th d.The 25.83%of alfalfa litter and 34.17%of smooth bromegrass litter were significantly different between mixed decomposition and single decomposition.The RIP of alfalfa litter was significantly greater than zero and for smooth bromegrass litter it was significantly less than zero,which gradually increased with the decomposition time.Among three litter mixtures,decomposition was promoted at 50%MS+50%BI treatment,but80%MS+20%BI treatment was just the opposite.20%MS+80%BI treatment showed alfalfa and smooth bromegrass litter decomposition were most inhibited and least promoted,respectively,but the magnitudes were close and canceled each other out,resulting in an additive effect during the decomposition.There were interactions among litter components during the decomposition of litter mixture,which was quite different from the decomposition of single species.3.The effects of average soil temperature,average soil water content,near-surface cumulative radiation,soil mineral nitrogen content and soil microorganism on litter decomposition varied a lot at the five decomposition sites.The average soil temperature was the highest in smooth bromegrass grassland?BI?.The average soil water content in alfalfa grassland?MS?was the highest,while the BI was the lowest,and in 80%MS:20%BI and 20%MS:80%BI grasslands it was significantly higher than that in 50%MS:50%BI grassland.The near-ground cumulative radiation in BI grassland was the highest,and in 80%MS:20%BI grassland,the lowest.With the increase in decomposition time,soil ammonium N content first decreased and then increased,while soil nitrate N content first decreased and then increased,and then repeated again.During the decomposition of litter mixture,the dominant phyla of soil bacteria were Proteobacteria,Acidobacteria and Actinobacteria,and the dominant phylum of soil fungi was Ascomycota.From 276 to 340 d of decomposition,the differences in soil bacterial communities among grasslands gradually appeared,while the differences in soil fungal communities gradually disappeared.At the genus level,the species number of soil bacteria gradually increased,while the species number of soil fungi had no change.Soil temperature and water content only indirectly affected litter decomposition through soil microorganism,while near-surface radiation and soil mineral nitrogen affected litter decomposition both through direct and indirect effects.The dominant groups of soil microorganisms gradually changed from fungi to bacteria due to different decomposition abilities and preferences. |
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