| The coastal sand is a transitional zone where human life and various economic activities are frequent between land and oceans,and it is also a fragile environmental zone.Coastal protective forest is an important part of China's forest resources and a special type of forest.It plays an important role in preventing wind and sand,maintaining soil and water,regulating the climate,and improving the ecological environment.Due to the decline of soil fertility,the lack of coastal protection afforestation tree species,and the pure forest structure of the coastal shelter forest are likely to cause the degradation of artificial forest.Establishing a reasonable mixed forest in coastal sand has become an urgent problem to be solved in the process of coastal shelterbelt management and construction.In view of this,this study takes the Eucalyptus urophylla × E.grandis and Casuarina equisetifolia as the research objects with litter bag decomposition method.Placed E.urophylla x E.grandis litter(A-A,M-A),C.equisetifolia(A-M,M-M),E.urophylla x E.grandis:C.equisetifolia 1:1 mixed litter(A-Hl,M-H1),E.urophylla x E.grandis:C.equisetifolia mixture 1:2 mixed litter(A-H2,M-H2,M-H3),E.urophylla x E.grandis:C.equisetifolia 2:1 mixed litter(A-H3)five treatments under the Eucalyptus urophylla x E.grandis and Casuarina equisetifolia experimental forests.The litter decomposition rate,the release of nutrients(C,N,P,K,Ca,Mg,etc.)and the changes in litter enzyme activity(acid phosphatase,polyphenol oxidase,cellulase.)during decomposition were analyzed,exploring the decomposition mechanism of litterfall of different tree species under sandy conditions and the decomposition mechanism of different mixed litters can provide theoretical basis for the future construction of different mixed proportion shelterbelts along the coast,maintenance of protective forestland strength,and improvement of coastal shelterbelt ecosystem stability.The main results are as follows:(1)The annual litterfall(15.16 t hm-2 a-1)in the C.equisetifolia plantation on the coastal sandy land was significantly higher than that in the E.urophylla × E.grandis plantation(8.43 t hm-2 a-1).The monthly dynamic changes of litterfall in the two plantations were similar,with multi-peak curves.The maximum value of Eucalyptus urophylla was in May(1.78 t hm-2 a-1),and the maximum value of Casuarina equisetum was in July(2.521 hm'2 a-1).The amount of litter in the two plantations showed seasonal characteristics that were significantly higher in summer than in winter,and was dominated by litter,accounting for more than 54%of the total litter.(2)In the litter decomposition tests,the remaining rate of dry weight of litter under this two kinds of forests decreased first,and then decreased gradually.After the decomposition of 360d,the dry weight remaining rate was 31.52%?44.55%.The time required for the half-life of each litter to reach 50%decomposition(t0.s)was 0.55 a?0.94 a,and the time required for 95%decomposition(t0.05)was 2.38 a?4.04 a.The time required to decompose 95%of the litter of Pinus massoniana and C.equisetifolia is about 4 times as the time needed to decompose 50%,showing that the decomposition speed of litter was faster in earlier stage and slower in later stage.In this research,litter was placeed on the bottom of woodland,so its decompose was faster.(3)After the litter of E.urophylla x E.grandis and C.equisetifolia were mutually displaced.The decomposition coefficient of E.urophylla x E.grandis was 0.897 a-1 and 1.051 a-1 respectively under the E.urophylla x E.grandis experimental forests and C.equisetifolia experimental forests;The decomposition coefficient of C.equisetifolia was 1.020 a-1 and 0.877 a-1 respectively under the C.equisetifolia experimental forests and E.urophylla x E.grandis experimental forests.The results showed that in the test of the migration of E.urophylla x E.grandis and C.equisetifolia,the E.urophylla x E.grandis and C.equisetifolia were both decomposed faster under C.equisetifolia,and C.equisetifolia was decomposed faster in its own habitat.(4)The effect of different litter mixing ratios on the decomposition rate differed.In this study,under the E.urophylla x E.grandis experimental forests,the higher the ratio of decomposition of the E.urophylla x E.grandis leaves in the mixing process,the higher the decomposition rate,and the decomposition rate of C.equisetifolia showed no regularity with the ratio.Under the two forest stand environments,the dry weight remaining rate of the litter was the smallest in each stage of the mixed treatment of A-H3 and M-H3.The combined forecast value was significantly lower than the actual value of the dry weight remaining rate of the litter,indicating that the mixed treatment of 2:1 litter between E.urophylla × E.grandis and C.equisetifolia significantly promoted litter decomposition.The actual production of C.equisetifolia is about twice as much as the amount of litterfall of E.urophylla × E.grandis,so the best effect is to mix 1:1 with E.urophylla × E.grandis and C.equisetifolia.(5)In the process of decomposition,The variation trend of each nutrient element content differed.The variation of carbon content was basically consistent with the decomposition rate.The nutrient release in the early stage of decomposition was less than the later stage of decomposition as a whole.For the same treatments under different stands,The amount of forest litter released by E.× E.grandis was less than that of C.equisetifolia most of the time.Nitrogen concentration of litter in different treatments was significantly enriched in the early stages of decomposition.The nitrogen of litterfall under the E.urophylla × E.grandis experimental forests showed the process of enrichment-release-enrichment,and the nitrogen accumulation under the C.equisetifolia experimental forests only experienced enrichment-release process.The final release of nitrogen was greater under the C.equisetifolia experimental forests than that under the E.urophylla x E.grandis.experimental forests.In the same decomposition environment,the nitrogen content in the litter of E.x E.grandis was more active than that of C.equisetifolia.So it was concluded that different mixed litters had different effects on the release of nitrogen.(6)In this study,the initial phosphorus content of C.equisetifolia.litter was significantly higher than that of E.urophylla x E.grandis.The two kinds of litter showed that phosphorus was released first and then enriched in the main field decomposition.The giant scorpion showed nutrient release when decomposed under the C.equisetifolia.C.equisetifolia showed nutrient enrichment when decomposed under the E.urophylla x E.grandis.The mixed litter showed an enrichment trend in general.Potassium is a relatively unstable element and is easily transferred during litter decomposition.In the litter decomposition experiment of the coastal sand,the potassium content changed greatly.The calcium element showed a small amount of enrichment in the decomposition.While magnesium did not change much during litter decomposition.(7)The activities of acid phosphatase,polyphenol oxidase and cellulase were decomposed for 360 days(in summer)were lower than those for 180 days(in winter)in the litter of pure E.urophylla × E.grandis and pure C.equisetifolia.The litter enzyme activity did not show obvious seasonal differences under different mixed conditions.significant or no significant difference existed in litterfall enzyme activity under different treatments,so did in the same treatment under different stands.Pearson correlation analysis showed that there was no significant positive correlation between acid phosphatase and polyphenol oxidase and cellulase while there was a very significant negative correlation between polyphenol oxidase and cellulase. |
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