摘要
桉树连栽引起的立地衰退制约了速生人工丰产林的发展，因此，如何有效地提高人工林的产量、改善林地土壤肥力是当前林业生产中急需解决的问题。本论文研究在广东雷州半岛进行了桉树-相思的混交和N施肥试验，旨在研究混交和施肥后林地土壤养分的变化、养分和树木生长的关系；同时在广东、广西选择了4个不同的立地条件进行施肥研究，进一步探索桉树在不同的立地、不同的施肥条件下的生长情况，为合理施肥提供依据。本文得出的结论如下：
1 桉树、相思混交后不同的混交方式其凋落枝、叶以及总量都有不同的差异。其中以桉树纯林的凋落枝最大，达4.818t／hm~2yr，以相思纯林的凋落叶最大，达17.73t／hm~2yr。凋落物总量以相思纯林最大，桉树纯林最小，其余各混交方式差别不大。单位重量的凋落枝、叶的养分含量以相思叶含N量最大，U6枝最小，分别为11.18g／kg、4.44gkg。P、K、B都以桉树叶最大，分别为0.96g／kg、0.31g／kg、64.85mg／kg。从N、P、K、B总量来看，相思纯林含K量最大、1株桉树1株相思混交含P最大外，1行桉树1行相思混交含N、B最大。桉树凋落枝多，相思凋落叶多这种生理特性以及两者的混交方式决定了各混交处理凋落物、养分含量的差异，以1株桉树1株相思、1行桉树1行相思混交效果较好。凋落物能增加林地有机质含量，有利于提高林地土壤肥力。
2 相思纯林土壤中的硝态氮、氨态氮、有效K含量均最大，桉树纯林土壤中有效P的含量最大，1株桉树1株相思、1行桉树1行相思混交各养分含量相差不大。因此桉树与相思混交，能提高林地土壤N供应能力，对改良土壤养分、维持林地长期生产力有一定的作用。
3 桉树、相思混交方式不同导致了其树高、胸径、单株蓄积量的差异，但总蓄积量各混交方式没有明显相差。桉树纯林的林分总蓄积量比1行桉树
中文摘要
1行相思混交的要低6.71%，树高、胸径差异性不显著;比1株按树1株相
思的树高、总蓄积量要低8.7%、6.99%，胸径差异性不显著。在各种混交方
式综合来看，1株按树1株相思、l行按树1行相思的混交效果较好。
4施氮肥对按树林地土壤的硝态氮、氨态氮的影响较大。半年后，不施
肥处理的硝态氮、氨态氮比造林前分别低308.3%、35.5%;而施肥处理只比
造林前低87.3%、22.3%。这说明施氮肥后及时补充了按树早期生长所需的
土壤N素。
5施氮肥对按树的生物量有显著的影响，施肥处理的树干、树皮、枝条、
根生物量都要比不施肥高。施1009、巧0g氮肥处理的总生物量比不施肥要
分别高144.3%、953%。施1009氮肥处理的生物量比施1509的要高，可能
当氮肥达到一定量后对按树的生长有抑制作用。
6不同的立地条件下不同的配方施肥对按树的高、地径生长有显著的影
响。以广西防城港的N15P15K15配方施肥地径最大，为4.96cm;高生长以
广东唐家的N5P18K8配方最大，达3.slm。校树高、地径在不同立地条件下
的差异，除受施肥配方影响外，还跟土壤性质、当地的气候条件有密切的关
系。
Land degradation of eucalypt plantations after successive rotations limits the development of plantation industry in south China. Therefore, it is important to increase the productivity of the eucalypt plantations by increasing soil nutrient availability. Trails on mixed plantation of Eucalyptus urophylla (clone U6) with Acacia crassicarpa, N fertilizer application to Eucalyptus urophylla (clone U6) were established in Leizhou, Guangdong to determine the relationship between soil N supply and tree growth, and the change of soil nutrient status. At the same time, 4 different sites in Guangdong and Guangxi were selected in order to determine the growth of Eucalyptus urophylla (clone U6) with the application of different fertilizers. The conclusions of the thesis are as follows:
1 There was a significant difference among mixing treatments in amount of litterfall. Pure eucalypt treatment had the highest amount of branch litterfall (4.818t/hm2 yr) and the pure acacia treatment had the highest amount of leaf litterfall (17.73 t/hm2 yr). The total amount of litterfall of pure acacia was the highest and pure eucalypt was the smallest among all treatmensts. Concentration of N in acacia leaf litterfall was highest (11.18 g/kg) and branch of eucalypt was lowest (4.44 g/kg) among different components of litterfall. Concentrations of P, K, B in eucalypt leaf litterfall was 0.96 g/kg, 0.31 g/kg, 64.85 mg/kg, respectively, and they were higher than in acacia leaf litterfall. Pure acacia treatment had the
highest total K content in litterfall among all treatments. The treatment with one eucalypt and one acacia tree in the same row had the highest P content in litterfall and the treatment of one row eucalypt with one row acacia had the highest N, B contents in litterfall among all treatments. Intercroping acacia with eucalypt increased litterfall and the best mixing treatments were a eucalypt and an acacia tree in the same row or a row eucalypt with a row acacia.
2 The pure acacia forest had the highest concentration of NO3--N, NH4+-N and available K in topsoil, and pure eucalyptus forest had the highest available P in top soil. There was no difference between the 2 treatments of a eucalypt and an acacia tree in the same row and a row eucalypt with a row acacia in available nutrients in top soil. It was proved that mixed plantation increased N availability in top soil in comparison with pure eucalypt plantation.
3 Height, diameter and stand volume of trees were different among treatments, but the total stand volume was not significantly different. Comparing to the treatment of a row eucalypt with a row acacia, the stand volume of pure eucalypt was 6.71% lower, but the tree height and diameter were not significant different. Comparing to the treatment of a eucalypt and an acacia tree in the same row, height and stand volume of eucalypt trees in pure eucalypt treatment was 8.7% and 6.99% lower, respectively, but diameter of eucalypt trees was not significantly different. The 2 treatments of a eucalypt and an acacia tree in the same row and a row eucalypt with a row acacia were much better than other mixed treatments.
4 Application of N fertilizer to eucalypt plantation remarkably increased NO3--N and NH4+-N in top soil. After half year, concentrations of NO3--N and NH4+-N in top soil were 308.3% and 35.5% lower than those in topsoil before
tree planting without application of N fertilizer. But with application of N fertilizer, they were only 87.3% and 22.3% lower than before planting. The application of N fertilizer increased N supply to tree growth in the early plantation establishment.
5 There was a significant effect of N application on biomass of eucalypt trees. The biomass of tree trunk, bark, branch and root in the treatments with N application were much higher than those without N application. Comparing to the treatment without N application, the total biomass of the 2 treatments with 100g and 150g N fertilizer per tree was 144.3%, 95.3% higher, respectively. The lower biomass in the treatment with 1
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