磷对中亚热带森林细根分解的影响及作用机制

Many studies have shown that nitrogen (N) availability directly controls terrestrial C uptake and losses in mid and high-latitude ecosystems. Highly weathered soil is widespread in tropics and other low-latitude ecosystems. However, it is still not clear whether phosphorus (P) is the dominant control of litter decomposition in this low soil P availability area. Based on our previous researches, we choose two natural evergreen broadleaved stands with similar tree species composition and significant different soil P availability, which have developed on granite and limestone, respectively. Fine root litters of Castanopsis carlesii (high P concentration) and Cunninghamia lanceolata (low P concentration) are collected, then the two roots and their mixture form high, middle and low P concentration gradient. Part of collected fine roots are buried in both stands for decomposition in the field by means of litter-bag method. The dry mass loss, nutrient release rates and dynamics of different organic carbon fractions are assayed in the leaf litter decomposition process. By comparing the mass loss rates for the three roots in the same stand and the same roots in the different stand, we test our hypothesis that roots with higher P concentration decompose faster on the same soil and the same roots in the lower P availability soil decompose slower. With P or lime addition to elevate soil P availability, we test our hypothesis that the stimulating effect of P on root litter decomposition is much stronger in roots with low P concentration or in soil with low P availability. For indoor control experiment, fine roots with different N/P ratios are incubated in petri dishes to test our hypothesis that mixture of the two fine roots show rather balanced nutrient supply which can decompose faster than single root does. With P addition, we test our hypothesis that the stimulating effect is stronger in fine roots with higher N/P ratio. Fine root samples are assayed for six extracellular enzyme activities which are mainly engaged in degrading cellulose and lignin, and microbial community compositions for the samples are determined by PCR-DGGE. By ways of above mentioned methods, we try to reveal the mechanism of P regulation of fine root decomposition in subtropical forests.

N有效性调控温带生态系统碳的光合吸收和分解损失。在土壤风化严重、P有效性低的热带亚热带地区，凋落物分解调控机制尚不明确,影响准确预测全球环境变化情景下生态系统碳循环。本项目基于前期研究结果，选取两片树种相似、土壤P有效性有显著差异，由花岗岩和石灰岩发育的天然林为样地；选取米槠(高P)和杉木(低P)细根，并把米槠+杉木细根混合，构成底物N/P比梯度。野外采用网袋法测定分解过程凋落物干重、有机碳组分损失和N、P养分释放速率；比较同种细根在不同林分，不同细根在相同林分的分解速率，验证底物或土壤Ｐ有效性控制细根分解的假说；通过外源P添加和施石灰，验证P有效性提高对低P底物和低P土壤的分解促进作用更强的假说。室内把不同细根进行培养，验证细根混合使N、P养分均衡而促进分解，添加P促进低N/P比细根分解加速的假说。测定分解过程降解纤维素和木质素的微生物和胞外酶活性，揭示P对中亚热带细根分解的调控机制。

磷是限制森林生产力和其他生态系统过程的重要元素，然而磷对凋落物分解的影响及其作用机制尚不明确。本项目在磷有效性普遍较低的中亚热带地区，探讨磷有效性对森林细根分解的作用机制。.主要研究内容分为以下四个部分：1.采用尼龙网袋，把构成底物养分有效性梯度的米槠、米槠+杉木和杉木细根分别置于土壤磷有效性不同的花岗岩和石灰岩发育的天然常绿阔叶林中分解，测定细根的干质量、有机碳组分损失和养分释放速率；2. 在室内把米槠、米槠+杉木和杉木细根置于石英砂培养皿中分解，比较在添加和不添加 P 情况下，三种细根干质量损失速率；3. 比较在外源 P 添加情况下，米槠、米槠+杉木和杉木细根分解的干质量、有机碳组分损失和养分释放速率；4. 测定三种细根分解过程凋落物和土壤中 6 种胞外酶活性以及微生物生物量的动态，分析 P 添加情况下细根分解速率的差异与酶活性变化的关系。.研究发现：1.高P有效性、石灰岩发育的宁化天鹅洞森林中细根分解速率低于低P有效性、花岗岩发育的三明三元区格氏栲森林中的细根分解；2.不同的底物在相同的环境下分解，高P含量底物，如米槠根比杉木根分解快；3.室内培养的情况下，加P促进了细根分解，且对P含量较低的杉木细根分解促进作用更强；4.野外控制情况下，加P对低P底物分解促进作用更强；5. 在2年分解期内，各处理细根中纤维素水解酶（CBH）、酸性磷酸酶（AP）、β-葡萄糖苷酶（BG）、β-N-乙酰氨基葡萄糖苷酶（NAG）活性的变化呈先上升后下降的趋势，而过氧化物酶（PerOx）和酚氧化酶（PhOx）活性呈上升的趋势。不同处理中P添加提高了CBH、BG、NAG、PhOx和PerOx活性，而抑制了AP活性。相关分析表明细根分解速率与AP活性呈显著的负相关，与PerOx、PhOx活性呈显著的正相关（P <0.05）。.研究结果充分表明，P有效性限制了亚热带森林凋落物的分解速率。研究同时表明AP活性与PerOx、PhOx活性能指示凋落物的分解速率。研究结果加深了对热带亚热带地区凋落物分解的认识，有利于对区域生态系统碳循环的准确预测。