氮沉降对不同人工林生态系统中磷循环关键过程的影响

The increase in atmospheric nitrogen (N) deposition can alter ecosystem phosphorus (P) cycling and lead to the imbalance between N and P supply in ecosystems, which is an important way by which N deposition affects ecosystem structure and functions. However, the pathways through which N deposition affects P cycling in forests, and the mechanisms regulating these pathways, are largely unknown. Three forest plantations with contrasting soil nutrient status will be selected for this study: a sub-tropic Cunninghamia lanceolata plantation with low soil P availability, a temperate Larix gmelinii plantation with low soil N availability, and a semi-arid Pinus sylvestris var. mongolica plantation with low availability of both N and P. N addition experiments have been conducted for more than 5 years in these plantations. Another short-term N addition experiment with root exclusion and litter removal will be conducted in these plantations to separate the role of trees and soils in regulating the effects of N addition on P cycling. We will examine the key properties and processes of P cycling, including soil available P and its δ18O-PO4 value, transformations of soil inorganic and organic P, P resorption and return of P through litter decomposition, as well as soil microbial community composition and physicochemical properties related to soil P transformation at all study sites. The objectives of this study are to: (1) make clear the effects of N deposition on key properties and processes of P cycling in forests and pathways through which N deposition affects P cycling; (2) compare the differences in effects of N deposition on P cycling among these forest plantations and identify the mechanisms underlying these differences. Results of the present study can improve our understanding of the consequences of N deposition on forest ecosystems and the coupling of forest N and P cycles, as well as guide the management practices of these plantations under the background of increasing N deposition.

随着全球氮沉降的加剧，氮沉降引起的磷循环和氮磷平衡的改变已成为其影响森林生态系统结构功能的重要途径。然而，目前对氮沉降影响森林磷循环的作用过程和调控机制并不清楚。本项目拟选取土壤养分条件差别明显的亚热带杉木人工林（磷缺乏）、温带落叶松人工林（氮缺乏）和半干旱区樟子松人工林（氮和磷均缺乏）为研究对象，利用原有的氮添加（> 5年）试验，结合短期的断根和凋落物移除条件下的氮添加试验（以区分植物和土壤因素在调控氮沉降影响磷循环中的作用），研究氮添加对不同人工林土壤中有效磷含量及其δ18O-PO4值、有机和无机磷转化及相关土壤生物化学性质、林木对磷的需求、回收和归还的影响。本研究旨在明确氮沉降对森林生态系统磷循环关键过程的影响及其作用途径，揭示氮沉降对磷循环的影响在不同人工林中的差异性及其调控机制。研究结果有助于加深对森林生态系统氮沉降效应和氮磷耦合机制的认识，并为氮沉降条件下人工林的管理提供依据。

为探究不同人工林中氮沉降对磷循环关键过程影响的差异及其机制，本项目选取了已开展长期氮沉降试验的杉木人工林（磷缺乏）、落叶松人工林（磷缺乏）和樟子松人工林（氮和磷缺乏）为研究对象，结合林下植被去除、地上凋落物移除和根系去除试验，测定了土壤磷转化的相关指标和微生物性质。结果表明，（1）断根显著降低了磷酸酶（58%）、微生物量磷(50%)、活性有机磷（65%）和各无机磷组分（25%-42%）含量。凋落物移除对磷组分无显著直接作用，但显著增强了断根对无机磷组分的影响。（2）氮添加显著降低了樟子松和落叶松林土壤酸性磷酸酶（18%-21%）和植酸酶活性（42%-51%），显著提高了杉木林土壤酸性磷酸酶活性（23%），显著降低了所有林分中16S rRNA、phoD，phoC，appA基因丰度。氮添加显著增加了樟子松林土壤中编码16S sRNA拟杆菌门的相对丰度，降低了链霉菌属的相对丰度，增加了落叶松林中厚壁菌门的相对丰度，增加了杉木林绿弯菌门的相对丰度，而降低了慢生根瘤菌属相对丰度。（3）氮添加显著降低了樟子松林中活性无机磷和残留磷含量（42%），和南瓮河落叶松林土壤中的无机磷组分含量。林下植被去除和氮沉降的影响存在显著的负交互作用。（4）氮添加显著提高了樟子松针叶秋季磷的回收(55%)。在落叶松林中，氮添加显著降低了成熟叶易降解磷和全磷含量和凋落叶残余磷浓度。从以上研究结果可见，根系活动驱动着土壤中有机磷和无机磷的活化。地上凋落物对土壤磷转化的短期直接作用很小，但其能通过补充土壤钙和维持土壤pH值促进根系对无机磷的活化。林下植被能有效调节氮沉降对土壤磷的影响。氮添加抑制了樟子松和落叶松林中有机磷的矿化，促进了杉木林中有机磷的矿化。杉木林中氮添加不仅改变了土壤微生物量和群落结构，还提高了功能基因的表达强度。氮添加促进了土壤中难溶性无机磷的溶解。氮添加引起的土壤pH值和有机碳的变化是决定其对土壤磷转化影响的主要因素。树木能够通过重新利用和分配磷组分、提高磷的季节性回收来响应氮添加引起的磷需求的变化。