樟子松对氮沉降与干旱胁迫下土壤氮磷有效性变化的适应策略

Soil nitrogen (N) and phosphorus (P) availability is greatly influenced by global change factors, such as N deposition and altered precipitation regimes. Considering that N and P are essential macronutrients for tree growth, the adaptability of trees to soil N and P availability is the key to maintain the health and stability of forest ecosystems. However, it remains unclear how trees regulate nutrient conservation via nutrient resorption, and nutrient acquisition via fine roots, mycorrhizal fungi and rhizosphere effects, to adapt to altered soil nutrient availability. Moreover, it is still unknown whether the three nutrient acquisition strategies listed above work in synergy or tradeoff to adapt to altered soil nutrient availability. To bridge these gaps, this proposed project will be conducted to study how trees adapt to changes in soil N and P availability induced by N deposition and drought in a Mongolian pine plantation in Keerqin Sandy Lands. Soil N and P availability, the gradual process of nutrient resorption with leaf aging, root length density, extramatrical mycelia of ectomycorrhizal fungi and root exudates will be measured by the methods of ingrowth cores and static culture-based systems used to collecting root exudates in the field. These results will be useful in exploring the response of nutrient resorption to altered soil N and P availability, identifying the relationships among the three nutrient acquisition strategies in nutrient uptake, elucidating the mechanisms underlying how the Mongolian pine adapts to altered soil N and P availability. This project will provide theoretical basis from the perspective of nutrient conservation and nutrient acquisition for sustainable management of forest plantations in ecologically vulnerable regions.

以氮沉降和降水格局变化为标志的全球变化正深刻地影响着森林土壤氮磷有效性。氮和磷是树木生长的必需养分元素，树木对土壤氮磷有效性变化的适应能力是维持森林生态系统健康与稳定的关键。然而，树木如何调控叶片养分回收过程（养分保存策略），如何从根系、菌根真菌和根际效应等三种养分获取策略来适应土壤养分有效性的改变，以及三种养分获取策略上的协同或权衡关系，仍不清楚。为此，本项目以科尔沁沙地樟子松人工林为研究对象，依托模拟氮沉降与干旱胁迫试验平台，运用根系功能分级、内生长法和根系分泌物原位收集装置，测定土壤氮磷有效性、养分由老叶向新叶的逐步转移过程、根长密度、根外菌丝密度、根系分泌物量和化学组成，探讨养分回收对土壤养分有效性变化的响应特征，明确三种养分获取策略之间的相互关系，阐明樟子松对氮沉降和干旱胁迫下土壤养分有效性变化的养分保存与养分获取策略，为生态脆弱区人工林可持续经营提供科学依据。

以氮沉降和降水格局变化为标志的全球变化影响着森林生态系统结构和功能。了解树木对氮沉降和降水格局变化导致的土壤氮磷有效性变化的适应性及其机制可为维持森林生态系统健康提供科学依据。本项目以科尔沁沙地樟子松人工林为对象，依托模拟氮沉降与干旱胁迫试验平台，测定了土壤氮磷有效性、养分由老叶向新叶的逐步转移过程、根长密度、根外菌丝密度、根系生物量等，探讨了樟子松对干旱胁迫与氮沉降导致土壤养分有效性变化的响应与适应机制。研究发现：（1）干旱胁迫下，樟子松保持稳定的细根生物量、生产力和周转、吸收根比根长和直径以及外生菌根菌丝生物量，增加根际土壤有机碳、全氮和酸性磷酸酶活性。（2）氮添加条件下，樟子松通过提高深层细根生物量分布、增加表层细根周转和外生菌根菌丝生物量来提高养分吸收。（3）氮添加降低樟子松针叶氮回收能力，对氮回收效率和所有叶龄针叶氮素季节性转移并未产生影响。樟子松通过一年生和二年生针叶的夏季磷素转移来应对磷素供应的不足。（4）减少降雨不影响樟子松针叶的N:P，说明樟子松的养分化学计量比相对稳定。减少降雨处理下樟子松针叶氮磷浓度的变化与土壤氮磷养分供应无关。（5）叶片养分浓度的年份变化与年降雨量呈显著正相关，与土壤有效养分浓度的年际变化无关。樟子松养分回收的年际间变化与土壤养分和针叶养分浓度呈负相关。氮添加对氮回收效率的影响在不同年份间存在显著差异，但其余大部分植物养分指标对氮添加的响应在所有年份间具有一致性。年降雨量的变化决定氮添加对氮回收效率影响的时间变化。相比于当前多数研究试验只开展一年，考虑年际间降雨变化的多年研究试验为理解和预测氮添加对半干旱生态系统养分循环的影响提供了更加全面的视角。本研究从细根功能性状和叶片养分回收角度探讨了樟子松对干旱胁迫和氮沉降的响应，为理解植物对全球气候变化的适应机制提供了科学基础，对干旱易发地区和高氮沉降地区造林时树种选择具有重要意义。