黄土高原植被演替中凋落物和根系对土壤有机碳激发效应影响机制

Priming effects of soil organic carbon (SOC) are important mechanisms linking the input and output of soil carbon pool, and their strength and direction are regulated by the biotic and abiotic factors. However, the mechanisms of SOC priming effects caused by the input variations of litter and root during vegetation succession currently remained unclear. Therefore, in this project, Populus davidiana and Quercus liaotungensis forests at two successional stages on the Loess Plateau are to be chosen as research objects. By applying the methods of field dynamic monitoring, in-situ control experiments, and laboratory simulation experiments combined with stable isotope 13C labeled tracer, the investigation on the annual and seasonal variations of litter and roots productivity as well as soil respiration of the two forests were carried out. In addition, the differences of soil temperature, moisture, active organic carbon components, microbial parameters and SOC priming effects among different litter and root treatments will be analyzed to explore the main factors affecting the output dynamics of SOC pool, and therefore investigate the response characteristics of SOC priming effects on litter and root control. Moreover, the allocation pattern of input carbon among soil carbon, microbial biomass carbon and CO2-carbon were quantified by 13C isotope mass balance equation, and then the relative contributions of input carbon and original soil organic carbon were evaluated. The project aims to reveal the mechanism of carbon input on the SOC priming effects to promote the understanding of the changes of litter and root during vegetation succession on the stability of soil organic carbon pool, and to provide a scientific basis on the regulation of soil carbon processes on the Loess Plateau.

土壤激发效应是联系土壤有机碳输入和输出的重要机制，其发生强度和方向受外界生物和非生物因素调控。然而，目前关于植被演替中凋落物和根系输入变化对土壤有机碳激发效应影响机制依然缺乏清晰认识。因此, 本项目拟以黄土高原不同演替阶段山杨林和辽东栎林为研究对象，采用野外动态监测、原位控制试验和室内模拟结合13C稳定同位素标记示踪的方法，研究植被演替中凋落物和根系生产力、土壤呼吸等的年、季变化规律，辨析不同凋落物和根系处理土壤温湿度、活性有机碳、微生物指标及有机碳激发效应的差异，明确凋落物和根系输入对土壤有机碳输出的影响动态及主控因素，探讨有机碳激发效应对凋落物和根系输入的响应特征，阐明输入碳在土壤碳库、微生物碳库和矿化CO2碳库的分配格局，量化输入碳与原土壤有机碳对矿化CO2的贡献，揭示碳输入对土壤有机碳激发效应的影响机制，为深化认识黄土高原植被演替中土壤有机碳库稳定性机制及其调控过程提供科学依据。

本研究以黄土高原不同演替阶段山杨林和辽东栎林为研究对象，通过野外监测、原位控制、同位素示踪等手段，明确了该区森林演替中土壤碳库输入和输出特征的变化，阐明了凋落物和根系添加、去除对土壤碳库及其激发效应影响，揭示了土壤有机碳激发效应对该区森林演替中土壤碳过程的驱动机制。主要发现如下：（1）山杨林年均凋落物和根系碳库输入量分别为0.59和1.11 Mg C/ha/yr；辽东栎林年均凋落物和根系碳库输入量为0.77和1.59 Mg C/ha/yr。两种林分均表现为根系碳库输入大于凋落物碳库，并且，演替后期辽东栎林年碳库输入量明显高于演替前期山杨林。（2）山杨林和辽东栎林土壤呼吸总体呈现一致的年、季变化，土壤呼吸最高值出现在雨热条件较好的6-8月份，两种林分年均土壤呼吸没有显著差异。从年际尺度看，两种林分土壤呼吸主要受土壤温度控制，而受土壤水分影响较小。（3）凋落物和根系的添加或去除对山杨林和辽东栎林土壤呼吸和理化性质均产生显著影响。凋落物加倍通过增加更多的外源碳输入，从而提高了土壤有机碳、全氮、溶解性有机碳、微生物量碳、氮等含量，进而促进了土壤呼吸。山杨林根系和凋落物呼吸分别占年异养呼吸的20.9%和70.9%，激发效应占异养呼吸的8.2%。辽东栎林根系和凋落物呼吸分别占年异养呼吸的51.7%和69.3%，激发效应占异养呼吸的-20.9%。（4）山杨林向辽东栎林演替中凋落物生物量增加促进了表层SOC的积累，可用碳源的增加支持了更多土壤微生物，进而提高表层SCO周转速率。辽东栎下层SOC的下降可能归因于强烈的历史侵蚀和土壤激发效应。另一方面，SOC的增加提升了土壤CO2分压，促进表土中碳酸盐的溶解和浸出，并导致SIC降低。相反，降低的SOC会降低CO2分压，有利于下层土壤SIC的增加。本研究为深化认识黄土高原植被演替过程中土壤有机碳库稳定性机制及其调控过程提供科学依据。