Abstract:

Tree root respiration is a major contributor to soil CO2 pools, and 2/3 of total forest soil respiration is from root respiration. Information about root respiration is important for understanding the implications of environmental change on soil carbon cycling and carbon sequestration, and for the development of carbon models of forest ecosystem dynamics. Furthermore, the global CO2 flux from root respiration is estimated to be about 18 Pg C·a-1, which is an order of magnitude larger than that produced by anthropogenic sources of CO2. Therefore, changes in forest tree root respiration could have a significant impact on the future global carbon balance. Ecological research on tree root respiration is relatively recent and still very little research on this topic has been conducted in China. In this paper, We review the characteristics, methodologies and factors affecting tree root respiration for the purpose of stimulating new, domestic research on this topic. Tree root respiration is composed of maintenance respiration and growth respiration. Maintenance respiration, the dominant component of total of total root respiration, is used to maintain the living biomass, and growth respiration, which is used to construct new biomass, is proportional to the amount of new dry matter synthesized. Root respiration rates vary significantly among forest types. The proportion of the total soil carbon flux that is attributable to live root respiration appears to be very high in cold, northern biomes, ranging from 50% to 93% in the arctic tundra and from 62% to 89% in boreal forests. In temperate zones, estimated proportions of the total soil respiration flux that is derived from live root respiration range from 33% to 50% in broad-leaved forests and from 35% to 62% in pine forests. Root respiration typically is 40%-60% of total soil respiration in most forests. Tree root respiration has significant seasonal dynamics with respiration greatest during the growing season and lowest during the dormant periods of the year. . Methods used to measure tree root respiration include root exclusion methods, in vitro root techniques, stable or radioactive isotope methods and in situ cuvette methods. Each approach has advantages and disadvantages. The first two methods are relatively simple and inexpensive and are commonly used in forest ecosystems. Isotope based methods provide quantitative answers with the least amount of disturbance to the soil and roots, but the complexity of the experimental setup and the high costs associated with the analysis of radioactive or stable C isotopes are major disadvantages. The in situ cuvette method is considered an important method for future studies. Critical factors influencing rates of tree root respiration include soil temperature, root diameter size, ambient CO2 concentration, soil moisture, and nutrient availability. . Despite intensive research in recent years, many uncertainties remain in this dynamic and important field of research. Topics of particular importance include: 1) Discussion and comparison of the appropriate methods for accurately measuring tree root respiration; 2) Application of effective methods for separating root respiration and rhizosphere respiration in the field; 3) Long-term research on the dynamics of tree root respiration in forest ecosystems; 4) Studies of tree root respiration in different ecosystems and climatic zones; 5) How to scale up from small chambers to the stand level, ecosystem level, regional or global level; 6) Understanding the mechanistic response of tree root respiration to global climate change; and, 7) Inter-disciplinary research on tree root respiration to understand its role in the global carbon cycle.