落叶松人工林细根动态与土壤资源有效性关系研究

树木细根在森林生态系统C和养分循环中具有重要的作用。由于温带土壤资源有效性具有明显的季节变化, 导致细根生物量、根长密度 (Rootlengthdensity, RLD) 和比根长 (Specificrootlength, SRL) 的季节性变化。以 17年生落叶松 (Larixgmelini) 人工林为研究对象, 采用根钻法从 5月到 10月连续取样, 研究了不同土层细根 (直径≤ 2mm) 生物量、RLD和SRL的季节动态, 以及这些根系指标动态与土壤水分、温度和N有效性的关系。结果表明 :1) 落叶松细根年平均生物量 (活根 +死根 ) 为 189.1g·m-2 ·a-1, 其中 5 0 %分布在表层 (0~ 10cm), 33%分布在亚表层 (11~ 2 0cm), 17%分布在底层 (2 1~ 30cm) 。活根和死根生物量在 5~ 7月以及 9月较高, 8月和 10月较低。从春季 (5月 ) 到秋季 (10月 ), 随着活细根生物量的减少, 死细根生物量增加 ;2 ) 土壤表层 (0~ 10cm) 具有较高的RLD和SRL, 而底层 (2 1~ 30cm) 最低。春季 (5月 ) 总RLD和SRL最高, 分别为 10 6 2 1.4 5m·m-3 和 14.83m·g-1, 到秋季 (9月 ) 树木生长结束后达到最低值, 分别为 2 198.2 0m·m-3 和 3.77m·g-1;3) 细根生物量、RLD和SRL与土壤水分、温度和有效N存在不同程度的相关性。从单因子分析来看, 土壤水分和有效N对细根的影响明显大于温度, 对活根的影响大于死根。由于土壤资源有效性的季节变化, 使得C的地下分配格局发生改变。各土层细根与有效性资源之间的相关性反映了细根功能季节性差异。细根 (生物量、RLD和SRL) 的季节动态 (5 8%~ 73%的变异 ) 主要由土壤资源有效性的季节变化引起。

SEASONAL DYNAMICS OF FINE ROOT BIOMASS, ROOT LENGTH DENSITY,SPECIFIC ROOT LENGTH AND SOIL RESOURCE AVAILABILITY IN A LARIX GMELINI PLANTATION

Fine root turnover is a major pathway for carbon and nutrient cycling in terrestrial ecosystems and most likely is sensitive to many global change factors. Despite its importance in plant C allocation, nutrient cycling dynamics and the tremendous research efforts that have been made in the past, our understanding of fine root turnover remains limited, because dynamic fine root processes associated with soil resource availability still remains poorly understood. Soil moisture, temperature and available nitrogen are the most important soil resources that impact fine root growth and mortality at both the individual root branch and ecosystem level. In temperate forest ecosystems, seasonal changes of soil resource availability will alter the pattern of carbon allocation to below ground; therefore, fine root biomass, root length density (RLD) and specific root length (SRL) vary during the growing season. Studying seasonal changes of fine root biomass, RLD and SRL associated with soil resource availability will help us understand the mechanistic controls of carbon to fine root longevity and turnover.The objective of this study was to understand whether seasonal variations of fine root biomass, RLD and SRL were associated with soil resource availability, such as moisture, temperature and nitrogen, and to understand how these soil components impacted fine root dynamics in a Larix gmelini plantation. We used a soil coring method to obtain fine root (≤2 mm in diameter) samples every month from May to October in 2002 from a 17 year old Larix gmelini plantation in Maoershan Experiment Station, Northeast Forestry University. Seventy-two soil cores (Inside diameter 60 mm; depth intervals: 0-10, 11-20, and 21-30 cm) were sampled randomly from three replicate 25 m×30 m plots to estimate fine root biomass (live and dead), and calculate root length density (RLD) and specific root length (SRL). Soil moisture, temperature, and nitrogen (ammonia and nitrate) at the three depth intervals also were analyzed in the plots. The results showed that the average standing fine root biomass (live and dead) was 189.1 g·m-2·a-1, and 95.4 g·m-2·a-1 (50%) was distributed in the surface soil layer (0-10 cm), 61.5 g·m-2·a-1 (33%) and 32.2 g·m-2·a-1 (17%) were in middle (11-20 cm) and deep layer (21-30 cm), respectively. Live and dead fine root biomass was the highest from May to July and in September, but lower in August and October. The live fine root biomass decreased and dead biomass increased during the growing season. Mean RLD (7 411.56 m·m-3·a-1) and SRL (10.83 m·g-1·a-1) in the surface layer were greater than the RLD (1 474.68 m·m-3·a-1) and SRL (8.56 m·g-1·a-1) in the deep soil layer. Root length density and SRL in May were the highest (10 621.45 m·m-3 and 14.83 m·g-1) as compared to other months, and RLD was the lowest in September (2 198.20 m·m-3) and SRL the lowest in October (3.77 m·g-1). Seasonal dynamics of fine root biomass, RLD and SRL had a close relationship with changes in soil moisture and nitrogen availability, and, to a lesser extent, temperature, as determined by regression analysis. Fine roots in the upper soil layer have a function of absorbing water and nutrients, while the main function of fine roots in the deeper soil may be water uptake rather than nutrient acquisition. Therefore, carbon allocation to roots in the upper soil layer and deeper soil layers was different. Multiple regression analysis showed that variation in soil resource availability can explain 71%-73% of the seasonal variation of RLD and SRL and 58% of the variation in fine root biomass. These results suggest a greater metabolic activity of fine roots living in soils with high resource availability, resulting in an increased allocation of carbohydrates to fine roots in resource rich soils but lower allocation to roots in soils with lower resource availability.