Abstract:

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.

Key words: Larix gmelini, Fine root biomass, Root length density, Specific root length, Soil resource availability, Seasonal dynamics