Soil Respiration in Bonanza Creek Experimental Forest Floodplain Black Spruce Sites

Fine root processes play a prominent role in the carbon and nutrient cycling of boreal ecosystems due to the high proportion of biomass allocated belowground and the rapid decomposition of fine roots relative to aboveground tissues. To examine these issues in detail, major components of ecosystem carbon flux were studied in three mature black spruce forests in interior Alaska, where fine root production, respiration, mortality and decomposition, and aboveground production of trees, shrubs and mosses were measured relative to soil CO2 fluxes.
Fine root production, measured over a 2-year period using minirhizotrons, varied from 0.004 ? 0.001 mm cm-2 d-1 over winter, to 0.051 ? 0.015 mm cm-2 d-1 during July, with peak growing season values comparable to those reported for many temperate forests using similar methods. On average, 84% of this production occurred within 20 cm of the moss surface, although the proportion occurring in deeper profiles increased as soils gradually warmed throughout the summer. Monthly rates of production and mortality were somewhat asynchronous because mortality tended to peak during fall and be minimal during periods of peak production. Production and mortality were, however, positively correlated across all tubes and time periods (r2 = 0.42, P < 0.0001). Annual fine root production averaged 2.45 ? 0.31, 8.01 ? 1.39, and 2.53 ? 0.27 mm cm-2 yr-1 among the three sites, when averaged across years.
Fine root survival and decomposition were measured by tracking and analyzing the fate of individual fine roots using mark-recapture techniques. Fine root survival was greatest during periods of peak root growth, and least over winter (?time). Roots first appearing in the middle of the growing season had higher survival rates than those first appearing early or late in growing season, or over winter (?cohort), and risk of mortality decreased with root age (?age). Survival estimates translate to mean life spans of 108 ? 4 days during the growing season. While these values are in striking contrast to needle longevity and rates of aboveground litter decomposition, they are similar to many values found for temperate systems, supporting the notion that there are basic morphological and physiological traits of first-order roots that are common to most woody plant root systems. During the growing season, monthly fine root decomposition rates averaged 0.46 ? 0.01 month-1, while decomposition rates over winter averaged 0.73 ? 0.01 winter-1. These growing season estimates translate to 49 ? 2 days from the time a root was first observed as dead, to the time it disappeared. For roots that decomposed during the growing season, those with longer life spans decomposed more slowly after death. Comparing these results with other minirhizotron studies suggests that life-history traits of black spruce first-order roots are similar to those from temperate (and perhaps most) forest ecosystems.
Annual production of fine roots averaged 228 ? 75 g biomass m-2 yr-1, constituting approximately 56% of total stand production. Aboveground production of trees (50 ? 14 g biomass m-2 yr-1, 13%) and shrubs (40 ? 2 g biomass m-2 yr-1, 11%) contributed similarly to total production, while mosses (73 ? 14 g biomass m-2 yr-1, 20%) accounted for the largest component of aboveground production. Soil temperature had a strong control over both soil respiration (Q10 = 2.21 ? 0.31) and root respiration (Q10 = 2.30 ? 0.37). During the growing season (15 May to 15 September), approximately 56% of soil CO2 efflux (580 ? 40 g C m-2) was derived from fine root respiration (329 ? 54 g C m-2). Although apparent rates of heterotrophic respiration (May - September) and total production did not differ, definitive estimates of net ecosystem production are impossible given the potentially large, unmeasured components of NPP, such as root exudation and mycorrhizal production. Nevertheless, rates of fine root production, mortality, and decomposition indicate that in these black spruce ecosystems, fine roots are much more dynamic than would be predicted from patterns of aboveground processes, and that carbon, and presumably nutrients, are cycling through fine roots at rates several orders of magnitude faster than through aboveground tissues.

由于地下生物量分配占比高，且细根相较于地上组织分解更快，细根过程在寒带生态系统的碳与养分循环中发挥着核心作用。为深入解析上述科学问题，研究团队在阿拉斯加内陆的3处成熟黑云杉林分中开展了生态系统碳通量主要组分的观测研究，同步测定了细根的生产、呼吸、死亡与分解过程，以及树木、灌木和苔藓的地上部生产，并将其与土壤二氧化碳通量进行关联分析。
研究采用微根管技术开展了为期2年的细根生产量观测，结果显示其日生长速率在冬季为0.004 ± 0.001 mm·cm⁻²·d⁻¹，7月可达0.051 ± 0.015 mm·cm⁻²·d⁻¹；生长季峰值水平与诸多温带森林采用同类方法测得的结果相当。平均而言，84%的细根生产发生在苔藓表层以下20 cm范围内，但随着夏季土壤逐步升温，深层土壤中的细根生产占比逐渐升高。细根生产与死亡的月动态存在一定异步性：死亡峰值多出现于秋季，而在生产高峰期死亡率最低。不过在所有观测管与观测时段内，细根生产与死亡均呈显著正相关（决定系数r²=0.42，P<0.0001）。跨2年平均后，3个样地的年细根生产量分别为2.45 ± 0.31、8.01 ± 1.39和2.53 ± 0.27 mm·cm⁻²·yr⁻¹。
研究采用标记-重捕法追踪并分析单根细根的命运，以此测定细根存活与分解过程。细根存活率在根系生长高峰期最高，冬季最低。生长季中期萌发的细根，其存活率高于生长季早、晚期萌发的细根，亦高于冬季萌发的细根；且死亡率随根系年龄增大而降低。存活估算结果显示，生长季内细根平均寿命为108 ± 4天。尽管该数值与针叶寿命及地上凋落物分解速率差异显著，但与诸多温带生态系统的观测值相近，这支持了“一级根存在多数木本植物根系共有的基本形态与生理特征”这一观点。生长季内细根月均分解速率为0.46 ± 0.01 month⁻¹，冬季分解速率则为0.73 ± 0.01 winter⁻¹。上述生长季分解速率对应：细根从首次被观测为死亡至完全消失的平均时长为49 ± 2天。对于生长季内发生分解的细根，其寿命越长，死亡后的分解速率越慢。将本研究结果与其他微根管研究对比可知，黑云杉一级根的生活史特征与温带（乃至多数）森林生态系统的同类根系相似。
细根年生产量平均为228 ± 75 g生物量·m⁻²·yr⁻¹，约占林分总生产量的56%。树木地上部生产量为50 ± 14 g生物量·m⁻²·yr⁻¹（占比13%），灌木地上部生产量为40 ± 2 g生物量·m⁻²·yr⁻¹（占比11%），二者对总生产量的贡献相近；而苔藓地上部生产量为73 ± 14 g生物量·m⁻²·yr⁻¹（占比20%），是地上部生产的最大组分。土壤温度对土壤呼吸（Q10=2.21 ± 0.31）与根系呼吸（Q10=2.30 ± 0.37）均具有显著调控作用。生长季（5月15日至9月15日）期间，约56%的土壤二氧化碳排放通量（580 ± 40 g C·m⁻²）来自细根呼吸（329 ± 54 g C·m⁻²）。尽管异养呼吸的表观速率（5月至9月）与总生产量并无显著差异，但由于净初级生产力（Net Primary Productivity, NPP）存在诸多潜在且未被测定的组分（如根系分泌物与菌根生产），因此无法得到生态系统净生产的准确估算值。尽管如此，细根生产、死亡与分解的速率表明，在这些黑云杉生态系统中，细根的周转速率远高于基于地上部过程所预测的水平，且碳以及养分通过细根循环的速率较地上组织快数个数量级。