RAPID: Do wildfire effects on roots and rhizosphere activity control both short- and long- term soil C stabilization?

这一RAPID提案是为了研究受大型灾难性野火影响的林分土壤碳动态，这场野火烧毁了俄勒冈州H.J.安德鲁斯实验森林的三个流域和周围森林。美国的原始森林在树木生物量和土壤中都含有大量的碳(C)。正因为如此，森林采伐或火灾等干扰可能会从植被或土壤中释放C到空气中，从而有可能影响大气。在过去的几十年里，太平洋西北地区野火的数量和规模都在增加，预计温度的升高会延长火灾季节。由于预计在不久的将来，美国西部的许多生态系统将经历越来越大、越来越严重的火灾，因此了解野火如何影响生态系统的碳循环对未来全球温度的预测越来越重要。虽然火对地表植被碳收支的影响相对容易观察和测量，但火对地表以下土壤有机质C的影响不太容易测量或预测。考虑到土壤C库比大气C库大3倍以上，了解土壤对极端火灾干扰的响应机制对于建立不同环境变化情景下全球C浓度的精确模型至关重要。这项研究的结果不仅将为全球地球系统科学模型提供信息，还将帮助生态系统管理者了解不同火灾管理方案的潜在收益和成本。本研究将检验关于根/根际活动以及地上和地下碎屑输入对灾难性扰动后土壤C稳定和不稳定影响的新假设。这些假设基于一种新的土壤C动力学模型，该模型认为，在未受干扰的生态系统中，土壤C是根际源土壤C损失和根源土壤C增加之间的平衡。当不稳定的碳源（通常来自活根渗出物）的输入增加了稳定土壤有机质（SOM）库（如矿物相关有机质（MAOM））的呼吸（= C损失）时，土壤C就会引发；然而，微生物对优质碎屑的有效利用，如细根凋落物，以MAOM的形式保留在土壤中（= C增益）。这两个相互竞争的过程使MAOM水平保持稳定并低于真正的矿物饱和度。然而，当树木死亡干扰发生时，不仅有大量死亡细根物质涌入（通过微生物利用和微生物周转帮助土壤碳稳定），而且根际注入也随着根系挥发性溶解有机碳的停止而停止（消除土壤碳的不稳定）。该模型预测，由于根际活动的立即减少加上碎屑根物质的持续流入，在碎屑输入受到严重破坏后，土壤C库和C动态的变化非常迅速。这与目前的收获或火灾后土壤C模型相反，在这些模型中，最稳定的土壤C库可能会增加，直到森林再生。将根据安德鲁斯森林假日农场火灾中整个地区的树木死亡率、烧伤严重程度、火灾前地上生物量和激光雷达导出的地球物理变量建立采样网格。土壤岩心将在确定的采样点内沿样带采集，并进行密度分馏，以分离出根、土壤颗粒碳和MAOM。整个土壤和土壤物理组分中的SOM将使用固态13C核磁共振进行表征，这也使我们能够检查热原C在POM或MAOM池中的任何运动。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This RAPID proposal is to examine soil carbon dynamics in forest stands affected by a large and catastrophic wildfire that burned three watersheds and surrounding forest in the H.J. Andrews Experimental Forest, OR. Old-growth forests in the USA hold significant amounts of carbon (C) in both tree biomass and in soils. Because of this, disturbances such as forest harvest or fire that might release C into the air from vegetation or soil have the potential to impact the atmosphere. Both the number and the sizes of wildfires in the Pacific Northwest have been increasing in the last several decades, and increases in temperature are projected to lengthen fire seasons. Because so many Western U.S. ecosystems are expected to experience increasingly large and severe fires in the near future, understanding how wildfire impacts ecosystem C cycling is increasingly critical to future projections of global temperatures. While fire effects on the C budgets of above ground vegetation are relatively easy to observe and measure, fire effects on soil organic matter C below the surface is less easy to measure or predict. Given that the soil C pool is over 3 times greater than the atmospheric C pool, it is critical to understand the mechanisms of soil response to extreme fire disturbance in order to have accurate models of global C concentrations under different scenarios of environmental change. Results from this research will not only inform global earth system science models, but will also help inform ecosystem managers about potential benefits and costs of different fire management scenarios.This research will test novel hypotheses about the influences of root/rhizosphere activity and above and belowground detrital inputs on soil C stabilization and destabilization after catastrophic disturbance. Hypotheses are based on a new model of soil C dynamics that starts with the argument that soil C in undisturbed ecosystems is a balance between rhizosphere-derived priming losses and root-derived soil C gains. Priming of soil C occurs when inputs of labile C sources, usually from live root exudation, fuels increased respiration (= C loss) of stable soil organic matter (SOM) pools such as mineral-associated organic matter (MAOM); however, efficient microbial use of high-quality detrital inputs such as fine root litter are retained (= C gain) in soil as MAOM. These two competing processes keep MAOM levels both stable and below true mineral saturation. However, when tree-mortality disturbance occurs, not only is there a significant influx of dead fine root material (aiding soil C stabilization through microbial use and microbial turnover), but rhizosphere priming ceases as root exudation of labile dissolved organic carbon ceases (removing soil C destabilization). This model predicts that change to soil C pools and C dynamics after significant disruption to detrital inputs is remarkably rapid due to immediate reduction in rhizosphere activity coupled with continued influx of detrital root materials. This is contrary to current models of soil C after harvest or fire in which the most stable pools of soil C could increase until the forest regenerates. A sampling grid will be established based on tree mortality, burn severity, pre-fire aboveground biomass and LiDAR-derived geophysical variables throughout the region burned in the Holiday Farm fire at the Andrews Forest. Soil cores will be taken along transects within identified sampling sites and density fractionated to separate out roots, particulate soil carbon, and MAOM. SOM in whole soils and soil physical fractions will be characterized using solid-state 13C nuclear magnetic resonance also allowing us to examine any movement of pyrogenic C into either POM or MAOM pools.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.