Water Resources Research Act Program

Details for Project ID 2016MT306B

Student Fellowship: Snowpack controls on nitrogen availability and nitrogen uptake in a Rocky Mountain conifer forest

Institute: Montana
Year Established: 2016 Start Date: 2016-03-01 End Date: 2017-02-28
Total Federal Funds: $1,000 Total Non-Federal Funds: $440

Principal Investigators: Claire Qubain

Project Summary: Mountain snowpack has decreased in the Rocky Mountains since 1950 due to human activity[1]. Higher temperatures, earlier snowmelt, and more precipitation falling as rain instead of snow are projected to further alter terrestrial and aquatic productivity[1, 2]. Because most precipitation falls in the form of snow in temperate mountainous systems, decreased snowpack could have drastic social, economic, and ecological implications: climate studies have measured increases in wildfire severity and frequency[3, 4], decreases in soil moisture during the growing season[5, 6], and changes in plant community structure as snowpack declines[7]. While studies have demonstrated widespread ecosystem responses to climate change, the affect of decreased snowpack and earlier snowmelt on nutrient cycles, and the nitrogen cycle in particular, remains unclear. This study will relate topographic and climatic controls on N availability to N uptake in conifer forests in order to clarify biochemical regulation of N dynamics. Topography is a first order control on N availability. It directly mediates N availability through erosion and leaching processes. Topographic heterogeneity indirectly influences N availability by regulating water movement, precipitation distribution, and snowpack accumulation[11-13].Snowpack is also an important driver of N availability in western conifer forests[13-15]. Topographically defined snowpack accumulation influences soil moisture during the growing season[16]. Further, water availability affects plant distribution and microbial activity[17, 18], and so drives spatial patterns of N availability in the soil. Snowpack adds an additional environmental factor influencing N availability by insulating soil. This insulation prevents trees’ roots from freezing during the winter so they can maintain N uptake function during the growing season[19]. Insulation from snowpack also promotes microbial N mineralization throughout the winter, so during snowmelt, a pulse of N from this winter microbial activity is available in soil[20]. Linking physical controls to N availability in soil is necessary in order to understand how and when plants use this limiting nutrient. While topographic and climatic controls on N availability are well studied, temporal variation in plant N uptake has received less attention. We know that the largest pulse of available N occurs during snowmelt in snow dominated, N limited systems, so plants would be expected to take N from this large pool. However, studies have found that in conifers, the N used for the current year’s growth is actually from the previous year[21], suggesting that conifer trees may not be using this readily available N. This potential asynchrony between N availability and N uptake is difficult to detect because few studies have examined N uptake throughout an entire growing season. Studies examining timing of N uptake in Western conifer forests have received little attention. This study addresses how topography, snowpack, and soil moisture control N dynamics and be one of the first to examine these dynamics throughout an entire growing season. Refining our understanding of N availability and uptake in conifer forests will become even more crucial as anthropogenic N deposition increases, snowpack decreases, and climate continues to change.