Charaterizing soil properties and biogeochemical processes in a mountain peatland

Abstract

Northern peatlands are important to the global carbon (C) and nitrogen (N) cycles. Peat profiles in Rocky Mountain areas commonly show complex stratigraphy with underlying and/or interbedded mineral sediments, referred to as stratified mineral horizons. Stratified mineral horizons usually have lower hydraulic conductivity and more electron acceptors, which influences biogeochemical processes. To study the effect of mineral sediments on pedological and biogeochemical processes, I conducted a field study and a microcosm study. The field study was located in a mountain peatland in the foothills of the Canadian Rocky Mountains with three different organic soil types: sedge peat/silty sediments/calcareous sediments (PMC), sedge peat/silty sediments/moss peat (PMP) and sedge peat/moss peat (PP). Soil samples were tested for spatial distribution of total organic C (TOC), total N (TN), pH, volumetric water content (θv), C and N cycling rates, C composition and microbial community structure. A microcosm study was designed to mimic climate warming conditions with four temperature-water table treatments: current temperature/current water table, higher temperature/current water table, current temperature/lower water table, and higher temperature/lower water table. In the microcosm study, PMC and PP soils were incubated for 28 days and tested for GHG emissions and concentrations, biogeochemical process rates, apparent enzyme activation energy (Ea) and bacterial community structure. In the field study, results indicated mineral sediments mainly affect pedological and biogeochemical processes in subsurface peat rather than surface peat. Mineral sediments affected the spatial distributions of total organic C (TOC), total N (TN), pH and volumetric water content (θv) via elevating the pH adjacent to calcareous sediments and slowing water infiltration to lower depths. The pH and θv further affected TOC and TN distribution by regulating organic matter decomposition during the peatland’s geomorphic history. At the same time, mineral sediments also affected C and N cycling processes, though depth had an even greater effect. The effect of mineral sediments on N cycling was mainly due to high pH from calcareous sediments, which promoted net nitrification but lowered net ammonification in the PMC. Moreover, mineral sediment mitigated the lag phase of N cycling in deeper layers. The effect of mineral sediments on C cycling was reflected in two aspects, geomorphic history and hydrological conditions. During the peatland’s geomorphic history, mineral horizons promoted decomposition by increasing pH and providing electron acceptors in overlying peat. Enhanced decomposition in the past resulted in more recalcitrant materials in peat at present. This, combined with physicochemical protection of C by mineral sediments, further restricted C mineralization in the PMP and PMC. Hydrologically, stratified mineral horizons slowed water infiltration and resulted in higher θv above the mineral horizon and lower θv below the mineral horizon. This restricted C mineralization in peat above mineral sediment and encouraged C mineralization in peat below mineral sediment in PMP. In addition, these factors also affected microbial community structure, with the highest Stress and Bacteria:Fungi ratio in peat above mineral sediment and different microbial community structure in peat below mineral sediments. In the microcosm study, I found that high temperature increased GHG emission and GHG concentration – especially at depth – in most samples. Soil types affected CO2 and N2O concentrations from subsurface horizons: PP had higher CO2 and N2O than PMC. Importantly, N2O concentration and production rates were affected by interaction of soil types and temperature near the water table: N2O production in PP was more enhanced by high temperature. This was possibly because PP had greater labile C and lower pH. In addition, compared with PP, the Ea for N2O generation in PMC was increased more by high temperature incubation and microbial community structures were quite different in the two soils, especially the lower relative abundance of copiotrophs in PMC. Overall, the findings highlight that stratified mineral sediment affected spatial distribution of key soil properties, which influenced biogeochemical processes in this mountain peatland. Elevated pH due to calcareous sediment promoted nitrification and C mineralization. In addition, stratified mineral sediment affected θv, which then affected microbial community structure and C mineralization. Under a warming climate, compared with a continuous peat profile, peat with mineral sediments tends to have less labile C and higher pH, which could potentially result in less CO2 and N2O emission and mitigate N2O production proximal to the water table.