Impact of projected land conversion on water balance of boreal soils in western Newfoundland

Conversion of boreal forest into agricultural land is likely to occur due to the shift of climatic zones and increasing food demand. However, any land conversion will affect the water balance and hence solute fluxes within the soil column and connected ecosystems. Understanding the consequences of land conversion on soil hydrology is essential to support an economically viable agriculture while minimizing its environmental footprint. Hydrological models can simulate these effects based on regionally adjusted climate scenarios. Here, we combined a local climate analysis with hydrological simulations (Hydrus-1D) of boreal soils before and after agricultural conversion. Historical climate analysis showed increasing temperatures and growing degree days while precipitation remains stable. Hydrological simulations revealed lower saturation and higher infiltration rates for unconverted soils, indicating lower runoff and increased infiltration and deep percolation. In contrast, agricultural soils have slower infiltration rates, particularly in the upper horizon. Over the long term, agricultural conversion consequently increases erosion risk and nutrient loss by runoff. This might further progressively limit groundwater recharge, affect hydrological processes and functions and future drought/flood conditions at catchment levels. Hence, conversion of boreal soils demands a primary identification of suitable areas to minimize its impacts.

Temporal variation in carbon and nitrogen sequestration rates in boreal soils across a variety of ecosystems

Boreal soils play a critical role in the global carbon (C) cycle; therefore, it is important to understand the mechanisms that control soil C accumulation and loss for this region. Examining C & nitrogen (N) accumulation rates averaged over decades to centuries may provide additional understanding of the dominant mechanisms for their storage, which can be masked by seasonal and interannual variability when investigated over the short-term. We examined longer-term accumulation rates, using 210Pb and 14C to date soil layers, for a wide variety of boreal ecosystems: a black spruce forest, a shrub ecosystem, a tussock grass ecosystem, a sedge dominated ecosystem, and a rich fen. All ecosystems had similar decadal C accumulation rates, averaging 84 ± 42 gC m−2 yr−1. Long-term (century) C accumulation rates were slower than decadal rates, averaging 14 ± 5 gC m−2 yr−1 for all ecosystems except the rich fen, for which the long-term C accumulation rates was more similar to decadal rates (44 ± 5 gC m−2 yr−1 and 76 ± 9 gC m−2 yr−1, respectively). The rich fen also had significantly higher long-term N accumulation rates (2.66 gN m−2 yr−1). The lowest N accumulation rate, on both a decadal and long-term basis, was found in the black spruce forest (0.22 and 1.4 gN m−2 yr−1, respectively). Our results suggest that long-term C and N cycling at the rich fen is fundamentally different from the other ecosystems, likely due to differences in the predominant mechanisms for nutrient cycling (for C) and reduced amounts of disturbance by fire (for C & N). This result implies that most shifts in ecosystem vegetation across the boreal region, driven by either climate or succession, will not significantly impact regional C or N dynamics over years to decades. However, ecosystem transitions to or from a rich fen will promote significant shifts in soil C and N storage.