Abstract
Short-lived pulses of soil nitrous oxide (N2O) emissions during freeze-thaw periods can dominate annual cumulative N2O fluxes from temperate managed and natural soils. However, the effects of freeze thaw cycles (FTCs) on dinitrogen (N2) emissions, i.e., the dominant terminal product of the denitrification process, and ratios of N2/N2O emissions have remained largely unknown because methodological difficulties were so far hampering detailed studies. Here, we quantified both N2 and N2O emissions of montane grassland soils exposed to three subsequent FTCs under two different soil moisture levels (40 and 80% WFPS) and under manure addition at 80% WFPS. In addition, we also quantified abundance and expression of functional genes involved in nitrification and denitrification to better understand microbial drivers of gaseous N losses. Our study shows that each freeze thaw cycle was associated with pulse emissions of both N2O and N2, with soil N2 emissions exceeding N2O emissions by a factor of 5–30. Increasing soil moisture from 40 to 80% WFPS and addition of cow slurry increased the cumulative FTC N2 emissions by 102% and 77%, respectively. For N2O, increasing soil moisture from 40 to 80% WFPS and addition of slurry increased the cumulative emissions by 147% and 42%, respectively. Denitrification gene cnorB and nosZ clade I transcript levels showed high explanatory power for N2O and N2 emissions, thereby reflecting both N gas flux dynamics due to FTC and effects of different water availability and fertilizer addition. In agreement with several other studies for various ecosystems, we show here for mountainous grassland soils that pulse emissions of N2O were observed during freeze-thaw. More importantly, this study shows that the freeze-thaw N2 pulse emissions strongly exceeded those of N2O in magnitude, which indicates that N2 emissions during FTCs could represent an important N loss pathway within the grassland N mass balances. However, their actual significance needs to be assessed under field conditions using intact plant-soil systems.
Highlights
In the last century, human activities have more than doubled the amount of reactive nitrogen (Nr) in the biosphere, mainly through fossil fuel combustion and fertilization (Fowler et al 2013)
Available approaches for quantifying the gaseous end-products (N2 + N2O) of denitrification are limited and can be classified into (1) the acetylene blockage method, which exploits acetylene inhibition of the reduction of N2O to N2 (Groffman et al 2006; Felber et al 2012), (2) the 15N gas flux technique, which requires 15NO3− application to soil followed by the measuring of 15N2O and/or 15N2 in the gaseous end-products (Kulkarni et al 2014; Sgouridis and Ullah 2015), and (3) the gas-flow soil core (GFSC) method, which allows the direct simultaneous measurement of N2O and N2 fluxes from soil cores when the soil atmosphere is replaced by a mixture of He/ O2 (Butterbach-Bahl et al 2002; Wang et al 2011)
Over the entire incubation period including all three freeze-thaw cycles (FTCs), increasing soil moisture from 40% to 80% WFPS and the addition of slurry increased the cumulative N2 emissions by 101.9% and 76.8%, respectively (Table 2)
Summary
Human activities have more than doubled the amount of reactive nitrogen (Nr) in the biosphere, mainly through fossil fuel combustion and fertilization (Fowler et al 2013). Available approaches for quantifying the gaseous end-products (N2 + N2O) of denitrification are limited and can be classified into (1) the acetylene blockage method, which exploits acetylene inhibition of the reduction of N2O to N2 (Groffman et al 2006; Felber et al 2012), (2) the 15N gas flux technique, which requires 15NO3− application to soil followed by the measuring of 15N2O and/or 15N2 in the gaseous end-products (Kulkarni et al 2014; Sgouridis and Ullah 2015), and (3) the gas-flow soil core (GFSC) method, which allows the direct simultaneous measurement of N2O and N2 fluxes from soil cores when the soil atmosphere is replaced by a mixture of He/ O2 (Butterbach-Bahl et al 2002; Wang et al 2011) Among these approaches, the GFSC method has the advantage that it does not require addition of isotopically labeled substrates or inhibitors to the soil (Chen et al 2015; Wen et al 2016)
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