Abstract
Considerable research has shown that modifications in global temperature regimes can lead to changes in the interactions between soil respiration and the sequestration of C and N into soil organic matter (SOM). We hypothesized that despite the interconnected nature of respiration, net N mineralization, and nitrification processes, there would be differences in their thermodynamic responses that would affect the composition of inorganic soil N and the potential for retention of N in SOM. To test this hypothesis, soil respiration, N mineralization and nitrification responses were evaluated during constant temperature incubations at seven temperatures (4–42°C) in tilled and no-till soils from two major agroecological zones in Oregon; Willamette Valley, and Pendleton located in the Columbia River Basin. We observed (1) significant thermodynamic differences between the three processes in all soils, (2) a distinctly different thermodynamic profile in Willamette vs. Pendleton, and (3) a dynamic response of Topt (optimal temperature for activity), and Tsmax (temperature of greatest rate response to temperature), and temperature sensitivity (Δ) over the incubation time course, resulting in shifts in the thermodynamic profiles that could not be adequately explained by changes in process rates. We found that differences in contributions of ammonia oxidizing archaea and bacteria to nitrification activity across temperature helped to explain the thermodynamic differences of this process between Willamette and Pendleton soils. A two-pool model of SOM utilization demonstrated that the dynamic thermodynamic response of respiration in the soils was due to shifts in utilization of labile and less-labile pools of C; and that the respiration response by Pendleton soils was more dependent upon contributions from the less-labile C pool resulting in higher Topt and Tsmax than Willamette soils. Interestingly, modeling of N mineralization using the two-pool model suggested that only the less-labile pool of SOM was contributing to N mineralization at most temperatures in all soils. The difference in labile and less-labile SOM pool utilization between respiration and N mineralization may suggest that these processes may not be as interconnected as previously thought.
Highlights
Considerable research has shown that modifications in global temperature regimes can lead to changes in the interactions between soil respiration and the sequestration of C and N into soil organic matter (SOM) (Davidson and Janssens, 2006; Sierra, 2012; Sierra et al, 2015; Blagodatskaya et al, 2016) (Figure 1).Warming of soils increases the rate of decomposition of SOM catalyzed by heterotrophic soil microorganisms for energy generation (CO2 respiration) and growth (C sequestration)
Beginning at 14 d rates of CO2 production slowed over time in all soils, declining significantly at some temperatures (Supplementary Figures 2, 3); with the exception of Pendleton soils incubated at 42◦C in which respiration rates increased significantly after 7 d
Regardless of differences in climatic regimes, or soil C contents, there were no significant differences in the thermodynamic parameters describing respiration of the four soils used in this study
Summary
Considerable research has shown that modifications in global temperature regimes can lead to changes in the interactions between soil respiration and the sequestration of C and N into soil organic matter (SOM) (Davidson and Janssens, 2006; Sierra, 2012; Sierra et al, 2015; Blagodatskaya et al, 2016) (Figure 1).Warming of soils increases the rate of decomposition of SOM catalyzed by heterotrophic soil microorganisms for energy generation (CO2 respiration) and growth (C sequestration). A portion of the mineralized soil NH+4 released during respiratory processes is immobilized into microbial biomass, and the remaining NH+4 becomes available for plant uptake or nitrification and denitrification processes (De Neve et al, 2003; Miller and Geisseler, 2018). It is well-documented that the activity of SOM degrading microorganisms continues to increase in response to temperature up to ≥50◦C, yet microbial growth is generally optimal at ∼30◦C (He et al, 2014; Dan et al, 2019); resulting in greater vulnerability of soil C and N loss above 30◦C. The activity of nitrifiers controls the portion of N that is potentially vulnerable to leaching from the rooting zone in response to precipitation or irrigation, and contributes to the efficiency with which mineral N is converted into plant N by controlling the availability of reduced (NH+4 ) or oxidized (NO−3 ) forms of N
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