Temperature Sensitivity of Nitrogen Dynamics of Agricultural Soils of the United States

Soil temperature controls gaseous nitrogen losses through nitrous oxide (N 2 O) and ammonia (NH 3 ) fluxes. Eight surface soils from agricultural fields across the United States were incubated at 10˚C, 20˚C, and 30˚C, and N 2 O and NH 3 flux were measured twice a week for 91 and 47 d, respectively. Changes in cumulative N 2 O and NH 3 flux and net N mineralization at three temperatures were fitted to calculate Q 10 using the Arrhenius equation. For the majority of soils, Q 10 values for the N 2 O loss ranged between 0.23 and 2.14, except for Blackville, North Carolina (11.4) and Jackson, Tennessee (10.1). For NH 3 flux, Q 10 values ranged from 0.63 (Frenchville, Maine) to 1.24 (North Bend, Nebraska). Net soil N mineralization-Q 10 ranged from 0.96 to 1.00. Distribution of soil organic carbon and total soil N can explain the variability of Q 10 for N 2 O loss. Understanding the Q 10 variability of soil N dynamics will help us to predict the N loss.


Introduction
Gaseous losses of nitrogen (N), nitrous oxide (N 2 O) denitrification and ammonia (NH 3 ) volatilization, reduce fertilizer-N use efficiency and may cause environmental degradation [1]. Global estimates suggest approximate N losses of generally expressed as the function of the increase in metabolic rate with 10˚C rise in temperature or Q 10 . For most modeling approaches, Q 10 value was assumed to be close to 2, irrespective of soil type, climate and management practices [6] [8]. However, researchers reported a wide range of Q 10 values ranging from 1 to 17.1 for denitrification [9], 1.4 to 5.0 for volatilization [10], and 1.67 to 2.43 for soil N mineralization [11].
A laboratory incubation study was conducted to determine the Q 10 value of N 2 O and NH 3 flux, volatilization and N mineralization for eight soil samples collected across agricultural systems of the United States. If Q 10 value is not affected by climate, soil type, or cropping system, measurements of Q 10 will be equal to 2 regardless of soil evaluated. To test this hypothesis, we measured cumulative N 2 O and NH 3 flux and net N mineralization with incubation temperature, and temperature sensitivity or Q 10 of N 2 O and NH 3 flux and net N mineralization at 10˚C, 20˚C, and 30˚C. We then calculated the temperature sensitivity, or Q 10 of N 2 O and NH 3 flux and net N mineralization for these agricultural soils.

Materials and Methods
Surface soil samples of 0 -15 cm depth were collected from eight agricultural fields across the United States ( Figure 1, Table 1). Soil samples were air-dried  and grounded to pass through 2 mm sieve. Soil pH and electrical conductivity were measured of 1:2.5 soil slurry with Oakton PC700 pH and EC meter. Soil organic carbon and total N were determined by automated dry combustion method [12]. Soil inorganic N concentration was measured by extracting soils with 2 M KCl and determining NH + 4 and NO − 3 concentrations using Timberline Ammonia (TL-2800) analyzer (Boulder, CO). Field capacity (at 0.33 bar) was determined using the pressure plate apparatus as described by [13].
Soil samples were incubated at 10˚C, 20˚C and 30˚C using an incubation chamber. For incubation, 30 g soils moistened at field capacity level were placed in a 1-L clear jar ( servation day, first headspace air sample was collected using a 10 mL syringe, followed by the removal of the acid trap, then the jar was aerated for half an hour, and soil moisture was readjusted to field capacity and then jars were capped and returned to the incubator. The N 2 O concentration of headspace air samples was determined using a Shimadzu GC-2014 (Shimadzu Scientific Instruments Inc., Houston, TX) fitted with 63Ni-electron capture detector. The GC oven was operated at 80˚C and ECD was operated at 325˚C, and N 2 carrier gas was supplied at 20 PSI. Instrument was calibrated using analytical N 2 O standards of: 0, 1, 5, 50, 100, 500, and 1000 µmole•ml −1 . Compound peak was recorded and analyzed with Lab Solutions software (LabSolutions, Atlanta, Georgia). The N 2 O concentration was converted to mass unit using ideal gas equations and expresses as micrograms of N 2 O produced between sampling days per kg of soil [14] [15]. Soil-emitted NH 3 was trapped and replaced with fresh phosphoric acid solution at the same intervals as N 2 O flux measured. The collected acid solution was extracted with 25 mL of 2 M KCl with half an hour shaking the mixture in reciprocal shaker [14]. The extracts were then analyzed for NH + 4 concentrations using an automated ammonia analyzer (TL 2800, Timberline Instruments, Boulder, CO). The amount of volatilization during each incubation interval was expressed in the form of microgram NH 3 per gram soil. Cumulative NH 3 -N loss (mg NH 3 -N kg soil) during the entire incubation was computed from the summation of NH 3 emission during all sampling periods.
After 91 days of incubation, soil samples from each jar were analyzed for inorganic N concentration (NH + alization percentage and Q 10 values were compared for different sites using the completely randomized design (CRD) with a mean separation at 95% significance level using SAS 9.4. For each site, incubation temperature effect on cumulative N 2 O and NH 3 flux were also determined using CRD with a mean separation at 95% significance level. Correlation coefficient and regression analyses were conducted to determine the relationship between soil properties and Q 10 values using SAS 9.4.

Results and Discussion
Cumulative N 2 O flux increased with temperature for most soils except those collected from Frenchville, Bismarck, and Pendleton ( 14. Blackville and Jackson had lower soil organic C than other sites; low soil organic C or high recalcitrance of substrates should generally be more sensitive to temperature changes than that of more labile substrates, which could, in turn, increase the Q 10 value. Researchers have also found that additions of C and N substrates reduced Q 10 of N 2 O due to increased soil microbial C and N use efficiency [15]. Increasing temperature reduced cumulative NH 3 flux except for Downer, and North Bend, sites (Table 2). Soils from Blackville had the highest and Frenchville, had the lowest cumulative NH 3 flux at all three temperatures. Soils from North Bend had the highest Q 10 value for NH 3 , but similar to Downer, and Pendleton. For the rest of the sites, Q 10 value for NH 3 ranged from 0.63 to 0.70.

Most researchers observed an increase in volatilization loss with temperature [7]
[16]. Researchers [7] reported a two-fold increase when temperature increased from 5˚C to 25˚C but a threefold when temperature increased from 25˚C to 45˚C. They concluded that greatly enhanced NH 3 volatilization at 45˚C compared with 25˚C was related to the inhibition of nitrification at high temperature, which increased the supply of ammoniacal N for NH 3 volatilization for a prolonged time. Our maximum incubation temperature of (30˚C) was comparatively lower than the threshold for the inhibition of nitrification. Further, researcher [17] found that high temperatures (32˚C) increased the initial rates of NH 3 -N loss and they were proportionally reduced at later stages; on the contrary, the lowest temperature (12˚C) resulted in the lowest initial NH 3 -N loss rate but became highest for the last 76 hours. Open Journal of Soil Science  10 . Other researchers found that Q 10 values of N mineralization varied from 1.03 to 11.89 with an average of 2.21 [11]. The Pearson relationship between soil organic C and total N showed a significant negative relationship with Q 10 value of N 2 O (−0.82 and −0.72, respectively), but did not show any relationship with volatilization or N mineralization. Linear regression relationships showed that SOC and TN explained the 68 and 52 percent of the variation in Q 10 of N 2 O. With the rise in each unit (g•kg −1 ) of SOC and total N, Q 10 value of N 2 O declines by 0.67 and 6.0, respectively. Similarly, other researchers [15] also observed a significant inhibition of pulse N 2 O emissions following C addition, they hypothesized that C addition facilitates the microbial growth and in turn accelerates N immobilization rate.

Conclusion
This study clearly indicates a wide variation in Q 10 for N 2 O (0.23 to 11.4), and small variations in Q 10 for NH 3 (0.63 to 1.24) and for the net N mineralization (0.96 to 1.00). Distribution of soil organic C can explain the spatial variation of Q 10 for N 2 O flux. Future research should explore the spatial variation in Q 10 for soils within sensitive regions.