High soil temperatures alter the rates of nitrification, denitrification and associated N 2 O emissions

Abstract

Purpose

The responses of nitrification and denitrification are not well characterised at temperatures above 35 °C, which is the focus of our study.

Materials and methods

Soils collected from two dairy pastures (Victoria, Australia) were incubated at 10 to 45 °C in the dark for 5 to 10 days following amendment with 100 μg N g⁻¹ either as NH₄NO₃, ¹⁴NH₄¹⁵NO₃ or ¹⁵NH₄¹⁵NO₃ (10 atom% ¹⁵N excess) at 50% water-filled pore space. To detect N₂O from heterotrophic nitrification, acetylene (0.01% v/v) was used in a subset of samples amended with ¹⁵NH₄¹⁵NO₃. Atom% ¹⁵N enrichments of NO₃ˉ, N₂O and N₂ were measured during the experiment to evaluate the responses of nitrification and denitrification to temperature.

Results and discussion

N₂O production from the two soils increased with rising temperature and peaked between 35 and 40 °C. N₂O production from nitrification and denitrification both had similar thermal responses, which were different to N₂ production. The N₂O/N₂ ratio decreased from > 4 at 35–40 °C to 0.5 at 45 °C, due to greater N₂ than N₂O production in the Dermosol. Heterotrophic nitrifiers oxidised NH₄⁺ and released N₂O at 35–40 °C, suggesting a role for heterotrophs in N cycling under warm climates. T_opt for nitrification was between 35 and 40 °C, which is higher than reported previously. A short-term effect of high temperatures could provide NH₄⁺ for the growth of crops but may also decrease soil C pools.

Conclusions

Increasing temperature above 35 °C altered the rates of nitrification, denitrification associated N₂O and N₂ production. Nitrification and denitrification peaked at 35–40 °C in the Chromosol and Dermosol. The production of N₂ increased rapidly above 40 °C, which may be related to high soil respiration rates that likely decreased O₂ availability, thus expanding the anaerobic microsites; such circumstances increased the reduction of N₂O to N₂ production from the Dermosol.

from Energy Ecology Environment Ambio via Terpsi Hori on Inoreader http://bit.ly/2QECiYk