Abstract. Understanding the production pathways of potent greenhouse gases, such as
nitrous oxide (N2O), is essential for accurate flux prediction and for
developing effective adaptation and mitigation strategies in response to
climate change. Yet there remain surprising gaps in our understanding and
precise quantification of the underlying production pathways – such as the
relationship between soil moisture and N2O production pathways. A
powerful, but arguably underutilized, approach for quantifying the relative
contribution of nitrification and denitrification to N2O production
involves determining 15N2O isotopomers and 15N site
preference (SP) via spectroscopic techniques. Using one such technique, we
conducted a short-term incubation where N2O production and
15N2O isotopomers were measured 24 h after soil moisture
treatments of 40 % to 105 % water-filled pore space (WFPS) were established
for each of three soils that differed in nutrient levels, organic matter,
and texture. Relatively low N2O fluxes and high SP values indicted
nitrification during dry soil conditions, whereas at higher soil moisture,
peak N2O emissions coincided with a sharp decline in SP, indicating
denitrification. This pattern supports the classic N2O production
curves from nitrification and denitrification as inferred by earlier
research; however, our isotopomer data enabled the quantification of source
partitioning for either pathway. At soil moisture levels < 53 %
WFPS, the fraction of N2O attributed to nitrification (FN)
predominated but thereafter decreased rapidly with increasing soil moisture
(x), according to FN=3.19-0.041x, until a WFPS of 78 % was reached. Simultaneously, from WFPS of 53 % to
78 %, the fraction of N2O that was attributed to denitrification
(FD) was modelled as
FD=-2.19+0.041x; at moisture levels of > 78 %, denitrification completely
dominated. Clearly, the soil moisture level during transition is a key
regulator of N2O production pathways. The presented equations may be
helpful for other researchers in estimating N2O source partitioning when
soil moisture falls within the transition from nitrification to
denitrification.
Source partitioning using N2O isotopomers and soil WFPS to establish dominant N2O production pathways from different pasture sward compositions
