Response to Comment on “Oxygen Regulates Nitrous Oxide Production Directly in Agricultural Soils”

X et al. addressed two important questions concerning the reduced soil gas diffusivity when intensive precipitation or irrigation occurred: 1) it contributed to the decrease in soil O2 concentration ([O2]soil) and increase in soil N2O concentration ([N2O]soil) due to the prevention of soil gaseous exchange with the atmosphere under conditions of surface water ponding or soil saturation; 2) it significantly affected transportation and consumption of N2O within the soil profile and therefore might cause a discrepancy between the [N2O]soil and N2O flux and their responses to the [O2]soil variations. We know that a complexity of processes are involved in the formation of N2O and its subsequent diffusion in the soil matrix prior to emission to the atmosphere. N2O emissions from the soil surface are ultimately the net result of production, consumption, and transportation of N2O within soil profile. Our paper aimed to explain how [N2O]soil variations corresponded to changes in [O2]soil in the real field conditions of upland soils and how this relationship was affected by the complicated and interacting factors of climate, soil, and agricultural management. This can contribute to an understanding of the mechanism by which the diverse processes of N2O production including nitrification, denitrification, and coupled nitrification denitrification respond to the varying [O2]soil considering the contrasting yield of N2O by these processes. Undoubtedly, N2O production would be stimulated by increasingly anaerobic conditions (decreased [O2]soil) when intensive precipitation or irrigation occurs, and surface water ponding would indeed prevent diffusion of the produced N2O to be emitted from the soil surface, which may cause N2O accumulation thus increasing [N2O]soil to some extent, rather than increasing [N2O]soil solely by enhanced N2O production. It is still difficult to differentiate between these two processes of an increase in [N2O]soil. However, it is noteworthy that upland soils with a loamy texture such as our studied soil have good drainage ability. Water infiltration through the top soil layer (0−20 cm) could be generally finished within 4 h following irrigation as indicated by our hourly monitoring of soil [O2]soil and [N2O]soil during these events (data unpublished). It is consistent with the conceptual infiltration model that demonstrates that soil surface water can infiltrate through 20 cm within about 4 h. The initial infiltration rates of the normally dry upland soils could be very high once flooded by intensive precipitation or irrigation and decrease to a steady infiltration rate after 2−3 h generally ranging between 5 and 10 mm/h in the loamy soil. Thus, the theoretical infiltration time for 60 mm irrigation should be less than 6−12 h in loamy soil. Our results showed that soil water-filled pore space (WFPS) only reached 60−68%, even immediately following extreme rainfall on 21 July, 2016, and then dropped slightly to 55−62% in the next week (Figure S4). This also illustrates a rapid infiltration of water through the top soil layer and left the pore space not completely blocked by water but still having some air-filled porosity (around 20%). Under these circumstances, gas exchange between the atmosphere and soil profile was not totally blocked thus allowing O2 diffusion into soil and N2O emission out of soil. The maximum N2O emissions from soils occurred at 67−80% WFPS with a bulk density ranging between 1.1 and 1.5 g cm−3, in which the relative soil gas diffusivity (Dp/D0) consistently fell to a critical value of 0.006, indicating an unrestricted gas diffusion under this relatively high range of soil moisture. Therefore, the ponded soil surface or saturated soil layer after heavy rainfall or irrigation would not last long enough to incur any severe restrictions on gas transport in these upland soils, unlike the situation in rice paddy or wetland soils. Soil water holding capacity, that is, the maximum water content that could be stably held by soil aggregates after saturation and a thorough infiltration, is far smaller when measured using intact soils under field conditions (around 25%) than measured using sieved soils in the laboratory (around 45%), which explains the relatively low WFPS in the in situ structured soils even under waterlogged conditions. Nevertheless, how the reduced gas diffusivity affects N2O transportation and consumption in the soil profile and subsequent emission to the atmosphere is also a key question we are interested in investigating. In our ongoing work, we are simultaneously measuring the N2O flux at the soil surface, [N2O]soil, and [O2]soil in the soil matrix. The preliminary results show that the dynamics of [N2O]soil and N2O flux are relatively consistent in timing and patterns which both respond to the changes in [O2]soil in the upland soils (data unpublished). The effect of the reduced gas diffusivity was weaker when gas concentrations were high and did not cause significant inconsistency between [N2O]soil, N2O flux and their responses to the varying [O2]soil. However, the comment by Xu et al. highlights the importance of combining the [N2O]soil Correspondence/Rebuttal pubs.acs.org/est