Low pH inhibits soil nosZ without affecting N 2 O uptake

Purpose Nitrous oxide (N 2 O) is an important greenhouse gas. Soils are the main source and an important sink of N 2 O. Gross rates of soil N 2 O emission and uptake determine the net flux and concentration of N 2 O in the atmosphere. Denitrification is generally considered a key pathway of N 2 O consumption, during which N 2 O is reduced to N 2 by nitrous oxide reductase (N 2 OR) encoded by nosZ genes. Although the soil pH is an important factor in denitrification, its effect on N 2 O uptake across the soil‒atmosphere interface is still unclear. Materials and methods This study subjected acid red soil to eight different pH treatments (including 3.80, 4.47, 4.98, 6.08, 6.69, 6.99, 7.15, and 7.32) via lime application. An incubation experiment was conducted using a 15 N 2 O dilution technique to quantify soil gross N 2 O emission, N 2 O uptake, and net N 2 O flux. Results and discussion There are two contrasting gross N 2 O emission patterns in the tested pH range. When pH ≤ 4.98, the cumulative gross N 2 O emission decreased as the soil pH increased. In contrast, when pH > 4.98, the increasing pH promoted gross N 2 O emission. The significant positive correlation between gross N 2 O emission and net N 2 O flux indicated that the net flux of N 2 O is driven by gross emission rather than uptake. There was no observable effect on the cumulative N 2 O uptake in the 3.80–6.99 pH range, but it decreased at 7.15 and 7.32 pH. However, the copies of nosZ genes (both nosZ clades I and II) were inhibited at low pH and considerably increased with increasing soil pH. In addition, the cumulative N 2 O uptake/gross emission ratio was negatively correlated with soil pH. Conclusions Our findings show that low pH inhibits soil nosZ without affecting N 2 O uptake. However, this study provides direct evidence for N 2 O absorption across the atmospheric soil interface.