Incubation experiments

Soil samples obtained from four aggregate fractions from different land use types (i.e., CK, GG, and GC) were used in incubation experiments. Each soil sample (treatment) repeated four times. An aliquot of a 30-g soil sample was placed in a glass jar (250 ml) and covered with porous film. All glass jars were incubated for 56 d at aerobic conditions (25 °C). Soil moisture was maintained 70% of water holding capacity (WHC) by adding distilled water every two to three days. Prior to soil incubation, all samples were pre-incubated for 3 d (25 °C) to activate microbial activity. Moreover, there were three blanks filled with quartz sand instead of soil samples.

The gaseous samples were collected at soil incubation times of 1, 3, 5, 7, 9, 16, 26, 36, 46, and 56 d, respectively. At each sampling time, all jars were sealed by a butyl rubber septa for 24 h before sampling, subsequently, a syringe (100 ml) was employed to collect approximately 50 ml of gaseous sample from each jar. Then, the concentrations of CO₂ and N₂O were immediately measured by a gas chromatograph (Agilent 7890A; Santa Clara, CA, USA). The concentrations of CO₂ and N₂O were determined by an electron capture detector (ECD) and a flame ionization detector (FID), respectively. The detector temperatures were 250 °C for FID and 330 °C for ECD. The column temperature was maintained at 55 °C. The carrier gas flow rate was 25 ml min⁻¹ N₂ (Beijing AP BAIF Gases Industry Co., Ltd, China). After each sampling cycle was completed, the butyl rubber septa was removed to let the jars flush with ambient fresh air, and then re-sealed by a porous film again for the next measurement.

The fluxes of CO₂ and N₂O emission were calculated using the following formula (Ding, Sun & Huang, 2019).

where F is the emission flux of CO₂(ng CO₂–C g⁻¹ soil d⁻¹) or N₂O (ng N₂O–N g⁻¹ soil d⁻¹), V is the jar headspace volume (L), C_a and C_b are the CO₂(ppb) or N₂O (ppb) concentrations in the soil sample and blank jars, respectively, T is the incubation temperature (°C), M is the molecular weight of CO₂–C (12) or N₂O-N (28), W is the soil dry weight (g), and t is the sealing time of the jars (d).

Cumulative flux (CF) of CO₂ and N₂O during the whole incubation was computed as follows:

where F is the emission flux of CO₂ (mg kg⁻¹ soil d⁻¹) or N₂O (ug kg⁻¹ soil d⁻¹), i is the i-th measurement, and (D_i+1 −D_i) is the number of days between two consecutive measurements.