Effects Of Soil Moisture On Greenhouse Gas Flux Of Pinus Tabulaeformis Secondory Forest At Different Slope Positions In Qin Ling Mountains

In China,large areas of forests distribute in mountains,which make forest soil greenhouse gas?GHG?fluxes have large spatial variations and are strongly regulated by topographic factors.We conducted an incubation experiment with soils from four slope positions?lower,middle,upper and top?in natural secondary Pinus tabulaeformis forest of Huoditang Area in Qinling Mountains.According to the dynamics of WFPS in field plots,all soils were incubated in three moisture treatments?Low,Middle and High?.The fluxes of greenhouse gases were measured in 6,12,24,48,and 72 h after incubated,and the denitrification potentials of soils were measured using the acetylene inhibition method.The results showed that:?1?At top slope position,CO₂ emissions?360.13 ± 31.79 ?g·g-1?of middle WFPS treatment was significantly higher than the other treatments?P<0.05?.At middle slope position,CO₂ emissions increased with the increasing of WFPS.In high WFPS treatments,top slope position CO₂ emission was significantly higher than the other slope positions?P<0.05?.Generally,CO₂ emissions rate decreased with the increasing of incubation time both at middle and lower slope positions.But at top,upper slope positions,CO₂ emissions rates increased again in the mid-late incubation due to the increasing of dissolved soil organic carbon.?2?At high WFPS treatments,denitrification was the dominant process for N₂O production.When soil NO₃^-content was limited,N₂O production rate decreased with the increasing of incubation time.However,the converting process of N₂O to N2 was not affected by NO₃^-content.This led to the rate of N₂O consumption was faster than that N₂O production.Therefore,the N₂O consumption appeared in the mid-late incubation.At the same slope position,N₂O emissions in high WFPS treatments was significantly higher than the other treatments?P<0.05?.At different slope positions,N₂O emission rate of middle slope position was significantly higher than the other slope positions?P<0.05?.In the middle and low WFPS treatments,nitrification was the dominant process for N₂O production.At the same slope position,there was no significant difference among the treatments.At different slope positions,N₂O emissions from lower slope position was highest?46.98 ± 35.18 ?g·g-1and 36.27 ± 29.16 ?g·g-1?,while N₂O emissions from middle slope position was lowest?17.85 ± 0.79 ?g·g-1 and 17.07 ± 0.57 ?g·g-1?.?3?At the same slope position,CH₄ emissions were not significantly different among three treatments.At different slope positions,CH₄ emissions from middle slope position was significantly lower than the other slope positions?P<0.05?in high WFPS treatments.The inhomogeneous distribution of soil moisture resulted in CH₄ emitted during the early stage of incubation.However,during the later stage of incubation,high CO₂ concentration led to the decreasing of soil p H,which inhibited the activity of methanogenic bacteria.At the same time,the activity of methane oxidizing bacteria was promoted in an appropriate soil moisture condition.All of this resulted in the appearance of CH₄ consumption in the experiment.Overall,in low and middle WFPS treatments,N₂O emissions was not significantly different among four slope positions,CO₂ emissions from top slope positions was highest;In high WFPS treatments,N₂O sink occurred in top and upper slope positions due to the lack of NO₃^-;The CH₄ emissions was not affected by soil moisture.