Effects Of Forest Regenerations After Clear-cutting On The Fluxes Of CO₂ And N₂O In Chinese Fir Plantation

Reforestation after clear-cutting is used to facilitate rapid establishment of new stands.However,reforestation may cause additional soil disturbance by affecting soil attributes,thus potentially influencing the fluxes of soil CO₂and N₂O.As the plantation tree species with the largest area in China,Chinese fir plays a significant role in carbon sequestration and mitigating climate change because of its rapid growth and strong carbon fixed capacity.Some problems have emerged in pure Chinese fir plantation during continuous planting,such as stand productivity declining and soil degradation.Its ability of carbon uptake is weakened.Preserving or introducing broadleaved species in the clear-cutting site of pure Chinese fir plantations has been proved as an effective way to improve stand structure,production and ecological service function.However,these kinds of forest regeneration practices may affect the fluxes of soil CO₂and N₂O,there are few researches focusing on such effects of subsequent forest practices after clear-cutting.The effects of clear-cutting followed by first year's getting rid of the residues and second year's overall brush clearing(CC?O treatment)and clear-cutting with first year's slash burning followed by second year's soil preparation,overall brush clearing and replanting(CCB?SO treatment)in Chinese fir plantations on the fluxes of soil CO₂ and N₂O in Lin'an,Hangzhou,Zhejiang,China.The fluxes were measured by using the method of static chamber-gas chromatography.The results are the followings after two year's monitoring.(1)The monthly and seasonal changes in soil temperature at 5cm between treatments were consistent.Compared with the control plots without clear cutting,the soil temperature in the updated plot after clear cutting significantly increased,showing UC<CC?O<CCB?SO.Clear cuts can significantly increase soil temperature.Clear-cutting can significantly increase soil temperature over unburned hills.(2)The monthly variation of 0-5 cm soil moisture in all treatments is consistent with the annual dynamic change.Compared with the control plots without clear cutting,the 5 cm soil temperature in the updated plot after clear cutting was significantly reduced,showing UC>CC?O>CCB?SO.The clear cutting can significantly reduce the soil moisture,and soil moisture in the plots received the fire after clear cutting was significantly lower than the non-fired one.(3)Soil CO₂ fluxes between treatments varied in opposite directions during the two years of the test period.In the first year,UC>CC?O>CCB?SO.The CO₂ fluxes of CC?O and CCB?SO decreased by 14.3%and 25.1%,respectively,compared with the controls without clear cutting.In the second year,CCB?SO>CC?O>UC.Soil CO₂ fluxes of CC?O and CCB?SO increased by 37.1%and 73.5%,respectively,compared to the controls without clear cutting.The results showed that the clear cutting without residue significantly reduced the CO₂ flux of soil,and the CO₂ flux was significantly reduced by the fire treatment compared with the unfired treatment.Clearing and weeding after clear cutting significantly increased the CO₂flux of the soil,and the soil preparation,A series of management measures like digging and planting,and raising are significantly increased the soil CO₂ flux.(4)Soil N₂O fluxes showed different results during the testing period.In the first year,UC=CC?O<CCB?SO,the average annual flux of N₂O in the three treated soils was 25.7,26.4,and 35.6?g m² h^-1,respectively,and the N₂O flux increased by 38.5%under the treatment of CCB?SO.In the second year,CCB?SO>CC?O>UC,the average annual flux of N₂O in the three treatments was 23.4,36.5,and 44.6?g m² h^-1,and the N₂O fluxes in the soil treated with CC?O and CCB?SO increased by 56.0%and 90.6%,respectively.Clear-cutting without remaining residues did not affect the N₂O flux of the soil.After clear cutting,the fire treatment significantly increased the N₂O flux of the soil.Clear cutting and weeding significantly increased the N₂O flux,management measures like soil preparation,digging,planting and tending has also significantly increased the soil N₂O flux.(5)Clear-cutting and non-retention of residues and only cutting and weeding have no significant effect on soil p H in the short term;the fire after clear cutting significantly increases soil p H.There is no significant effect on soil p H.A series of management measures such as site preparation,digging,seedling planting and tending carried out in the artificially updated plot will increase soil p H.(6)Clear cutting without remaining residues or clear cutting with slash burning can significantly reduce soil carbon stocks(0-20 cm)in the short term,and the fires that are taken after clear cutting will not reduce more carbon stocks.There was no significant effect on the soil carbon storage(0-20 cm)with the comprehensive cutting and weeding carried out in the main plot of the renewal and regeneration,and a series of operational measurements like soil preparation,digging,planting,and tending in the artificial renewal plot will reduce soil carbon stocks.(7)Under the three treatments,soil CO₂ emission was exponentially correlated with soil temperature at 5 cm(p<0.001),and the soil moisture content was significantly correlated with the linear model of soil CO₂ flux(p<0.05 or p<0.01).Different soil regeneration patterns changed the soil temperature sensitivity of soil CO₂ flux after clear cutting.The trend of UC>CC?O>CCB?SO was present in two years,indicating that the higher the level of human disturbance,the lower the soil temperature sensitivity.Compared with soil temperature and humidity,soil temperature is a more important factor and soil moisture is not a limiting factor.(8)There was a highly significant positive correlation between soil N₂O flux and soil temperature in three treatments without clear cut(UC),clear cut(CC?O)and clear cut fire(CCB?SO)(p<0.001).In the first year,there was no correlation between soil N₂O flux and soil moisture.In the second year,there was a significant positive correlation between soil N₂O flux and soil moisture.(9)In order to reduce the CO₂ and N₂O emissions from the soil,unnecessary disturbances to the soil should be minimized and management intensity should be reduced as much as possible in the management and management of planted forests.