| Larix Princicpis-rupprechtii is a major constructive species of needle leaf forests in the mountainous areas of North China,and an important tree species for afforestation.With the increase of planting area and the expansion of management scale,L.Princicpis-rupprechtii is a substantial forest resource in China,plays a vital role in timber production,soil and water conservation and ecological environmental regulation in North China.There are almost one million acres of L.Princicpis-rupprechtii plantations which are in urgent need of proper thinning.Only a few researches with unclear results studied the effect of thinning on soil respiration.The controlling factors and how are they regulate soil respiration are still very controvatial.Therefore,this study is significant to evaluate the effect of thinning intensity on soil respiration in L.Princicpis-rupprechtii plantations.Accordingly,this study selected young,middle-aged and near-mature L.Princicpis-rupprechtii plantations as research objects,applied the advanced and proven IRGA method with the Li-8100 Automated Soil CO2 Flux System.This study measured soil respiration,root respiration,litter respiration and SOM respiration comprehensively in three L.Princicpis-rupprechtii plantations form 2012 to 2014.This study separated the source components and their contributions to soil respiration,ascertained how soil respiration changes on different time scales,explored the relationships among soil respiration and its abiotic and biotic controlling factors,and determined the effects of thinning intensity on soil respiration.The results showed that:(1)There were significant variations on different time scales in three L.Princicpis-rupprechtii plantations.The diurnal variation of soil respiration basically showed as a single curve model,which had a maximum appeared at 14:00?16:00 and a minimum at 2:00-6:00.The seasonal variation of soil respiration also took the form of single curve with a maximum in July or August of summer and a minimum in December or January of winter.All components of soil respiration in three L.Princicpis-rupprechtii plantations increased year by year with different ranges during 2012?2014.(2)This study confirmed the optimal sampling time(OST)in different growing period of three L.Princicpis-rupprechtii plantations.The OST for the young plantation was 10:00?12:00 during the growing season and non-growing season,and was 8:00?10:00 during the early growing season.The OST for the middle-aged plantation was 10:00?12:00 during the early growing season and growing season,and was 8:00?10:00 during the non-growing season.The OST for near mature plantation was 10:00?12:00 during the growing season,and was 8:00?10:00 during the early growing season and non-growing season.At the OST,the sampling errors were comparably lower(-4.6%?9.0%)than those sampling during the whole day time.This optimal sampling time was more time and labor saving and more efficient when numerous plots needs to be measured.(3)The winter soil respiration was a vital proportion for the annual soil carbon efflux.Winter soil respiration averaged at 0.67 ?mol m-2 s-1 contributed 10.85?13.13%to the annual CO2 efflux.Soil respiration had a significant positive correlation with soil temperature at 10 cm depth in winter.When the soil temperature at 10 cm depth lowered than 0?,the relationship between them was y=0.460e0.231x,which explained 76.3%of the variations in soil respiration.The sensitivity of soil respiration Q10 was 10.07.When the soil temperature at 10 cm depth was higher than 0?,the relationship between them was y=0.801e0.231x,which explained 71.7%of the variations in soil respiration.The sensitivity of soil respiration Q10 was 3.90.Because of the "cycle of freezing and thawing" happening at the end of winter and the beginning of spring,soil respiration had a pulsed increase.It increased by 2?8 times,sharply from 0.18?0.71 ?mol m-2 s-1 to 1.18?1.50 ?mol m-2 s-1.(4)All components of soil respiration increased with different extent along with the increase of stand ages.In young,middle-aged and near-mature stands,the means of soil respiration separately were 1.97,2.14 and 2.57 ?mol m-2 s-1.Root respiration were 0.66?0.74 and 0.88 ?mol m-2 s-1,separately.Litter respiration were 0.48,0.54 and 0.57 ?mol m-2 s-1,seperately.SOM respiration were 0.82?0.86 and 1.10 ?mol m-2 s-1,separately.Combining the trenching method and litter removing method,the contributions of root,litter and SOM to soil respiration were determined as 32.5%,22.9%and 44.6%,separately.In three L.Princicpis-rupprechtii plantations,the contributions of autotrophic respiration and heterotrophic respiration had a ratio of 3:7 approximatly.(5)Non-linear complex model of soil temperature and moisture,Rs=aebTWc,could increase the predictability for soil respiration than the single-factor models,and explained 78?93%of variation in soil respiration.These models separately wereRs=1.66e0.15TW0.75,Rs=1.88e0.16TW0.80 and Rs=2.25e0.13TW0.08 in young,middle-aged and near mature L.Princicpis-rupprechtii plantations.The exponential model with larger R2 values was the best one to describe the relationship between soil respiration and temperature.In young,middle-aged and near-mature L.Princicpis-rupprechtii plantations,soil temperature explained 75.8-82.3%of the variations in soil respiration,separately.Q10 increased with stand age,were 2.34,3.56 and 3.89 in young,middle-aged and near-mature stands.A linear model symptomatized that there was somewhat correlation between soil respiration and soil moisture at 10 cm depth,and explained only 23.3%,23.5%and 22.4%of variations in soil respiration.(6)There were different controlling factors for each component of soil respiration.87.4%of variation in total soil respiration was explained by 5 controlling factors.The importance of these 5 controlling factors had an order of Soil temperature>SOC in 0-10 cm soil layer>Fine root biomass>Soil moisture>capillary porosity.76.4%of variation in root respiration could be explained by 4 controlling factors,the standard coefficient of these factor had an order of Fine root biomass>Soil temperature>Soil moisture>LAI.Litter respiration was controed by soil microbe,soil temperature and litter production.3 controlling factor which had the order of Soil temperature>Soil microbe>Litter production could explain 68.3%of variation in litter respiration.78.2%of SOM respiration was controlled by soil temperature,SOC in 0?10cm soil layer,soil microbe and soil enzyme.These four controlling factors had an order of Soil temperature>Soil microbe>Soil enzyme>SOC in 0-10 cm soil layer.(7)In the young L.Princicpis-rupprechtii plantations,soil respiration decreased along with the increase of thinning intensity.Averages of soil respiration in different thinning plots were 1.97?1.95?1.89?1.79 and 1.73 ?mol m-2 s-1.The sensitivity of soil respiration Qio were alerted by thinning intensities.Fitted Q10 were 2.44 and 2.59 in T1 and T2 treatments respectively,larger than that in CK(2.34).Soil carbon outputs coorelated negatively with thinning intensity.The minimum of carbon output was 6.55 t C hm-2 yr-1 in T4 treatment and the maximum was 7.46 t C hm-2 yr-1 in T3 treatment.Our result suggested a reasonable thinning intensity of 35%should be adopted in young L.Princicpis-rupprechtii plantations.It could reduce 1.79 t C hm-2 yr-1 of carbon output released to atmosphere from forest soil.(8)In the middle-aged L.Princicpis-rupprechtii plantations,Averages of soil respiration in different thinning plots were 2.24,2.02,1.90,1.79 and 1.86?mol m-2 s-1.Thinning decreased Soil respiration and the Q10 values.The carbon output through soil respiration in thinned plots was 6.48?8.76 t C hm-2 yr-1.The maximum was 8.67 t C hm-2 yr-1 in CK and the minimu1 was 7.81 t C hm-2 yr-1 in very heavy thinning plots.This suggested very heave thinning(40%)could reduce 2.28 t C hm-2 yr-1 of carbon output in middle-aged L.Princicpis-rupprechtii plantations.In actual forest management,we still should consider the specific stands' condition to adopt an optimal thinning intensity.(9)In the near mature L.Princicpis-rupprechtii plantations,soil respiration had no significant difference in thinned plots(P>0.05).Averages of soil respiration in different thinning plots were 2.39,2.36,2.39,2.39 and 2.14 ?mol m-2 s-1.There was a significant correlation between soil respiration and soil temperature.84.2%?87.9%of variation in soil respiration could be singly explained by soil temperature.Qio of soil respiration negatively correlated with thinning intensity,were 3.90,3.78,3.28,3.12 and 2.69.The carbon output through soil respiration in thinned plots was 8.28?9.33 t C hm-2 yr-1.The miniUum was 7.81 t C hm-2 yr-1 in very heavy thinning plots.As commercial forest,a very heavy thinning'intensity of 35%or clear cutting should be adopted according to actual needs.In summary,the effects of thinning intensity on soil respiration varied significantly in three L.Princicpis-rupprechtii plantations and in different time scales.Combining both thinning intensity and soil carbon output and doing deep and systemic research is vital to measure carbon budget in ecosystem and estimate the carbon sequestration capacity,to improve and optimize the models of carbon cycle prediction,and to simulate the response of ecosystem to global change and the interaction between them. |
PDF Full Text Request