Dynamics And Controls Factors Of Soil Respiration In A Subalpine Shrub Stands And Meadow, China

Soil respiration is the main source of carbon efflux from terrestrial ecosystems to theatmosphere, which indicates the rates of soil microbial activity and soil organic carbonmineralization. As a complicate ecological process, soil respiration is affected not only bybiological factors, but also by environmental factors. However considerable uncertainties existabout the mechanisms underlying the soil respiration in the present studies. So a goodunderstanding of temporal-spatial dynamic variation and driving factors of day and nocturnalsoil respiration will help us to better comprehend the mechanism of soil respiration variation.At present, studies on soil respiration were more concentrated in the subtropical mid-tolow-elevation areas and less in the subtropical high-altitude subalpine forest and meadow, andwithout knowledge of nocturnal soil respiration. Therefore, this study was designed to examinesoil respiration along with its controlling factors in sclerophyllous evergreen broad forest andmeadow at the southeastern margin of the Qinghai-Tibet Plateau in order to assess the regionalcarbon budget.The Quercus aquifolioides shrub with the eco-hydrological functions distributes in highaltitude areas. Subalpine meadow is one of important vegetation types in the western Sichuan.In this study we measured RS, soil temperature and water content with a LI-8100A SoilRespiration System in three altitudinal plots (3549m，3091m and2551m respectively) inBanlang Mountain in Wolong Nature Reserve from Sept.2010to Dec.2011and sampled soilthere and analyzed their contents of soil active organic carbon and soil nitrogen in order toexplore the seasonal variation of day-and night-time soil respiration in relation to theenvironmental factors in the Q.aquifolioides stand. The results indicate that:(1) Both RHand RRsignificantly and similarly varied with the shifts of seasons. Duringthe study period the mean rates of day-and night-time RHwere1.57±1.06(mean±Std.) and1.13±0.96μmol m-2s-1respectively, and the RRvalues were0.60±0.61and0.47±0.41μmolm-2s-1. The annual cumulative RHand RR(295.02and200.46gCm-2yr-1) in the day were obviously higher than those (125.43and121.20gCm-2yr-1) at night in the primary stand. Soiltemperature and soil water content are the main factors affecting RHand RRof the day and thenight. The annual cumulative RHand soil total respiration highly correlated with soil microbialbiomass nitrogen (MBN), soil labile organic carbon (LOC) and light fraction organic carbon(FLOC) at0~30cm soil depth; RRhighly correlated with FLOC but there was little correlationbetween RRand the fine root biomass (root diameter<0.5cm).(2) The temperature sensitivity values (Q10) of soil heterotrophic respiration and total soilrespiration in the daytime were lower than those at night. The annual Q10values of soilheterotrophic respiration and total soil respiration in the daytime measured by T5were3.77±0.25(mean±standard error) and4.96±0.45, and those at night were4.51±0.41and5.28±0.79, respectively.(3) Soil surface CO2effluxes in the different stands showed similar variation patternsbetween day and night. Namely, soil surface CO2effluxes were greater in the daytime and inthe growing seasons. Soil temperature, soil water content and their interaction were the majorenvironmental factors affecting soil surface CO2effluxes. The model constructed by thesefactors explained the variance of soil surface CO2effluxes (83.2%in the daytime and96.6%at night). The temperature sensitivity values (Q10) in the growing seasons were higher thanthose in the non-growing seasons and those at night were higher than those in the daytime.During the whole measurement period their day Q10ranged from4.35to4.48(mean4.44)and their nocturnal Q10ranged from5.37to6.42(mean4.44). However, we found their Q10values increased with the decrease of soil temperature, and reached their maximums in thenon-growing seasons and minimum in the growing seasons. The cumulative CO2effluxhighly correlated with MBN and LOC stocks (P<0.05) and slightly correlated with LFOC.The estimated annual soil surface CO2efflux values for the Q.aquifolioides stand ranged from371.08to951.55Cm-2yr-1(mean684.75Cm-2yr-1) in2011, and the cumulative flux in thegrowing seasons was as4.1times as that in the non-growing seasons, and the mean ratio ofnocturnal soil surface CO2efflux to annual flux was42.46%(ranging from39.82to44.60%). So if only daytime data are used to estimate the annual soil surface CO2efflux, the efflux willbe overestimated by12.96%in the non-growing seasons and by3.19%the growing seasonsrespectively. We conclude that biotic and abiotic factors influence soil respiration differentlyat the different time scales. Seasonal variations of soil CO2efflux strongly depends on soiltemperature, soil moisture and their interactions. The variances of soil CO2efflux betweenday and night maybe mainly result from quantity and quality of soil substrates. Therefore,apart from soil temperature, soil moisture and their synthetic effect and components of soilactive organic carbon, we should also take into consideration other abiotic factors andhighlight the measurement of the soil nocturnal CO2efflux so as to estimate daily and annualsoil surface CO2effluxes more accurately.(4) The RSand RHof the burned stand and the control Q.aquifolioides stand showedsimilar parabolic curve patterns, which were highest in the growing seasons and lowest in thenon-growing seasons. The RSand RHof the burned stand were obviously higher than those ofthe controlled stand. There was a significant exponential correlation between the soiltemperature and the RSand RHof the burned stand and the controlled stand during the wholemeasurement period whether in the growing seasons or in the non-growing seasons. However,there was a significant power function correlation between the soil water content and the RSand RHin the non-growing seasons and the whole measurement period, but no correlation inthe growing seasons. So the double-factor model of soil temperature and soil water contentcan be used to explain the variance of rhizospheric and heterotrophic respiration as timebeing. In the whole study period, the RSand RHvalues (Q10) of the burned stand were3.00and3.56, respectively, which were lower than those (4.23and5.33, respectively) of thecontrolled stand. Linear regression analyses suggested that soil light fraction organic carbon(LFOC), particulate organic carbon (POC) and nitrate nitrogen (NO3-N) were importantparameters in soil respiration studies. The annual RSand RHof the burned area were768.23and655.60gCm-2yr-1respectively, lower by13.57%and higher by20.59%than those of thecontrolled stand correspondingly. Hence, we conclude that it was the fire which interfered with the soil temperature and the supply of soil organic carbon fractions and soil nitrogen thataffected the RSand RH.(5) Soil rhizospheric respiration was reduced by plot girdling. In specific, it was loweredby about10.6%and16.5%in the growing seasons and the non-growing seasons, and byabout11.96%annually.(6) Soil respiration rate (RS) in the alpine meadow showed large seasonal variations inthe daytime and the nighttime in the measurements period, with a two-peak curve in thedaytime and parabola curve at the nighttime. In the whole period (April to November), the RSof the nighttime accounted for46.7%of the total; the RSin the growing seasons was muchgreater than that in the non-growing seasons. Compared with the soil respiration rates in theQ.aquifolioides stand at the same altitude, those in the alpine meadow were higher by4.7%and20.5%respectively in the daytime and at night. The average Q10values in the daytimeand night-time were3.90and3.74respectively. The results showed that in estimating theseasonal and annual effluxes of soil respiration we should intensify the measurement ofnocturnal soil respiration.Further, during experimental time it was observed that the monthly mean soiltemperature at5cm soil depth linearly dropped by0.22℃with the elevation of each100meters in the Q.aquifolioides stand of sunny slope at the altitudes between2551m and3549mfrom January to December2011. But it is very interesting that the soil temperature at5cm inthe Q.aquifolioides primary stand was the significantly lowest at the altitude of3091m in thenon-growing seasons, which was2.41℃,2.28℃and2.74℃lower than those in contiguousburned stand, at3549m and2551m primary stands respectively. However, the soiltemperature increased with the increase of altitude in the growing seasons (May throughOctober2011). The result implies that in constructing a general soil respiration model weshould take the forest type into account for vegetation differences can lead to the timedifferences.By all accounts, for an accurate estimation of seasonal and annual soil carbon fluxes in Q.aquifolioides stands and in subalpine meadows, measurements of the soil nocturnal CO2efflux and in the non-growing seasons were essential. At the same time, soil physicalenvironments and substrate availability should be taken into consideration in analyzing andinterpreting measurements of soil surface respiration and its components.