Soil Organic Carbon Mineralization Response To Temperature Change In The Typical Farmland Soil In The Purple Hilly Region

The purple soil and paddy soil of representative farmland soil in the Purple Hilly Region were used as the test soils.Soil sampled in the field and an incubation experiment in the laboratory were conducted to investigate the response of typical farmland soil organic carbon mineralization to temperature variation in the purple hilly region.The study consisted of two incubation experiment.The first experiment,for two kinds of test soil,set two moisture levels,that is,aerobic?70%water holding capacity,70%WHC?,Submersion?soil:water=1:2?,referred to as M1 and M2,and three temperature treatments,that is,constant temperature?15??20??25??,cyclical temperature fluctuations?15/25?,the temperature change order is 15?—20?—25?—20?—15??.Entering the next one immediately after the end of the previous temperature cycle,all the treatments were incubated for 66 d.The second experiment,the purple soil was used as the test soil,moisture adjusted 70%WHC and consisted of three temperature treatments,that is,treatment HW?constant temperature 20??,treatment BW1?a sudden cooling from 20?to 10??and treatment BW2?a sudden warming from 20?to 30??.In treatment BW1 and treatment BW2,the sudden temperature change both happened when soil organic carbon?SOC?mineralization rate was basically stable and maintained a relatively lower level at the temperature of20?(29^th day).After the sudden temperature change,treatment BW1,treatment BW2and treatment HW was cultivated for another 47 d.The CO₂ emission in the incubation experiment was used to analyze the effects of temperature and moisture on mineralization characteristics of typical farmland soil organic carbon,and the SOC mineralization kinetic model of two-pool first-order and temperature sensitivity coefficient were analysed to study dynamic characteristics and temperature sensitivity of typical farmland soil organic carbon in the purple hilly region.These researches provided scientific reference for comprehensive understanding the mechanism of soil organic carbon mineralization of typical farmland soil,correct assessing environmental effects of temperature change in the purple hilly region.The main results showed asfollows:?1?Under the 70%WHC regime,there was a sharp downward trend of SOC mineralization rate of test soils with different temperature,while a relatively small decline or even a slight rise then fall in submersion were observed in the beginning 10days.In 10³9 days,mineralization rate decreased gradually,then remained relatively stable after 39 days.With same incubation temperature regime,the SOC mineralization rates of two test soils at 70%WHC were significantly greater than submersion in first day,but the results were opposite with the extension of culture time that the mineralization rates of purple soil and paddy soil were higher in submersion than those in aerobic environment after 2 and 5 days,respectively.And the time of the mineralization rate of SOC reached stationary under submersion regime was more lagged than that of aerobic regime.?2?During the incubation period,the amount of cumulative SOC mineralization of the two test soils with the same moisture regimes followed:25?>15/25?>20?>15?,and the amount of SOC mineralization cultured at 25°C was significantly more than that at 15??P<0.05?.Under the same moisture,there was no significant difference in the accumulation of SOC in the incubation period of the purple soil at the incubation period of 20 C and the periodic temperature?15/25 C?and the constant temperature,but there was no significant difference in the rest period of the paddy soil under the submergence culture of 0³ d.This indicates that under the condition of aerobic and flooded paddy soil,the influence of variable temperature?15/25 C?SOC mineralization can be evaluated at 20 constant temperature.In addition,the interaction of temperature and moisture on SOC mineralization in purple soil was only in the early stage of culture?0³d?,whereas in paddy soil it was in the middle stage?10³9 d?.?3?Under aerobic regime,the temperature sensitivity of SOC mineralization of purple soil and paddy soil was significantly rose with the increase of incubation temperature,but there was no significant difference.Under submersion regime,the SOC mineralization of purple soil at 15?was more sensitive to the rising in temperature,while that of paddy soil was at 20?.?4?During the sixty-six days of culture period,the soil Microbial biomass carbon?MBC?of each treatment showed a peak in 1¹0 day and 25⁶6 day respectively.In the peak of culture period,the MBC of purple soil and paddy soil with high temperature 25?were significantly greater than 15?.The cyclical temperature fluctuations had a significant effect on the MBC of purple soil only medium-term?5²5 day?of 70%WHC,but it had no obvious influence on the purple soil under the submersion and the paddy soil under the each moisture treatments.It was obvious that the influence of temperature on the MBC was prominent in the peak of microbial growth.The warmer temperature could influence SOC mineralization through changing the microbe,but the cyclical temperature fluctuations could not influence that.?5?Under each moisture regime,the amount of the labile SOC fraction would increase with the increase of culture temperature,which was consistent with the variation of total cumulative mineralization of SOC.And when the condition was aerobic,C₀ at 25?was significantly greater than the other tread,there was also difference of C₀ between 25?and 15/25?and 15?of the 2 tested soils under submersion regime?P<0.05?.But between cyclical temperature fluctuations and constant 20?,the C₀ of purple soil and paddy soil did not have any difference.And at the temperature?20?,there were significant differences of C₀ between submersion and aerobic,but no differences when the temperature was low of purple soil.As for paddy soil,only when the temperature was 25?,C₀ of submersion condition was obviously greater than that of 70%WHC?P<0.05?.Meanwhile,the amount of the labile SOC fraction had important effect on the SOC mineralization process of soil,there was no significant difference between each treatment of C₀/C₆₆,but were more than 60%.This showed that the effect of high temperature and high moisture on C₀was the most prominent,and temperature and moisture can affect the SOC mineralization by changing the amount of labile SOC fraction.Temperature had a significant influence on mineralization rate constants of recalcitrant SOC?h?.Under aerobic regime,h of tested soils increased with increase of temperature.There was no differences between different temperature of h value of purple soil,as for the paddy soil h of 25?and cyclical temperature fluctuations was greater that of low temperature?15??.When it was submersion,h of 15?was significantly lower than that of other temperature treatments of purple,the h of paddy soil at high temperature of 25?was 2.6,2.9 and 3 times of 15/25?,20?and 15?,which reached significant difference?P<0.05?.Which meant the influence of temperature on h was also the cause of the difference of SOC mineralization in each treatment.?6?Sudden temperature change had a notable effect on purple soil SOC mineralization?P<0.05?.More specifically,treatment BW2 had significantly promoted SOC mineralization,however,treatment BW1 dramatically impaired that.Compared with the constant temperature?HW?treatment,the SOC mineralization rates of the two were increased by 225.1%and decreased by 38.5%.There was a significant difference in the amount of SOC accumulated mineralization between each temperature treatment?P<0.05?within initial stage after sudden temperature change(30^th d⁴4^th d).And compared to accumulated soil SOC mineralization in treatment HW,that in treatment BW2 and treatment BW1 increased by 40.5%and decreased by55.3%respectively.But with the extension of the culture time,the amount of SOC accumulation in each treatment did not find a significant difference.Indicating that the effect of sudden temperature change on purple soil SOC mineralization was time-bounded,which remained around two weeks.Combined with mineralization kinetic data,the results showed that sudden temperature change affect SOC mineralization by changing the labile SOC pool size.