Response Of Soil "Birch Effect" To Simulated Rainfalls In The Dry Cropland

The precipitation event controls ecosystem process in the arid and semi-arid region, and the rewetting of dry soil due to precipitation could stimulate the soil carbon mineralization and a short pulse of CO2 release, so as called "Birch effect". Two mechanisms including "substrate supply", which means that soil aggregate structure would be broken leading an increase in carbon substrate for mineralization, as well as "microbial stress", that the rapid changes of soil water potential stimulate microbial metabolism, have been used to explain the Birch effect during rewetting dry soil, which can be elucidated by different measurable soil C fractions:ⅰ) K2SO4-extracted soil C (EOC); ⅱ) microbial biomass C (MBC); or ⅲ) chloroform-fumigated extracted C (CFE). However, the relative importance of the two mechanisms under different precipitation patterns was still not clear.Based on a field rainfall simulation experiment, we designed three experiment of different precipitation models to studied the response of soil carbon fraction on the effects of soil CO2 release process and its responding mechanism in combination with the analysis of soil respiration rate and soil carbon fraction. The first experiment is that we conducted precipitation simulation experiments in a winter wheat field to measure the short-term response of precipitation levels (1-32 mm) on soil respiration from 0 to 72 h both at jointing stage and during summer fallow on the Loess Plateau. The results showed that such effects were lower during the wheat growing season than during the fallow period due to wheat root respiration. Higher precipitation delayed but increased soil respiration compared to lower precipitation, probably by influencing soil microbial activity. Root respiration during the crop growing season can influence the pattern and amount of soil respiration. Stepwise correlation analysis between soil carbon fractions and soil respiration rate showed that soil MBC was significantly correlated with soil respiration rate at jointing stage, indicating that a "microbial stress" mechanism dominated "Birch effect" during growing season. However, during summer fallow, soil respiration rate was significantly correlated with MBC referring to "microbial stress" when the precipitation amount was lower than 8 mm, but correlated with CFE referring to both mechanisms when precipitation amount was equal to 8 mm. The mechanism for "Birch effect" shifted into "Substrate supply" when precipitation amount was equaled to or higher than 16 mm;The second experiment is that we designed three different rainfall frequencies during summer fallow in the dry cropland.The frequency includs twelve times(Ⅰ 5),six times(Ⅰ 10) and two times.Result shows that with the increased frequency of rainfall, the dynamic intensity of EOC decreases, the value of EOC concentration ([EOC]) was linearly correlated with rainfall frequency([EOC]=-2.8I-2.3, R2=0.97**), the value of EOC concentration ([MBC]) was correlated with rainfall frequency in a quadratic way as [MBC]=-7.0I2+107 I-52 (R2=0.86**),a quadratic correlations was also found between the peak value of CFE([CFE]=-2.8 I2+43I-31,R2=0.73**). Stepwise correlation analysis between soil carbon fractions and soil respiration rate showed that with the increased frequency of rainfall, the dominant mechanism for "Birch effect" shifted from "Substrate supply" into "Substrate supply" into "microbial stress";The third experiment is that we increase rainfall on the basis of natural precipitation in winter jointing stage (May-June,2004). The experiment have five kinds of precipitation treatment 0 mm(CK),8 mm (P8),16 mm (P16),32 mm (P32),64 mm (P64).The results of increasing rainfall experiment on the basis of natural precipitation in jointing stage (July-June)show that both mechanism dominated "Birch effect" that inspired natural precipitation, a "microbial stress" mechanism dominated "Birch effect" with the Soil respiration is related to MBC under during with P8、P16、P32、P64.In deal with CK,soil total organic carbon (SOC) in Surface before the precipitation decreases, other 4 kinds of processing increases, this is good to verify the response mechanism of the soil "Birch effect" impact on soil carbon library. Soil C/N under CK have no significant change after the experiment, but after adding different rainfall experiment, C/N appeared obvious growth trend. C/N have a quadratic collectation with increasing rainfall (C/N=0.0013P2 +0.0974+0.25P,R=0.74**), a linear positive correlation with soil respiration rate after increasing rainfall is found.(C/N=1.17+13.25x,R=0.68**).