Nitrogen Forms And Availability In Plough Layer Of Black Soil Under Simulated Freezing And Thawing Conditions

In this paper,the surface soils were selected as the research object in a typical black soil region.We simulated the freezing and thawing circulation in laboratory,in order to analyze the effect of freeze-thaw cycle-index(0,3,6,9,12 times),freezing temperature(-5?,-15?,-25?)and antecedent soil moisture content(natural moisture content,saturated moisture content)on the mainly forms of soil nitrogen,such as total nitrogen(TN),ammonium(NH4+-N),nitrate(NO3--N),the ratios of ammonium nitrogen to nitrate(ANR),microbial biomass nitrogen(MBN),soluble organic nitrogen(SON)and any organic nitrogen fractions.The results showed that:(1)Freezing and thawing affected the soil nitrogen forms,at-25?,the contents of ammonium,nitrate,MBN,and SON were significantly increased;at-5? and-15? the ANR was increased with the cycle-index remarkably,while it turned to be decline at-25? condition.At natural moisture content,the NH4+-N was significantly reduced by 31.76%~52.99%;while the NO3--N was increased by 89.95%-495.32%after freezing and thawing.At natural or saturated moisture content,the MBN was both decreased with the cycle-index significantly,and ANR was the same.At-5?,-15?,-25?,or saturated moisture content,the total nitrogen contents were all increased obviously.Compared with antecedent soil moisture content,the freeze-thaw cycle-index and freezing temperature had a greater influence on soil nitrogen.Freeze-thaw cycle-index was mainly reduced the NH4+-N and MBN,and increased the NO3--N and SON.Freezing temperature was basically affected inorganic nitrogen and SON,and a lower freezing temperature can significantly increase the ammonium,nitrate and SON,but diminish ANR.While,the increase of antecedent soil moisture was mostly added nitrate and ANR,but reduced ammonium.In addition,the effect of freezing and thawing on soil nitrogen availability had a strong correlation with MBN,and the increase of SON and inorganic nitrogen can improve the nitrogen availability effectively.(2)The effects of freezing and thawing on organic nitrogen fractions were various.At-15? or natural moisture,AN(ammonium nitrogen)was both reduced after freezing and thawing,meanwhile,AN and total acid hydrolysable nitrogen(TAHN)were decreased under saturated moisture.At-5? and-25 ?,the amino sugar nitrogen(ASN)was increased by 171.71%?279.14%and 85.71%?309.14%,respectively.At-15?,-25? or saturated moisture,the hydrolysable unknown nitrogen(HUN)and non-hydrolysable nitrogen(NHN)was significantly increased,and HUN was also enhanced under natural moisture.Compared with freezing temperature,the freeze-thaw cycle-index and antecedent soil moisture content had a bigger effect on soil organic nitrogen fractions.Freeze-thaw cycle-index was reduced AN,but increased amino acid nitrogen(AAN),ASN,HUN,and NHN.While,the increase of antecedent soil moisture was mainly enhanced AN,ASN,TAHN and NHN.Moreover,freezing and thawing raised organic nitrogen fractions through increased AAN,ASN,HUN,and NHN,thereby increasing the soil nitrogen reserve,promoting nitrogen mineralization,and improving nitrogen availability.This research could provide a reference for evaluating the effect of freezing and thawing on the nitrogen supply capacity scientifically in black soil region.