| This study investigated characteristics of soil organic carbon mineralization in grasslands fenced to exclude livestock for different periods of time in the dessert steppes of Ningxia,and explore the response of soil organic carbon decomposition to changes of temperature and moisture.Grasslands that were unenclosed,5-year enclosed,9-year enclosed and 12-year enclosed were selected as investigative plots by adopting spatial sequences instead of time series.The effects of temperature(5,15,25,and 35?),moisture(25,50,75,and 100%SSM)and dead leaves from the canopy(Lespedeza potaninii,Agropyron mongolicum,Stipa breviflora,or mixed litter)added to surface litter,and soil aggregates(>2,2-0.25,0.25-0.053,and<0.053 mm)on soil mineralization in desert steppe were investigated.Changes in soil aggregate structure and the content and composition of soil organic carbon were analyzed.In addition,the relationships between mineralization and stability coefficient(Kos)of soil organic carbon were explored.The results were as follows:The content of SOC in the desert steppe increased with enclosure time.Twelve year enclosure had greater SOC than unenclosed grassland.SMBC content of 5-year and 12-year enclosed grassland was also higher than unenclosed.There was no difference in SMBN between the unenclosed and enclosed grasslands(P>0.05).The SOC content of<0.053 mm aggregates of was the highest,followed by>2 mm aggregates and 2-0.025 mm aggregates.The SOC content of 0.25-0.053 mm was the least.There were three stages in the release of soil organic carbon(CO2)during the incubation period under 5,15,25 and 35 ?.The 0-4 d period was the rapid generation stage(rapid decomposition of carbon library),5-62 d was the slow generation stage(chronic carbon pool decomposition),63-76 d was the third stage(inert carbon pool decomposition).There was no difference in mineralization rate or cumulative mineralization between the unenclosed and enclosed grasslands at 15 and 25?(P>0.05),but mineralization rate and cumulative mineralization were lower in 12-year enclosed grassland at 5? and 35 ?.The Q10 decreased with the enclosure time.During the whole incubation,the Q10 of unenclosed grassland was significantly higher than others grassland(P<0.05).There were no diference in cumulative mineralization among the grasslands when moisture was 50%or 75%SSM.When the moisture was 100%SSM,the cumulative mineralization was highest in the unenclosed grassland,and it higher than others.When the moisture was 100%SSM,cumulative mineralization was lower in the 5-year enclosed grassland.The average mineralization rate of soil organic carbon increased with the decrease of soil aggregates size in the whole incubation period.The cumulative carbon mineralization of>2 mm and 2-0.25 mm aggregates increased firstly and then decreased,with 5-year enclosed grassland was demonstrating the largest mineralization.As for 0.25-0.053 mm and<0.053 mm,the cumulative organic carbon mineralization of soil aggregates decreased first and then increased,with the lowest in the 5-year enclosed and 12-year enclosed grasslands.Adding litter significantly increased mineralization rates and cumulative mineralization,resulting in a strong excitation effect.When the Lespedeza potaninii litter was added,the soil organic carbon mineralization was the strongest in the 9-year enclosed grassland,and it was 2.68 times higher than untreated soil,and higher than other fencing year treatments(P<0.05).Unenclosed and 9-year enclosed grasslands were not different(P>0.05),but these treatments had higher mineralization than 5-year enclosed and 12-year enclosed(P<0.05)after adding Stipa breviflora and mixed litter.There was no difference in soil organic carbon mineralization between 5-year enclosed and 12-year enclosed grasslands(P>0.05).In summary,enclosure favored soil organic carbon storage in desert steppe in Ningxia.The mineralization of soil organic carbon was lower in 12-year enclosed grassland,which encouraged accumulation of soil organic carbon. |
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