| Plant litter is the main source of soil organic carbon(SOC)in grassland,while litter quality would affect soil organic carbon content and its stability.Early researches suggested that the decomposition of recalcitrant litter(low-quality)would have a greater contribution to the formation of soil organic carbon than the labile litter(highquality),because recalcitrant litter can accumulate in soil by resisting degradation.However,recent studies believe that the contribution of easily decomposing litters to soil organic carbon is greater,because after the action of microorganisms,the easily decomposing litters are associated with the soil organic carbon.The <0.053 mm aggregates combine to form stable organic carbon in the mineral-bound state,which can be stored in the soil for a longer time.The thicketization in grassland mainly occurs in arid and semi-arid areas,and can lead to changes in vegetation community structure,and then affect SOC contents and its stability.In addition,the changes of soil temperature and water content are also the main factors affecting grassland shrub.To explore how the changes of vegetation community structure affect soil organic carbon content and its stability under the background of the climate change,four plant materials in Longzhong loess plateau were selected: stem of Artemisia gmelinii(TS),root of Stipa bungeana Trin(CR),leaf of Stipa bungeana Trin(CL)and leaf of Sophora alopecuroides L.(KL)and incubated with soil for 12 months.The aims of the study were to study the effects of litter quality,soil temperature and soil water content on litter decomposition,and on the changes of SOC and soil aggregate SOC during litter decomposition.The main conclusions are as follows:1.The decomposition rate and the residual rate of TS,CR,CL and KL decreased with the decomposition time.The decomposition constant(k)of TS,CR,CL and KL after 2 months were 2.34?3.15 times as much as that after 12 months,respectively.The decomposition rates of high-quality litters(KL and CL)were higher than that of lowquality litter(TS and CR).The organic C contents in the litters under different treatments also decreased significantly with the decomposition time.The organic N in the low-quality litters was affected by the N fixation behavior of microbes,and the organic N in the low-quality litters increased first and then decreased with the decomposition time.Compared with the low-quality litters(TS and CR),the hemicellulose and cellulose contents in the high-quality litters(KL and CL)decreased significantly,while the lignin content increased significantly in a short period.2.High-quality litters(KL and CL)were more beneficial to the formation of<0.053 mm aggregates.After decomposition for one year,the SOC contents of the four treatments were significantly higher than that of the control(CK)(the increase rate was5.13%-17.98%),while the contributions of high-quality litters(KL and CL)to SOC contents were significantly higher than that of low-quality litters(TS and CR).Litter quality had no effects on the aggregates proportion of 0.25mm?2mm and 0.053-0.25 mm sizes.The organic C in 0.25mm?2mm and 0.053 mm ?0.25 mm aggregates can be easily decomposed,C input of low-quality litters did not lead to a significant increase in SOC content of these two particle size aggregates under the lower decomposition rate.Because of higher content of labile components and lower C/N ratio,high-quality litters could be more efficiently utilized and transformed by microbes.under the treatment of KL,its decomposition process significantly increases the aggregate C of each particle size.Since the litters are more likely to form mineral-bound SOC after microbial decomposition and utilization,the impact of the litter on the SOC is significant <0.053 mm aggregate content contributed the most(KL: 40% increase;CL:32% increase).In the experiment,high quality litters(KL and CL)had higher new C formation efficiency than low quality litters.3.Within a certain range,the increase of soil temperature and soil water content accelerated the decomposition of all litters,especially the decomposition of soluble substances.The lignin content increased significantly with the increase of soil temperature and soil water content during the one-year's decomposition.For one thing,this could be attributed to the recalcitrance of lignin,for another,the increase of soil temperature and water content might increase the activity of lignin synthetase,which might promote microbial metabolism to form lignin-like substances.4.The 0.25-2mm and 0.053-0.25 mm aggregates can protect SOC mainly through unstable physical adsorption and occlusion,so the different soil temperature and soil water content have not caused significant changes in the proportion of 0.25-2mm and0.053-0.25 mm aggregates,while the increase of water content led a significant increase in the proportion of <0.053 mm aggregates.The net increase of organic C in 0.25?2mm,0.053?0.25 mm and <0.053 mm aggregates reached the maximum value(0.36?0.78g/80g)under 25?W2 treatment after decomposition for one year,and there was no significant differences in most cases under other treatments.The new C formation efficiency of 0.25-2mm and 0.053-0.25 mm aggregates was promoted with the increase of soil temperature and soil water content.During the study period,with the protection of stable biochemical mechanism,the increases of soil temperature and soil water content significantly increased the new C formation efficiency in <0.053 mm aggregates,namely,under the condition of higher temperature and soil moisture,the SOC contents in all aggregate's sizes increased significantly during litter decomposition process,which was more beneficial to the formation SOC in <0.053 mm aggregate.In conclusion,compared with low-quality litters,higher quality of litters,higher soil temperature and higher soil moisture are beneficial to the decomposition of litters,which can promote the increase of SOC content and the formation of stable SOC during the one-year's litter decomposition. |
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