| The agroforestry system is conductive to solve the problem of more people and less land,agriculture and forestry compete for land,ecology restoration and improving land utilization,which is important to solve food security and resource and environmental problems.As a typical fragile habitat area,the Loess Plateau has gradually formed an agroforestry ecosystem since the implementation of the project of returning farmland to forest and grass.As an important part of the agroforestry ecosystem,abandoned agricultural land accounts for a large proportion of the natural recovery process.Moreover,the dead sediment,as an important source of carbon input,affects the soil quality and function.In order to investigate the effect of natural recovery on soil quality after agricultural land abandonment,this study selected the abandoned land with soybean crop cultivated before and used the decomposition bag method to carry out field decomposition experiments to investigate the differences in decomposition characteristics and quality changes of dead litter of two typical plants(Artemisia annua and Artemisia ferox)during the natural recovery process of abandoned land in loess hills.And then investigated the decomposition characteristics and quality changes of soil carbon pool during the process of decomposition were investigated.Meanwhile,we investigated the changes of soil carbon content and enzyme activity during the decomposition of deadfall,clarified the correlation between soil carbon content,enzyme activity,environmental factors and carbon pool stability under the change of deadfall quality.In addition,this paper clarified the main driving factors of soil carbon pool stability change in the abandoned land,so as to provide scientific basis for soil quality control and sustainable development after the abandonment of farmland in loess hilly areas.The main results of this study are as follows:(1)The quality of the litter substrate significantly affected litter decomposition.Cellulose content,lignin/N,and C/N ratio were all good predictors of decomposition rate,and the lower their values,the faster the decomposition.The decomposition process of litter dynamically varied with the decomposition time,and the rate of mass lossing during decomposition was faster in the early stage of decomposition while slower in the late stage of decomposition.The total carbon and phosphorus content of the litter showed a releasing pattern with decomposition,and the total nitrogen of the litter showed a short-term slight enrichment and then fluctuating release pattern with the decomposition.(2)The quality of litter substrate also caused the changes in enzyme activity,with low C/N ratio litter being superior to high C/N ratio litter in increasing soil enzyme activity.The decomposition of dead leaves and dead roots of both plants in the abandoned grassland significantly increased the soil microbial enzyme activity.Soil β-1,4-glucosidase(BG),cellobiose hydrolase(CBH),and β-N-acetylamino glucosidase(NAG)activities were all highest at the late stage of decomposition,with the highest BG enzyme activity under Artemisia ferox dead leaves(AL)treatment,which was 47.43%-67.62% higher than the control,and the highest activity of CBH enzyme was achieved under the dead leaves of Artemisia annua(AL)treatment,which was higher than that of the control by 62.80%-70.71%.Similarly,the highest activity of NAG enzyme was achieved under the dead leaves of Artemisia annua(AL)treatment,which was higher than that of the control by 80.76%-88.29%while Leucine aminopeptidase(LAP)has the highest activity in the early stage of decomposition,which is 14.51%-16.83% higher than the control.(3)The input of deadfall increased the content of soil organic carbon and its components,and the increase was greatest in the late stage of decomposition.Moreover,compared with deadfall roots,the decomposition rate of deadfall leaves was faster and had a more significant effect on the increase of soil organic carbon and its components.The input of litter increased the soil organic carbon content,with the highest values occurring at the late stage of decomposition(360 d)in both soil layers,increasing by 21.06%-35.29% compared to the control.Soil reactive organic carbon(LOC),inert organic carbon(ROC),particulate organic carbon(POC),mineral bound organic carbon(MAOC),soluble organic carbon(DOC),and microbial mass carbon(MBC)increased by 33.27%-35.72%,6.19%-12.48%,32.28%-73.18%,and 44.92%-45.07%,19.90%-39.38%,and 2.51%-2.29%,respectively after the decomposition(450d).The correlation analysis showed that SOC,LOC,ROC,and POC content had a significant negative correlation with litter total carbon(LTC)content,while the above mentioned showed a significant positive correlation with litter loss rate,which implies that the increase in soil organic carbon during litter decomposition originated from the decomposition of litter.Soil enzyme activity was also a key factor in the change of soil carbon fraction content,and there was a significant positive correlation between soil enzyme activity and accumulation of soil active carbon during litter decomposition,while it was less correlated with inert carbon fraction.(4)The input of litter significantly improved the soil carbon pool management index,and the litter leaves with higher C/N ratio were more effective than the litter roots in enhancing the soil carbon pool management index.Active carbon(LOC)and inert carbon(ROC)contents are decisive indicators to characterize the stability of carbon pools.The input of litter increased the soil carbon pool management index and enhanced the proportion of soil active carbon.Soil carbon fraction,environmental factors and enzyme activity together explained 39.4%-47.3% of the variation in carbon pool stability.Changes in soil carbon fraction content was the main controlling factor affecting carbon pool stability in abandoned grassland,and both explained the highest percentage(43.5% and 30.7%)of carbon pool stability in the two soil layers respectively.Among them,the content of reactive carbon(LOC)and inert carbon(ROC)were the decisive indicators to characterize the stability of carbon pools,with explanations reaching 47.2% and 41.1% in the 0-5 cm soil layer and 35.4%and 27.8% in the 5-10 cm soil layer.In summary,the initial quality of deadfall(cellulose,lignin/N,and C/N ratio)are important indicators to determine its decomposition rate in the ecological restoration process of ecologically fragile abandoned wastelands in the loess hills,which means the lower the quality of deadfall,the faster it decomposes.Compared with deadfall roots,low quality deadfall input was more significant in promoting soil microbial activity,increasing soil carbon pool management index significantly compared with the control.This experiment clarified the influence mechanism of deadfall decomposition on the change of soil carbon pool stability during the revegetation of abandoned land,which can provide scientific basis for promoting the synergistic and precise policy of food security and ecological security of agroforestry system in loess hilly areas. |
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