Dual Isotopes Tracing Carbon And Nitrogen Dynamics During Leaf Litter Decomposition Under Different Precipitation Regimes

As the basic carrier of nutrients,litter is the link between plants and soil in nutrient cycle,and its decomposition process plays an important role in the composition and cycle of carbon（C）and nitrogen（N）pools in terrestrial ecosystems.Loess Plateau of China is one of the most eroded regions in the world.Since the 1980s,the afforestation policy has been implemented.Among them,the leguminous Robinia pseudoacacia,which grows rapidly and can fix N in the atmosphere,and the salicaceae Populus tomentosa,which has the potential to conserve water and soil,are considered as the main tree species for afforestation.Vegetation restoration is beneficial to soil carbon storage and turnover,and litter cover can also reduce surface runoff and soil erosion.Litter becoming the main source of soil organic matter and nutrients,maintaining ecosystem stability and improving soil texture.Therefore,it is very important to study the decomposition characteristics of Robinia pseudoacacia and Populus tomentosa leaf litter in the Loess Plateau region to improve the regional ecological environment.However,few studies have used dual isotope labeling,and the effect of precipitation on the fractions of litter-derived C and N in soil on the Loess Plateau is unclear.Therefore,we selected Robinia pseudoacacia and Populus tomentosa leaves on the Loess Plateau as the research object,and adopted ¹³C and ¹⁵N double isotope in situ labeling and artificial simulated culture experiments,and set different precipitation（400 mm,600 mm,800 mm）and different decomposition media（sandy soil and soil）treatments.We investigated the changes in C and N fluxes during the decomposition of legume and non-legume leaf litter,and the responses of these processes to different amounts of precipitation.We mainly measured the mass loss,C content,N content,δ¹³C andδ¹⁵N values of leaf litter during decomposition（2,4,6,8,12,16 months）.The C and N content,δ¹³C andδ¹⁵N values of different soil profiles were also analyzed.And the p H value,electrical conductivity（EC）value,nitrate nitrogen（NO₃^--N）,ammonium nitrogen（NH₄⁺-N）,soluble organic carbon（DOC）,soluble organic nitrogen（DON）contents in soil leaching solution.The main results are as follows:（1）Theδ¹⁵N andδ¹³C values of Robinia pseudoacacia leaves were 3604‰and 55‰,respectively,and theδ¹⁵N andδ¹³C values of Populus tomentosa leaves were 8115‰and452‰,respectively.Theδ¹⁵N andδ¹³C values of the labeled of Robinia pseudoacacia leaf litter was significantly lower than that of Populus tomentosa.It shows that the double labeling method can be used to label C and N simultaneously.（2）Leaf litter mass loss and C loss were higher in soil than in sand,while N loss was independent of decomposition medium.The mass loss of leaf litter mainly occurred in the first 2 months of decomposition,and a large number of water-soluble unstable compounds were lost due to precipitation in the early rainy season.With the extension of decomposition time,theδ¹³C values of leaf litter decreased gradually,theδ¹⁵N values of Robinia pseudoacacia leaf litter increased gradually,and theδ¹⁵N values of Populus tomentosa leaf litter decreased gradually.N content was negatively correlated withδ¹³C value andδ¹⁵N value.（3）The decomposition of litter results in the accumulation of OC and TN on the surface of the soil/sand,and this accumulation effect is significant in sand,and the content of C and N in soil is significantly higher than that in sand.The decomposition of leaf litter enrichedδ¹³C andδ¹⁵N values in 0-2 cm soil layer,and the enrichment degree in Robinia pseudoacacia leaf litter treatment was significantly greater than that in Populus tomentosa leaf litter treatment.On the contrary,theδ¹³C values of the control soil are more evenly distributed on the profiles（around-21‰）.The fractions of litter-derived C and N in soil was significantly lower than that in sand,accounting for about 30%in sand.The fractions of litter-derived C in soil were significantly higher than litter-derived N,and the litter-derived of C and N from Robinia pseudoacacia leaf litter was higher than that from Populus tomentosa.（4）The mass loss of leaf litter was the least under 400 mm precipitation treatment.Theδ¹³C value of Robinia pseudoacacia leaf litter was the highest at 400 mm precipitation.There were no significant differences of C content,N content andδ¹⁵N value of leaf litter in different precipitation treatments.In addition,the C content,N content,δ¹³C value andδ¹⁵N value of leaf litter treated soil were not significantly affected by different precipitation treatments.800 mm precipitation was the most suitable for leguminous Robinia pseudoacacia leaf litter C and N to enter soil,while 600 mm precipitation was the most suitable for non-leguminous Populus tomentosa leaf litter C and N to enter soil.（5）The contents of C and N in leaf litter and in soil/sand,as well as the fractions of litter-derived C and N were coupled under different decomposition times,different soil layers and different precipitation gradients,and this coupling relationship was strongest at the early stage of decomposition.Litter cover weakened the response of N to C in soil,but uncoupling did not occur.The coupling degree of C and N of Robinia pseudoacacia leaf litter was significantly stronger than that of Populus tomentosa.The degree of coupling between C and N in litter is independent of whether litter is decomposed into soil.High precipitation will enhance the coupling of the fractions of litter-derived C and N.（6）The p H value of leaching solution was significantly higher in sand than that in soil,but the EC value and NO₃^--N content in sand were significantly lower than that in soil.In addition,the concentration of NH₄⁺-N,DOC and DON in the leaching solution had no significant difference between the sand and the soil,and the concentrations were all small.The p H value,EC value,NH₄⁺-N and DOC concentration of the leaching solution were less affected by the change of precipitation.With the process of decomposition,the decrease of EC value,NH₄⁺-N,NO₃^--N and DOC concentration mainly occurred in the first 3 months.Among them,the NO₃^--N concentration is the highest,and the decrease is the largest in the first month of decomposition（the decrease can reach 50%in 2-4 weeks）.In conclusion,the simultaneous ¹³C and ¹⁵N labeling methods can be applied to broad-leaved vegetation in the Loess Plateau.The abundance of ¹³C and ¹⁵N labelled in leaves with high N content was lower than that in leaves with low N content.Compared with sand,leaf litter decomposed faster on soil.The mass loss of leaf litter of broadleaf vegetation in the Loess Plateau region should be mainly measured by N/P ratio,and N/P ratio is negatively correlated with leaf litter mass loss.The soluble material of leaf litter was mainly lost in the first 60 days,and only the insoluble material remained after 8months,which was slowly decomposed under the joint action of soil enzyme system.The fractions of litter-derived C and N in soil from Robinia pseudoacacia was higher than that of Populus tomentosa.From the perspective of improving soil nutrients and carbon sequestration and reducing emissions,800 mm precipitation was the most suitable for Robinia pseudoacacia leaf litter C and N to enter soil,while 600 mm precipitation was the most suitable for Populus tomentosa leaf litter C and N to enter soil.The degree of coupling between C and N in litter is independent of whether litter is decomposed into soil.High precipitation will enhance the coupling of leaf litter sources C and N.The results provided a scientific basis for accurately predicting C and N cycles in terrestrial ecosystems.