| Nitrogen（N）is the main nutrient regulating the growth rate of trees.The mineral N absorbed and utilized by trees comes from the mineralization of soil organic N.Meanwhile the chemical properties of litter are the key factors affecting the mineralization of soil organic N.In forest ecosystems,although many studies have been carried out on the effect of single litter decomposition on soil N mineralization,there are only few studies investigating the effect of mixed litter decomposition.In this study,Fraxinus mandshurica（S）and Larix olgensis（L）and Juglans mandshurica（H）were selected as they are the main tree species for afforestation in northeast China.Leaf litters were collected from each species and incubated with soils for 175d to determine the decomposition rate of single and mixed litter additions（S×L,H×L）and investigate the influence of litter properties on soil net N mineralization rate.The results are as follows:（1）At the end of incubation,the mass residual rate in the S×L treatment was 64.06%whereas it was 67.22%in the H×L treatment,but there was no significant difference between these two treatments（P>0.05）.Meanwhile,no significant difference between mixed litter and single litter residue rate（P>0.05）was observed.During incubation,the decomposition of mixed litters had a significant non-additive effect,which was an antagonistic effect in the early stage but turned to a synergistic effect in the late stage.During incubation,the total N content of mixed litters and single litters showed to be enriched in early stage and consumed in late stage,while the total P content showed to be enriched and consumed then enriched and consumed again,whereas litter lignin content gradually increased and cellulose and hemicellulose content both gradually decreased.Moreover,there was a negative correlation（P<0.05）between litter decomposition and total N content,and a positive correlation（P<0.05）between litter decomposition and C/N,cellulose/N,lignin/N ratio respectively.（2）During the whole incubation period,in all litter-treated and control soils the concentrations of NH4+-N,NO3--N and inorganic N（NH4+-N+NO3--N）decreased at first,reached the lowest value on the 7th day of incubation,subsequently increased and reached the highest value at the end of incubation.The average soil mineral N concentration of all litter treatments was higher than that of controls,indicating that litter addition promoted the increase of mineral N concentration.The average soil mineral N concentrations of mixed litter treatments were significantly lower than those of the broad-leaved treatments（S and H）,but significantly higher than those of L litter treatments（P<0.05）.During incubation,the net ammonification rate,net nitrification rate and net N mineralization rate in all litter treatments and the control showed an"M"shaped trend,and the average soil net N mineralization rate in all litter treatments was higher than control.The average soil net N mineralization rate in mixed litter treatments was lower than that of the corresponding broadleaf litter treatments（S and H）,but higher than that in L litter treatment.（3）Soil organic C,N and P concentrations were not significantly affected by single and mixed litter treatments（P>0.05）.During incubation,the MBC and MBN concentrations in all litter-treated and control soil increased first and then decreased,and the average MBC and MBN concentrations in litter-treated soil were higher than those in control soil.Correlation analysis showed that mineral N concentration was positively correlated with litter N content,and negatively correlated with litter C/N,cellulose/N,lignin/N,soil MBC,MBN（P<0.05）.Meanwhile,the net ammonification rate was negatively correlated with the lignin/N ratio（P<0.05）.The net nitrification rate and net N mineralization rate were positively correlated with litter N content and negatively correlated with litter C/N,cellulose/N,and lignin/N（P<0.05）,respectively.In conclusion,mixed litter（S×L and H×L）treatment didn’t affect how soil net N mineralization changed over time comparing with corresponding single litter treatment.However,the average net N mineralization rate in mixed litter treatments was higher than that in the corresponding broadleaf litter treatments（S and H）,and greater than that in the L treatment.Based on the above,the mixed forest of Larix olgensis and Fraxinus mandshurica and Juglans mandshurica was more conducive to soil organic N mineralization than the pure Larix olgensis forest. |
