Effects Of Nitrogen Addition On Litter Decomposition In Artificial Pinus Tabulaeformis Forests

Litter decomposition is an important part of the nutrient cycle in forests ecosystem through releasing nutrients to regulate the formation of soil organic carbon.Increasing nitrogen(N)deposition is significantly affecting the material cycle and energy flow in forest ecosystems.However,it is not clear how N deposition affects litter decomposition,which restricts our understanding about the carbon cycle model.This study was conducted in an artificial Pinus tabulaeformis forests and simulated litter decomposition by using litter bag method from November 2015 to September 2016.Three N treatments,namely,N3(3 g N m-2 a-1),N6(6 g N m-2 a-1),and N9(9 g N m-2 a-1),and a control treatment with no N added were applied for three years,and each was replicated four times.The main conclusions are as following:(1)Litter decomposition had obviously staggered feature.In the early stage(0-60 days),the loss of litter was fast,and the decomposition rate was slow in the middle stage(60-180 days),then increased at the later stage(180-300 days).After 300 days of decomposition,the mass remaining rate is 69.08%-72.66%.The litter residual rate of CK was greatest,then N3 and N9,and N6 was lowest.The decomposition coefficients of each treatment were obtained by using the Olson exponential decay model.The decomposition coefficients of each treatment were 0.368(CK),0.341(N3),0.424(N6)and 0.388(N9),respectively.In general,N6 and N9 treatment could promote litter leaf decomposition.(2)N addition increased the content of pine leaf initial N,P,and non-structural carbohydrate(NSC),and had no significant effect on initial C and lignin content.C content fluctuated during litter decomposition,and varied with different treatments.At the end of sampling,C content of CK was highest,and N3 was lowest.The total N content firstly increased to the highest level at 2 months,and then showing down significantly afer that.N content of N3 treatment was significantly higher than CK and N6 during decomposition process.There was no significant difference in P content between different N treatments.The content of NSC in litter was degraded rapidly at the early stage of decomposition,and the CK treatment was higher than N addition treatment at the end of decomposition.Except for N9 treatment,the lignin content firstly increased,then decreased with the decomposition time.After 10 months,the lignin content of N6 treatment was significantly lower than others.C/N,C/P and lignin/N in N addition treatment were lower than CK during decomposition.(3)There was no significant effect of short-term N addition on surface soil aggregates,while soil organic carbon was dramatically affected by N addition.Soil organic carbon content in the 0-10 cm soil layer was higher in the three N addition treatments and was highest in the N6 treatment.Additionally,the change in organic carbon content in macro-and micro-aggregates generally followed the pattern of soil organic carbon,and moderately,highly labile organic carbon also followed.The results showed that short-term N addition affected surface soil organic carbon content by changing the amount of moderately labile organic carbon in the surface soil macro-and micro-aggregates.The results of a principal components analysis showed that soil total N and pH were affected by short-term N addition and it also caused a change in soil texture and total P.The changes of soil physical and chemical properties may result in an increase in root biomass and litter decomposition which brought a significantly change on active organic carbon content in surface soil aggregates.(4)Path analysis showed that the factors influencing litter decomposition under different treatments were different.Lignin content is the main factor affecting litter decomposition.The effect of N addition on lignin degradation was related to the amount of N added and the litter matrix quality and decomposition environment.The degradation of lignin in CK and N3 treatment was inhibited by N reacted with lignin.The results showed that low N(3 g N·m-2·a-1)addition had no obviously effect on the decomposition' of litter leaf,and the medium N(6 g N-m·2·a-1)and high N(9 g N·m-2·a-1)addition could promote the litter decomposition.The initial N content,C/N?C/P?and lignin/N could not be used to predict the litter decomposition process.Lignin content is the main influencing factor.However,due to the interference from other factors,the effects of lignin on litter decomposition would show different results.