Effect Of Nitrogen Addition On Litter Decomposition And Nutrient Release In Grasslands

Litter is the main pathway,through which above-ground plants become decomposer subsystems and return nutrients to the soil.Litter decomposition is the key process of nutrient cycling in the ecosystems.Nitrogen(N)deposition can indirectly affect litter decomposition by changing environmental conditions such as soil nutrient status,and directly by increasing its N content.Although many studies have been conducted to examine the regulation of N addition on the litter decomposition in grasslands,there is a lack of deep understanding of the nutrient release pattern and its interaction with litter decomposition.Therefore,it is important to study the role of N addition in regulating nutrient release during litter decomposition and understanding the response mechanisms of grassland ecosystem functions to global changes and human activities.In this paper,we studied the effect of N addition and litter quality on mass loss and nutrient release of litter decomposition with a long-term control experiment(2009-2022),and analyzed the impacts of nutrient release on litter mass loss,and obtained the following main results and conclusions:(1)The mass loss of litter exhibited distinct stages,and nutrient release differed among different nutrients.The mass loss of the litter with low and high N content changed similarly with time,rapidly in the early 10 months and stablely in the later periods.Litter C,K,Na and Mg were released during decomposition,and N was concentrated and released in the early and later periods,respectively.Fe and Zn were always concentrated.The release of P,Ca and Mn differed between the two types of litter.P in the low N litter changed from be released to be concentrated,Ca changed from be concentrated to be released,and Mn was always concentrated.In the high N litter,P and Ca was concentrated,and Mn was released.The litter cellulose was released,while the lignin was concentrated throughout the experimental periods.(2)The correlation between litter mass loss and nutrient release changed with time.Except Na,release of Ca and Mn,all other elements were significantly correlated with litter mass loss.Release of C and cellulose was most strongly correlated with mass loss.The negative correlations with N release increased and the negative correlations with P release decreased with time,while the correlations with Ca,Fe,Ze and lignin changed from negative in the early period to positive in the later period.Release of K,Na and Mn had positive correlation in the early and end of period,but had strong negative correlation in the mid-period.The synergistic and trade-off relationships among nutrients were strengthened and then weakened over time.In the first growing season,they were weakened and then strengthened,while in the second year,they were higher in the middle than in the early and later period of the growing season.In general,the relationships among the nutrients increased and then decreased during the decomposition process.(3)N addition promoted litter mass loss and its effect on nutrient release was elemental dependent.N addition increased the release rate of cellulose,which directly and indirectly increased litter mass loss by inhibiting the release of nutrients.Thus,N addition promoted the litter mass loss by enhancing cellulose release.N addition significantly increased the litter N and P content and promoted Mg release.N addition significantly increased the K content,promoted Mn release and inhibited the enrichment of Zn,but had no significant effect on the release of Na,Ca and Fe.(4)High N litter showed a home-field advantage in the decomposition process,but not for low N litter.The high N litter had greater decomposition rate in the home environments than in the guest environments,while the low N litter had greater decomposition rate in the guest environments.The effect of home-field advantage(HFA)on nutrient release changed with time and differed among nutrients.The home-field advantage index(HFAI)of litter mass loss and carbon decreased and then increased with time,and was always negative for N release.K and Na had greater HFAI at the later stage than the early stage,while the HFAI of Ca,Mg,Zn,Fe and Mn were always negative and stronger at the later stage than the early stage.The HFAI of cellulose and lignin was negative at the early stage and almost despeared at the later stage,indicating that there was almost no HFA effect on cellulose and lignin at the later stage of decomposition.In summary,the mass loss and nutrient release of litter exhibited distinct stages,and the correlation between litter mass loss and nutrient release changed with time and was elemental dependent.N addition increased litter mass loss through affecting nutrient release,and promoted litter decomposition.The results of this study also indicated that there was a homefield advantage effect in the decomposition process of high N litter.This study showed the patterns of mass loss and nutrient release during litter decomposition,and revealed the mechanisms about how N addition affected litter decomposition.It will improve the understanding of the response of grassland structure and function to global changes and human activities,and also guide the formulation of scientific and rational grassland management policies.