The Effects Of Nitrogen And Phosphorus Additions On The Quality Of Chinese Fir Leaf Litter And Its Decomposition Processes

Chinese fir?Cunninghamia lanceolata?is an important afforestation species in the subtropical China.The decline of soil fertility and slow return of nutrients are important factors limiting the sustained productivity of Chinese fir plantation.The nutrient release from litter decomposition is an important way for the nutrient return to soil of Chinese fir plantation.Under the intensive management mode of plantations,fertilization?nitrogen and phosphorus addition?is an important measure to improve the nutrient supply and maintain productivity of forest land,yet fertilization can change both the internal and external conditions of litter decomposition at the same time and affect the process of decomposition.However the effect of nitrogen?N?and phosphorus?P?addition on the litter decomposition process through changing of litter quality is less researched,and the mechanism of microbial regulation is still unclear.In this study,Chinese fir plantation was selected as the research object.Since 2011,the plots with N and P addition were laid in Taihe,Jiangxi Province,specifically CK?contrast?,N1(50 kg N·hm^-2·a^-1),N2(100 kg N·hm^-2·a^-1),P(50 kg P·hm^-2·a^-1),N1P and N2P,in which N is added as NH₄NO₃,P is added as NaH₂PO₄.The litter of Chinese fir in the plots was collected to analyze the effects of N and P addition on litter leaves quality after N and P addition treatment for 5 years,and all litters were put into CK plots for 2 years for decomposition test.Through analyzing the quality attenuation,release rules of N and P elements,and changes in structural carbohydrate content during the decomposition process of litters,and combined with changes in microbial groups,the influence mechanism of the quality change of litters caused by N and P addition on their decomposition was investigated.The main results are:?1?N and P addition significantly affected the content of N and P in litters,but had no significant effect on the content of C.N content in litters was significantly increased after N addition or N conbined with P addition.P content in litters was increased by P addition,but P content was not affected by N addition.The contents of P in litters treated with P,N1P and N2P were 75.8%,84.8%and 103.0%higher than CK.At the same time,N addition significantly reduced the C/N,while P addition significantly reduced the C/P and N/P.The contents of lignin and cellulose in litters were not affected by the addition of N and P,but hemicellulose significantly decreased by P addition.?2?The mass remainning rate of litters decreased slowly at 0¹80 d,rapidly at 180³60 d,and gradually slowed down at 360⁷20 d.At 360 d and 720 d,the mass remainning rate decreased by 39.4%and 50.2%,respectively.At 360 d,the mass remainning rate of litters treated with N2P decreased by 14.8%.At 720 d,N2,P,N1P and N2P treatments were significantly different from CK,with decreases of 10.6%,12.4%,11.8%and 22.3%,respectively.?3?The N release process of litters treated with N and P addition was different.After CK,P,N1 and N1P treatment,N of litters exhibited a short enrichment process on 90 d before release.However,the litters treated with N2 and N2P showed a release process in the whole decomposition stage.At 720 d,compared with CK,the N remainning rates of N1,N2,N1P and N2P increased by 11.2%,16.0%,10.7%and 11.3%,respectively.There was an obvious enrichment process in the decomposition process of litters P content,and the enrichment process lasted until 180 d of decomposition.At 180 d,the average residual P rate of litters was 138.1%,while at 720 d,the average residual P rate of litters was 55.7%.On the whole,P content of litters treated with N and P addition showed a process of enrichment-release,but non-P addition treatment?CK,N1 and N2?showed stronger enrichment at 180 d,the P addition treatment?P,N1P and N2P?of was weak.When decomposing 720 d,the P remainning rates of litters treated with P,N1P and N2P were significantly lower than those of CK.?4?The remainning rates of lignin,cellulose and hemicellulose decreased with the decomposition.At 720 d of decomposition,the lignin remainning rate in litters treated with P,N1P and N2P was significantly lower than that of CK.At 360 d of decomposition,the cellulose remainning rate of litters treated with P,N1P and N2P was significantly lower than that of CK,but at 540 d and 720 d of decomposition,there was no significant difference in the cellulose remainning rate under the treatments.?5?At 360 d and 720 d of decomposition,N and P addition had a significant effect on the content of PLFAs in litters,and the content of PLFAs at 360 d of decomposition was significantly higher than that at 720 d.At 360 d,P and N1P treatment significantly increased the content of total PLFAs in litters,and at 720 d,P,N1P and N2P treatment significantly increased the content of total PLFAs in litters.There was no significant difference in fungi biomass among the treatments at 360 d after decomposition,while N1,P,N1P and N2P treatments significantly increased the fungi biomass in litters at 720 d after decomposition.At 360 d,P,N1P and N2P treatment was significantly increased fungi biomass,and at 720 d,N1P and N2P treatment was significantly increased bacteria biomass in litters.Meanwhile,the fungi/bacterial was 0.48 and 0.16 at 360 d and 720 d,respectively.At 720 d of decomposition,N1P and N2P treatment significantly increased the ratio of fungi/bacteria.In conclusion,the addition of N and P significantly changed the substrate quality of litters of Chinese fir,which mainly showed that the content of nutrients in litters increased and C/N,C/P and hemicellulose/N was decreased,but the content of lignin and cellulose in litters were not affected by the addition of N and P.The substrate quality change of litters significantly affected the decomposition rate and the release of N and P nutrients,and the decomposition rate was mainly regulated by P addition treatment.After P addition treatment,the decomposition of litters was accelerated,and the release rate of P was accelerated.At the same time,the substrate quality of litters will affect the decomposition of lignin and cellulose by changing the biomass and community structure of microorganisms during the decomposition process.It follows that N and P addition on forest litter substrate quality change in the long run will affect the decomposition of litter and nutrient release,and then affect the nutrient cycle of the entire ecosystem.