Effects Of Nitrogen Addition On Litter Decomposition In Poplar Plantations In A Coastal Area,China

With nitrogen deposition globalization, it may have had a significant influence on the function of forest ecosystem. Forest litter decomposition is a crucial process to maintain the material and energy cycling normally, which the microorganisms play a key role in. The experiment was conducted in the Dongtai National Forest Farm, during Jan. 2015 to Dec. 2015. We added the NH4NO3(N0(0 g N/(m2a)), N1(5 g N/(m2a)), N2(10 g N/(m2a)), N3(15 g N/(m2a)), N4(30 g N/(m2a))) to simulate the nitrogen deposition and set the in situ litter bags to simulate the litter decomposition in an 11-year-old poplar(Populus deltoides CL‘35') plantation. We focused on the physical and chemical changes during decomposition of litter leaf and twig under N addition. The main results are shown below:1. The higher the concentration of the N addition, the deeper the decomposition of litter leaf was suppressed. On the contrary, the decomposition of litter twig was promoted except N3. We found that decomposition coefficients of leaf and twig were between 0.670 and 1.696 by Olson models. Time needed for the decomposition proportion in fifty percent and ninety-five percent in litter leaf and twig was 0.68~0.91 a, 2.04~3.25 a, and 0.91~1.19 a, 3.37~4.62 a, respectively.2. The lignin content accumulated at first, and then maintained at a high level during the litter leaf decomposition, and lignin decomposition was inhibited significantly under the high concentration N addition. The cellulose content increased first then dropped during the leaf decomposition, and it increased first then decreased under the addition N concentration increasing at the 365 th d. The higher the addition N concentration was, the more lignin content the litter twigs had. The cellulose content had no apparent change at first then declined during the twig decomposition. And the higher the addition N concentration was, the longer the time that cellulose content kept stable was.3. The total nitrogen content progressive increased during the leaf decomposition, and it showed enrichment under adding low N concentration but released under high N concentration. The ratios of C/N progressive decreased during the leaf decomposition. During the decomposition of litter twig, the total nitrogen content felled before the 115 th d, and then it increased. It showed enrichment under high N concentration adding but released under low N concentration at the 365 th d. The ratio of C/N increased first then decreased during the twig decomposition, and it dropped down under the N adding.4. The ratio of SMBC/SMBN increased during litter leaf decomposition under N adding, but N0 increased first then decreased. And the ratio of SMBC/SMBN increased during litter twig decomposition. The leaf and twig ratio order was N2 > N1 > N4 > N0 > N3 and N4 > N3 > N2 > N0 > N1, respectively at the 365 th d. The ratio of SMBC/SMBN were linear negative correlation with both dry weight remains and the ratio of C/N of leaf and twig, and were linear positive correlation with lignin/cellulose of leaf and twig.5. The dry weight remains of the litter leaf were positively correlated with the ratio of lignin/C(p<0.05); decomposition rate were significant negatively correlated with the lignin content and the ratio of lignin/C(p<0.01); lignin content were negatively correlated with the cellulose and holocellulose content(p<0.05). The weight remains of the litter twig were negative correlation with lignin content and the ratio of lignin/C(p<0.05), and were significant negatively correlated with the ratio of lignin/cellulose(p<0.01), and were significant positively correlated with the cellulose and holocellulose content(p<0.05), and were positive correlation with the ratio of C/N(p<0.05); decomposition rate were significant negatively correlated with cellulose and holocellulose content(p<0.01), and were positive correlation with the ratio of lignin/C(p<0.05), and were negative correlation with the ratio of C/N(p<0.05); the ratio of C/N were significant negatively correlated with ratio of lignin/cellulose and lignin/holocellulose(p<0.01).The soil pH increased during both litter leaf and twig decomposition. It declined during growing season under N adding, but it recovered to normal level during non-growing season.6. The soil water content changed without obvious regular pattern during both litter leaf and twig decomposition, and only the soil water content under leaf was influenced under N adding. The dry weight remains of litter leaf and twig and soil pH were linear negative correlation, and there was a linear positive correlation between the dry weight remains of twig and soil water content.In a word, only the degree not the trends of variation was happened during litter leaf and twig decomposition under N adding. The decomposition of litter leaf and twig was inhibited and promoted, respectively, which may influence the normal cycling of the matter and energy in poplar plantations. In this condition, it may have an influence on the carbon sequestration in the ecosystem.