| Evaluating the effects of nitrogen (N) deposition on forest soil carbon (C) and N cycling is important for estimating regional C and N pools and its responses to environmental changes. Simulated N deposition experiments were conducted with four different N supply levels, including control (CK,0 kg N ha-1 yr-1), low (L,50 kg N ha-1 yr-1), intermediate (M,100 kg N ha-1 yr-1) and high level (H,150 kg N ha-1 yr-1), in four forest stands with different tree species compositions(i.e., Betula platyphylla, Fraxinus mandschurica, Pinus koraiensis, Larix olgensis) in Maoershan region of Heilongjiang Province. Soil C and N concentration, C and N input and output rate were measured.1. N deposition treatments had significant impacts on vagetation growth, and soil pH value. N treatments significantly increased average annual increment of DBH in four forest stands (B. platyphylla, F. mandschurica, P. koraiensis, L. olgensis). Analysis on forest growth under N deposition indicated the N critical load was more than 150 kg N ha-1 yr-1 in the research area. However, N treatments significantly decreased vegetable coverage in four forest stands. Analysis on vegetable coverage under N deposition indicated the N critical load was less than 50 kg N ha-1 yr-1 in the research area. N treatments significantly decreased soil pH value, which have decreased with increasing simulated N deposition in four forest stands.2. N deposition treatments had different impacts on soil C pools in different forest stands. Soil organic C concentration under L treatment were the largest compared with other treatments. Soil organic C concentration in broad-leaved forest (B. platyphylla and F. mandschurica) were inhibited more easily than that in coniferous forest (P. koraiensis and L. olgensis) under N treatments. L treatment significantly increased soil microbial biomass C concentration in four forest stands (P<0.05), H treatment were lower than L treatment. The dissolved organic C concentration and labile C concentration in soil under L treatment were higher than that under H treatment in four forest stands. The less root biomass density was in forest stand, the microbial biomass C concentration and dissolved organic C concentration were inhibited more easily by N treatments.3. N deposition treatments had different impacts on C release rate from litter decomposition and soil respiration rate in different forest stands. C release rate from litter decomposition in broad-leaved forest were inhibited more easily than that in coniferous forest under N treatments. Leaf and root litter C release rate in broad-leaved forest under L treatment were increased compared with CK treatment (P<0.05), but decreased under H treatment (P<0.05). Leaf litter C release rate in coniferous forest were not significant under all N treatments. Root litter C release rate in coniferous forest under L and M treatments were increased compared with CK treatment (P<0.05), but insignificant under H treatment (P<0.05). Soil respiration rate in broad-leaved forest were more vulnerable to the impact of N deposition than that in coniferous forest. Soil respiration rate in coniferous forest were increased under L treatment, while that were not affected in broad-leaved forest. Compared with CK treatment, Soil temperature sensitivity coefficient in 5 cm and 10 cm under L treatment was increased by 2.65% and 3.12%, but that under H treatment decreased by 6.29% and 5.46%, respectively.4. N deposition treatments had significant impacts on N pools and N cycling processes in soil. Compared with CK, N treatments significantly increased soil total N concentration and microbial biomass N concentration, respectively, which have decreased with increasing N treatments. Soil dissolved N concentration have increased with increasing N treatment. Soil microbial biomass N concentration in coniferous forest were inhibited more easily than that in broad-leaved forest under N treatments. Compared with CK treatment, N release rate from leaf and root litter decomposition were insignificant under L treatment, while decreased under H treatment in four forest stands. Compared with CK treatment, soil N nitrification rate under L, M and H treatment were increased by 6.20%(P>0.05),33.00%(P<0.05), and 34.42%(P<0.05), respectively. N deposition treatments mainly effected on soil N nitrification processes in Summer and Fall. N treatments improved variation range of soil N nitrification rate.5. N deposition treatments had significant impacts on ecological stoichiometry characteristics of forest ecosystem. In short N treatments, N treatments mainly effected on forest litters (leaf and root litter) and tree nutrient absorption organ (leaf and root) ecological stoichiometry characteristics, but did not effected on tree tissue organ (Stem and branch) ecological stoichiometry characteristics. N:P ratio in soil, litter, root were more strong effect than C:N ratio under N treatments. Compare with easily degradable litter, C:N ratio in recalcitrant litter were more strong effect under N treatments. Soil C cycling processes were not affected by C:N ratio under N deposition treatments, but soil N mineralization rate and N nitrification rate were effected significantly by C:N ratio under N deposition treatments. Soil C and N cycling processes with microorganism, which included litter C release rate, heterotrophic respiration rate, and soil N nitrification rate, was effected by N:P ratio under N treatments. Ecological stoichiometric analysis found the limitation of forest ecosystem might be switched from N to P under N treatments. Soil C and N concentration, and cycling processes were analyzed synthetically under N deposition treatments, which indicated the critical loads for N deposition was less than 100 kg N ha-1 yr-1 in the research area, might be close to 50 kg N ha-1 yr-1. |
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