| Nitrogen (N), as a fundamental and key element in global biogeochemical systems, is one of the most important factors in controlling carbon (C) and N processes in terrestrial ecosystems. In this meta-analysis study, after reviewed more than 2000 published papers about N addition, we extracted data from 266 independently papers that studied the effects of long-term manipulative N addition on terrestrial ecosystem C and N sequestrations. The experiments we chose included traditional N fertilization and simulated N deposition studies. The compiled database included 27 variables which contained C and N pools of plant (both aboveground and belowground parts), litter, soil and microbial biomass, root:shoot ratio, soil available N. the soil bulk density, soil pH value, soil respiration, N mineralization, N immobilization, nitrification, denitrification and the C:N ratios of plants, litter, soil and microbes.Overall. our results indicated that N fertilization not only had different effects on the C and N pools of plant. litter, microbe and soil. but significantly influenced terrestrial ecosystem C and N fluxes. Compared with the control groups, averaged plant C and N pool sizes significantly increased in N fertilization groups, ranging from 35.7% increase in aboveground plant C pool and 44.2% in aboveground plant N pool, to 23.1% increase in aboveground plant N pool and 53.2% increase in belowground plant N pool. The C and N contents in litter pool sizes were consistently higher in N fertilization groups than control groups by 20.9% and 23.9%, respectively. In summary. N addition significantly increased soil C and N pools across ecosystems by 2.2% and 6.2%. In nonagricultural ecosystem, however, N fertilization had minor effects on the soil C pool size but significantly increased the soil N pool size. The soil C pool had a 0.26% decline and the soil N pool soil had a 4.45% increase. Both soil microbial biomass C (MBC) and microbial biomass N (MBN) significantly decreased by 6.4% and 5.8%, respectively, in response to N addition. Averaged C:N ratios of plants, litter, microbes and soil all significantly declined. Averaged soil respiration, N mineralization, nitrification and denitrification all significantly increased by 5.3%, 22.9%,194.2% and 112.2%, respectively. The soil C and N pool sizes of different ecosystems (e.g., cropland, forest, grassland, others) responded differently to long-term N fertilization. N addition significantly increased the soil C pool size of croplands by 3.48%, but did not significantly alter the soil C pool sizes of forest, grassland and tundra ecosystems. In addition, the increments of soil N pools in cropland, forest and grassland are 6.98%,2.93% and 8.17%, respectively, in response to N fertilization. In agricultural ecosystem, N addition decreased soil C turnover rate and increased soil C turnover time so that could lead to the accumulation of soil C pool size. On the other hand, in nonagricultural ecosystem, N addition stimulated soil C turnover rate and decreased soil C turnover time, which led to the minor change of soil C pool. Both soil N turnover rates increased and soil N turnover time decreased in agricultural ecosystem and nonagricultural ecosystem. However, the increment of N turnover rate and decrease of turnover time in nonagricultural ecosystem are more than that in agricultural ecosystem. The significant increases of soil inorganic N and soil N pool, the rising soil N turnover rate and decline of N turnover time, the stimulation of N mineralization, nitrification and denitrification processes indicated that N addition significantly drove the N cycling. Thus, in different ecosystems N addition consistently stimulated plant growth, inhibited microbial biomass, decreased C:N ratios, but had different effects on soil C and N sequestrations. The relationships between the alteration of C and N pools in plant, litter, microbe and soil indicated that ecosystem C and N cycling are highly coupled. In addition, our results also showed that N application rate, experimental duration, cumulative N application amount, soil depth, latitude, mean annual temperature, and mean annual precipitation could influence the effects of N addition on ecosystem C and N sequestrations.In conclusion, even the soil C pool size in nonagricultural ecosystems had minor response to long-term N addition, the net C accumulation in agricultural ecosystem and the increments of both aboveground plant and belowground plant biomass in response to N fertilization or N deposition could help to sequestrate C in terrestrial ecosystem and buffer against the rise of atmospheric CO2.
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