Nitrogen Fertilization Enhances Soil Organic Carbon Accumulation by Improving Photosynthetic C Assimilation and Root Exudation Efficiency in a Salt Marsh
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Titel:
Nitrogen Fertilization Enhances Soil Organic Carbon Accumulation by Improving Photosynthetic C Assimilation and Root Exudation Efficiency in a Salt Marsh
Beschreibung:
Although salt marshes cover only a small area of the Earth, their contribution to long term carbon (C) burial is comparable to C sinks in many more dominant terrestrial ecosystem types. Continuous nitrogen (N) loading alters plant growth and subsequently has the potential to impact soil organic carbon (SOC) accumulation in salt marshes. However, there is presently little information concerning the input and allocation of photosynthesized C in plant-soil-microbial systems. This knowledge gap hampers the quantification of C fluxes and the clarification of the mechanisms controlling the C budget under N loading in salt marsh ecosystems. To address this, we conducted an N fertilization field observation combined with a five hour 13C-pulse labeling experiment in a salt marsh dominated by Suaeda. salsa (S. salsa) in the Yellow River Delta, China. N fertilization increased net 13C assimilation of S. Salsa by 177.37%, which was primarily allocated to aboveground biomass and SOC. However, N fertilization had little effect on 13C allocation to belowground biomass. Correlation analysis showed that 13C incorporation in soil was significantly and linearly correlated with 13C incorporation in shoots rather than in roots both in a 0N (0 g N m-2 yr-1) and +N (20 g N m-2 yr-1) group. The results suggest that SOC increase under N fertilization was mainly due to an increased C assimilation rate and more efficient downward transfer of photosynthesized C instead of root lysate and detritus. In addition, N fertilization strongly improved the 13C amounts in the chloroform-labile SOC component by 315%. However, the absolute increment of newly fix 13C mainly existed in the form of residual SOC, which had more tendency for burial in the soil. Thus, N fertilization enhanced SOC accumulation although C loss increased via belowground respiration. These results have important implications for predicting the carbon budget under further human-induced N loading