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Weizmann Institute of Science (2008)

Factors controlling availability and dynamics of nitrogen in a semi-arid forest ecosystem

Gelfand, Ilya

Titre : Factors controlling availability and dynamics of nitrogen in a semi-arid forest ecosystem

Auteur : Gelfand, Ilya

Université de soutenance : Weizmann Institute of Science

Grade : Dissertations (Ph.D.) 2008

Land use and afforestation are important factors in the contemporary carbon cycle and influence the airborne fraction of anthropogenic CO 2 emissions. Here we studied ecosystem-scale changes in nitrogen dynamics associated with afforestation in a semiarid shrubland in southern Israel. This was motivated by the previous observations that the 40 years old pine afforestation system resulted in 2.5 fold increase in carbon stock, and mean annual NEE (last 6 years) of 2.6 tC ha -1 , similar to the global mean in the FluxNet monitoring system. The results of the present study show that there were only minor changes in ecosystem nitrogen stocks. However, there was almost 5 fold increase in the nitrogen use efficiency (NUE), compared to the surrounding shrubland (406.4 vs. 86.9, gr/gr based on ANPP, N stock and N turnover rate). Leaf C/N ratios increased from 22 in the shrubland to 53 in the forest, and litter C/N increased from 38 to 146, consistent with the increase in NUE and indicating efficient N reallocation in the forest. Mineralization and decomposition rates were slowed down by ∼50% (with large variability), and ∼20% respectively. These reductions in N recycling rates were accompanied with ∼65% decrease in annual nitric oxide emission in the forest ecosystem (with large seasonal cycle), and accumulation of soil NO 2- in the beginning of the wet period. The nitrite accumulation indicated differential recovery from the long dry season of microbial populations that oxidize ammonium to nitrate, which improved synchronization of nitrate supply to peak plant activities. The range of adjustments observed in the ecosystem N cycle help explain the significant carbon sequestration and biomass storage in this semiarid forest, without significant changes in N inputs. This, in turn, indicated that longterm warming and drying trends predicted for the entire Mediterranean, and other regions, may have milder effects on ecosystem productivity and carbon sequestration than predicted from short term drought or heat episodes in temperate regions.

Mots clés : Carbon sequestration, Pine afforestation, Environmental science, Semiarid forests, Nitrogen cycle, Health and environmental sciences Biomass storage


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