| 摘要: |
Cropland expansion and fertilizer applications are among the most important substantial effects of human actions on the global nitrogen (N) cycle. However, questions remain over the fate of anthropogenic N inputs, particularly whether a significant fraction of N-based fertilizers have been lost to inert N-2 or reactive N forms. Here, we combine natural N isotope constraints on the pre-industrial N cycle with global mass-balance modeling to investigate the role of cropland conversion on gaseous N emissions and hydrological N leaching fluxes. We estimate that cropland expansion has been accompanied by >9-fold increase in N input rates to cropping systems, roughly doubling the baseline N budget of the terrestrial biosphere. As a consequence, approximately 10 times more N is exported from modem croplands to the hydrosphere than in 1860, with a five-fold increase in cropland N gases emission to the atmosphere. Atmospheric NH3, NO, N(2)0 and N-2 fluxes increased from 8.6, 16.6, 11.7 and 31.9 Tg N yr(-1), respectively, in 1860 to 17.7, 23.6,152 and 39.7 Tg N yr(-1), respectively, by 2000. Thus, the growth in N-2 accounted for similar to 20% of cropland-driven N losses (dissolved plus gaseous pathways), with the remaining 80% exported as reactive N forms. Although the increase in N-2 emissions has mitigated some of the unwanted side-effects of N fertilizer applications on human health, the economy, and climate change, this inert sink has been unable to keep pace with the increase in N inputs for enhanced food production. Our results imply that, unless new management steps are taken, an increasing fraction of N fertilizers will mobilize to reactive N forms in the global land, air and water systems, thus further accelerating the negative consequences of human modifications of the N cycle this century. |
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