An overabundance of reactive nitrogen (N) as a result of anthropogenic activities has led to multiple human health and environmental concerns. Efforts to address these concerns require an accurate accounting of N inputs. Here we present a novel synthesis of data describing N inputs to the US including the range of estimates spatial patterns and uncertainties. This analysis shows that human-mediated N inputs are ubiquitous across the country but are spatially heterogeneous ranging from < 0.1 to 34.6 times the background N input for individual water-resource units (8-digit Hydrologic Unit Codes). The Midwest Mid-Atlantic central California and portions of the Columbia River valley currently receive the highest N loads. Major opportunities to advance our understanding of N sources can be achieved by: (1) enhancing the spatial and temporal resolution of agricultural N input data (2) improving livestock and human waste monitoring and (3) better quantifying biological N fixation in non-cultivated ecosystems.

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【从土壤有机质Δ14C值估计森林土壤CO2通量的组成】Mirco Rodeghiero Galina Churkina Cristina Martinez Thomas Scholten Damiano Gianelle Alessandro Cescatti. Components of forest soil CO₂efflux estimated from Δ¹⁴C values of soil organic matter.Plant and Soil 2013 364(1-2): 55-68

Abstract

Aims

The partitioning of the total soil CO₂ efflux into its two main components: respiration from roots (and root-associated organisms) and microbial respiration (by means of soil organic matter (SOM) and litter decomposition) is a major need in soil carbon dynamics studies in order to understand if a soil is a net sink or source of carbon.

Methods

The heterotrophic component of the CO₂ efflux was estimated for 11 forest sites as the ratio between the carbon stocks of different SOM pools and previously published (Δ¹⁴C derived) turnover times. The autotrophic component including root and root-associated respiration was calculated by subtracting the heterotrophic component from total soil chamber measured CO₂ efflux.

Results

R

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