There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar d 15 N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha À1 yr À1 , while the throughfall covered a larger range of 11.7-65.1 kg N ha À1 yr À1 . Foliar N concentration, NO 3 À leaching to stream, and soil NO 3 À concentration were low and NO 3 À production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO 3 À leaching to stream, and negatively with pH values in soil and stream water. Foliar d 15 N was between À6.6% and 0.7%, and was negatively correlated with soil NO 3 À concentration and NO 3 À leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest d 15 N values. We proposed several potential mechanism that could contribute to the d 15 N pattern, including (1) increased plant uptake of 15 N-depleted soil NO 3 À , (2) foliage uptake of 15 N-depleted NH 4 1 , (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.
This study experimentally determined the contribution of ferric iron (Fe(III)) associated with humic substances (HS) to light absorption by chromophoric dissolved organic matter (CDOM). The associations between Fe(III) and HS (HS‐Fe) were generated by mixing HS standards with Fe(III) in acidic conditions and adjusting pH to 8. HS‐associated Fe was separated from total Fe by filtering (0.7 µm and 0.2 µm filters) and by removing the free Fe ions by cation exchange chromatography. The maximum Fe‐binding capacities (at pH 8) of Suwannee River humic acid, Suwannee River fulvic acid, and Pony Lake (Antarctica) fulvic acid were 13.0, 13.5, and 7.64 µmol Fe [mg C]−1, respectively, indicating that wetland‐derived HS had a higher Fe‐binding capacity than plankton‐derived HS. HS‐associated Fe increased the absorption coefficient of CDOM by up to several fold in the visible range of the spectrum and reduced the spectral slope coefficient of CDOM. The Fe‐induced increase in light absorption was spectrally similar among different HS examined. The Fe‐specific absorption coefficient spectrum for HS‐associated Fe (aλ,Fe*) was calculated from the Fe‐induced increase in light absorption by normalizing it with the concentration of Fe in the HS pool. The aλ,Fe* was adopted in estimation of the contribution of HS‐associated Fe to light absorption by CDOM in 13 circum‐neutral natural waters collected from a spring, 10 major rivers, a lake, and a coastal area. HS‐associated Fe was calculated to be responsible for from 0.6% (Mississippi River) to 56.4% (Löytynlähde spring) of light absorption by CDOM at a wavelength of 410 nm. This study shows that HS‐associated Fe can be an important component in light absorption by CDOM and also influence the spectral slope coefficient of CDOM.
Knowledge of latitudinal patterns in plant defense and herbivory is crucial for understanding the mechanisms that govern ecosystem functioning and for predicting their responses to climate change. Using a widely distributed species in East Asia, Quercus variabilis, we aim to reveal defense patterns of trees with respect to ontogeny along latitudinal gradients. Six leaf chemical (total phenolics and total condensed tannin concentrations) and physical (cellulose, hemicellulose, lignin and dry mass concentration) defensive traits as well as leaf herbivory (% leaf area loss) were investigated in natural Chinese cork oak (Q. variabilis) forests across two ontogenetic stages (juvenile and mature trees) along a ~14°-latitudinal gradient. Our results showed that juveniles had higher herbivory values and a higher concentration of leaf chemical defense substances compared with mature trees across the latitudinal gradient. In addition, chemical defense and herbivory in both ontogenetic stages decreased with increasing latitude, which supports the latitudinal herbivory-defense hypothesis and optimal defense theory. The identified trade-offs between chemical and physical defense were primarily determined by environmental variation associated with the latitudinal gradient, with the climatic factors (annual precipitation, minimum temperature of the coldest month) largely contributing to the latitudinal defense pattern in both juvenile and mature oak trees.
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