Nitrogen isotope pattern in Mongolian larch stands at the southern Eurasian boreal forest boundary.

In the last decades a drastic increase in air temperature but a stable precipitation regime in Mongolia has led to gradual drying conditions. Thus, we evaluated the effect of spatial and climatic characteristics on the soil-plant nitrogen dynamics in three representative larch stands (Larix sibirica) with different geographical and climatic conditions using stable nitrogen isotopes. The results showed significant differences in the soil inorganic N content among sites and consequently a different isotopic composition in the plant-soil system. Litter, bark and wood had the lowest δ15N values for all sites, slightly higher δ15N values for needles, while the highest δ15N values were observed for roots and soil. These differences could be the result of the larch stands age themselves, but were in agreement with the spatial and climatic characteristics of the sites. Based on the δ15N value a higher reliance on ectomycorrhizal fungi (ECMF) was observed in the warmest and driest site, while lower dependency was shown in the cooler northern site with higher soil inorganic N content. In both sites, the rate of air temperature increase has been similar in the last decades; however, their soil-plant N dynamics showed different characteristics.

Evaluation of the effect of the 2011 Tsunami on coastal forests by means of multiple isotopic analyses of tree-rings.

The March 2011 Mega-Tsunami in eastern Japan damaged at different degrees the black pine (Pinus thunbergii) forests along the coast. In order to evaluate the recovery of black pine four years later, tree-ring samples from 9 trees for the period 2002-2014 were analyzed for ring growth and stable isotopes (δ13C, δ15N and δ18O). The results showed that annual tree-ring width decreased approximately 70 % from the year 2011 to 2014 compared to the period previous to the tsunami (2002-2010). The multiple isotopic analyses showed that the reduction in growth was caused by soil salinity that prompted stomatal closure and an abrupt increase of tree-ring δ13C. Sea water deposition in the soil did not affect tree-ring δ18O values. Two years after the tsunami, decreasing tree-ring δ13C values caused by apparently photosynthetic recovery did not translate into radial tree-growth, indicating a possible shift in carbon allocation to foliage and mainly roots as a defense mechanism to sodium toxicity. The dual δ13C-δ18O model explains neither the limited growth nor the subsequent recovery in δ13C. Similarly tree-ring δ15N indicated that there was no difference in nitrogen availability before and after the tsunami, suggesting that nutrients were not a limitation but rather soil salinity.

Effects of pre-treatment on the nitrogen isotope composition of Japanese black pine (Pinus thunbergii) tree-rings as affected by high N input.

Temporal changes in the acquisition of nitrogen (N) are recorded in tree-rings together with unique N isotopic values. Some debate continues regarding the importance of wood pre-treatment in isotope analysis and, thus, this study focuses on the removal of labile components to determine the intrinsic nature of N in tree-rings. The total concentration and stable isotopic value of N in annual tree-rings were determined for two cores from Japanese black pine (Pinus thunbergii) from areas colonized by black cormorant (Phalacrocorax carbo). One core sample was also collected from a control site, without cormorants. Sharp increases in tree-ring δ(15)N values associated with migration of the cormorant population indicate positive incorporation of N from soils, whereas a less pronounced trend was observed for ring samples for periods without or substantially less migration, and for those obtained from the control site. All labile N components were removed by repeated extraction with toluene/ethanol (1:1) solution. Radial translocation of labile N is limited in tree-rings from Japanese black pine, providing intrinsic records on N acquisition. The difference in N isotopic values (up to 7.0‰) following pre-treatment was statistically significant for trees affected by the avian colony, whereas the pre-treatment of the control samples did not influence N values. The implication is that in agreement with previous studies pre-treatment is not necessary when trees are exposed to natural N concentrations in the soil but the removal of enriched δ(15)N labile components is necessary when woody plants are exposed to unusually high inputs of N into soils. However, the temporal trend in tree-ring δ(15)N series of the avian N affected trees did not change. Thus, if the priority is not the value but the trend then pre-treatment is not necessary.

Differential response of two Pinus spp. to avian nitrogen input as revealed by nitrogen isotope analysis for tree rings.

Temporal variations in N concentration and δ(15)N value of annual tree rings (1 year of time resolution) of two Japanese Black Pine (Pinus thunbergii) and three Japanese Red Pine (Pinus densiflora) trees under current breeding activity of the Great Cormorant (Pharacrocorax carbo) and the Black-tailed Gull (Larus crassirostris), respectively, in central and northeastern Japan were studied. Both species from control sites where no avian input occurs show negative values (δ(15)N = around -4 ‰ to -2 ‰) which are common among higher plants growing under high rainfall regimes. The δ(15)N values of P. densiflora show uniformly positive values several years before and after the breeding event, indicating N translocation that moved the absorbed N of a given growth year to tree rings of the previous year while a clear historical value of soil N dynamics was kept intact in the annual rings of P. thunbergii. Long-term N trends inferred from tree rings must take into account tree species with limited translocation rates that can retain actual N annual acquisition.