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

Abstract

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 δN values for all sites, slightly higher δN values for needles, while the highest δN 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 δN 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.

Nitrogen resorption and fractionation during leaf senescence in typical tree species in Japan

“…Differences in leaf δ 15 N among tree species could also be due to differences in rooting depth, although beech and larch are characterized by shallower rooting, or physiological factors such as different leaf-out dates, which might give one species access to a different N pool, but that seems unlikely. Therefore, the enriched δ 15 N values for larch seemed to be due mainly to preferential uptake of ammonium and physiological differences in transport, and assimilation of N between these species (Pardo et al 2006(Pardo et al , 2013, because leaf δ 15 N values in the present study agree with values found in larch forests of Mongolia (Hayashi et al 2018). Leaf δ 15 N in black locust and cedar were similar to tree-ring δ 15 N values of black locust (close to atmospheric δ 15 N = 0) and black pine in coastal areas (ranging from − 2.0 to − 4.0‰) of the Shonai Region (Lopez et al 2014), respectively.…”

“…Another influence on the δ 15 N value in the leaves could be mycorrhizal associations of the tree species; plants associated with ectomycorrhizal fungi (ECM) such as beech and larch have depleted δ 15 N compared to those with arbuscular mycorrhizal fungi (AM) (Michelsen et al 1996;Schmidt and Stewart 2003) such as cedar. However, larch leaves were in general substantially more enriched in δ 15 N compared to the other two species, which could indicate a lower reliance on ECM during N uptake (Hayashi et al 2018). Differences in leaf δ 15 N among tree species could also be due to differences in rooting depth, although beech and larch are characterized by shallower rooting, or physiological factors such as different leaf-out dates, which might give one species access to a different N pool, but that seems unlikely.…”

Increasing tree density accelerates stand‐level nitrogen cycling at the taiga–tundra ecotone in northeastern Siberia

“…Furthermore, during senescence in September, foliar δ 15 N decreased with increasing tree density and greater proportion of N resorbed. A declining trend in δ 15 N from pre‐ to post‐abscission has been documented for congeneric L. laricina (Chapin III & Kedrowski, 1983) and L. sibirica needles with a similar magnitude of resorption (Hayashi et al, 2018), and across many other species (Enta et al, 2020). These patterns in foliar δ 15 N coupled with the high resorption levels confirm that these TTE forests are low‐N environments but indicate that N limitation does not increase with density, potentially because of shifts in allocation and turnover of N that support productivity.…”

A preliminary study into the use of tree-ring and foliar geochemistry as bio-indicators for vehicular NOxpollution in Malta

“…However, in all cases, top-soils are a few permille heavier than the tree-rings, which are in turn slightly heavier than the recent foliage, meaning that isotopic fractionation between different N reservoirs at each site is conserved, but the starting compositions were likely distinct [50]. Given the similarity in climate and bedrock geology, it is expected that processes contributing to isotopic shifts in the top-soils sampled at S1 and S2 are also occurring at S3 [51], and that the only major factor which differs is the proximal presence of NOx pollution at the former two sites. Hence, we suggest that the top-soil δ 15 N trends reported in this study can be explained by the deposition of traffic-related NOx and particulates which are 15 N-enriched, similarly to the results observed and interpreted by Xu et al [36].…”