Carbon and nitrogen dynamics along the log bark decomposition continuum in a mesic old-growth boreal forest

Narrowing the uncertainties in carbon (C) and nitrogen (N) dynamics during decomposition of coarse woody debris (CWD) can significantly improve our understanding of forest ecosystem functioning. We examined C, N and pH dynamics in the least studied CWD component—tree bark in a 66-year-long decomposition chronosequence. The relative C concentration decreased by ca. 32% in pine bark, increased by ca. 18% in birch bark and remained stable in spruce and aspen bark. Nitrogen increased in bark of all tree species. In conifer bark, it increased along with epixylic succession. Over 45 years, the relative C/N ratio in bark decreased by 63 and 45% for coniferous and deciduous species, respectively. Bark pH did not change. Due to bark fragmentation, the total C and N amounts in bark of individual logs of aspen, birch, pine and spruce decreased at average rates of 0.03, 0.02, 0.26 and 0.05 year^?1, and 0.02, 0.02, 0.03 and 0.03 year^?1, respectively. At the forest stand level, the total amounts of C and N in log bark were 853 and 21 kg ha^?1 or 11.2 and 45.5% of the C and N amounts stored in downed logs and ca. 2.3–3.8 and 2.2–2.4%, respectively, of total C and N amounts stored in forest litter. In boreal forests, decomposing log bark may act as a long-term source of N for wood-inhabiting communities.     

Potentially toxic elements in urban soil catenas of W-Mo (Zakamensk,Russia) and Cu-Mo (Erdenet,Mongolia) mining areas

Purpose

The aim of this study was to characterize the conditions of the lateral migration of potentially toxic elements (PTEs) and to identify the zones of their accumulation in soil catenas of ore-mining landscapes in the cities of Zakamensk (Buryat Republic, Russia) and Erdenet (Mongolia) which are situated in the basin of the Selenga River.

Materials and methods

Eight soil catenas across the river valley with 31 pits in Zakamensk area and five catenas with 15 pits in Erdenet area were studied. Soil samples were collected in four landscape–geochemical positions: autonomous, on tops of the hills; trans-eluvial, on slopes of river valleys; trans-accumulative, at the footslopes; and superaqual, on the river floodplains. The total contents of 16 PTEs in the 148 samples were determined by mass spectrometry and by atomic emission spectrometry with inductively coupled plasma.

Results and discussion

It is found that the chemical composition of undisturbed soils is greatly affected by the widespread distribution of volcanic rocks enriched with W, Mo, Cd, Bi, and Sr in Zakamensk and with V, Co, Sr, and As in Erdenet. The difference between geochemical positions is relatively small. Under the influence of technogenic loads in the cities, the concentrations of PTE change along soil catenas increases by an order of magnitude. For chalcophile elements—Mo, Bi, Pb, Sb, and Cu in Zakamensk and Cu, Zn, Mo, Cd, Sn, and W in Erdenet—the type of their lateral distribution in the soil catenas changes because of the intense migration from the tailings and facilities with water and air flows. The accumulation of PTEs takes place on the chemisorptive (V, Co, Ba, As, Cr, and Ni), gley (Mo and Sb), organomineral (Sn), and alkaline (Sr and W) lateral geochemical barriers (LGBs) confined to subordinate positions. The artificial technological LGBs are formed due to the increased content of sand derived from the tailings by water erosion in Zakamensk and the input of silt particles windblown from the technogenic sources in Erdenet. This causes the rise in the concentrations of major ore elements (Mo, W, and Cu) and the accompanying elements (Pb, Bi, As, Sr, Cr, and Ni).

Conclusions

The results of soil–geochemical studies using the catenary approach and concept of LGBs made it possible to understand the main features of migration and fixation of PTEs in soils of the mining centers. They indicate the formation of a system of LGBs with intensive accumulation of major ore elements and associated ones in the subordinate landscapes of Zakamensk and Erdenet.

     

Humic substances alter the uptake and toxicity of nanodiamonds in wheat seedlings

Purpose

Detonation synthesis nanodiamonds (ND) are among the most widely applied nanoparticles due to their low cost of production and broad scope of applications. However, the fate and behavior of NDs in the environment are largely unknown. The behavior of NDs is greatly affected by humic substances (HSs), which comprise 50 to 80 % of natural organic matter in water and soil ecosystems. The uptake of detonation NDs by wheat seedlings and its toxicity were evaluated in the presence of seven HSs of different origins, including humic acids (HA, HS fraction soluble in alkali and insoluble in acid) and fulvic acids (FA, soluble in both alkali and acid).

Materials and methods

To monitor the uptake of NDs by plants, tritium-labeled NDs were produced. Liquid scintillation spectrometry and autoradiography were used to determine the amount of NDs absorbed by plants. The photosynthetic activity of the plants was measured using light response curves.

Results and discussion

After a 24-h exposure period, the ND content in the plant roots was 1720 μg g^?1. The introduction of HSs decreased the ND contents in the plant roots to 680–1570 μg g^?1 (except for peat FA, for which the ND content did not differ from the blank value). The observed phenomenon was probably related mainly to the influence of HSs on the zeta potential of the NDs, which shifted from positive to negative. Based on chlorophyll fluorescence evaluation, the toxicity of NDs did not inhibit photosynthesis during illumination in the physiological range. However, NDs were slightly toxic to wheat plants under excessive light, likely due to the inhibition of electron transport between Q _A and Q _B and the disruption of the formation of a thylakoid transmembrane potential.

Conclusions

The introduction of HA in a suspension of NDs obviously reduced the inhibiting effect of the NDs; however, the mitigating activities of FA were not so apparent. Our results demonstrate the urgent need for further studies of the influences of NDs on plant growth and development.

     

CLIMATE-CHANGE-INDUCED TEMPORAL VARIATION IN PRECIPITATION INCREASES NITROGEN LOSSES FROM INTENSIVE CROPPING SYSTEMS: ANALYSIS WITH A TOY MODEL

A simple ‘toy’ model of productivity and nitrogen and phosphorus cycling was used to evaluate how the increasing temporal variation in precipitation that is predicted(and observed) to occur as a consequence of greenhouse-gasinduced climate change will affect crop yields and losses of reactive N that can cause environmental damage and affect human health. The model predicted that as temporal variability in precipitation increased it progressively reduced yields and increased losses of reactive N ...