Soil carbon and nitrogen stocks in forests along an altitudinal gradient in the eastern Himalayas and a meta‐analysis of global data

High‐altitude soils potentially store a large pool of carbon (C) and nitrogen (N). The assessment of total C and N stocks in soils is vital to understanding the C and N dynamics in terrestrial ecosystems. In this study, we examined effects of altitude and forest composition on soil C and N along a transect from 317 to 3300 m a.s.l. in the eastern Himalayas. We used meta‐analysis to establish the context for our results on the effects of altitude on soil C, including variation with depth. Total C and N contents of soils significantly increased with altitude, but decreased with soil depth. Carbon and N were similarly correlated with altitude and temperature, and temperature was seemingly the main driver of soil C along the altitudinal gradient. Altitude accounted for 73% of the variation in C and 47% of the variation in N stocks. Soil pH and cation exchange capacity were correlated with both soil C and N stocks. Increases in soil C and N stocks were related to forest composition, forest basal area as well as quantity of leaf litter that were in turn influenced by altitude and temperature. Concentrations of C in foliage increased by 2.1% for every 1000 m rise in altitude, while that in leaf litter increased by 2.3%.     

Improved ε‐poly‐l‐lysine productivity partly resulting from rapid cell growth in cultures using a glucose–glycerol mixed carbon source

Productivity enhancements with mixed carbon sources are usually accompanied by simultaneous improvement of cell growth. However, whether the enhanced cell growth in mixed carbon sources influences the biochemical productivity of ε‐poly‐l ‐lysine (ε‐PL) still remains unclear. In this study, we investigated the effect of growth rate on the ε‐PL productivity in a glucose–glycerol mixed carbon source. Based on the typical ε‐PL fed batch fermentation, chemostat culture and relevant physiological analyses were carried out. The ε‐PL productivity was positively correlated to the growth rate ranging from 0.02 to 0.06 h^?1. The primary metabolism activity was enhanced at higher growth rate, providing sufficient precursor l ‐lysine and energy for ε‐PL production. Meanwhile, these two key elements were equally important for biomass production, which could be quickly produced when the cells were fast growing. In addition, rapid growth also strengthened the antioxidant capacity of cells to defend potential oxidative stress. The positive correlation between the growth rate and ε‐PL productivity indicated that the improvement of ε‐PL productivity in the mixed carbon source was partly attributed to the simultaneously enhanced cell growth. Information obtained may provide references for further studies on other secondary biochemicals’ production using mixed carbon sources.     

Holocene boundary dynamics of a northern Australian monsoon rainforest patch inferred from isotopic analysis of carbon, (14C and δ13C) and nitrogen (δ15N) in soil organic matter

Abstract Soil organic matter (SOM) was sampled from lateritic soil profiles across an abrupt eucalypt savanna–monsoon rainforest boundary on the north coast of Croker Island, northern Australia. Accelerator mass spectrometry dating revealed that SOM that had accumulated at the base of these 1.5 m profiles had a radiocarbon age of about 5000 years. The mean carbon and nitrogen stable isotope composition of SOM from 10 cm deep layers from the surface, middle and base of three monsoon rainforest soil profiles was significantly different from the means for these layers in three adjacent savanna soil profiles, suggesting the isotopic ‘footprint’ of the vegetation boundary has been stable since the mid Holocene. Although there were no obvious environmental discontinuities associated with the boundary, the monsoon rainforest was found to occur on significantly more clay rich soils than the surrounding savanna. Tiny fragments of monsoon rainforest and abandoned ‘nests’ (large earthen mounds) of the orange‐footed scrubfowl, an obligate monsoon rainforest species, occurred in the savanna, signalling that the rainforest was once more extensive. Despite episodic disturbances, such as tropical storm damage and fires, the stability of the boundary is probably maintained because clay rich soils enable monsoon rainforest tree species to grow rapidly and achieve canopy closure, thereby excluding grass and reducing the risk of fire. Conversely, slower tree growth rates, grass competition and fire on the savanna soils would impede the expansion of the rainforest although high rainfall periods with shorter dry seasons may enable rainforest trees to grow sufficiently quickly to colonize the savanna successfully.