Xiangyin Ni

The responses of early foliar litter humification to reduced snow cover during winter in an alpine forest

Ni, X., Yang, W., Li, H., Xu, L., He, J., Tan, B. and Wu, F. 2014. The responses of early foliar litter humification to reduced snow cover during winter in an alpine forest. Can. J. Soil Sci. 94: 453-461. Snow cover can be reduced by ongoing winter warming in alpine biomes, affecting foliar litter humification, but few reports are available. To quantitatively clarify how early foliar litter humification responds to reduced snow cover in winter, a field litterbag experiment was conducted in an alpine forest in southwestern China. Mass losses, ΔlogK, E4/E6, degrees of humification and humification rates of six typical local foliar litters were investigated at the snow formation, snow cover and snow melt stage under snowpack levels differing in depth (deep snowpack, medium snowpack, thin snowpack, no snowpack) from November 2012 to April 2013. The results indicated that 14-15% of willow (Salix paraplesia), 8-9% of fir (Abies faxoniana), 6-7% of birch (Betula albo-sinensis), 5-8% of cypress (Sabina saltuaria), larch (Larix mastersiana) and azalea (Rhododendron lapponicum) foliar litter was humified, which was about 50% of what decomposed during the first winter. Moreover, the early humification of foliar litter (except for fir and birch) responded positively to the reduced snow cover, but mass loss exhibited negative responses. Such results suggest that reduced snow cover in winter would increase soil carbon or other material sequestration in the scenario of climate change.

Grass and forbs respond differently to nitrogen addition: a meta-analysis of global grassland ecosystems

Nitrogen (N) deposition has increased globally and has profoundly influenced the structure and function of grasslands. Previous studies have discussed how N addition affects aboveground biomass (AGB), but the effects of N addition on the AGB of different functional groups in grasslands remain unclear. We conducted a meta-analysis to identify the responses of AGB and the AGB of grasses (AGBgrass) and forbs (AGBforb) to N addition across global grasslands. Our results showed that N addition significantly increased AGB and AGBgrass by 31 and 79%, respectively, but had no significant effect on AGBforb. The effects of N addition on AGB and AGBgrass increased with increasing N addition rates, but which on AGBforb decreased. Although study durations did not regulate the response ratio of N addition for AGB, which for AGBgrass increased and for AGBforb decreased with increasing study durations. Furthermore, the N addition response ratios for AGB and AGBgrass increased more strongly when the mean annual precipitation (MAP) was 300–600 mm but decreased with an increase in the mean annual temperature (MAT). AGBforb was only slightly affected by MAP and MAT. Our findings suggest that an acceleration of N deposition will increase grassland AGB by altering species composition.

Forest gaps slow the sequestration of soil organic matter: a humification experiment with six foliar litters in an alpine forest

Humification of plant litter containing carbon and other nutrients greatly contributes to the buildup of soil organic matter, but this process can be altered by forest gap-induced environmental variations during the winter and growing seasons. We conducted a field litterbag experiment in an alpine forest on the eastern Tibetan Plateau from November 2012 to October 2014. Six dominant types of foliar litter were placed on the forest floor in various forest gap positions, including gap centre, canopy gap, expanded gap and closed canopy. Over two years of incubation, all foliar litters were substantially humified especially during the first winter, although the newly accumulated humic substances were young and could be decomposed further. The forest gaps exhibited significant effects on early litter humification, but the effects were regulated by sampling seasons and litter types. Compared with the litter under the closed canopy, humification was suppressed in the gap centre after two years of field incubation. The results presented here suggest that gap formation delays the accumulation of soil organic matter, and reduces soil carbon sequestration in these alpine forests.

Copper and zinc dynamics in foliar litter during decomposition from gap center to closed canopy in an alpine forest

ABSTRACT Forest gap in alpine forests may redistribute the hydrothermal conditions in winter and growing season, which may affect the releases of copper and zinc in foliar litter during decomposition. However, the details of this process are largely unknown. Foliar litters of willow (Salix paraplesia), larch (Larix mastersiana), fir (Abies faxoniana), azalea (Rhododendron lapponicum), birch (Betula albosinensis) and cypress (Sabina saltuaria) were selected in an alpine forest of eastern Tibetan Plateau. The litterbags were placed on the forest floor from gap center, canopy gap edge and expanded gap edge to closed canopy. Zinc and copper contents were studied as litter decomposition proceeded. After one year of decomposition, zinc accumulated in all foliar litters regardless of gap positions, but copper accumulated in the litters of fir, azalea and cypress. Separately, copper was released from all foliar litters in winter, whereas zinc in litters of larch, azalea, birch and cypress was released in winter. Moreover, both copper and zinc accumulated during the growing season regardless of litter species. Nevertheless, higher accumulation rates were observed under closed canopy compared with other gap positions. These results suggest that forest gap slows the releases of copper and zinc in foliar litter during forest regeneration in these cold biomes.

Carbon, nitrogen and phosphorus stocks in soil organic layer as affected by forest gaps in the alpine forest of the eastern Tibet Plateau

Carbon (C) and nutrients storages in soil organic layer play an important role in forest productivity in the alpine-gorge area. However, forest gaps during forest regeneration could regulate carbon and nutrients stocks in soil organic layer by controlling hydro-thermal environment conditions, but little information has been available on it. Therefore, concentrations and stocks of carbon, nitrogen (N) and phosphorus (P) in fresh litter layer (LL), fragmented litter layer (FL) and humified litter layer (HL) were investigated from gap center, canopy gap and expanded gap to closed canopy in an alpine forest of eastern Tibetan Plateau in later fall (October, 2013). The results showed that the forest gaps reduced the thicknesses of LL and FL, but increased the thickness of HL. Carbon concentrations in LL and FL decreased as affected by forest gaps, whereas both N and P concentrations increased regardless of soil organic layers. Moreover, forest gaps reduced C, N and P stocks in LL and FL, although which in HL increased. In addition, C, N and P stocks were significantly related to negative accumulated temperature regardless of soil organic layers. These results suggest that forest gap could promote carbon and nutrient releases from fresh litters, but limit carbon and nutrients output from humified litters in these alpine forests.

Individual and combined effects of multiple global change drivers on terrestrial phosphorus pools: A meta-analysis

Human activity-induced global change drivers have dramatically changed terrestrial phosphorus (P) dynamics. However, our understanding of the interactive effects of multiple global change drivers on terrestrial P pools remains elusive, limiting their incorporation into ecological and biogeochemical models. We conducted a meta-analysis using 1751 observations extracted from 283 published articles to evaluate the individual, combined, and interactive effects of elevated CO2, warming, N addition, P addition, increased rainfall, and drought on P pools of plant (at both single-plant and plant-community levels), soil and microbial biomass. Our results suggested that (1) terrestrial P pools showed the most sensitive responses to the individual effects of warming and P addition; (2) P pools were consistently stimulated by P addition alone or in combination with simultaneous N addition; (3) environmental and experimental setting factors such as ecosystem type, climate, and latitude could significantly influence both the individual and combined effects; and (4) the interactive effects of two-driver pairs across multiple global change drivers are more likely to be additive rather than synergistic or antagonistic. Our findings highlighting the importance of additive interactive effects among multiple global change drivers on terrestrial P pools would be useful for incorporating P as controls on ecological processes such as photosynthesis and plant growth into ecosystem models used to analyze effects of multiple drivers under future global change.

Temporal dynamics of phosphorus during aquatic and terrestrial litter decomposition in an alpine forest

Plant litter decomposition in forested soil and watershed is an important source of phosphorus (P) for plants in forest ecosystems. Understanding P dynamics during litter decomposition in forested aquatic and terrestrial ecosystems will be of great importance for better understanding nutrient cycling across forest landscape. However, despite massive studies addressing litter decomposition have been carried out, generalizations across aquatic and terrestrial ecosystems regarding the temporal dynamics of P loss during litter decomposition remain elusive. We conducted a two-year field experiment using litterbag method in both aquatic (streams and riparian zones) and terrestrial (forest floors) ecosystems in an alpine forest on the eastern Tibetan Plateau. By using multigroup comparisons of structural equation modeling (SEM) method with different litter mass-loss intervals, we explicitly assessed the direct and indirect effects of several biotic and abiotic drivers on P loss across different decomposition stages. The results suggested that (1) P concentration in decomposing litter showed similar patterns of early increase and later decrease across different species and ecosystems types; (2) P loss shared a common hierarchy of drivers across different ecosystems types, with litter chemical dynamics mainly having direct effects but environment and initial litter quality having both direct and indirect effects; (3) when assessing at the temporal scale, the effects of initial litter quality appeared to increase in late decomposition stages, while litter chemical dynamics showed consistent significant effects almost in all decomposition stages across aquatic and terrestrial ecosystems; (4) microbial diversity showed significant effects on P loss, but its effects were lower compared with other drivers. Our results highlight the importance of including spatiotemporal variations and indicate the possibility of integrating aquatic and terrestrial decomposition into a common framework for future construction of models that account for the temporal dynamics of P in decomposing litter.

Forest Gaps Alter the Total Phenol Dynamics in Decomposing Litter in an Alpine Fir Forest.

The total phenol content in decomposing litter not only acts as a crucial litter quality indicator, but is also closely related to litter humification due to its tight absorption to clay particles. However, limited attention has been focused on the total phenol dynamics in foliar litter in relation to forest gaps. Here, the foliar litter of six representative tree species was incubated on the forest floor from the gap center to the closed canopy of an alpine Minjiang fir (Abies faxoniana) forest in the upper reaches of the Yangtze River and eastern Tibetan Plateau. The dynamics of total phenol concentration in the incubated litter was measured from November 2012 to October 2014. Over two-year incubation, 78.22% to 94.06% of total phenols were lost from the foliar litter, but 52.08% to 86.41% of this occurred in the first year. Forest gaps accelerated the loss of total phenols in the foliar litter in the winter, although they inhibited the loss of total phenols during the growing season in the first year. In comparison with the effects of forest gaps, the variations of litter quality among different species were much stronger on the dynamics of total phenols in the second year. Overall, the loss of total phenols in the foliar litter was slightly higher in both the canopy gap and the expanded gap than in the gap center and under the closed canopy. The results suggest that the predicted decline in snow cover resulting from winter warming or vanishing gaps caused by forest regeneration will retard the loss of total phenol content in the foliar litter of alpine forest ecosystems, especially in the first decomposition year.

Immobilization of heavy metals during aquatic and terrestrial litter decomposition in an alpine forest

Plant litter decomposition is an important pathway of heavy metal cycling in forested soil and watershed ecosystems globally, but is so far an overlooked aspects in the existing literature. To investigate the temporal dynamics of heavy metals in decomposing litter, we conducted a two-year field experiment using litterbag method across aquatic and terrestrial ecosystems in an alpine forest on the eastern Tibetan Plateau. Using multigroup comparisons of structural equation modeling with different litter mass-loss intervals, we assessed the direct and indirect effects of several biotic and abiotic factors on the release rates of lead (Pb), cadmium (Cd), and chromium (Cr). Results suggested that both the concentrations and amounts of Pb, Cd, and Cr increased during litter decomposition regardless of ecosystem type and litter species, showing an immobilization pattern. The release rates of Pb, Cd, or Cr shared a common hierarchy of drivers across aquatic and terrestrial ecosystems, with environmental factors and initial litter quality having both direct and indirect effects, and the effects of initial litter quality gained importance in the late decomposition stages. However, litter chemical dynamics and microbial diversity index have significant effects on release rates throughout the decomposition process. Our results are useful for better understanding heavy metal fluxes in aquatic and terrestrial ecosystems, and for predicting anthropogenic heavy metal pollution impacts on ecosystems. In addition, our results indicated that not only spatial but also temporal variability should be taken into consideration when addressing heavy metal dynamics accompanying litter decomposition process.

The effect of canopy exchange on input of base cations in a subalpine spruce plantation during the growth season

Canopy exchange is one of the most important processes involved in the internal transfer of elements in forest ecosystems. However, little information is available on how canopy exchange influences the input of base cations in subalpine forests. Therefore, the concentrations and fluxes of base cations in throughfall and stemflow were investigated from August 2015 to July 2016 (except for the frozen season) in a representative subalpine spruce plantation in the eastern Tibet Plateau. Our results showed that the mean concentrations of K, Ca, Na and Mg were higher in the stemflow than in the throughfall and precipitation. The total input fluxes of K, Ca, Na and Mg in the internal forest were lower than those in the non-forest. Moreover, the results from the canopy budget model indicated that the canopy exchange fluxes of K, Ca and Mg were higher than the dry deposition fluxes, and Ca and Mg were uptaken, whereas K was leached when precipitation passed through the canopy. Therefore, the results suggested that the input of base cations is mainly controlled by canopy exchange during precipitation in subalpine forest ecosystems, and the canopy could alter the sinks and sources of base cations from precipitation.