Search

Nitrogen (N) enrichment is well-known to cause declines in plant diversity, yet it remains a challenge to predict which species would be winners or losers. Here, we examined whether traits and their plasticity could predict plant performance under N enrichment, and how changes in plant performance and trait expression could affect community functional property. Relative cover, six leaf traits, and maximum plant height of eight common vascular plants were measured after six-year N addition (0, 3, 6, 12 g N m−2 year−1) in a boreal peatland in Northeast China. Plant performance responding to N enrichment and trait plasticity were separately calculated as changes in relative cover and trait values between control and N addition treatments. Moreover, we quantified relative contributions of species turnover and intraspecific trait variation to changes in functional property. Changes in plant performance were significantly related to plasticity in specific leaf area (SLA) and leaf N concentration (LN) under low N addition, but to maximum plant height under high N addition. When scaled up to the community level, intraspecific trait variation significantly contributed to changes in community-weighted mean leaf area, SLA, leaf tissue density, and LN, and functional diversity of all six leaf traits. These findings highlight the significance of trait plasticity in predicting responses of plant performance and community functional property to N enrichment, and suggest that trait-based approaches are useful in predicting ecological consequences of environmental changes in boreal peatlands.

Soil respiration (Rs) is expected to positively feedback to climate warming. The strength of this feedback is uncertain as numerous environmental factors, such as precipitation and soil moisture, can moderate the warming response of Rs. We combined seven-year Rs measurements in a warming experiment in the Tibetan alpine grassland with a meta-analysis on grassland warming experiments globally to investigate how precipitation and soil moisture influences the warming response of Rs. We further analyzed the warming responses of heterotrophic (Rh) and autotrophic (Ra) components of Rs. Warming enhanced growing-season Rs in the wet years but decreased it in the dry years in the field experiment at the Tibetan grassland. Precipitation modulated the warming responses of growing-season Rs via Rh, but not Ra. Consistent with the field experiment, a positive relationship between precipitation and the warming response of growing-season Rs was also observed in the global-scale meta-analysis on grassland warming experiments. Precipitation influences the warming effects on Rs and could result in variation in warming response of Rs across years and experimental systems. Empirical functions provided by this study could be used to reduce the uncertainty in predicting Rs in a warmer future.

Nitrogen (N) addition could affect litter decomposition through its direct effects on soil N availability and indirect effects on initial litter chemistry. The aim of this study was to evaluate the relative contribution of these direct and indirect effects to the decomposition of fine roots with different diameter classes. A two-year reciprocal replant–transplant field experiment was conducted in a Mongolian pine (Pinus sylvestris var. mongolica) plantation to examine the relative effect of exogenous and endogenous N enrichment induced by N addition (10 g N m−2 yr.−1) on the decomposition of fine roots with different diameter classes: < 0.5 mm (small fine root, SFR) and 0.5–2 mm (large fine root, LFR). The LFR had significantly higher decomposition rates (k: 0.315–0.397 yr.−1) than the SFR (0.245–0.274 yr.−1) after 2 years of incubation. Exogenous N (i.e., increased soil N availability due to N addition) had no significant effect on the decomposition rates of fine roots, whereas endogenous N (i.e. increased N concentration in litter due to N addition) inhibited and accelerated the decomposition of SFR and LFR, respectively. Endogenous N decreased the net release of N but both endogenous and exogenous N increased the net release of phosphorus (P) from SFR. By contrast, exogenous and endogenous N decreased the net release of N and P from LFR. Our results suggest that N addition affected fine root decomposition indirectly by changing the chemical traits of fine roots rather than directly through changing soil N availability. Elevated input and decreased net N release of fine roots might be a potential mechanism explaining the increases of total organic carbon and total N in the semi-arid forest soil under N addition. Our study also suggests that SFR may be a more important source of stable soil organic matter relative to LFR.

While growing evidence has shown consistently positive plant diversity effects on primary productivity, diversity effects on litter decomposition remain inconclusive. Our aim was to evaluate the relative importance of functional identity (FI), functional diversity (FD) and species richness (SR) for litter mixture decomposition and litter mixing effects. 32 mixtures of leaf litter from seven tree species were constructed by creating two independent gradients of FD and SR. Decomposition of these mixtures was measured using laboratory microcosms. Moreover, component litter was separated from mixtures to assess species-specific responses to mixing. FI but not FD or SR significantly influenced litter mixture decomposition. Six and nine litter mixtures separately exhibited synergistic and antagonistic non-additive effects with litter mixing effects ranging from −8.97% to 16.82%, which did not significantly relate to FD or SR. Mass loss of two fast-decomposing species was inhibited and that of two out of five slow-decomposing species was accelerated by the presence of other species. Chemical traits of a given component species and trait differences between this species and its neighbors significantly influenced its response to litter mixing. These results suggest that relationship between FD and litter mixing effect is far from generalizable and highlight that analyzing species-specific responses to mixing is useful in exploring mechanisms underlying interactions among component litter.

Variation in tree litter inputs and understory vegetation caused by human disturbances and climate change in forest plantations can extend to alter forest stability and productivity over time. Here, we explore how tree litter inputs interact with understory plant management to influence belowground processes in a managed forest plantation. We conducted a two-factor nested experimental manipulation of pine litter and understory vegetation in a nutrient-poor Pinus sylvestris var. mongolica plantation. Three levels of tree litter manipulation (ambient litter, litter removal and litter addition) were nested in two levels of understory manipulation (understory intact and understory removal). After two years of manipulation, mineral soils were analyzed for total and extractable C, N and P concentrations, N mineralization, enzyme activities, as well as the microbial community structure (as indicated by phospholipid fatty acids). Litter removal had little impact on C and nutrient cycling as well as microbial biomass and community structure in this low nutrient pine plantation; however, litter addition and the removal of the understory vegetation had large impacts on these processes. Litter addition elevated soil microbial biomass, acid phosphatase and β-1, 4-glucosidase activities, by a much greater degree when the understory vegetation was intact than when the understory was removed. Litter addition also reduced soil available P by 39% when the understory vegetation was intact, and reduced soil available P by 74% and NO3 −–N by 45% when the understory was removed. Litter addition significantly reduced the ratio of Gram-positive to Gram-negative bacteria as well as the ratio between PLFA markers cy17:0 and 16:1ω7. Understory removal reduced the ratio of PLFA markers cy17:0 to 16:1ω7. Our study results show that, in this managed pine plantation, soil microbial community structure and function were more sensitive to an increase rather than to a decrease in pine litter inputs. Further, we found that the presence of understory vegetation can increase soil microbial biomass and alleviate the reduction in available N and P concentrations induced by pine litter addition. Thus, preservation of the understory vegetation is an effective way to maintain the functional stability of managed forests on nutrient-poor soils.

Litter chemical properties may be changed by species mixing and consequently affect litter decomposition; however, the effects of species mixing on chemical properties of litter mixtures during decomposition are poorly understood. In this context, we test how changing species diversity may influence the species-mixing effects on chemical properties as well as mass loss of decomposing litter mixtures. We assessed the effects of species mixing on C/N ratio, concentrations of N, lignin and polyphenol as well as mass loss of litter mixtures in a Mongolian pine plantation of Northeast China, using a full factorial design containing 15 possible litter combinations of Mongolian pine, Artemisia scoparia, Setaria viridis and Phragmites communis during 182 days of incubation. In most instances, species mixing produced non-additive effects on mass loss, C/N ratio and release of N and lignin, and additive effects on polyphenol release of litter mixtures. Among the non-additive effects, synergisms were more common than antagonisms, and both of them changed with incubation time. Meanwhile, species composition rather than species richness explained the non-additive effects of species mixing on mass loss, while they both explained the non-additive effects of species mixing on release of N and lignin and C/N ratio of litter mixtures. Furthermore, the coefficients of variation of mass loss significantly decreased with increasing species richness. Our results suggest that species composition and richness can mediate the decomposition and most chemical properties of mixed-species litter, which are not likely to be accurately predicted from component species decaying alone.

Background and Aims Given that plant growth is often water limited in drylands, it has been proposed that water seems to influence productivity by altering physiological/ metabolic responses and nutrient availability in short term. It is unclear, however, whether water mediates a positive plant-soil feedback and whether the feedback drives variations in plant productivity. Methods A 4-year field experiment was performed to examine the effects of water and nitrogen (N) addition on nutrient concentrations in soil and plant, nutrient resorption and potential return, in a temperate grassland in northern China. Results Water addition enhanced plant N and phosphorus (P) concentrations but reduced plant N and P resorption efficiency, leading to the increased potential N and P return to soil via litterfall. Enhanced nutrient potential return likely contributed to an increase of plant productivity in the following year. These “fertilization effects” caused by water addition were similar to those by N addition. Conclusions Our study suggests that the positive plantsoil feedback induced by increased precipitation may have a role in water-induced increases in productivity, and highlights the “fertilization effect” of water addition in a semiarid grassland in short term.

Leaf litter decomposes on the surface of soil in natural systems and element transfers between litter and soil are commonly found. However, how litter and soil organic matter (SOM) interact to influence decomposition rate and nitrogen (N) release remains unclear. Leaf litter and mineral soil of top 0-5 cm from six forests were incubated separately, or together with litter on soil surface at 25 A degrees C for 346 days. Litter N remaining and soil respiration rate were repeatedly measured during incubation. Litter carbon (C) and mass losses and mineral N concentrations in litter and soil were measured at the end of incubation. Net N transfer from soil to litter was found in all litters when incubated with soil. Litter incubated with soil lost more C than litter incubated alone after 346 days. For litters with initial C: N ratios lower than 52, net N-min after 346 days was 100 % higher when incubated with soil than when incubated alone. Litter net N-min rate was negatively related to initial C: N ratio when incubated with soil but not when incubated alone. Soil respiration rate and net N-min rate did not differ between soil incubated with litter and soil incubated alone. We conclude that soils may enhance litter decomposition rate by net N transfer from soil to litter. Our results together with studies on litter mixture decomposition suggest that net N transfer between decomposing organic matter with different N status may be common and may significantly influence decomposition and N release. The low net N-min rate during litter decomposition along with the small size of litter N pool compared to soil N pool suggest that SOM rather than decomposing litter is the major contributor to plant mineral N supply.

Understanding changes in soil N cycling with stand development is critical for forest management as tree growth is affected by soil N availability. The aim of this study was to evaluate the changes in soil N availability and loss with stand development in trembling aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lamb.) in northeastern Alberta, Canada. Soil inorganic N availability (measured as N supply rate) and foliar N chemistry (N concentration and δ15N) in trembling aspen stands ranged from 52 to 70 years old (n = 7) and jack pine stands 43 to 78 years old (n = 8) were investigated in 2008 and 2009. The relationships among the ratios of NO3 --N to total inorganic N (NO3 --N/TIN), foliar N concentration, and foliar δ15N with stand age were also explored by regression analyses. Total inorganic N supply rates did not systematically change with stand age across stand types, soil layers and measurement periods; whereas NO3 --N/TIN showed a decreasing tendency with stand age, suggesting that nitrification and associated N loss potential became smaller in older stands with greater limitation in soil N availability. Foliar δ15N decreased with stand age from −1.7 to −4.7‰ for aspen and from −4.1 to −7.1‰ for jack pine, and there were positive correlations between foliar δ15N and soil NO3 --N/TIN, suggesting that decreased soil N loss led to less 15N-depletion in the inorganic N available for tree uptake in older stands. However, foliar N concentration did not significantly change with stand age, suggesting that there were other N sources such as organic N in the forest floor, in addition to the inorganic N, available for plant uptake. Our results suggest that soil inorganic N availability became more limited as stand age increased. In addition, the ratio of NO3 --N/TIN and its relationship with foliar δ15N indicated decreased soil N loss potential and shifted N sources with stand age in boreal forests that are typically N-limited. Our study demonstrated that declining nitrification with increasing stand age might be one of the mechanisms mediating N-limitation in the studied boreal forests.

Litter decomposition is a major process in the carbon (C) flow and nutrient cycling of terrestrial ecosystems, but the effects of litter type, microsite, and root diameter on decomposition are poorly understood. Litterbags were used to examine the decomposition rate of leaf litter and roots at three soil depths (5, 10 and 20 cm) over a 470-day period in Pinus sylvestris plantations in northern China. Leaves and the finest roots decomposed more quickly at 5 cm depth and coarser roots (>1-mm) decomposed more quickly at 10 and 20 cm depth. Roots generally decomposed more quickly than leaf litter, except at 5 cm deep; leaves decomposed more quickly than the coarsest roots (>5-mm). Root decomposition was strongly influenced by root diameter. Leaves experienced net nitrogen (N) immobilization and coarse roots (>2-mm) experienced more N release than fine roots. Significant heterogeneity was seen in N release for fine-roots (

Grassland conversion to cropland (GCC) may result in loss of a large amount of soil organic carbon (SOC). However, the assessment of such loss of SOC still involves large uncertainty due to shallow sampling depth, soil bulk density estimation and spatial heterogeneity. Our objectives were to quantify changes in SOC, soil total nitrogen (STN) and C:N ratio in 0–100 cm soil profile after GCC and to clarify factors influencing the SOC change. A nest-paired sampling design was used in six sites along a temperature gradient in Northeast China. SOC change after GCC ranged from −17 to 0 Mg ha−1 in 0–30 cm soil layer, recommended by IPCC, across the six sites, but ranged from −30 to 7 Mg ha−1 when considering 0–100 cm. We found a linear relationship between SOC change in 30–100 cm and that in 0–30 cm profile (ΔC30−100 = 0.35ΔC0−30, P

Background and aims Litter decomposition is a key process controlling flows of energy and nutrients in ecosystems. Altered biodiversity and nutrient availability may affect litter decomposition. However, little is known about the response of litter decomposition to co-occurring changes in species evenness and soil nutrient availability. Methods We used a microcosm experiment to evaluate the simultaneous effects of species evenness (two levels), identity of the dominant species (three species) and soil N availability (control and N addition) on litter decomposition in a Mongolian pine (Pinus sylvestris var. mongolica) plantation in Northeast China. Mongolian pine needles and senesced aboveground materials of two dominant understory species (Setaria viridis and Artemisia scoparia) were used for incubation. Results Litter evenness, dominant species identity and N addition significantly affected species interaction and litter decomposition. Higher level of species evenness increased the decomposition rate of litter mixtures and decreased the incidence of antagonistic effects. A. scoparia-dominated litter mixtures decomposed faster than P. sylvestris var. mongolica-andS. viridis-dominated litter mixtures. Notably, N addition increased decomposition rate of both single-species litters and litter mixtures, and meanwhile altered the incidence and direction of nonadditive effects during decomposition of litter mixtures. The presence of understory species litters stimulated the decomposition rate of pine litters irrespective of N addition, whereas the presence of pine litters suppressed the mass loss of A. scoparia litters. Moreover, N addition weakened the promoting effects of understory species litters on decomposition of pine litters. Conclusions Pine litter retarded the decomposition of understory species litters whereas its own decomposition was accelerated in mixtures. Nitrogen addition and understory species evenness altered species interaction through species-specific responses in litter mixtures and thus affected litter decomposition in Mongolian pine forests, which could produce a potential influence on ecosystem C budget and nutrient cycling.

Background and aims Afforestation on croplands can help sequester atmospheric CO2 through increased carbon (C) storage in the soil and vegetation. However, the dynamics of soil organic C (SOC) and rootC stocks, particularlythose inthedeepersoillayers, following afforestation are not well documented for semi-arid regions. The aim of this study was to investigate the dynamics of soil and root C stocks to 1 m depth following afforestation with poplar (Populus×xiaozhuanica W. Y. Hsu & Liang) on croplands at the Keerqin Sandy Lands in northeast China. Methods Forest floor, root and mineral soil samples were collected from 23 paired plots of poplar plantations with different stand basal areas (SBA, ranging from 0.2 m 2 ha �1 to 32.6 m 2 ha �1 ) and reference croplands using a paired-site design. Changes of SOC concentration and content, and root C content were analyzed using paired t tests, and the relationships between forest floor C content, soil and root ΔC (ΔC refers to the difference in C stocks between a poplar plantation and the paired cropland) and SBA were tested with a polynomial regression analysis. Results Afforestation resulted in linear increases of ΔC in the forest floor and 0–10 cm mineral soil with SBA (R 2 =0.67, p

Background and aims Roots of the lowest branch orders have the highest mortality rate, and may contribute predominately to plant carbon (C) and nutrient transfer into the soil. Yet patterns and controlling factors of the decomposition of these roots are poorly understood.

Fresh tree root decomposition induced by tillage is an important source of soil nutrients in agroforestry systems. Here we examined the effects of tree species, root size and soil N enrichment on fresh root decomposition under laboratory conditions. Fresh roots with two diameters (

Loss of functional diversity has been dem- onstrated to have a variety of impacts on ecosystem functioning. However, most studies have been imple- mented in artificially assembled communities by removing the original vegetation and seeding with desired species or functional group compositions. Such approaches could significantly disturb belowground biomass, especially roots, making it difficult to examine belowground responses to diversity manipu- lations. To circumvent this issue, plant diversity gradients were established by in situ removal of aboveground biomass of different plant functional groups (PFGs) in a typical steppe, and belowground processes related to roots and soil were examined. Root nutrient pools exhibited contrasting patterns, with the phosphorus (P) pool decreasing linearly upon increased PFG removal while the nitrogen (N) pool had a hump- shaped response. Soil NO 3� increased while net N mineralization decreased with PFG removal. In con- trast, soil P showed little response to PFG removal. Furthermore, both the identity and number of PFG removed had a significant influence on root and soil properties. The results of this study showed that loss of a combination of PFGs was important in natural grassland, and an approach with minimal influence on belowground processes is promising in studying diversity loss effects in natural ecosystems.

Foliar traits are often interpreted to reflect strategies for coping with water and nutrient supply limitations. In this study, we measured several important leaf traits for 147 species sampled from a remnant, temperate deciduous broad-leaved forest in Keerqin Sandy Lands, Northeast China to test whether these traits are 'invariant' or dependent on water supply limitations. Our data show that average specific leaf area (SLA), nitrogen (N) and phosphorus (P) concentrations, leaf C/N, C/P and N/P were 273 cm 2 g �1 ,1 8.1 mgg �1 ,1 .60 mgg �1 ,2 8.2, 343 and 12.4, respectively. However, most of these traits were significantly different (P

Soil microbial properties play a key role in belowground ecosystem functioning, but are not well understood in forest ecosystems under nitrogen (N) enrichment. In this study, soil samples from 0-10 cm and 10-20 cm layers were collected from a Dahurian larch (Larix gmelinii Rupr.) plantation in Northeast China after six consecutive years of N addition to examine changes in soil pH, nutrient concentrations, and microbial biomass and activities. Nitrogen addi- tion significantly decreased soil pH and total phos- phorus, but had little effect on soil total organic carbon (TOC) and total N (TN) concentrations. The NO3 � -N concentrations in the two soil layers under N addition were significantly higher than that in the control, while NH4 + -N concentrations were not

We compared soil moisture content, pH, total organic carbon (C org), total nitrogen (TN), total phosphorus (TP) and inorganic N (NH4 +–N, NO3 −–N) concentrations, soil potential C and N mineralization rates, soil microbial biomass C (C mic), soil metabolic quotient (qCO2), soil microbial quotient (C mic/C org) and soil enzyme (urease and invertase) activities in semiarid sandy soils under three types of land cover: grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, and elm (Ulmus punila)–grass savanna in southeastern Keerqin, in northeast China. Soil C org, TN and TP concentrations (0–10, 10–20, 20–40 and 40–60 cm) were lower while soil C/N and C/P ratios were higher in the plantation than in grassland and savanna. The effects of land cover change on NH4 +–N and NO3 −–N concentrations, soil potential nitrification and C mineralization rates in the surface soil (0–10 cm) were dependent on sampling season; but soil potential N mineralization rates were not affected by land cover type and sampling season. The effects of land cover change on C mic and qCO2 of surface soil were not significant; but C mic/C org were significantly affected by land cover change and sampling season. We also found that land cover change, sampling season and land cover type × sampling season interaction significantly influenced soil enzyme (urease and invertase) activities. Usually soil enzyme activities were lower in the pine plantations than in grassland and savanna. Our results suggest that land cover change markedly influenced soil chemical and biological properties in sandy soils in the semiarid region, and these effects vary with sampling season.