Belowground Plant Biomass Research Articles

The interplay between abiotic factors and below-ground biological interactions regulates carbon exports from peatlands

Climate change projections indicate increases in CO2 emissions and DOC release from peatlands, thereby potentially transforming them from sinks to carbon sources. The individual effects of abiotic and biotic factors on peatland carbon exports and the interactions among all these factors are still poorly understood. Therefore, we conducted a field study over two years in a peatland complex by selecting four habitats differing in their dominant vegetation (shrub, grass, sedge and moss) in order to assess the links between abiotic and biotic factors and carbon release at each habitat. Accounting for the wide variability across habitats allowed us to examine the direct and indirect effects of abiotic and biotic factors on carbon cycling at the ecosystem level (i.e. the whole peatland complex). Results showed that the habitats dominated by vascular plants had the highest microbial and above-ground plant biomass, as well as the highest values of Acari and Collembola biomass, whereas Enchytraeidae biomass was dominant in the moss habitat. Furthermore, at ecosystem level, the SEM explained 75% of the total soil respiration variation and 33% of the DOC variation. Accordingly, soil respiration was directly linked to enchytraeid biomass, with these invertebrates being positively related to soil moisture. In contrast, both soil temperature and soil moisture had direct effects on microbial and Acari biomass, and the interaction between Acari biomass and below-ground plant biomass was responsible for DOC release. These findings provide new insights into the role of soil biota in carbon dynamics in peatlands, by showing that the direct effects of abiotic factors on biotic communities (both plants and soil organisms) determine how much carbon is lost from these systems (either as CO2 e or DOC).

Large differences in biomass quantity and quality between invasive Reynoutria japonica and resident vegetation are not reflected in topsoil physicochemical properties

Biological invasions are an important element of ongoing global change. Reynoutria japonica is one of the most invasive plant species in Europe and North America, potentially affecting recipient ecosystems. However, little is known about the relationships between its senescing biomass and element concentrations/pools in soil. The aim of this study was to compare the quantity and quality of aboveground and belowground plant biomass, and the quality of topsoil between plots with invasive R. japonica and resident plant communities. The study was conducted at 25 paired R. japonica-native plots in southern Poland. Harvested biomass was dried and weighted, and characterized in terms of C, Ca, K, Mg, N and P concentrations and pools, as well as C/N and C/P ratios. These parameters were also measured in soil along with texture, bulk soil density, moisture and pH. R. japonica produced greater amounts of aboveground and belowground biomass relative to resident species. The concentrations and pools of most elements in plant biomass were affected by invasion (R. japonica vs. resident), biomass type (aboveground vs. belowground) and/or their interactions. Senescing aboveground biomass of R. japonica was characterized by poor quality – lower N, P, K, greater C, Ca concentrations and greater C/N, C/P ratios compared to resident vegetation. Belowground R. japonica biomass had either greater or similar element concentrations (with the exception of P) or ratios. Element pools were greater in R. japonica than in resident species, with the exception of N and P in aboveground biomass. Despite these differences, topsoil generally did not differ in physicochemical properties when analyzed across all study sites; only total N and H2O-extractable K pools were lower under R. japonica than resident vegetation. Our data suggest that ecosystem response to invasion may be site-specific and depend on initial ecosystem properties.

Marsh Plants Enhance Coastal Marsh Resilience by Changing Sediment Oxygen and Sulfide Concentrations in an Urban, Eutrophic Estuary

Despite considerable efforts to restore coastal wetlands, the ecological mechanisms contributing to the success or failure of restoration are rarely assessed. Accumulation of hydrogen sulfide in sediments may accelerate rates of marsh loss in eutrophic estuaries and is likely driven by complex feedbacks between wetland plant growth and microbial redox reactions. We used a chronosequence of restored marshes in urbanized and eutrophic Jamaica Bay (New York City, USA) to assess how sediment redox conditions change among seasons and over the lifetime of restored marshes. We also compared a stable extant marsh to one that has deteriorated over the past 50 years. We collected seasonal sediment cores from each marsh, and used a motorized microprofiling system to measure the vertical distribution of oxygen and sulfide. We fit a logistic function to each profile to estimate (1) maximum concentrations, (2) rates of increase/decline, and (3) depths of maximum increase/decline. We quantified sediment density, porosity, organic content, and belowground plant biomass, and estimated differences in daily tidal inundation among sites using water-level loggers. We found that minimum oxygen and maximum sulfide concentrations occur during summer. Sulfide concentrations were highest in sites that experienced the longest daily tidal inundation, including the degraded extant marsh and the oldest restored marsh. Spatial patterns in oxygen and sulfide were related to belowground plant biomass, supporting our hypothesis that root growth increases sediment oxygen and partially alleviates sulfide stress. Our data support the growing body of evidence that belowground plant growth may enhance the resilience of marshes to sea-level rise by increasing marsh elevation and facilitating oxygen diffusion into marsh sediments.

Synthetic Humic Acids Solubilize Otherwise Insoluble Phosphates to Improve Soil Fertility.

Artificial humic acids (A‐HA) made from biomass in a hydrothermal process turn otherwise highly insoluble phosphates (e.g. iron phosphate as a model) into highly available phosphorus, which contributes to the fertility of soils and the coupled plant growth. A detailed electron microscopy study revealed etching of the primary iron phosphate crystals by the ‐COOH and phenolic groups of humic acids, but also illustrated the importance of the redox properties of humic matter on the nanoscale. The combined effects result in the formation of then bioavailable phosphate nanoparticles stabilized by humic matter. Typical agricultural chemical tests indicate that the content of total P and directly plant‐available P improved largely. Comparative pot planting experiments before and after treatment of phosphates with A‐HA demonstrate significantly enhanced plant growth, as quantified in higher aboveground and belowground plant biomass.

Synthetic Humic Acids Solubilize Otherwise Insoluble Phosphates to Improve Soil Fertility

AbstractArtificial humic acids (A‐HA) made from biomass in a hydrothermal process turn otherwise highly insoluble phosphates (e.g. iron phosphate as a model) into highly available phosphorus, which contributes to the fertility of soils and the coupled plant growth. A detailed electron microscopy study revealed etching of the primary iron phosphate crystals by the ‐COOH and phenolic groups of humic acids, but also illustrated the importance of the redox properties of humic matter on the nanoscale. The combined effects result in the formation of then bioavailable phosphate nanoparticles stabilized by humic matter. Typical agricultural chemical tests indicate that the content of total P and directly plant‐available P improved largely. Comparative pot planting experiments before and after treatment of phosphates with A‐HA demonstrate significantly enhanced plant growth, as quantified in higher aboveground and belowground plant biomass.

An investigation of plant-induced suction and its implications for slope stability

Both above- and below-ground biomass of plants provide additional strength to soil through different mechanisms, which help to increase the stability of a vegetated slope. Nonetheless, shallow landslides on steep slopes covered with vegetation still occur, often being triggered above the groundwater table, due to loss of suction subsequent to rainfall. Therefore, it is essential to know to what extent vegetation enhances slope stability, and to quantify the contribution of vegetation to the shear strength of soil to determine factors of safety. Results of large-scale direct shear experiments on root-permeated soils and slope geometry from a landslide database were synthesised through an infinite slope analysis under partially saturated conditions to find critical combinations of slope angle and suction stress. Monte Carlo simulations yielded a clear separation of stable and unstable zones, which can be used to define the susceptibility of a slope to near surface failure. This method, based on the simulations, has the potential to be used as a regional early warning system.

Changes in abundance and composition of nitrifying communities in barley (Hordeum vulgare L.) rhizosphere and bulk soils over the growth period following combined biochar and urea amendment

To understand the effects of biochar and urea on soil N availability and plant growth, we conducted a pot experiment growing barley (Hordeum vulgare L.) under six treatments: control (N0), soil with 30 g kg−1 biochar (N0B), soil with 0.23 g kg−1 urea (N1), soil with 0.23 g kg−1 urea and 30 g kg−1 biochar (N1B), soil with 0.46 g kg−1 urea (N2), and soil with 0.46 g kg−1 urea and 30 g kg−1 biochar (N2B). The nitrifying community abundance and compositions in rhizosphere and bulk soils were analyzed using quantitative polymerase chain reaction (qPCR) and amplicon-based Illumina Hiseq sequencing. Adding urea with biochar (N1B) produced the greatest increase in above- and belowground plant biomass, followed by doubling the amount of urea with biochar (N2B); both treatments raised pH (p < 0.001) and lowered extractable N in the rhizosphere (p < 0.05). N1B treatment produced the greatest increase in ammonia-oxidizing bacteria (AOB) amoA gene copies, presumably because the combined amendment raised soil pH, which favored AOB access to NH4+. Nitrifier sequences were selected after blasting with reported nitrifiers in NCBI (similarity ≥ 97%). Nitrosospira dominated AOB communities during the plant seedling stage; however, during the mature stage, Nitrosomonas dominated over Nitrosospira and the nitrite-oxidizing bacteria (NOB) community became diverse. Redundancy analysis indicated that nitrifying community composition was affected by multiple soil properties, including N availability (i.e., exchangeable NH4+ and NO3−) and soil chemistry (i.e., pH, dissolved organic C, and exchangeable base cations). Our research suggests a positive application of combining biochar with urea in improving N bioavailability and promoting plant growth in the acidic soil.

Below-ground biomass of plants, with a key contribution of buried shoots, increases foredune resistance to wave swash.

Sand dunes reduce the impact of storms on shorelines and human infrastructure. The ability of these ecosystems to provide sustained coastal protection under persistent wave attack depends on their resistance to erosion. Although flume experiments show that roots of perennial plants contribute to foredune stabilization, the role of other plant organs, and of annual species, remains poorly studied. Furthermore, it remains unknown if restored foredunes provide the same level of erosion resistance as natural foredunes. We investigated the capacity of three widespread pioneer foredune species (the perennial Ammophila arenaria and the annuals Cakile maritima and Salsola kali) to resist dune erosion, and compared the erosion resistance of Ammophila at natural and restored sites. Cores collected in the field were tested in a flume that simulated a wave swash. A multi-model inference approach was used to disentangle the contributions of different below-ground compartments (i.e. roots, rhizomes, buried shoots) to erosion resistance. All three species reduced erosion, with Ammophila having the strongest effect (36 % erosion reduction versus unvegetated cores). Total below-ground biomass (roots, rhizomes and shoots), rather than any single compartment, most parsimoniously explained erosion resistance. Further analysis revealed that buried shoots had the clearest individual contribution. Despite similar levels of total below-ground biomass, coarser sediment reduced erosion resistance of Ammophila cores from the restored site relative to the natural site. The total below-ground biomass of both annual and perennial plants, including roots, rhizomes and buried shoots, reduced dune erosion under a swash regime. Notably, we show that (1) annual pioneer species offer erosion protection, (2) buried shoots are an important plant component in driving sediment stabilization, and (3) management must consider both biological (plants and their traits) and physical (grain size) factors when integrating dunes into schemes for coastal protection.

Plant species natural abundances are determined by their growth and modification of soil resources in monoculture

The abundance of a plant species in a diverse community may depend on two aspects of a plant’s resource niche: its ability to garner limiting resources for its own growth, and its ability to reduce resources available for other species. Given that these two aspects of niche can be quantified in monoculture, we tested whether plant growth or plant modification of soil resources in monoculture relate to plant abundance in naturally-assembled California grassland communities. We grew 18 native and exotic grassland plant species in replicated monocultures, measured plant biomass and soil resources in these monocultures, and then assessed how well the measured variables for each species in monoculture correlated with natural abundances of the species across the local landscape. Both aboveground and belowground plant biomass in monoculture were positively correlated with species abundances in naturally-assembled communities; aboveground monoculture biomass alone accounted for 63% of the natural variability in species abundances, whereas shallow and subsurface root biomass accounted for 28 and 38%, respectively. Nitrogen concentrations in shallow monoculture soils were also positively correlated with a species’ natural abundance in the field, accounting for 43% of the natural variability in species abundances. Our results suggest that the performance of species when grown alone—e.g., biomass production and impacts on soil resources—can inform their performance in diverse communities and across heterogeneous landscapes.

The impacts of biological control on the performance of Lythrum salicaria 20 years post-release

Biological control has the potential to be an effective tool in the control of invasive plants. However, because it involves the introduction of a novel species to an ecosystem, predicting the potential impacts and overall effectiveness can be difficult, especially at the outset. For this reason, long term follow up of programs is essential. Unfortunately, few programs include official follow up, and those that do are often not collecting all the information necessary to assess the benefits and risks. Here we describe a study where we performed plant population surveys to estimate the impacts of a large scale release of two species of chrysomelid beetles, Neogalerucella calmariensis and N. pusilla on the control of the invasive wetland plant, Lythrum salicaria in Ontario, Canada. We surveyed 18 populations that varied in their colonization history, including a set that are among the original sites of the Ontario government’s planned release program initiated in 1992. We discovered that L. salicaria plants at the original release sites exhibited more herbivore damage and fewer primary inflorescences relative to those at naïve (never colonized) or recent (those apparently colonized via migration) sites. We also discovered that, across sites, quadrats with a heavier load of Neogalerucella spp. tended to have lower overall plant species richness, which was likely due to the fact that beetles tend to congregate in areas with high target plant density. Surprisingly, many of the factors associated with a successful release program, such as reduced target plant density, reduced target plant fruit production, and reduced above- and/or belowground plant biomass at release sites were not detected in our study. We discuss the potential reasons for our findings and make recommendations for the application of biocontrol and follow up programs.

Ecological biomass allocation strategies in plant species with different life forms in a cold desert, China

Biomass allocation patterns among plant species are related to their adaptive ecological strategies. Ephemeral, ephemeroid and annual plant life forms represent three typical growth strategies of plants that grow in autumn and early spring in the cold deserts of China. These plants play an important role in reducing wind velocity in the desert areas. However, despite numerous studies, the strategies of biomass allocation among plant species with these three life forms remain contentious. In this study, we conducted a preliminary quadrat study during 2014–2016 in the southern part of the Gurbantunggut Desert, China, to investigate the allocation patterns of above-ground biomass (AGB) and below-ground biomass (BGB) at the individual level in 17 ephemeral, 3 ephemeroid and 4 annual plant species. Since ephemeral plants can germinate in autumn, we also compared biomass allocation patterns between plants that germinated in autumn 2015 and spring 2016 for 4 common ephemeral species. The healthy mature individual plants of each species were sampled and the AGB, BGB, total biomass (TB), leaf mass ratio (LMR) and root/shoot ratio (R/S) were calculated for 201 sample quadrats in the study area. We also studied the relationships between AGB and BGB of plants with the three different life forms (ephemeral, ephemeroid and annual). The mean AGB values of ephemeral, ephemeroid and annual plants were 0.806, 3.759 and 1.546 g/plant, respectively, and the mean BGB values were 0.106, 4.996 and 0.166 g/plant, respectively. The mean R/S value was significantly higher in ephemeroid plants (1.675) than in ephemeral (0.154) and annual (0.147) plants. The mean LMR was the highest in annual plants, followed by ephemeroid plants and ephemeral plants, reflecting the fact that annual plants allocate more biomass to leaves, associated with their longer life span. Biomass of ephemeral plants that germinated in autumn was significantly higher than those of corresponding plants that germinated in spring in terms of AGB, BGB and TB. However, the R/S value was similar in plants that germinated in autumn and spring. The slope of regression relationship between AGB and BGB differed significantly among the three plant life forms. These results support different biomass allocation hypotheses. Specifically, at the individual level, the AGB and BGB partitioning supports the allometric hypothesis for ephemeroid and annual plants and the isometric hypothesis for ephemeral plants.

Above- to Belowground Vegetation Biomass Ratio in Temperate North-East Atlantic Saltmarshes Increases Strongly with Soil Nitrogen Gradient

Saltmarshes provide a broad range of ecosystem services, but have been degraded globally and are further threatened by sea-level rise. Changes in biomass partitioning between above- and belowground components of plant community biomass as a consequence of nutrient enrichment can impact those services profoundly. However, the evidence for such impacts is largely confined to North America, leaving a gap in global understanding of coastal ecosystem resilience. Using field surveys across 15 saltmarshes along the south and east coasts of Ireland, we tested whether aboveground (AG) and belowground (BG) plant biomass allocation across all plant assemblages combined and within individual plant assemblages is related to soil nutrients. AG biomass and AG/BG biomass ratio were positively related, and BG biomass was negatively related to NOx−, both across all assemblages combined and within individual assemblages. AG biomass and AG/BG ratio were also positively related to NH4+ within individual assemblages, but not across all of them combined. The relationships between biomass and labile P were weaker, and their direction varied across the whole P gradient and among individual assemblages. These findings demonstrate broad, community-wide links between biomass allocation and soil nutrients in saltmarshes. The up to 6.6-fold increase in AG/BG biomass ratio along the N gradient, coupled with differential responses among functionally contrasting plants, could have significant implications for multiple saltmarsh functions such as energy fixation for consumers, wave attenuation, sediment binding and carbon storage. Improved understanding of the underlying causalities would help promoting saltmarsh resilience.

The effect of arbuscular mycorrhizal fungi Rhizophagus intraradices and soil microbial community on a model plant community in a post-mining soil

The aim of this study was to assess the effect of arbuscular mycorrhizal fungi Rhizophagus intraradices and soil microbial groups and their interactions on a simple plant community in a microcosm experiment. The experiment was performed with two grass species (Poa compressa, Festuca rubra) and two herb species (Centaurea jacea, Lotus corniculatus) which are characteristic of intermediate succession stages in post-mining sites. Three months before the start of the experiment, bacteria, saprophytic fungi, protists, and their combined treatments were inoculated into the soil. At the start of the experiment, half of the pots were inoculated with mycorrhiza. After 60 days, plants were harvested and shoot and root biomass and microbial respiration and biomass were assessed. Above- and belowground plant biomass was significantly lower in the treatments with mycorrhiza. The effect was significant for aboveground biomass of grasses, especially that of Poa compressa, and for grass/herb ratio but not for herbs. Microbial respiration was also lower with mycorrhiza. Among microbial community treatments, saprophytic fungi showed significant effects on plant growth. The results showed the importance of mycorrhizal fungi on plant biomass and its interaction with different plant species and microbial groups which would be useful when extrapolating these results to a natural environment.

Biochar influences nitrogen and phosphorus use efficiency of tomato and soil nutrients under groundwater and wastewater irrigation

Wastewater irrigation is a common practice in developing countries, while the application of biochar in agricultural lands is gathering momentum worldwide. However, it is not known how biochar and wastewater in concert influence the uptake and concentration of nutrients in plants and soil, respectively. The influence was investigated of biochar on the growth performance of tomato plants grown in pots and the concentration of mineral N and soluble P in soil under wastewater and groundwater irrigation. Two biochars (wood-derived and manure-derived) were applied to soil as 3% and 7% amendments and tomato plants were grown. Both biochar types tended to increase the aboveground and belowground biomass and water use efficiency of plants at the higher application rate under both irrigation treatments (p < 0.05). The N contents in plant tissues were significantly higher for almost all treatments of biochar application rate under both irrigation treatments while under wastewater irrigation, biochar significantly reduced the P contents. The nitrogen use efficiency and phosphorus use efficiency of plants tended to increase (p < 0.05) at higher biochar application rates under both irrigation treatments. Amendment using biochars increased the concentration of mineral N and soluble P at both application rates under groundwater irrigation while under wastewater irrigation, only the P contents were increased in response to both biochar types at both application rates (p < 0.05).

Field Experiments and Meta-analysis Reveal Wetland Vegetation as a Crucial Element in the Coastal Protection Paradigm

Increasing rates of sea-level rise and wave action threaten coastal populations. Defense of shorelines by protection and restoration of wetlands has been invoked as a win-win strategy for humans and nature, yet evidence from field experiments supporting the wetland protection function is uncommon, as is the understanding of its context dependency. Here we provide evidence from field manipulations showing that the loss of wetland vegetation, regardless of disturbance size, increases the rate of erosion on wave-stressed shorelines. Vegetation removal (simulated disturbance) along the edge of salt marshes reveals that loss of wetland plants elevates the rate of lateral erosion and that extensive root systems, rather than aboveground biomass, are primarily responsible for protection against edge erosion in marshes. Meta-analysis further shows that disturbances that generate plant die-off on salt marsh edges generally hasten edge erosion in coastal marshes and that the erosion protection function of wetlands relates more to lateral than vertical edge-erosional processes and is positivelycorrelated with the amount of belowground plant biomass lost. Collectively, our findings substantiate a coastal protection paradigm that incorporates preservation of shoreline vegetation, illuminate key context dependencies in this theory, and highlight local disturbances (e.g., oil spills) that kill wetland plants as agents that can accelerate coastal erosion.

Natural and Regenerated Saltmarshes Exhibit Similar Soil and Belowground Organic Carbon Stocks, Root Production and Soil Respiration

Saltmarshes provide many valuable ecosystem services including storage of a large amount of ‘blue carbon’ within their soils. To date, up to 50% of the world’s saltmarshes have been lost or severely degraded primarily due to a variety of anthropogenic pressures. Previous efforts have aimed to restore saltmarshes and their ecosystem functions, but the success of these efforts is rarely evaluated. To fill this gap, we used a range of metrics, including organic carbon stocks, root production, soil respiration and microbial communities to compare natural and a 20-year restoration effort in saltmarsh habitats within the Sydney Olympic Park in New South Wales, Australia. We addressed four main questions: (1) Have above- and belowground plant biomass recovered to natural levels? (2) Have organic carbon stocks of soils recovered? (3) Are microbial communities similar between natural and regenerated saltmarshes? and (4) Are microbial communities at both habitats associated to ecosystem characteristics? For both soil organic carbon stocks and belowground biomass, we found no significant differences between natural and regenerated habitats (F(1,14) = 0.47, p = 0.5; F(1,42) = 0.08, p = 0.76). Aboveground biomass was higher in the natural habitat compared to the regenerated habitat (F(1,20) = 27.3, p < 0.0001), which may result from a site-specific effect: protection from erosion offered by a fringing mangrove forest in the natural habitat but not the regenerated habitat. Our microbial community assessment indicated that restored and natural saltmarsh habitats were similar at a phylum level, with the exception of a higher proportion of Proteobacteria in the rhizosphere of saltmarshes from the regenerated habitat (p < 0.01). Abundance of both Desulfuromonas and Geobacter was associated with high carbon and nitrogen densities in soils indicating that these genera may be key for the recovery of ecosystem characteristics in saltmarshes. Our restored and natural saltmarsh soils store at 30 cm depth similar levels of organic carbon: 47.9 Mg OC ha−1 to 64.6 Mg OC ha−1. Conservation of urban saltmarshes could be important for ‘blue carbon’ programmes aimed at mitigating atmospheric carbon dioxide.

Root Decomposition of Grazed Signalgrass in Response to Stocking and Nitrogen Fertilization Rates

Belowground root biomass can respond differently to grazing management affecting grass sward quality and longevity. Linking grazing management with belowground responses is scarce in warm‐climate grassland literature. The objective of this research was to investigate the effect of different stocking rates (SRs) and N fertilization on root chemical composition and decomposition in signalgrass pastures [Urochloa decumbens (Stapf.) R. D. Webster]. The experiment consisted of a factorial arrangement of three SRs (2.0, 3.9, and 5.8 animal units [AU] ha−1; 1 AU = 450 kg live weight) and three N fertilization rates (0, 150, and 300 kg N ha−1 yr−1). Response variables included root remaining biomass and N, root N and lignin concentrations, root C/N ratio, and root lignin/N ratio. Response variables were not affected by SR and N fertilization, except for remaining root biomass. All response variables were affected by incubation time. Root decomposition fitted a single exponential decay model. Remaining biomass was 30% of initial biomass (k = 0.0022 g g−1 d−1) after 512 d. Remaining N was 31% of initial N (k = 0.0007 g g−1 d−1) at the end of the incubation. Remaining root biomass was reduced with increasing N levels for 3.9 AU ha−1, but not for 2.0 and 5.8 AU ha−1. Root decomposition released ∼65 kg N ha−1 during 512 d trough mineralization. Grassland ecosystems have important C stock in their belowground plant biomass, which is relevant for the global C cycle. Therefore, practices that maintain or increase belowground C stocks should be preferred.

Saturated N2O emission rates occur above the nitrogen deposition level predicted for the semi-arid grasslands of Inner Mongolia, China

Nitrous oxide (N2O) is one of the most important greenhouse gases emitted by the semi-arid grasslands of Northern China. The majority of previous studies focused on nitrogen (N) deposition and its impacts on N2O emission rates in this region, while the mechanisms and controls of N2O emission and the evidence for a “balance point” following increasing N deposition remained unclear. In this study, we investigated during 2013–2015 the environmental and plant controls over N2O emission rates within a long-term N addition experiment with nitrogen levels of 1, 2, 4, 8, 16, 32, 64 g N m−2 yr−1, respectively, by using an in situ static chamber method. The results showed that N2O emission rates increased with increasing nitrogen addition rates. The emission rates showed significantly positive linear correlations with soil temperature, NO3−-N, NH4+-N, inorganic N, microbial biomass carbon, microbial biomass N, total soluble N, below-ground plant biomass and above-ground plant biomass, but a significantly negative correlation with soil pH. A structural equation modeling analysis showed that N addition affected in particular the pH value and subsequently N cycling and soil N2O emission. Meteorological factors impacted N2O emission rates through affecting soil environment and N cycling processes. The formulated “N2O emission balance hypothesis” was supported and the balance point at which the N2O emission rate became saturated was somewhere between 32 and 64 g N m−2 yr−1. The plant N usage efficiency was highest when the rate of N addition was 2 g N m−2 yr−1. The increased knowledge of environmental and plant control over soil N2O emission provides a better understanding of N cycling within the semi-arid temperate grassland ecosystem and will be fundamental for quantifying N2O budgets at various scales.

Relationships between plant diversity and biomass production of alpine grasslands are dependent on the spatial scale and the dimension of biodiversity

There is no consensus about the relationship between plant species diversity and productivity in grasslands. Lack of consensus might be caused by different diversity metrics used and different spatial scales examined. In particular, the impacts of different biodiversity indices on the plant diversity–plant productivity relationship of alpine grasslands at different spatial scales of the Qinghai–Tibetan Plateau (QTP) require investigation. We investigated the relations of multi-dimensional diversity indices (species richness, Shannon–Wiener diversity, and Pielou evenness) with aboveground and belowground plant biomass in the alpine grasslands of the QTP by linear mixed model (LMM). We assessed the plant diversity and biomass of alpine meadow, alpine steppe, and alpine desert steppe from 5 sites across the vast landscape of the QTP by using 6 randomly placed plots in each site and 27 replicated quadrats in each plot. The species richness of the studied alpine grasslands ranged from 2 to 35, aboveground plant dry biomass ranged from 17 to 838 g m−2, and belowground plant dry biomass ranged from 398 to 3945 g m−2. The species richness and Shannon–Wiener indices were positively correlated with aboveground plant biomass and belowground plant biomass at the landscape scale, whereas the Pielou evenness index was negatively correlated with aboveground plant biomass and belowground plant biomass at the landscape scale. Less than 20% sites showed significant relationships between plant diversity indices and plant biomass at site scale. Plant biomass partitioning between the root and shoot was not correlated with the species richness, Shannon–Wiener and Pielou evenness indices at all scales. The plant diversity–biomass relationships of the alpine grassland ecosystems in the QTP depend on both the type (or dimension) of the biodiversity index and the spatial scale of sampling. Thus, the design and implementation of plant diversity conservation should consider different dimensions of plant diversity. It is not enough to increase species richness only in the process of grassland restoration and conservation, but also to reduce species evenness so as to promote productivity more fully, for example, in planting artificial grassland.

Carbon and nitrogen availability in paddy soil affects rice photosynthate allocation, microbial community composition, and priming: combining continuous 13C labeling with PLFA analysis

Carbon (C) and nitrogen (N) availability in soil change microbial community composition and activity and so, might affect soil organic matter (SOM) decomposition as well as allocation of plant assimilates. The study was focused on interactions between C and N availability and consequences for rhizodeposition and microbial community structure in paddy soil. Rice continuously labeled in a 13CO2 atmosphere was fertilized with either carboxymethyl cellulose (CMC) (+C), ammonium sulfate (+N), or their combination (+CN), and unfertilized soil was used as a control. 13C was traced in aboveground and belowground plant biomass, soil organic matter, and microbial biomass. Microbial community composition was analyzed by phospholipid fatty acids (PLFAs). +CN application led to a higher yield and lower root C and N content: 13C assimilated in shoots increased by 1.39-fold and that in roots decreased by 0.75-fold. Correspondingly, after +CN addition, 13C from rhizodeposits incorporated into SOM and microorganisms decreased by 0.68-fold and 0.53-fold, respectively, as compared with that in the unfertilized soil. The application of +C or + N alone resulted in smaller changes. CMC led to a 3% of total N mobilized from SOM and resulted in a positive priming effect. Both fertilizations (+C, +N, or + CN) and plant growth stages affected soil microbial community composition. With decreasing microbial biomass C and N, and PLFA content under +CN amendment, +CN fertilization decreased Gram-positive (G+)/ Gram-negative (G-) ratios, and resulted in lower G+ bacteria and fungi abundance, whereas G- and actinomycetes were stimulated by N fertilization. Organic C fertilization led to a positive N priming effect. Organic C and mineral N application decreased C input by rhizodeposition associated with lower 13C recovery in SOM and microbial incorporation. C and N addition also altered microbial community composition, as +CN decreased content of microbial groups, such as G+ bacteria and fungi, but +N stimulated G- bacteria and actinomycetes.