Belowground Root Research Articles

Effects of grazing and mowing on vertical distribution of soil nutrients and their stoichiometry (C: N: P) in a semi-arid grassland of North China

Grazing and mowing are increasingly recognized as a major driver of variations in soil nutrient levels during grassland management process. However, relatively few studies have examined the effects of grazing and mowing on the variations in concentrations, stocks, and stoichiometry of soil organic carbon (SOC), total nitrogen (TN), and total phosphorous (TP) in the deepsoil layer. In this study, a 6-year trial was conducted under grazing and mowing in a semi-arid grassland of North China to compare the concentrations, stocks, and stoichiometry of SOC, TN and TP in the top 1 m soil layer. The results indicated that both grazing and mowing increased SOC and TN stocks in the topsoil layer (0–30 cm), while mowing decreased SOC stock in the subsoil layer (30–100 cm). Meanwhile, neither treatment had any significant effects on TP concentrations and stock in the entire 0–100 cm profile. However, both grazing and mowing caused a decline in C:N and C:P ratios at 50–70 cm depth. The shifts in plant community composition regulated the changes of SOC and TN stocks in the topsoil layer, while resource allocation between aboveground plants and belowground roots regulated the changes of SOC stock and C:N:P stoichiometry in the subsoil layer. It is speculated that low-frequency grazing might be a sustainable grassland management regime, as it conserves both above- and belowground parts and maintains healthy ecosystem functions in semi-arid grasslands.

Root biomass and productivity in subtropical arid mangroves from the Gulf of California

Root biomass and productivity research provide valuable information for understanding the ecological processes that occur within mangroves. The data reported by different studies on belowground biomass suggest a high spatial variability, which local factors can control. In this study, the productivity and biomass of belowground roots and pneumatophores of arid mangroves were measured along with sediment and water physical-chemical characteristics in three mangrove zones of the arid region of the Gulf of California, Mexico. Core extraction was performed to measure belowground biomass; in addition, the root productivity was determined using the in-growth core technique, and pneumatophores were sampled at the soil level. Sediment samples were collected monthly to determine texture, pH, bulk density, moisture content, and organic matter. The flood pattern was estimated using piezometers. The study ran from June 2018 to May 2019. An average root productivity was 213.7 g m−2•year−1, with belowground biomass (74.05 ± 15.2 ton•ha−1) and pneumatophores of 5.69 ± 0.92 ton•ha−1. We conclude that the amount of root biomass in arid regions is inverse to the scrub-type mangrove's structural development, which stores up to 70% more than the mangroves in tropical regions. Thus mangroves in arid regions play a role in belowground carbon storage, which depends on multiple variables simultaneously (i.e.: salinity, flood pattern, soil depth, interstitial ORP).

Competing Processes Drive the Resistance of Soil Carbon to Alterations in Organic Inputs

Protecting existing soil carbon (C) and harnessing the C sequestration potential of soils require an improved understanding of the processes through which soil organic matter accumulates in natural systems. Currently, competing hypotheses exist regarding the dominant mechanisms for soil C stabilization. Many long-standing hypotheses revolve around an assumed positive relationship between the quantity of organic inputs and soil C accumulation, while more recent hypotheses have shifted attention toward the complex controls of microbial processing and organo-mineral complexation. Here, we present the observed findings of soil response to 20 years of detrital manipulations in the wet, temperate forest of the H.J. Andrews Experimental Station. Annual additions of low-quality (high C:N content) wood litter to the soil surface led to a greater positive effect on observed mean soil C concentration relative to additions of higher-quality (low C:N content) needle litter over the 20-year study period. However, high variability in measurements of soil C led to a statistically non-significant difference in C concentration between the two treatments and the control soil. The observed soil C responses to these two addition treatments demonstrates the long timescale and potential magnitude of soil C responses to management or disturbance led changes in forest litter input composition. Detrital input reduction treatments, including cutting off live root activity and the aboveground removal of surface litter, led to relatively small, non-significant effects on soil C concentrations over the 20-year study period. Far greater negative effects on mean soil C concentrations were observed for the combined removal of both aboveground litter and belowground root activity, which led to an observed, yet also non-significant, 20% decline in soil C stocks. The substantial proportion of remaining soil C following these dramatic, long-term reductions in above- and belowground detrital inputs suggests that losses of C in these forest soils are not readily achieved over a few decades of reductions in detrital input and may require far greater periods of time or further perturbations to the environment. Further, the observed soil C responses to detrital manipulations support recent hypotheses regarding soil C stabilization, which emphasize litter quality and mineral stabilization as relevant controls over forest soil C.

GPR-Based Automatic Identification of Root Zones of Influence Using HDBSCAN

The belowground root zone of influence (ZOI) is fundamental to the study of the root–root and root–soil interaction mechanisms of plants and is vital for understanding changes in plant community compositions and ecosystem processes. However, traditional root research methods have a limited capacity to measure the actual ZOIs within plant communities without destroying them in the process. This study has developed a new approach to determining the ZOIs within natural plant communities. First, ground-penetrating radar (GPR), a non-invasive near-surface geophysical tool, was used to obtain a dataset of the actual spatial distribution of the coarse root system in a shrub quadrat. Second, the root dataset was automatically clustered and analyzed using the hierarchical density-based spatial clustering of applications with noise (HDBSCAN) algorithm to determine the ZOIs of different plants. Finally, the shape, size, and other characteristics of each ZOI were extracted based on the clustering results. The proposed method was validated using GPR-obtained root data collected in two field shrub plots and one simulation on a dataset from existing literature. The results show that the shrubs within the studied community exhibited either segregated and aggregated ZOIs, and the two types of ZOIs were distinctly in terms of shape and size, demonstrating the complexity of root growth in response to changes in the surrounding environment. The ZOIs extracted based on GPR survey data were highly consistent with the actual growth pattern of shrub roots and can thus be used to reveal the spatial competition strategies of plant roots responding to changes in the soil environment and the influence of neighboring plants.

Early indicators of stress in mangrove forests with altered hydrology in Tampa Bay, Florida, USA

Mangroves experience stress or mortality when gas exchange by aerial root structures is hindered by sediment burial or constant inundation. The resulting loss of belowground roots and increased decomposition can lead to a loss of surface elevation through peat collapse, furthering the degree of inundation. This study identified ten live mangrove forests across Tampa Bay (Florida, USA) with stagnant water and/or abundant adventitious root growth (a sign of stress) and compared vegetative, water, and soil characteristics of stressed and reference mangrove forests to identify early indicators of stress and potential peat collapse. Two stressed sites were characterized by sandy, high-density soils and were hypothesized to have been stressed by a burial event. The remaining eight stressed locations had stagnant pools of discolored water with low dissolved oxygen (1.1 ± 1.3 mg L−1). Soil from the hydrologic stress sites had significantly lower dry bulk density than reference mangrove sites (0.24 ± 0.14 g cm−3 vs. 0.40 ± 0.11 g cm−3), greater organic matter composition (49.3 ± 17.2% vs. 26.1 ± 9.0%), and a higher proportion of root material. Salinity and porewater dissolved organic carbon varied widely but were not significantly different between reference and hydrologic stress sites. Altered hydrology due to anthropogenic alteration can often be improved; thus, proactive restoration is recommended to prevent mortality and peat collapse in affected mangrove forests.

Climate and intertidal zonation drive variability in the carbon stocks of Sri Lankan mangrove forests

Sri Lanka is at the forefront of global mangrove conservation. It is the first country to officially protect all its remaining mangrove forests and has embarked on an ambitious plan to restore 10,000 ha of wetland during the United Nations Decade of Ecosystem Restoration. One incentive for this conservation effort is a recognition, based on research mostly done elsewhere, of the importance of mangroves for carbon sequestration and storage. However, a lack of data on Sri Lankan mangrove carbon pools, especially on soil organic carbon, has been recognized as a major impediment to national climate change mitigation strategies. The current work examined both above and below-ground carbon stocks of five important mangrove forests in Sri Lanka (Rekawa, Puttalam-Kalpitiya, Pambala-Chilaw, Batticaloa and Negombo) which are situated in the three major climate zones (dry, intermediate and wet) and therefore sample the main climatic drivers of spatial variability. Above-ground carbon, below-ground root carbon and soil carbon stocks of mangroves in Sri Lanka ranged from 75.5 to 189.1 Mg C ha−1, 7.9 to 14.3 Mg C ha−1 and 643.6 to 1253.6 Mg C ha−1, respectively. The highest total mangrove carbon stock was recorded from the Rekawa lagoon which is in the intermediate climate zone (1455.4 Mg C ha−1) while the lowest was found in the Batticaloa lagoon in the dry zone (734.7 Mg C ha−1). Soil carbon stocks were substantially higher in the places where vegetation biomass and stand densities are high. Soil comprised 83–90% of the total mangrove carbon stocks at all sites, highlighting the large potential for release into the atmosphere as carbon dioxide if these habitats are disturbed. Overall, our study contributes important data that broadens our current understanding of how mangrove organic carbon pools vary spatially and with climatic zone.

Dynamics of soil reinforcement by roots in a regenerating coppice stand of Quercus serrata and effects on slope stability

Regeneration by sprouting in secondary forests is a necessary technique not only for the utilization of forest resources, but also for maintaining forest health by regrowth and avoiding slope instability caused by over-sized aboveground biomass. The strength of soil reinforcement by roots generally decreases during the tree death process in deforestation, such as clear-cutting, whereas root system strength temporarily decreases in trees that regenerate sprouting after cutting. However, quantitative evaluation of decreasing soil reinforcement by roots and slope stability has not been conducted during regeneration. In this study, we evaluated changes in the number of roots and root distribution up to 3 years after cutting in the regeneration process of Quercus serrata, a major dominant species of secondary forests in warm and cool temperate zones in Japan. We also estimated soil reinforcement by roots: additional cohesion from roots using the root bundle model, and slope stability based on a factor of safety by dividing the effect of the tree root system into vertical and horizontal cohesion with soil physical properties. After cutting at the height of approximately 0.2 m, the aboveground sprouting branches immediately regenerated, whereas the belowground root system gradually decayed. The additional cohesion from roots in trees before cutting was calculated as 34.6 kPa, whereas it decreased significantly to 21.7 kPa in living stumps 3 years after cutting. The value for the factor of safety with a saturated soil thickness of 0.7 m was calculated as 1.20 in soil under trees before cutting, whereas it decreased significantly to 1.11 in soil under living stumps in the 3 years after cutting. The safety factor's value to 0.99 in soil under dead stumps, which did not regenerate sprouts within 3 years after cutting. We concluded that the probability of slope failure was significantly greater in the dead stumps than in the living stumps and could vary depending on the dynamics of above- and below-ground parts, including fine roots.

Remarkable Similarity in Timing of Absorptive Fine-Root Production Across 11 Diverse Temperate Tree Species in a Common Garden.

Long-term minirhizotron observations of absorptive fine roots provide insights into seasonal patterns of belowground root production and carbon dynamics. Our objective was to compare root dynamics over time across mature individuals of 11 temperate trees species: five evergreen and six deciduous. We analyzed the timing and growth on 1st-and 2nd-order roots in minirhizotron images down to a vertical depth of 35 cm, as well as monthly and total annual length production. Production patterns were related to total annual precipitation of the actual and previous year of root production over 6 years. The main or largest peak of annual fine-root production occurred between June and September for almost all species and years. In most years, when peaks occurred, the timing of peak root production was synchronized across all species. A linear mixed model revealed significant differences in monthly fine-root length production across species in certain years (species x year, P < 0.0001), which was strongly influenced by three tree species. Total annual root production was much higher in 2000–2002, when there was above-average rainfall in the previous year, compared with production in 2005–2007, which followed years of lower-than-average rainfall (2003–2006). Compared to the wetter period all species experienced a decline of at least 75% in annual production in the drier years. Total annual root length production was more strongly associated with previous year’s (P < 0.001) compared with the actual year’s precipitation (P = 0.003). Remarkably similar timing of monthly absorptive fine-root growth can occur across multiple species of diverse phylogeny and leaf habit in a given year, suggesting a strong influence of extrinsic factors on absorptive fine-root growth. The influence of previous year precipitation on annual absorptive fine-root growth underscores the importance of legacy effects in biological responses and suggests that a growth response of temperate trees to extreme precipitation or drought events can be exacerbated across years.

Root architecture characteristics of differing size-controlling rootstocks and the influence on the growth of ‘Red Fuji’ apple trees

High-density cultivation using dwarfing rootstocks is widely practiced worldwide. The root is an essential organ for apple rootstocks, especially for dwarfing rootstocks. However, few studies about the root traits of apple rootstocks have been published. In this study, the root growth characteristics of the 'Red Fuji' apple cultivar scions grafted onto Shao series no. 40 (RF/SH.40) and Baleng Crab (RF/BC) rootstocks, which have apparent size-controlling differences, were investigated. Subsequently, the tree morphological and leaf photosynthetic parameters were measured, and the correlation between the roots and branches was studied. Thus, the root system of RF/BC was more widely distributed than that of RF/SH.40, with higher volume, projected area, surface area, root number, and length of the root system. Multiple comparisons revealed that the number of roots might be the critical factor contributing to differences in the two root systems. Furthermore, RF/BC had higher leaf photosynthetic rates, water use efficiency, and branch numbers and length than RF/SH.40. RF/BC had a wider branch distribution in a vertical orientation; however, a wider branch distribution in a horizontal orientation and a higher rate of branches shorter than 15 cm was observed for RF/SH.40. The above-ground tree architecture and below-ground root traits tend to be positively correlated. The parameters that primarily affected the correlation between above- and below-ground were branch volume, branch number, total projected, and total surface area of the root system.

PGPR Mediated Alterations in Root Traits: Way Toward Sustainable Crop Production

The above ground growth of the plant is highly dependent on the belowground root system. Rhizosphere is the zone of continuous interplay between plant roots and soil microbial communities. Plants, through root exudates, attract rhizosphere microorganisms to colonize the root surface and internal tissues. Many of these microorganisms known as plant growth promoting rhizobacteria (PGPR) improve plant growth through several direct and indirect mechanisms including biological nitrogen fixation, nutrient solubilization, and disease-control. Many PGPR, by producing phytohormones, volatile organic compounds, and secondary metabolites play important role in influencing the root architecture and growth, resulting in increased surface area for nutrient exchange and other rhizosphere effects. PGPR also improve resource use efficiency of the root system by improving the root system functioning at physiological levels. PGPR mediated root trait alterations can contribute to agroecosystem through improving crop stand, resource use efficiency, stress tolerance, soil structure etc. Thus, PGPR capable of modulating root traits can play important role in agricultural sustainability and root traits can be used as a primary criterion for the selection of potential PGPR strains. Available PGPR studies emphasize root morphological and physiological traits to assess the effect of PGPR. However, these traits can be influenced by various external factors and may give varying results. Therefore, it is important to understand the pathways and genes involved in plant root traits and the microbial signals/metabolites that can intercept and/or intersect these pathways for modulating root traits. The use of advanced tools and technologies can help to decipher the mechanisms involved in PGPR mediated determinants affecting the root traits. Further identification of PGPR based determinants/signaling molecules capable of regulating root trait genes and pathways can open up new avenues in PGPR research. The present review updates recent knowledge on the PGPR influence on root architecture and root functional traits and its benefits to the agro-ecosystem. Efforts have been made to understand the bacterial signals/determinants that can play regulatory role in the expression of root traits and their prospects in sustainable agriculture. The review will be helpful in providing future directions to the researchers working on PGPR and root system functioning.

Root Biomass Estimation in Natural and Planted Mangrove

Belowground root biomass plays an essential role in the mangrove ecosystem as potential carbon storage, nutrient gain, or sediment accumulation yet is still overlooked. Therefore, estimation of root biomass is necessary. The objective was to determine and compare the belowground root biomass at the natural and planted mangrove ecosystem. We also measured the forest structure and the aboveground biomass. This study was conducted at four stations in Pramuka Island, Pari Island, Rambut Island (2 stations), Seribu Islands, Jakarta. The first island represented planted mangroves, whereas the latter two as natural stems. Rhizophora stylosa was the dominant species in three stations except for Rambut Island 2 that dominated by Ceriops Tagal. Stand densities were 2730±5 stems ha-1, 1733±5 stems ha-1, 1311±2 stems ha-1, and 1300±2 stems ha-1 for Pramuka Island, Pari Island, Rambut Island 1 and 2. The highest pile density was counted in Rambut Island 2 for 1612±8 stems ha-1 and the lowest in Pramuka Island (780±5 stems ha-1). The aboveground biomass was estimated as follows 109.13±11.91 Mg ha-1 in Pari Island, 89.45±19.53 in Rambut Island 2, 68.41±17.12 Mg ha-1 in Rambut Island 1, and 14.05±5.74 Mg ha-1 in Pramuka Island. Belowground root biomass in planted mangrove Pramuka Island (3.63±0.82 Mg ha-1) and the other two natural mangroves Pari Island (6.56±0.47 Mg ha-1) and Rambut Island 1 (7.17±0.81 Mg ha-1), were similar except for Rambut Island 2 (49.23±6.77 Mg ha-1). The contribution of root biomass ranges from 6-24% of the total biomass of mangrove trees, whereas the fine roots (&lt; 2 mm) biomass occurred 28-74% in the entire root biomass. The results confirm the high contribution of belowground root biomass to the total biomass pool and the importance of maintaining the planted mangrove as a part of ecosystem rehabilitation.

Root trait-microbial relationships across tundra plant species.

Summary Fine roots, and their functional traits, influence associated rhizosphere microorganisms via root exudation and root litter quality. However, little information is known about their relationship with rhizosphere microbial taxa and functional guilds.We investigated the relationships of 11 fine root traits of 20 sub‐arctic tundra meadow plant species and soil microbial community composition, using phospholipid fatty acids (PLFAs) and high‐throughput sequencing. We primarily focused on the root economics spectrum, as it provides a useful framework to examine plant strategies by integrating the co‐ordination of belowground root traits along a resource acquisition–conservation trade‐off axis.We found that the chemical axis of the fine root economics spectrum was positively related to fungal to bacterial ratios, but negatively to Gram‐positive to Gram‐negative bacterial ratios. However, this spectrum was unrelated to the relative abundance of functional guilds of soil fungi. Nevertheless, the relative abundance of arbuscular mycorrhizal fungi was positively correlated to root carbon content, but negatively to the numbers of root forks per root length.Our results suggest that the fine root economics spectrum is important for predicting broader groups of soil microorganisms (i.e. fungi and bacteria), while individual root traits may be more important for predicting soil microbial taxa and functional guilds.

Surface elevation change evaluation in mangrove forests using a low‐cost, rapid‐scan terrestrial laser scanner

Mangrove forests have adapted to sea level rise (SLR) increases by maintaining their forest floor elevation via belowground root growth and surface sediment deposits. Researchers use surface elevation tables (SETs) to monitor surface elevation change (SEC) in mangrove forests, after which this information is used to assess SLR resiliency or to dictate active forest management for vulnerable systems. This method requires significant investments in terms of time and human resources and is limited in the number of points it can measure per plot. We use a low‐cost, portable terrestrial laser scanning (TLS) system to assess SEC for three mangrove forests on Pohnpei Island (Federated States of Micronesia). Cloth simulation filtering was used for ground detection, after which results were refined by filtering points using angular orientation. Digital elevation models then were generated via kriging interpolation for data collected in 2017 and 2019, after which the heights of corresponding points were compared across years. Extreme elevation changes, due to disturbances such as footprints or fallen logs, were removed using interquartile range analysis. The TLS‐obtained average SEC ranged between −6.92 and +6.01 mm, which exhibited an average consistency of 72% when compared to simultaneously collected SET data (root mean square error = 1.36 mm). We contend that this approach represents an improvement over the manual method, where very few points typically are used, that is, ≅ 36 points vs. ≅ 30,000 points in the case of TLS, and could contribute to improved monitoring and management of these rapidly changing forest environments.

Scaling relations and substrate conditions controlling the complexity of Rhizophora prop root system

The complex structures of the prop root system of the mangrove genus Rhizophora attenuate storm surges and tsunamis and promote sedimentation. While quantification of the mangrove's drag force is essential for assessing the mangroves' ability to reduce the risk of disaster and vulnerability to rising sea-level, the projected area of prop root system having multiple orders of prop roots (e.g., first-, second-, and third-order prop roots, where the root order indicates the level of branching from the stem) are still unexplored. To contribute to the quantitative evaluation of the mangrove's drag force, we investigated the vertical profile of whole-tree prop root projected area and the number of prop roots of 156 trees sampled from Indonesia, the Philippines, and Japan. Our results showed that prop roots above the first-order contribute up to 80% of the whole-tree prop root projected area, highlighting the importance of the presence of second-, third-, and even higher-order prop roots on the drag force exerted by mangroves. Based on field data, an empirical model for the prop root system, described by a scaling factor (S), and maximum and minimum root heights (HRmax, HRmin), was developed by assuming that the size distribution of prop roots follows a constant scaling factor S. S and HRmax showed significant correlations with stem diameter at breast height suggesting the scaling relations in the prop root system. The model, which employs the site- and species-specific regression models for S and HRmax, reproduced the vertical distribution of the number of prop roots – a good predictor of the prop root projected area – with reasonable accuracy. However, the site- and species-specific scaling relations are highly variable depending on the environment, suggesting morphological plasticity in the prop root system. Rhizophora trees that grow on a shallow sediment layer (around 0.1 m thickness) tend to produce more prop roots compared with those on a thicker sediment layer suggesting a morphological response of the prop root system to increase below-ground root biomass where below-ground root development is physically constrained. Multivariate analysis indicated that hard substrate also possibly contributes to higher complexity of prop root system. However, prediction of the site- and species-specific scaling relations from these environmental variables still needs to be improved. Further research is needed to explain the mechanisms of morphological response of prop root systems to environmental gradients and to establish a generalized model for predicting the prop root system in various environments.

Cover crop diversity improves multiple soil properties via altering root architectural traits

Cover crops may influence soil health and functioning. However, little is known about the role of belowground root architectural traits in linking cover crop diversity with rhizosphere soil ecosystem properties. We hypothesize that cover crop diversity may improve root traits, which in return, could influence its effects on essential indicators of soil physicochemical heterogeneity, such as the composition of soil aggregate-size classes and nutrients, and the soil organic matter (SOM) and soil organic carbon (SOC) contents. We studied the impact of plant diversity on root traits and soil properties. The four soil aggregate-size classes, such as large macro- (>2000 μm), small macro- (<2000-500 μm), meso- (<500-250 μm), and micro-aggregates (<250 μm) were separated by the dry sieving. Root traits such as surface area (cm2) and length (cm) were quantified by image analysis using Winrhizo. The soil nutrient, SOM, and SOC contents were determined by standard methods. We used Fox's (2005) method for partitioning the selection and complementarity effects on plant productivity. Plant diversity improved productivity, root architectural traits, root coverage, the composition of soil aggregate-size classes and nutrients, SOM and SOC contents across the soil depth. It increased and decreased the relative abundance of meso- and micro-aggregates, respectively. Both root architectural traits and SOC contents positively correlated with several soil edaphic properties essential for physicochemical heterogeneity, health, and functioning. Our results suggest that competition among plant roots in species-rich than poor communities may improve rhizosphere soil carbon storage, the composition of soil aggregate-size classes, and nutrients.

In-Situ CO2 Partitioning Measurements in a Phragmites australis Wetland: Understanding Carbon Loss through Ecosystem Respiration

Common reed (Phragmites australis) is dominant vegetation of temperate coastal wetlands in northeast China. To studying the link between ecosystem respiration (Reco) and its influential factors, a multi-year in-situ experiment was carried out in a newly restored wetland during the growing seasons of 2012 to 2014. Total in-situ Reco was separated into soil microbial and belowground root respiration (Rs + r) and plant respiration (Rplant). The soil microbial respiration rate (Rs) was isolated from Rs + r, making it easier to understand each component of Reco. With the wetland restoration process, the seasonal average aboveground biomass (dry mass) increased from 411.5 g m−2 to 2048.1 g m−2 and the corresponding Reco increased from 751.78 mg CO2 m −2 h−1 to 2612.41 mg CO2 m −2 h−1. Rplant contributed averagely 69% ~ 71% to Reco on the whole seasonal scale and the plant activity was strongly seasonal. With 1 g of aboveground common reed biomass (dry weight), approximately 3.6 mg CO2 would be produced per hour during the sprouting period while it could be as low as 0.3 mg CO2 during plant senescence period. Inundation regime dominated the contribution of Rs to Reco and the flooded contribution would lower the Rs contribution to as low as 11%.

Sea-level rise and the emergence of a keystone grazer alter the geomorphic evolution and ecology of southeast US salt marshes

Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Here, we reveal that a historically innocuous grazer-the marsh crab Sesarma reticulatum-is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). Aerial-image analyses reveal that resulting "Sesarma-grazed" creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Field experiments further demonstrate that Sesarma-grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions.

Nutrient Dynamics in Switchgrass as a Function of Time

There are wide variations in fertilizer recommendations for switchgrass (Panicum virgatum L.) as biofuel feedstock or forage. Inconsistent yield responses to fertilization are common. Nutrient translocation, from aboveground leaves and stems to belowground roots, is a contributor to variable, and sometimes the absence of, fertilizer response. A field study evaluated how major nutrients are cycled within switchgrass during the growing season. Aboveground (AG) and belowground (BG) biomasses were harvested and analyzed separately for yield (AG) and nutrient concentrations (AG and BG). Maximum yields were 26.3 (2008), 17.5 (2009) and 29.3 (2010) Mg ha−1 until senescence. In all years, the N concentration of the AG biomass decreased as the season progressed (p &lt; 0.0001). The belowground biomass N concentration increased over time in 2008 and 2010 (p &lt; 0.05). Phosphorus (P) and potassium (K) displayed similar trends to nitrogen concentration. Temporal changes of other nutrients in the AG and BG biomass concentrations were inconsistent and varied compared with N. The dynamics of the macronutrients suggested translocation from AG to BG as the switchgrass matured. Nutrients stored in roots can be beneficial for regrowth in the following growing season, which may reduce response to fertilizers. A greater understanding of nutrient cycling and harvest timing is needed to better manage different switchgrass production systems.

Genotype-by-environment interaction for dual-purpose traits in sweetpotato

ABSTRACT Development of high-yielding dual-purpose sweetpotato (Ipomoea batatas [L.] Lam) genotypes is useful for cultivation in mixed crop-livestock farming systems. The objective of this study was to determine genotype-by-environment interaction (GEI) for storage root yield (SRY), vine yield (VY), root: vine ratio (RVR), total above-ground and below-ground biomass (TABGB) and root dry matter content (RDMC) among newly developed sweetpotato clones to select promising dual-purpose clones. Forty-five sweetpotato genotypes, of which 32 were newly developed candidate clones, were evaluated across six diverse environments in Rwanda. Additive main effects and multiplicative interaction (AMMI) and genotype and genotype-by-environment (GGE) biplot analyzes were used to analyze GEI. AMMI analysis revealed a relatively larger proportion of the GEI effect for SRY (31.21%), VY (37.17%), RVR (41.73%), TABGB (35.24%) and RDMC (36.23%) than for genotype and environmental effects, suggesting differential response of test genotypes across environments for the assessed traits. GGE-biplot analysis explained total variation of 72.98% for SRY, 83.26% for VY, 74.03% for RVR, 78.82% for TABGB and 56.78% for RDMC. Clone 21 with enhanced SRY and TABGB, and clone 42 for SRY had broad adaptation. For better VY, clone 733 was the best performer in environments E2 and E6 which are designated as mega-environment (MGE) 2, and clone 321 in E1, E3 and E4 (MGE 3). For RVR, clone 577 had best adaptation in E3, E4 and E5 (MGE 1), while clone 460 in E2 and E6 (MGE 2). Clones 42 and 381 were best performers for RDMC with specific adaptation in environments E3 and E1, respectively.

Carbon management practices regulate soil bacterial communities in response to nitrogen addition in a pine forest

Carbon (C) and nitrogen (N) represent the commonest limiting nutrients for microbial growth in terrestrial ecosystems. However, most of our understanding of how C:N ratios modulate microbial growth comes from short-term growth assays under controlled conditions. Four levels of N additions including control (CK, 0 kg·hm− 2·a− 1), low N (LN, 50 kg·hm− 2·a− 1), medium N (MN, 100 kg·hm− 2·a− 1), and high N (HN, 150 kg·hm− 2·a− 1) inputs were applied monthly in a pine forest (Pinus tabulaeformis). Three C management practices were further conducted in each N additions plot, namely aboveground litter and belowground root removal (LRR), aboveground litter removal (LR), and intact soils (non-removal, NR). Soil bacterial richness, diversity, community composition and soil properties were measured. MN and HN significantly increased the relative abundance of copiotrophic taxa, but decreased that of oligotrophic taxa. Bacterial richness and diversity were not altered by N enrichment. LRR significantly increased the relative abundance of Gemmatimonadetes while decreased that of Actinobacteria. C management did not affect bacterial richness and community structure but LR significantly decreased bacterial diversity. Structural equation modeling showed that N addition induced the decrease in soil pH was responsible for the changes in the bacterial community structure. C management exhibited a direct negative effect on bacterial diversity and had an indirect positive effect on it via increasing soil moisture and microbial biomass C:N ratio. The findings highlight the contrasting impacts of N addition and C management on soil bacterial communities and emphasize the regulation of above- and below-ground C substrate supplies on the N responses of soil bacterial communities.