Aboveground Plant Productivity Research Articles

Mixed planting mode is the best measure to restore soil quality in alpine mines

In ecologically fragile areas heavily affected by human disturbance, it is imperative to restore soil quality to preserve nutrient cycling and ensure ecosystem stability. However, our understanding about the underlying mechanisms of dynamic change in soil quality associated with revegetation in alpine areas remains incomplete. In this study, data from 96 plots were collected to reveal the response of soil quality to vegetation restoration in alpine mines. The soil quality index (SQI) was developed utilizing 19 measured indicators to assess differences in soil quality between various vegetation restoration modes (natural restoration (CK), planting with Elymus dahuricus (ED), Medicago sativa (MS), and multi-plant mixed (Avena fatua L. + Elymus dahuricus + Medicago sativa + Oxytropis coerulea, MM)) and restoration periods (1, 3, 4, and 6 years). A partial least squares structural equation model (PLS-SEM) was constructed to recognize the impact of 14 variables related to topographical features, plant biodiversity and community stability, aboveground plant productivity characteristics, root functional traits, and soil erosion on soil quality. The results revealed significant variations in soil quality among different vegetation restoration modes. Within the initial four years following vegetation restoration, the discernible discrepancies in SQI values were as follows: ED > MM > MS > CK; by the sixth year, the order shifted to MM > ED > MS > CK. Aboveground plant productivity characteristics and functional root traits significantly contribute to soil quality. In contrast, plant biodiversity and community stability had no significant effects on soil quality. Topographical features and soil erosion adversely affected soil quality, with soil erosion exerting the most prominent effect.

Vegetation responses to snow cover removal and rainfall reduction in subalpine grasslands: Insights from a 4-year field experiment in the Spanish Pyrenees

ABSTRACT Reduced precipitation as well as warming may result in less snow accumulation in seasonally snow-covered areas, leading to lower minimum soil temperatures and more frequent and severe soil frosts. Therefore, plant stress is increased not only by drought in warmer months, but also by increased exposure to frost in cooler periods. We conducted a 4-year field experiment to evaluate the effects of diminished snowpack accumulation (through snow removal) and rainfall reduction (through rain-out shelters) on aboveground plant productivity, diversity and species composition of two subalpine grassland plant communities from the central region of the Spanish Pyrenees. We found that the snow removal treatment decreased minimum soil temperature by 0.5°C. Plant diversity decreased by 16 percent, although this effect was only observed in one of the grasslands studied. Aboveground primary productivity seemed to be unaffected. In contrast, we found that the rainfall reduction treatment negatively affected aboveground productivity of leguminous forb species, yet no effect on plant diversity was observed. Both treatments were important drivers of changes in plant species composition. Overall, our results suggest that the resilience of subalpine grasslands to snow cover removal and rainfall reduction treatments may depend on the specific community composition and dominant plant groups.

Mineral fertilization impacts microbial activity and endophytic fungi but not microbial biomass in semiarid grasslands

Applications of mineral fertilizer to grasslands have become more frequent in recent decades to increase forage production. However, the impacts of mineral fertilizer on the soil microbiome is poorly understood in cold semiarid grassland ecosystems of Southern Patagonia, Argentina. Therefore, our objective was to analyze experimentally the influence of mineral nutrient fertilization (N, P, K, and NPK in combination) on soil microbial community attributes, such as microbial biomass carbon (MBC) and nitrogen (MBN), soil basal respiration (SBR), microbial metabolic coefficients, the colonization of endophytic fungi such as arbuscular mycorrhizal (AM) fungi, and dark septate endophytes (DSE), and aboveground plant productivity. Mineral fertilization with macronutrients (N, P, K, and NPK) decreased the SBR, qCO2, AM fungi and DSE fungi, but did not generate changes in MBC and MBN. The magnitude of these responses depends on years after fertilization. We found that soil microbiome was strongly dependent on a range of biotic and abiotic factors, such as growing season precipitation, aboveground plant biomass the relationship between the microbial biomass and microbial respiration, and between endophytic fungi and plants. This work improved our understanding of the soil microorganisms’ response to mineral fertilizer application and provides new insights into soil nutrient dynamics and ecosystem functioning.

Cereals rhizosphere microbiome undergoes host selection of nitrogen cycle guilds correlated to crop productivity

Sustainable transformation of agricultural plant production requires the reduction of nitrogen (N) fertilizer application. Such a reduced N fertilizer application may impede crop production due to an altered symbiosis of crops and their rhizosphere microbiome, since reduced N input may affect the competition and synergisms with the plant. The assessment of such changes in the crop microbiome functionalities at spatial scales relevant for agricultural management remains challenging. We investigated in a field plot experiment how and if the N cycling guilds of the rhizosphere of globally relevant cereal crops - winter barley, wheat and rye - are influenced by reduced N fertilization. Crop productivity was assessed by remote sensing of the shoot biomass. Microbial N cycling guilds were investigated by metagenomics targeting diazotrophs, nitrifiers, denitrifiers and the dissimilatory nitrate to ammonium reducing guild (DNRA). The functional composition of microbial N cycling guilds was explained by crop productivity parameters and soil pH, and diverged substantially between the crop species. The responses of individual microbial N cycling guild abundances to shoot dry weight and rhizosphere nitrate content was modulated by the N fertilization treatments and the crop species, which was identified based on regression analyses. Thus, characteristic shifts in the microbial N cycling guild acquisition associated with the crop host species were resolved. Particularly, the rhizosphere of rye was enriched with potentially N-preserving microbial guilds – diazotrophs and the DNRA guild – when no fertilizer was applied. We speculate that the acquisition of microbial N cycling guilds was the result of plant species-specific acquisition strategies. Thus, the investigated cereal crop holobionts have likely different symbiotic strategies that make them differently resilient against reduced N fertilizer inputs. Furthermore, we demonstrated that these belowground patterns of N cycling guilds from the rhizosphere microbiome are linked to remotely sensed aboveground plant productivity.

Reducing plant-derived ethylene concentrations increases the resistance of temperate grassland to drought

Model predictions indicate that extreme drought events will occur more frequently by the end of this century, with major implications for terrestrial ecosystem functions such as plant productivity and soil respiration. Previous studies have shown that drought-induced ethylene produced by plants is a key factor affecting plant growth and development, but the impact of drought-induced ethylene on ecosystem functions in natural settings has not yet been tested. Here, we reduced the amount of plant-derived ethylene concentrations by adding the ethylene inhibitor aminoethoxyvinylglycine (AVG), and investigated in situ plant productivity, soil respiration and ethylene concentrations for two years in a semi-arid temperate grassland in Inner Mongolia, China. Drought significantly reduced plant productivity and soil respiration, but the application of AVG reduced ethylene concentrations and significantly increased aboveground plant productivity and soil respiration, effectively enhancing resistance to drought. The reason for this could be that AVG application increased the activity of 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase and abundance of the acdS gene (the key gene for ACC deaminase), facilitating reduced ACC concentrations in the plant tissue and reduced in planta ethylene synthesis. In addition, there was a significant correlation between soil ACC deaminase activity and plant productivity. Given the global distribution of arid and semi-arid areas, and the expected increases in the frequency and intensity of drought stress, this is a significant concern. These results provide novel evidence of the impact of drought-induced plant ethylene production on ecosystem functions in semi-arid temperate grassland ecosystems.

The complexity of the bacterial community in response to fertilization determines forage production in a semiarid grassland

The role of soil microbial diversity and interactions in controlling plant productivity after fertilization remains unknown in grassland systems. This knowledge gap hampers our understanding of how shifts in soil microbial diversity and interactions can modify soil functioning and grassland production under changing environments. In this study, we evaluated the influence of short-term field fertilization on soil microbial diversity, particularly the complexity of microbial ecological networks in controlling aboveground plant productivity under five nitrogen addition rates (0, 6, 12, 20, 32 g N m−2 year−1). The N fertilization resulted in higher forage production. The bacterial and fungal alpha diversity and taxonomic composition were relatively stable across the N addition rates. Network analysis showed that the complexity of the bacterial network increased gradually with increasing N addition rates, peaking at 20 g N m−2 year−1, while modularity and |negative|: positive cohesion had contrasting patterns across the N addition rates. Additionally, the changes of bacterial interactions were predominantly driven by the keystone taxa richness and abundance. Compared with the bacterial co-occurrence network, the fungal co-occurrence network remained relatively stable. Strong positive linkages between plant production and the complexity of the bacterial network were also observed. Furthermore, the increment of forage production was highly related to the keystone taxa abundance. Together, our results highlighted that the complexity of the soil bacterial network plays a vital role in enhancing the connection between soil microbial diversity and plant production. Our findings exemplify a significant progress in projecting the effects of soil microbial diversity on ecosystem functions in grassland ecosystems.

Predicting Soil Respiration from Plant Productivity (NDVI) in a Sub-Arctic Tundra Ecosystem

Soils represent the largest store of carbon in the biosphere with soils at high latitudes containing twice as much carbon (C) than the atmosphere. High latitude tundra vegetation communities show increases in the relative abundance and cover of deciduous shrubs which may influence net ecosystem exchange of CO2 from this C-rich ecosystem. Monitoring soil respiration (Rs) as a crucial component of the ecosystem carbon balance at regional scales is difficult given the remoteness of these ecosystems and the intensiveness of measurements that is required. Here we use direct measurements of Rs from contrasting tundra plant communities combined with direct measurements of aboveground plant productivity via Normalised Difference Vegetation Index (NDVI) to predict soil respiration across four key vegetation communities in a tundra ecosystem. Soil respiration exhibited a nonlinear relationship with NDVI (y = 0.202e3.508 x, p < 0.001). Our results further suggest that NDVI and soil temperature can help predict Rs if vegetation type is taken into consideration. We observed, however, that NDVI is not a relevant explanatory variable in the estimation of SOC in a single-study analysis.

Testing the efficacy of existing USDA Rhizobium germplasm collection accessions as inoculants for guar

Legume inoculants are products that introduce legume-specific rhizobia bacteria to the soil and promote the process of biological nitrogen fixation. Guar (Cyamopsis tetragonoloba L. Taub.), also known as clusterbean, is a legume produced in semi-arid regions around the world, primarily for industrial uses of guar gum, but there is only one known commercial inoculant products for the crop, which is not available worldwide. The USDA-ARS National Rhizobium Germplasm Collection has four Rhizobium strains listed as compatible with guar: USDA 3089, USDA 3385, USDA 3386, and USDA 3595. The objective of this research was to test the efficacy of four peat-based, seed-applied inoculants derived from these strains, as well as a mixture of all four strains, compared with un-inoculated plants in controlled greenhouse conditions. Among the tested strains, USDA 3089 and USDA 3386 had the greatest positive impacts, as they increased nodule weight, total assimilated nitrogen, biomass production, and protein production per plant relative to the un-inoculated control. USDA 3385 and the Mix likewise increased nodule weight. No inoculant affected other measured aboveground plant morphological and productivity parameters or nodule number. These Rhizobium strains, particularly USDA 3089 and 3386, were identified as beneficial and recommended for further testing in the field in development of an effective inoculant for guar.

Using a meta-analysis approach to understand complexity in soil biodiversity and phosphorus acquisition in plants

Current soil phosphorus (P) management is neither environmentally nor economically sustainable. Soil biodiversity has been offered as a solution to unsustainable land management and to promote ecosystem service provision. We know soil biology is instrumental in plant access to soil P, but specific effects of biological complexity, (used here to describe the number of links between different organisms), under different P levels on plant productivity are not well understood. We conducted a meta-analysis on relevant literature, which reported the response of terrestrial plants of economic and anthropogenic importance to P conditions, and controlled for biological treatments across different land-uses (arable, grassland and woodland). We hypothesised that: 1) in arable systems increased biological complexity will enhance plant productivity; 2) in perennial systems such as grassland and woodlands, increasing biological complexity will have no effect; 3) increasing the fertility of the system by addition of P fertiliser will reduce any benefits of biological complexity. We found that soil organisms are not always beneficial to plant shoot biomass, but that the effects of - and interaction among - bacteria, protozoa, nematodes, mycorrhizae, collembola and earthworms differ in their impact on plant biomass (positive or negative) dependent on the presence of other community members, P-level status and time. These findings bring into question existing frameworks that link below-ground biodiversity with above-ground plant productivity. We recommend further experimental work be conducted, which controls for land-use, P status, and soil biological composition and complexity. Such work should be followed by future systematic reviews, which could pragmatically inform more tailored biological management for plant P requirements, land-use and ecosystem service provision. To enable further meta-analyses of this type we recommend habitual inclusion of sufficient experimental detail and data, as a prerequisite for publication and a useful way to utilise increased online publication space.

Direct and indirect effects of long-term fertilization on the stability of the persistent seed bank

If and how eutrophication influences the persistent soil seed bank is poorly understood. Here, we hypothesized that eutrophication alters the composition of the persistent seed bank indirectly through changes in the soil characteristics and aboveground plant community and productivity. We also hypothesized that changes in the persistent seed bank will consequently impact the aboveground vegetative composition. We tested these hypotheses using data from a 9-year nitrogen and phosphorus fertilization experiment in an alpine meadow ecosystem on the eastern Qinghai-Tibet plateau. We found that long-term nitrogen and phosphorus fertilization indirectly impacted the composition of the persistent seed bank through changes in soil pH, aboveground vegetation composition and annual net primary productivity (ANPP). Changes in the composition of the persistent seed bank, however, were relatively minor in comparison to changes in aboveground vegetation composition. Finally, changes in the persistent seed bank did not feedback on aboveground vegetation composition. Our findings demonstrate the importance of soil pH, ANPP and vegetation composition in regulating the persistent seed bank under eutrophication. Our results also highlight the relative stability of the persistent seed bank to long-term eutrophication and their important contribution to the sustainability of grassland ecosystems.

Consistent responses of surface- and subsurface soil fungal diversity to N enrichment are mediated differently by acidification and plant community in a semi-arid grassland

Soil fungal communities are critical to decomposition, nutrient cycling and the maintenance of plant diversity and abundance. However, despite global increases in reactive nitrogen (N) inputs to terrestrial ecosystems, due to anthropogenic activities, an explicit evaluation of the direct (resource availability) and indirect (acidification and plant community changes) effects of N enrichment on soil fungal communities in grassland ecosystems remains largely unexplored. In this study, we used Illumina sequencing of the ITS1 barcode region to elucidate the responses of soil fungal communities using a 7-year simulated N deposition experiment that spanned a broad range of N addition rates and made a systematic evaluation of the role and relative importance of N availability, plant community and soil acidification as drivers of soil fungal diversity in a semi-arid grassland ecosystem. Our results showed that N enrichment led to significant declines in soil fungal alpha diversity and promoted strong shifts in beta diversity (community composition) in both surface and subsurface soil layers. We found that N addition-induced soil acidification overwhelmed the effects of increased N availability and plant community changes, and played a primary role in shaping the observed changes in fungal alpha and beta diversity in surface soil. Conversely, in the subsurface soil layer, both fungal alpha and beta diversity were primarily controlled by N addition-induced changes in plant community attributes (i.e., aboveground plant productivity and plant community composition). Thus, our work illustrates the consistent responses of surface and subsurface soil fungal diversity (both alpha and beta diversity) to N addition that are mediated by different mechanisms and provides an integrated insight into how N enrichment could alter soil fungal diversity in semi-arid grassland in future scenarios of elevated N deposition.

Using APAR to Predict Aboveground Plant Productivity in Semi-Arid Rangelands: Spatial and Temporal Relationships Differ

Monitoring of aboveground net primary production (ANPP) is critical for effective management of rangeland ecosystems but is problematic due to the vast extent of rangelands globally, and the high costs of ground-based measurements. Remote sensing of absorbed photosynthetically active radiation (APAR) can be used to predict ANPP, potentially offering an alternative means of quantifying ANPP at both high temporal and spatial resolution across broad spatial extents. The relationship between ANPP and APAR has often been quantified based on either spatial variation across a broad region or temporal variation at a location over time, but rarely both. Here we assess: (i) if the relationship between ANPP and APAR is consistent when evaluated across time and space; (ii) potential factors driving differences between temporal versus spatial models, and (iii) the magnitude of potential errors relating to space for time transformations in quantifying productivity. Using two complimentary ANPP datasets and remotely sensed data derived from MODIS and a Landsat/MODIS fusion data product, we find that slopes of spatial models are generally greater than slopes of temporal models. The abundance of plant species with different structural attributes, specifically the abundance of C4 shortgrasses with prostrate canopies versus taller, more productive C3 species with more vertically complex canopies, tended to vary more dramatically in space than over time. This difference in spatial versus temporal variation in these key plant functional groups appears to be the primary driver of differences in slopes among regression models. While the individual models revealed strong relationships between ANPP to APAR, the use of temporal models to predict variation in space (or vice versa) can increase error in remotely sensed predictions of ANPP.

Why does mineral fertilization increase soil carbon stocks in temperate grasslands?

Globally, grasslands are a major land cover type and store significant amounts of soil organic carbon (SOC). Fertilization with the major plant nutrients nitrogen (N), phosphorus (P), and potassium (K) may affect SOC stocks, e.g., by altering aboveground and belowground plant productivity, species composition, litter composition and decomposition, and microbial metabolism. However, belowground responses to fertilization in grasslands are not fully understood, hampering accurate predictions of SOC dynamics. In this study, seven different long-term grassland fertilization experiments (16–58 years) in Germany and the Netherlands were sampled to determine the effects of mineral fertilization (i.e., N, P, K, PK, and NPK) compared with unfertilized plots (Control) on SOC stocks and root C stocks. Soils were sampled to a depth of 100 cm or to the maximum depth possible. Potential litter decomposition was assessed using Lipton Rooibos and Lipton Green teas as standardized and well-characterized litter materials. In the topsoil (0–30 cm depth), PK, NPK, and NPK+ (increased NPK) fertilization had significant positive effects on SOC stocks, with annual sequestration rates of 0.28, 0.13, and 0.37 Mg ha−1 yr−1, respectively, within an average time span of 34 (PK, NPK) or 20 (NPK+) years. For NPK fertilization, 1.15 kg of N was needed to sequester 1 kg SOC. Increased SOC stocks could not be explained by altered belowground C inputs or decomposition rates, since root C stocks were not affected by fertilization and potential litter decomposition was unchanged. The highly significant increases in dry matter yield with PK and NPK fertilization and resulting higher aboveground C inputs were also unlikely to explain the observed SOC stock changes, since increases were only observed at soil depths >10 cm. However, significantly narrower root C:N ratios were observed for the N (26.9), PK (36.5), NPK (36.8), and NPK+ (33.2) treatments than for the Control (41.8), which may have caused increased microbial C use efficiency, positively affecting SOC storage. The narrower C:N ratios in PK treatments were explained by significantly increased abundance of legumes. We concluded that the carbon footprint of fertilization-induced SOC sequestration needs to be considered when the latter should be accounted for as a climate mitigation measure. Thereby, PK fertilization has much lower CO2 costs than NPK fertilization due to N input via legumes.

Long-term effects of grazing and topography on extra-radical hyphae of arbuscular mycorrhizal fungi in semi-arid grasslands.

Grazing and topography have drastic effects on plant communities and soil properties. These effects are thought to influence arbuscular mycorrhizal (AM) fungi. However, the simultaneous impacts of grazing pressure (sheepha-1) and topography on plant and soil factors and their relationship to the production of extra-radical AM hyphae are not well understood. Our 10-year study assessed relationships between grazing, plant species richness, aboveground plant productivity, soil nutrients, edaphic properties, and AM hyphal length density (HLD) in different topographic areas (flat or sloped). We found HLD linearly declined with increasing grazing pressure (1.5-9.0 sheepha-1) in sloped areas, but HLD was greatest at moderate grazing pressure (4.5 sheepha-1) in flat areas. Structural equation modeling indicates grazing reduces HLD by altering soil nutrient dynamics in sloped areas, but non-linearly influences HLD through plant community and edaphic changes in flat areas. Our findings highlight how topography influences key plant and soil factors, thus regulating the effects of grazing pressure on extra-radical hyphal production of AM fungi in grasslands. Understanding how grazing and topography influence AM fungi in semi-arid grasslands is vital, as globally, severe human population pressure and increasing demand for food aggravate the grazing intensity in grasslands.

Circular linkages between soil biodiversity, fertility and plant productivity are limited to topsoil at the continental scale.

The current theoretical framework suggests that tripartite positive feedback relationships between soil biodiversity, fertility and plant productivity are universal. However, empirical evidence for these relationships at the continental scale and across different soil depths is lacking. We investigate the continental-scale relationships between the diversity of microbial and invertebrate-based soil food webs, fertility and above-ground plant productivity at 289 sites and two soil depths, that is 0-10 and 20-30cm, across Australia. Soil biodiversity, fertility and plant productivity are strongly positively related in surface soils. Conversely, in the deeper soil layer, the relationships between soil biodiversity, fertility and plant productivity weaken considerably, probably as a result of a reduction in biodiversity and fertility with depth. Further modeling suggested that strong positive associations among soil biodiversity-fertility and fertility-plant productivity are limited to the upper soil layer (0-10cm), after accounting for key factors, such as distance from the equator, altitude, climate and physicochemical soil properties. These findings highlight the importance of surface soil biodiversity for soil fertility, and suggest that any loss of surface soil could potentially break the links between soil biodiversity-fertility and/or fertility-plant productivity, which can negatively impact nutrient cycling and food production, upon which future generations depend.

Influence of the Wax Lake Delta sediment diversion on aboveground plant productivity and carbon storage in deltaic island and mainland coastal marshes

Coastal Louisiana is experiencing a significant loss of coastal wetland area due to increasing sea level rise, subsidence, sediment starvation and marsh collapse. The construction of large scale Mississippi River sediment diversions is currently being planned in an effort to help combat coastal wetlands losses at a rate of >50 km−2 y−1. The Wax Lake Delta (WLD) is currently being used as a model for evaluating potential land gain from large scale diversions of Mississippi River water and sediment. In this study, we determine the impact of the WLD diversion on plant production at newly formed islands within the delta and adjacent, mainland freshwater marshes. Plant aboveground productivity, sediment nutrient status and short term accretion were measured at three locations on a transect at each of three fresh water marsh sites along Hog Bayou and at six newly formed emerging island sites in the delta. Spring flooding has resulted in a greater increase in plant production and consequently, greater carbon sequestration potential in adjacent mainland marshes compared to the newly formed island sites, which contain less total carbon (C), nitrogen (N), and phosphorus (P) in the sediment. While sediment diversions are predicted to create land, as seen in island formation in the WLD, the greatest benefit of river sediment diversions from a carbon credit perspective might be to the adjacent freshwater mainland marshes for several reasons. Both greater plant production and sediment C accumulation are two important factors for marsh stability, while perhaps even more critical, is the prevention of the loss of stored sediment C in the marsh profile. This stored C would be lost without the introduction of freshwater, nutrients and sediment through river sediment diversion efforts.

Functional composition rather than species richness determines root characteristics of experimental grasslands grown at different light and nutrient availability

Background and aims Results from many biodiversity experiments have established evidence for positive effects of diversity on aboveground plant productivity. However, less is known about the relationships between plant diversity and belowground plant community characteristics and their consistency at altered environmental conditions.

Multiple soil nutrient competition between plants, microbes, and mineral surfaces: model development, parameterization, and example applications in several tropical forests

Abstract. Soil is a complex system where biotic (e.g., plant roots, micro-organisms) and abiotic (e.g., mineral surfaces) consumers compete for resources necessary for life (e.g., nitrogen, phosphorus). This competition is ecologically significant, since it regulates the dynamics of soil nutrients and controls aboveground plant productivity. Here we develop, calibrate and test a nutrient competition model that accounts for multiple soil nutrients interacting with multiple biotic and abiotic consumers. As applied here for tropical forests, the Nutrient COMpetition model (N-COM) includes three primary soil nutrients (NH4+, NO3− and POx; representing the sum of PO43−, HPO42− and H2PO4−) and five potential competitors (plant roots, decomposing microbes, nitrifiers, denitrifiers and mineral surfaces). The competition is formulated with a quasi-steady-state chemical equilibrium approximation to account for substrate (multiple substrates share one consumer) and consumer (multiple consumers compete for one substrate) effects. N-COM successfully reproduced observed soil heterotrophic respiration, N2O emissions, free phosphorus, sorbed phosphorus and NH4+ pools at a tropical forest site (Tapajos). The overall model uncertainty was moderately well constrained. Our sensitivity analysis revealed that soil nutrient competition was primarily regulated by consumer–substrate affinity rather than environmental factors such as soil temperature or soil moisture. Our results also imply that under strong nutrient limitation, relative competitiveness depends strongly on the competitor functional traits (affinity and nutrient carrier enzyme abundance). We then applied the N-COM model to analyze field nitrogen and phosphorus perturbation experiments in two tropical forest sites (in Hawaii and Puerto Rico) not used in model development or calibration. Under soil inorganic nitrogen and phosphorus elevated conditions, the model accurately replicated the experimentally observed competition among nutrient consumers. Although we used as many observations as we could obtain, more nutrient addition experiments in tropical systems would greatly benefit model testing and calibration. In summary, the N-COM model provides an ecologically consistent representation of nutrient competition appropriate for land BGC models integrated in Earth System Models.

Carbon storage in seagrass soils: long-term nutrient history exceeds the effects of near-term nutrient enrichment

Abstract. The carbon sequestration potential in coastal soils is linked to aboveground and belowground plant productivity and biomass, which in turn, is directly and indirectly influenced by nutrient input. We evaluated the influence of long-term and near-term nutrient input on aboveground and belowground carbon accumulation in seagrass beds, using a nutrient enrichment (nitrogen and phosphorus) experiment embedded within a naturally occurring, long-term gradient of phosphorus availability within Florida Bay (USA). We measured organic carbon stocks in soils and above- and belowground seagrass biomass after 17 months of experimental nutrient addition. At the nutrient-limited sites, phosphorus addition increased the carbon stock in aboveground seagrass biomass by more than 300 %; belowground seagrass carbon stock increased by 50–100 %. Soil carbon content slightly decreased ( ∼ 10 %) in response to phosphorus addition. There was a strong but non-linear relationship between soil carbon and Thalassia testudinum leaf nitrogen : phosphorus (N : P) or belowground seagrass carbon stock. When seagrass leaf N : P exceeded an approximate threshold of 75 : 1, or when belowground seagrass carbon stock was less than 100 g m−2, there was less than 3 % organic carbon in the sediment. Despite the marked difference in soil carbon between phosphorus-limited and phosphorus-replete areas of Florida Bay, all areas of the bay had relatively high soil carbon stocks near or above the global median of 1.8 % organic carbon. The relatively high carbon content in the soils indicates that seagrass beds have extremely high carbon storage potential, even in nutrient-limited areas with low biomass or productivity.

Impact of Lowland Rainforest Transformation on Diversity and Composition of Soil Prokaryotic Communities in Sumatra (Indonesia).

Prokaryotes are the most abundant and diverse group of microorganisms in soil and mediate virtually all biogeochemical cycles in terrestrial ecosystems. Thereby, they influence aboveground plant productivity and diversity. In this study, the impact of rainforest transformation to intensively managed cash crop systems on soil prokaryotic communities was investigated. The studied managed land use systems comprised rubber agroforests (jungle rubber), rubber plantations and oil palm plantations within two Indonesian landscapes Bukit Duabelas and Harapan. Soil prokaryotic community composition and diversity were assessed by pyrotag sequencing of bacterial and archaeal 16S rRNA genes. The curated dataset contained 16,413 bacterial and 1679 archaeal operational taxonomic units at species level (97% genetic identity). Analysis revealed changes in indigenous taxon-specific patterns of soil prokaryotic communities accompanying lowland rainforest transformation to jungle rubber, and intensively managed rubber and oil palm plantations. Distinct clustering of the rainforest soil communities indicated that these are different from the communities in the studied managed land use systems. The predominant bacterial taxa in all investigated soils were Acidobacteria, Actinobacteria, Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. Overall, the bacterial community shifted from proteobacterial groups in rainforest soils to Acidobacteria in managed soils. The archaeal soil communities were mainly represented by Thaumarchaeota and Euryarchaeota. Members of the Terrestrial Group and South African Gold Mine Group 1 (Thaumarchaeota) dominated in the rainforest and members of Thermoplasmata in the managed land use systems. The alpha and beta diversity of the soil prokaryotic communities was higher in managed land use systems than in rainforest. In the case of bacteria, this was related to soil characteristics such as pH value, exchangeable Ca and Fe content, C to N ratio, and extractable P content. Archaeal community composition and diversity were correlated to pH value, exchangeable Fe content, water content, and total N. The distribution of bacterial and archaeal taxa involved in biological N cycle indicated functional shifts of the cycle during conversion of rainforest to plantations.