Amendment Experiments Research Articles

Puzzling out the ecological niche construction for nitrogen fixers in a coastal upwelling system

Abstract Diazotrophs are a diverse group of microorganisms that can fertilize the ocean through biological nitrogen fixation (BNF). Due to the high energetic cost of this process, diazotrophy in nitrogen-replete regions remains enigmatic. We use multidisciplinary observations to propose a novel framework for the ecological niche construction of nitrogen fixers in the upwelling region off NW Iberia — one of the most productive coastal regions in Europe — characterized by weak, and intermittent wind-driven upwelling and the presence of bays. The main diazotroph detected (UCYN-A2) was more abundant and active during summer and early autumn, coinciding with relatively high temperatures (>16°C), low nitrogen:phosphorus ratios (N:P < 7.2), and a large contribution of ammonium (>75%) to the total dissolved inorganic nitrogen available. Furthermore, nutrient amendment experiments showed that BNF is detectable when phytoplankton biomass and productivity are nitrogen limited. Seasonally recurrent biogeochemical processes driven by hydrography create an ecological niche for nitrogen fixers in this system. During the spring–summer upwelling, non-diazotroph autotrophs consume nitrate and produce organic matter inside the bays. Thereafter, the combined effect of intense remineralization on the shelf and sustained positive circulation within the bays in late summer-early autumn, conveys enhanced ammonium content and excess phosphate into the warm surface layer. The low N:P ratio confers a competitive advantage to diazotrophs since they are not restricted by nitrogen supply. The new nitrogen supply mediated by BNF could extend the productivity period and may be key reason why upwelling bays are more productive than upwelled offshore waters.

Influence of Biomass Amendments on Soil CO2 Concentration and Carbon Emission Flux in a Subtropical Karst Ecosystem

Soil in karst areas is rare and precious, and karst carbon sinks play an important role in the global carbon cycle. Therefore, the purpose of karst soil improvement is to improve soil productivity and a carbon sink effect. Biomass amendment experiments in this study included three schemes: filter mud (FM), filter mud + straw + biogas slurry (FSB), and filter mud + straw + cow manure (FSC). The characteristics of soil CO2 production, transport, and the effect on soil respiration carbon emissions in two years were compared and analyzed. The results were as follows: 1. The rate, amount, and depth of CO2 concentration were affected by the combinations with biogas slurry (easy to leach) or cow manure (difficult to decompose). 2. The diurnal variation curves of soil respiration in the FSB- and FSC-improved soils lagged behind those in the control soil for three hours. While the curves of FM-improved soil and the control soil were nearly the same. 3. Soil–air carbon emissions increased by 35.2 tCO2/(km2·a−1) under the FM scheme, decreased by 212.9 tCO2/(km2·a−1) under the FSB scheme, and increased by 279.5 tCO2/(km2·a−1) under the FSC scheme. The results were related to weather CO2 accumulation in the deep or surface layers under different schemes.

Optimization of soil hydrological properties in degraded grasslands by soil amendments

Soil amendments facilitate the restoration of degraded soil fertility and vegetation productivity. Soil hydrological functions are crucial for assessing soil degradation and restoration in grassland ecosystems. However, the manner in which soil amendments drive changes in the hydrological properties of grassland ecosystems at different soil depths remains unclear. In this study, we conducted a five-year soil amendment experiment in a semi-arid grassland of the Loess Plateau to evaluate the impact of aluminum sulfate (AS) and biochar (BC), both individually and in combination, on the hydrological properties of soil profiles. The results showed that AS significantly increased saturated water-holding capacity (SWHC) by 19.04 %, field capacity (FC) by 27.39 %, soil water storage (SWS) by 1.57 %, and saturated hydraulic conductivity (Ks) by 19.03 % in the upper soil layer (0–40 cm). BC application increased SWHC by 20.09 %, FC by 17.56 %, SWS by 12.94 %, and Ks by 16.66 % in the 0–40 cm soil layers. The combined effects of AS and BC optimized the soil hydrological properties by increasing SWHC, FC, SWS, and Ks by 25.74 %, 35.45 %, 18.47 %, and 25.80 %, respectively. These improvements were driven by significant changes in the soil organic matter, fine root biomass, total porosity, clay content, and soil bulk density. Notably, the soil amendments did not significantly affect the hydrological properties of the deep soil layer (40–80 cm). Our study demonstrated that the strategic use of AS and BC, particularly in combination, effectively enhanced soil fertility and hydrological functions, thereby increasing grassland ecosystem productivity. These findings offer critical insights for sustainable grassland management and highlight the potential of tailored soil amendments to restore and improve soil fertility and plant productivity in degraded grassland ecosystems.

Evaluation on the Effects of Organic Amendment on Soil Productivity of Selected Watersheds in Nnamdi Azikiwe University Awka Campus, Nigeria

This study was done to investigate soil physicochemical properties on the prevalence of biodiversity at Nnamdi Azikiwe University Awka and Amansea and to compare the performance of the different study areas (abandoned farmland and heavily grazed land) with respect to their productivity using different soil amendments. A soil amendment experiment was carried out using CRBD with four treatment levels of poultry manure for abandoned farmland and heavily grazed land. The study results revealed that soil physicochemical properties vary significantly across the study area. The difference between nitrogen and potassium levels were found to be significant at (P < 0.05), indicating favourable conditions for plant growth in specific locations. Conversely, other parameters, such as moisture content, phosphorus, organic matter, and pH, were not significant at (P > 0.05), suggesting suboptimal conditions for certain plant species. Roadside ecosystems exhibited the highest soil chemical properties, particularly potassium, making them conducive for plant growth. The perennial watershed ecosystem also offered favourable conditions, fostering a variety of plant species. In contrast, annual watershed areas exhibit the least soil nutrient support due to potential leaching effects. There was a notable positive correlation was observed between soil moisture and soil pH, percentage nitrogen, phosphorus, and potassium content (p<0.05). Specifically, significant positive correlations were identified between soil pH and phosphorus, as well as between soil pH and potassium (p < 0.05).More so, analysis of variance showed a significant difference between sites in all the physiochemical parameters examined (p<00.05).

Functional regimes define the response of the soil microbiome to environmental change.

The metabolic activity of soil microbiomes plays a central role in carbon and nitrogen cycling. Given the changing climate, it is important to understand how the metabolism of natural communities responds to environmental change. However, the ecological, spatial, and chemical complexity of soils makes understanding the mechanisms governing the response of these communities to perturbations challenging. Here, we overcome this complexity by using dynamic measurements of metabolism in microcosms and modeling to reveal regimes where a few key mechanisms govern the response of soils to environmental change. We sample soils along a natural pH gradient, construct >1500 microcosms to perturb the pH, and quantify the dynamics of respiratory nitrate utilization, a key process in the nitrogen cycle. Despite the complexity of the soil microbiome, a minimal mathematical model with two variables, the quantity of active biomass in the community and the availability of a growth-limiting nutrient, quantifies observed nitrate utilization dynamics across soils and pH perturbations. Across environmental perturbations, changes in these two variables give rise to three functional regimes each with qualitatively distinct dynamics of nitrate utilization over time: a regime where acidic perturbations induce cell death that limits metabolic activity, a nutrient-limiting regime where nitrate uptake is performed by dominant taxa that utilize nutrients released from the soil matrix, and a resurgent growth regime in basic conditions, where excess nutrients enable growth of initially rare taxa. The underlying mechanism of each regime is predicted by our interpretable model and tested via amendment experiments, nutrient measurements, and sequencing. Further, our data suggest that the long-term history of environmental variation in the wild influences the transitions between functional regimes. Therefore, quantitative measurements and a mathematical model reveal the existence of qualitative regimes that capture the mechanisms and dynamics of a community responding to environmental change.

Growth and mortality of aerobic anoxygenic phototrophs in the North Pacific Subtropical Gyre.

Aerobic anoxygenic phototrophic (AAP) bacteria harvest light energy using bacteriochlorophyll-containing reaction centers to supplement their mostly heterotrophic metabolism. While their abundance and growth have been intensively studied in coastal environments, much less is known about their activity in oligotrophic open ocean regions. Therefore, we combined in situ sampling in the North Pacific Subtropical Gyre, north of O'ahu island, Hawaii, with two manipulation experiments. Infra-red epifluorescence microscopy documented that AAP bacteria represented approximately 2% of total bacteria in the euphotic zone with the maximum abundance in the upper 50 m. They conducted active photosynthetic electron transport with maximum rates up to 50 electrons per reaction center per second. The in situ decline of bacteriochlorophyll concentration over the daylight period, an estimate of loss rates due to predation, indicated that the AAP bacteria in the upper 50 m of the water column turned over at rates of 0.75-0.90 d-1. This corresponded well with the specific growth rate determined in dilution experiments where AAP bacteria grew at a rate 1.05 ± 0.09 d-1. An amendment of inorganic nitrogen to obtain N:P = 32 resulted in a more than 10 times increase in AAP abundance over 6 days. The presented data document that AAP bacteria are an active part of the bacterioplankton community in the oligotrophic North Pacific Subtropical Gyre and that their growth was mostly controlled by nitrogen availability and grazing pressure.IMPORTANCEMarine bacteria represent a complex assembly of species with different physiology, metabolism, and substrate preferences. We focus on a specific functional group of marine bacteria called aerobic anoxygenic phototrophs. These photoheterotrophic organisms require organic carbon substrates for growth, but they can also supplement their metabolic needs with light energy captured by bacteriochlorophyll. These bacteria have been intensively studied in coastal regions, but rather less is known about their distribution, growth, and mortality in the oligotrophic open ocean. Therefore, we conducted a suite of measurements in the North Pacific Subtropical Gyre to determine the distribution of these organisms in the water column and their growth and mortality rates. A nutrient amendment experiment showed that aerobic anoxygenic phototrophs were limited by inorganic nitrogen. Despite this, they grew more rapidly than average heterotrophic bacteria, but their growth was balanced by intense grazing pressure.

DNA sequencing, microbial indicators, and the discovery of buried kimberlites

Population growth and technological advancements are placing growing demand on mineral resources. New and innovative exploration technologies that improve detection of deeply buried mineralization and host rocks are required to meet these demands. Here we used diamondiferous kimberlite ore bodies as a test case and show that DNA amplicon sequencing of soil microbial communities resolves anomalies in microbial community composition and structure that reflect the surface expression of kimberlites buried under 10 s of meters of overburden. Indicator species derived from laboratory amendment experiments were employed in an exploration survey in which the species distributions effectively delineated the surface expression of buried kimberlites. Additional indicator species derived directly from field observations improved the blind discovery of kimberlites buried beneath similar overburden types. Application of DNA sequence-based analyses of soil microbial communities to mineral deposit exploration provides a powerful illustration of how genomics technologies can be leveraged in the discovery of critical new resources.

Phosphate burial in aquatic sediments: Rates and mechanisms of vivianite formation from mackinawite

Excess phosphorus abundance often drives eutrophication and affects surface water quality. Formation of vivianite (Fe3(PO4)2 • 8H2O) in aquatic sediments acts as a significant sink for phosphate (P), crucial for resorting surface waters. Authigenic vivianite formation, however, can be limited by other ferrous iron containing phases, in particular iron sulfides. Although thermodynamically feasible under suitable conditions, the formation of vivianite from mackinawite has been widely disregarded for authigenic phosphate mineral formation. Here we investigated the formation of vivianite from mackinawite (FeS) in batch experiments in which dissolved sulfide was continuously removed, at P levels between 0 – 5 mM in a pH of 6 to 8. Solid characterizations by electron microscopy, X-ray diffraction as well as Mössbauer and X-ray absorption spectroscopy demonstrates that vivianite was formed at all pH values in P amended experiments. The temporal evolution of dissolved Fe(II) concentrations indicates that the transformation proceeds via the release of the dissolved Fe(II) by FeS dissolution and subsequent vivianite precipitation, over time scales of days. The kinetics of the transformation are controlled by the dissolution rates of FeS. Aging and transformation of FeS, however, compete with vivianite formation. Aging is more pronounced at higher pH but is inhibited by P adsorption. Hence, the effect of pH and P concentration on aging is the main reason for the influence on these parameters on the rates and extent of vivianite formation. Our findings demonstrate that FeS can be an effective iron source for vivianite formation in aquatic sediments when sulfide concentrations decrease due to, for example, changes in external forcing or microbial sulfide oxidation. Formation of vivianite from FeS as an Fe source can also open new perspectives in P recovery in water treatment, for example when Fe is added to digesters to bind H2S.

Differential effects of clay mineralogy on thermal maturation of sedimentary n-alkanes

Clay minerals serve as important catalysts for the degradation of a range of biomolecules during diagenesis, yet their effects are seldom considered when interpreting organic molecular and isotopic data in the context of paleoenvironmental reconstruction. We investigated the thermal maturation of immature sedimentary organic matter in the presence of two clay minerals (kaolinite and montmorillonite) using a series of laboratory-based anhydrous pyrolysis experiments. Organic matter was extracted from a modern terrestrial sediment, mixed with each clay mineral, and heated at 100–300 °C for up to 30 days – in this way, initial organic matter was identical in each system. Abundances, molecular distributions, and paired compound-specific stable isotopic compositions (δDn-alkane and δ13Cn-alkane) of n-alkanes were measured as a function of heating time. For both kaolinite and montmorillonite amended experiments, the shifts in molecular distributions are highly controlled by significant secondary production of n-alkanes from molecularly large lipid precursors, as evidenced by significant increases in the total amount of n-alkanes accompanying increased thermal maturation. Significant decreases in carbon preference index (CPI) are observed before minor decreases in average chain length (ACL), leading to behavior in CPI-ACL space that is fundamentally different from identical systems free of clay minerals. Furthermore, distinct distributions of secondary n-alkanes between the clay mineral systems indicate differential mechanisms and sites of precursor degradation. In the presence of clay minerals, δ13Cn-alkane of immature bituminous organics initially decreases with increasing thermal maturation, opposite most data reported from experimental alteration of relatively mature sedimentary matter. This shift is similarly driven by secondary production of n-alkanes, arising from kinetic fractionation associated with preferential cleavage of 12C12C bonds from parent molecules – the kinetics of this process vary per clay mineralogy but are controlled by the same process. Conversely, we see an enrichment in δDn-alkane with increasing maturation, indicating control of the two isotopic systems by separate mechanisms. Moreover, kaolinite and montmorillonite exhibit differential control on δDn-alkane, leading to separate domains in dual isotopic space. Overall, the changes in δ13Cn-alkane and δDn-alkane are relatively small over the timescales studied, even at temperatures as high as 300 °C (less than ∼0.6–1‰ for δ13Cn-alkane and ∼8–15‰ for δDn-alkane). Considering these factors on a geological timescale, variations in clay mineralogy through a buried sedimentary sequence may impart matrix effects on molecular and isotopic signatures that can bias interpretation of paleoenvironmental conditions.

Interactions between arsenic migration and CH4 emission in a soil bioelectrochemical system under the effect of zero-valent iron

Dissimilatory soil arsenic (As) reduction and release are driven by microbial extracellular electron transfer (EET), while reverse EET mediates soil methane (CH4) emission. Nevertheless, the detailed biogeochemical mechanisms underlying the tight links between soil As migration and methanogenesis are unclear. This study used a bioelectrochemical-based system (BES) to explore the potential effects of zero-valent iron (ZVI) addition on “As migration–CH4 emission” interactions from chemical and microbiological perspectives. Voltage and ZVI amendment experiments showed that dissolved As was efficiently immobilized with increased CH4 production in the soil BES, As release and CH4 production exhibited a high negative exponential correlation, and reductive As dissolution could be entirely inhibited in the methanogenic stage. Gene quantification and bacterial community analysis showed that in contrast to applied voltage, ZVI changed the spatial heterogeneity of the distribution of electroactive microorganisms in the BES, significantly decreasing the relative abundance of arrA and dissimilatory As/Fe-reducing bacteria (e.g., Geobacter) while increasing the abundance of aceticlastic methanogens (Methanosaeta), which then dominated CH4 production and As immobilization after ZVI incorporation. In addition to biogeochemical activities, coprecipitation with ferric (iron) contributed 77–93% dissolved As removal under ZVI addition. This study will enhance our knowledge of the processes and microorganisms controlling soil As migration and CH4 emission.

Effect of labile and recalcitrant carbon on heterotrophic nitrification in a subtropical forest soil

AbstractCarbon (C) is an important factor controlling heterotrophic nitrification in soil, but the effect of individual C components (e.g., labile and recalcitrant C) is largely unclear. We carried out a C amendment experiment in which either labile C (glucose) or a recalcitrant C (cellulose and biochar) was added to a subtropical forest soil. A 15N‐, 13C‐tracing and MiSeq sequencing study was performed to investigate soil gross heterotrophic nitrification rates, carbon utilization for soil respiration and microbial biomass production and microbial composition, respectively. After 2 days, results showed a significant increase of gross heterotrophic nitrification rate in glucose (GLU) (on average 3.34 mg N kg−1 day−1), cellulose (CEL) (on average 0.21 mg N kg−1 day−1) and biochar (BIO) (on average 0.13 mg N kg−1 day−1) amendment in comparison with the unamended soil (CK) (on average 0.01 mg N kg−1 day−1; p < 0.05). The contribution of heterotrophic nitrification to total soil nitrification was significantly larger in GLU (average 85.86%), CEL (average 98.52%) and BIO (average 81.25%) treatments compared with CK (average 33.33%; p < 0.01). After 2‐month amendment, the gross rates remarkably decreased in GLU (average 0.02 mg N kg−1 day−1), and the contribution to total nitrification (average 8.73%) were significantly lower than that in CK (p < 0.05). A decrease in the proportion of heterotrophic nitrification to total nitrification in soil was also observed in CEL (average 38.40%) and BIO (6.74%) treatments. Nevertheless, BIO amendment (compared to CK, GLU and CEL) showed the highest gross heterotrophic nitrification rate, accompanied by a notably higher abundance of specific heterotrophic nitrifiers, i.e. Trichoderma, Aspergillus and Penicillium. These results point to a stimulatory effect of C addition on soil heterotrophic nitrification in the short term, while the stimulatory impact of C amendment diminishes with the decline in easily available C. In addition, a shift of the microbial composition in the long term can possibly be sustained for longer if additional recalcitrant C is available to heterotrophic nitrifiers. The dynamic response of heterotrophic nitrification to labile and recalcitrant C in this study offered an explanation for the positive effect of plantation and plant root exudation on the process.

Compositional changes and ecological characteristics of earthworm mucus under different electrical stimuli

Earthworm mucus is rich in nutrients that can initiate the mineralization and humification of organic matter and is of great importance for contaminated soil remediation and sludge reutilization. In this study, six voltage and current combinations were utilized to promote earthworm mucus production (5 V and 6 V at 10, 20 and 30 mA, respectively), to explore the compositional changes of the mucus produced under different electrical stimuli, and to propose the best electrical stimulation group and mucus fraction applicable to soil heavy metal pollution remediation and sludge reutilization. The results showed that the mucus produced by the six electrical stimuli was mainly composed of proteins, amino acids, carbohydrates, fatty acids, and polysaccharides, with small amounts of alcohol, phenol, and ester organic substances. Under different electrical stimuli, each component changed significantly (P < 0.05). pH and conductivity were higher at 6 V 20 mA, total nitrogen and phosphorus contents reached their maximum at 5 V 30 mA, and total potassium at 6 V 10 mA. Protein, amino acids, and carbohydrates were most abundant in the mucus produced at 5 V 10 mA, while trace metal elements reached their lowest values at 5 V 10 mA. Finally, based on principal component analysis and combined with previous studies, it was concluded that the mucus produced at 5 V 10 mA was weakly alkaline, high in amino acids and nutrients and low in trace metal elements, and most suitable for sludge and straw composting experiments, soil remediation and amendment experiments.

Sufficient potassium improves inorganic phosphate-limited photosynthesis in Brassica napus by enhancing metabolic P fractions and Rubisco activity.

Crop photosynthesis (A) and productivity are often limited by a combination of nutrient stresses, such that changes in the availability of one nutrient may affect that of another nutrient, in turn influencing A. In this study, we examined the synergistic effects of phosphorus (P) and potassium (K) on leaf A in a nutrient amendment experiment, in which P and K were added individually or in combination to Brassica napus grown under P and K co-limitation. The data revealed that P addition gradually removed the dominant limiting factor (i.e., P) and improved leaf A. Strikingly, the addition of K synergistically improved gross P uptake, mainly by boosting plant growth, and compensated the physiological demand for P by prioritising investment in metabolic P pools (P-containing metabolites and inorganic phosphate, Pi). The enlarged pool of metabolically active P was partially associated with upregulation of Pi regeneration through release from triose phosphates rather than replacement of P-containing lipids. This process mitigated P restrictions on A by maintaining the ATP/NADPH and NADPH/NADP+ ratios and increasing the content and activity of Rubisco. Our findings demonstrate that sufficient K increased Pi-limited A by enhancing metabolic P fractions and Rubisco activity. Thus, ionic synergism may be exploited to mitigate nutrient-limiting factors to improve crop productivity.

Diatom nutrient requirements change with lake nutrient limitation and enrichment in New Zealand dune lakes

ABSTRACT Nutrients are important determinants of diatom growth in lakes, and diatoms are considered reliable indicators of changing lake nutrient concentrations and eutrophication. However, diatom ecologies are not static, nor are they linked to single environmental variables, leading to imprecise diatom nutrient inferences. Anthropogenic nutrient enrichment is the principal issue facing New Zealand’s lakes. Thus, knowledge of diatom responses to nutrients in New Zealand lakes will be important for understanding contemporary and past changes in nutrient availability. Using a nutrient amendment experiment and diatom communities from dune lakes, here we show that the response of specific diatom species is not universal among lakes and is partly determined by lake nutrient concentrations and limitation status. The response of focal diatom species to nutrient additions differed from previously reported nutrient requirements, and did not align with published, assigned trophic statuses. This study highlights that the response of diatoms to nutrient enrichment is context-dependent, and that intraspecific generalisations of diatom ecologies between geographic locations or through time should be made with caution. To apply diatoms to making nutrient inferences, more work focusing on how physiochemical and biological factors influence diatom nutrient requirements is required.

Remediation of Pb-contaminated soil using modified bauxite refinery residue

This study examines amendment of Pb-contaminated soil with modified bauxite refinery residue (MBRR) to decrease soil Pb mobility and bioaccessibility. Amendment experiments were conducted using four soils contaminated with Pb from various sources, including smelting, shooting-range activities and Pb-based paint waste. Lead L3-edge X-ray absorption spectroscopy (XAS) indicated that Pb speciation in these soils was a mixture of Pb sorbed to Fe (hydr)oxide and clay minerals, along with Pb bound to organic matter. Amendment with MBRR decreased water-soluble Pb and/or Toxicity Characteristic Leachate Procedure (TCLP) Pb concentrations. Lead L3-edge XAS and X-ray diffraction (XRD) indicated that Pb retention by MBRR occurred via sorption to Fe- and Al-(hydr)oxides at low Pb loadings, in addition to formation of hydrocerussite (Pb3(CO3)2(OH)2) at high loadings. Soil amendment with MBRR had relatively little effect on gastric-phase Pb bioaccessibility; as quantified via the Solubility/Bioavailability Research Consortium, SBRC, in vitro assay. In contrast, amendment with MBRR caused substantial decreases in relative intestinal-phase Pb bioaccessibility (Rel-SBRC-I) due to increased Pb sorption by MBRR’s Fe- and Al-hydr(oxide) minerals as simulated GI tract conditions shifted from the gastric- to the intestinal-phase. These decreases in Rel-SBRC-I point to the potential efficacy of using amendment with MBRR to decrease soil Pb bioavailability.

Temperature, redox, and amendments alter wetland soil inorganic phosphorus retention dynamics in a Laurentian Great Lakes priority watershed

Surface water phosphorus loading must be reduced to improve water quality and decrease harmful algal blooms. Many wetlands have a natural capacity to retain inorganic reactive PO43− via soil sorption. However, soil PO43− retention capacity is finite and may be limited by soil legacy phosphorus effects in agricultural and urban areas. This study evaluated soil PO43− retention in soils from a wetland constructed on former agricultural land in the Lake Erie, Maumee River watershed targeted for nutrient load reduction. Soil PO43− sorption isotherms were evaluated under cool (10 °C), warm (22°), aerobic, and anaerobic treatments to determine changes in PO43− retention due to environmental conditions and estimate seasonal changes in PO43− sorption. The soils displayed a strong capacity for PO43− retention by sorption. However, results indicate that cooler temperatures and anaerobic conditions decreased PO43− sorption and lowered retention rates at PO43− concentrations observed in the region. Soil amendment experiments investigated opportunities to increase PO43− retention because many soils display elevated phosphorus concentrations due to historic land use, limiting their ability to adsorb additional PO43−. Amendments increased PO43− retention capacity compared to unamended soils in the presence of high PO43− concentrations, suggesting soil PO43− retention can be improved in areas where natural storage capacity has been exhausted. Results from this study can inform natural resources managers in the Laurentian Great Lakes and elsewhere when identifying potential nutrient reduction wetland locations and assist with developing operational guidelines to optimize PO43− retention and water quality improvements using wetlands.

Effect of Long-Term Organic Amendment Application on the Vertical Distribution of Nutrients in a Vertisol

Soil nutrients in deep soils are important for nutrient cycling and plant growth. Organic amendments have been widely used for enhancing soil health and crop yield. However, little is known about the effects of organic amendments on the vertical distributions of soil nutrients. Based on a 32-year long-term organic amendment experiment, the objective of this study was to evaluate changes in the vertical distribution of nutrients in a soybean–wheat system Vertisol. The results showed that NPK with manure or straw application significantly increased soil organic carbon (SOC), total N, total P, alkali-hydrolyzable N, available P and available K above the 40 cm soil layer. Variations in soil micronutrients primarily occurred above the 20 cm soil layer, and the highest contents were observed for NPKWS and NPKPM, respectively. Nevertheless, large amounts of NO3−−N contents accumulated in the 120–200 cm depth with manure but not straw application, indicating a high potential risk of nitrate leaching in manure treatments. These findings suggested that the application of organic amendment (manure or straw) could be recommendable for improving soil nutrients along the soil profile. Straw incorporation could be used as an alternative option for sustainable agriculture in regions with inadequate manure resources or severe nitrate leaching.

Changes in Microbiome Activity and Sporadic Viral Infection Help Explain Observed Variability in Microcosm Studies.

The environmental conditions experienced by microbial communities are rarely fully simulated in the laboratory. Researchers use experimental containers (“bottles”), where natural samples can be manipulated and evaluated. However, container-based methods are subject to “bottle effects”: changes that occur when enclosing the plankton community that are often times unexplained by standard measures like pigment and nutrient concentrations. We noted variability in a short-term, nutrient amendment experiment during a 2019 Lake Erie, Microcystis spp. bloom. We observed changes in heterotrophic bacteria activity (transcription) on a time-frame consistent with a response to experimental changes in nutrient availability, demonstrating how the often overlooked microbiome of cyanobacterial blooms can be altered. Samples processed at the time of collection (T0) contained abundant transcripts from Bacteroidetes, which reduced in abundance during incubation in all bottles, including controls. Significant biological variability in the expression of Microcystis-infecting phage was observed between replicates, with phosphate-amended treatments showing a 10-fold variation. The expression patterns of Microcystis-infecting phage were significantly correlated with ∼35% of Microcystis-specific functional genes and ∼45% of the cellular-metabolites measured across the entire microbial community, suggesting phage activity not only influenced Microcystis dynamics, but the biochemistry of the microbiome. Our observations demonstrate how natural heterogeneity among replicates can be harnessed to provide further insight on virus and host ecology.

Effects of Added Humic Substances and Nutrients on Photochemical Degradation of Dissolved Organic Matter in a Mesocosm Amendment Experiment in the Gulf of Finland, Baltic Sea.

Humic substances, a component of terrestrial dissolved organic matter (tDOM), contribute to dissolved organic matter (DOM) and chromophoric DOM (CDOM) in coastal waters, and have significant impacts on biogeochemistry. There are concerns in recent years over browning effects in surface waters due to increasing tDOM inputs, and their negative impacts on aquatic ecosystems, but relatively little work has been published on estuaries and coastal waters. Photodegradation could be a significant sink for tDOM in coastal environments, but the rates and efficiencies are poorly constrained. We conducted large-scale DOM photodegradation experiments in mesocosms amended with humic substances and nutrients in the Gulf of Finland to investigate the potential of photochemistry to remove added tDOM and the interactions of DOM photochemistry with eutrophication. The added tDOM was photodegraded rapidly, as CDOM absorption decreased and spectral slopes increased with increasing photons absorbed in laboratory experiments. The insitu DOM optical properties became similar among the control, humic- and humic+nutrients-amended mesocosm samples toward the end of the amendment experiment, indicating degradation of the excess CDOM/DOM through processes including photodegradation. Nutrient additions did not significantly influence the effects of added humic substances on CDOM optical property changes, but induced changes in DOM removal.

Investigating potential hydrological ecosystem services in urban gardens through soil amendment experiments and hydrologic models

Among the ecosystem services provided by urban greenspace are the retention and infiltration of stormwater, which decreases urban flooding, and enhanced evapotranspiration, which helps mitigate urban heat island effects. Some types of urban greenspace, such as rain gardens and green roofs, are intentionally designed to enhance these hydrologic functions. Urban gardens, while primarily designed for food production and aesthetic benefits, may have similar hydrologic function, due to high levels of soil organic matter that promote infiltration and water holding capacity. We quantified leachate and soil moisture from experimental urban garden plots receiving various soil amendments (high and low levels of manure and municipal compost, synthetic fertilizer, and no inputs) over three years. Soil moisture varied across treatments, with highest mean levels observed in plots receiving manure compost, and lowest in plots receiving synthetic fertilizer. Soil amendment treatments explained little of the variation in weekly leachate volume, but among treatments, high municipal compost and synthetic fertilizer had lowest leachate volumes, and high and low manure compost had slightly higher mean leachate volumes. We used these data to parameterize a simple mass balance hydrologic model, focusing on high input municipal compost and no compost garden plots, as well as reference turfgrass plots. We ran the model for three growing seasons under ambient precipitation and three elevated precipitation scenarios. Garden plots received 12–16% greater total water inputs compared to turfgrass plots because of irrigation, but leachate totals were 20–30% lower for garden plots across climate scenarios, due to elevated evapotranspiration, which was 50–60% higher in garden plots. Within each climate scenario, difference between garden plots which received high levels of municipal compost and garden plots which received no additional compost were small relative to differences between garden plots and turfgrass. Taken together, these results indicate that garden soil amendments can influence water retention, and the high-water retention, infiltration, and evapotranspiration potential of garden soils relative to turfgrass indicates that hydrologic ecosystem services may be an underappreciated benefit of urban gardens.