Increased Nitrogen Mineralization Research Articles

Effects of biochar on the transformation and utilization of nitrogen fertilizer in the black soil region of Northeast China

Nitrogen (N) fertilizer is often used in production practice to effectively maintain crop productivity; however, low nitrogen use efficiency (Nue) has always been a problem. Specifically, the transformation and utilization of nitrogen fertilizer by biochar and the driving mechanisms remain unclear. We used four biochar application rates (0, 3750, 7500, and 11,250 kg·ha−1) and analyzed the effects of biochar on nitrogen fertilizer utilization, residue, and loss over three years using 15N isotope tracer technology. The results showed that (1) biochar improved the nitrogen use efficiency of maize plants, reduced total nitrogen loss, and increased the maize yield. Compared to the control treatment in the same year, the application of 7500 kg·ha−1 biochar increased the nitrogen use efficiency by 24.27 %, 27.77 %, and 35.82 %, and the yield increased by 21.1 %, 26.7 %, and 24.5 %, respectively. (2) Biochar increased the proportion of mineral nitrogen supplied by fertilizer in the mineral nitrogen pool. The application of 7500 kg·ha−1 biochar increased mineral nitrogen by 3.05 %, 3.22 %, and 3.8 %, respectively, compared to the control treatments in the same year. Biochar promoted the transformation of nitrogen in the 0–40 cm soil layer to three different soil nitrogen pools, especially the organic nitrogen pool. (3) Biochar significantly improved the soil bacterial community and increased the abundances of N transformation functional genes. The redundancy analysis (RDA) showed that the gdhA mineralization gene was the driving factor of nitrogen fertilizer transformation, contributing 43.6 % of the variance. In summary, the application of 7500 kg·ha−1 of biochar for two consecutive years was conducive to maintaining farmland soil fertility, while its use would not be recommended for more than three consecutive years.

Hot moment of N2O emissions in seasonally frozen peatlands.

Since the start of the Anthropocene, northern seasonally frozen peatlands have been warming at a rate of 0.6 °C per decade, twice that of the Earth's average rate, thereby triggering increased nitrogen mineralization with subsequent potentially large losses of nitrous oxide (N2O) to the atmosphere. Here we provide evidence that seasonally frozen peatlands are important N2O emission sources in the Northern Hemisphere and the thawing periods are the hot moment of annual N2O emissions. The flux during the hot moment of thawing in spring was 1.20 ± 0.82 mg N2O m-2 d-1, significantly higher than that during the other periods (freezing, -0.12 ± 0.02 mg N2O m-2 d-1; frozen, 0.04 ± 0.04 mg N2O m-2 d-1; thawed, 0.09 ± 0.01 mg N2O m-2 d-1) or observed for other ecosystems at the same latitude in previous studies. The observed emission flux is even higher than those of tropical forests, the World's largest natural terrestrial N2O source. Furthermore, based on soil incubation with 15N and 18O isotope tracing and differential inhibitors, heterotrophic bacterial and fungal denitrification was revealed as the main source of N2O in peatland profiles (0-200 cm). Metagenomic, metatranscriptomic, and qPCR assays further revealed that seasonally frozen peatlands have high N2O emission potential, but thawing significantly stimulates expression of genes encoding N2O-producing protein complexes (hydroxylamine dehydrogenase (hao) and nitric oxide reductase (nor)), resulting in high N2O emissions during spring. This hot moment converts seasonally frozen peatlands into an important N2O emission source when it is otherwise a sink. Extrapolation of our data to all northern peatland areas reveals that the hot moment emissions could amount to approximately 0.17 Tg of N2O yr-1. However, these N2O emissions are still not routinely included in Earth system models and global IPCC assessments.

The Effect of Mealworm Frass on the Chemical and Microbiological Properties of Horticultural Peat in an Incubation Experiment.

Insect farming is growing in popularity, and in addition to insect meal, it generates waste products such as exuviae and frass, which can be recycled in agriculture. The aim of this incubation experiment was to evaluate the effect of Tenebrio molitor L. frass on selected chemical and biological properties of deacidified peat, which is widely used in horticulture. The optimal rate of frass fertilizer in peat for growing vegetables and ornamental plants was determined, with special emphasis on mineral nitrogen levels. Peat was fertilized with five nitrogen rates, 0, 50, 100, 200, and 400 mg dm-3, and supplied with frass or urea. The study demonstrated that frass can be used as organic fertilizer. An increase in the nitrogen rate significantly increased mineral nitrogen content and electrical conductivity and decreased Ca content in peat. Both frass and urea increased the ammonification rate at the beginning of incubation and the nitrification rate from the second week of the experiment. Higher frass rates (5 and 10 g dm-3) increased the content of plant-available nutrients (nitrogen, phosphorus, magnesium, potassium, and sodium) in peat as well as the abundance of microorganisms supporting organic matter mineralization. Unlike frass, urea increased the counts of nitrogen-fixing bacteria in peat.

Nitrogen dynamics and carbon sequestration in soil following application of digestates from one- and two-step anaerobic digestion

Anaerobic digestion (AD) is an important tool for reducing greenhouse gas emissions from agricultural production. A prolonged retention time by adding an extra anaerobic digestion step can be utilized to further degrade the digestates, contributing to increased nitrogen mineralisation and reducing decomposable organic matter. These modifications could influence the potential N fertiliser value of the digestate and soil carbon sequestration after field application. This study investigated the effects of prolonging retention time by implementing an additional anaerobic digestion step on carbon and nitrogen dynamics in the soil and soil carbon sequestration. Two digestates obtained from two biogas plants operating at contrasting hydraulic retention times, with and without an additional digestion step, were applied to a loamy sand soil. N mineralisation dynamics were measured during 80 days and C mineralisation during 212 days. After 80 days of incubation, the net inorganic N release from digestates obtained from a secondary AD step increased by 9–17 % (% of the N input) compared to corresponding digestates obtained from a primary AD step. A kinetic four-pool carbon model was used to fit C mineralisation data to estimate carbon sequestration in the soil. After 212 days of incubation, the net C mineralisation was highest in undigested solid biomass (68 %) and digestates obtained from the primary AD step (59–65 %). The model predicted that 26–54 % of C applied is sequestered in the soil in the long-term. The long-term soil C retention related to the C present before digestion was similar for one- and two-step AD at 12–16 %. We conclude that optimizing the anaerobic digestion configurations by including a secondary AD step could potentially replace more mineral N fertiliser due to an improved N fertiliser value of the resultant digestate without affecting carbon sequestration negatively.

Chemical attributes of percolate from degraded sand soil irrigated with treated industrial wastewater

AbstractApplying treated industrial tannery wastewater to soil can be an important practice for its disposal and aid in the recovery of degraded soils. This study aimed to evaluate the chemical attributes of the percolate of degraded soil irrigated with treated industrial wastewater (TIW). The experiment was carried out in a greenhouse, using soil‐filled polyvinyl chloride columns. The treatments consisted of TIW at increasing rates (0%, 25%, 50%, 75%, and 100%) and organomineral and organic fertilization. Irrigation with treated wastewater on soil degraded by clay extraction increased mineral nitrogen, biochemical oxygen demand (BOD), electrical conductivity, sodium, and sodium adsorption ratio in percolate under maize cultivation. Irrigation with TIW increased mineral nitrogen and BOD in the percolate by 34% and 25%, respectively, relative to the control treatment (plain water). Concentrations of total nitrogen and BOD must be less than 100 and 65 mg L–1, respectively, to minimize the potential contamination of water bodies. The degraded soil and maize (Zea mays L.) cultivation reduced the total mineral nitrogen and BOD in the percolate by 96% and 84%, respectively. The sodium adsorption ratio had a high removal efficiency of 79% of the percolate when compared to TIW.

Prediction of the effects of management practices on discharge and mineral nitrogen yield from paddy fields under future climate using APEX-paddy model

This study is to evaluate the BMPs in the reduction of surface discharge and mineral nitrogen yield from paddy cultivation for three future time slices (e.g., the 2010s, 2040s, and 2070s) using APEX-Paddy (Agricultural Policy/Environmental eXtender-Paddy) model. The model was calibrated and validated for surface discharge and mineral nitrogen yield using 3-year monitoring data (2013–2015) from the conventional paddy management field (CMP-1). For surface discharge and mineral nitrogen yield estimates, the future projections of 29 GCMs (General Circulation Model) were bias-corrected and applied to the calibrated APEX-Paddy model. We investigated five specific management strategies related to paddy drainage outlet regulation and new fertilization methods, as the BMPs minimize the mineral nitrogen yield and surface discharges due to climate change. The modeling results indicated that the effects of BMPs would vary by future climate scenarios (i.e., RCP4.5, RCP8.5) and periods (i.e., the 2010s, 2040s, 2070s). It was generally expected that the surface discharge and mineral nitrogen yields would increase in the future. The combination of raising drainage outlets and soil test-based fertilization (DOR-STF) showed a substantial reduction in surface discharge in both scenarios (RCP4.5 and 8.5); the highest reduction rate was observed in the 2010s and was estimated at 21.9 % under RCP4.5. Soil test-based fertilization (STF) showed a substantial reduction in mineral nitrogen yield by 31.0 and 28.3 % during the 2010s under RCP8.5 and RCP4.5, respectively followed by DOR-STF, as compared to conventional management practice (CMP-1). However, the combination of drainage outlet raising, and fertilizer application before outlet weir installation (DOR-FABWI) management resulted in increased mineral nitrogen yield of up to 31.0 % under RCP4.5 and 36.7 % under RCP8.5. The study findings indicate that climate change will increase exports of mineral nitrogen from paddy fields. Nevertheless, appropriate BMPs may play a vital role in reducing the mineral nitrogen yields for the production of paddy rice in future climates, and these effects may vary according to future climate conditions.

Above- and Below-ground Cascading Effects of Wild Ungulates in Temperate Forests

Ungulates have become abundant in many temperate forests, shifting tree species composition by browsing and altering soil physical conditions by trampling. Whether these effects cascade down to other trophic levels and ecosystem processes is poorly understood. Here, we assess the paths through which ungulates have cascading effects on other trophic levels (regeneration, litter, invertebrates, rodents and organic matter decomposition). We compared ungulate effects by comparing 15 response variables related to different trophic levels between paired fenced and unfenced plots in twelve temperate forest sites across the Netherlands, and used pathway analysis model to identify the (in)direct pathways through which ungulates have influenced these variables. We found that plots with ungulates (that is, unfenced) compared to plots without (that is, fenced) had lower litter depth, sapling diversity, sapling density, rodent activity, macro-invertebrate biomass, decomposition rate of tea bags, pine and birch litter and higher soil compaction. These findings were used in a path analysis to establish potential causal relationships, which showed that ungulate presence: decreased sapling density, which indirectly decreased rodent activity; decreased litter depth, which indirectly reduced invertebrate diversity; increased soil compaction, which also decreased invertebrate diversity. Soil pH decreased invertebrate biomass, which also increased nitrogen mineralization. Yet, we did not find cascading effects of ungulates on decomposition rates. Importantly, an increase in ungulate abundance strengthens the cascading effects in this system. Our results suggest that ungulates can trigger cascading effects on lower trophic levels, yet decomposition and mineralization rates are resilient to ungulate browsing and trampling. Therefore, temperate forests conservation could benefit by limiting ungulate abundance.

Effect of ash application on the decomposer food web and N mineralization in a Norway spruce plantation.

In the face of global climate change there is an increasing demand for biofuel, which exerts pressure on production and thus management of biofuel plantations. The intensification of whole-tree harvest from biofuel plantations increases export of nutrients. Returning ash from biofuel combustion to the forest plantations can amend the soil nutrient status and thus facilitate sustainable forest management. However, ash affects the forest floor decomposer food web, potentially changing organic matter turnover, carbon sequestration and nitrogen availability. Our aim was to examine the response of decomposer organisms, food web structure and nitrogen mineralization function after ash application. In a coniferous forest plantation amended with 0, 3, 4.5 or 6t ash ha-1, we sampled in several depths of the forest floor for key organisms of the decomposer food web (fungal biomass, 0-12cm; bacteria, protozoa, nematodes and enchytraeids, 0-3cm and 3-6cm; microarthropods and earthworms, 0-5cm), 2, 14 and 26months after ash application. We used structural equation modelling (SEM) to detangle the direct and indirect effects of ash application on organisms in the decomposer food web and on nitrogen availability. We found that ash increased the abundance of bacteria and protozoa, as well as the inorganic nitrogen pool at 0-3cm depth, whereas the effect of ash was negligible at 3-6cm depth. Earthworm abundance increased, whereas enchytraeid abundance decreased 2years after ash application. The structural equation modelling showed that ash application stimulated the bacterial feeding pathway and increased nitrogen mineralization. Contrary, ash had a negative effect on fungal biomass at the first sampling, however, this effect subdued over time. Our results suggest that as the soil decomposer food web is resilient to ash application, this is a viable option for sustainable management of biofuel plantations.

N2O, CH4, and CO2 Emissions from Continuous Flooded, Wet, and Flooded Converted to Wet Soils

Fluctuations in soil water content, either anthropogenic or natural, can remarkably influence the C and N pools in arable lands. The dynamics of C and N are directly related in regulation of greenhouse gases (GHGs) emissions. The present study was conducted to explore the effects of water regimes (continuous flooding, continuous wet, and flooding converted to wet) on soil GHGs emissions. Nitrous oxide (N2O) emissions from continuous flooding treatment were lower as compared with the continuous wet soil, but significantly (p ≤ 0.01) enlarged in the flooding converted to wet soil treatment through increased mineral nitrogen. Methane (CH4) emissions were significantly (p ≤ 0.01) higher in the continuous flooding when compared with the wet soil conditions but substantially decreased in the flooding converted to wet soil treatment. Carbon dioxide (CO2) emissions were significantly (p ≤ 0.01) larger in the flooding converted to wet soil treatment as compared with the continuous flooding and wet soil treatments owing to increased mineralization and C contents. The results suggest that converting flooding to wet soil can substantially triggers the C and N pools and thus influences the GHGs emissions from soil.

Biochar addition persistently increased soil fertility and yields in maize-soybean rotations over 10 years in sub-humid regions of Kenya

Application of biochar has been shown to increase soil fertility and enable soil carbon sequestration, indicating potential for agricultural and environmental benefits from using locally produced biochar on African smallholder farms. However, previous studies have been rather short-term and little is known about the longer-term effects of biochar application on crop yields. Biochar contains ash, but the potential liming effect and nutrient release from ash may be short-lasting. To investigate long-term effects, we set up a series of field trials replicated at three sites in Kenya in 2006. The trials are still on-going and are possibly the longest biochar trials in sub-Saharan Africa. Here, we report effects on crop yield and soil properties over 10 years after applying biochar, produced mainly from Acacia spp., at a rate of 50 + 50 Mg ha−1 during the first two seasons. Maize (Zea mays) and soybean (Glycine max) were grown in rotation, with or without inorganic fertiliser, and crop yield was monitored. For comparison of soil properties, additional plots were kept in bare fallow. Biochar addition slightly increased soil porosity, pH, plant-available phosphorus and soil water-holding capacity. Crop yield responded positively to biochar at all sites and yield responses were similar with and without mineral fertiliser, i.e., the effects of biochar and mineral fertiliser were additive. The seasonal yield increase due to biochar application was in average around 1.2 Mg ha−1 for maize and 0.4 Mg for soybean, independently of fertilisation, over seasons and sites. Application of mineral fertiliser to maize increased maize yield by 1.6 Mg ha−1 and the subsequent, unfertilized soybean yield by 0.6 Mg ha−1, illustrating a carry-over effect. Most importantly, the effect on maize and soybean yield of adding biochar to soil persisted over the whole 10-year period. Analysis of the carbon (C) balance in topsoil indicated that about 40% of biochar C was apparently lost through mineralization, erosion or vertical translocation. Moreover, changes in soil carbon/nitrogen ratios indicated that biochar application increased nitrogen mineralization from native soil organic matter.

Successive impact of the nitrogen fertilization of basket willow (Salix viminalis L.) on the canopy architecture in 9th and 10th year of regrowth of shoots

The purpose of the present paper was an assessment of the successive impact of fertilization with nitrogen on the regrowth dynamics of the shoots of 10 genotypes (three clones and seven varieties) of basket willow (Salix viminalis L.) in the 9th and 10th year of cultivation. In 2008- 2015, mineral nitrogen fertilization was applied in the whole experiment in four doses. The measurements of height and thickness of willow shoots, of the quantity of live and dead shoots in the snag and live and dead snags on the plot were performed in the experiment realized in 2016-2017. Biometric measurements showed that increased mineral nitrogen fertilization in the year of its application intensified shoots growth in height and thickness, yet in the successive impact, in the 9th and 10th year of willow vegetation weakening of shoot regrowth in height and thickness is observed, and the number of live shoots in the snag and live snags on the plot have reduced. In particular, negative successive impact of the nitrogen fertilization on the willow canopy architecture was demonstrated on the objects that were mowed twice in the first 4-year rotation and on the varieties that do not tolerate this treatment.

CHANGES IN THE CHEMICAL PROPERTIES OF GROUNDWATER IN SOILS UNDER MEAT AND BONE MEAL, NATURAL AND MINERAL FERTILIZATION REGIMES

The objective of this study was to evaluate the effect of different doses of meat and bone meal (MBM) applied as organic fertilizer on the quality of groundwater in arable fields. The effect of meat and bone meal fertilizer applied at rates of 1.0, 1.5, 2.0 and 2.5 t ha-1 was compared with no fertilization, mineral fertilization, and manure fertilization (10 t ha-1). Groundwater samples for chemical analyses were collected every month during the growing season using piezometers installed in the experimental plots. Groundwater samples were analyzed to determine the concentrations of mineral nitrogen NO3-, NO2-and NH4+, total phosphorus, phosphate and potassium. The application of meat and bone meal at the rate of 1.0 to 2.0 t ha-1 with supplemental potassium fertilization does not lead to higher contamination of groundwater than mineral (NPK) fertilization. The use of meat and bone meal fertilizers at a rate of 2.5 t ha-1 significantly increased mineral nitrogen and phosphate concentrations in groundwater samples. The results of our analyses suggest that meat and bone meal can be successfully used at a rate of 2.0 t ha-1 to fertilize crops grown in a five-year crop rotation sequence on lessive soils. The application of higher doses of meat and bone meal intensifies the flow of biogenic elements into groundwater.

Reduced turning frequency and delayed poultry manure addition reduces N loss from sugarcane compost

Composting is an effective method to recycle biodegradable waste as soil amendment in smallholder farming systems. Although all essential plant nutrients are found in compost, a substantial amount of nitrogen is lost during composting. This study therefore investigated the potential of reducing N losses by (i) delaying the addition of nitrogen-rich substrates (i.e. poultry manure), and (ii) reducing the turning frequency during composting. Furthermore, we tested the effect of compost application method on nitrogen mineralization. Sugarcane-waste was composted for 54days with addition of poultry manure at the beginning (i.e. early addition) or after 21days of composting (delayed addition). The compost pile was then turned either every three or nine days. Composts were subsequently applied to soil as (i) homogeneously mixed, or (ii) stratified, and incubated for 28days to test the effect of compost application on nitrogen mineralization. The results showed that delayed addition of poultry manure reduced total nitrogen loss by 33% and increased mineral nitrogen content by >200% compared with early addition. Similarly, less frequent turning reduced total N loss by 12% compared with frequent turning. Stratified placement of compost did not enhance N mineralization compared to a homogeneous mixing. Our results suggested that simple modifications of the composting process (i.e. delayed addition and/or turning frequency) could significantly reduce N losses and improve the plant-nutritional value of compost.

Phytoplankton Response to Saharan Dust Depositions in the Eastern Mediterranean Sea: A Mesocosm Study

The response of phytoplankton populations from surface ultra-oligotrophic waters of the Eastern Mediterranean Sea to Saharan dust additions was studied during a 10-day mesocosm experiment in May 2014. A set of triplicate mesocosms entitled ‘Single Addition’ treatment (SA) was amended with Saharan dust once, while another triplicate set entitled ‘Repetitive Addition’ treatment (RA) received the same amount of dust divided into three consecutive daily doses administered within the first three experimental days, both simulating patterns of dust deposition events taking place in the field. In both treatments, dust particles released small amounts of dissolved inorganic nitrogen and phosphorus which stimulated by two-fold both chlorophyll-a concentration and primary production for a time period of six days, as compared to a set of control mesocosms carried out without dust addition. Phytoplankton response was similar in both treatments, regardless of the dust addition pattern, and it evolved through two distinct phases in both cases. The first phase (i.e. 1 to 2 days after initial addition) was characterized by enhancement of picoplankton chlorophyll-normalized production rates as a result of elevated orthophosphate concentrations while the second phase (i.e. 3 to 4 days after initial dust addition), was characterized by elevated chlorophyll-normalized production rates corresponding to larger cells (> 5 μm) as a result of increased mineral nitrogen concentrations. The stimulated primary production of larger cells was not accompanied by a respective increase in carbon biomass suggesting important top-down control. The major phytoplankton taxa detected during the experiment were Synechococcus, Pelagophytes and Prymnesiophytes. Estimations of cellular pigment concentrations and carbon-to-chlorophyll ratios of identified groups and differences between prokaryotic and eukaryotic cells are discussed.

Nitrogen availability and mineralization in Pinus radiata stands fertilized mid-rotation at three contrasting sites

Fertilization of Pinus radiata plantations mid-rotation after thinning can alter soil nitrogen availability. However, the magnitudes and durations of tree and stand growth responses are not well understood across different soils with specific site conditions. Two mid-rotation fertilization trials in Pinus radiata plantations with unexpected sustained growth responses for more than 6 years and volume gains of 25 m3ha-1 and 50 m3 ha-1 in sandy and granitic soil, respectively, and one trial with no response to fertilization were selected to study the monthly dynamics of nitrogen availability and net mineralization using in situ core incubations. After 2 years, the results showed that fertilization increased nitrogen mineralization and availability until 6 years in sandy soils and until 7 years in granitic soil following fertilization. This result explained the sustained stand growth response observed at these sites. When considering the magnitude of the response, large increases in mineralization rates and soil N availability were observed in the granitic soil relative to the sandy soil. Our results suggest that stands with available N-(NH4+ + NO3-) levels less than 2 kg ha-1 during spring and fall months or with N-(NO3-) levels lower than 0.2 kg ha-1 during any month may respond to N fertilization.

Interactions Between Bacteria, Protozoa and Nematodes in Soil

Bacteria, protozoa and nematodes interact closely in soil ecosystems. Protozoa and nematodes eat bacteria (and occasionally each other), while bacteria defend themselves using chemical substances, resistant cell walls, irregular shapes and motility. Protozoa and nematodes are very different types of organisms, and hence apply very different feeding mechanisms; thus many protozoa can pick and choose individual bacterial cells, whereas nematodes ingest bacterial patches more uncritically. Protozoa and nematode are both aquatic organisms whose activity depends on available soil water, but differences in size, motility, resting stages and reproductive strategies mean that the soil physico-chemical environment influences the activity of protozoa and nematodes differently. For example, the relative importance of protozoa compared to nematodes may shift towards protozoa in very clay-rich soils. The interactions between the three organism groups have major ecological consequences such as modification of the bacterial communities and increased nitrogen mineralisation, both of which affect plant growth. Increased nitrogen mineralisation will usually be beneficial for plant growth, whereas the grazing induced changes in the bacterial communities can be both beneficial and detrimental to plants. Selective protozoan grazing can favour plant inhibiting bacteria. This may be a problem in clay rich soils where protozoa have better life conditions than nematodes.

Bacterial diversity amplifies nutrient‐based plant–soil feedbacks

Summary Plants foster diverse assemblages of bacteria in the rhizosphere serving important functions which may result in enhanced plant growth. Microbial diversity is increasingly recognized to shape the functionality of microbial communities. This leads to the assumption that there is a positive relationship between rhizosphere diversity and plant growth. Here we investigate how bacterial diversity affects the mineralization of organic matter and plant nutrient acquisition. We hypothesized that altered bacterial diversity will affect nitrogen mineralization, uptake by plants and ultimately plant growth. We set up a controlled model system with Arabidopsis thaliana colonized by defined assemblages of fluorescent pseudomonads, a well‐characterized plant‐beneficial rhizosphere taxon. The growth substrate contained casein as sole nitrogen source, making the plant nitrogen uptake dependant on breakdown by bacterial enzymes. Bacterial diversity was associated with a higher enzyme activity which increased nitrogen mineralization and enhanced plant growth. The effect of bacterial diversity on plant growth increased with time, pointing to a positive feedback between bacteria and plants: bigger plants associated with species‐rich bacterial communities supported more bacterial growth, which further enhanced the impact of bacteria on plant growth. We demonstrate that plant–soil feedbacks establish rapidly during one single growth season and that bacterial diversity modulates this interaction. Preserving soil microbial diversity therefore may improve positive plant–soil feedbacks and thereby plant growth.

Stoichiometric analysis of nutrient availability (N, P, K) within soils of polygonal tundra

Plant growth in arctic tundra is known to be commonly limited by nitrogen. However, biogeochemical interactions between soil, vegetation and microbial biomass in arctic ecosystems are still insufficiently understood. In this study, we investigated different compartments of the soil-vegetation system of polygonal lowland tundra: bulk soil, inorganic nutrients, microbial biomass and vegetation biomass were analyzed for their contents of carbon, nitrogen, phosphorus and potassium. Samples were taken in August 2011 in the Indigirka lowlands (NE Siberia, Russia) in a detailed grid (4 m × 5 m) in one single ice-wedge polygon. We used a stoichiometric approach, based on the N/P ratios in the vegetation biomass and the investigated soil fractions, to analyze limitation relations in the soil-vegetation system. Plant growth in the investigated polygonal tundra appears to be co-limited by nitrogen and phosphorus or in some cases only limited by nitrogen whereas potassium is not limiting plant growth. However, as the N/P ratios of the microbial biomass in the uppermost soil horizons were more than twice as high as previously reported for arctic ecosystems, nitrogen mineralization and fixation may be limited at present by phosphorus. We found that only 5 % of the total nitrogen is already cycling in the biologically active fractions. On the other hand, up to 40 % of the total phosphorus was found in the biologically active fractions. Thus, there is less potential for increased phosphorus mineralization than for increased nitrogen mineralization in response to climate warming, and strict phosphorus limitation might be possible in the long-term.

Substrate and nutrient limitation of ammonia-oxidizing bacteria and archaea in temperate forest soil

Ammonia-oxidizing microbes control the rate-limiting step of nitrification, a critical ecosystem process, which affects retention and mobility of nitrogen in soil ecosystems. This study investigated substrate (NH4+) and nutrient (K and P) limitation of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in temperate forest soils at Coweeta Hydrologic Laboratory, a long-term ecological research site in western North Carolina, USA. We investigated substrate and nutrient limitation by amending soils with either ammonium or a nutrient solution containing P and K, then assessing the growth of these organisms during in situ soil incubations. We found substantial growth of both AOA and AOB during all incubations including unamended control incubations. Our results demonstrate that substrate availability limits nitrification by AOB and that high levels of substrate addition inhibit the growth of AOA in these soils. We found no evidence for nutrient limitation of AOB, though nutrient addition indirectly stimulated nitrification by AOB through increased nitrogen mineralization. Our data did suggest nutrient limitation by AOA, though it is unclear whether AOA significantly contribute to ammonia oxidation in this system. Furthermore, we show that AOB are responsible for the majority of ammonia oxidation in high substrate, high nutrient conditions.

Functioning of a Shallow-Water Sediment System during Experimental Warming and Nutrient Enrichment

Effects of warming and nutrient enrichment on intact unvegetated shallow-water sediment were investigated for 5 weeks in the autumn under simulated natural field conditions, with a main focus on trophic state and benthic nitrogen cycling. In a flow-through system, sediment was exposed to either seawater at ambient temperature or seawater heated 4°C above ambient, with either natural or nutrient enriched water. Sediment–water fluxes of oxygen and inorganic nutrients, nitrogen mineralization, and denitrification were measured. Warming resulted in an earlier shift to net heterotrophy due to increased community respiration; primary production was not affected by temperature but (slightly) by nutrient enrichment. The heterotrophic state was, however, not further strengthened by warming, but was rather weakened, probably because increased mineralization induced a shortage of labile organic matter. Climate-related warming of seawater during autumn could therefore, in contrast to previous predictions, induce shorter but more intensive heterotrophic periods in shallow-water sediments, followed by longer autotrophic periods. Increased nitrogen mineralization and subsequent effluxes of ammonium during warming suggested a preferential response of organisms driving nitrogen mineralization when compared to sinks of ammonium such as nitrification and algal assimilation. Warming and nutrient enrichment resulted in non-additive effects on nitrogen mineralization and denitrification (synergism), as well as on benthic fluxes of phosphate (antagonism). The mode of interaction appears to be related to the trophic level of the organisms that are the main drivers of the affected processes. Despite the weak response of benthic microalgae to both warming and nutrient enrichment, the assimilation of nitrogen by microalgae was similar in magnitude to rates of nitrogen mineralization. This implies a sustained filter function and retention capacity of nutrients by the sediment.