The influence of 3-years of warming and N-deposition on ecosystem dynamics is small compared to past land use in subalpine meadows

Background and aims

Drivers of ecosystem dynamics that are under human influence range from local, land-management decisions to global processes such as warming temperatures and N deposition. The goal of this study was to understand how multiple, potentially interacting factors influence net primary production, N mineralization, and water and soil CO₂ fluxes.

Methods

Here I report on a three-year experiment that manipulated air temperature using ITEX passive warming cones and N deposition in a mountain meadow ecosystems that were historically grazed or protected from grazing.

Results

The strongest and most consistent effect was due to the legacy of grazing, with previously grazed sites having lower primary production, lower soil respiration rates, lower soil moisture, and lower soil C and N stocks than historically ungrazed sites. Warming increased soil respiration, but the effect was transient, and decreased over the 3-year study. Nitrogen addition increased primary production in the second and third year of the experiment but had no significant effect on soil respiration. The effect of historical grazing on primary production was approximately double the effect of N addition. Temperature and N deposition rarely interacted except for increasing N availability during the warm, wet growing season of 2004.

Conclusions

These findings indicate that the legacies of land use, with their influence on plant community composition and hydrologic processes, are locally more important than short-term step changes in temperature and nutrient availability.     

The effects of snowpack properties and plant strategies on litter decomposition during winter in subalpine meadows

Background and aims

Approximately 50 % of belowground organic carbon is present in the northern permafrost region and due to changes in climate there are concerns that this carbon will be rapidly released to the atmosphere. The release of carbon in arctic soils is thought to be intimately linked to the N cycle through the N cycle’s influence on microbial activity. The majority of new N input into arctic systems occurs through N₂-fixation; therefore, N₂-fixation may be the key driver of greenhouse gases from these ecosystems.

Methods

At Alexandra Fjord lowland, Ellesmere Island, Canada concurrent measurements of N₂-fixation, N mineralization and nitrification rates, dissolved organic soil N (DON) and C, inorganic soil N and surface greenhouse gas fluxes (CO₂, N₂O and CH₄) were taken in two ecosystem types (Wet Sedge Meadow and Dryas Heath) over the 2009 growing season (June-August). Using Structural Equation Modelling we evaluated the hypothesis that CO₂, CH₄ and N₂O flux are linked to N₂-fixation via the N cycle.

Results

The soil N cycle was linked to CO₂ flux in the Dryas Heath ecosystem via DON concentrations, but there was no link between the soil N cycle and CO₂ flux in the Wet Sedge Meadow. Methane flux was also not linked to the soil N cycle, nor surface soil temperature or moisture in either ecosystem. The soil N cycle was closely linked to N₂O emissions but via nitrification in the Wet Sedge Meadow and inorganic N in the Dryas Heath, indicating the important role of nitrification in net N₂O flux from arctic ecosystems.

Conclusions

Our results should be interpreted with caution given the high variability in both the rates of the N cycling processes and greenhouse gas flux found in both ecosystems over the growing season. However, while N₂-fixation and other N cycling processes may play a more limited role in instantaneous CO₂ emissions, these processes clearly play an important role in controlling N₂O emissions.     

Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, China

Aims

A 3-year field experiment (October 2004–October 2007) was conducted to quantify N₂O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

Methods

Static chamber-GC techniques were used to measure soil N₂O fluxes in three treatments (three replicates per treatment): CK (no fertilizer); N150 (300 kg N fertilizer ha^?1 yr^?1 or 150 kg N?ha^?1 per crop); N250 (500 kg N fertilizer ha^?1 yr^?1 kg or 250 kg N?ha^?1 per crop). Nitrate (NO ₃ ^? ) leaching losses were measured at nearby sites using free-drained lysimeters.

Results

The annual N₂O fluxes from the N fertilized treatments were in the range of 1.9 to 6.7 kg N?ha^?1 yr^?1 corresponding to an N₂O emission factor ranging from 0.12 % to 1.06 % (mean value: 0.61 %). The relationship between monthly soil N₂O fluxes and NO ₃ ^- leaching losses can be described by a significant exponential decaying function.

Conclusions

The N₂O emission factor obtained in our study was somewhat lower than the current IPCC default emission factor (1 %). Nitrate leaching, through removal of topsoil NO ₃ ^? , is an underrated regulating factor of soil N₂O fluxes from cropland, especially in the regions where high NO ₃ ^- leaching losses occur.     

Neutron imaging reveals internal plant water dynamics

Background and aims

Knowledge of plant water fluxes is critical for assessing mechanistic processes linked to biogeochemical cycles, yet resolving root water transport dynamics has been a particularly daunting task. Our objectives were to demonstrate the ability to non-invasively monitor individual root functionality and water fluxes within Zea mays L. (maize) and Panicum virgatum L. (switchgrass) seedlings using neutron imaging.

Methods

Seedlings were propagated for 1–3 weeks in aluminum chambers containing sand. Pulses of water or deuterium oxide were then tracked through the root systems by collecting consecutive radiographs during exposure to a cold-neutron source. Water flux was manipulated by cycling on a growth lamp to alter foliar demand for water.

Results

Neutron radiography readily illuminated root structure, root growth, and relative plant and soil water content. After irrigation there was rapid root water uptake from the newly wetted soil, followed by hydraulic redistribution of water through the root system to roots terminating in dry soil. Water flux within individual roots responded differentially to foliar illumination based on supply and demand of water within the root system.

Conclusions

Sub-millimeter scale image resolution revealed timing and magnitudes of root water uptake, redistribution within the roots, and root-shoot hydraulic linkages—relationships not well characterized by other techniques.     

Responses of CO2 efflux from an alpine meadow soil on the Qinghai Tibetan Plateau to multi-form and low-level N addition

Aims

To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai–Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO₂) exchange to N addition.

Methods

Based on a multi-form, low-level N addition experiment, soil CO₂ effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO₂ efflux.

Results

The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH ₄ ⁺ -N pool. N applications significantly increased aboveground biomass and soil CO₂ efflux; moreover, this effect was more significant from NH ₄ ⁺ -N than from NO ₃ ^? -N fertilizer. In addition, the soil CO₂ efflux was mainly driven by soil temperature, followed by aboveground biomass and NH ₄ ⁺ -N pool.

Conclusions

These results suggest that chronic atmospheric N deposition will stimulate soil CO₂ efflux in the alpine meadow on the Qinghai–Tibetan Plateau by increasing available N content and promoting plant growth.     

Soil warming accelerates decomposition of fine woody debris

Aims

There is evidence that increased N inputs to boreal forests, via atmospheric deposition or intentional fertilization, may impact negatively on ectomycorrhizal (ECM) fungi leading to a reduced flux of plant-derived carbon (C) back to the atmosphere via ECM. Our aim was to investigate the impact of N fertilization of a Pinus sylvestris (L.) forest stand on the return of recently photoassimilated C via the ECM component of soil respiration.

Methods

We used an in situ, large-scale, ¹³C-CO₂ isotopic pulse labelling approach and monitored the ¹³C label return using soil gas efflux chambers placed over three different types of soil collar to distinguish between heterotrophic (R_H), autotrophic (R_A; partitioned further into contributions from ECM hyphae and total R_A) and total (R_S) soil respiration.

Results

The impact of N fertilization was to significantly reduce R_A, particularly respiration via extramatrical ECM hyphae. ECM hyphal flux in control plots showed substantial spatial variability, resulting in mean flux estimates exceeding estimates of total R_A, while ECM contributions to R_A in N treated plots were estimated at around 30%.

Conclusion

Significant impacts on soil C cycling may be caused by reduced plant C allocation to ECM fungi in response to increased N inputs to boreal forests; ecosystem models so far lack this detail.     

Carbon allocation in Larrea tridentata plant-soil systems as affected by elevated soil moisture and N availability

Background and Aims

Global change will likely express itself in southwestern United States arid lands through changes in amounts and timing of precipitation in response to elevated CO₂ concentrations. In addition, increased nitrogen (N) deposition may occur due to increased urban development. This study addressed the effects of water and N availability on C allocation in arid land soil-plant systems.

Methods

Columns filled with Mojave Desert topsoil containing Larrea tridentata seedlings with two treatment levels each of N and soil moisture were labeled by exposure to ¹³C-enriched CO₂.

Results

Increased soil moisture increased plant biomass, total ¹³C uptake, ¹³C levels in leaves, soil organic matter, and soil respiration, decreased relative C allocation to stems but increased allocation to soil organic matter. Increased soil N availability increased N uptake but decreased C allocation to soil respiration presumably due to decreased substrate supply for microbes. There was no detectable label in carbonate C, suggesting that this pool does not significantly contribute to ecosystem C fluxes.

Conclusions

Our study indicates that increased water availability causes increased C uptake with increased C allocation to soil organic matter in Larrea tridentata-dominated communities while increased N deposition will have a minimal impact on C sequestration.     

Biochar’s role as an alternative N-fertilizer: ammonia capture

Background

Biochar’s role as a carbon sequestration agent, while simultaneously providing soil fertility improvements when used as an amendment, has been receiving significant attention across all sectors of society, ranging from academia, industry, government, as well as the general public. This has lead to some exaggeration and possible confusion regarding biochar’s actual effectiveness as a soil amendment. One sparsely explored area where biochar appears to have real potential for significant impact is the soil nitrogen cycle.

Scope

Taghizadeh-Toosi et al. (this issue) examined ammonia sorption on biochar as a means of providing a nitrogen-enriched soil amendment. The longevity of the trapped ammonia was particularly remarkable; it was sequestered in a stable form for at least 12?days under laboratory air flow. Furthermore, the authors observed increased ¹⁵N uptake by plants grown in soil amended with the ¹⁵N-enriched biochar, indicating that the ¹⁵N was not irreversibly bound, but, was plant-available.

Conclusions

Their observations add credence to utilizing biochar as a carrier for nitrogen fertilization, while potentially reducing the undesired environmental consequences through gas emissions, overland flow, and leaching.     

Plant and nutrient controls on microbial functional characteristics in a tropical Oxisol

Aims

Extracellular enzymes mediate the decomposition of organic matter and the release of plant-available nutrients. Current theory predicts that enzyme production by soil microbes is regulated by the stoichiometric demands of microbial biomass and the complexity of environmental resources, but most experiments ignore the potential effect of alleviated carbon limitation in the rhizosphere. Our objective was to investigate linkages between enzyme activities, soil nutrient availability and plant roots in a tropical Oxisol.

Methods

We conducted a greenhouse experiment using soils from the Luquillo Experimental Forest and seedlings of Tabebuia heterophylla. Planted and unplanted pots were fertilized with different combinations of phosphorus and either mineral nitrogen (ammonia chloride) or a nitrogen-rich organic compound (casein). We measured changes in plant and soil nutrients and five extracellular enzyme activities.

Results

Phosphatase activity declined by 28% in the P and 40% in the complex nitrogen treatment, while N-acetyl glucosaminidase increased 162% in the complex nitrogen treatment. Beta-glucosidase, beta-xylosidase, cellobiohydrolase and N-acetyl glucosaminidase all increased significantly over time in the simple nitrogen treatment (P?<?0.05).

Conclusions

Enzymatic responses support microbial resource allocation theory, that is, the concept that soil microbes regulate enzyme production based on scarcity of resources. However, we did not observe any additional effect of roots on extracellular enzyme activities. Enzymatic C:N, C:P and N:P ratios further support the notion that shifts in microbial stoichiometric demand drive responses to nutrients.     

Alfalfa-grass biomass, soil organic carbon, and total nitrogen under different management approaches in an irrigated agroecosystem

Background and aims

Management approach may influence forage production as well as soil organic carbon (SOC) and soil total nitrogen (STN) accrued beneath perennial grass-legume components of irrigated crop rotations. This study aimed to evaluate effects of conventional, certified organic, and reduced-tillage management approaches on above- and belowground biomass production and C and N content in alfalfa-grass mixture, and their relationships with SOC and STN.

Methods

An alfalfa-grass mixture was established in 2009 on four replications under a sprinkler irrigation system. Soil characteristics were analyzed at planting time in 2009. Aboveground biomass production, coarse and fine roots, SOC, STN, aboveground biomass C and N, and coarse- and fine-root C and N were quantified in samples collected during 2009–2011.

Results

Conventional management produced more aboveground biomass than reduced-tillage and organic, but production under organic matched conventional and exceeded reduced-tillage in the last two harvests of the study. Root production was constant under the three approaches, but resulted in more SOC accrued under reduced-tillage than under the other two approaches.

Conclusions

Biomass production was favored by conventional seedbed preparation and soil fertility management while SOC accrual was favored by minimum soil disturbance. In addition, aboveground biomass was influenced by seasonal air temperature, precipitation, and nutrient mineralization from the previous season, so above-/belowground allocation changed seasonally.     

Influence of organic residues and soil incorporation on temporal measures of microbial biomass and plant available nitrogen

Aims

Despite our current understanding of plant nitrogen (N) uptake and soil N dynamics in arable systems, the supply and demand of N are infrequently matched as a result of variable seasonal and soil conditions. Consequently, inefficiencies in N utilisation often lead to constrained production and can contribute to potential environmental impacts. The aim of this study was to examine the influence of plant residue quality (C/N ratio) and extent of residue incorporation into soil on temporal changes in soil mineral N and the associated plant N uptake by wheat in the semi-arid agricultural production zone of Western Australia.

Methods

Oat (Avena sativa); lupin (Lupinus angustifolius) and field pea (Pisum sativum) were incorporated into a Red-Brown Earth using varying degrees of mechanical disturbance (0 to 100% residue incorporated). Soil samples for inorganic N (NO ₃ ^? and NH ₄ ⁺ ) profiles (0?C50?cm), microbial biomass-C (0?C50?cm) and plant N uptake were taken throughout the growing season of the subsequent wheat (Triticum aestivum) crop. Grain yield and yield components were determined at harvest.

Results

Despite observed treatment effects for plant residue type and soil disturbance, fluctuations in inorganic N were more readily influenced by seasonal variability associated with wet-dry cycles. Treatment effects resulting from residue management and extent of soil disturbance were also more readily distinguished in the NO ₃ ^? pool. The release of N from crop residues significantly increased (p?=?0.05) with greater soil-residue contact which related to the method of incorporation; the greater the extent of soil disturbance, the greater the net supply of inorganic N. Differences in microbial biomass-C were primarily associated with the type of plant residue incorporated, with higher microbial biomass generally associated with legume crops. No effect of residue incorporation method was noted for microbial biomass suggesting little effect of soil disturbance on the microbial population in this soil.

Conclusions

Despite differences in the magnitude of N release, neither crop type nor incorporation method significantly altered the timing or pattern of N release. As such asynchrony of N supply was not improved through residue or soil management, or through increased microbial biomass in this semi-arid environment. N fluxes were primarily controlled by abiotic factors (e.g. climate), which in this study dominated over imposed agricultural management practices associated with residue management.     

Respiratory C fluxes and root exudation differ in two full-sib clones of Pinus taeda (L.) under contrasting fertilizer regimes in a greenhouse

Aims

We investigated whether changes in respiratory C fluxes, soil CO₂ efflux, or root exudate quantity or quality explained differences in growth rates between closely related clones of Pinus taeda (L.).

Methods

A factorial design with two clones, fertilized and control treatments, and four sequential harvests was installed in a greenhouse for 121 days.

Results

The two clones did show significant differences in respiratory C fluxes, soil CO₂ efflux, and root exudation quantity and quality. While the clones also differed in growth rates, the C fluxes assessed in this paper did not explain how seedlings were able to allocate more C to stem growth in the months following fertilizer application. Changes in root exudation were not consistent with reduced heterotrophic soil CO₂ efflux, which does not appear to be a plant-mediated process.

Conclusions

These results indicate that if single genotypes are deployed over large land areas in plantations, dramatic differences between clonal plant-soil interactions may require consideration in ecosystem C budgets. Further, the range of belowground fluxes observed implies that genotype-specific C allocation may make some clones better able to exploit a given resource environment than others.     

Plant genetic effects on soils under climate change

Background

Many biological questions about N availability and the N cycle require knowledge of the abundance and identity of molecules comprising the pool of organic N. Moreover, basic knowledge of the molecular composition of the soil solution can give rise to new hypotheses via data-driven or inductive reasoning.

Scope

This paper examines the composition of organic N molecules in the soil solution. Our perception of organic N in the soil solution is shaped by analytical approaches, and thus I briefly review approaches for sampling and analysis of the soil solution. I give examples of hypotheses generated by knowledge of the molecular composition of organic N and conclude by suggesting priorities for future research.

Conclusions

Studies of the molecular composition of organic N are very much in their infancy. Amino acids, their oligomers and polymers are consistently large components of the pool of organic N. The soil solution also contains organic N compounds from at least another 12 compound classes, but almost nothing is known about their functional significance. Uncovering the role of these other compounds in the N cycle would enrich our understanding of organic N and the N cycle, and place studies of amino acids and their polymers in a broader context.     

Positive feedbacks between decomposition and soil nitrogen availability along fertility gradients

Background and aims

We determined the relationship between site N supply and decomposition rates with respect to controls exerted by environment, litter chemistry, and fungal colonization.

Methods

Two reciprocal transplant decomposition experiments were established, one in each of two long-term experiments in oak woodlands in Minnesota, USA: a fire frequency/vegetation gradient, along which soil N availability varies markedly, and a long-term N fertilization experiment. Both experiments used native Quercus ellipsoidalis E.J. Hill and Andropogon gerardii Vitman leaf litter and either root litter or wooden dowels.

Results

Leaf litter decay rates generally increased with soil N availability in both experiments while belowground litter decayed more slowly with increasing soil N. Litter chemistry differed among litter types, and these differences had significant effects on belowground (but not aboveground) decay rates and on aboveground litter N dynamics during decomposition. Fungal colonization of detritus was positively correlated with soil fertility and decay rates.

Conclusions

Higher soil fertility associated with low fire frequency was associated with greater leaf litter production, higher rates of fungal colonization of detritus, more rapid leaf litter decomposition rates, and greater N release in the root litter, all of which likely enhance soil fertility. During decomposition, both greater mass loss and litter N release provide mechanisms through which the plant and decomposer communities provide positive feedbacks to soil fertility as ultimately driven by decreasing fire frequency in N-limited soils and vice versa.     

Exposure to warming and CO2 enrichment promotes greater above-ground biomass, nitrogen, phosphorus and arbuscular mycorrhizal colonization in newly established grasslands

Aims

In view of the projected increase in global air temperature and CO₂ concentration, the effects of climatic changes on biomass production, CO₂ fluxes and arbuscular mycorrhizal fungi (AMF) colonization in newly established grassland communities were investigated. We hypothesized that above- and below-ground biomass, gross primary productivity (GPP), AMF root colonization and nutrient acquisition would increase in response to the future climate conditions. Furthermore, we expected that increased below-ground C allocation would enhance soil respiration (R_soil).

Methods

Grassland communities were grown either at ambient temperatures with 375?ppm CO₂ (Amb) or at ambient temperatures +3°C with 620?ppm CO₂ (T+CO₂).

Results

Total biomass production and GPP were stimulated under T+CO₂. Above-ground biomass was increased under T+CO₂ while belowground biomass was similar under both climates. The significant increase in root colonization intensity under T+CO₂, and therefore the better contact between roots and AMF, probably determined the higher above-ground P and N content. R_soil was not significantly affected by the future climate conditions, only showing a tendency to increase under future climate at the end of the season.

Conclusions

Newly established grasslands benefited from the exposure to elevated CO₂ and temperature in terms of total biomass production; higher root AMF colonization may partly provide the nutrients required to sustain this growth response.     

Nitrate-use traits of understory plants as potential regulators of vegetation distribution on a slope in a Japanese cedar plantation

Background and aims

Plant physiological traits and their relation to soil N availability was investigated as regulators of the distribution of understory shrub species along a slope in a Japanese cedar (Cryptomeria japonica) plantation in central Japan.

Methods

At the study site, previous studies demonstrated that both net and gross soil nitrification rates are high on the lower slope and there are dramatic declines in different sections of the slope gradient. We examined the distributions of understory plant species and their nitrate (NO ₃ ^? -N) use traits, and compared the results with the soil traits.

Results

Our results show that boundaries between different dominant understory species correspond to boundaries between different soil types. Leucosceptrum stellipilum occurs on soil with high net and gross nitrification rates. Hydrangea hirta is dominant on soil with high net and low gross nitrification rates. Pieris japonica occurs on soil with very low net and gross nitrification rates. Dominant understory species have species-specific physiological traits in their use of NO ₃ ^? -N. Pieris japonica lacks the capacity to use NO ₃ ^? -N as a N source, but other species do use NO ₃ ^? -N. Lindera triloba, whose distribution is unrelated to soil NO ₃ ^? -N availability, changes the extent to which it uses NO ₃ ^? -N in response to soil NO ₃ ^? -N availability.

Conclusions

Our results indicate that differences in the physiological capabilities and adaptabilities of plant species in using NO ₃ ^? -N as a N source regulate their distribution ranges. The identity of the major form of available soil N is therefore an environmental factor that influences plant distributions.     

Comparison of methane, nitrous oxide fluxes and CO2 respiration rates from a Mediterranean cork oak ecosystem and improved pasture

Background and aims

During the recent decades, cork oak (Q. suber) mortality has been increasing in Mediterranean oak woodland endangering the economical and environmental sustainability of the “montado” ecosystem. This fact in combination with climate change and conversion of forestland to pasture may significantly affect the soil-atmosphere greenhouse gases (GHGs) exchange. Our study evaluates the impact of oak trees as compared to pasture on net ecosystem GHG (CH_4, N₂O, and CO₂) exchange as well as the main environmental factors influencing this exchange.

Methods

We used field chamber measurements for the collection of GHGs under three different conditions: 1) open area (OA), 2) under tree canopy area (UC) and 3) improved pasture (IP). Experiments were done under typical Mediterranean climate at central Portugal in 2010 and 2011.

Results

The UC had higher nitrification potential, soil C/N ratio, electrical conductivity, litter input and soil organic matter (SOM) than OA and IP. SOM positively correlated with soil CH₄ and N₂O fluxes but not with soil CO₂ respiration rates. Soil water content (SWC) drives both CH₄ and N₂O fluxes. Under certain conditions, when SWC reached a threshold (7 % for CH₄ and 3 % for N₂O) the result was net uptake and that net uptake increased with SWC. This was the case for the UC and OA. Conversely, for the IP soil water content above 4 % promoted net CH₄ release.

Conclusions

Our results show that cork oak influences soil properties and consequently GHGs fluxes. In the UC the input of litter for SOM together with soil moisture, favoured microbiological activity and related GHGs fluxes. Soil temperature is a secondary factor in the studied conditions. Our results also emphasized the potential impact posed by decreased cork oak tree density in the functioning of the “montado” ecosystem.     

Preferential use of root litter compared to leaf litter by beech seedlings and soil microorganisms

Background and aims

Litter decomposition is regulated by e.g. substrate quality and environmental factors, particularly water availability. The partitioning of nutrients released from litter between vegetation and soil microorganisms may, therefore, be affected by changing climate. This study aimed to elucidate the impact of litter type and drought on the fate of litter-derived N in beech seedlings and soil microbes.

Methods

We quantified ¹⁵N recovery rates in plant and soil N pools by adding ¹⁵N-labelled leaf and/or root litter under controlled conditions.

Results

Root litter was favoured over leaf litter for N acquisition by beech seedlings and soil microorganisms. Drought reduced ¹⁵N recovery from litter in seedlings thereby affecting root N nutrition. ¹⁵N accumulated in seedlings in different sinks depending on litter type.

Conclusions

Root turnover appears to influence (a) N availability in the soil for plants and soil microbes and (b) N acquisition and retention despite a presumably extremely dynamic turnover of microbial biomass. Compared to soil microorganisms, beech seedlings represent a very minor short-term N sink, despite a potentially high N residence time. Furthermore, soil microbes constitute a significant N pool that can be released in the long term and, thus, may become available for N nutrition of plants.