Input Of Fresh Organic Matter Research Articles

Source or decomposition of soil organic matter: what is more important with increasing forest age in a subalpine setting?

Afforestation has been the dominant land-use change in the Swiss Alps during the last decades which has not only the potential to increase soil organic carbon sequestration, but it has also the potential to alter soil organic matter (SOM) dynamics through the vegetation shift and change in organic matter (OM) input into soils. The effects of afforestation on SOM dynamics, however, are still not fully understood as specific sources of OM and modifications of soil processes influencing decomposition and preservation remain largely unknown on alpine to subalpine slopes. Within this study we aimed to identify the potential sources and the decomposition of OM in a subalpine afforestation chrono-sequence (0–130 years) with Norway spruce (Picea abies L.) on a former pasture by using a multi-proxy molecular marker approach. We observed that leaf-derived OM plays an essential role in the pasture areas, while root-derived OM only plays a minor role in pasture and forest areas. Needle-derived OM represents the dominant source of SOM with increasing forest age, while understory shrubs and moss also contribute to the OM input in younger forest stand ages. However, needle litter and buildup of organic layers and subsequently less input of fresh OM from organic horizons to mineral soil can result in increased OM decomposition in mineral soils rather than contributing to additional SOM stabilization in mineral soils. This was most pronounced in the oldest forest stand (130-year-old) in the investigated afforestation sequence, particularly in deeper soil horizons (10–45 cm). Thereby, our study provides new insights into SOM dynamics following afforestation, especially with respect to the long-term SOM sequestration potential of afforestation of subalpine pasture soils.

Factors determining the distribution of Erica patches on the Sanetti Plateau, Bale Mountains, Ethiopia

In the Bale Mountains, the ericaceous belt ranges between 3200 and 3800 m asl. Studies indicate an expansion on the Sanetti Plateau at the end of the Late Glacial and during the early Holocene. Currently, only patches of Erica growing between boulders are found on the Plateau, while most of the landscape above 3800 m asl is covered by afro-alpine plants. Driving factors for Erica patches above the upper ericaceous ecotone is a matter of debate. This study evaluates site variables and biogeochemical properties of soils under Erica patches and nearby Erica-free control to understand the environmental conditions responsible for the patchy occurrence of Erica on the Sanetti Plateau. Except for the boulder richness, Erica and control plots have comparable topography, soil texture, and electrical conductivity. However, soils below Erica patches have higher total organic carbon, nitrogen, carbon-to-nitrogen ratios, and black carbon contents than the control plots indicating fresh organic matter input and availability of combustible fuel. This implies that Erica did not fully cover the control plots in former times. Carbon and nitrogen stocks were slightly higher in control plots due to the lower stone contents of the profiles. In addition, soils of the Erica plots showed more positive δ13C values than the control soils, possibly attributed to water stress. In general, the relief and soil conditions of control plots may support the growth of Erica. However, Erica growing between boulders seems to benefit from the favorable microclimate and physical protection against grazing and fire.

Soil Organic Carbon and pH Dominate the Effects of Nitrogen Addition on Soil Microarthropods in a Poplar Plantation in Coastal Eastern China

Soil biodiversity and fuction have been altered by the increasing levels of nitrogen as a result of fertilization and atmospheric deposition. Although soil microarthropods are a crucial component of soil biodiversity and play a key role in a diverse range of soil functions, our understanding of the mechanisms by which N addition affects them remains limited. Using a long-term nitrogen addition experiment (2012–2016) in poplar plantations (Populus deltoides L. CL‘35′) located along the coast of Yellow Sea Forest Park in northern Jiangsu, eastern China (32°52′ N and 120°49′ E), where the soil was entisols, we examined the response of soil microarthropods across three soil depths (0–15 cm, 15–25 cm, 25–40 cm) to five N input levels (0, 5, 10, 15, 30 g N m−2 year−1) over four seasons. We found that the number of microarthropods per unit area initially grew and then dropped as more nitrogen was added to soils. Soil organic carbon (positive correlation, R2 = 0.53) and pH (negative correlation, R2 = 0.19) were the two dominant factors driving the effects of nitrogen addition on soil microarthropod densities at all soil depths. These results suggest that nitrogen input enhances the density of soil microarthropods via the increase in fresh organic matter input. However, the increase in organic matter may be offset by an indirect increase in acidity under high levels of N addition, providing one possible explanation for the reduced density of microarthropods in heavily fertilized soils.71

Loss of soil carbon and nitrogen indicates climate change-induced alterations in a temperate forest ecosystem

Climate warming is expected to influence terrestrial biogeochemical cycles by modifying the quality and quantity of plant litter input to soils. Although a growing number of studies recognize the importance of plant litter input in influencing the loss of soil organic matter (SOM) through a phenomenon called the priming effect (PE), the exact mechanisms behind PE are not well known. Importantly, most PE research is based on short term pot experiments in which fresh organic matter (FOM) input is represented by a single addition of compounds of unnaturally simple chemical composition. Furthermore, only a few studies exist in which the PE was explored in terms of organic C (SOC) and total N content in the soil. Here, we report results of a 3-year long litter manipulation study conducted under natural conditions in a broadleaved Korean pine forest in N-E China. We show that the extra supply (twice the normal input) of aboveground tree litter composing of conifer needles, leaves and small twigs was associated not only with slightly decreased SOC (by 5%) but especially that of soil total N (STN) (by 15%) content in the top soil (0–5 cm depth). In contrast, removal of litter resulted in an increased (ca. 15%) amount of both SOC and STN during the study when compared to control soils receiving natural litter input. Despite the enhanced leaf litter decomposition rate in the treatment receiving extra litter, the changes in SOC and STN were related neither to soil microbial biomass nor to community composition. The amount of N lost (40.0 g m−2) in the soil due to litter addition was ca. three times the amount of N added (12.3 g m−2) via the litter, while the amount of C lost (238 g m−2) was about one third of that added (940 g m−2), suggesting that soil N in our research site is more prone to the PE than soil C. As we did not manipulate belowground FOM input, our results suggest that input of aboveground litter rather than that by roots explained the PE in our study. Results of our long-term study conducted under natural conditions in undisturbed forest soils highlight the large potential of recalcitrant, aboveground litter to affect the PE, which should not go unnoticed when predicting the role of forest soils under conditions (such as climate warming) when these soils act as C sinks.

Mineral weathering is linked to microbial priming in the critical zone

Decomposition of soil organic matter (SOM) can be stimulated by fresh organic matter input, a phenomenon known as the ‘priming effect’. Despite its global importance, the relationship of the priming effect to mineral weathering and nutrient release remains unclear. Here we show close linkages between mineral weathering in the critical zone and primed decomposition of SOM. Intensified mineral weathering and rock-derived nutrient release are generally coupled with primed SOM decomposition resulting from “triggered” microbial activity. Fluxes of organic matter products decomposed via priming are linearly correlated with weathering congruency. Weathering congruency influences the formation of organo-mineral associations, thereby modulating the accessibility of organic matter to microbial decomposers and, thus, the priming effect. Our study links weathering with primed SOM decomposition, which plays a key role in controlling soil C dynamics in space and time. These connections represent fundamental links between long-term lithogenic element cycling (= weathering) and rapid turnover of carbon and nutrients (= priming) in soil.

Microscale spatial distribution and soil organic matter persistence in top and subsoil

The spatial distribution of organic substrates and microscale soil heterogeneity significantly influence organic matter (OM) persistence as constraints on OM accessibility to microorganisms. However, it is unclear how changes in OM spatial heterogeneity driven by factors such as soil depth affect the relative importance of substrate spatial distribution on OM persistence. This work evaluated the decomposition and persistence of 13C and 15N labeled water-extractable OM inputs over 50 days as either hotspot (i.e., pelleted in 1–2 mm-size pieces) or distributed (i.e., added as OM < 0.07 μm suspended in water) forms in topsoil (0–0.2 m) and subsoil (0.8–0.9 m) samples of an Andisol. We observed greater persistence of added C in the subsoil with distributed OM inputs relative to hotspot OM, indicated by a 17% reduction in cumulative mineralization of the added C and a 10% higher conversion to mineral-associated OM. A lower substrate availability potentially reduced mineralization due to OM dispersion throughout the soil. NanoSIMS (nanoscale secondary ion mass spectrometry) analysis identified organo-mineral associations on cross-sectioned aggregate interiors in the subsoil. On the other hand, in the topsoil, we did not observe significant differences in the persistence of OM, suggesting that the large amounts of particulate OM already present in the soil outweighed the influence of added OM spatial distribution. Here, we demonstrated under laboratory conditions that the spatial distribution of fresh OM input alone significantly affected the decomposition and persistence of OM inputs in the subsoil. On the other hand, spatial distribution seems to play a lower role in topsoils rich in particulate OM. The divergence in the influence of OM spatial distribution between the top and subsoil is likely driven by differences in soil mineralogy and OM composition.

Molecular diet analysis enables detection of diatom and cyanobacteria DNA in the gut of Macoma balthica.

Detritivores are essential to nutrient cycling, but are often neglected in trophic networks, due to difficulties with determining their diet. DNA analysis of gut contents shows promise of trophic link discrimination, but many unknown factors limit its usefulness. For example, DNA can be rapidly broken down, especially by digestion processes, and DNA provides only a snapshot of the gut contents at a specific time. Few studies have been performed on the length of time that prey DNA can be detected in consumer guts, and none so far using benthic detritivores. Eutrophication, along with climate change, is altering the phytoplankton communities in aquatic ecosystems, on which benthic detritivores in aphotic soft sediments depend. Nutrient-poor cyanobacteria blooms are increasing in frequency, duration, and magnitude in many water bodies, while nutrient-rich diatom spring blooms are shrinking in duration and magnitude, creating potential changes in diet of benthic detritivores. We performed an experiment to identify the taxonomy and quantify the abundance of phytoplankton DNA fragments on bivalve gut contents, and how long these fragments can be detected after consumption in the Baltic Sea clam Macoma balthica. Two common species of phytoplankton (the cyanobacteria Nodularia spumigena or the diatom Skeletonema marinoi) were fed to M. balthica from two regions (from the northern and southern Stockholm archipelago). After removing the food source, M. balthica gut contents were sampled every 24 hours for seven days to determine the number of 23S rRNA phytoplankton DNA copies and when the phytoplankton DNA could no longer be detected by quantitative PCR. We found no differences in diatom 18S rRNA gene fragments of the clams by region, but the southern clams showed significantly more cyanobacteria 16S rRNA gene fragments in their guts than the northern clams. Interestingly, the cyanobacteria and diatom DNA fragments were still detectable by qPCR in the guts of M. balthica one week after removal from its food source. However, DNA metabarcoding of the 23S rRNA phytoplankton gene found in the clam guts showed that added food (i.e. N. spumigena and S. marinoi) did not make up a majority of the detected diet. Our results suggest that these detritivorous clams therefore do not react as quickly as previously thought to fresh organic matter inputs, with other phytoplankton than large diatoms and cyanobacteria constituting the majority of their diet. This experiment demonstrates the viability of using molecular methods to determine feeding of detritivores, but further studies investigating how prey DNA signals can change over time in benthic detritivores will be needed before this method can be widely applicable to both models of ecological functions and conservation policy.

5300-Year-old soil carbon is less primed than young soil organic matter.

Soils harbor more than three times as much carbon (C) as the atmosphere, a large fraction of which (stable organic matter) serves as the most important global C reservoir due to its long residence time. Litter and root inputs bring fresh organic matter (FOM) into the soil and accelerate the turnover of stable C pools, and this phenomenon is termed the "priming effect" (PE). Compared with knowledge about labile soil C pools, very little is known about the vulnerability of stable C to priming. Using two soils that substantially differed in age (500 and 5300 years before present) and in the degree of chemical recalcitrance and physical protection of soil organic matter (SOM), we showed that leaf litter amendment primed 264% more organic C from the young SOM than from the old soil with very stable C. Hierarchical partitioning analysis confirmed that SOM stability, reflected mainly by available C and aggregate protection of SOM, is the most important predictor of leaf litter-induced PE. The addition of complex FOM (i.e., leaf litter) caused a higher bacterial oligotroph/copiotroph (K-/r-strategists) ratio, leading to a PE that was 583% and 126% greater than when simple FOM (i.e., glucose) was added to the young and old soils, respectively. This implies that the PE intensity depends on the chemical similarity between the primer (here FOM) and SOM. Nitrogen (N) mining existed when N and simple FOM were added (i.e., Glucose+N), and N addition raised the leaf litter-induced PE in the old soil that had low N availability, which was well explained by the microbial stoichiometry. In conclusion, the PE induced by FOM inputs strongly decreases with increasing SOM stability. However, the contribution of stable SOM to CO2 efflux cannot be disregarded due to its huge pool size.

ИЗМЕНЕНИЕ СОДЕРЖАНИЯ ГУМУСА И БИОЛОГИЧЕСКОЙ АКТИВНОСТИ АГРОТЕМНОГУМУСОВЫХ ПОДБЕЛОВ ПРИ РАЗЛИЧНЫХ АГРОТЕХНИЧЕСКИХ ПРИЕМАХ ОБРАБОТКИ ПОЧВ

Differences in the content of humus, microflora and catalase activity in agro-dark-humus podbels soils with different levels of agrotechnical impact and the use of environmentally friendly (phytomeliorative) methods of increasing soil fertility were established. The higher content of humus, indicators of biogenicity and enrichment in catalase were established in the soils of abandoned field. Bacteria using mineral nitrogen and oligonitrophils was dominated in composition of microflora. The reserves of humus in the meter layer of soils have been characterized as low. Тhe composition of the microflora showed the same regularity under the conditions of the phytomeliorative experiment. The content and reserves of humus decreased in comparison with the soil of abandoned field. Due to the process of nodule formation in the ELnn horizon, a high level of catalase activity was recorded (up to 9.9 cm3 O2/1 min 1 g of soil). This led to an increase in the average intraprofile distribution of catalase activity (Ka) and Кa reserves. Low levels of humus content were recorded in the variant with long-term use of organic fertilizers. At the same time, the humus reserves in the meter layer increased relative to the other variants of the experiment. Oligonitrophils and ammonifiers was dominated in the composition of the microflora, by while the proportion of actinomycetes sharply decreased. The value of Ka and its reserves decreased. Bacteria using mineral nitrogen and oligonitrophils are predominance in variants with with long-term use of mineral fertilizers. The intensification of microbiological activity caused an increase enrichment catalase and an increase in its reserves in soil. The lack of fresh organic matter input led to increased mineralization processes of specific soil organic compounds. This variant is characterized by the lowest humus content.

Fatty acid biomarkers as indicators of organic matter origin and processes in recent turbidites: The case of the terminal lobe complex of the Congo deep-sea fan

The Congo River is connected to its submarine canyon and supplies large quantities of terrestrial organic carbon via powerful turbiditic currents down to the Congo deep-sea fan. We investigated sediment cores from the terminal lobe complex of the Congo deep-sea fan (∼750 km offshore, ∼5000 m water depth), in order to assess the value of fatty acid biomarkers as indicators of organic matter (OM) origin and processes affecting its distribution and preservation in recent turbidites. Sediments from the Congo deep-sea fan are enriched in fatty acids compared to the surrounding abyssal plains and their composition closely resembles that of sediments recovered in the Congo River. Long chain fatty acid (LCFA) biomarkers in conjunction with the branched vs isoprenoid tetraether index (BIT) show that OM mostly originates from soil erosion and continental higher plants. This material has undergone limited reprocessing during transit certainly due to tight interactions with mineral particles and rapid transfer. The presence of phytoplankton biomarkers at the entrance of the terminal lobe area highlights that inputs of fresh OM, albeit limited, can reach the lobe complex. By combining fatty acid profiles and geochemical proxies in a multivariate analysis, we highlight that OM degradation is mostly limited to the oxic layer, and that fine soil-derived particles and the coarser higher plant detritus display distinct depositional dynamics. Finally, LCFA are remarkably well preserved under anoxic conditions at different time scales, in recent turbidites deposited during the last century and those deposited several thousand years ago.

Andic Soil Properties and Tephra Layers Hamper C Turnover in Icelandic Peatlands

AbstractDue to frequent volcanic activity and erosion of dryland soils, magnified by land use after human settlement (ca. 870 CE), peatlands in Iceland receive regular additions of mineral eolian deposits. Their soils may develop not only histic but also andic characteristics. Yet, mineral eolian deposition as an environmental determinant of peatlands in Iceland is still poorly understood, not least with regard to the peatlands carbon (C) stores. This study advances our understanding of the impact of tephra deposition on Histosols by elucidating interactions between C characteristics and andic soil properties. We compare Histosols from three Icelandic peatlands of different degrees of exposure to eolian deposition by evaluating data sets of their C structure derived by 13C NMR spectroscopy, andic soil properties based on selective extractions of Al, Fe, and Si, and decomposition proxies C/N, δ13C, and δ15N. By applying multivariate statistical methods, we are able to present several important patterns. Soil organic matter of Histosols with andic properties is less decomposed than that of Histosols without notable andic properties. Andic soil properties seem to impact their C structure by facilitating the formation of organo‐mineral complexes, which particularly hamper the decomposition of chemically more labile C groups. Tephra layers appear to hamper microbial activity in deeper soil layers by preventing input of fresh organic matter. The interaction of andic and histic soil properties and the protective role of major tephra deposits may enable an unusual potential for long‐term C stabilization in a natural peatland environment.

Total phosphorus records in coastal Antarctic sediments: Burial and evidence of anthropogenic influence on recent input

Total phosphorus (TP) records reflect variations in input and burial of organic matter (OM) in coastal and shelf sediments. At Antarctic regions, TP levels are often derived from natural sources; however, with emergent human pressure at these regions, anthropogenic sources of TP may play an important role. At the Antarctic Peninsula, Admiralty Bay attracts great scientific and touristic interest, especially during austral summer months, thus being vulnerable to human activities. Currently, only scarce spatial distributions of TP are available for this key region, whereas no vertical distributions have been determined. To fill this gap, we investigated short (< 20 cm) sediment cores in ten areas along Admiralty Bay for TP contents. We produced the first TP vertical distributions and established site-specific background values (from 492 ± 13 to 932 ± 17 μg g−1), which are related to hydrodynamics, sedimentology, and natural inputs of P. We observed a gradual surface TP enrichment at all sites, mainly due to input of fresh OM. Benefiting from our background values, we employ the Phosphorus Pollution Index (PPI) to assess possible human impacts. Generally, the increase of PPI suggests natural inputs of P. However, PPI ≥ 1.3 found at the nearby Comandante Ferraz Research Station (Brazil) can be linked to recent (past 2–3 decades) treated sewage inputs in Martel Inlet. We suggest PPI as a proxy for preliminary assessments of anthropogenic impacts in coastal Antarctic regions experiencing increased human pressure.

The magnitude and direction of priming were driven by soil moisture and temperature in a temperate forest soil of China

Increased temperature and extreme precipitation due to global climate change play a critical role in the soil carbon cycle. The priming effect describes the changes in soil organic carbon (SOC) mineralization after fresh organic matter (FOM) inputs and is a major component of the soil carbon cycle. However, further research is needed to improve understanding of the interactive influence of warming and soil moisture changes on soil carbon priming. In this study, we conducted a 57-day incubation experiment to investigate the interactive effects of soil moisture and temperature on priming on forest soils from a temperate region of China. 13C labeled Glucose was added to simulate the priming effect induced by rhizosphere exudates at three soil moisture levels (20 %, 55 %, and 90 % of field capacity (FC)) and three temperature levels (5, 15, and 25 °C). Soil priming increased significantly with the increase of moisture; the cumulative priming effects were positive at 55 % and 90 % FC, but were negative at 20 % FC. Warming significantly stimulated the magnitude of priming at all moisture levels, with the greatest negative priming under 20 %FC×25 °C (−165.9 mg C kg−1) and the greatest positive priming under 90 %FC×25 °C (588.0 mg C kg−1). Furthermore, priming considerably influenced the soil carbon balance, which resulted in maximum carbon storage under 20 %FC×25 °C (279.4 mg C kg−1) and the greatest carbon loss under 90 %FC×25 °C (−493.9 mg C kg−1). Furthermore, temperature sensitivity (Q10) of SOC mineralization was highly dependent on FOM inputs. In unprimed soils, Q10 was negatively and significantly related to moisture; however, no significant correlation was observed between Q10 and moisture after glucose addition because of the priming. The findings of this study contribute to the better understanding of the temperate forest soil carbon cycle under global climate change by highlighting the interactive effects of soil moisture and temperature on the priming effect.

Effects of Biochar on Soil Aggregation and Distribution of Organic Carbon Fractions in Aggregates

Soil aggregates are among crucial factors for determining both the quality and erosion resistance of soils. Biochar is a soil amendment that has seen increasing use to improve specific soil properties, mainly the physical structure and the preserving capacity of water and nutrients, as well as sequestration of soil organic carbon. In this study, we applied the rice husk biochar (RHB) and cattle manure compost (COM) in a sandy loam rural soil, which is widely distributed in southern Taiwan, to investigate the combined effects of the biochar and compost on soil aggregation and dynamic change of organic carbon fractions. Through our incubation experiment, both biochar and compost could promote the soil aggregation after eight weeks incubation. The total amounts of macroaggregates (MaAs, &gt;2.0 mm) and mesoaggregates (MeAs, 0.25–2.0 mm) increased by 1.3–9%. During aggregation processes, a considerably greater amount of the soil organic carbon was found to enrich mainly in MaAs and MeAs in all treatments. The COM addition in the soil further promotes organic carbon enrichment in microaggregates (MiAs, &lt;0.25 mm) + fine particles and MeAs after incubation. Increasing labile organic C (LOC) fractions were significantly found in MaAs and MeAs during aggregation processes, whereas decreasing LOC fractions were found in MiAs. The input of fresh organic matter (RHB and COM) initial acts as binding agents in MiAs, and then further enhances the formation of MeAs and MaAs gradually. In conclusion, RHB promotes the physical protection of organic C by increasing soil aggregation and is hence a management option to enhance the C sequestration potential.

The fate of primed soil carbon between biomass immobilization and respiration is controlled by nutrient availability

A positive Priming Effect (PE) is defined as an acceleration of the decomposition of soil organic matter (SOM) by a fresh organic matter (FOM) input. But in the literature many studies present this phenomenon as an extra loss of carbon from SOM to the atmosphere. SOM is actually a mixture of pools with different turnover rates, and microorganisms generating PE are heterotrophic. Therefore, whether PE is in fact an extra loss of C depends on the targeted SOM pool (high or low turnover) and the ratio between the incorporation of the primed carbon into microbial biomass vs its mineralization. A13 nutrient-poor tropical soil (enriched or not with a cocktail of nutrients) was incubated and submitted to two successive wheat-straw inputs with differential 13C enrichment, in order to observe the PE of one straw on the other, and to measure the specific respiration of the different carbon pools. In our nutrient-poor soil, nutrient enrichment intensified soil basal respiration while it lowered straw mineralization. Our study showed that, fresh and primed C allocation between microbial biomass and CO2 were similar and depended on the nutrient status of the soil. We did not observe any impact of freshly amended C on the fate of the previous amendment, but conversely we observed an impact of the previous organic input on the decomposition rate of the following one, as well as the allocation of the liberated C toward biomass rather than CO2.

Coupling of stable carbon isotopic signature of methane and ebullitive fluxes in northern temperate lakes.

Stable isotopic analysis is a popular method to understand the mechanisms sustaining methane (CH4) emissions in various aquatic environments. Yet, the general lack of concurrent measurements of isotopes and fluxes impedes our ability to establish a connection between the variation in the rates of CH4 emission and isotopic signature. Here, we examine the magnitude of CH4 ebullition (bubbling) and stable carbon isotopic signature (δ13C-CH4) of bubble CH4 in four northern temperate lakes and evaluate the in-lake processes shaping their variability. The ebullitive CH4 flux and bubble δ13C-CH4 varied from 0.01 to 37.0mmolm-2d-1 and between -71.0‰ and -50.9‰, respectively. The high emission lakes in general and high fluxing shallow zones within each lake consistently showed enriched δ13C-CH4 signature. Subsequently, in addition to the temperature dependence (1.4±0.1eV), the rates of ebullition strongly correlated with the variability of δ13C-CH4 across our study lakes. Our results suggest that higher ebullitive emissions are sustained by acetoclastic methanogenesis, likely fueled by fresh organic matter inputs. Further, the annual whole-lake estimate of bubble isotopic flux alone showed depleted δ13C-CH4 values (-64.6±0.6‰ to -60.1±3.2‰), yet the signature of the total CH4 emission (ebullition+diffusion) was relatively enriched (-60.7‰ to -52.6‰) due to high methanotrophic activity in the water column. We show that δ13C-CH4 signature of bubbles can be linked to the magnitude of ebullition itself, yet we suggest there is a need to account for different emission pathways and their isotopic signature to allocate CH4 source signature to northern lakes.

Keystone Perennial Grassland Species Control Soil Nitrogen Flows

Restoration of many grassland ecosystems is dependent on restoring native N cycling regimes, through methods such as nutrient stripping, C addition, and/or re-establishment of keystone species. We studied topsoil N pools and fluxes under five different understorey vegetation communities of different degradation states of grassy eucalypt woodlands across 11 sites in New South Wales, Australia, in order to identify whether high N flux but low standing N concentrations were responsible for the success of reference Themeda triandra communities. The presence of NO3− was the main differentiator between the annual exotic (AE) and reference states, which both demonstrated twofold-threefold increases N flux relative to the depleted states. Reference states were characterised by high proteolysis and dissolved organic N (DON) mineralisation rates, but also low δ15N and a high C:N ratio, which taken together indicate fresh organic matter inputs that turn over quickly. In quantifying fast rates of N cycling yet low NO3− pools in the reference states, our data highlight a productive ecosystem dominated by rapidly cycling DON rather than NO3− in its reference state. The higher NO3− concentrations coupled with fast N cycling in the degraded AE state indicate that although available N production is high in both systems, uptake is lower in the AE state, perpetuating a larger standing NO3− pool that may prevent re-establishment of native species. Steps that promote rapid DON cycling while inhibiting nitrification may lead to a soil biogeochemical status more suitable for the restoration of highly productive native grasslands.

Between-summer comparison of particulate organic matter in surface waters of a coastal area influenced by glacier meltwater runoff and retreat

We evaluated, in two consecutive austral summers, the biochemical composition, sources, distribution and degradation state of particulate organic matter (POM) in surface coastal waters of Collins Bay, King George Island, Antarctica. Bulk elemental and stable isotope analyses were combined with molecular-level proxies (fatty acids and amino acids) to obtain more comprehensive evidence about POM characteristics. Two different settings were recognized. In February 2016, there was a reduced glacier melting condition and meltwater runoff due to cold weather. Marine fresh phytoplankton and ice-associated POM with the dominance of diatoms, bacteria, zooplankton and ice-associated fauna were the major components of POM. In contrast, in January 2017 under increased glacier melting and meltwater runoff due to warm weather conditions, suspended POM was constituted by marine phytoplankton, Rhodophyta macroalgae detritus and an increased terrestrial contribution. The lower degradation state of suspended POM in February 2016 than in January 2017 and the early bacterial response to the input of fresh protein-rich organic matter derived from marine phytoplankton, mainly diatoms, was supported. Our results provide evidence of the negative impact of glacier retreat and increased meltwater runoff on POM features in Collins Bay, which can be relevant to other Antarctic coastal marine ecosystems.

Chernozems of Western Transbaikal: Distribution, Chemical Fertility, Organic Matter, Humus, and Yields

Along with the chestnut soils, Transbaikal chernozems are intensely involved in regional crop farming. The latter are deficient in humus (4.2%), feature a light granulometric composition, occur in patches commonly along various slopes, and are exposed to deflation. The biological activity is low due to deep freezing (4 m) and slow thawing. Microorganisms are distributed in the topsoil. The volume of the annual input of fresh organic matter into the soil is low and is slowly destroyed. Their humus is fulvic-humate in nature with a high content of nonhydrolyzable residue, which, along with humic acid, has a high proportion of fulvic acids. It is only with reduced agrotechnical use and the application of adaptive land-use practices that it is possible to attain a spring wheat yield of up to 21 quintal/ha, as compared to the yields of 12–13 quintal/ha obtained on regional farms.

Coarse mineral-associated organic matter is a pivotal fraction for SOM formation and is sensitive to the quality of organic inputs

Recent models of soil organic matter (SOM) formation propose that the coarse mineral-associated fraction (coarse-MAOM) could be a pivotal fraction bridging the gap between fresh organic matter inputs and stabilized, mineral-associated SOM. However, little is known about the effect of organic input chemistry and soil texture on the carbon (C) and nitrogen (N) dynamics of this fraction. We studied SOM dynamics in a factorial field experiment comparing 10 combinations of crop residue management (retained or exported) with different fertilizer sources, including solid poultry, liquid dairy and liquid swine manure, a mineral fertilizer (NPK), and a control without N (PK) in two soils with contrasted texture. After 9 y of management, we compared the distribution of C and N within the light fraction (LF), the coarse-MAOM, and the fine mineral-associated organic matter fraction (fine-MAOM) of surface (0–5 cm) soil. We found that retention of mature crop residues increased soil C content through an increase in LF and coarse-MAOM but did not influence N content in any fractions. Conversely, the fertilizer source influenced C and N accumulation in the coarse- and fine-MAOM in the silty clay soil where greater values were generally found with poultry and liquid dairy manures. Similar trends were found in the sandy loam soil, but statistically significant differences were only found for C in the coarse-MAOM fraction. The coarse-MAOM fraction had a much lower C:N ratio (15–16) than LF (26–36) and appeared generally responsive to the different organic inputs. Despite its limited size (∼10% of total soil organic carbon), coarse-MAOM appeared to represent a central location for the formation of new SOM and for the accumulation of C and N following fresh organic inputs. We therefore propose that it is considered in models as a pivotal fraction of SOM formation in arable soils.