Organic Matter Turnover Research Articles

A pulse of simulated root exudation alters the composition and temporal dynamics of microbial metabolites in its immediate vicinity

Root exudation increases the concentration of readily available carbon (C) compounds in its immediate environment. This creates ‘hotspots’ of microbial activity characterized by accelerated soil organic matter turnover with direct implications for nutrient availability for plants. However, our knowledge of the microbial metabolic processes occurring in the immediate vicinity of roots during and after a root exudation event is still limited.Using reverse microdialysis, we simulated root exudation by releasing a13C-labelled mix of low-molecular-weight organic C compounds at mm-sized locations in undisturbed soil. Combined with stable isotope tracing, we investigated the fine-scale temporal and spatial response of microbial metabolism, soil chemistry, and traced microbial respiration and uptake of exuded compounds.Our results show that a 9-h simulated root exudation pulse leads to i) a large local respiration event and ii) alteration of the temporal dynamics of soil metabolites over the following 12 day at the exudation spot. Notably, we observed a threefold increase in ammonium concentrations at 12 h and increased nitrate concentrations five days after the pulse. Moreover, various short-chain fatty acids (acetate, propionate, formate) increased over the following days, indicating altered microbial metabolic pathways and activity. Phospholipid and neutral lipid fatty acids (PLFAs, NLFAs) of all major microbial groups were significantly 13C-enriched within a 5 mm radius around the microdialysis probes, but not beyond. The highest relative 13C enrichment was observed in fungal NLFAs, indicating that a significant proportion of the exuded compounds had been incorporated into fungal storage compounds.Our findings indicate that the punctual release of low-molecular-weight organic C compounds into intact soil significantly changes microbial metabolism and activity in its immediate surroundings, enhancing mineralization of native organic nitrogen. This highlights the versatility of microbial metabolic pathways in response to rapidly changing C availability and their effectiveness in increasing nutrient availability near plant roots.

Contrasting impacts of plastic film mulching and nitrogen fertilization on soil organic matter turnover

Plastic film mulching and nitrogen (N) fertilization are two important field management practices used to increase crop yields in rain-fed agriculture in arid and semi-arid areas. These two practices, however, can have opposing effects on soil organic carbon (SOC) stock and turnover. To clarify and analyze the combined effects of these practices, we conducted a 7-year continuous maize cultivation experiment with four treatments: (1) no plastic film mulching without N fertilization (Control), (2) plastic film mulching without N fertilization (PFM), (3) N fertilization without plastic film mulching (N), and (4) plastic film mulching with N fertilization (PFM + N). The 13C natural abundance of in the light and heavy fractions of organic carbon (LFOC and HFOC) was used to differentiate “old” (>7 years) and “new” (<7 years) C in the soil after C3 to C4 vegetation change. PFM increased soil temperature and moisture content over 7 years and led to a 150 % and 108 % increase in the above-ground and root biomass of maize, respectively. Hot and wet conditions under PFM accelerated the decomposition of both labile C (i.e., LFOC) and persistent C (i.e., HFOC), as measured by CO2 efflux from the soil. PFM decreased the LFOC and HFOC pools due to the fast decomposition of “old” C and inadequate stabilization of “new” C. Furthermore, PFM accelerated HFOC decomposition to a greater extent than that of LFOC. The Q10 values of SOC decomposition remained similar both in the presence and absence of PFM. Nitrogen fertilization increased the aboveground and root biomass of maize by 54 % and 40 %, respectively. In the control soil, microorganisms decomposed organic N owing to limited availability of mineral N and in the absence of any from replenishment by fertilizer N. Thus, the increase in LFOC content under N fertilization, was primarily attributed to the reduced decomposition of “old” C. The use of N fertilization produced a similar HFOC content as that of the control soil, while also accelerating the decomposition of “old” C and promoting the stabilization of “new” C, resulting in a faster turnover rate. Consequently, the impact of N fertilization on SOC turnover was negligible owing to the contrasting effects on LFOC and HFOC. The application of N fertilization resulted in a reduction of the Q10 value as compared to control. The combination of PFM and N fertilization accelerated the decomposition of HFOC and SOC, while having absent effect on SOC, labile C and persistent C contents. This indicates that 7 years weren’t long enough to trigger a response to the combination of PFM and N fertilization from SOC and HFOC contents. In summary, PFM led to a decline in the SOC content by accelerating the decomposition of both LFOC and HFOC. Conversely, N fertilization maintained SOC content by inhibiting LFOC decomposition and enhancing HFOC decomposition.

Labile carbon-induced soil organic matter turnover in a subtropical forest under different redox conditions

Labile organic carbon (LOC) input strongly affects soil organic matter (SOM) dynamics, including gains and losses. However, it is unclear how redox fluctuations regulate these processes of SOM decomposition and formation induced by LOC input. The objective of this study was to explore the impacts of LOC input on SOM turnover under different redox conditions. Soil samples were collected in a subtropical forest. A single pulse of 13C-labeled glucose (i.e., LOC) was applied to the soil. Soil samples were incubated for 40 days under three redox treatments, including aerobic, anoxic, and 10-day aerobic followed by 10-day anoxic conditions. Results showed that LOC input affected soil priming and 13C-SOM accumulation differently under distinct redox conditions by altering the activities of various microorganisms. 13C-PLFAs (phospholipid fatty acids) were analyzed to determine the role of microbial groups in SOM turnover. Increased activities of fungi and gram-positive bacteria (i.e., the K-strategists) by LOC input could ingest metabolites or residues of the r-strategists (e.g., gram-negative bacteria) to result in positive priming. Fungi could use gram-negative bacteria to stimulate priming intensity via microbial turnover in aerobic conditions first. Reduced activities of K-strategists as a result of the aerobic to anoxic transition decreased priming intensity. The difference in LOC retention in SOM under different redox conditions was mainly attributable to 13C-particulate organic carbon (13C-POC) accumulation. Under aerobic conditions, fungi and gram-positive bacteria used derivatives from gram-negative bacteria to reduce newly formed POC. However, anoxic conditions were not conducive to the uptake of gram-negative bacteria by fungi and gram-positive bacteria, favoring SOM retention. This work indicated that redox-regulated microbial activities can control SOM decomposition and formation induced by LOC input. It is extremely valuable for understanding the contribution of soil affected by redox fluctuations to the carbon cycle.

Karacabey Kıyı Subasar Ormanlarında Kızılağaç ve Dişbudak yeşil yapraklarının biyokimyasal bileşimi üzerinde tuzlu su girişinin etkisi

Coastal forested wetlands provide substantial benefits to society, such as wave attenuation, erosion control, biodiversity support, and carbon sequestration. Many of these unique coastal ecosystems have been drained for various reasons, while those that remain are now threatened by salt water intrusion and sea level rise due to climate change. In this study, we aimed to investigate the effects of soil salinity on the biochemical components of the fresh leaves of alder (Alnus glutinosa L. Gaertn) and ash tree (Fraxinus angustifolia Vahl.) which are the dominant tree species in Karacabey coastal forested wetland next to the Sea of Marmara in Türkiye. For this purpose, fresh leaf and soil samples of alder and ash trees were collected from three zones (Z1: 0-1 km, Z2: 1 to 2 km and Z3: 2 to 3 km) from the inner border (Z3) of the forested wetland to the coastline (Z1) of the Sea of Marmara. The fresh leaf samples were analyzed for photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoids), anthocyanin, xanthophylls, free amino acids, total nitrate, proline, total polyphenols, total soluble tannins, total phenolic compounds, glucose, sucrose and total carbohydrates. The soil samples were analyzed for soil pH, electrical conductivity and soil texture. The results showed that the soil salinity decreased from the coastline (Z1) towards the inner border (Z3). Similarly, mean photosynthetic pigments and anthocyanin, xanthophyll also decreased from the Z1 towards Z3, whereas mean total polyphenols and total soluble tannin concentrations increased for the both tree species. The other biochemical compounds showed either an increase or a decrease according to the tree species. These pioneer results illustrate the important point that biotic or abiotic environment in which tree grows significantly change the specific biochemical components in the fresh leaves of alder and ash trees in the coastal forested wetlands. In turn, these changes may result in variation in nutrient cycling, carbon cycling, and organic matter turnover rates in these forest ecosystems.

Mangrove peat and algae leachates elicit rapid and contrasting molecular and microbial responses in coastal waters

As sea level rises, previously sequestered blue carbon can be exported offshore as particulate or dissolved organic matter where it may be re-mineralized or sequestered. The priming effect, or interactive effects of organic matter turnover with a mixed substrate, is well described in soils, but still debated in aquatic systems. Priming may contribute to enhanced blue carbon re-mineralization in coastal environments. Here we examined mangrove-derived dissolved organic matter turnover in a lab incubation, with leachates from mangrove peat, 13C-labeled algae, and peat+algae (primed). Particulate and dissolved organic matter were assessed; microbial metatranscriptomes were evaluated; and dissolved organic matter was characterized with high resolution mass spectrometry. Stable isotopes indicated rapid allocation of algal-derived dissolved organic matter into particulate organic matter. The algal treatment had the greatest increase in carbon dioxide, but primed and peat treatments had the greatest loss of dissolved organic carbon, greater RNA concentrations, and similar changes in total carbon dioxide. This suggests that, while total carbon dioxide did not increase under priming conditions, the addition of a peat substrate may promote microbial biomass production relative to carbon dioxide production. This work highlights that more targeted studies investigating the specific mechanisms of priming are necessary to address the molecular and microbial transformations associated with priming in aquatic systems.

Technogenic soil salinisation, vegetation, and management shape microbial abundance, diversity, and activity

The importance of the microbiome in the functioning of degraded lands in industrialised zones is significant. However, little is known about how environmental parameters affect microbial abundance, structure, diversity, and especially specific guilds involved in the nitrogen cycle in saline soils influenced by the soda industry. To address this knowledge gap, our research focused on assessing the microbiota in relation to soil properties and plant species composition across two transects representing different types of land use: saline wasteland and arable fields. Our findings show that the microbial communities were the most affected not only by soil salinity but also by pH and the composition of plant species. Taxonomic variability was the most shaped by salinity together with management type and CaCO3 content. The impact of salinity on the soil microbiome was manifested in a reduced abundance of bacteria and fungi, a lower number of observed phylotypes, reduced modularity, and a lower abundance of the nitrifying guild. Denitrification and nitrogen fixation were less affected by salinity. The last process was correlated with calcium carbonate. CaCO3 was also associated with microbial taxonomic variability and the overall microbial activity caused by hydrolases, which could aid organic matter turnover in saline but carbonate-rich sites. Bacterial genera such as Bacillus, Peanibacillus, and Rhodomicrobium, in addition to fungal taxa such as Cadophora, Mortierella globalpina, Preussia flanaganii, and Chrysosporium pseudomerdarium, show potential as favourable candidates for possible bioremediation initiatives. These results can be applied to future land reclamation projects. Funding informationThis research received no specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Biogeographical patterns of the soil fungal:bacterial ratio across France.

Soils are one of the major reservoirs of biological diversity on our planet because they host a huge richness of microorganisms. The fungal:bacterial (F:B) ratio targets two major functional groups of organisms in soils and can improve our understanding of their importance and efficiency for soil functioning. To better decipher the variability of this ratio and rank the environmental parameters involved, we used the French Soil Quality Monitoring Network (RMQS)-one of the most extensive and a priori-free soil sampling surveys, based on a systematic 16 km × 16 km grid and including more than 2,100 samples. F:B ratios, measured by quantitative PCR targeting the 18S and 16S rDNA genes, turned out to be heterogenously distributed and spatially structured in geographical patterns across France. These distribution patterns differed from bacterial or fungal densities taken separately, supporting the hypothesis that the F:B ratio is not the mere addition of each density but rather results from the complex interactions of the two functional groups. The F:B ratios were mainly influenced by soil characteristics and land management. Among soil characteristics, the pH and, to a lesser extent, the organic carbon content and the carbon:nitrogen (C:N) ratio were the main drivers. These results improved our understanding of soil microbial communities, and from an operational point of view, they suggested that the F:B ratio should be a useful new bioindicator of soil status. The resulting dataset can be considered as a first step toward building up a robust repository essential to any bioindicator and aimed at guiding and helping decision making. IMPORTANCE In the face of human disturbances, microbial activity can be impacted and, e.g., can result in the release of large amounts of soil carbon into the atmosphere, with global impacts on temperature. Therefore, the development and the regular use of soil bioindicators are essential to (i) improve our knowledge of soil microbial communities and (ii) guide and help decision makers define suitable soil management strategies. Bacterial and fungal communities are key players in soil organic matter turnover, but with distinct physiological and ecological characteristics. The fungal:bacterial ratio targets these two major functional groups by investigating their presence and their equilibrium. The aim of our study is to characterize this ratio at a territorial scale and rank the environmental parameters involved so as to further develop a robust repository essential to the interpretation of any bioindicator of soil quality.

BODIUM—A systemic approach to model the dynamics of soil functions

AbstractThe increasing demand for biomass for food, animal feed, fibre and bioenergy requires optimization of soil productivity, while at the same time, protecting other soil functions such as nutrient cycling and buffering, carbon storage, habitat for biological activity and water filter and storage. Therefore, one of the main challenges for sustainable agriculture is to produce high yields while maintaining all the other soil functions. Mechanistic simulation models are an essential tool to fully understand and predict the complex interactions between physical, biological and chemical processes of soils that generate those functions. We developed a soil model to simulate the impact of various agricultural management options and climate change on soil functions by integrating the relevant processes mechanistically and in a systemic way. As a special feature, we include the dynamics of soil structure induced by tillage and biological activity, which is especially relevant in arable soils. The model operates on a 1D soil profile consisting of a number of discrete layers with dynamic thickness. We demonstrate the model performance by simulating crop growth, root growth, nutrient and water uptake, nitrogen cycling, soil organic matter turnover, microbial activity, water distribution and soil structure dynamics in a long‐term field experiment including different crops and different types and levels of fertilization. The model is able to capture essential features that are measured regularly including crop yield, soil organic carbon, and soil nitrogen. In this way, the plausibility of the implemented processes and their interactions is confirmed. Furthermore, we present the results of explorative simulations comparing scenarios with and without tillage events to analyse the effect of soil structure on soil functions. Since the model is process‐based, we are confident that the model can also be used to predict quantities that have not been measured or to estimate the effect of management measures and climate states not yet been observed. The model thus has the potential to predict the site‐specific impact of management decisions on soil functions, which is of great importance for the development of a sustainable agriculture that is currently also on the agenda of the ‘Green Deal’ at the European level.

Pore scale modeling of the mutual influence of roots and soil aggregation in the rhizosphere

Investigating plant/root-soil interactions at different scales is crucial to advance the understanding of soil structure formation in the rhizosphere. To better comprehend the underlying interwoven processes an explicit, fully dynamic spatial and image-based modeling at the pore scale is a promising tool especially taking into account experimental limitations. We develop a modeling tool to investigate how soil aggregation, root growth and root exudates mutually interact with each other at the micro-scale. This allows the simultaneous simulation of the dynamic rearrangement of soil particles, the input and turnover of particulate organic matter, root growth and decay as well as the deposition, redistribution and decomposition of mucilage in the rhizosphere. The interactions are realized within a cellular automaton framework. The most stable configuration is determined by the amount and attractiveness of surface contacts between the particles, where organo-mineral associations preferably lead to the formation of soil aggregates. Their break-up can be induced by root growth or the degradation of gluing agents previously created after the decomposition of particulate organic matter and mucilage. We illustrate the capability of our model by simulating a full life cycle of a fine root in a two-dimensional, horizontal cross section through the soil. We evaluate various scenarios to identify the role of different drivers such as soil texture and mucilage. We quantify the displacement intensity of individual particles and the variations in local porosity due to the change in available pore space as influenced by the root growth and observe compaction, gap formation and a biopore evolution. The simulation results support that the deposition of mucilage is an important driver for structure formation in the rhizosphere. Although mucilage is degraded within a few days after exudation, it leads to a persistent stabilization of the aggregated structures for both textures in the vicinity of the root within a time frame of 1000 days. Local porosity changes are quantified for exudation periods of 1, 10 and 100 days and are already pronounced for short-term exudation of mucilage. This stabilization is significantly different from the structures encountered when only POM could trigger the evolution of gluing spots, and is still present after complete degradation of the root.

Major challenges in widespread adaptation of aerobic rice system and potential opportunities for future sustainability

Climate change and diminishing resources are the major challenges in sustainable rice production. The aerobic rice system can be a transformational approach to replace conventional rice in the face of climate change and scarcity of resources due to its high input-use efficiency. However, it could not be adapted widely yet due to several challenges like higher weed infestation and low nitrogen (N) use efficiencies. This transformed system can only be adapted at a wider scale through investigation of associated constraints and risks and provision of potential adjustive measures. Aerobic rice crop failure can be reduced if the availability of suitable and stress-resistant cultivars with early improved vigour, and short-duration characteristics are ensured for maintaining the reproductive capability under resource stressed environment. This transformed rice system exhibits limited response to applied N because of increased losses due to higher volatilization and rapid coupled nitrification-denitrification processes under alternate wetting and drying systems. Shifting from the flooded to aerobic rice system bringing changes in crop water demand, organic matter (OM) turnover, N dynamics, weed flora, and greenhouse gases (GHGs) emissions. Along with N-losses, continuous monocropping, higher weed infestation, weedy rice emergence, nitrous oxide (N2O) emissions, nutrient disorders, higher prevalence of pathogens and diseases, increased panicle sterility, and an increase in soilborne pathogens lodging are other major constraints negatively impacting the widespread adaptation of aerobic rice. This article reviews the climate change status, its associated impacts on the sustainability of traditional rice systems, and the rationale for transforming from traditional to aerobic rice systems. Furthermore, it describes the major challenges associated with the aerobic rice system, thereby presenting management strategies ensuring its wider adaptation for sustainable rice production in the face of projected climate change and scarcity of resources.

Uncovering the dominant role of root metabolism in shaping rhizosphere metabolome under drought in tropical rainforest plants

Plant-soil-microbe interactions are crucial for driving rhizosphere processes that contribute to metabolite turnover and nutrient cycling. With the increasing frequency and severity of water scarcity due to climate warming, understanding how plant-mediated processes, such as root exudation, influence soil organic matter turnover in the rhizosphere is essential. In this study, we used 16S rRNA gene amplicon sequencing, rhizosphere metabolomics, and position-specific 13C-pyruvate labeling to examine the effects of three different plant species (Piper auritum, Hibiscus rosa sinensis, and Clitoria fairchildiana) and their associated microbial communities on soil organic carbon turnover in the rhizosphere. Our findings indicate that in these tropical plants, the rhizosphere metabolome is primarily shaped by the response of roots to drought rather than direct shifts in the rhizosphere bacterial community composition. Specifically, the reduced exudation of plant roots had a notable effect on the metabolome of the rhizosphere of P. auritum, with less reliance on neighboring microbes. Contrary to P. auritum, H. rosa sinensis and C. fairchildiana experienced changes in their exudate composition during drought, causing alterations to the bacterial communities in the rhizosphere. This, in turn, had a collective impact on the rhizosphere's metabolome. Furthermore, the exclusion of phylogenetically distant microbes from the rhizosphere led to shifts in its metabolome. Additionally, C. fairchildiana appeared to be associated with only a subset of symbiotic bacteria under drought conditions. These results indicate that plant species-specific microbial interactions systematically change with the root metabolome. As roots respond to drought, their associated microbial communities adapt, potentially reinforcing the drought tolerance strategies of plant roots. These findings have significant implications for maintaining plant health and preference during drought stress and improving plant performance under climate change.

Prescribed Burning Alters Insects and Wood Decay in a Sagebrush-Steppe Rangeland in Southwestern Idaho, United States

Prescribed fall burning is commonly used worldwide on rangeland sites to enhance vegetation resources and restore disturbed ecosystems, but little is known about how it may alter microbial communities and insect activities. We used two site treatments (low- to moderate-burn severity plots and unburned control plots) at a high-elevation (2 100 m above sea level) shrub-steppe rangeland site. Four yr after the prescribed burn, captured insects primarily consisted of Coleoptera (beetles), Hymenoptera (ants, bees, and wasps), and Orthoptera (grasshoppers and crickets) species with similar or greater numbers captured on burned plots as compared with unburned plots. We used wood stakes to assess microbial population changes. Aspen (Populus tremuloides Michx.) and pine (Pinus taeda L.) wood stakes were placed horizontally on the soil surface and vertically into the mineral to determine microbially altered wood decay rates. In mineral soil, mass loss (wood decay) of both aspen and pine increased significantly in the burned plots after 5 yr. Surface aspen also had increased decay in the burned plots, but pine surface stakes were unaffected. Surprisingly, at this high-elevation site we also found subterranean termites (Reticulitermes tibialis Banks) feeding on both stake species, with greater numbers on aspen stakes in burned plots leading to greater wood mass loss and organic matter turnover rates. Our results suggest that that fall prescribed burning can enhance insect numbers and microbial decay in sagebrush-steppe ecosystems. Understanding these changes can help land managers predict the influence of fall prescribed burning operations on soil biological properties and insect communities.

Tracing sources and turnover of soil organic matter in a long-term irrigated dry forest using a novel hydrogen isotope approach

The contribution from foliage, roots, and fungi to soil organic matter (SOM) represents a key unknown in SOM dynamics. Since stable hydrogen isotope ratios (δ2Hn) can greatly differ among these sources, we explored δ2Hn to elucidate sources contribution to SOM. Moreover, we assessed whether addition of 2H-depleted water – providing 2H-signal that gets incorporated into organic matter – allows tracing of new organic H and thus assessing SOM turnover.In a 17-year-long irrigation experiment in a dry pine forest, we measured δ2Hn and stable carbon and nitrogen isotope ratios (δ13C, δ15N) in SOM sources, bulk SOM and particle-size fractions. Relative source contribution to SOM was estimated by Bayesian mixing models including all three isotopes.The δ2Hn increased from foliar litter (−153‰) to fine roots (−118‰) and fungal mycelia (−81‰). Mixing models indicated that foliar litter dominated organic layers (69 ± 15%) and coarse particulate organic matter (POM) at 0–5 cm (65 ± 12%). In contrast, roots dominated fine POM at 2–5 cm (58 ± 12%). Fungal mycelia contributed only to 1–5% of coarse and fine POM, but to 5–25% of mineral associated organic matter (MOM) at 0–5 cm.The soil water 2H-depletion with irrigation (−76‰ vs −68‰ at 0–10 cm) was paralleled by lower δ2Hn in coarse POM of irrigated vs dry plots (−122‰ vs −111‰ at 0–5 cm), indicating organic H turnover of less than 17 years. In contrast, δ2Hn in fine POM and MOM decreased only by 3‰ with irrigation, implying mean residence times of approximately 30 years.While δ13C and δ15N differed by a maximum of 1‰ between plant sources, δ2Hn were 30–39‰ higher in roots than foliar litter, thus improving SOM sources differentiation. Long-term 2H-labelling by irrigation indicated higher organic H turnover in coarse vs fine soil fractions, offering a novel tool to identify SOM cycling and microbial H incorporation into SOM pools.

New directions for the Tea Bag Index: Alternative teabags and concepts can advance citizen science

AbstractSince its first publication in 2013, the Tea Bag Index (TBI) has attained widespread ecological application, generating scientific data on stabilization and turnover of organic matter in soils. As a result of manufacturing changes (net structure, net material, tea varieties) in the past years, a critical examination of alternative tea bag products is necessary. The present study for the first time generated decomposition data on a wide range of potential alternative tea brands (four brands tested) and varieties (rooibos [RB], green [GT], black [BT], and peppermint tea [PM]). They were tested in both a laboratory and a field experiment lasting 90 days. Under controlled laboratory conditions GT, BT, and PM corresponded well to the kinetic assumptions of an asymptote model of the TBI, but RB did not. In the field experiment, the tea mass remaining after 90 days was determined for all tea bags across different land uses (arable land, grassland, and forest). BT imitated the decomposition behavior of Lipton's original GT even better than any of the GT tested. The finer structure of Lipton's new tea bag nets did not negatively affect the precision of resultant remaining masses and thus can be recommended for future studies. In conclusion, alternative tea bags should be explored across brands as well as varieties. Based on our findings, we proposed an update of the TBI, which is based on the readily feasible modeling of the decomposition curve. Crucial is the combined consideration of both multiple tea varieties and incubation intervals of different durations.

Direct Seeded Rice as Resource Efficient Technology

In Asia, seedlings are typically transplanted into puddled soil to cultivate rice (Oryza sativa L.). Due of the manpower, water, and energy requirements of this manufacturing system, it is becoming less lucrative as these resources become more scarce. Additionally, it impairs the soil's physical qualities, negatively impacts the performance of succeeding upland crops, and increases methane emissions. Different issues, such as a declining water table, a manpower shortage during peak seasons, and worsening soil quality, need the use of alternate establishment techniques to maintain both natural resource and rice yield. Due to its low input requirement, ability to reduce greenhouse gas emissions, and ability to adjust to climatic hazards, the direct seeded rice (DSR) technique has gained a lot of attention and popularity in recent years. Dry seed must be sown into a ready seedbed, while pre-germinated seed must be sown into standing water and puddles of soil. Many farmers have switched from transplanted to DSR culture as a result of the introduction of early maturing cultivars, the application of effective fertiliser management techniques, and increased adoption of integrated weed management strategies. In certain industrialised nations, such the USA, Australia, Japan, China, and Korea, DSR technology is heavily mechanised. By switching from conventional rice to DSR, crop water requirements, soil organic matter turnover, improved nutrient management, carbon sequestration, weed management, greenhouse gas emissions, and crop intensification will all be significantly reduced. The transition from PTR to DSR is hampered by a number of factors, including a high weed infestation, the development of weedy rice, an increase in soil-borne diseases (nematodes), nutritional disorders, poor crop establishment, lodging, the prevalence of blast, brown leaf spot, etc. By addressing these limitations, DSR may show to be a very viable, economically and technically viable substitute for PTR.

The patchiness of soil 13C versus the uniformity of 15N distribution with geomorphic position provides evidence of erosion and accelerated organic matter turnover

Farming on hillslopes often affects the accumulation and loss of soil organic matter (SOM) depending on slope position and cropping patterns. Most hillslope studies focus on soil movement to characterize SOM turnover under erosive conditions. In this study, we trace erosion and characterize agronomic practices erosive impacts on SOM translocation and transformation along geomorphic positions. To achieve this, we assessed the horizontal distribution (upper 15 cm) and vertical distribution (to 100 cm profiles) of soil δ15N and δ13C isotope abundance individually. We mapped the spatial distribution of δ13C, δ15N, and SOM turnover indices as a novel approach to tracing erosion and degradation of SOM in the field. Except for tillage (conventional vs. reduced tillage), other individual agricultural practices (residue removal with no cover crop vs. retaining residuals, cover cropping, and fertilizer 0, 40, and 80 kg ha-1 nitrogen) caused no significant shifts in δ15N and δ13C values in topsoil (0–15 cm). Among the evaluated factors, topography and depth predicted soil δ15N and δ13C profiles. Trends in δ13C vs. δ15N showed a wider range of δ13C values in topsoil of upslope plots under reduced tillage, while in the depositional location, conventional tillage had the same effect. This suggests erosion under reduced tillage occurred. Erosion and accelerated decomposition gradually slowed δ13C enrichment with soil depth. Digital soil mapping approach depicted low continuity of δ13C vs. high continuity of δ15N with geomorphic position We attributed the intermediate δ13C values, and steeper slope of δ13C against logarithm of soil organic carbon (SOC) across the slope to erosion and high SOM turnover, particularly of recently added plant inputs. Current results support the prediction of intensive vs. conservation practices’ effects on upslope soil stability and the fate of SOM in both topsoil and at depth of sloping farmlands.

Deciphering riverine dissolved organic matter biodegradation: Evidence from three-dimensional fluorescence

Biological degradation contributes significantly to aquatic dissolved organic matter (DOM) turnover. Yet to date, the specific biological pathways involved in DOM degradation are still obscure. Here, we show how DOM changes in response to 28-d incubation in a karst river, via the combined fluorescent excitation-emission matrix (EEM) with parallel factor analysis (PARAFAC) and peak picking. Tyrosine-, tryptophan- and UV humic-like components were identified as the primary DOM in original waters. Notable decreases of protein-like components were observed after 15 ℃ incubation, suggesting preferential consumption of biogenic DOM. Elevated temperature (30 ℃), however, caused significant decreases in visible humic-like and terrestrial fulvic-like components, but an increase in UV humic-like components in comparison to 15 ℃ incubation (p < 0.05). Humification index (HIX) decreased dramatically after 15 ℃ incubation, indicating that biological processes regulated river DOM properties. Our results highlight the significant biogenic signals in karst rivers, and evidence that fluorescence measurement can decipher riverine DOM biodegradation as a useful approach.

Ideas and perspectives: Alleviation of functional limitations by soil organisms is key to climate feedbacks from arctic soils

Abstract. Arctic soils play an important role in Earth's climate system, as they store large amounts of carbon that, if released, could strongly increase greenhouse gas levels in our atmosphere. Most research to date has focused on how the turnover of organic matter in these soils is regulated by abiotic factors, and few studies have considered the potential role of biotic regulation. However, arctic soils are currently missing important groups of soil organisms, and here, we highlight recent empirical evidence that soil organisms' presence or absence is key to understanding and predicting future climate feedbacks from arctic soils. We propose that the arrival of soil organisms into arctic soils may introduce “novel functions”, resulting in increased rates of, for example, nitrification, methanogenesis, litter fragmentation, or bioturbation, and thereby alleviate functional limitations of the current community. This alleviation can greatly enhance decomposition rates, in parity with effects predicted due to increasing temperatures. We base this argument on a series of emerging experimental evidence suggesting that the dispersal of until-then absent micro-, meso-, and macroorganisms (i.e. from bacteria to earthworms) into new regions and newly thawed soil layers can drastically affect soil functioning. These new observations make us question the current view that neglects organism-driven “alleviation effects” when predicting future feedbacks between arctic ecosystems and our planet's climate. We therefore advocate for an updated framework in which soil biota and the functions by which they influence ecosystem processes become essential when predicting the fate of soil functions in warming arctic ecosystems.

Biodegradable plastic mulch films increase yield and promote nitrogen use efficiency in organic horticulture

Plastic film mulches (PFM) are used extensively due to their ability to increase yield and suppress weed emergence. Their effects on plant-soil-microbial interactions, however, are less well understood. Organic systems rely on the supply of nutrients from organic sources (e.g., manures and fertility-building leys) and where poor N availability often limits yield. The issue is compounded by horticultural crops (e.g., lettuce) with a high N demand, but which are inefficient at recovering N from soil. The effect of PFM on the mineralisation of organic fertilisers and its interaction with other agronomic factors such as planting density is also less well-researched. We hypothesised that biodegradable PFM would be a useful tool to increase the efficiency of N management in organic horticulture by increasing the supply of available N leading to increased crop N uptake and crop yield, and simultaneously reducing N losses during the growing season. We conducted two field experiments under a temperate maritime climate with lettuces grown with either conventional (LDPE) PFM or a polylactic acid-based biodegradable PFM alongside un-mulched controls. The first experiment involved black or white coloured PFMs and two planting densities while the second experiment involved treatments with and without addition of poultry manure. Overall, yields were increased by 39% in both experiments with PFM, while soil mineral N concentrations were up to 5 times higher with PFM than without. Measurements of soil organic matter (SOM) turnover (Tea Bag Index) and soil CO2 efflux indicated a more rapid decay of SOM in the presence of the PFM. The use of PFM also promoted N use efficiency (NUE) by 300% in the presence of poultry manure. Denser planting with PFM resulted in proportionately higher yields. Higher yield and higher N concentrations (5-10%) in crop tissue in mulched plots resulted in higher total N uptake, however, total N uptake was low compared to soil concentrations: mulching with biodegradable PFM resulted in higher residual mineral N than un-mulched plots (77-147 mg kg-1 vs. 19 -70 mg kg-1). Our results are consistent with reduced N losses to the environment during the growing season and increased mineralisation under PFM. In conclusion, our findings support the adoption of PFM for organic horticulture and show that biodegradable PFM perform similarly to LDPE-based PFMs.

Mineral type and land-use intensity control composition and functions of microorganisms colonizing pristine minerals in grassland soils

Mineral surfaces in soil are an important interface for organic matter (OM) and nutrient cycling, with associated microorganisms contributing to the formation and turnover of mineral-associated OM (MAOM). However, the relevance of intrinsic (mineral type) versus extrinsic (land-use intensity) factors on the co-development of MAOM and microorganisms under natural conditions remains poorly understood. Mineral containers filled with mixtures of quartz-sand and pristine secondary minerals (goethite or illite) were exposed to 50 grassland topsoils of the Schwäbische Alb (Germany) along a land-use intensity gradient for five years. Mineral samples and soils were analyzed for organic carbon (OC) and nutrients (N and P), abundance and composition of major microbial groups based on phospholipid fatty acid profiles, as well as enzyme activities (β-glucosidase, β-xylosidase, N-acetylglucosaminidase, and acid phosphatase). Microorganisms colonized both mineral samples to the same extent, with goethite samples exhibiting greater MAOM accumulation and higher enzyme activities than illite samples. Both mineral samples differed from the overlying soils with greater relative abundances of fungi and Gram-negative bacteria and greater microbial acquisition of nutrients (N and P) relative to C as indicated by the stoichiometry of enzyme activities. Increasing land-use intensity was associated with decreasing C:N ratios and microbial abundances for goethite samples and increasing β-glucosidase activity for illite samples while the proportion of fungi was reduced in both mineral samples. We conclude that in the studied temperate grasslands the association of OM and microorganisms with secondary minerals is driven more by mineral type and reactivity than by differences in land-use intensity. The different minerals apparently formed distinct microhabitats with unique characteristics that differed in MAOM accumulation and microbial access to OC and nutrients, thus affecting microbial colonization and functionality.