Carbon Input Research Articles

REE + Y uptake in bioapatite revisited: Facies-controlled variability in coeval conodonts

REE + Y concentrations in bioapatite have been repeatedly shown to be ambiguous carriers of information for paleo-seawater reconstructions. The uptake of REE + Y begins rapidly after deposition, and bioapatite can, therefore, potentially record the pore water conditions during early diagenesis. In this study, we examined the REE + Y composition of conodonts belonging to a single species from three coeval facies across the Ordovician Baltic Paleobasin carbonate shelf. We show that the REE + Y concentrations, as well as Ce/Ce*, bell-shape index (BSI), Y/Ho and LaN/SmN ratios, vary systematically in conodonts from facies with different host rock compositions. Dissimilarly, REE + Y patterns and ratios of conodont dental elements from the same facies are uniform, indicating that the local depositional environment imposes principal control on the REE + Y uptake by bioapatite. Furthermore, our results show that even though REE + Y concentrations in different histological tissues of the same conodont vary significantly, no REE-specific fractionation occurs, resulting in similar REE + Y ratios in all tissue types. Our results illustrate that conodonts can record modified seawater-like characteristics of REE compositions if the input of FeMn oxyhydroxides and particulate organic carbon (POC) is low. In contrast, under elevated POC and FeMn oxyhydroxide loading, the conodonts show increasingly positive Ce anomalies and MREE enrichment. Collectively, these results suggest that, although primary seawater signatures are not captured in bioapatite, the conodont REE + Y patterns are controlled by local depositional environments and can record early diagenetic pore water conditions.

Plant carbon removal affects monthly temperature sensitivity of soil respiration during growing season in three typical plantations in the Loess Hilly Region, China

The temperature sensitivity of soil respiration (Q10) is primarily related to annual climate parameters and soil properties in regional models. Changes in climate and soil properties influence plant carbon (C) inputs, which in turn alter Q10 during the growing season. However, the magnitude of this influence remains uncertain. Herein, using meteorological data from small stations and field experiments across three plantations (Robinia pseudoacacia, Caragana korshinskii, and Bothriochloa ischaemum), we investigated the variations in Q10 with respect to the following four treatments: litter removal, root trenching, root trenching and litter removal, and a control group, for 6 months. We found that R. pseudoacacia had the highest relative contribution to atmospheric C release, followed by C. korshinskii, and then B. ischaemum. The Q10 components were month-dependent under C remove in three typical plantations. Litter removal significantly reduced Q10, but the effect of root trenching differed according to the plantation. The variations of Q10 with C removal were primarily controlled by interacting air temperature, air pressure, potential hydrogen, root C, root biomass, soil organic C, and relative humidity, explaining the variations of 81.29–99.33 % (R. pseudoacacia plantation), 89.1–99.76 % (C. korshinskii plantation), and 99.34–99.74 % (B. ischaemum) of the variation in Q10; among them, the cumulative contribution of climate (soil temperature, air pressure, and relative humidity), plants (root C and root biomass), and soil properties (potential hydrogen and soil organic C) accounted for 34.84 %–61.93 %, 12.15 %–32.75 %, and 8.07 %–32.11 %, respectively. These findings demonstrate that C removal variations influenced the Q10, whereas Q10 was also month-dependent. Overall, we recommend that the role of Q10 in the growing season should carefully consider C removal into the soil to improve the biosphere–atmosphere C exchange model in the context of climate change.

How water table level influences C balance under different fertilization regimes

Carbon sequestration in soil has been extensively sought in the agroecosystems through practices which increase organic carbon inputs and/or decrease soil organic carbon (SOC) degradation processes. Less is known about the extent of shallow water table influences in mineral soils, despite being soil moisture a major driver in modifying the C cycle. To examine its effects, a 4-yr lysimetric experiment was set up to measure the C balance components under free drainage and shallow water table at 60 and 120 cm depth. Two levels of N input (250 and 368 kg N ha−1 y−1) were also studied, using dry manure in 2011 and 2012 and fresh manure in 2013 and 2014. Carbon balance was estimated through the difference between inputs (C from organic inputs and root residues and exudates) and outputs (heterotrophic respiration, methane, and C leaching). A negative C balance was measured under all treatments (−3487 kg C ha-1), being respiration not compensated by the consistent C input of organic fertilizer. Furthermore, high N inputs increased SOC mineralization, decreasing the C balance. The role of soil was also observed by the SOC analyses, which confirmed the losses estimated through C balance. The study substantiated also the interacting effect between shallow water table and type of organic carbon, which was revealed crucial for C balance in mineral soils. To conclude, results suggested that water table level around 120-cm depth could limit SOC depletion.

Carbon Sequestration in Low Land Paddy Soils: Effect of Certain Cultural and Nutrient Management Practices: A Review

Carbon(C) is the only key to running in this worldly life and without carbon, nothing can be ensured, but the amount and form of C in different spheres of the earth make numerous changes. Changes in the carbon levels cause the lives of all living things. Soil carbon flux directly or indirectly affects the global climate and thus agriculture productivity. To ensuring the human rations, protection is intended for the rising populace worldwide, where the critical challenges in the agriculture sector are inevitable. Improved soil and nutrient supervisions and cultural practices are very imperative to tackling these troubles. Augmenting the productivity of various agro-ecosystems, soil productiveness, and carbon accretion via certain approaches become a must concern towards sustainable food production. “Paddy soils form the huge area of artificial swamplands on the earth, and serves as food basket for the world population also responsible for sequestering soil organic carbon potentially”. Rice accounts for around 9-10 % of the total cropland area globally, and their environmental conditions are responsible for storing organic carbon in soil, methane (CH4) production, and emit nitrous oxide (N2O) in meager amount. The present review signifies the present and future potential agricultural management practices, particularly soil and plant nutrition and their effects on soil organic carbon storage (SOCS) and carbon sequestration (CS) by paddies grown under submerged conditions compared to other crops. Increasing carbon inputs and reducing SOC losses in low land paddy soils need attention as its concern with GHGs that implies direct causes of global climate. As future direction, life-cycle assessments of certain practices in low land paddy soils helps in assessing the carbon footprints and sustaining the crop productivity consequently mitigating climate change. With this view, this review study was taken to the life of carbon in the terrestrial ecosystem and its accumulation in low land paddy soils moderated by cultural and nutrient management practices adapted for rice production in low lands.

The fusion of multiple scale data indicates that the carbon sink function of the Qinghai-Tibet Plateau is substantial

BackgroundThe Qinghai-Tibet Plateau is the “sensitive area” of climate change, and also the “driver” and “amplifier” of global change. The response and feedback of its carbon dynamics to climate change will significantly affect the content of greenhouse gases in the atmosphere. However, due to the unique geographical environment characteristics of the Qinghai-Tibet Plateau, there is still much controversy about its carbon source and sink estimation results. This study designed a new algorithm based on machine learning to improve the accuracy of carbon source and sink estimation by integrating multiple scale carbon input (net primary productivity, NPP) and output (soil heterotrophic respiration, Rh) information from remote sensing and ground observations. Then, we compared spatial patterns of NPP and Rh derived from the fusion of multiple scale data with other widely used products and tried to quantify the differences and uncertainties of carbon sink simulation at a regional scale.ResultsOur results indicate that although global warming has potentially increased the Rh of the Qinghai-Tibet Plateau, it will also increase its NPP, and its current performance is a net carbon sink area (carbon sink amount is 22.3 Tg C/year). Comparative analysis with other data products shows that CASA, GLOPEM, and MODIS products based on remote sensing underestimate the carbon input of the Qinghai-Tibet Plateau (30–70%), which is the main reason for the severe underestimation of the carbon sink level of the Qinghai-Tibet Plateau (even considered as a carbon source).ConclusionsThe estimation of the carbon sink in the Qinghai-Tibet Plateau is of great significance for ensuring its ecological barrier function. It can deepen the community’s understanding of the response to climate change in sensitive areas of the plateau. This study can provide an essential basis for assessing the uncertainty of carbon sources and sinks in the Qinghai-Tibet Plateau, and also provide a scientific reference for helping China achieve “carbon neutrality” by 2060.

Accelerated organic matter decomposition in thermokarst lakes upon carbon and phosphorus inputs.

Mineralization of dissolved organic matter (DOM) in thermokarst lakes plays a non-negligible role in the permafrost carbon (C) cycle, but remains poorly understood due to its complex interactions with external C and nutrient inputs (i.e., aquatic priming and nutrient effects). Based on large-scale lake sampling and laboratory incubations, in combination with 13 C-stable-isotope labeling, optical spectroscopy, and high-throughput sequencing, we examined large-scale patterns and dominant drivers of priming and nutrient effects of DOM biodegradation across 30 thermokarst lakes along a 1100-km transect on the Tibetan Plateau. We observed that labile C and phosphorus (P) rather than nitrogen (N) inputs stimulated DOM biodegradation, with the priming and P effects being 172% and 451% over unamended control, respectively. We also detected significant interactive effects of labile C and nutrient supply on DOM biodegradation, with the combined labile C and nutrient additions inducing stronger microbial mineralization than C or nutrient treatment alone, illustrating that microbial activity in alpine thermokarst lakes is co-limited by both C and nutrients. We further found that the aquatic priming was mainly driven by DOM quality, with the priming intensity increasing with DOM recalcitrance, reflecting the limitation of external C as energy sources for microbial activity. Greater priming intensity was also associated with higher community-level ribosomal RNA gene operon (rrn) copy number and bacterial diversity as well as increased background soluble reactive P concentration. In contrast, the P effect decreased with DOM recalcitrance as well as with background soluble reactive P and ammonium concentrations, revealing the declining importance of P availability in mediating DOM biodegradation with enhanced C limitation but reduced nutrient limitation. Overall, the stimulation of external C and P inputs on DOM biodegradation in thermokarst lakes would amplify C-climate feedback in this alpine permafrost region.

Excluding Roots or Mycorrhizal Hyphae Alters the Microbial Community and Function by Decreasing Available C and N in a Subtropical Chinese Fir Forest

Carbon (C) inputs, primarily from roots and associated mycorrhizal hyphae, serve as crucial energy sources for microbial-driven C and nitrogen (N) cycling in the soil. However, our understanding of how soil microbial diversity, function, and associated soil properties respond to the exclusion of roots and their associated mycorrhizal hyphae remains limited. In our study, we conducted an experiment with no exclusion of roots or mycorrhizal hyphae (Control), exclusion of roots and retention of mycorrhizal hyphae (NR), and exclusion of roots and mycorrhizal hyphae (NRH) in a Chinese fir (Cunninghamia lanceolata) forest, the most important plantation in China. The soil properties, microbial community diversity and composition, and microbial function were investigated after 2 years of experiment exclusion. We found that exclusion of roots and hyphae significantly decreased DOC, DON, NH4+-N, and NO3−-N, but not SOC, TN, and TP, indicating that the exclusion of roots and mycorrhizal hyphae mainly reduced available C and N concentrations. Meanwhile, the species richness and Chao1 of bacteria and fungi were significantly reduced, primarily due to the decrease in available C and N levels. These findings suggest that the removal of roots and mycorrhizal hyphae results in a decrease in C and N availability, subsequently leading to a loss of microbial diversity. Compared to after the CT treatment, the relative abundances of Proteobacteria and Actinobacteria phyla were reduced after exclusion of roots and hyphae. However, the relative abundances of the phyla Acidobacteria, WPS2, Rozellomycota, and Glomeromycota showed an increase in exclusion treatments. Furthermore, the relative abundances of genes for C degradation (e.g., malQ, malZ, chi, rfbB, bglX, and ablA), C fixation (e.g., accA, icd, korA, and korB), and N fixation (nifS) were increased; conversely, the N degradation genes (e.g., nasA, nirB, ureC, and gdh2) were decreased in treatments involving excluding roots and hyphae. These results, in conjunction with the strong relationships between functional genes and DOC, DON, NH4+-N, and NO3−-N, suggest that microorganisms regulate functional genes to enhance C and N fixation or organic matter decomposition in response to C or N limitation resulting from root and mycorrhizal hypha exclusion. Collectively, our study revealed that the changes in roots-derived C directly altered available C and N in soil, which influenced the microbial community and function, and, in turn, regulated microbial-driven nutrient cycling in forest soils.

Patterns and driving factors of soil organic carbon sequestration efficiency under various manure regimes across Chinese croplands

The carbon sequestration efficiency of manure (CSE-manure) is critical for the accurate estimation of manuring-induced changes in soil organic carbon (SOC). However, the mechanisms of large-scale spatiotemporal changes in CSE-manure and their crucial drivers are unclear. To investigate these under various manure regimes, we performed field observation and collected relevant data using 1023 fields across China’s main cropland, as well as collected data on high-resolution climate and soil properties. The experimental durations significantly (P < 0.001) affected CSE-manure. The CSE-manure values were 31.4% ± 13.4 (mean ± SD), 19.2% ± 10.3%, and 16.1% ± 9.4 at the experimental durations of < 10, 10–20, and > 20 years, respectively. In addition, the manure sources significantly affected CSE-manure. The CSE-manure values of the manures from horse (27.9% ± 19.0) and sheep (26.9% ± 11.0) and farmyard manure (25.7% ± 14.8) were higher than those of the manures from pig (19.8% ± 15.4), poultry (19.1% ± 10.58), and cattle (23.1% ± 13.1). The CSE-manure values varied as per the region; the values were higher in North China than in South China. Of all the investigated variables, carbon and nitrogen input from manure, soil bulk density, annual average air temperature and precipitation, and initial SOC and soil clay content were the most important drivers of CSE-manure. The results suggested that carbon sequestration potential was higher in the northeast region. The spatiotemporal CSE-manure values under various climate conditions were predicted using trained machine learning models. Simulation results suggested that by 2100, the rate of decrease in CSE-manure will be higher at RCP (Representative Concentration Pathways) 8.5, and climate change will have a great impact on CSE-manure due to high warming effects.

Crop Residues Management: A Viable Tool for Sustainable Agriculture

The crop residue, traditionally considered as agricultural waste, is increasingly being viewed as a valuable resource. If the current trend continues, crop residue will be a “co-product” of grain production where both the grain and the residue have significant value. Potentially gross quantities of over 500 Mt crop residues are available in India on an annual basis and generated by various crops. Due to the scarcity of alternative organic amendments, the retention of crop residue in fields can be considered key in promoting physical, chemical, and biological attributes of soil in the agricultural systems of developing countries. The stems, leaves, chaffs, husks, etc., that remain in the fields after crops are harvested, play a critical role in soil quality and environmental issues since they are primary inputs of elemental carbon (C) into the soil system. About 25% N, 25% P, 75% K and 50% S uptake by cereal crops are retained in residues, making them valuable sources of nutrients.

Natural and anthropogenic carbon input affect microbial activity in salt marsh sediment.

Salt marshes are dynamic, highly productive ecosystems positioned at the interface between terrestrial and marine systems. They are exposed to large quantities of both natural and anthropogenic carbon input, and their diverse sediment-hosted microbial communities play key roles in carbon cycling and remineralization. To better understand the effects of natural and anthropogenic carbon on sediment microbial ecology, several sediment cores were collected from Little Sippewissett Salt Marsh (LSSM) on Cape Cod, MA, USA and incubated with either Spartina alterniflora cordgrass or diesel fuel. Resulting shifts in microbial diversity and activity were assessed via bioorthogonal non-canonical amino acid tagging (BONCAT) combined with fluorescence-activated cell sorting (FACS) and 16S rRNA gene amplicon sequencing. Both Spartina and diesel amendments resulted in initial decreases of microbial diversity as well as clear, community-wide shifts in metabolic activity. Multi-stage degradative frameworks shaped by fermentation were inferred based on anabolically active lineages. In particular, the metabolically versatile Marinifilaceae were prominent under both treatments, as were the sulfate-reducing Desulfovibrionaceae, which may be attributable to their ability to utilize diverse forms of carbon under nutrient limited conditions. By identifying lineages most directly involved in the early stages of carbon processing, we offer potential targets for indicator species to assess ecosystem health and highlight key players for selective promotion of bioremediation or carbon sequestration pathways.

Black carbon, soil organic matter molecular signatures under different land uses in Shenyang, China and relationship with PAHs

The content, composition and molecular signatures of soil organic matter (SOM) have important influences on the cycle of soil organic carbon (SOC) and the partitioning of polycyclic aromatic hydrocarbons (PAHs) in soil. Seventy-nine soil samples from farmland, forest and urban areas were collected in Shenyang, China to investigate black carbon (BC) content, SOM molecular signatures varied with land use patterns, as well as the relationship with PAHs. The content of BC in urban soils was significantly higher than that of farmland and forest. BC was a key contributor of urban SOM which accounted for 0.35 ± 0.31 of SOC in urban soil. Based on BC/SOC ratio, the main sources of BC were identified as fossil fuel combustion for urban soils, while for farmland and forest soils, it is the mixed results of fossil fuel combustion and biomass burning. All categories of PAHs in urban soils showed the highest level compared to farmland and forest soils. Pearson's correlation analysis results showed there were significant positive correlations between BC and PAHs categories in urban soils, indicating the important role of BC in the accumulation of PAHs in soil. SOM from each of the two different land use patterns can be distinguished by molecular signatures. Urban SOM had abundant molecular markers derived from condensed organic carbon inputs, which was consistent with the BC/SOC value. Farmland SOM had abundant carbon from vegetation and microorganisms, and forest SOM was rich in organic carbon from fresh plant materials. The markers enriched in urban SOM showed significant correlations with most PAHs categories, highlighting the affinity of urban SOM for PAHs at the molecular level. This study contributed to understanding the impact of land management methods on SOM molecular composition signatures and its influence on PAHs occurrence in soil, providing a theoretical basis for regional soil pollution management.

Harvest index has increased over the last 50 years of maize breeding

Quantifying historical changes in maize harvest index (HI), the fraction of above-ground biomass allocated to grain yield, can enhance our ability to explain grain yield trends and estimate stover carbon inputs for sustainability assessments. However, the HI genetic gain has not been the primary focus of previous era studies. The aim of this study is to enhance our knowledge of maize HI genetic gain. Our first objective is to quantify HI genetic gain in Bayer Crop Science Legacy hybrids and investigate the contribution of breeding and agronomic management. Our second objective is to develop a general-use model to describe the temporal evolution of maize HI. We studied 54 commercial hybrids (103-day and 111-day relative maturities) released from 1983 to 2020 across 13 environments, including plant density (current and historical increasing rate) and N-fertilizer (low and sufficient N rates) treatments. The HI was estimated at physiological maturity by destructively sampling plants. Then we synthesize new experimental data with literature findings (n = 16) to provide a robust HI genetic gain estimate. Results showed that HI has increased over the years from 0.516 to 0.571 in 103-day hybrids and from 0.537 to 0.584 in 111-day hybrids. The genetic gains were similar across environments and management treatments within the studied range, indicating that this increase is attributed to maize breeding. The N-fertilizer treatments affected the magnitude of the HI, but plant density did not. Our results, combined with 16 literature datasets, revealed a 0.26% year−1 relative increase in HI since 1964. We estimated that the increase in HI accounts for ca. 15% of the historical maize yield increase in the US Corn Belt over the past 50 years. The maize HI has increased over the last 50 years, and this increase was attributed to breeding, not to management. Our findings enhance our knowledge of maize HI, will support robust estimations of carbon inputs in sustainability studies, and inform crop models to better capture historical yield increases.

Canopy Leaching Rather than Desorption of PM2.5 From Leaves Is the Dominant Source of Throughfall Dissolved Organic Carbon in Forest

AbstractDissolved organic carbon (DOC) in throughfall plays a vital role in providing carbon and energy to organisms in forests. We utilized carbon isotope analysis to allocate the sources of throughfall DOC, including leaching from plant tissues, PM2.5 deposition on plant foliage, and precipitation. Rainwater, PM2.5, and throughfall samples were collected from pine and oak forests between March and November 2021. The mean concentration of throughfall DOC was 7.9 mg L−1, approximately six times higher than that of rainfall. The mean Δ14C of throughfall DOC was −38.2‰, ∼200‰ higher than that of rainwater or PM2.5. Mass balance estimates revealed that canopy leaching contributed to ∼83% of throughfall DOC, while desorption of PM2.5 and rainwater accounted for only ∼3% and ∼14% of throughfall DOC, respectively. These results clearly highlight canopy leaching as the primary source of carbon input to the forest floor, with a relatively minor contribution from PM2.5 desorption on leaves.

Effect of temperature on carbon accumulation in northern lake systems over the past 21,000 years

Introduction: Rising industrial emissions of carbon dioxide and methane highlight the important role of carbon sinks and sources in fast-changing northern landscapes. Northern lake systems play a key role in regulating organic carbon input by accumulating carbon in their sediment. Here we look at the lake history of 28 lakes (between 50°N and 80°N) over the past 21,000 years to explore the relationship between carbon accumulation in lakes and temperature changes.Method: For this study, we calculated organic carbon accumulation rates (OCAR) using measured and newly generated organic carbon and dry bulk density data. To estimate new data, we used and evaluated seven different regression techniques in addition to a log-linear model as our base model. We also used combined age-depth modeling to derive sedimentation rates and the TraCE-21ka climate reanalysis dataset to understand temperature development since the Last Glacial Maximum. We determined correlation between temperature and OCAR by using four different correlation coefficients.Results: In our data collection, we found a slightly positive association between OCAR and temperature. OCAR values peaked during warm periods Bølling Allerød (38.07 g·m−2·yr−1) and the Early Holocene (40.68 g·m−2·yr−1), while lowest values occurred during the cold phases of Last Glacial Maximum (9.47 g·m−2·yr−1) and Last Deglaciation (10.53 g·m−2·yr−1). However, high temperatures did not directly lead to high OCAR values.Discussion: We assume that rapid warming events lead to high carbon accumulation in lakes, but as warming progresses, this effect appears to change as increased microbial activity triggers greater outgassing. Despite the complexity of environmental forcing mechanisms affecting individual lake systems, our study showed statistical significance between measured OCAR and modelled paleotemperature for 11 out of 28 lakes. We concluded that air temperature alone appears to drive the carbon accumulation in lakes. We expected that other factors (catchment vegetation, permafrost, and lake characteristics) would influence accumulation rates, but could not discover a conclusive factor that had a statistical significant impact. More data available on long-term records from northern lake systems could lead to more confidence and accuracy on the matter.

A time-space window between Eocene karst bauxite genesis and the first molasse deposition in the Dinaric Foreland Basin in the North Dalmatia, Croatia

Karst bauxite deposits in the North Dalmatian piggyback basin (NDPGB) are a part of the Mediterranean bauxite belt, which is the largest European bauxite deposit zone; however, there is a general lack of information regarding the genesis, age, and precursor of the bauxite deposits in this region. In this study, we combined detrital zircon U–Pb geochronology with compositional, mineralogical, and morphological data from four bauxite locations in the NDPGB to provide a new palaeogeographical and palaeoenvironmental evolution model for the Lutetian–Rupelian timeframe of the NDPGB. The Eocene climatic conditions began with the Palaeocene–Eocene Thermal Maximum event (∼56 Ma), followed by the Early Eocene Climatic Optimum (∼49 Ma) and Middle Eocene Climatic Optimum (∼40 Ma), and were completed as a cooling trend culminating around the Eocene/Oligocene boundary (∼34 Ma), with a shift towards an icehouse climate. These events were coeval with the continuous drift of the African continent towards Eurasia and the subsequent closure of the western part of the former Neo-Tethys Ocean associated with massive volcanic activity. Based on the bauxite deposits of the NDPGB, Early Eocene limestones formed in the last phase of the long-lasting Adriatic Carbonate Platform. The Middle Eocene orogenic activity resulted in an elevation in this area. High average temperatures, accelerated hydrological cycles and precipitation, and intensive continental weathering with increased volcanic carbon input resulted in favourable conditions for the development of karst bauxites at this time. Further Upper Eocene tectonic deformation of the NDPGB area resulted in the development of bauxite traps and enabled redeposition of the initial bauxite material. Subsequently, the bauxite deposits were covered with clastic carbonate molasse derived from the intensive erosion of the young Dinaric orogeny. The implications of this study are as follows. First, it provides new information on the timing of bauxitisation in the area by providing the first radiometric zircon geochronology, which refined and restricted the time window for bauxite formation in this region. Additionally, our results provide a new perspective on the possibility of aeolian precursors in karst bauxite formation and provide new constraints on the first tectonic marks of the initial Dinaric orogeny.

Increased Soil Organic Carbon Losses Due to Soybean Monoculture in the Argentine Pampas: A Meta-Analysis

ABSTRACT Agricultural soils can act as a repository for atmospheric carbon and hence mitigate global warming. One of the best management practices that can increase the soil organic carbon (SOC) is the selection of high carbon input crops. Some crops like soybean would lead to SOC decrease due to its low residue production. This is the main crop in the Pampas region of Argentina. The objective of the present study was to determine how soybean monoculture impacts the SOC in the region. Data were collected from 11 long-term field experiments. Results of nine SOC comparisons of soybean monoculture with rotations and seven comparisons of other crop monocultures with rotations were extracted. Meta-analytic methods were used for the analysis. SOC stock in the 0–20 cm layer was 10% lower under monoculture of soybean than under rotations. The average duration of the experiments was 10 years. On the contrary, no differences of the SOC were detected when monocultures of wheat, corn or sunflower were compared with rotations. When the initial and final SOC stocks were compared in each treatment, it was estimated that in nearly all cases there were losses of SOC. The average loss of SOC was 8.1 t ha−1 under soybean monoculture and 3.9 t ha−1 under the corresponding rotations. Consequently, the reviewed experiments indicate losses of SOC due to agriculture in the Pampas that double with soybean monoculture.

High-resolution record of temporal change in organic matter burial over the past ∼8,600 years on the northwestern continental slope of the South China Sea

Sedimentary organic matter (SOM) on continental slopes in marine regions can sensitively record climatic and environmental changes. In this study, total organic carbon content (TOC), total nitrogen content (TN), and their stable isotope compositions (δ13C and δ15N) for sediments of core G02 were investigated (at ∼24.2-year resolution) to reveal the temporal variations in organic matter sources and the main controls on the sources and distribution of buried organic matter on the northwestern continental slope of the South China Sea over the last ∼8600 years. Results of a δ13C binary mixing model reveal that ∼82.3 ± 3% of SOM is derived from marine autochthonous sources. We suggest that the carbon and nitrogen contents and compositions of SOM are governed by distinct factors. The more positive δ15N values before the Pulleniatina Minimum Event occurrence are ascribed to stronger subsurface water intrusion by the Kuroshio Current, which led to enhanced subsurface denitrification and in turn counteracted the effect of mixing with surface water caused by the East Asian winter monsoon. Sedimentary δ13C values show a fluctuant decrease during ca. 8.6–3.0 cal kyr BP and a conspicuous increase during ca. 3.0–1.4 cal kyr BP. These changes are attributed to the decrease of marine productivity induced by the continuous weakening East Asian monsoon effect and the decrease of terrigenous organic carbon input induced by the weakened Indian summer monsoon precipitation, respectively. Since ca. 1.4 cal kyr BP, human activities have become the dominant factor in controlling the production and distribution of organic carbon. The results provide an important basis for understanding of source-sink processes of organic matter and the factors influencing these processes on continental slopes in low-latitude marginal seas.

Soil organic carbon priming co-regulated by labile carbon input level and long-term fertilization history

Labile carbon (C) input and fertilization have important consequences for soil organic matter (SOM) decomposition via the priming effect (PE), thereby impacting soil fertility and C sequestration. However, it remains largely uncertain on how the labile C input levels interact with long-term fertilization history to control PE intensity. To clarify this question, soil samples were collected from a 38-year fertilization field experiment (including five treatments: chemical nitrogen fertilizer, N; chemical fertilizer, NPK; manure, M1; 200 % manure, M2; NPK plus M2, NPKM2), with strongly altered soil physiochemical properties (i.e., soil aggregation, organic C and nutrient availability). These soil samples were incubated with three input levels of 13C-glucose (without glucose, control; low, 0.4 % SOC; high, 2.0 % SOC) to clarify the underlying mechanisms of PE. Results showed that the PE significantly increased with glucose input levels, with values increasing from negative or weak (−2.21 to 3.55 mg C g−1 SOC) at low input level to strongly positive (5.62 to 8.57 mg C g−1 SOC) at high input level across fertilization treatments. The increased PE intensity occurred along with decreased dissolved total nitrogen (DTN) contents and increased ratios of dissolved organic C to DTN, implying that the decline in N availability largely increased PE via enhanced microbial N mining from SOM. Compared to N and NPK treatments, the PE was significantly lower in the manure-amendment treatments, especially for low input level, due to more stable SOM by aggregate protection and higher N and phosphorus availability. These results suggested that manure application could alleviate SOM priming via increased soil C stability and nutrient availability. Collectively, our findings emphasize the importance of long-term fertilization-driven changes in labile C inputs, SOM stability, and nutrient availability in regulating PE and soil C dynamics. This knowledge advances our understanding of the long-term fertilization management for soil C sequestration.

Warming inhibits the priming effect of soil organic carbon mineralization: A meta-analysis

Fresh organic carbon (C) input will accelerate or inhibit the mineralization of native soil organic carbon (SOC), which is called positive or negative priming effect (PE), respectively. However, little is known about how warming affects the PE. Here, we adopted a widely-used ratio of SOC mineralization between substrate-added and unadded-control treatments to represent PE intensity and used the PE difference between ambient-control temperature and elevated temperature to indicate the effect of warming on PE (ΔPE). By conducting a meta-analysis of 146 observations from 57 independent soils worldwide, we found that experimental warming significantly decreased the PE by 0.26 (unitless). Among ecosystems, warming significantly suppressed the PE of cropland and grassland soils by 0.43 and 0.21 respectively, but did not change the PE of forest soils. Moreover, we found significant positive correlations of ΔPE with the initial soil C/N ratio and the effect size of warming on microbial biomass. Between substrate types (i.e., containing N or not), warming significantly decreased the PE induced by N-containing substrates. These results suggested that the response of PE to warming is likely regulated by soil N availability and warming-induced changes in microbial biomass. As such, we proposed a conceptual framework—the microbial N mining hypothesis dominates in soils with low C/N ratio where warming inhibits PE by promoting N mineralization, while the stoichiometric decomposition hypothesis dominates in soils with high C/N ratio where warming stimulates PE by promoting N mineralization. Collectively, these findings provide important insights into how warming affects SOC dynamics via inhibiting PE, which may weaken the positive feedback between soil C emission and climate warming.

Spatial variability of the modern radiocarbon reservoir effect in the high-altitude lake Laguna del Peinado (southern Puna Plateau, Argentina)

Abstract. The high-altitude lakes of the Altiplano–Puna Plateau in the Central Andes commonly have large radiocarbon reservoir effects. This, combined with the general scarcity of terrestrial organic matter, makes obtaining a reliable and accurate chronological model based on radiocarbon ages a challenge. As a result, age–depth models based on radiocarbon dating are often constructed by correcting for the modern reservoir effect, but commonly without consideration of spatial and possible temporal variations of reservoir ages within the lake and across the basin. In order to get a better constraint on the spatial variability of the radiocarbon reservoir effects, we analyse 14C ages of modern terrestrial and aquatic plants from the El Peinado basin in the southern Puna Plateau, which hosts Laguna del Peinado fed by hydrothermal springs. The oldest 14C ages of modern samples (&gt; 18 000 and &gt; 26 000 BP) were found in hot springs discharging into the lake, likely resulting from the input of 14C-depleted carbon from old groundwater and 14C-free magmatic CO2. In the littoral and central part of Laguna del Peinado, 14C ages of modern samples were several thousand years younger (&gt; 13 000 and &gt; 12 000 BP) compared to the inflowing waters as a result of CO2 exchange with the atmosphere. Altogether, our findings reveal a spatial variability of up to 14 000 14C years of the modern reservoir effect between the hot springs and the northern part of the Peinado lake basin. Temporal changes of reservoir effects in sediment records are more difficult to quantify, but 14C ages from a short core from Laguna del Peinado may suggest temporal reservoir age variations of a few thousand years. This study has implications for accurate 14C-based chronologies for palaeoclimate studies in the Altiplano–Puna Plateau and similar settings. Our results highlight the need to consider spatial and likely also temporal variations in the reservoir effects when constructing age–depth models.