Is soil carbon disappearing? The dynamics of soil organic carbon in Java

Research in the soil of the tropics mostly has demonstrated the decline of soil organic carbon (SOC) after conversion of primary forest to plantation and cultivated lands. This paper illustrates the dynamics of SOC on the island of Java, Indonesia, from 1930 to 2010. We used 2002 soil profile observations containing organic carbon (C) analysis in the topsoil, which were collected by the Indonesian Center for Agricultural Land Resources Research & Development from 1923 to 2007. Results show the obvious decline of SOC values from around 2% in 1930–1940 to 0.8% in 1960–1970. However, there has been an increase of SOC content since 1970, with a median level of 1.1% in the year 2000. Our analysis suggests that the human influence and agricultural practices on SOC in Java have been a stronger influence than the environmental factors. SOC for the top 10 cm has shown a net accumulation rate of 0.2–0.3 Mg C ha^?1 yr^?1 during the period 1990–2000. These findings give rise to optimism for increased soil C sequestration in the tropics.     

Distribution of soil phytolith-occluded carbon in the Chinese Loess Plateau and its implications for silica–carbon cycles

Background and aims

Plants absorb and carry soluble silica from soils and then deposit SiO₂?·?nH₂O within themselves producing amorphous silica particles known as phytoliths. Trace amount of organic carbon is occluded during phytolith formation referred to as phytolith-occluded carbon (PhytOC). This carbon fraction has been recognized as an important way of carbon biosequestration. Previous studies have investigated the PhytOC contents of many crop plants and their contribution to global carbon sink. However, the PhytOC in soil is less focused. In this study, we investigated the distribution of soil PhytOC in the Chinese Loess Plateau (CLP).

Methods

Twenty-six soil profiles were collected in the Chinese Loess Plateau. A wet oxidation method was used for phytolith extraction. Occluded carbon was determined by element analyzer.

Results

Our results showed that the soil PhytOC density (SPCD) ranged from 0.757 to 23.110 g/m² among different soil profiles. The SPCD of profiles in the Southern CLP was generally higher than that in the Northern CLP. It was estimated that 5.35 Mt of PhytOC was stored in the upper soil of the CLP. We also estimated the annual phytolith flux into the Yellow River from the CLP by soil erosion and about 2.5 Mt of phytoliths eroded and transported into rivers per year.

Conclusions

Our study indicated that PhytOC was one of the potential biosequestration way and phytoliths had an important influence on biogeochemical cycle of silica. Our results suggested that the soil PhytOC was mainly influenced by different plant communities.     

Effects of cropland reclamation on soil organic carbon in China's black soil region over the past 35 years

The long-term use of cropland and cropland reclamation from natural ecosystems led to soil degradation. This study investigated the effect of the long-term use of cropland and cropland reclamation from natural ecosystems on soil organic carbon (SOC) content and density over the past 35 years. Altogether, 2140 topsoil samples (0–20 cm) were collected across Northeast China. Landsat images were acquired from 1985 to 2020 through Google Earth Engine, and the reflectance of each soil sample was extracted from the Landsat image that its time was consistent with sampling. The hybrid model that included two individual SOC prediction models for two clustering regions was built for accurate estimation after k-means clustering. The probability hybrid model, a combination between the hybrid model and classification probabilities of pixels, was introduced to enhance the accuracy of SOC mapping. Cropland reclamation results were extracted from the land cover time-series dataset at a 5-year interval. Our study indicated that: (1) Long-term use of cropland led to a 3.07 g kg⁻¹ and 6.71 Mg C ha⁻¹ decrease in SOC content and density, respectively, and the decrease of SOC stock was 0.32 Pg over the past 35 years; (2) nearly 64% of cropland had a negative change in terms of SOC content from 1985 to 2020; (3) cropland reclamation track changed from high to low SOC content, and almost no cropland was reclaimed on the “Black soils” after 2005; (4) cropland reclamation from wetlands resulted in the highest decrease, and reclamation period of years 31–35 decreased when SOC density and SOC stock were 16.05 Mg C ha⁻¹ and 0.005 Pg, respectively, while reclamation period of years 26–30 from forest witnessed SOC density and stock decreases of 8.33 Mg C ha⁻¹ and 0.01 Pg, respectively. Our research results provide a reference for SOC change in the black soil region of Northeast China and can attract more attention to the area of the protection of “Black soils” and natural ecosystems.     

High uncertainties detected in the wetlands distribution of the Qinghai–Tibet Plateau based on multisource data

Landscape and Ecological Engineering - Twenty wetland-related data products (including remote sensing datasets, compilation datasets and model simulation datasets) were collected to evaluate the...     

Effect of tillage and straw return on carbon footprints,soil organic carbon fractions and soil microbial community in different textured soils under rice–wheat rotation: a review

Reviews in Environmental Science and Bio/Technology - Measuring the influence of long-term agricultural tillage practices on soil organic carbon (SOC) is of great importance to farmers and...     

Will changes in soil organic carbon act as a positive or negative feedback on global warming?

The world's soils contain about 1500 Gt of organic carbon to a depth of 1m and a further 900 Gt from 1--2m. A change of total soil organic carbon by just 10% would thus be equivalent to all the anthropogenic CO₂ emitted over 30 years. Warming is likely to increase both the rate of decomposition and net primary production (NPP), with a fraction of NPP forming new organic carbon. Evidence from various sources can be used to assess whether NPP or the rate of decomposition has the greater temperature sensitivity, and, hence, whether warming is likely to lead to an increase or decrease in soil organic carbon.Evidence is reviewed from laboratory-based incubations, field measurements of organic carbon storage, carbon isotope ratios and soil respiration with either naturally varying temperatures or after experimentally increasing soil temperatures. Estimates of terrestrial carbon stored at the Last Glacial Maximum are also reviewed. The review concludes that the temperature dependence of organic matter decomposition can be best described as: d(T) = exp[3.36 (T – 40)/(T + 31.79)] where d(T) is the normalised decomposition rate at temperature T (in °C). In this equation, decomposition rate is normalised to 1 at 40 °C.The review concludes by simulating the likely changes in soil organic carbon with warming. In summary, it appears likely that warming will have the effect of reducing soil organic carbon by stimulating decomposition rates more than NPP. However, increasing CO₂ is likely to simultaneously have the effect of increasing soil organic carbon through increases in NPP. Any changes are also likely to be very slow. The net effect of changes in soil organic carbon on atmospheric CO₂ loading over the next decades to centuries is, therefore, likely to be small.     

Dissolved organic carbon dynamics in the peat–streamwater interface

A series of experiments were conducted to address the fate of dissolved organic carbon (DOC) in the peat–stream interface zone linking a minerotrophic poor fen and an ombrotrophic mire with surrounding stream water in the drainage area of Lake Örträsket in northern Sweden. Transport and mineralisation of DOC were quantified in peat–stream interface cores in response to variations in pore water velocity, DOC concentration and the molecular size and source of DOC. Mineralisation and CH₄ production were positively correlated with pore water velocity at rates between 0.08 and 0.20cmh^–1 and negatively correlated at rates between 0.20 and 0.40cmh^–1. The DOC concentration of the effluent from the peat cores was independent of the pore water velocity but proportional to the DOC concentration of the source water. Higher concentrations of DOC were exported from than imported to the peat cores, and the cores exported DOC molecules of smaller average molecular size than received. Carbon mineralisation in the peat, assessed in a static system, was independent of the concentration of DOC. DOC with a nominal cutoff at 100Da was mineralised faster by streamwater bacteria than DOC dialysed with a cutoff at 3500Da, and their mineralisation rate was positively correlated with the DOC concentration. Streamwater bacteria mineralised streamwater DOC at a lower rate than the peat–stream interface zone pore water DOC. The pattern of velocity dependence of mineralisation was the same for both sources of peat DOC but the mineralisation rates, average molecular size, and bioavailability of DOC were different, emphasising the importance of the compositional heterogeneity of the peat–stream interface zone for the DOC budget of streamwater.     

How long before a change in soil organic carbon can be detected?

When planning sampling in an experiment where soil organic carbon (SOC) content is expected to change, it is necessary to know how many samples will need to be taken to demonstrate a change in SOC and after how long this change will be detectable. Much has been published on the number of samples required to demonstrate the minimum detectable difference in SOC, but less on how long it takes for this change to be detectable. In this paper, a model of SOC dynamics is used to estimate the minimum time taken for a change in total SOC content to become measurable under different carbon inputs, land uses and soil types. For free air carbon dioxide enrichment (FACE), and other experiments in which SOC is expected to increase, relationships between the percentage change in C inputs and the time taken to measure a change in SOC are presented, for two levels of sampling intensity corresponding to the maximum that is practically possible in most experiments (～100 samples) and that used regularly in field experiments (10–20 samples). In FACE experiments, where C inputs increase by a maximum of about 20–25%, SOC change could be detected with 90% confidence after about 6–10 years if a sampling regime allowing 3% change in background SOC level (probably requiring a very large number of samples) were used, but could not be detected at all if a sampling regime were used that allowed only a 15% change in background SOC to be detected. If increases in C inputs are much below 15%, it might not be possible to detect a change in soil C without an enormous number of samples. Relationships between the change in C inputs and the time taken to measure a change in SOC are robust over a range of soil types and land uses. The results demonstrate how models of SOC dynamics can be used to complement statistical power analyses for planning when, and how intensively, to sample soils during experiments. An advantage of the modelling approach demonstrated here is that estimates of the minimum time taken for a change in soil carbon to become detectable can be made, even before any detailed soil samples are taken, simply from estimates of the likely increase in carbon inputs to the soil (via expected changes in net primary production).     

Long-term manuring and fertilization effects on soil organic carbon pools under a wheat–maize cropping system in North China Plain

The effects of organic manure and chemical fertilizer on total soil organic carbon (C _T), water-soluble organic C (C _WS), microbial biomass C (C _MB), labile C (C _L), C mineralization, C storage and sequestration, and the role of carbon management index (CMI) in soil quality evaluation were studied under a wheat–maize cropping system in a long-term experiment, which was established in 1989 in the North China Plain. The experiment included seven treatments: (1) OM: application of organic manure; (2) 1/2OMN: application of half organic manure plus chemical fertilizer NPK; (3) NPK: balanced application of chemical fertilizer NPK; (4) NP: application of chemical fertilizer NP; (5) PK: application of chemical fertilizer PK; (6) NK: application of chemical fertilizer NK; and (7) CK: unfertilized control. Application of organic manure (OM and 1/2OMN) was more effective for increasing C _T, C _WS, C _MB, C _L, C mineralization, and CMI, as compared with application of chemical fertilizer alone. For the chemical fertilizer treatments, balanced application of NPK (treatment 3) showed higher C _T, C _WS, C _MB, C _L, C mineralization, and CMI than the unbalanced use of fertilizers (treatments 4, 5, and 6). The C storage in the OM and 1/2OMN treatments were increased by 58.0% and 26.6%, respectively, over the NPK treatment, which had 5.9–25.4% more C storage than unbalanced use of fertilizers. The contents of C _WS, C _MB, and C _L in organic manure treatments (treatments 1 and 2) were increased by 139.7–260.5%, 136.7–225.7%, and 150.0–240.5%, respectively, as compared to the CK treatment. The CMI was found to be a useful index to assess the changes of soil quality induced by soil management practices due to its significant correlation with soil bulk density and C fractions. The OM and 1/2OMN treatments were not a feasible option for farmers, but a feasible option for sequestering soil carbon, especially for the OM treatment. The NPK treatment was important for increasing crop yields, organic material inputs, and soil C fractions, so it could increase the sustainability of cropping system in the North China Plain.     

Cultivation of a perennial grass for bioenergy on a boreal organic soil – carbon sink or source?

The area under the cultivation of perennial bioenergy crops on organic soils in the northern countries is fast increasing. To understand the impact of reed canary grass (RCG, Phalaris arundinaceae L.) cultivation on the carbon dioxide (CO₂) balance of an organic soil, net ecosystem CO₂ exchange (NEE) was measured for four years in a RCG cultivated cutover peatland in eastern Finland using the eddy covariance technique. There were striking differences among the years in the annual precipitation. The annual precipitation was higher during 2004 and 2007 and lower during 2005 and 2006 than the 1971–2000 regional mean. During wet growing seasons, moderate temperatures, high surface soil moisture and low evaporative demand favoured high CO₂ uptake. During dry seasons, owing to soil moisture and atmospheric stress, photosynthetic activity was severely restricted. The CO₂ uptake [gross primary productivity (GPP)] was positively correlated with soil moisture, air temperature and inversely with vapour pressure deficit. Total ecosystem respiration (TER) increased with increasing soil temperature but decreased with increasing soil moisture. The relative responses of GPP and TER to moisture stress were different. While changes in TER for a given change in soil moisture were moderate, variations in GPP were drastic. Also, the seasonal variations in TER were not as conspicuous as those in GPP implying that GPP is the primary regulator of the interannual variability in NEE in this ecosystem. The ecosystem accumulated a total of 398 g C m^?2 from the beginning of 2004 until the end of 2007. It retained some carbon during a wet year such as 2004 even after accounting for the loss of carbon in the form of harvested biomass. Based on this CO₂ balance analysis, RCG cultivation is found to be a promising after‐use option on an organic soil.     

Soil hydraulic manipulation and organic amendment for the enhancement of selenium volatilization in a soil–pickleweed system

Biological volatilization of selenium (Se) in contaminated areas represents an environmentally friendly phytoremediation approach. Implementation of phytovolatilization technology for the remediation of Se-contaminated soils or sediments is oftentimes limited by its low remediation efficiency under field conditions. This greenhouse study determined the feasibility of manipulating soil organic content and hydraulic conditions in a soil–pickleweed (Salicornia bigelovii) system for the enhancement of Se volatilization. Based on annual shoot biomass production rate under field conditions (approximately 1.5 kg m⁻²), the addition of pickleweed shoot tissues to the soil surface resulted in 2.2-fold more biogenic volatile Se than the control, up to 251.6 ± 140.5 μg m⁻² d⁻¹. Selenium volatilization was significantly reduced at a soil water potential of −25 kPa, but substantially increased after re-irrigation to 0 kPa. In a 42-day experiment, the rate of Se volatilization was significantly correlated with soil water potential (P < 0.0001). Findings from this study demonstrate that Se volatilization be substantially enhanced by amending soil with pickleweed residues and by creating wetting and drying cycles that can be monitored with soil water potential probes in the field.     

Diatom–salinity relationships in wetlands: assessing the influence of salinity variability on the development of inference models

We determined in situ feeding rates of three co-occurring coastal mysid species using [methyl-³H]-thymidine-labelled algal detritus (Lessonia corrugata), NaH¹⁴CO₃-labelled phytoplankton (Isochrysis galbana) and zooplankton (Artemia sp. nauplii). All three species showed a wide and overlapping range of feeding rates on the three food types, suggesting they were broadly omnivorous. However, selectivity studies often showed a strong preference for animal prey. Although there was an overlap in the types of food the mysids ingested, some degree of feeding niche partitioning was demonstrated. Paramesopodopsis rufa tended to be more carnivorous, Tenagomysis tasmaniae fed least on zooplankton and phytoplankton, and largely on algal detritus, and Anisomysis mixta australis ingested few zooplankters, and moderate amounts of algal detritus and phytoplankton. Handling editor: P. Viaroli     

Structure and ecological function of the soil microbiome affecting plant–soil feedbacks in the presence of a soil-borne pathogen

Interactions between plants and soil microbes are important for plant growth and resistance. Through plant–soil-feedbacks, growth of a plant is influenced by the previous plant that was growing in the same soil. We performed a plant–soil feedback study with 37 grass, forb and legume species, to condition the soil and then tested the effects of plant-induced changes in soil microbiomes on the growth of the commercially important cut-flower Chrysanthemum in presence and absence of a pathogen. We analysed the fungal and bacterial communities in these soils using next-generation sequencing and examined their relationship with plant growth in inoculated soils with or without the root pathogen, Pythium ultimum. We show that a large part of the soil microbiome is plant species-specific while a smaller part is conserved at the plant family level. We further identified clusters of plant species creating plant growth promoting microbiomes that suppress concomitantly plant pathogens. Especially soil inocula with higher relative abundances of arbuscular mycorrhizal fungi caused positive effects on the Chrysanthemum growth when exposed to the pathogen. We conclude that plants differ greatly in how they influence the soil microbiome and that plant growth and protection against pathogens is associated with a complex soil microbial community.     

Shrub (Prosopis velutina) encroachment in a semidesert grassland: spatial–temporal changes in soil organic carbon and nitrogen pools

Recent trends of increasing woody vegetation in arid and semiarid ecosystems may contribute substantially to the North American C sink. There is considerable uncertainty, however, in the extent to which woody encroachment alters dryland soil organic carbon (SOC) and total nitrogen (TN) pools. To date, studies assessing SOC and TN response to woody plant proliferation have not explicitly assessed the variability caused by shrub age or size and subcanopy spatial gradients. These factors were quantified via spatially intensive soil sampling around Prosopis velutina shrubs in a semidesert grassland, using shrub size as a proxy for age. We found that bulk density increased with distance from the bole (P < 0.005) and decreased with increasing shrub size (P= 0.056), while both SOC and TN increased with shrub size and decreased with distance from the bole (P < 0.001 for both). Significant (and predictable) spatial variation in bulk density suggests that use of generic values would generate unreliable estimates of SOC and TN mass, and subcanopy SOC pools could be overestimated by nearly 30% if intercanopy bulk density values were applied to subcanopy sites. Predictive models based on field-documented spatial patterns were used to generate integrated estimates of under-shrub SOC and TN pools, and these were compared with results obtained by typical area-weighting protocols based on point samples obtained next to the bole or at a specified distance from the bole. Values obtained using traditional area-weighting approaches generally overestimated SOC pools relative to those obtained using the spatially integrated approach, the discrepancy increasing with increasing shrub size and proximity of the point sample to the bole. These discrepancies were observed at the individual plant scale and for landscapes populated by various shrub size classes. Results suggest that sampling aimed at quantifying shrub encroachment impacts on SOC and TN pools will require area-weighting algorithms that simultaneously account for shrub size (age) and subcanopy spatial patterns.     

Aspects of carbon and nitrogen cycling in soils of the Bornhöved Lake district II. Modelling the influence of temperature increase on soil respiration and organic carbon content in arable soils under different managements

Based on field measurements in two agriculturalecosystems, soil respiration and long-term response ofsoil organic carbon content (SOC) was modelled. Themodel predicts the influence of temperature increaseas well as the effects of land-use over a period ofthirty years in a northern German glacial morainelandscape. One of the fields carried a maizemonoculture treated with cattle slurry in addition tomineral fertilizer (maize monoculture), the otherwas managed by crop rotation and recieved organicmanure (crop rotation). The soils of both fieldswere classified as cambic Arenosols. The soilrespiration was measured in the fields by means of theopen dynamic inverted-box method and an infrared gasanalyser. The mean annual soil respiration rates were 268 (maizemonoculture) and 287 mg CO₂ m^-2 h^-1(crop rotation). Factors controlling soil respirationwere soil temperature, soil moisture, root respirationand carbon input into the soil. Q₁₀-valuesof soil respiration were generally higher in winterthan in summer. This trend is interpreted as anadaptive response of the soil microbial communities.In the model a novel mathematical approach withvariable Q₁₀-values as a result oftemperature and moisture adjustment is proposed. Withthe calibrated model soil respiration and SOC werecalculated for both fields and simulations over aperiod of thirty years were established. Simulationswere based on (1) local climatic data, 1961 until1990, and (2) a regional climate scenario for northernGermany with an average temperature increase of 2.1 K.Over the thirty years period with present climateconditions, the SOC pool under crop rotation wasnearly stable due to the higher carbon inputs, whereasabout 16 t C ha^-1 were lost under maizemonoculture. Under global warming the mean annualsoil respiration for both fields increased and SOCdecreased by ca. 10 t C ha^-1 under croprotation and by more than 20 t C ha^-1 undermaize monoculture. It was shown that overestimationof carbon losses in long-term prognoses can be avoidedby including a Q₁₀-adjustment in soilrespiration models.     

Carbon flow in the rhizosphere: carbon trading at the soil–root interface

The loss of organic and inorganic carbon from roots into soil underpins nearly all the major changes that occur in the rhizosphere. In this review we explore the mechanistic basis of organic carbon and nitrogen flow in the rhizosphere. It is clear that C and N flow in the rhizosphere is extremely complex, being highly plant and environment dependent and varying both spatially and temporally along the root. Consequently, the amount and type of rhizodeposits (e.g. exudates, border cells, mucilage) remains highly context specific. This has severely limited our capacity to quantify and model the amount of rhizodeposition in ecosystem processes such as C sequestration and nutrient acquisition. It is now evident that C and N flow at the soil–root interface is bidirectional with C and N being lost from roots and taken up from the soil simultaneously. Here we present four alternative hypotheses to explain why high and low molecular weight organic compounds are actively cycled in the rhizosphere. These include: (1) indirect, fortuitous root exudate recapture as part of the root’s C and N distribution network, (2) direct re-uptake to enhance the plant’s C efficiency and to reduce rhizosphere microbial growth and pathogen attack, (3) direct uptake to recapture organic nutrients released from soil organic matter, and (4) for inter-root and root–microbial signal exchange. Due to severe flaws in the interpretation of commonly used isotopic labelling techniques, there is still great uncertainty surrounding the importance of these individual fluxes in the rhizosphere. Due to the importance of rhizodeposition in regulating ecosystem functioning, it is critical that future research focuses on resolving the quantitative importance of the different C and N fluxes operating in the rhizosphere and the ways in which these vary spatially and temporally.     

Can the restoration of natural vegetation be accelerated on the Chinese Loess Plateau? A study of the response of the leaf carbon isotope ratio of dominant species to changing soil carbon and nitrogen levels

For the heavily degraded ecosystem on the Chinese Loess Plateau, it would be of great significance if vegetation restoration could be accelerated anthropogenically. However, one major concern is that if the late successional species were planted or sown in degraded habitats, would they still be competitive in terms of some critical plant traits associated with specific habitats? Water use efficiency (WUE) is a major plant trait shaping the pattern of species turnover in vegetation secondary succession on the Loess Plateau. We hypothesized that if late successional stage plants could still hold a competitive advantage in terms of WUE, the prospects for an acceleration of succession by sowing these species in newly abandoned fields would be good. We tested this hypothesis by comparing the leaf C isotope ratio (δ¹³C) value (a surrogate of WUE) of dominant species from different successional stages at given soil C and N levels. Results indicated that leaf δ¹³C of the two dominant species that co-dominated in the second and third stages were significantly more positive than that of the dominant species from the first stage regardless of changing soil C and N. Yet the dominant species from the climax stage is a C₄ grass assumed to have the highest WUE. In addition, increasing soil nutrition had no effects on leaf δ¹³C of two dominant species in the late successional stage, indicating that dominant species from the late successional stages could still have a competitive advantage in terms of WUE in soil C- and N-poor habitats. Therefore, from the perspective of plant WUE, there are great opportunities for ecosystem restoration by sowing both dominant species and other species that co-occur in late successional stages in newly abandoned fields, for the purpose of enhancing species diversity and optimising species composition.