high-C Soils Research Articles

Change of soil carbon storage in monoculture tree plantations across wide environmental gradients: Argentina as a case study

Forest plantations (through afforestation or reforestation) have been proposed as a valuable option to mitigate carbon (C) emissions. However, high spatiotemporal variation has been observed in soil organic carbon (SOC) pools associated to these systems. It has been proposed that SOC stock (Mg/ha) changes under forest plantations (ΔSOC) are related to the SOCstock in the natural system being replaced with positive ΔSOC in the sites with lowest initial SOCstock and negative ΔSOC in sites with initially high SOCstock. Although there is some debate related with statistical artifacts, the slope of this relationship may depend on environmental, biological and anthropic variables. In this context, we took advantage of a recent effort to quantify soil organic C stocks in forest plantations and contiguous land uses along 136 sites across a wide climatic and edaphic gradient in Argentina, South-America, to explore the patterns and drivers of SOC change due to this land-use change. We also added 183 data from a systematic bibliographic survey. Average ΔSOC of all the studied paired sites was low (average of −3.85 ± 29.97 Mg ha−1), with 57 % of the paired sites showing a negative change. After applying a diagnosis method to detect statistical regression-to-the-mean bias and considering the pooled data and the different forest plantation groups (Pinusspp.,Eucalyptusspp.,Salicaceaespp, and native species), only Eucalyptusplantations showed a negative relationship betweenΔSOC and SOCstock in the control situation (i.e. a significant and negative impact on SOC in high-C soils). In general, most of the sites where forest plantations were introduced in Argentina have a relatively low baseline SOC (SOCstock < 200 Mg ha−1), and 98 % of the studied sites showed SOC deficit (Csat-def) estimated as the difference between the observed vs. the theoretical maximal C storage potential based on silt and clay content. SOCstock and the Csat-def variation in forest plantations across regions were explained mainly by the SOCstock of the natural situation where the plantation was installed, followed by edaphic and some climatic variables. Within the silvicultural variables evaluated, only the forest species cultivated had a significant effect at the scale evaluated and within the range of stand densities (medium to high) and ages (close to rotation period in each region) considered. Our results indicate that -along a broad range of environmental situations but within a limited set of silvicultural conditions- in average forests plantations decrease soil carbon stock. However, there are situations -that could be coarsely predicted with a model with multiple variables and their interactions- where the change is positive bringing opportunities to increase the C sequestration service of planted forests.

Effects of whole-orchard recycling on nitrate leaching potential in almond production systems.

Inefficient nitrogen (N) fertilization and irrigation have led to unhealthy nitrate levels in groundwater bodies of agricultural areas in California. Simultaneously, high commodity prices and drought have encouraged perennial crop growers to turnover less-productive orchards, providing opportunities to recycle tree biomass in situ and to use high-carbon (C) residues to conserve soil and water resources. Although climate change adaptation and mitigation benefits of high-C soil amendments have been shown, uncertainties remain regarding the benefits and trade-offs of this practice for N cycling and retention. We used established almond [Prunus dulcis (Mill.) D. A. Webb] orchard trials on Hanford fine sandy loam with short-term and long-term biomass recycling legacies to better understand the changes in N dynamics and retention capacity associated with this practice. In a soil column experiment, labeled N fertilizer was added and traced into various N pools, including microbial biomass and inorganic fractions in soil and leachate. Shifts in microbial communities were characterized using the abundance of key N cycling functional genes regulating nitrification and denitrification processes. Our findings showed that, in the short term, biomass recycling led to N immobilization within the orchard biomass incorporation depth zone (0-15cm) without impacts on N leaching potential. However, this practice drastically reduced nitrate leaching potential by 52%, 10 yr after biomass incorporation without an increase in N immobilization. Although the timing of these potential benefits as a function of microbial population and C and N biogeochemical cycles still needs to be clarified, our results highlight the potential of this practice to meaningfully mitigate nitrate discharges into groundwater while conserving soil resources.

Soil freezing-thawing induces immediate shifts in microbial and resource stoichiometry in Luvisol soils along a postmining agricultural chronosequence in Western Germany

Freeze-thaw (FT) events exert a great physiological stress on soil microorganisms and hence impact biogeochemical processes in soils. As numerous environmental factors affect microbial and chemical responses to FT, a better understanding of the leverage factors that regulate the responses to FT events is required. To date, FT-induced shifts and transformations in microbial and resource stoichiometry have received particularly little attention. We exposed fifteen Luvisol soils with different time after restoration and corresponding differences in soil organic C contents from a postmining agricultural chronosequence to a single FT event and analysed changes in soil chemistry and microbial stoichiometry one hour and eighteen hours after thawing. FT considerably altered soil biochemical attributes within the first hours of thawing. Microbial biomass C declined substantially after FT, and its relative losses were positively correlated with enhanced dissolved organic C contents. Thus, microbial cell lysis likely led to the significant increase of dissolved organic C. Moreover, microbial biomass C losses were disproportionally higher in C-rich soils, suggesting that soil microorganisms in high-C soils might be particularly prone to FT stress. Microbial biomass N marginally decreased one hour after thawing, yet returned to initial levels eighteen hours after thawing. The alternating responses of microbial biomass C and N caused a strong stoichiometric reduction of the microbial C:N ratio. The resulting microbial oversaturation with N relative to C is likely the first step in the chain of processes that generally lead to the high N losses commonly recorded in agricultural soils in the aftermath of FT events. Metabolic activity of the soil microbial community increased with the relative decline of the microbial biomass C:N ratio eighteen hours after thawing, suggesting increased levels of microbial metabolic expenditure due to stoichiometric shifts. The strength of the FT-driven biochemical responses was strongly dependent on soil organic C content, indicating that high-C soils might be especially vulnerable to initial C and N losses due to shifts in microbial stoichiometry.

Can char carbon enhance soil properties and crop yields in low-carbon soils?

Restoring soil carbon (C) lost due to intensive farming is a long-term endeavor under current conservation management practices. Application of coal combustion residue (293g C kg-1 ) from a sugar beet (Beta vulgaris L.) processing factory, hereafter referred to as char, could rapidly restore soil C and productivity in degraded croplands, but data on this potential strategy are unavailable. We assessed the impacts of char application to two relatively low-C soils (10.1 and 12.2g C kg-1 ) and one relatively high-C soil (17.3g C kg-1 ) on soil C, soil physical and fertility properties, and crop yields in no-till systems in the Great Plains after 2yr. Char was disked to 15cm soil depth at char-C application rates ranging from 0 to 19.7 Mg C ha-1 , corresponding to char application rates ranging from 0 to 67.3 Mg ha-1 . The highest char rate increased C concentration in all soils but increased C stock only in low-C soils. Char did not affect soil penetration resistance, available water, aggregate stability, most nutrients, and crop yields. Char application at high rates increased sulfate, Ca, Mg, and Na concentrations but did not influence other properties. Carbon recovery of the char applied at the highest rate varied among soils from 50 to 85%, but the mechanisms for such differences need further investigation. Short-term duration, low char C concentration, and low application rates may explain the limited char effects. Overall, char application at 19.7 Mg char-C ha-1 (i.e., 67.3 Mg char ha-1 ) increased soil C concentration but had negligible effects on other soil properties and crop yields after 2yr.

Phosphorus Dynamics Near Bald Cypress Roots in a Restored Wetland

Core Ideas Previous tree exposure to saturated conditions limited root death after ponding. Root growth and death had no apparent effect on concentrations of Fe2+, DOC, or DTP. Concentrations of Fe2+ were related to water table levels and redox status. Phosphorus concentrations were controlled by iron reduction and oxidation. Phosphorus (P) dissolution occurs commonly in wetland soils restored from agricultural land. Associated with P release are high concentrations of dissolved organic carbon (DOC) and Fe2+. This field study evaluated the effect of a fluctuating water table on the root dynamics of bald cypress (Taxodium distichum L. Rich.) to determine whether root death created soil reduction microsites, potentially contributing to P dissolution. The study site is a restored Carolina bay wetland with organic soils. Root growth and death were monitored on 16 6‐yr‐old bald cypress using minirhizotrons. Root dynamics, water table levels, and soil porewater chemistry and redox potential in the root zone were monitored for 2 yr. Soil solution samples were analyzed for Fe2+, pH, DOC, and P. High rates of root growth occurred during dry conditions, whereas root death occurred during sustained periods of saturation, particularly within 20 cm of the surface. Cyclic changes in concentrations of Fe2+, DOC, and dissolved total P (DTP) were related to water table position but not to changes in root numbers. After sustained periods of saturated conditions, redox potential decreased to 0 mV, Fe2+ increased to 1.75 mg Fe2+ L–1, and DOC increased to 350 mg L–1, resulting in peak DTP concentrations of 750 μg L–1, compared with 100 μg L–1 during dry periods. This study showed that in these high‐C soils (∼20% organic C) rooting dynamics had minimal impact on changes in P concentrations and that P dissolution was largely controlled by Fe reduction processes occurring within the C‐rich soil matrix.

The short-term effects of liming on organic carbon mineralisation in two acidic soils as affected by different rates and application depths of lime

Two acidic soils (initial pH, 4.6) with contrasting soil organic C (SOC) contents (11.5 and 40 g C kg−1) were incubated with 13C-labelled lime (Ca13CO3) at four different rates (nil, target pH 5, 5.8 and 6.5) and three application depths (0–10, 20–30 and 0–30 cm). We hypothesised that liming would stimulate SOC mineralisation by removing pH constraints on soil microbes and that the increase in mineralisation in limed soil would be greatest in the high-C soil and lowest when the lime was applied in the subsoil. While greater SOC mineralisation was observed during the first 3 days, likely due to lime-induced increases in SOC solubility, this effect was transient. In contrast, SOC mineralisation was lower in limed than in non-limed soils over the 87-day study, although only significant in the Tenosol (70 μg C g−1 soil, 9.15%). We propose that the decrease in SOC mineralisation following liming in the low-C soil was due to increased microbial C-use efficiency, as soil microbial communities used less energy maintaining intracellular pH or community composition changed. A greater reduction in SOC mineralisation in the Tenosol for low rates of lime (0.3 and 0.5 g column−1) or when the high lime rate (0.8 g column−1) was mixed through the entire soil column without changes in microbial biomass C (MBC) could indicate a more pronounced stabilising effect of Ca2+ in the Tenosol than the Chromosol with higher clay content and pH buffer capacity. Our study suggests that liming to ameliorate soil acidity constraints on crop productivity may also help to reduce soil C mineralisation in some soils.

Bacterial Community Composition Associated with Pyrogenic Organic Matter (Biochar) Varies with Pyrolysis Temperature and Colonization Environment.

Microbes that colonize pyrogenic organic matter (PyOM) (also called biochar) play an important role in PyOM mineralization and crucially affect soil biogeochemical cycling, while the microbial community composition associated with PyOM particles is poorly understood. We generated two manure-based PyOMs with different characteristics (PyOM pyrolyzed at the low temperature of 300°C [i.e., PyOM300] and at the high temperature of 700°C [i.e., PyOM700]) and added them to high-carbon (4.15%) and low-C (0.37%) soil for microbial colonization. 16S rRNA gene sequencing showed that Actinobacteria, particularly Actinomycetales, was the dominant taxon in PyOM, regardless of the PyOM pyrolysis temperature and soil type. Bacterial communities associated with PyOM particles from high-C soils were similar to those in non-PyOM-amended soils. PyOM300 had higher total microbial activity and more differential bacterial communities than PyOM700. More bacterial operational taxonomic units (OTUs) preferentially thrived on the low-pyrolysis-temperature PyOM, while some specific OTUs thrived on high-pyrolysis-temperature PyOM. In particular, Chloroflexi species tended to be more prevalent in high-pyrolysis-temperature PyOM in low-C soils. In conclusion, the differences in colonized bacterial community composition between the different PyOMs were strongly influenced by the pyrolysis temperatures of PyOM, i.e., under conditions of easily mineralizable C or fused aromatic C, and by other properties, e.g., pH, surface area, and nutrient content. IMPORTANCE Pyrogenic organic matter (PyOM) is widely distributed in soil and fluvial ecosystems and plays an important role in biogeochemical cycling. Many studies have reported changes in soil microbial communities stimulated by PyOM, but very little is known about the microbial communities associated with PyOM. The microbes that colonize PyOMs can participate in the mineralization of PyOM, so changing its structure affects the fate of PyOMs and contributes to soil biogeochemical cycling. This study identified the bacterial community composition associated with PyOMs on the basis of high-throughput sequencing and demonstrated that both PyOM pyrolysis temperature and the colonization environment determined the bacterial community composition. Our work increases our understanding of the dominant phylogenetic taxa associated with PyOMs, demonstrates mechanisms mediating microbial metabolism and growth in PyOMs, and expands a new research area for pyrogenic organic matter. This study identified the bacterial community composition associated with PyOM, which is widely distributed in the environment. Most bacterial OTUs preferentially thrived on PyOM pyrolyzed at low temperature, while some specific OTUs thrived on PyOM pyrolyzed at high temperature.

Modelling the influence of soil carbon on net greenhouse gas emissions from grazed pastures

Sequestering carbon (C) in soil organic matter in grassland systems is often cited as a major opportunity to offset greenhouse gas (GHG) emissions. However, these systems are typically grazed by ruminants, leading to uncertainties in the net GHG balance that may be achieved. We used a pasture model to investigate the net balance between methane (CH4), nitrous oxide (N2O) and soil C in sheep-grazed pasture systems with two starting amounts of soil C. The net emissions were calculated for four soil types in two rainfall zones over three periods of 19 years. Because of greater pasture productivity, and consequent higher sheep stocking rates, high-rainfall sites were associated with greater GHG emissions that could not be offset by C sequestration. On these high-rainfall sites, the higher rate of soil organic carbon (SOC) increase on low-SOC soils offset an average of 45% of the livestock GHG emissions on the modelled chromosol and 32% on the modelled vertosol. The slow rate of SOC increase on the high-SOC soils only offset 2–4% of CH4 and N2O emissions on these high-rainfall sites. On low-rainfall sites, C sequestration in low-SOC soils more than offset livestock GHG emissions, whereas the modelled high-C soils offset 75–86% of CH4 and N2O emissions. Greater net emissions on high-C soils were due primarily to reduced sequestration potential and greater N2O emissions from nitrogen mineralisation and livestock urine. Annual variation in CH4 and N2O emissions was low, whereas annual SOC change showed high annual variation, which was more strongly correlated with weather variables on the low-rainfall sites compared with the high-rainfall sites. At low-soil C concentrations, with high sequestration potential, there is an initial mitigation benefit that can in some instances offset enteric CH4 and direct and indirect N2O emissions. However, as soil organic matter increases there is a trade-off between diminishing GHG offsets and increasing ecosystem services, including mineralisation and productivity benefits.

Relationships between Biological and Thermal Indices of Soil Organic Matter Stability Differ with Soil Organic Carbon Level

Thermal analysis techniques can provide an important addition to our understanding of soil organic matter (SOM) composition and stability. Several recent studies have linked thermal and biological stability of SOM; however, contrasting results have been reported. The objective of this study was to characterize the relationships between thermal and biological SOM stability for a wide range of mineral soils. Soils were collected from 28 sites from across the United States and analyzed by thermogravimetry (TG) and differential scanning calorimetry (DSC) coupled with CO2 evolved gas analysis (CO2–EGA). We compared thermal analysis results to mean soil respiration rates during incubation at 20°C for 365 d (R20). For soils with <30 g C kg−1 (low C), R20 was negatively correlated with the temperature at which half of the DSC energy is released and the temperature at which half of the CO2–EGA is evolved. Conversely, for soils with >30 g C kg−1 (high C), R20 was positively correlated with CO2–EGA and DSC energy released between 345 and 460°C and to SOM energy density (in J mg−1 C). Differences between low-C and high-C soils indicate the relative importance of mineral association of SOM in low-C soils and the abundance of intact plant debris that is relatively energy dense and thermally resistant but relatively easy to decompose in high-C soils. Above all, thermal analysis proved to be a useful technique for interpreting SOM stability, but sample C concentration must be considered because it affects the dominant SOM stabilization mechanisms and thermal analysis results.

Agricultural management affects earthworm and termite diversity across humid to semi-arid tropical zones

Earthworm and termite diversity were studied in 12 long-term agricultural field trials across the sub-humid to semi-arid tropical zones of Eastern and Western Africa. In each trial, treatments with high and low soil organic C were chosen to represent contrasts in long-term soil management effects, including tillage intensity, organic matter and nutrient management and crop rotations. For each trial, a fallow representing a relatively undisturbed reference was also sampled. Earthworm taxonomic richness decreased in the direction fallow>high-C soil>low-C soil and earthworm abundance was higher in fallow than under continuous crop production. Termite abundance was not significantly different between fallow and high and low-C treatments and termite taxonomic richness was higher in fallow soil than in the two cropping systems. We concluded that fewer species of earthworms and termites were favored under agricultural management that led to lower soil C. Results indicated that the soil disturbance induced by continuous crop production was more detrimental to earthworms than to termites, when compared to the fallow.

Soil C balances in Swedish agricultural soils 1990–2004, with preliminary projections

Swedish agricultural land comprises about 3 Mha and its topsoil contains about 270 Mt C (0–25 cm depth). Based on daily climate data, annual yield data and a soil database, we calculate the topsoil C dynamics for Swedish agricultural land 1990–2004, using a soil C balance model, ICBM. Losses from high C (organic) soils are calculated from subsidence, which in turn is calculated from soil properties, cropping system and weather conditions. We also present scenarios and projections into the future. Mineral soils are close to balance in all of the eight agricultural regions investigated. Average soil C mass roughly increases from South to North, since the lower yields and thus C inputs in Northern regions are more than balanced by the higher decomposition rates due to warmer climate in the South. The higher proportion of grass leys in the North also contributes to higher C mass. High C soils (>7% C, corresponding to 12% soil organic matter content) lose 2–6 t C ha−1 year−1, depending on weather and cropping system, and total annual loss from Swedish agricultural high-C soils is about 1 Mt year−1. This loss is discussed in the context of plant production and remedial actions. Projections into the future, assuming that a temperature increase leading to increased decomposition rates also will lead to higher yields, indicate a potential to at least maintain soil C mass in Swedish agricultural mineral soils. Growing crops with residues more resistant towards decomposition would be an efficient way to increase soil C mass. See also http://www-mv.slu.se/vaxtnaring/olle.