Soil Organic Carbon Measurements Research Articles

Bulk density of mineral and organic soils in the Canada’s arctic and sub-arctic

Bulk density is an indicator of soil compaction subject to anthropogenic impact, essential to the interpretation of any nutrient budgets, especially to perform carbon inventories. It is so expensive to measure bulk density in arctic/sub-arctic and there are relatively very few field measurements are available. Therefore, to establish a bulk density and SOC empirical relationship in Canada’s arctic and sub-arctic ecosystems, compiled all the bulk density and SOC measurements that are available in Northern Canada. In addition an attempt has been made for bulk density and SOC field measurement in Yellowknife and Lupin, to develop an empirical relationship for Canada’s arctic and sub-arctic.Relationships between bulk density (BD) and soil organic carbon (SOC) for mineral soil and organic soils (0–100cm depth) were described by exponential functions. The best fit model, predictive bulk density (BDp), for mineral soil, (BDp=0.701+0.952 exp(−0.29 SOC), n=702, R2=0.99); for organic soil (BDp=0.074+2.632 exp(−0.076 SOC), n=674, R2=0.93). Different soil horizons have different bulk densities and may require different predictive equations, therefore, developed predictive best fit exponential equation for both mineral and organic soils together (BDp=0.071+1.322 exp(−0.071 SOC), n=1376, R2=0.984), where X is a dummy variable with a value of 0 for surface peat (0–25cm depth) and 1 for subsurface peat (25–175cm). We recommend using the soil organic carbon density approach to estimate BD from SOC because it allows BD to be predicted without significant bias.

Radiocarbon-Based Assessment of Heterotrophic Soil Respiration in Two Mediterranean Forests

The amount of soil organic carbon (SOC) released into the atmosphere as carbon dioxide (CO2), which is referred to as heterotrophic respiration (Rh), is technically difficult to measure despite its necessity to the understanding of how to protect and increase soil carbon stocks. Within this context, the aim of this study is to determine Rh in two Mediterranean forests dominated by pine and oak using radiocarbon measurements of the bulk SOC from different soil layers. The annual Rh was 3.22 Mg C ha−1 y−1 under pine and 3.13 Mg C ha−1 y−1 under oak, corresponding to 38 and 31% of the annual soil respiration, respectively. The accuracy of the Rh values was evaluated by determining the net primary production (NPP), as the sum of the Rh and the net ecosystem production measured by eddy covariance, then comparing it with the NPP obtained through independent biometric measurements. No significant differences were observed, which suggested the suitability of our methodology to infer Rh. Assuming the C inputs to soil to consist exclusively of the aboveground and belowground litter and the C output exclusively of the Rh, both soils were C sinks, which is consistent with a previous modeling study that was performed in the same stands. In conclusion, radiocarbon analysis of bulk SOC provided a reliable estimate of the average annual amount of soil carbon released to the atmosphere; hence, its application is convenient for calculating Rh because it utilizes only a single soil sampling and no time-consuming monitoring activities.

Differential Effects of Conservational Management on SOC Accumulation in the Grasslands of China.

Conservational management practices in grasslands have been considered one of the efficient options to enhance the soil organic carbon (SOC) accumulation. However, the SOC changes after the conservational management practices vary significantly under different grassland vegetation types and the environmental conditions. At present, it is not clear how the SOC accumulation changes along the soil profile if conservational management practice was adopted. In this study, we collected 663 paired observational data of SOC changes with and without conservational management practices in grasslands of China from 176 published literatures that has both the surface (0‒20 cm) and subsurface (to 40 cm depth) SOC measurements. The differences of SOC density (SOCD) between pre‒management and post‒management in the vertical soil layers were analyzed in order to establish a quantitative relationship of the SOC changes between the subsurface and the surface. The results revealed that in all grasslands, conservational management practices benefits the SOC accumulation by enhancing 0.43‒1.14 Mg C ha–1 yr–1. But the SOC increment weakened downwards along the soil profile. While the surface SOC was enhanced by 17% after conservational management, the subsurface SOC was enhanced by only 7%. The SOC accumulation was closely correlated with restoration duration, pre-management SOCD and the environmental factors and differed greatly among different grasslands and the practices adopted. The alpine and mountain grassland showed a higher annual SOC increment than the temperate grassland with the annual rate of 1.62 and 0.72 Mg C ha-1 yr-1, respectively. The SOC increment caused by the artificial plantation and the grazing exclusion conservational management was more than 2-fold that of the cropland abandonment and the extensive utilization. With the quantitative relationship of the SOC changes between soil layers, we provide a methodological option to estimate SOC changes to layers deeper than the recommendation of IPCC when only the surface layer SOC increment is available.

Observed and predicted changes in soil carbon stocks under export and diversified agriculture in the Caribbean. The case study of Guadeloupe

Export agriculture in the Caribbean is often blamed for pollution of soils and water resources. At the same time, the reduction of preferences in the international markets for major agricultural exports from the Caribbean induced a partial reorientation of the agriculture towards the local markets, which included crop diversification. This study was carried out to assess the sustainability of that agricultural switch with respect to maintaining, increasing or decreasing soil organic carbon (SOC) stocks. We analysed the impact of export crops (sugarcane and banana monocultures) and diversified agriculture (as monoculture or in rotation with export crops) on SOC stocks using the case study of Guadeloupe (Lesser Antilles). Agriculture in Guadeloupe involves a mosaic of soils, climates, crops and farming practices, which well represent the tropical conditions of export and diversified agriculture in the region. The study was based on: (i) a soil database including information on the SOC stocks of numerous cropping system—agro-ecological region (AER) situations, (ii) a survey of farming practices performed on a network of 382 farmers (e.g. crop rotations and yields, management of residues and organic amendments), and (iii) the development of a simple model of annual C inputs and outputs to assess SOC dynamics at the AER scale. The model was calibrated and evaluated using 253 plots and included 827 SOC measurements selected from the soil database. The model produced satisfactory estimates of changes in SOC stocks and provided an explanation for differences between cropping systems and AERs in terms of the C inputs and outputs. While sugarcane and banana monocultures were able to preserve or increase SOC, diversification was likely to reduce it. These differences were due to higher C inputs from crop residues together with lower C outputs for export agriculture. Lower SOC outputs by mineralization were mainly associated with the longer cycle of these pluriannual crops (5yr), which decreased the impact of soil tillage at planting. Banana monoculture in the most humid AERs was the only cropping system that displayed a clear pattern of C sequestration (+0.5MgCha−1yr−1). The highest SOC losses were observed with vegetable crops in the same AERs (e.g. −2.0MgCha−1yr−1). Calculations made using the model indicated that the sustainability of diversified agriculture in the Caribbean might be reinforced by adopting reduced soil tillage, organic amendments and the liming of the more acid soils to increase yields and crop residues. The results of this study suggest that the implementation of new agricultural policies to reduce the negative environmental impacts of export crops should involve measures to preserve their positive effect on SOC storage.

Land use change decreases soil carbon stocks in Tibetan grasslands

Land use is an important factor affecting soil organic carbon (SOC) dynamics and can produce positive C climate feedback, but its effects remain unknown for Tibetan ecosystems. Recent land use changes have converted the traditional winter Kobresia pastures of nomads in the northeastern Tibetan Plateau to Elymus pastures or even to cropland. Detailed SOC measurements up to 30-cm depth were combined with analysis of δ13C, δ15N, bulk density, microbial C, and N contents in three land use types. Bulk density was decreased by conversion from Kobresia pasture to cropland but increased by conversion to Elymus pasture. The loss of 1 % of SOC caused by land use change leads to δ13C increase of 0.8 ‰. Conversion to cropland significantly decreased SOC stocks (10 %) and microbial biomass C, but the C loss (1.6 %) was insignificant in Elymus pasture. Land use changes strongly increased soil δ15N in the top 5 cm. Conversion to Elymus pasture did not change the C stocks, but conversion to cropland decreased C stocks by 10 % within 10 years. Soil δ13C and δ15N data indicate acceleration of C and N cycling due to the replacement of Kobresia pasture by Elymus pasture and cropland.

Vis‐Near IR Reflectance Spectroscopy for Soil Organic Carbon Content Measurement in the Canadian Prairies

The objective of this study was to measure soil organic carbon (SOC) content for Canadian prairies soils using Vis‐near IR reflectance (VisNIR) spectroscopy. Specifically, the effects of the spectral data pretreatment method and number of latent variables on SOC prediction were determined. In addition, the capability of VisNIR spectroscopy to capture SOC variability was evaluated. For these purposes, 491 soil samples of 0–30 cm from two perpendicular 1600 m long sampling transects (NS and WE transects with 239 and 252 samples, respectively) in the Canadian prairies were scanned by VisNIR spectroscopy. SOC content at one transect was predicted by models calibrated at the other transect. The potential of VisNIR spectroscopy in predicting SOC was verified in this area. Smoothing using cubic smoothing spline outperformed other pretreatment methods. The performance of SOC prediction improved and then worsened with the increase in the number of latent variables in the models. Wavelet transform indicated a similar pattern of high variances in the scale‐location domains between the predicted and measured SOC; the predicted SOC showed a decreased structured variability at the NS transect and increased nugget effect at the WE transect. At both transects, weaker, but with the same levels, spatial dependency and greater correlation length were observed for the predicted SOC compared with the measured SOC. The obtained results can be applicable for SOC measurements with VisNIR spectroscopy.

Prediction of volumetric soil organic carbon from field‐moist intact soil cores

SummaryThis study investigated the potential for visible–near‐infrared (vis–NIR) spectroscopy to predict locally volumetric soil organic carbon (SOC) from spectra recorded from field‐moist soil cores. One hundred cores were collected from a 71‐ha arable field. The vis–NIR spectra were collected every centimetre along the side of the cores to a depth of 0.3 m. Cores were then divided into 0.1‐m increments for laboratory analysis. Reference SOC measurements were used to calibrate three partial least‐squares regression (PLSR) models for bulk density (ρb), gravimetric SOC (SOCg) and volumetric SOC (SOCv). Accurate predictions were obtained from averages of spectra from those 0.1‐m increments for SOCg (ratio of performance to inter‐quartile (RPIQ) = 5.15; root mean square error (RMSE) = 0.38%) and SOCv (RPIQ = 5.25; RMSE = 4.33 kg m−3). The PLSR model for ρb performed least well, but still produced accurate results (RPIQ = 3.76; RMSE = 0.11 Mg m−3). Predictions for ρb and SOCg were combined to compare indirect and direct predictions of SOCv. No statistical difference in accuracy between these approaches was detected, suggesting that the direct prediction of SOCv is possible. The PLSR models calibrated on the 10‐cm depth intervals were also applied to the spectra originally recorded on a 1‐cm depth increment. While a bigger bias was observed for 1‐cm than for 10‐cm predictions (1.13 and 0.19 kg m−3, respectively), the two populations of estimates were not distinguishable statistically. The study showed the potential for using vis–NIR spectroscopy on field‐moist soil cores to predict SOC at high depth resolutions (1 cm) with locally derived calibrations.

Edaphic controls on soil organic carbon stocks in restored grasslands

Cultivation of undisturbed soils dramatically depletes organic carbon stocks at shallow depths, releasing a substantial quantity of stored carbon to the atmosphere. Restoration of native ecosystems can help degraded soils rebuild a portion of the depleted soil organic matter. However, the rate and magnitude of soil carbon accrual can be highly variable from site to site. Thus, a better understanding of the mechanisms controlling soil organic carbon stocks is necessary to improve predictions of soil carbon recovery. We measured soil organic carbon stocks and a suite of edaphic factors in the upper 10cm of a series of restored tallgrass prairies representing a range of drainage conditions. Our findings suggest that factors related to soil organic matter stabilization mechanisms (texture, polyvalent cations) were key predictors of soil organic carbon, along with variables that influence plant and microbial biomass (available phosphorus, pH) and soil moisture. Exchangeable soil calcium was the strongest single predictor, explaining 74% of the variation in soil organic carbon, followed by clay content, which explained 52% of the variation. Our results demonstrate that the cumulative effects of even relatively small differences in these edaphic properties can have a large impact on soil carbon stocks when integrated over several decades.

Changes in soil carbon stocks under perennial and annual bioenergy crops

AbstractBioenergy crops are expected to provide biomass to replace fossil resources and reduce greenhouse gas emissions. In this context, changes in soil organic carbon (SOC) stocks are of primary importance. The aim of this study was to measure changes in SOC stocks in bioenergy cropping systems comparing perennial (Miscanthus × giganteus and switchgrass), semi‐perennial (fescue and alfalfa), and annual (sorghum and triticale) crops, all established after arable crops. The soil was sampled at the start of the experiment and 5 or 6 years later. SOC stocks were calculated at equivalent soil mass, and δ13C measurements were used to calculate changes in new and old SOC stocks. Crop residues found in soil at the time of SOC measurements represented 3.5–7.2 t C ha−1 under perennial crops vs. 0.1–0.6 t C ha−1 for the other crops. During the 5‐year period, SOC concentrations under perennial crops increased in the surface layer (0–5 cm) and slightly declined in the lower layers. Changes in δ13C showed that C inputs were mainly located in the 0–18 cm layer. In contrast, SOC concentrations increased over time under semi‐perennial crops throughout the old ploughed layer (ca. 0–33 cm). SOC stocks in the old ploughed layer increased significantly over time under semi‐perennials with a mean increase of 0.93 ± 0.28 t C ha−1 yr−1, whereas no change occurred under perennial or annual crops. New SOC accumulation was higher for semi‐perennial than for perennial crops (1.50 vs. 0.58 t C ha−1 yr−1, respectively), indicating that the SOC change was due to a variation in C input rather than a change in mineralization rate. Nitrogen fertilization rate had no significant effect on SOC stocks. This study highlights the interest of comparing SOC changes over time for various cropping systems.

Spatial variability of surface soil saturated hydraulic conductivity in a small karst catchment of southwest China

Soil saturated hydraulic conductivity (Ks) is one of the most important parameters that determine soil water redistribution. However, it is little known in a karst area with thin, discontinuous, and rocky soils. The purpose of this paper was to investigate the spatial variability of Ks and its influencing factors in a small karst catchment with high heterogeneity in southwest China. Undisturbed surface (0–10 cm) soil samples were collected with metal cylinders from an 80 m × 80 m grid for measuring Ks, bulk density (ρb), non-capillary porosity (NCP), and capillary porosity (CP). Disturbed surface samples were taken with a shovel to measure soil organic carbon (SOC) and rock fragment content (RC). Environmental factors including topographical location (slope and depression), land-use type (forestland, shrubland, shrub-grassland, and farmland), slope gradient, slope exposure, and elevation were investigated for each sampling point. The results showed that Ks value was relatively high (9.10 m/d) with a moderate variation, and had a strong spatial dependence (nugget/sill value = 0.06 %). NCP showed a very significantly (p 0.05), but their interaction effects were significant. This suggested that the influencing factors were more complex and they should be considered together in a karst area. The mean Ks value was the lowest in the forestland on the hillslope and in the farmland in the depression, indicating where the surface runoff may appear easily. Such results could help to better understand the soil hydrological processes and contribute to the building of hydrological models in small karst catchments.

Estimating upper soil horizon carbon stocks in a permafrost watershed of Northeast Siberia by integrating field measurements with Landsat-5 TM and WorldView-2 satellite data

Arctic soils contain three times as much carbon (C) as all aboveground biomass distributed globally, much of which is stored in permafrost soils. Here, we (1) determine the predictability of estimating soil organic carbon (SOC) using different satellite data, classifications, and methods; (2) estimate the quantity and distribution of SOC for the top 10 cm for the Ambolikha River watershed (~121 km2) in northeast Siberia, a sub-watershed of the Kolyma River; and (3) produce a hybrid SOC map through data fusion, combining the strengths of each data type. Land cover maps were produced using a pixel-based classification with Landsat-5 Thematic Mapper (TM) data and an object-based classification using WorldView-2 data. Spectral mixture analysis (SMA) was performed on both data types to calculate the fraction of four vegetation types in each pixel, and land cover maps were combined with field measurements of SOC in the top 10 cm. The overall classification accuracy was 69% for Landsat, 81% for WorldView-2, and 82% for the hybrid map. The hybrid map estimated 490.5 Gg of SOC for the top 10 cm within the Ambolikha River watershed, with less severe over- or under-estimated than the Landsat or WorldView-2 maps alone. The results suggest that (a) higher spatial resolution satellite data and object-based classification should be used for land cover classification in the Arctic; (b) integrating multiple sensors maximizes the strengths of different sensors for estimating SOC; and (c) previous studies performed at the regional or pan-Arctic scale using lower spatial resolution data likely underestimate total SOC.

Comparison of Three Methods for Measurement of Soil Organic Carbon

Quantification of soil organic carbon (SOC) is an important element in the assessment of the carbon sequestration potential of soils in tree-based intercropping (TBI) systems. The organic carbon (OC) concentrations of soils in TBI systems often differ from those in conventional agricultural systems due to the additional C inputs from litter fall and roots. However, the presence of soil inorganic carbon (SIC) can confound the measurements of SOC. This study compared three methods of measuring SOC: (i) measurement of the total soil C (TC) in one subsample and, after treatment in a muffle furnace (575 °C) for 24 h, measurement of SIC in another subsample; (ii) SOC measured after fumigation with 12 M hydrochloric acid (HCl) to remove SIC; and (iii) SOC measured after digestion with 0.73 M H2SO3 to remove SIC. The TC, SOC, and SIC concentrations were determined by combustion. A correction factor was applied to express SIC and SOC concentrations on an original, untreated soil basis. Measurement of SOC by the muffle furnace method resulted in the greatest SOC concentrations for Populus spp. (hybrid poplar) for samples from two of the three depths (0–10 and 20–40 cm). Measurement of SOC by the HCl fumigation and H2SO3 digestion methods were highly correlated, suggesting complete removal of SIC with minimal oxidation of SOC. These results have implications for the method of measuring SOC in calcareous soils under coniferous and deciduous tree species to a depth of 40 cm.

Measuring soil organic carbon: which technique and where to from here?

This review compares and contrasts analytical techniques for the measurement of total soil organic carbon (TOC). Soil TOC is seen to be a highly important health and quality indicator for soils, as well as having the potential to sequester atmospheric carbon. Definition of the form of organic carbon measured by a given method is vital to the selection of appropriate methodology, as well as the understanding of what exactly is being measured. Historically, studies of TOC have ranged from basic measures, such as colour and gravimetric analyses, to dry and wet oxidation techniques. In more recent times, various spectroscopic techniques and the application of remote or mobile approaches have gained prominence. The different techniques, even the oldest ones, may have their place in current research depending on research needs, the available time, budget and access to wider resources. This review provides an overview of the various methods, highlights advantages, limitations and research opportunities and provides an indication of what the method actually measures so that meaningful comparisons can be made.

Plant-soil feedbacks and the partial recovery of soil spatial patterns on abandoned well pads in a sagebrush shrubland.

Shrub-dominated arid and semiarid ecosystems are characterized by spatail patterns in vegetation and bare ground (e.g., resource islands). Modern oil and gas well pad construction entails complete removal of vegetation and upper soil layers, followed by replacement of soils and attempts at revegetation; historically, many pads were merely abandoned. Feedbacks between soil and vegetation are required for the recovery of ecosystem functions in these catastrophically disturbed systems. We measured soil organic carbon (SOC), employing a spatially explicit sampling protocol, on two sites in undisturbed big sagebrush communities and a chronosequence of eight recovering well pads. Sites in undisturbed communities exhibited significant spatial autocorrelation of SOC at the plot level that was absent from all of the well pad sites. Incorporating shrub presence as a covariate revealed three additional cases of SOC spatial autocorrelation on well pads. These results, along with SOC patterns between and- under plants, suggest resource island development. These findings support the hypothesis that species identity as well as functional group need to be taken into account in restoration. Restoration of ecosystem functions, including those associated with resistance and resilience to disturbance, may be enhanced when characteristic soil heterogeneity and vegetation spatial patterns recover.

不同林龄白桦天然次生林土壤碳通量和有机碳储量

Soil respiration,known as the second largest componentof carbon dioxide flux between terrestrial ecosystems and the atmosphere,is sensitive to climate,vegetation type as well as forest age. In order to simulate the long-term forest carbon dynamics and its impact on climate system,we need to understand the successional status of carbon dynamics more than learn the responses of forest ecosystems to the changing climate. However,in spite of its significant role in determining the distribution of carbon pools and fluxes in different forest ecosystems,the effect of age on forest carbon fluxes great is uncertain. Soil temperature and soil water content are recognized as the main factors controlling the temporal variation of soil respiration. In this paper,soil carbon dynamic was measured in different age stands of Secondary Betula platyphylla( 20 a,36a,82a) in Xiaoxing'an Mountain,China. Soil respiration measurements were conducted using a LI-6400-09 soil CO2 flux system from May to October in 2008. Besides,We measured soil organic carbon content( g / kg) and bulk density( g / cm3)at four layers across the chronosequence in August 2009. Our specific objectives were to:( 1) compare the cumulative soil respiration during the growing season for the different age stands of Secondary Betula platyphylla,( 2) quantify soil organic carbon( SOC) contents and SOC density and( 3) establish quantitative relationships between soil carbon fluxes and SOC density for the three forest ecosystems. Our results showed that the seasonality of soil respiration was driven mainly by soiltemperature with the peak appeared in August,and was roughly consistent with that of soil temperature. Soil temperature at10 cm could explain 86% —92% of the variation of soil respiration for the 3 different age stands of Secondary Betula platyphylla. And the soil volumetric water content was not significantly correlated with soil respiration rate and it was not a key factor to soil respiration. We fitted site-based models and used continuous measurements of soil temperature to estimate cumulative soil respiration for the growing season of 2008( days 132—295). Cumulative soil respiration in the growing season was estimated to be 740、768 and 809 C m- 2a-1in the 20 a,36a and 82 a Betula platyphylla,respectively. Both,cumulative soil respiration and Q10 increased during stand establishment. Q10 was estimated to be 2.64,2.91 and 3.35 in the20 a,36a and 82 a Betula platyphylla,respectively. Both,total soil organic carbon( SOC) contents and SOC density at 0—50 cm increased during stand establishment,but decreased with soil depth. SOC was estimated to be 43. 75,47. 72 and55. 96 g / kg for the three forest ecosystems. Meanwhile,the figures were 14.7,18.1 and 18.7 kg / m2 for SOC density in the20 a,36a and 82 a Betula platyphylla,respectively. Soil surface carbon flux was positively correlated to SOC content( P 0. 01),and the significance level of the correlation depended on soil depth,and the soil carbon flux was more positively correlated to SOC concentration at the depth of 0—10 cm( R2= 0.908).

Evaluation of methods for quantifying organic carbon in mangrove soils from semi-arid region

The objective of this work was to evaluate methods for the determination of soil organic carbon (SOC) in mangrove samples in order to improve economically feasible and precise analytical alternatives Total SOC was quantified through elemental analyzer (EA), considered as the standard method, and compared to (a) a chemical method, based on the oxidation of SOC by potassium dichromate under various conditions: wet and dry samples, with or without external heat source and variations in sulfuric acid concentrations (3.6, 6, 9, and 12 mol L−1); (b) spectral reflectance; and (c) thermogravimetry. Regarding the chemical analysis, the use of dried samples improved the accuracy; external heat sources led to higher interference, with a lower correlation to EA SOC; and the higher correlation with the EA was obtained between a 6 M concentration of sulfuric acid (r = 0.733). The spectral reflectance method showed a low correlation between the EA (−0.492 < r < 0.252) while thermogravimetry showed the largest (r = 0.905). SOC determination through chemical methods using dry samples with a sulfuric acid concentration of 6 M shows the best accuracy. As for the use of the spectral reflectance method in mangrove soils, further studies are needed to investigate the influence of pre- and postprocessing techniques. Thermogravimetry is characterized as the most adequate alternative method for SOC measurement when EA are not available since a correction factor is used (f = 0.27) to convert the soil organic matter content to SOC.

Distinguishing recent and fossil organic matter — A critical step in evaluation of post-mining soil development – using near infrared spectroscopy

Content of soil organic carbon (SOC) is an important parameter for monitoring of reclamation success. In coal mine soils, measurements of SOC are affected by fossil of organic matter in the overburden, such as kerogen and coal.The objective of this study was to assess the potential of near infrared spectroscopy (NIRS) coupled with partial least square analysis (PLS) for quantifying recently formed and fossil organic matter in post-mining soils near Sokolov, Czech Republic. NIR data were acquired from soil samples (n=14), where recent SOC was quantified by 14C-AMS analysis, and from artificial mixtures (n=125) of overburden, coal and material from fermentation (Oe) layer. 12 PLS analyses were performed on spectra, each separately calibrated to total C (Ctot), recent C (Crec), coal C (Ccoal) and kerogen C (Cker). All models produced correlation coefficient of cross-validation RCV>0.97. When validated using the independent validation set of soils, addition of soil samples into the calibration set improved soil predictions of Ctot and Crec (obtained from cross-validation) considerably. All four models for Ctot and Crec were classified as successful as indicated by root mean square error ranging from 0.74 to 1.11. Results showed no clear effect of grinding on predictive accuracy; thus, we conclude that particle size <2mm is sufficient for NIRS. Our results demonstrate the ability of NIRS–PLS to quantify fossil and recent carbon in soils.

Long-term effect of contrasted tillage and crop management on soil carbon dynamics during 41 years

Although numerous studies have been conducted on the effect of tillage on soil organic carbon (SOC), there is still no consensus on the importance of sequestration which can be expected from reduced tillage. Most studies have used a synchronic approach in fields or long-term experiments which were often poorly characterized with respect to initial conditions. In this paper, we used a diachronic approach to quantify SOC changes in a 41 years experiment comparing no-till (NT), shallow till (ST) and full inversion tillage (FIT) combined with crop managements (residues removal, rotation and catch crops). It included SOC measurements at time 0 and every 4 years, calculations at equivalent soil mass within or below the old ploughed layer. Results show that tillage or crop management had no significant effect on SOC stocks after 41 years both in the old ploughed layer (ca. 0–28cm) and deeper (ca. 0–58cm). Tillage had no effect on crop yields and residues. In the reduced tillage treatments (ST and NT), SOC accumulated in the surface layer (0–10cm), reaching a plateau after 24 years but declined continuously in the lower layer (10–28cm) at a rate of 0.42–0.44% yr−1. The difference in SOC stocks (ST or NT minus FIT) over the old ploughed layer followed a non-monotonic pattern over time. Reduced tillage caused a rapid SOC sequestration during the first 4 years which remained more or less constant (mean=2.17 and 1.31tha−1, resp.) during the next 24 years and disappeared after 28 years. The drop was attributed to the higher water balance recorded during years 24–28. In the reduced tillage treatments, the changes in SOC over time were negatively correlated with the water balance, indicating that sequestration rate was positive in dry periods and negative in wet conditions. This study highlights the interest of diachronic approaches to understand the effect of tillage and its interaction with environmental and management factors.

Modelling soil carbon and nitrogen dynamics using measurable and conceptual soil organic matter pools in APSIM

Measurable soil organic carbon (SOC) fractions are recommended to be used in soil organic matter (SOM) models to replace the conceptual pools in order to reduce uncertainty related to model initialization. However, changes in SOM pools require derivation of the decomposition rate for the new pools and also impact on simulation of nutrient cycling in soil. Here, we used the Agricultural Production Systems sIMulator (APSIM) to explore the impact of adopting measurable SOM pools on simulated SOC and nitrogen dynamics under three agro-ecosystems at Brigalow, Tarlee and Wagga Wagga in Australia. The decomposition rates for the measurable pools were derived based on the data in the top 30cm soil at Brigalow. With those decomposition rates, the model could predict the variation in observed total SOC, and also captured the dynamics of the measured SOC fractions at the three sites. However, the re-parameterized model overestimated soil nitrogen. Extending the model simulations to centurial scales led to the distinct difference in simulated soil carbon and nitrogen dynamics, due to the difference in pool sizes, decomposition rates and C:N ratios between the two approaches. Sensitivity analysis based on measurable pool approach indicated that simulated soil carbon dynamics was most sensitive to the partitioning of carbon in the sub-pools of new plant material, decomposition rates of particulate and humic organic matter. The simulated soil nitrogen dynamics was most sensitive to the C:N ratio of humic organic matter. In order to accurately simulate the long-term carbon and nitrogen dynamics, more detailed information on the composition of the measurable SOM pools, particularly their C:N ratios and the underlying mechanisms controlling their decomposition and transformation among different pools across different environments are needed.

Response of soil organic carbon and nutrients to simulated nitrogen deposition in typical mixed broadleaved-Korean pine forest.

In order to determine the effects of increased nitrogen deposition on soil organic carbon and nutrients in mixed broadleaved-Korean pine forest in Xiaoxing'an Mountains,China,a simulated nitrogen deposition field experiment was conducted from June 2008 to August 2010. Four nitrogen treatments(control,low,medium and high nitrogen) used CO( NH 2) 2 solution of 0,30,60 and 120 kg·hm- 2·a- 1. We collected 0- 20,20-40, 40- 60 cm soil samples and measured soil organic carbon(SOC),total nitrogen(TN),alkali-hydrolyzable N(HN),available phosphorus(AP) and available potassium(AK) in June and October,2009 and 2010. For two years experiments,we found that SOC and TN contents decreased with the increase of soil depth. Low and medium nitrogen treatments significantly increased SOC,HN and AK contents( P 0. 05). While,medium, high nitrogen treatments significantly reduced AP content(P 0. 05),and there was no significant changes of the TN before and after simulated nitrogen deposition field experiment(P 0. 05). Regression analysis showed that significant positive associations were found between the SOC and TN,HN,AP,AK( P 0. 001). The study suggests that continuous nitrogen deposition may increase the soil organic carbon pool and soil fertility in the mixed broadleaved-Korean pine forest ecosystem.