Soil Organic Carbon Change Rates Research Articles

Analysis of Soil Carbon Stock Dynamics by Machine Learning—Polish Case Study

A simplified differential equation for the dynamics of soil organic carbon (SOC) that describes the rate of SOC change (dSOC/dt) was constructed using the LASSO regression—a regularized linear regression machine learning method. This method selects the best predefined explanatory variables and empirically evaluates the relevant parameters of the equation. The result, converted into a formula for the long-term equilibrium level of soil carbon, indicates the existence of carbon sequestration potential in the studied regions of Poland. In particular, the model predicts high SOC content in regions with a high Topographic Wetness Index (TWI), such as river valleys or areas with high cattle density, as expected.

Changes in soil organic carbon across an atmospheric CO2 gradient under natural and artificial vegetation of semi-arid lands

Soil organic carbon (SOC) was studied across a gradient of 400–550 ppm CO2 in calcareous contaminated semi-arid areas. An experiment was performed under natural and artificial vegetation cover of Atriplex canescens (Pursh) Nutt. SOC and vegetation cover were investigated every 2 years from 2012 to 2018. Findings disclosed that there was a polynomial relation between SOC and CO2 concentration. The SOC change rate had the highest positive relationship with CO2 concentration in the uppermost 20 cm of soil: R2 = 0.96 and 0.47 in plantation and control areas, respectively. Moreover, plant life forms had a significant effect on the SOC in CO2 contaminated areas (P < 0.01). The highest correlation (R2 = 0.93–0.99) was observed between SOC rate changes and perennial cover percentage at a depth of 0–20 and 20–40 cm, respectively, through the time.

The State of Soil Organic Carbon in Vineyards as Affected by Soil Types and Fertilization Strategies (Tri Morave Region, Serbia)

Due to specific soil properties and management practices, soils in vineyards are sensitive to degradation. The aims of this study were to examine (i) the state of soil organic carbon (SOC) in vineyards compared to other agricultural land, (ii) the influence of different fertilization strategies and soil type on SOC content and (iii) the rate of SOC change over time and potential of deep tillage for SOC preservation in subsoil. The study was carried out at 16 representative vineyard locations of the Tri Morave region, which represents the largest vine growing region in Serbia. The analyzed area included 56 vineyard plots. Results showed that SOC stocks in the topsoil and subsoil were lower than the average for agricultural land in Serbia. The soil type was an important predictor of carbon storage in the topsoil. An adequate application of inorganic fertilizers or green manure combined with farmyard manure initially resulted in the highest SOC contents. Continuous application of inorganic fertilizer without organic amendments has led to a decrease of SOC in topsoil. High rates of SOC stock change in topsoil accompanied a rapid reduction of SOC in the earlier stage of cultivation. In all investigated subsoils, SOC increased, except for unfertilized vineyards. Deep tillage has the potential to preserve SOC in the deeper soil layer and prevent carbon loss from the topsoil. More attention should be paid to the soil conservation practices to meet environmental sustainability of viticulture.

Evaluation of the Potential for Soil Organic Carbon Content Monitoring With Farmers

Increasing soil organic carbon (SOC) content is crucial for soil quality and climate change mitigation. SOC monitoring is indispensable to the corresponding policies and should provide results at farm scale to allow for incentives. In Switzerland farmers perform mandatory analyses of the SOC content of the 0-20 cm topsoil of every field, based on a composite sample, at least every ten years. The corresponding results are stored in a database in canton of Geneva. These data may be relevant for topsoil SOC monitoring, in particular for carbon sequestration policies, provided that they show appropriate quality, which is analyzed in this study. The minimum detectable change (MDC) of past results calculated based on the observed SOC changes was 0.013 % g g-1 at canton scale (2700 fields). Based on extended sampling of three representative fields different sampling strategies were simulated to determine the best future sampling guidelines for farmers. Collecting 20 aliquots with a gouge on the field diagonals was considered the best sampling compromise with field MDC of about 0.1 % g g-1 and a sampling duration of 20 mn. Compared to this procedure, former farmers’ sampling was not biased in average but showed a variance of 0.22% g g-1 due to smaller number of aliquots and varying sampling depths. Based on the best sampling results and assumptions on farm-scale SOC variance or SOC differences, the MDCs at farm scale ranged from 0.21 to 0.12 % g g-1 (5 fields) and 0.09 to 0.05 % g g-1 (30 fields), respectively. These MDCs are small compared to published monitoring networks MDCs, and allow determining SOC change rates at farm scale, thus offering perspectives for inexpensive and efficient monitoring in the frame of soil quality or climate mitigation incentives. For the latter, however, additional information with equivalent soil mass and deeper layers carbon content would be necessary.

Loss of soil organic carbon in Swiss long-term agricultural experiments over a wide range of management practices

Soil carbon sequestration (SCS) is one of the cheapest and technically least demanding carbon dioxide (CO2) removal (CDR) or negative CO2 emission technologies. For a realistic assessment of SCS, it is critical to evaluate how much carbon (C) can be stored in soil organic matter under actual agricultural practices. This includes typical crop rotations and fertilization strategies, depends on resources that are available (e.g. farmyard manure (FYM)) and are affordable for farmers. Furthermore, it is important to assess SCS based on given climatic and soil conditions. Here, we evaluate changes in soil C storage for Switzerland using data from eleven long-term field experiments on cropland and permanent grassland that include common local practices.At all sites, changes in soil organic carbon (SOC) stocks were measured in topsoil (∼0-0.2 m) in response to a total of 80 different treatments including different types of mineral or organic fertilization (e.g. FYM, slurry, peat, compost) or soil management (tillage vs. no-till). The treatments were applied to different, diverse crop rotations or grass mixtures that are representative for Switzerland. We found that topsoils lost C at an average rate of 0.29 Mg C ha−1 yr−1, although many of the investigated treatments were expected to lead to SOC increases. Based on a linear mixed effects model we showed that SOC change rates (ΔSOC) were driven by C inputs to soil (harvest residues and organic fertilizer), soil cover and initial SOC stocks. The type of land use or soil tillage had no significant effect. Our analysis suggests that current efforts to manage soils sustainably need to be intensified and complemented with further techniques if Switzerland wants to achieve the goal of the 4 per 1000 initiative.

An alternative approach to reduce algorithm-derived biases in monitoring soil organic carbon changes.

Quantifying soil organic carbon (SOC) changes is a fundamental issue in ecology and sustainable agriculture. However, the algorithm‐derived biases in comparing SOC status have not been fully addressed. Although the methods based on equivalent soil mass (ESM) and mineral‐matter mass (EMMM) reduced biases of the conventional methods based on equivalent soil volume (ESV), they face challenges in ensuring both data comparability and accuracy of SOC estimation due to unequal basis for comparison and using unconserved reference systems. We introduce the basal mineral‐matter reference systems (soils at time zero with natural porosity but no organic matter) and develop an approach based on equivalent mineral‐matter volume (EMMV). To show the temporal bias, SOC change rates were recalculated with the ESV method and modified methods that referenced to soils at time t1 (ESM, EMMM, and EMMV‐t1) or referenced to soils at time zero (EMMV‐t0) using two datasets with contrasting SOC status. To show the spatial bias, the ESV‐ and EMMV‐t0‐derived SOC stocks were compared using datasets from six sites across biomes. We found that, in the relatively C‐rich forests, SOC accumulation rates derived from the modified methods that referenced to t1 soils and from the EMMV‐t0 method were 5.7%–13.6% and 20.6% higher than that calculated by the ESV method, respectively. Nevertheless, in the C‐poor lands, no significant algorithmic biases of SOC estimation were observed. Finally, both the SOC stock discrepancies (ESV vs. EMMV‐t0) and the proportions of this unaccounted SOC were large and site‐dependent. These results suggest that although the modified methods that referenced to t1 soils could reduce the biases derived from soil volume changes, they may not properly quantify SOC changes due to using unconserved reference systems. The EMMV‐t0 method provides an approach to address the two problems and is potentially useful since it enables SOC comparability and integrating SOC datasets.

Soil organic carbon dynamics jointly controlled by climate, carbon inputs, soil properties and soil carbon fractions.

Soil organic carbon (SOC) dynamics are regulated by the complex interplay of climatic, edaphic and biotic conditions. However, the interrelation of SOC and these drivers and their potential connection networks are rarely assessed quantitatively. Using observations of SOC dynamics with detailed soil properties from 90 field trials at 28 sites under different agroecosystems across the Australian cropping regions, we investigated the direct and indirect effects of climate, soil properties, carbon (C) inputs and soil C pools (a total of 17 variables) on SOC change rate (rC , Mg C ha-1 yr-1 ). Among these variables, we found that the most influential variables on rC were the average C input amount and annual precipitation, and the total SOC stock at the beginning of the trials. Overall, C inputs (including C input amount and pasture frequency in the crop rotation system) accounted for 27% of the relative influence on rC , followed by climate 25% (including precipitation and temperature), soil C pools 24% (including pool size and composition) and soil properties (such as cation exchange capacity, clay content, bulk density) 24%. Path analysis identified a network of intercorrelations of climate, soil properties, C inputs and soil C pools in determining rC . The direct correlation of rC with climate was significantly weakened if removing the effects of soil properties and C pools, and vice versa. These results reveal the relative importance of climate, soil properties, C inputs and C pools and their complex interconnections in regulating SOC dynamics. Ignorance of the impact of changes in soil properties, C pool composition and C input (quantity and quality) on SOC dynamics is likely one of the main sources of uncertainty in SOC predictions from the process-based SOC models.

Simulation of soil organic carbon changes in Vertisols under conservation tillage using the RothC model

The purpose of this study was to determine the measured and simulated rates of soil organic carbon (SOC) change in Vertisols in short-term experiments when the tillage system is changed from traditional tillage (TT) to conservation tillage (CT). The study was conducted in plots in four locations in the state of Michoacán and two locations in the state of Guanajuato. In the SOC change simulation, the RothC-26.3 carbon model was evaluated with different C inputs to the soil (ET1-ET5). ET was the measured shoot biomass (SB) plus estimated rhizodeposition (RI). RI was tested at values of 10, 15, 18, 36 and 50 % total biomass (TB). The SOC changes were simulated with the best trial where ET3 = SB + (0.18*TB). Values for model efficiency and the coefficient of correlation were in the ranges of 0.56 to 0.75 and 0.79 to 0.92, respectively. The average rate of SOC change, measured and simulated, in the study period was 3.0 and 1.9 Mg ha−1 yr−1, respectively; later, in a simulation period of 45 years, SOC change was 1.2 ± 0.8. In particular, without making adjustments in the RothC parameters and with information on measured plant residue C inputs to the soil, it was possible to simulate changes in SOC with RothC and estimate trends over a period of more than 45 years.

Effects of long-term fertilization and residue management on soil organic carbon changes in paddy soils of China: A meta-analysis

Quantification of changes in soil organic carbon (SOC) and SOC stock as influenced by fertilization practices is needed for the improvement of carbon (C) sequestration and soil quality. A meta-analysis of 582 paired-treatment data from 95 long-term field experiments published from 1980 to 2012 was used to characterize the changes in SOC under different fertilization treatments and residue management practices in China’s paddy soils. All treatments sequestrated significant amounts of C compared with the control (CK, no fertilizer application). The greatest mean difference in SOC change rates was measured in the NPKM (mineral nitrogen, phosphorus and potassium plus manure) treatment, i.e., 0.401gkg−1yr−1, followed by the M (manure only) treatment (0.36gkg−1yr−1), while the N (mineral nitrogen only) treatment caused the lowest rate of 0.046gkg−1yr−1. The rates of SOC change in the fertilization treatments with C incorporation (e.g., manure application or straw return) were noticeably higher than those treated with inorganic chemical fertilizers only. The mean difference in the SOC change rates decreased with time and duration of the experiments. The effects of the initial SOC content on the rates of SOC change were not significant, and the effects of SOC accumulation originating from the fertilization treatments depended on the cropping systems. Potential SOC sequestration estimates were more accurate when two factors (sequestration duration and SOC saturation level) were taken into consideration. Our results indicate that the application of manure (e.g., M or NPKM) showed the greatest potential for C sequestration in agricultural soil and produced the longest SOC sequestration duration (45–51 yr).

Effect of the Application of Carbonized Biomass from Crop Residues on Soil Organic Carbon Retention

This study was conducted to investigate the effect of carbonized biomass from crop residues on soil carbon storage during soybean cultivation. The carbonized biomass was made by field scale mobile pyrolyzer. The treatments consisted of control without input and three levels of carbonized biomass inputs as 59.5 kg 10a -1 , C-1 ; 119 kg 10a - 1, C-2 ; 238 kg 10a -1 , C-3. Soil samples were collected during the 113 days of experimental periods, and analyzed soil pH and moisture contents. Soil carbon contents and soybean yield were measured at harvesting period. For the experimental results, soil pH ranged from 6.8 to 7.5, and then increased with increasing carbonized material input. Soil moisture contents were slightly higher by 0.1~1.5% than the control, but consistent pattern was not observed among the treatments. Soil carbon and organic carbon contents in the treatments increased at 24 and 15% relative to the control at 15 days after sowing, respectively. Loss rate of SOC (soil organic carbon) relative to its initial content was 7.2% in control followed by C-1, 6.8%> C-2, 3.5%>C-3, 1.1% during the experimental periods. The SOC change rate decreased with increasing carbonized biomass rate. It was appeared that soybean yields were 476.9 kg 10a -1 in the control, and ranged from 453.6 to 527.3, kg 10a -1 in the treatments. However, significant difference was not found among the treatments. It might be considered that the experimental results will be applied to soil carbon sequestration for future study.

Soil organic carbon sequestration in upland soils of northern China under variable fertilizer management and climate change scenarios

AbstractWe determined the historical change in soil organic carbon (SOC) stocks from long‐term field trials that represent major soil types and climatic conditions of northern China. Soil carbon and general circulation models were validated using these field trial data sets. We then applied these models to predict future change in SOC stocks to 2100 using two net primary production (NPP) scenarios (i.e., current NPP or 1% year−1NPP increase). The conversion rate of plant residues to SOC was higher in single‐cropping sites than in double‐cropping sites. The prediction of future SOC sequestration potential indicated that these soils will be a net source of carbon dioxide (CO2) under no fertilizer inputs. Even when inorganic nutrients were applied, the additional carbon input from increased plant residues could not meet the depletion of SOC in parts of northern China. Manure or straw application could however improve the SOC sequestration potential at all sites. The SOC sequestration potential in northern China was estimated to be −4.3 to 18.2 t C ha−1by 2100. The effect of projected climate change on the annual rate of SOC change did not differ significantly between climate scenarios. The average annual rate of SOC change under current and increased NPP scenarios (at 850 ppm CO2) was approximately 0.136 t C ha−1yr−1in northern China. These findings highlight the need to maintain, and where possible increase, organic carbon inputs into these farming systems which are rapidly becoming inorganic fertilizer intensive.

A synthesis of change in deep soil organic carbon stores with afforestation of agricultural soils

Afforestation is known as an available mitigation activity to climate change because it causes sequestration of CO2 from the atmosphere and stores it as the living biomass and the dead organic matter. However, the response of soil organic carbon (SOC) to afforestation in deep soil layers is still poorly understood. We surveyed previously published literature for changes in deep SOC (defined as at least 10cm deeper than the 0–10cm layer) after afforestation of croplands and grasslands (total 63 sites from 56 literature), in order to examine changes in deep SOC and quantify the relationship between SOC change rates in topsoil and subsoil. The results of the meta analysis indicated that the responses of SOC to afforestation were opposite for cropland than grassland. The SOC in soil depth layers of 0–10, 10–20, 20–40, 40–60 and 60–80cm were reduced with afforestation of grassland but not significantly (p>0.05), while conversion of cropland to forests (trees or shrubs) increased SOC significantly for each soil depth layer up to 60cm depth (p<0.05). Significant relationships of SOC change rate were found between topsoil (0–20cm) and deeper soil layers (20–40 and 40–60cm). The linear regression showed that SOC change rate in 0–40cm, 0–60cm, and 0–100cm soil profiles was 1.33, 1.49, and 1.55 times greater, respectively than the change rates in the corresponding 0–20cm depth profile. Partial correlation analysis revealed that stand age and initial SOC content were determinants of deep soil SOC change after afforestation of agricultural soils. This study also showed that the O horizon can play an important role in carbon sequestration after afforestation of agricultural sites. We concluded that subsoil carbon must be taken into account when evaluating of SOC change with afforestation and, therefore, recommended that the soil sampling depth for afforested soils be set to at least 60cm in mineral soils and include the O horizon. However, due to poor study designs and lack of standardized sampling protocols in the literature, these results were high in uncertainty.

Simulación de los cambios de carbono orgánico del suelo en sistema de cultivo con higuerilla por el modelo RothC

El objetivo del presente trabajo fue la simulación de los cambios del carbono orgánico del suelo (COS), por el modelo RothC, en razón del cambio de uso de suelo del sistema tradicional - asociación maíz-calabaza (TMC) - a sistemas con higuerilla (Ricinus communis): multiestratos (MUL), callejones (CALL) y monocultivo de higuerilla (HIG). Las simulaciones del COS se hicieron para las profundidades de suelo 0-20 y 0-40 cm, para el periodo de 1980-2040, considerándose como línea base al sistema TMC. Las tasas de cambio de COS estimadas con el RothC, en ambas profundidades de suelo, estuvieron en 0,5-1,2, 0,4-0,8, 0,3-0,5 y 0,04-0,1 Mg ha-1 de C por año en los sistemas HIG, MUL, CALL y TMC, respectivamente, y fueron consistentes con las reportadas en la literatura. El desempeño del RothC tuvo 89% de eficiencia (EF), con R²=0,9, lo que muestra que este modelo puede usarse con información temporal del COS escasa, información de la historia de uso de suelo y mediciones de la entrada de residuos vegetales aéreos y subterráneos en el suelo.

An analysis of soil carbon dynamics in long-term soil fertility trials in China

Soil carbon dynamics would be influenced by fertilization management in the agro-ecosystem. In this study, we analyze carbon inputs and soil organic carbon (SOC) dynamics under wheat-corn double cropping system based on four long-term experimental sites in different climate zones of China. We examine soil carbon responses to various carbon inputs by using linear (S = aA − b) and non-linear (\( S = S_{M} \frac{A}{{A + K_{S} }} - S_{L} \)) equations. The term S is the SOC change rate; a, the proportion of C inputs incorporated into soil; b, minimum change rate of SOC; SM, the asymptotic maximum value at SOC change rate approaching infinity (Mg C ha−1 year−1); SL, the decomposition rate of SOC substrates, and KS, a constant value (or ‘half-saturation constant’). The S value is fitted using linear equation with SOC data over the duration of the experiment. The annual C input (A) is estimated by measured crop biomass and C input from manure. Different amounts of balanced fertilization show little impact on the C inputs derived by plants, reaching to ~3.5 Mg C ha−1 year−1. The SOC change rate is much higher under the manure application than treatments with chemical fertilizers only. Statistical analysis shows that the linear and non-linear equations perform equally well (p < 0.01) within the experimental data interval. But the non-linear equation is more suitable for specific purpose. Using the non-linear equation, we can predict that minimum C input to maintain the current SOC level would be 0.33–1.32 Mg C ha−1 year−1 at the most sites but only 0.03 Mg C ha−1 year−1 at the Changping site. The chemical nitrogen and phosphate fertilization yield sufficient carbon biomass inputs to maintain the current SOC levels. However, to increase SOC at 1 Mg C ha−1 year−1, soils need over 10 Mg C ha−1 year−1 at most sites. Our results suggest that the increment of SOC stocks would be mainly related to the additional carbon inputs for the long-term perspectives.

Soil carbon dynamics under different cropping and pasture management in temperate Australia: Results of three long-term experiments

In addition to its important influence on soil quality and therefore crop productivity, soil organic carbon (SOC) has also been identified as a possible C sink for sequestering atmospheric carbon dioxide. Limited data are available on the impact of management practices on the rate of SOC change in agricultural soils in Australia. In this paper, results of three long-term trials (13–25 years) located near Wagga Wagga in temperate Australia were used to assess C dynamics under different tillage and stubble management practices, and under cropping intensities in pasture/crop rotations. Experimental results confirm the importance of management practices and pasture in determining first the steady-state SOC concentrations that are characteristic of given rotations and crop management systems, and second the rates of change of SOC concentrations as they approach steady-state concentrations in agricultural soils of this agro-ecological zone. A long-term crop/pasture experiment at a site with initial high SOC showed that the rate of SOC change in different treatments ranged from –278 to +257 kg C/ha.year over 0–0.3 m soil depth. Under continuous cropping, even under conservation agriculture practices of no-tillage, stubble retention, and crop rotation, the high initial SOC stock (0–0.3 m) present after a long-term pasture phase was, at best, maintained but tended to decrease with increased tillage or stubble burning practices. The effect of tillage was greater than that of stubble management. Increases in SOC were observed only in rotations incorporating a pasture phase. Our results suggest that improved soil nutrient and grazing management of permanent pasture can lead to an increase of 500–700 kg C/ha.year where the initial SOC concentrations are well below steady-state concentrations that could be expected after long periods of improved management. No difference was found between perennial pasture and annual pasture to the depth measured (0–0.3 m). Our results suggest that pasture holds the key to maintaining, and even increasing, SOC under crop/pasture in this environment.

The role of chemical and organic fertilizers on yield, yield variability and carbon sequestration— results of a 19-year experiment

Fertilization practice in the North China Plain has been changing since the late 1970s. To evaluate how organic and chemical fertilizers contribute to yield, yield variability and soil carbon sequestration, we analyzed wheat (Triticum aestivum L.) yield data in a long-term fertilization experiment that began in 1989, conducted pot experiments using soils from the long-term fertilization experiment plots, and simulated the soil organic carbon (SOC) dynamics of individual treatments in the long-term experiments. Wheat yield results showed that when organic fertilizer was used as an alternative nutrient source for chemical fertilizers, it was neither directly beneficial to crop yield, nor decreased yield variability when compared to a balanced chemical fertilizer. However, there was a linear relationship between yield trend and SOC change rate (r = 0.951, P < 0.01). The use of organic fertilizer increased SOC and soil fertility and consequently resulted in a larger yield trend when compared to a balanced chemical fertilizer. Roth-C model simulation and pot experimental results indicated that soils with higher SOC had a higher root/shoot ratio. Therefore, the long-term use of organic fertilizer not only directly increases SOC, but indirectly contributes to carbon sequestration by favoring root development. We found that yield variability was determined by the relative contributions of soil fertility and fertilizer to yield (the contribution of fertilizer to yield is the yield difference between fertilized and unfertilized treatments). The contribution of balanced chemical fertilizer to yield was higher than that of organic fertilizer, resulting in less yield variability in balanced chemical fertilizer treatment. However, if organic fertilizer was used as a complementary nutrient source with chemical fertilizers, it would increase the contribution of fertilizers to yield, thus decreasing yield variability.

Predicted soil organic carbon stocks and changes in Kenya between 1990 and 2030

Under the United Nations Framework Convention on Climate Change (UNFCCC), Non-Annex 1 countries such as Kenya are obliged to report green house gas (GHG) emissions from all sources where possible, including those from soils as a result of changes in land use or land management. At present, the convention encourages countries to estimate emissions using the most advanced methods possible, given the country circumstances and resources. Estimates of soil organic carbon (SOC) stocks and changes were made for Kenya using the Global Environment Facility Soil Organic Carbon (GEFSOC) Modelling System. The tool conducts analysis using three methods: (1) the Century general ecosystem model; (2) the RothC soil C decomposition model; and (3) the Intergovernmental Panel on Climate Change (IPCC) method for assessing soil C at regional scales. The required datasets included: land use history, monthly mean precipitation, monthly mean minimum and maximum temperatures for all the agro-climatic zones of Kenya and historical vegetation cover. Soil C stocks of 1.4–2.0 Pg (0–20 cm), compared well with a Soil and Terrain (SOTER) based approach that estimated ∼1.8–2.0 Pg (0–30 cm). In 1990 48% of the country had SOC stocks of <18 t C ha −1 and 20% of the country had SOC stocks of 18–30 t C ha −1, whereas in 2000 56% of the country had SOC stocks of <18 t C ha −1 and 31% of the country had SOC stocks of 18–30 t C ha −1. Conversion of natural vegetation to annual crops led to the greatest soil C losses. Simulations suggest that soil C losses remain substantial throughout the modelling period of 1990–2030. All three methods involved in the GEFSOC System estimated that there would be a net loss of soil C between 2000 and 2030 in Kenya. The decline was more marked with RothC than with Century or the IPCC method. In non-hydric soils the SOC change rates were more pronounced in high sandy soils compared to high clay soils in most land use systems.

The net flux of carbon from agricultural soils in Canada 1970–2010

SummaryChanges in soil organic carbon (SOC) in agricultural soils influence soil quality and greenhouse gas concentrations in the atmosphere. Land use, management practices, soil characteristics, and climate influence such changes. Using the Century model we estimated the rate of SOC change in agricultural soils of Canada for the period 1970 to 2010. This estimation was based on the estimated SOC change for 15% of the 1250 agriculturally designated soil landscape of Canada (SLC) polygons. Simulations were carried out for two to five crop rotations and for conventional and no‐tillage. The results indicate that the agricultural soils in Canada, whose SOC are currently very close to equilibrium, will stop being a net source of CO2 and will become a sink by the year 2000. Rates of carbon change for the years 1970, 1990, and 2010 were estimated to be −67, − 39, and 11 kgC ha−1. The rate of decline in the carbon content of agricultural soils in Canada has slowed considerably in the 1990s as a result of an increase in the adoption of no‐tillage management, a reduction in the use of summer fallowing, and an increase in fertilizer application. We estimate that the proportion of agricultural land storing SOC will have increased from 17% in 1990 to 53% by the year 2000.

The rate of carbon change in agricultural soils in Canada at the landscape level

The Century model [a computer simulation of the dynamics of soil organic carbon (SOC)] was used to estimate the rate of SOC change in agricultural soil in Canada. The analysis was carried out on 180 Soil Landscapes of Canada (SLC) polygons, representing 15% of the SLC polygons within agricultural regions. The analysis was stratified into soil zones and into soil textural classes. For each sampled polygon, Century was run for 1 to 5 types of crop rotations under conventional-tillage as well as no-tillage, providing that no-till was used on at least 5% of the land. From the Century simulations, it was estimated that the overall rate of SOC loss from agricultural soils in Canada for 1990 was 39.1 kg ha−1 yr−1 This implies that 1.93 Mt of SOC (7.08 Mt of CO2) was lost from agricultural soils in Canada. Compared to 1990, the SOC loss was estimated to have been greater by 11.9 kg ha−1 yr−1 in 1980 and 9.1 kg ha−1 yr−1 in 1985. The lower loss in 1990 was primarily due to the incorporation of no-till practices and reduction of summer fallow in the mid 1980s. In 1990, at the provincial level, Alberta had the highest rate of SOC loss at 74.5 kg ha−1 yr−1 followed by Manitoba with 66.1 kg ha−1 yr−1 In Ontario, Quebec, and the Atlantic Provinces the average provincial rate of SOC loss was less than 35 kg ha−1 yr−1 Higher SOC loss rates were typically found in soils with coarser texture and greater native SOC content. Key words: Carbon dioxide, greenhouse gas, Century model