Soil phosphorus fractions after 111 years of animal manure and fertilizer applications

Accumulated soil P in agricultural soils is a major source of soluble and particulate forms of P entering water resources and degrading water quality. However, few research sites are currently available to evaluate the long-term effects of different cropping systems and fertility practices on soil inorganic and organic P accumulation. The objectives of this study were: (1) to compare the forms and quantity of different inorganic and organic soil P fractions in plots on Sanborn Field, which has been cultivated for 111 years; and (2) to assess the use of standard soil test P extractants for determining changes in soil P dynamics over time. A modified sequential P extraction procedure was used to separate labile and stable inorganic and organic P pools from surface soils collected on Sanborn Field in 1915, 1938, 1962, and 1999 from plots in continuous corn, continuous wheat, continuous timothy, and a corn-wheat-clover rotation amended with either manufactured fertilizers, horse or dairy manure or receiving no fertilization since 1888. Additional samples were collected from a native grass prairie site of a similar soil series to estimate soil characteristics at Sanborn Field before initial cultivation in 1888. Observed accumulation of Bray-1 P among fertilizer and manure treatments was attributed to over-application of P due to unrealistically high yield goals for each cropping system. Long-term cultivation of Sanborn Field increased soil bulk density and lowered soil pH and total organic C compared with native prairie. Fertilization either by addition of manufactured fertilizer or manure significantly increased inorganic resin-P and inorganic NaOH-extractable P. Applications of animal manure also significantly increased most organic P fractions compared with the unfertilized treatment. The native prairie had a larger proportion of total P in organic forms compared with cultivated plots, especially in organic NaOH-extractable P, but no significant decreases in either residual or total P were observed due to cultivation. This study confirms that soil P availability in cropping systems that are amended with predominantly organic P amendments may differ from conventional cropping systems relying on manufactured P fertilizers. However, no direct evidence was found to support the hypothesis that any individual inorganic or organic soil P fraction has a better relationship than conventional soil test P extractants with plant P uptake under contrasting organic and conventional fertility practices.     

Long-term fertilization effects on carbon pools and carbon management index of loamy soil under grass–forage legumes mixture in semi-arid environment

ABSTRACT

Soil organic carbon (SOC) is a key component for sustaining crop production. A field experiment was conducted during 2004–2018 to assess the changes in soil carbon fractions under different fertilization practices in grass-legumes mixture. The result indicates that application of farmyard manure (FYM) at 80 Mg ha^–1 has increased SOC concentration leading to carbon sequestration rate of 4.2 Mg ha^–1 year^–1. Further, it has increased the proportion of labile carbon in the total SOC and have accumulated 126, 60, 83 and 95% higher very labile, labile, less labile and non-labile C stock than that of control plot, respectively, in top 30 cm soil layer. Inorganic fertilization and FYM 20 Mg ha^–1 influenced SOC concentration, SOC stock and C sequestration rate similarly. The highest carbon management index (264) was found in the treatment receiving FYM 80 Mg ha^–1 and it was positively correlated with SOC (r = 0.84**). The sensitivity index of the SOC varied from 26 to 152% and the differences were greatest in FYM treatments. The result indicates that grass-legumes mixture build-up the SOC in long term and the addition of FYM further increases it.     

Management effects on soil C storage on the Canadian prairies

The Canadian prairie, which accounts for about 80% of Canada's farmland, has large reserves of soil organic carbon (SOC). Changes in the size of the SOC pool have implications for soil productivity and for atmospheric concentrations of CO₂, an important ‘greenhouse gas'. We reviewed recent findings from long-term research sites to determine the impact of cropping practices on SOC reserves in the region. From this overview, we suggest that: (1) the loss of SOC upon conversion of soils to arable agriculture has abated; (2) significant gains in SOC (typically about 3 Mg C ha⁻¹ or less within a decade) can be achieved in some soils by adoption of improved practices, like intensification of cropping systems, reduction in tillage intensity, improved crop nutrition, organic amendments, and reversion to perennial vegetation; (3) changes in SOC occur predominantly in ‘young' or labile fractions; (4) the change in SOC, either gain or loss, is of finite duration and magnitude; (5) estimates of SOC change from individual studies are subject to limitations and are best viewed as part of a multi-site network; and (6) the energy inputs into agroecosystems need to be included in the calculation of the net C balance. The long-term sites indicate that Canadian prairie soils can be a net sink for CO₂, though perhaps only in the short term. These sites need to be maintained to measure the effects of continued agronomic evolution and predicted global changes.     

Effects of reduced tillage,crop residue management and manure application practices on crop yields and soil carbon sequestration on an Andisol in northern Japan

Abstract

Soil carbon sequestration in agricultural lands has been deemed a sustainable option to mitigate rising atmospheric CO₂ levels. In this context, the effects of different tillage and C input management (residue management and manure application) practices on crop yields, residue C and annual changes in total soil organic C (SOC) (0–30 cm depth) were investigated over one cycle of a 4-year crop rotation (2003–2006) on a cropped Andisol in northern Japan. For tillage practices, the effects of reduced tillage (no deep plowing, a single shallow harrowing for seedbed preparation [RT]) and conventional deep moldboard plow tillage (CT) were compared. The combination of RT, residue return and manure application (20 Mg ha^?1 in each year) increased spring wheat and potato yields significantly; however, soybean and sugar beet yields were not influenced by tillage practices. For all crops studied, manure application enhanced the production of above-ground residue C. Thus, manure application served not only as a direct input of C to the soil, but the greater crop biomass production engendered enhanced subsequent C inputs to the soil from residues. The SOC contents in both the 0–5 cm and 5–10 cm layers of the soil profile were greater under RT than under CT treatments because the crop residue and manure were densely incorporated into the shallow soil layers. Comparatively, neither tillage nor C input management practices had significant effects on annual changes in SOC content in either the 10–20 cm or 20–30 cm layers of the soil profile. When soil C sequestration rates, as represented by annual changes in total SOC (0–30 cm), were assessed on a total soil mass basis, an anova showed that tillage practices had no significant effect on total C sequestration, but C input management practices had significant positive effects (P ≤ 0.05). These results indicate that continuous C input to the soil through crop residue return and manure application is a crucial practice for enhancing crop yields and soil C sequestration in the Andisol region of northern Japan.     

Enhancing crop productivity for recarbonizing soil

Plants capture atmospheric carbon dioxide (CO₂) for carbon (C) assimilation through photosynthesis, with the photosynthates stored as plant biomass (above- and below-ground plant parts). The C stored as living biomass is a short-term C sequestration strategy, whereas soil organic carbon (SOC) is a long-term C sequestration strategy. In this regard, plant roots are the primary route of C entry into the SOC pool. Through establishing a recalcitrant SOC pool, long-term sequestration can potentially offset the C losses caused by soil degradation in industrial and pre-industrial eras. Over the next 50–100 years, implementing effective agricultural practices could sequester 80–130 GT (10⁹) C as SOC. Carbon, as the primary elemental component of soil organic matter, plays a significant role in shaping the soil’s physical, chemical, and biological properties, ultimately influencing soil biomass productivity. By enhancing crop productivity and biomass production, farmers can increase C sequestration, creating a positive feedback loop that contributes to overall C sequestration. Carbon sequestration has numerous co-benefits, including climate change mitigation, ecosystem health, food security, and farm profitability. Adopting conservation agriculture and site-specific practices and developing crop and pasture genotypes with high yields and C sequestration potential should significantly improve crop productivity and C sequestration simultaneously. This opinion article delves into the nexus between photosynthesis and soil C sequestration, highlighting its significance in enhancing farm productivity while mitigating climate change.     

In situ and potential CO2 evolution from a Fluventic Ustochrept in southcentral Texas as affected by tillage and cropping intensity

Quality of agricultural soils is largely a function of soil organic matter. Tillage and crop management impact soil organic matter dynamics by modification of the soil environment and quantity and quality of C input. We investigated changes in pools and fluxes of soil organic C (SOC) during the ninth and tenth year of cropping with various intensities under conventional disk-and-bed tillage (CT) and no tillage (NT). Soil organic C to a depth of 0.2 m increased with cropping intensity as a result of greater C input and was 10% to 30% greater under NT than under CT. Sequestration of crop-derived C input into SOC was 22±2% under NT and 9±4% under CT (mean of cropping intensities ± standard deviation of cropping systems). Greater sequestration of SOC under NT was due to a lower rate of in situ soil CO₂ evolution than under CT (0.22±0.03 vs. 0.27±0.06 g CO₂–C g⁻¹ SOC yr⁻¹). Despite a similar labile pool of SOC under NT than under CT (1.1±0.1 vs. 1.0±0.1 g mineralizable C kg⁻¹ SOC d⁻¹), the ratio of in situ to potential CO₂ evolution was less under NT (0.56±0.03) than under CT (0.73±0.08), suggesting strong environmental controls on SOC turnover, such as temperature, moisture, and residue placement. Both increased C sequestration and a greater labile SOC pool were achieved in this low-SOC soil using NT and high-intensity cropping.     

Different impacts of native and exotic earthworms on rhizodeposit carbon sequestration in a subtropical soil

Earthworms are known to regulate the sequestration of soil and leaf litter carbon (C). However, their impacts on the more accessible rhizospheric C, which represents a major energy source for soil food webs and an essential factor for C sequestration, are still unclear. Previous studies indicate that earthworms regulate the dynamics of SOC and leaf litter-C by increasing C accessibility to microbiota. However, in the case of labile rhizodeposit-C, microbiota might not require any pre-conditioning by earthworms and may rapidly metabolize most of this root-derived C. Consequently, potential pathways by which earthworms may affect the fate of rhizodeposit-C would be to regulate the biomass and/or activity of rhizosphere microbiota and, further, to mineralize/stabilize microbial products. A ¹³CO₂ labelling experiment was performed to determine the impacts of four different earthworm species on the fate of tree rhizodeposit-C in a subtropical soil. We hypothesized that endogeic earthworm species, representing primarily geophagous species, would closely interact with soil microbiota and sequester the microbially metabolized rhizodeposit-C more efficiently than epigeic and anecic earthworm species. We found that irrespective of ecological group affiliation, the three native earthworms did not affect rhizodeposit-C sequestration. In contrast, the exotic endogeic species stimulated the immobilization of rhizodeposit-C in the biomass of root-associated bacteria and/or arbuscular mycorrhizal fungi and, further, accessed the microbiota-metabolized rhizodeposit-C more efficiently. As a consequence, the exotic endogeic earthworm species transiently tripled rhizodeposit-C retention in soil. We propose that the weak linkages between native earthworms and rhizodeposits-related microbiota limit earthworm impacts on rhizodeposit-C sequestration. However, the exotic endogeic species Pontoscolex corethrurus may potentially alter rhizodeposit-C dynamics in invaded areas by shifting rhizosphere microbial community composition. This work highlights a distinct mechanism by which earthworms can regulate C dynamics and indicates a significant contribution of invasive earthworm species to belowground processes.     

Re‐evaluating the biophysical and technologically attainable potential of topsoil carbon sequestration in China's cropland

To assess the topsoil carbon sequestration potential (CSP) of China's cropland, two different estimates were made: (i) a biophysical potential (BP) using a saturation limit approach based on soil organic carbon (SOC) accumulation dynamics and a storage restoration approach from the cultivation‐induced SOC loss, and (ii) a technically attainable potential (TAP) with a scenario estimation approach using SOC increases under best management practices (BMPs) in agriculture. Thus, the BP is projected to be the gap in recent SOC storage to either the saturation capacity or to the SOC storage of uncultivated soil, while the TAP is the overall increase over the current SOC storage that could be achieved with the extension of BMPs. The recent mean SOC density of China's cropland was estimated to be 36.44 t/ha, with a BP estimate of 2.21 Pg C by a saturation approach and 2.95 Pg C by the storage restoration method. An overall TAP of 0.62 Pg C and 0.98 Pg C was predicted for conservation tillage plus straw return and recommended fertilizer applications, respectively. This TAP is comparable to 40–60% of total CO₂ emissions from Chinese energy production in 2007. Therefore, carbon sequestration in China's cropland is recommended for enhancing China's mitigation capacity for climate change. However, priority should be given to the vast dry cropland areas of China, as the CSP of China is based predominantly on the dry cropland.     

The priming potential of biochar products in relation to labile carbon contents and soil organic matter status

Recognition of biochar as a potential tool for long-term carbon sequestration with additional agronomic benefits is growing. However, the functionality of biochar in soil and the response of soils to biochar inputs are poorly understood. It has been suggested, for example, that biochar additions to soils could prime for the loss of native organic carbon, undermining its sequestration potential. This work examines the priming potential of biochar in the context of its own labile fraction and procedures for their assessment. A systematic set of biochar samples produced from C4 plant biomass under a range of pyrolysis process conditions were incubated in a C3 soil at three discrete levels of organic matter status (a result of contrasting long-term land management on a single site). The biochar samples were characterised for labile carbon content ex-situ and then added to each soil. Priming potential was determined by a comparison of CO₂ flux rates and its isotopic analysis for attribution of source. The results conclusively showed that while carbon mineralisation was often higher in biochar amended soil, this was due to rapid utilisation of a small labile component of biochar and that biochar did not prime for the loss of native organic soil organic matter. Furthermore, in some cases negative priming occurred, with lower carbon mineralisation in biochar amended soil, probably as a result of the stabilisation of labile soil carbon.     

Impacts of Plant Community Changes on Soil Carbon Contents in Northeastern Illinois

Land-cover changes not only affect regional climates through alteration in surface energy and water balance, but also affect key ecological processes, such as carbon (C) cycling and sequestration in plant ecosystems. The object of this study was to investigate the effects of land-cover changes on the distribution of soil organic carbon (SOC) contents under four plant community types (deciduous forests, pine forests, mixed pine-deciduous forests, and prairies) in northeastern Illinois, USA. Soil samples were collected from incremental soil depths (0–10, 10–20, 20–30, and 30–50 cm) under the studied plant communities. The results showed that SOC concentration decreased with increases of soil depth in the studied forests and prairies. No significant differences of SOC concentrations were found at the upper soil layers (0–10 cm) among the four plant types. However, SOC concentrations were statistically higher at the lower soil depth (30–40 cm) in prairies than in other three forest types. The SOC storage (0–40 cm soil depth) was reduced in an order prairies (250.6) > mixed pine-deciduous forests (240.7) > pine forests (190.1) > deciduous forests (163.4 Mg/ha). The characteristics of relative short life cycle, restively high turnover rate of roots, and large partition of photosynthetic production allocated to belowground were likely attributed to the higher accumulation of C in soils in tallgrass prairies than in forests. Our data indicated the conversion of native tallgrass prairies to pure forest plantations resulted in a considerable decline of SOC storage. Results suggest that land-cover changes have a significant impact on SOC storage and sequestration in plant ecosystems.     

Impacts of agricultural management practices on C sequestration in forest-derived soils of the eastern Corn Belt

Soil organic matter has recently been implicated as an important sink for atmospheric carbon dioxide (CO₂). However, the relative impacts of various agricultural management practices on soil organic matter dynamics and, therefore, C sequestration at spatial scales larger than a single plot or times longer than the typical three year experiment have rarely been reported. Results of maintaining agricultural management practices in the forest-derived soils of the eastern Corn (Zea mays L.) Belt states of Kentucky, Michigan, Ohio and Pennsylvania (USA) were studied. We found annual organic C input and tillage intensity were the most important factors in affecting C sequestration. The impact of rotation on C sequestration was primarily related to the way it altered annual total C inputs. The removal of above-ground plant biomass and use of cover crops were of lesser importance. The most rapid changes in soil organic matter content occurred during the first five years after a management practice was imposed with slower changes occurring thereafter. Certain management practices, e.g. no-tillage (NT), increased the soil's ability to sequester atmospheric CO₂. The impact of this sequestration will be significant only when these practices are used extensively on a large percentage of cropland and when the C-building practices are maintained. Any soil C sequestered will be rapidly mineralized to CO₂ if the soil organic matter building practices are not maintained.     

Evaluation of microbial methods as potential indicators of soil quality in historical agricultural fields

In agricultural ecosystems that have had consistent cropping histories, standard microbial methods may be used to evaluate past and present practices. Our objective was to evaluate several microbial methods that best indicate cropping histories and soil quality on long-term plots. We selected soil microbial carbon (C), phospholipid analyses, direct counts of total fungal and bacterial biomass, and soil enzymes (phosphatases) to measure direct and indirect microbial activity on the Sanborn Field and Tucker Prairie. The Sanborn Field has been under various cropping and management practices since 1888 and the Tucker Prairie is an uncultivated site. Seven different plots were chosen on the Sanborn Field and random samples were taken in the summit area on the Tucker Prairie, which represented a reference site. Soil microbial biomass C, phospholipids, and enzyme activity were reflective of the cropping and management histories observed on the Sanborn Field. Enzymatic activity was highly correlated to soil organic matter. The direct counts of fungal and bacterial biomass showed that fungal populations dominated these soils, which may be attributed to soil pH. Soil microbial biomass C and enzyme assays seemed to be better potential indicators of cropping histories than the other methods tested in the long-term plots.This paper has been assigned by the Missouri Agricultural Experiment Station to Journal Series no. 12043     

Distribution and amount of soil organic C in long-term management systems in Texas

Soil organic carbon (SOC) distribution is altered by residue management practices, but the effect on total C mass is not well understood, especially in warm regions. The objective of this study was to determine the effect of residue management practices on SOC distribution and amount across an 1100 km transect (northwest to southeast) of Texas. Long-term (>10 years) continuous cropping rotation and residue management plots located near Bushland, Temple, and Corpus Christi, Texas, were sampled incrementally with depth for SOC distribution and mass. The mass of SOC varied among locations depending on management, and climatic conditions. No-tillage management resulted in increased SOC concentration and mass in the surface 0.07 m in comparison to more intensive tillage management (e.g., sweep, chisel plow, moldboard plow). Fertilization had little effect on C sequestration at any site. Carbon sequestration decreased as mean annual temperature increased. Carbon may be sequestered in soil under Texas climatic conditions, but the amounts may be quite small.     

Soil Carbon Sequestration in Long-Term Fertilization Under Jute-Rice-Wheat Agro-Ecosystem

Soil organic carbon (SOC) sequestration in response to long-term fertilizer management practices under jute-rice-wheat agro-ecosystem in alluvial soils was studied using a modeling approach. Fertilizer management practices included nitrogen (N), phosphorus (P) and potassium (K) fertilization, manure application, and root-stubble retention of all three crops. Soil carbon (C) model RothC was used to simulate the critical C input rates needed to maintain initial soil C level in long timescale (44 years). SOC change was significantly influenced by the long-term fertilizer management practices and the edaphic variable of initial SOC content. The effects of fertilizer combination “100%NPK+FYM” on SOC changes were most significant over “100%NPK” fertilization. If the 100% NPK fertilizer along with manure applied with stubble and roots retention of all crops, alluvial soils of such agro-ecosystem would act as a net C sink, and the average SOC density kept increasing from 18.18 Mg ha^?1 during 1972 to the current average of ～22 Mg ha^?1 during 2065 s. On an average, the critical C input was estimated to be 5.30 Mg C ha^?1 yr^?1, depending on local soil and climatic conditions. The critical C input could be effectively estimated using a summary model driven by current SOC level, mean annual temperature, precipitation, and soil clay content. Such information will provide a baseline for assessing soil C dynamics under potential changes in fertilizer and crop residues management practices, and thus enable development of management strategies for effectively mitigating climate change through soil C sequestration.     

Stability and saturation of soil organic carbon in rice fields: evidence from a long-term fertilization experiment in subtropical China

Purpose

Soil organic carbon (SOC) sequestration in croplands plays a critical role in climate change mitigation and food security, whereas the stability and saturation of the sequestered SOC have not been well understood yet, particularly in rice (Oryza sativa L.) fields. The objective of this study was to determine the long-term effect of inorganic fertilization alone or combined with organic amendments on SOC stability in a double rice cropping system, and to characterize the saturation behavior of the total SOC and its fractions in the paddy soil.

Materials and methods

Soils were collected from a long-term field experiment in subtropical China where different fertilization regimes have been carried out for 31 years. The total SOC pool was separated into four fractions, characteristic of different turnover rates through chemical fractionation. Annual organic carbon (C) inputs were also estimated by determining the C content in crop residues and organic amendments.

Results and discussion

Relative to the initial level, long-term double rice cropping without any fertilizer application significantly increased SOC concentration, suggesting that double rice cropping facilitates the storage and accumulation of SOC. The partial substitution of inorganic fertilizers with organic amendments significantly increased total SOC concentration compared to the unfertilized control. Total SOC increased significantly with greater C inputs and did not show any saturation behavior. Increased SOC was primarily stored in the labile fraction with input from organic amendments. However, other less labile SOC fractions showed no further increase with greater C inputs exhibiting C saturation.

Conclusions

While the paddy soil holds a high potential for SOC sequestration, stable C fractions saturate with increasing C inputs, and thus, additional C inputs mainly accumulate in labile soil C pools.     

Soil organic carbon in some land uses of Costa Rica

The native vegetation in the Tropics is increasingly replaced by crops, pastures, tree plantations, or settlements with contradictory effects on soil organic carbon (SOC). Therefore, the general objective was to estimate the SOC stock depth distribution to 100-cm depth in soils of Costa Rica and to assess their theoretical carbon (C) sink capacity by different management practices. A study was established in three ecoregions of Costa Rica: the Isthmian-Atlantic Moist Forest (AM), the Pacific Dry Forest (PD), and the Montane Forest (MO) ecoregions. Within each ecoregion, three agricultural land uses and a mature forest were sampled to 100-cm depth. The SOC stock in 0–100 cm depth was 114–150 Mg C ha^?1 for AM, 76–165 Mg C ha^?1 for PD, and 166–246 Mg C ha^?1 for MO. Land use had only weak effects on SOC concentrations and stocks except at PD where both were lower for soils under mango (Mangifera indica) and pasture. This may indicate soil degradation which was also supported by data on SOC stratification. However, it was generally unclear whether differences among land uses within each ecoregion already existed particularly at deeper depths before land-use change, and whether the sampling approach was sufficient to investigate them. Nevertheless, about 26–71% of Costa Rica's total C emissions may be offset by SOC sequestration in agricultural and forest soils. However, ecoregion-specific practices must be implemented to realize this potential.     

Long-term effects of recommended management practices on soil carbon changes and sequestration in north-eastern Italy

Management practices can have significant implications for both soil quality and carbon (C) sequestration potential in agricultural soils. Data from two long‐term trials (one at field scale and the other at lysimeter scale), underway in north‐eastern Italy, were used to evaluate the dynamics of soil organic carbon (SOC) and estimate the impact of recommended management practices (RMPs) on soil carbon sequestration. Potential SOC sequestration was calculated as the differences between the change in SOC of treatments differing only for the specified RMP for a period of at least 25 years. The trials compared the following situations: (a) improved crop rotations versus monoculture; (b) grass versus improved crop rotations; (c) residue incorporation versus residue removal; (d) high versus low rates of inorganic fertilizers; (e) integrated nutrient management/organic manures versus inorganic fertilizers. At the lysimeter scale, some of these treatments were evaluated in different soils. A general decrease in SOC (1.1 t C ha^?1 year^?1) was observed after the introduction of intensive soil tillage, evidencing both the worsening of soil quality and the contribution towards global CO₂ emissions. Initial SOC content was maintained only in permanent grassland, complex rotations and/or with the use of large quantities of livestock manure. SOC sequestration reached a maximum rate of 0.4 t C ha^?1 year^?1 comparing permanent grassland with an improved crop rotation. Crop residue incorporation and rates of inorganic fertilizer had less effect on SOC sequestration (0.10 and 0.038 t C ha^?1 year^?1, respectively). The lysimeter experiment highlighted also the interaction between RMPs and soil type. Peaty soil tended to be a source of C independent of the amount and quality of C input, whereas a proper choice of tillage practices and organic manures enhanced SOC sequestration in a sandy soil. The most promising RMPs in the Veneto region are, therefore, conversion to grassland and use of organic manures. Although some of these RMPs are already supported by the Veneto Region Rural Development Plan, their more intensive and widespread implementation requires additional incentives to become economically feasible.     

Dissolved and Soil Organic Carbon after Long‐Term Conventional and No‐Tillage Sorghum Cropping

Abstract

Distribution of dissolved (DOC) and soil organic carbon (SOC) with depth may indicate soil and crop‐management effects on subsurface soil C sequestration. The objectives of this study were to investigate impacts of conventional tillage (CT), no tillage (NT), and cropping sequence on the depth distribution of DOC, SOC, and total nitrogen (N) for a silty clay loam soil after 20 years of continuous sorghum cropping. Conventional tillage consisted of disking, chiseling, ridging, and residue incorporation into soil, while residues remained on the soil surface for NT. Soil was sampled from six depth intervals ranging from 0 to 105 cm. Tillage effects on DOC and total N were primarily observed at 0–5 cm, whereas cropping sequence effects were observed to 55 cm. Soil organic carbon (C) was higher under NT than CT at 0–5 cm but higher under CT for subsurface soils. Dissolved organic C, SOC, and total N were 37, 36, and 66%, respectively, greater under NT than CT at 0–5 cm, and 171, 659, and 837% greater at 0–5 than 80–105 cm. The DOC decreased with each depth increment and averaged 18% higher under a sorghum–wheat–soybean rotation than a continuous sorghum monoculture. Both SOC and total N were higher for sorghum–wheat–soybean than continuous sorghum from 0–55 cm. Conventional tillage increased SOC and DOC in subsurface soils for intensive crop rotations, indicating that assessment of C in subsurface soils may be important for determining effects of tillage practices and crop rotations on soil C sequestration.     

耕作、残茬及施肥管理对土壤有机质动态变化影响的情景分析

Based on data from 10-year field experiments on residue/fertilizer management in the dryland farming region of northern China, Century model was used to simulate the site-specific ecosystem dynamics through adjustment of the model＇s parameters, and the applicability of the model to propose soil organic carbon （SOC） management temporally and spatially, in cases such as of tillage/residue/fertilization management options, was identified v/a scenario analysis.Results between simulations and actual measurements were in close agreement when appropriate applications of stover,manure and inorganic fertilizer were combined. Simulations of extreme C/N ratios with added organic materials tended to underestimate the measured effects. Scenarios of changed tillage methods, residue practices and fertilization options showed potential to maintain and enhance SOC in the long run, while increasing inorganic N slowed down the SOC turnover rate but did not create a net C sink without any organic C input. The Century model simulation showed a good relationship between annual C inputs to the soil and the rate of C sequestration in the top 20 cm layer and provided quantitative estimations of changes in parameters crucial for sustainable land use and management. Conservation tillage practices for sustainable land use should be integrated with residue management and appreciable organic and inorganic fertilizer application, adapted according to the local residue resource, soil fertility and production conditions. At least 50% residue return into the soil was needed annually for maintenance of SOC balance, and manure amendment was important for enhancement of SOC in small crop-livestock systems in which crop residue land application was limited.     

Modeling soil organic carbon dynamics in Oxisols of Ibirubá (Brazil) with the Century Model

Introduction of conservation practices in degraded agricultural land will generally recuperate soil quality, especially by increasing soil organic matter. This aspect of soil organic C (SOC) dynamics under distinct cropping and management systems can be conveniently analyzed with ecosystem models such as the Century Model. In this study, Century was used to simulate SOC stocks in farm fields of the Ibirubá region of north central Rio Grande do Sul state in Southern Brazil. The region, where soils are predominantly Oxisols, was originally covered with subtropical woodlands and grasslands. SOC dynamics was simulated with a general scenario developed with historical data on soil management and cropping systems beginning with the onset of agriculture in 1900. From 1993 to 2050, two contrasting scenarios based on no-tillage soil management were established: the “status quo” scenario, with crops and agricultural inputs as currently practiced in the region and the “high biomass” scenario with increased frequency of corn in the cropping system, resulting in about 80% higher biomass addition to soils. Century simulations were in close agreement with SOC stocks measured in 2005 in the Oxisols with finer texture surface horizon originally under woodlands. However, simulations in the Oxisols with loamy surface horizon under woodlands and in the grassland soils were not as accurate. SOC stock decreased from 44% to 50% in fields originally under woodland and from 20% to 27% in fields under grasslands with the introduction of intensive annual grain crops with intensive tillage and harrowing operations. The adoption of conservation practices in the 1980s led to a stabilization of SOC stocks followed by a partial recovery of native stocks. Simulations to 2050 indicate that maintaining “status quo” would allow SOC stocks to recover from 81% to 86% of the native stocks under woodland and from 80% to 91% of the native stocks under grasslands. Adoption of a “high biomass” scenario would result in stocks from 75% to 95% of the original stocks under woodlands and from 89% to 102% in the grasslands by 2050. These simulations outcomes underline the importance of cropping system yielding higher biomass to further increase SOC content in these Oxisols. This application of the Century Model could reproduce general trends of SOC loss and recovery in the Oxisols of the Ibirubá region. Additional calibration and validation should be conducted before extensive usage of Century as a support tool for soil carbon sequestration projects in this and other regions can be recommended.