Potential Mineralizable Carbon Research Articles

Soil carbon and nitrogen responses to forage cropping systems following irrigation retirement

AbstractLimited annual precipitation and increasing scarcity of irrigation water are forcing farmers in arid and semi‐arid regions to convert irrigated fields into dryland farming, yet their impacts on soil carbon (C) and nitrogen (N) fractions are not fully understood. This study evaluated changes in soil C and N fractions and forage production under four cropping systems after irrigation retirement in a semi‐arid condition. Treatments included an annual winter wheat forage (Triticum aestivum L.) (AWW), a perennial wheat (PW) forage (T. aestivum L. × Thinopyrum spp.) (PW), a mixture of native grasses (NG), and a pasture crop (PC = AWW and NG mixture). Evaluation of various soil C and N fractions, forage dry matter yield, and nutritive value parameters suggested that soil inorganic N and potentially mineralizable N (PMN) were 38%–45% and 36%–44% greater in autumn 2020 and 62%–68% and 46%–55% greater in spring 2021 under AWW and PW, respectively, than NG and PC. Soil potentially mineralizable carbon (PMC) was 58%–129% and 64%–138% greater under NG and PW than AWW and PC. The responses of other soil C and N fractions varied with soil sampling dates but not with crops or crop × sampling date interaction. Total forage dry matter yield was 4016, 7849, 12,517, and 13,869 kg ha−1 for AWW, PW, PC, and NG, respectively, and PW had the best forage nutritive value. These results suggest soil C and N mineralization could be enhanced by adopting perennial systems, including NG, PW, and PC, while PW maintains good‐quality forage production.

Effect of Exogenous Nutrient Addition on Soil Organic Carbon Mineralization and Stabilization

Soil organic carbon (SOC) pools have the potential to attain sustainable agriculture goals under climate change. External organic and inorganic nutrient inputs considerably affect SOC mineralization and SOC pools. Therefore, a laboratory-based, three-factor soil incubation experiment was conducted to investigate the impact of different exogenous nutrient additions on the mineralization and stability of SOC. The study investigated the effects of three fertilizer types (inorganic, organic, and a combination of inorganic and organic) and three rates of fertilizer addition (low, medium, and high) while considering two temperature levels (15 °C and 25 °C). At 25 °C, the application of fertilizer at a high rate significantly increased the SOC mineralization (2.84–19.97%) compared to fertilizer applied at a low rate, while, at 15 °C, different fertilizer types had no significant impact. Overall, fertilization resulted in an increase in the total potential mineralizable carbon (7.87–84.50%), while the rate of decomposition was decreased. The priming effect of inorganic fertilizer decreased over time, with the main effect observed during the initial 14 days. The addition of organic fertilizer resulted in a lesser increment in the soil activity index while simultaneously yielding a higher Q10 compared to inorganic fertilizer. Overall, the mixed application of organic and inorganic fertilizers was suggested to improve SOC stabilization and promote sustainable agricultural development.

A tale of two systems: Does reducing tillage affect soil health differently in long-term, side-by-side conventional and organic agricultural systems?

Tillage reduction is an increasingly common goal of farmers worldwide to reduce soil erosion and improve agroecosystem sustainability. Reduced tillage is usually achieved by different strategies in conventional and organic agricultural systems, yet it is unclear if these different strategies have contrasting effects on soil health. To compare how reduced-till strategies affect soil health under conventional and organic management, we evaluated soil health of a long-term field crop trial in Pennsylvania, USA. This trial was established in 1981 with three side-by-side management systems: an agrochemical-based conventional maize-soybean system (CNV), a legume-based low-input organic grain system (LEG), and a manure-based organic grain and forage system (MNR). Ten years after reduced-till treatments were introduced in 2008, herbicide-based no-till in the CNV system did not affect soil organic matter (SOM), cation exchange capacity, permanganate oxidizable carbon (POXC), or autoclaved-citrate extractable (ACE) protein content under conventional field crop production. Reducing tillage in the conventional system significantly increased potentially mineralizable carbon (PMC) by 23 ± 11%, and led to more severe surface compaction, reflected by 23 ± 9% shallower penetration resistance at 300 psi. In the LEG and MNR systems, tillage was reduced through cover crop-based rotational no-till, where maize and soybeans were no-till planted following the use of a roller-crimper to terminate fall-planted cover crops. In these two organic systems, moderately reducing tillage did not cause significant changes in individual soil health indicators except for a 17 ± 8% increase of PMC in the MNR system. Individual soil health indicators and Comprehensive Assessment of Soil Health (CASH) scores generally did not differ between the CNV and LEG systems, and were significantly higher in the longer-rotation, manure-based MNR system. These results suggest that soil health in organic systems was determined more by diversified crop rotations and adequate organic inputs than by reducing tillage frequency, whereas in conventional systems other co-adapting soil health practices might be necessary to alleviate surface compaction and realize the full benefits of reduced tillage.

The Development and Utilization of Saline–Alkali Land in Western Jilin Province Promoted the Sequestration of Organic Carbon Fractions in Soil Aggregates

Soil samples from T (0~20 cm) and S (20~40 cm) layers of four saline–alkali rice fields (R5, R15, R20, and R35) with different reclamation years were selected to study the distribution of soil aggregates and the contents of readily oxidizable organic carbon (ROC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), potentially mineralizable carbon (PMC), and soil organic carbon (SOC). The effects of large macroaggregate (>2 mm, LMA), small macroaggregate (0.25 to 2 mm, SMA), and microaggregate (<0.25 mm, MA) particle size, soil layer, and soil physicochemical properties on SOC fractions were also analyzed. The results showed that the LMA size in saline–alkali paddy fields were easily decomposed and was unstable due to the influence of the external environment. With the increase in reclamation years, the proportion of LMA in the S layer decreased gradually. The ROC, DOC, MBC and TOC contents of aggregates in the T and S layers gradually increased with the increase in reclamation years, and SOC fractions contents of aggregates in different grain sizes were SMA > LMA > MA. An effective way to increase carbon sink and improve the ecological environment in western Jilin Province is to change the soil environment by planting rice in saline–alkali land.

Cover Crops for Resilience of a Limited-Irrigation Winter Wheat–Sorghum–Fallow Rotation: Soil Carbon, Nitrogen, and Sorghum Yield Responses

Cover crops can improve soil health by maintaining soil organic carbon (SOC) and nitrogen (N) contents, yet their dynamics in relation to crop yield in a semi-arid cropping system are poorly understood. The main objective of this study was to evaluate the response of diverse winter cover crop species and their mixture on SOC and N fractions and their relationship with sorghum (Sorghum bicolor L. Moench) yield in a winter wheat (Triticum aestivum L.)–sorghum–fallow rotation with limited irrigation management. Cover cropping treatments included pea (Pisum sativum L.), oat (Avena sativa L.), canola (Brassica napus L.), and mixtures of pea+oat (POM), pea+canola (PCM), peat+oat+canola (POCM), and a six-species mixture (SSM) of pea+oat+canola+hairy vetch (Vicia villosa Roth)+forage radish (Raphanussativus L.)+barley (Hordeum vulgare L.) as cover crops and a fallow. Soil samples were analyzed for residual inorganic N, potentially mineralizable carbon (PMC) and nitrogen (PMN), SOC, and total N. Response of labile inorganic N, PMC, and PMN varied with cover crop treatments. The SOC and total N contents did not differ among treatments but were 20% and 35% higher in 2020 than in 2019, respectively. Sorghum grain yield was 25% and 40% greater with oats than with PCM and canola cover crops in 2019, while it was 33–97% greater with fallow and oats than other treatments in 2020. Oat as a cover crop could improve the resilience of limited-irrigation cropping systems by increasing SOC, soil N, and crop yield in semi-arid regions.

Evaluation of Humic Fertilizers Applied at Full and Reduced Nitrogen Rates on Kentucky Bluegrass Quality and Soil Health

Soil health and sustainable management practices have garnered much interest within the turfgrass industry. Among the many practices that enhance soil health and sustainability are applying soil additives to enhance soil biological activity and reducing nitrogen (N) inputs—complimentary practices. A two-year study was conducted to investigate if reduced N fertilizer rates applied with humic substances could provide comparable turfgrass quality as full N rates, and whether humic fertilizers would increase biological aspects of soil health (i.e., microbial biomass and activity). Treatments included synthetic fertilizer with black gypsum (SFBG), poly-coated humic-coated urea (PCHCU; two rates), urea + humic dispersing granules (HDG; two rates), urea, stabilized nitrogen, HDG, and a nontreated control. Reduced rates of N with humic substances maintained turfgrass quality and cover, and reduced clipping biomass compared to full N rates. There were no differences in soil physical and chemical properties besides soil sulfur (S) concentration. SFBG resulted in the highest soil S concentration. Fertilizer treatments had minimal effect on microbial biomass and other plant-available nutrients. However, PCHCU (full rate) increased potentially mineralizable carbon (PMC) and N (PMN) by 68% and 59%, respectively, compared to the nontreated control. Meanwhile SFBG and stabilized nitrogen also increased PMC and PMN by 77% and 50%, and 65% and 59%, respectively. Overall, applications of reduced N fertilizer rates with the addition of humic substances could be incorporated into a more sustainable and environmentally friendly turfgrass fertilizer program.

Temporal variability of soil organic carbon in paddies during 13‐year conservation tillage

AbstractDecreasing temporal variability of soil organic carbon (SOC) can avoid its temporary loss, which is positive to SOC accumulation and mitigation of climate change. Thus, this study was designed to analyze the temporal variability of SOC stock during 13‐year tillage practices and its driving factors in rice paddies of Southern China. Four tillage practices were studied: no‐till (zero‐tillage) with residues retained on the soil surface (NTS, conservation tillage), rotary tillage with incorporated residues (RTS), moldboard plow tillage with incorporated residues, or with residues removed. The SOC stock at 0–30 cm soil depth varied widely between 2007 and 2017, partly due to the annual temperature and precipitation variations. The temporal variation of SOC was lower under NTS than other tillage practices (p < .05). This was associated with the lower variation of heavy SOC and mineral associated SOC as well as with lower (p < .05) potential mineralizable carbon under NTS. Despite SOC saturated in all treatments, NTS increased (p < .05) the SOC content by 12.1%–35.3% at 0–5 cm soil depth. Thus, NTS in double rice paddies has been proved to be a promising option to maintain high levels of soil C accumulation with low temporal variability, especially at the upper 0–5 cm depth.

Linking Soil Health to Sustainable Crop Production: Dairy Compost Effects on Soil Properties and Sorghum Biomass

Dairy compost is utilized in agricultural fields to supplement nutrients, yet its role in optimizing nutrient supply and health of semiarid soils is not clear. A greenhouse study was conducted over two months to evaluate soil properties and forage sorghum production under various compost rates. The study had six treatments and four replications. Treatments included compost application rates at 6.7 (C1), 13.5 (C2), 20.2 (C3), 26.9 (C4), and 33.6 Mg ha−1 (C5) and a control (C0). Soil samples were analyzed for soil organic carbon (SOC), potentially mineralizable carbon (PMC), total nitrogen (N), inorganic N, potentially mineralizable N (PMN), available phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). Plant biomass production and biomass C, N, and lignin contents were also estimated. High compost rates improved soil properties significantly (p < 0.05) indicated by increased SOC, N, P, K, Ca, and cation exchange capacity (CEC). Sorghum biomass production did not increase significantly with compost rate, while shoot N content increased at higher rates of compost. A nutrient management plan that integrates dairy compost application has potential to improve soil health and support sustainable forage production in semiarid regions.

Soil carbon pools in two natural grasslands of Serbian highlands

Grasslands are a major player in the global carbon cycle, although carbon stocks in grasslands are influenced by human activities and natural disturbances. The aim of this study is to determine differences in carbon stock on two test areas of grassland ecosystem in the highlands of Stara Planina and Zlatar Mountains (Serbia). The investigated sites are natural mountain grasslands of the same vegetation community (Agrostietum capillarisPavl. 1955) and soil type (Umbric Leptosol (Dystric) and Haplic Cambisol (Dystric)), but with different grazing intensity. Aboveground and belowground biomasses were measured in each sample plot, and soil was sampled at fixed depths of 0-10, 10-20 and 20-40 cm. The estimation of C stock and the rate of soil C accumulation were determined by the Tier 2 method IPCC (2003). Carbon mineralization potentials were determined via sequential incubation procedure in the laboratory conditions. According to the obtained results, the greater amount of precipitation on Mt. Stara Planina resulted in a greater accumulation of aboveground biomass, which was subjected to a greater decomposition in situ, thus showing a lower amount of PMC in vitro. In addition, potentially mineralizable carbon (PMC) among the sample plots from both sites indicates that the mineralization of soil organic matter was more influenced by the factors related to the soil characteristics, climatic conditions and grazing.

Conservation Systems for Positive Net Ecosystem Carbon Balance in Semiarid Drylands

Core Ideas The soil organic C dynamics and net ecosystem C balance of five dryland cropping systems were compared. Conservation systems stored up to 15% more soil organic C than conventional system. Net ecosystem C balance was positive with cover cropping. Cover crops and conservation tillage are crucial for soil C storage in drylands. Biomass C inputs often limit agroecosystem C dynamics, nutrient cycling, and soil organic carbon (SOC) storage in semiarid drylands. This study evaluated SOC and net ecosystem carbon balance (NECB) of five cropping systems in the drylands of the Southern Great Plains. Cropping systems evaluated included corn (Zea mays)–sorghum [Sorghum bicolor (L.) Moench] rotation with conventional tillage without cover cropping (CTNC), strip tillage with and without cover cropping (STCC and STNC, respectively), and no tillage with and without cover cropping (NTCC and NTNC, respectively). After 4 yr of experimental tillage, we measured CO2 emissions, soil and soil surface air temperatures, soil moisture content, potentially mineralizable carbon (PMC), total SOC, total nitrogen (TN), and net primary productivity (NPP). Conservation systems (any treatments including no‐till, strip till, or cover crops) had 5 to 6°C lower soil temperature and 2.8 to 4.9°C lower soil surface air temperature and stored 2.3 to 3.9% more soil moisture content than CTNC. Conservation systems also stored 15.2% more SOC than CTNC. Cropping systems that integrated cover crops in the rotation (STCC and NTCC) had greater NPP and positive NECB. Regardless of tillage management, cover cropping had a greater NECB, including SOC (NECBSOC) than CTNC. Reducing tillage and diversifying cropping systems through cover cropping can benefit semiarid dryland agroecosystems by increasing SOC storage and maintaining positive NECB.

Soil organic matter dynamics under irrigated perennial forage–annual crop rotations

AbstractReduced soil disturbance and increased organic matter inputs in forage production systems may improve soil quality and crop production through their effects on soil organic matter (SOM) dynamics and nutrient cycling. We evaluated the effects of conventional (CV), organic (OR) and reduced‐tillage (RT) management systems on SOM pools in perennial forage–annual crop rotations established on soils previously under long‐term monocropped corn (Zea mays L.) field. The SOM pools evaluated include potentially mineralizable carbon (PMC) and nitrogen (PMN), dissolved organic carbon (DOC) and nitrogen (DON), microbial biomass carbon (MBC) and nitrogen (MBN), inorganic nitrogen (inorganic‐N), soil organic carbon (SOC) and soil total nitrogen (STN). The crop rotation was 3 years of alfalfa (Medicago sativa L.)–grass mixture (2009‐2011) followed by a fourth year of corn (2012) across all management systems. While soils under the RT system had the highest rate of labile carbon (C) and nitrogen (N) accrual, all management systems accumulated SOC and STN during 2009‐2011. The response of different C and N pools was not consistent in 2012 when the fields were transitioned from alfalfa‐grass mixture to corn production. Soil PMN and inorganic‐N contents were significantly greater under CV than other management systems, whereas PMC and MBC remained greater under RT than other management systems. This suggests CV may provide immediate benefits to crop production by increasing N availability, but RT has potential to increase SOC accumulation and improve soil quality in the long‐term under perennial forage–annual crop rotations.

Comprehensive analysis of grazing intensity impacts soil organic carbon: A case study in typical steppe of Inner Mongolia, China

Overgrazing is a primary cause of grassland degradation, including the loss of soil carbon, but comprehensive analysis of the mechanism by which grazing affects soil organic carbon (SOC) is limited. To investigate this mechanism, we measured the contents of total soil organic carbon (TSOC) and active SOC fractions in grazed and ungrazed soil, and we examined the influence of grazing on soil microenvironments, soil carbon input, transformation and utilization, and soil respiration. The experiment was conducted in typical steppe in Inner Mongolia, China, under light, moderate, and heavy grazing plus a no-grazing control. Grazing changed the contents and stocks of the TSOC and active SOC fractions as well as the composition of the SOC. After three years, light and moderate grazing increased TSOC content by 3.44% and 5.43%, respectively, while heavy grazing decreased it by 4.30%. The changes in active SOC fractions were not uniform under different grazing intensities. All grazing intensities increased the stocks of microbial biomass carbon (MBC) and dissolved organic carbon (DOC) compared with the control, but their magnitudes did not rise with increased grazing intensity. Moderate grazing increased the stocks of potentially mineralizable carbon (PMC) and particulate organic carbon (POC), whereas light and heavy grazing decreased them; this may indicate likely long-term changes in SOC following changes in grazing intensity. Light grazing resulted in a larger C input to soil and faster transformation, more emission, and less accumulation of SOC. Moderate grazing led to a smaller C input to soil, and its transformation, utilization and emission rates were between those associated with light grazing and heavy grazing, and there was more accumulation. Heavy grazing resulted in the smallest C input, faster transformation and utilization, less change in emission, and less accumulation. Based on these results, moderate grazing is likely the most practical grazing intensity for the sustainable utilization of this grassland ecosystem to feed livestock. However, because this was a short-term study, the results did not reach significance, and long-term studies are needed.

Soil Organic Matter Mineralization under Different Temperatures and Moisture Conditions in Kõzõldağ Plateau, Turkey

Drought by climate change in East Mediterranean Region will change soil temperature and moisture that lead to alter the cycling of biological elements like carbon and nitrogen. However, there are few studies that show how sensitivity of soil organic matter mineralization to temperature and/or moisture can be modified by changes in these parameters. In order to study how these changes in temperature and moisture affect soil carbon and nitrogen mineralization, a laboratory experiment was carried out in two depths (0-5 and 5-15 cm) of soils of Onobrychis beata and Trifolium speciosum being common annual plants in Turkey that was taken from Kizildag Plataeu (Adana city). Some soil physical and chemical properties and as well as rate of carbon and nitrogen mineralizations were determined for both depths of soils. These soils were incubated for 42 days under different field capacities (FC 60, 80 and 100%) and temperatures (24, 28 and 32°C). Cumulative carbon mineralization (Cm), potential mineralizable carbon (C0) and rate of carbon mineralization of all soils were increased with rising temperatures. Rate of carbon mineralization in O. beata soil were lower than T. speciosum soil. NH4-N and NO3-N contents at 42nd day were higher than initial levels of soils and also increased with temperatures and field capacities. In summary, sensitivity of soil organic matter mineralization to temperature was higher at 32°C in upper layer and lower at 24°C in deeper layer of both soils.

Potentially mineralizable carbon in soils under defferent land management

Soil organic matter pools provide insight into the potential stabilization or degradation of the soil resource by various long-term land management systems. Potentially mineralizable carbon (PMC) in soil is a measure of easily decomposable carbon and considered an important pool of soil organic matter. PMC was measured in soil samples taken from six different land management types viz. grass, legume, cinnamon, coconut, vegetable and a planted forest located in the low country wet zone of Sri Lanka. Soil samples taken from the different fields were incubated and carbon mineralization during 0-3, 3-10 and 10-24 day periods were determined as total CO2 evolved using alkali (NaOH) traps. Land management significantly influenced the total carbon mineralized during the incubation. Soils from the grass and coconut fields recorded the significantly high PMC compared to other land management types. The PMC in soils ranged from 298 to 428 mg/kg of soil in different land management treat ments. PMC was also calculated based on the carbon mineralized as a fraction of total organic carbon (TOC) in soil. Planted forest had the lowest PMC as a fraction of TOC (1.6%) compared to the highest recorded in the coconut field (5.2%). Results of this study suggest that PMC is related to the vegetation type and the nature of plant litter that supply organic matter to soil. It also appeared that labile carbon is more prevalent under grass vegetation compared to other land management types. PMC in soil could be used as an indicator to assess the amount of labile and easily decomposable carbon in soil.

Analysis of Spatial and Temporal Variation of Soil Organic Carbon Budget in Northern Kazakhstan

Farm-regional evaluation of soil organic carbon (SOC) dynamics for the chernozem zone in northern Kazakhstan is now vitally important for making agro-environmental policy. In this study, the author intends to evaluate spatial and temporal variation of the SOC budget and to seek alternative ways of sustainable land use in northern Kazakhstan. To achieve this final goal, this study established the following objectives: to (1) clarify the spatial patterns of soil organic matter content, (2) clarify the influence of land use on the dynamics of soil organic carbon in situ for the three differential soil classes, and (3) clarify spatial and temporal variability of SOC budget in farm-regional scale. The highest SOC content was found in the northern region and the lowest content in the southern region of the study area. Potentially mineralizable carbon (PMC) content, however, was highest in the midwestern region. Furthermore, fluctuations in CO 2 emission from the soils showed a similar pattern to temperature fluctuations. Using PMC, soil temperature, precipitation and/or dryness factor, 40-80% of the variation of CO 2 emission could be estimated. From farm-regional evaluation, it is concluded that the spatial and temporal variation of the SOC budget was significantly affected by the crop rotation system in the area. In addition, the fallow- and barley-based crop rotation systems accelerate SOC depletion in the study area.

Predicting Phytotoxicity of Compost-Amended Soil from Compost Stability Measurements

Composting can divert waste from landfills and provide a fertilizer for use in agriculture. Variability in the performance of applied composts associated with phytotoxicity, however, often hinders compost use. We propose a sensitive assay for evaluating compost phytotoxicity when amended to soil, and develop a sigmoidal model to relate phytotoxicity (measured as seed germination) with compost stability (measured as carbon mineralization rate and potential mineralizable carbon). Carbon mineralization experiments were performed with compost-amended soil mixtures consisting of sandy loam and commercial potting soils amended with 0, 5, and 50% (v/v) food-waste and green-waste composts, and incubated between 20 and 45°C. Sensitivity of garden cress (Lepidium sativum), radish (Raphanus sativus), Chinese cabbage (Brassica rapa var. Pekinensis), and lettuce (Lactuca sativa) seeds to compost phytotoxic compounds was evaluated by direct seeding into mixtures. Cress was found to be the most sensitive of the four species tested. Phytotoxicity of compost-amended soil mixtures measured by cress germination was related to compost carbon mineralization rate and potential mineralizable compost carbon remaining in the mixtures using sigmoidal models. The mean biases calculated using model predictions and data from independent mineralization experiments were less than 10% phytotoxicity, suggesting either estimate of stability could be used as an indicator of phytotoxicity of compost-amended soil.

Spatial prediction of soil organic matter in northern Kazakhstan based on topographic and vegetation information

This study aimed to improve the accuracy of spatial prediction for soil organic matter, potential mineralizable carbon (PMC) and soil organic carbon (SOC), using secondary information, namely topographic and vegetation information, in northern Kazakhstan. Secondary information included elevation (ELEV), mean curvature (MEANC), compound topographic index (CTI) and slope (SLOPE) obtained from a digital elevation model, and enhanced vegetation index (VI) values obtained from a moderate resolution imaging spectroradiometer (MODIS). The prediction methods were statistical (multiple linear regression between soil organic matter and secondary information) and geostatistical algorithms (regression-kriging Model-C and simple kriging with varying local means [SKlm]). The VI, ELEV and MEANC were selected as the independent variables for predicting PMC and SOC. However, MEANC showed an opposite effect on PMC and SOC accumulation patterns. Model validity revealed that SKlm was the most appropriate method for predicting PMC and SOC spatial patterns because model validity revealed the smallest errors for this method. Maps from the kriged estimates showed that a combination of secondary information and geostatistical techniques can improve the accuracy of spatial prediction in study areas.

Soil carbon dioxide fluxes in established switchgrass land managed for biomass production

Switchgrass ( Panicum virgatum L.) grown for biomass feedstock production has the potential to increase soil C sequestration, and soil CO 2 flux in grassland is an important component in the global C budget. The objectives of this study were to: (1) determine the effects of N fertilization and harvest frequency on soil CO 2 flux, soil microbial biomass carbon (SMBC), and potentially mineralizable carbon (PMC); and (2) evaluate the relationship of soil CO 2 flux with soil temperature, soil moisture, SMBC, and PMC. Two N rates (0 and 224 kg ha −1) were applied as NH 4NO 3 and cattle ( Bos Taurus L.) manure. Switchgrass was harvested every year at anthesis or alternate years at anthesis. The data were collected during growing season (May–October) 2001–2004 on switchgrass-dominated Conservation Reserve Program (CRP) land in east-central South Dakota, USA. Manure application increased soil CO 2 flux, SMBC, and PMC during the early portion of the growing season compared with the control, but NH 4NO 3 application did not affect soil CO 2 flux, SMBC, and PMC. However, seasonal variability of soil CO 2 flux was not related to SMBC and PMC. Estimated average soil CO 2 fluxes during the growing periods were 472, 488, and 706 g CO 2–C m −2 for control, NH 4NO 3–N, and manure-N plots, respectively. Switchgrass land with manure application emitted more CO 2, and approximately 45% of the C added with manure was respired to the atmosphere. Switchgrass harvested at anthesis decreased soil CO 2 flux during the latter part of the growing season, and flux was lower under every year harvest treatment than under alternate years harvest. Soil temperature was the most significant single variable to explain the variability in soil CO 2 flux. Soil water content was not a limiting factor in controlling seasonal CO 2 flux.