Switchgrass (Panicum virgatum L.) is a native warm-season (C4) grass that has the potential to be used as a bioenergy crop and reduce increases in atmospheric carbon dioxide (CO2). Continuous production and removal of switchgrass, however, may deplete soil fertility. A strategy of returning plant components higher in nutrients to the field during harvest may help maintain soil fertility. In this study, nutrient partitioning in switchgrass parts over time and their C and N mineralization patterns in soil were determined. Switchgrass (cv. Alamo) was harvested on a biweekly schedule from June to October, with plants from each harvest separated into six parts (top, middle, and bottom leaves and stems) for mineral and fiber analysis. Plant materials from three harvests were used in an incubation study to determine effects of plant component, age, and composition on carbon (C) and nitrogen (N) mineralization. Results indicated that a strategy of returning specific plant parts to the field would not substantially conserve soil nutrients without proportionally decreasing materials available for bioenergy production. Structural components (cell wall and cellulose) were dominant factors affecting the quantity of C mineralized. Approximately 50% of C added as switchgrass was mineralized after 100 days of incubation. Soil N immobilization was observed in all switchgrass plant part treatments.
Abstract Soil properties were assessed that may have contributed to the failure of Coastal bermudagrass [Cynodon dactylon (L. Pers.)] to respond to application of K on Darco fine sand for 5 consecutive years. The depth to which plant roots absorbed K was assessed by field observation and by determination of K levels in control and treatment plots. Potassium minerals were determined by chemical methods to assess potential nutrient sources. Potassium removal was indicated to at least 160 cm by soil solution content and to 235 cm by exchangeable K concentration and Gapon selectivity values. Depth of bermudagrass rooting was in agreement with chemical indicators of K removal. Mica and interstratified mica in the fine silt and clay fractions correlated best with exchangeable K (r 2 = 0.97) suggesting a direct influence of the finer micaceous materials in Darco soils upon K retained in an exchangeable form and possibly upon K release. Inclusion of feldspar‐K with mica‐K reduced the r 2 value from 0.97 to 0.83 suggesting that K‐feldspar does not contribute appreciable K to exchangeable form. Feldspar was present mostly in the sand and two coarser silt fractions which expose little surface area to weathering forces. The large rooting volume of Coastal bermudagrass in Darco soil in conjunction with the mica and appreciable interstratified mica present may explain why no plant response was obtained to K application.
No-tillage (NT) has the potential to enhance C and N sequestration in agricultural soils of the southern USA, but results may vary with crop species. The objectives of this study were to investigate the impacts of NT, conventional tillage (CT), and crop species on soil organic carbon (SOC) and nitrogen (SON) sequestration and distribution within aggregate-size fractions in a central Texas soil after 20 yr of management. No-tillage increased SOC over CT at the 0- to 5-cm depth by 97, 47, and 72%, and SON by 117, 56, and 44% for continuous grain sorghum [Sorghum bicolor (L.) Moench], wheat (Triticum aestivum L.), and soybean [Glycine max (L.) Merr.], respectively. Crop species had significant impacts on SOC and SON sequestration. On average, the wheat monoculture had greater SOC (9.23 Mg C ha−1) at the 0- to 5-cm depth than sorghum (6.75 Mg C ha−1) and soybean (7.05 Mg C ha−1). No-tillage increased the proportion of >2-mm and 250-μm to 2-mm macroaggregate fractions in soil compared with CT. At the 0- to 5-cm depth, NT increased SOC compared with CT by 158% in macroaggregate fractions, but only 40% in <250-μm fractions. No-tillage increased SON compared with CT by 300, 94, 41, and 39% for >2-mm, 250-μm to 2-mm, 53- to 250-μm, and <53-μm fractions, respectively. Long-term impacts of NT included a greater proportion of macroaggregates and increased C and N sequestration, but impacts were dependent on crop species and varied with soil depth.
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. Keywords: Carbon sequestrationdissolved organic carbonsoil organic carbonsorghumtillage Acknowledgments This research was partially funded by the Consortium for Agricultural Soils Mitigation of Greenhouse Gases (CASMGS), through the cooperation of the State Research, Education, and Extension Service, United States Department of Agriculture, Agreement No. 2001-38700‐11092.
Oilseed crops are being widely evaluated for potential biodiesel production. Seed meal (SM) remaining after extracting oil may have use as bioherbicides or organic fertilizers. Brassicaceae SM often contains glucosinolates that hydrolyze into biologically active compounds that may inhibit various pests. Jatropha curcas SM contains curcin, a phytoxin. A 14-day greenhouse study determined that Sinapis alba (white mustard), Brassica juncea (Indian mustard), Camelina sativa , and Jatropha curcas applied to soil at varying application rates [0, 0.5, 1.0, and 2.5% (w/w)] and incubation times (1, 7, and 14 d) prior to planting affected seed emergence and seedling survival of cotton [ Gossypium hirsutum (L.)], sorghum [ Sorghum bicolor (L.) Moench], johnsongrass ( Sorghum halepense ), and redroot pigweed ( Amaranthus retroflexus ). With each species, emergence and survival was most decreased by 2.5% SM application applied at 1 and 7 d incubations. White mustard SM incubated for 1 d applied at low and high rates had similar negative effects on johnsongrass seedlings. Redroot pigweed seedling survival was generally most decreased by all 2.5% SM applications. Based on significant effects determined by ANOVA, results suggested that the type, rate, and timing of SM application should be considered before land-applying SMs in cropping systems.
Crop species and conservation tillage may enhance carbon (C) and nitrogen (N) sequestration potential in subsurface soils. The objectives of this study were to determine the effects of crop species and tillage on soil organic C (SOC) and total N distribution in six soil depth intervals from 0 to 105 cm after 20 years of treatment imposition. Tillage had the most influence on soil C and N at 0 to 5 cm, and impacts extended to the 15- to 30-cm depth for wheat and sorghum. Overall, SOC and total N for wheat were 18 and 15% higher than sorghum and soybean. Dissolved organic C (DOC) depth distribution was similar to SOC and total N. The proportion of SOC as DOC ranged from 1.3 to 3.3% and increased with soil depth. The highest soil C and N levels occurred for wheat under no tillage. The depth of soil impacted by crop species was shallower for conventional tillage than no tillage, and the depth distribution exhibited a logarithmic pattern. Soil organic C, total N, and DOC decreased 404, 507, and 205%, respectively from 0-5 to 80-105 cm. The maximum depth interval below which no further decreases in SOC and total N occurred was 30 to 55 cm for soybean, 55 to 80 cm for wheat, and 80 to 105 cm for sorghum, demonstrating the importance of subsurface soils for C sequestration. Crop management impacts below the depth of tillage demonstrate the importance of crop rooting and belowground biomass, or translocation of dissolved organic matter, to subsoil C sequestration.
Management practices, such as no‐tillage (NT) and high‐intensity cropping sequences, have the potential to enhance C and N sequestration in agricultural soils. The objectives of this study were to investigate the impacts of conventional‐tillage (CT), NT, and multiple cropping sequences on soil organic carbon (SOC) and nitrogen (SON) sequestration and on distribution within aggregate‐size fractions in a southcentral Texas soil after 20 yr of treatment imposition. No‐tillage management increased soil aggregation compared with CT, with the bulk of SOC and SON storage present in larger aggregate‐size fractions (>2 mm, 250 μm to 2 mm) at both soil depths. Multiple cropping systems, such as a grain sorghum [ Sorghum bicolor (L.) Moench] / wheat ( Triticum aestivum L.)/soybean [ Glycine max (L.) Merr] (SWS) rotation and a wheat/soybean (WS) doublecrop had the highest SOC and SON storage, while the continuous monoculture soybean treatment had the lowest storage. Soil organic C and SON storage were significantly greater under NT than CT for all cropping sequences at 0 to 5 cm and for SWS and WS at 5 to 15 cm. At the 0‐ to 5‐cm depth, NT increased SOC storage by 64% and SON storage by 76% compared with CT. However, at 5 to 15 cm, NT only increased SOC storage by 28% and SON storage by 40%. The use of NT showed a greater impact for increasing SON storage than for SOC storage, suggesting that N cycling is an important factor related to soil C sequestration potential.
