• Authors:
    • Houghton, R. A.
  • Source: Global Change Biology
  • Volume: 1
  • Issue: 4
  • Year: 1995
  • Summary: Changes in land use between 1850 and 1980 are estimated to have increased the global areas in croplands, pastures, and shifting cultivation by 891, 1308, and 30 x 10^6 ha, respectively, reducing the area of forests by about 600 x 10^6 ha, releasing about 100 PgC to the atmosphere, and transferring about 23 PgC from live vegetation to dead plant material and wood products. Another 1069 X 10^6 ha are estimated to have been logged during this period, and the net release of carbon from the combined processes of logging and regrowth contributed 23 PgC to the 100-PgC release. Annual rates of land-use change and associated emissions of carbon have decreased over the last several decades in temperate and boreal zones and have increased in the tropics. The average release of carbon from global changes in land use over the decade of the 1980s is estimated to have been 1.6 +/- 0.7 PgC y-1 almost entirely from the tropics. This estimate of carbon flux is higher than estimates reported in recent summaries because it is limited here to studies concerned only with changes in land use. Other recent analyses, based on data from forest inventories, have reported net accumulations of carbon as high as 1.1 PgC y-1 in temperate and boreal zones. Because the accumulation of carbon in forests may result from natural processes unrelated to land-use change, estimates based on these inventories should be distinguished from estimates based on changes in land use. Both approaches identify terrestrial sinks of carbon. The argument is made here, however, that differences between the two approaches may help identify the location and magnitude of heretofore 'missing' sinks. Before different estimates can be used in this way, analyses must consider similar geographical regions and dates, and they must account for the accumulation and loss of carbon in forest products in a consistent fashion.
  • Authors:
    • Sweeney, D. W.
    • Moyer, J. L.
  • Source: Communications in Soil Science and Plant Analysis
  • Volume: 26
  • Issue: 1-2
  • Year: 1995
  • Summary: Legumes provide benefit in crop rotations, but data are limited on soil inorganic nitrogen (N) and soil strength responses to spring- or fall-seeded legumes as green manures for grain sorghum [Sorghum bicolor (L.) Moench] production on the prairie soils of the eastern Great Plains of the United States. With increased emphasis on conservation tillage, information is also needed on combining conservation tillage with the use of legume cover crops. This experiment was established to examine the effects of i) red clover (Trifolium pratense L.) and hairy vetch (Vicia villosa Roth.) as previous crops to grain sorghum compared with continuous grain sorghum, ii) reduced or no-tillage, and iii) fertilizer N rate on changes in soil inorganic N and soil strength. At two adjacent sites (Parsons silt loam; fine, mixed thermic Mollic Albaqualf) differing in initial pH and phosphorus (P) and potassium (K) fertility, soil nitrate-nitrogen (NO3-N) was as much as fourfold higher following kill-down of red clover or hairy vetch than following continuous grain sorghum. At the higher fertility site, soil total inorganic N [TIN: sum of ammonium-nitrogen (NH4-N) and NO3-N] shortly following kill-down of red clover exceeded TIN following hairy vetch by more than 35% and that with continuous grain sorghum by 110%; however, at the lower fertility site, the trend for soil TIN to be higher following legumes was not significant. Tillage did not affect soil NO3-N levels in the spring following legume kill-down. However, subsequent soil NO3-N levels under no-tillage tended to be higher in the spring, but lower in the fall than with reduced tillage. Increases in soil TIN by legumes and fertilizer were related to grain sorghum yield, but likely were not the only factors affecting yield. Legumes and tillage used in grain sorghum production may also provide other non-N benefits as suggested by soil penetration resistance measured at the end of the study.
  • Authors:
    • Unger, P. W.
  • Source: Journal of Soil and Water Conservation
  • Volume: 50
  • Issue: 3
  • Year: 1995
  • Summary: Crop residue management was chosen as a key practice to help control erosion on nearly 75% of the highly erodible land covered by conservation plans. This study determined the effects of treatments that involved retaining all residues on the surface (NT+Res), removing some residues at harvest (NT-ResH) or at planting (NT-ResP), and conventional tillage (ConvT) on soil water storage and use, and yields of continuous winter wheat (Triticum aestivum L.) produced with limited irrigation. Water storage between crops was greater with NT+Res (95 mm) and NT-ResH (100 mm) than with ConvT (79 mm), but soil water depletion was not affected by treatments. Grain yield was greater with NT+Res (4.56 Mg ha(-1)), than with ConvT (4.26 Mg ha(-1)) and NT-ResH (4.18 Mg ha(-1)), but straw yield was not affected by treatments. Grain and straw yield differed among crops. Continuous wheat production with limited irrigation resulted in an estimated 2.2 Mg ha(-1) of residues on the surface at planting with the NT-ResH and NT-ResP treatments. The initial amount was 9.0 Mg ha(-1) with the NT+Res treatment, and much of this remained on the surface at planting of the next crop. In all cases, the residue amounts provided considerably more (a minimum of about 70%) than the 30% surface cover usually required to control erosion on highly erodible land. Hence, use of limited irrigation and no-tillage can help producers meet the surface residue requirements established for their conservation plans for highly erodible lands in the southern Great Plains.
  • Authors:
    • Martin, R. J.
    • Marcellos, H.
    • Felton, W. L.
  • Source: Australian Journal of Experimental Agriculture
  • Volume: 35
  • Issue: 7
  • Year: 1995
  • Summary: Four experiments were commenced after a 1980 wheat crop, and a fifth after the 1981 crop, at different sites representing the major soil types of northern New South Wales in the 550-700 mm rainfall zone, to examine the influence of 3 fallow management practices [no tillage (NT); stubble retention after harvest, cultivation (SM); stubble burning after harvest, cultivation (SB)] on wheat production. Data considered in this paper cover the continuous wheat subtreatments of the 5 experiments (1981-90). Nitrogen applied at 50 kg N/ha in addition to the basal treatment was included as a treatment from 1986 to 1988. Across all sites and seasons, grain yields were in the order SB>SM approximate to NT, stubble retention having a greater effect than tillage. In some years at some sites, differences in grain yield and grain N yield were not significant. In others, when significant yield differences occurred, variations in grain yield and grain N yield were highly correlated with differences in soil N available for the crop. The data show that the influence of fallow management interacted with season and crop nutrition, and required long-term study for proper assessment.
  • Authors:
    • Mullins, G. L.
    • Williams, J. C.
    • Delaney, D. P.
    • Wood, C. W.
    • Kingery, W. L.
  • Source: Journal of Environmental Quality
  • Volume: 23
  • Issue: 1
  • Year: 1994
  • Summary: The largest portion of Alabama's rapidly growing poultry industry is geographically concentrated in the Sand Mountain region of northern Alabama. The result is that large amounts of waste are applied to relatively small areas of agricultural soils. A study was conducted to determine the effects of long-term broiler waste (litter) application on environmentally related soil conditions in the region. The region has an average annual rainfall of 1325 mm, which is evenly distributed throughout the year, a thermic temperature regime, and soils in the region are of the Ultisol order. In each of four major broiler-producing counties, three pairs of sites consisting of long-term (15-28 yr) littered and nonlittered fields on matching soil series and maintained under perennial tall fescue (Festuca arundinacea Schreb.) were sampled. Soil cores were taken to 3 m or lithic contact and depth-incremented samples (0-15, 15-30, and each subsequent 30-cm interval) were analyzed for organic C, total N, NO3-N, pH, electrical conductivity, and acid-extractable P, K, Ca, Mg, Cu, and Zn. Litter application increased organic C and total N to depths of 15 and 30 cm, respectively, as compared with nonlittered soils, whereas pH was 0.5 units higher to a depth of 60 cm under littered soils. Significant accumulation of NO3-N was found in littered soils to or near bedrock. Extractable P concentrations in littered soils were more than six times greater than in nonlittered soils to a depth of 60 cm. Elevated levels of extractable K, Ca, and Mg to depths greater than 60 cm also were found as a result of long-term litter use. Extractable Cu and Zn had accumulated in littered soils to a depth of 45 cm. These findings indicate that long-term land application of broiler litter, at present rates, has altered soil chemical conditions and has created a potential for adverse environmental impacts in the Sand Mountain region of Alabama.
  • Authors:
    • Schroeder, P. E.
    • Lee, J. J.
    • Andrasko, K. J.
    • Winjum, J. K.
    • Dixon, R. K.
  • Source: Climatic Change
  • Volume: 27
  • Issue: 1
  • Year: 1994
  • Summary: Degraded or sub-standard soils and marginal lands occupy a significant proportion of boreal, temperate and tropical biomes. Management of these lands with a wide range of existing, site-specific, integrated, agroforest systems represents a significant global opportunity to reduce the accumulation of greenhouse gases in the atmosphere. Establishment of extensive agricultural, agroforest, and alternative land-use systems on marginal or degraded lands could sequester 0.82-2.2 Pg carbon (C) per year, globally, over a 50-year time-frame. Moreover, slowing soil degradation by alternative grassland management and by impeding desertification could conserve up to 0.5-1.5 Pg C annually, A global analysis of biologic and economic data from 94 nations representing diverse climatic and edaphic conditions reveals a range of integrated land-use systems which could be used to establish and manage vegetation on marginal or degraded lands. Promising land-use systems and practices identified to conserve and temporarily store C include agroforestry systems, fuelwood and fiber plantations, bioreserves, intercropping systems, and shelterbelts/windbreaks. For example, successful establishment of low-intensity agroforestry systems can store up to 70 Mg C/ha in boreal, temperate and tropical ecoregions. The mean initial cost of soil rehabilitation and revegetation ranges from $500-3,000/ha for the 94 nations surveyed. Natural regeneration of woody vegetation or agro-afforestation establishment costs were less than $1000/ha in temperate and tropical regions. The costs of C sequestration in soil and vegetation systems range from $1-69/Mg C, which compares favorably with other options to reduce greenhouse gas emissions to the atmosphere. Although agroforestry system projects were recently established to conserve and sequester C in Guatemala and Malaysia, constraints to wide-spread implementation include social conditions (demographic factors, land tenure issues, market conditions, lack of infrastructure), economic obstacles (difficulty of demonstrating benefits of alternative systems, capital requirements, lack of financial incentives) and, ecologic considerations (limited knowledge of impacts and sustainability of some systems).
  • Authors:
    • Polley, H. W.
    • Mayeux, H. S.
    • Johnson, H. B.
    • Gebhart, D. L.
  • Source: Journal of Soil and Water Conservation
  • Volume: 49
  • Issue: 5
  • Year: 1994
  • Summary: ABSTRACT The land use change from cropland to perennial grass cover associated with The Conservation Reserve Program (CRP) may sequester atmospheric CO, back into the soil carbon pool, thereby changing formerly cultivated soils from sources to sinks,for atmospheric carbon. To evaluate the effect of CRP on soil organic carbon (SOC] levels, samples from adjacent cropland, native pasture, and five year old CRP sites in Texas, Kansas, and Nebraska were analyzed. Across all locations, SOC levels for cropland, CRP, and native pasture were 59.2, 65.1, and 90.8 metric tons c-1 ha-1 in the surface 300 cm, respectively. CRP lands gained an average of 1.1 tons C ha-1 yr-1 suggesting that the 17 million hectares of land enrolled in CRP may have the potential to sequester about 45% of the 38. 1 million tons of carbon released annually into the atmosphere from US agriculture. These findings illustrate that agricultural CO2 emissions may be effectively controlled through changes in land use and management systems.
  • Authors:
    • Lindwall, C. W.
    • Roman, E. S.
    • Moyer, J. R.
    • Blackshaw, R. E.
  • Source: Crop Protection
  • Volume: 13
  • Issue: 4
  • Year: 1994
  • Summary: Soil erosion by wind or water is a serious problem in North and South America. When no-till or reduced tillage is used to control erosion, the density of certain annual and perennial weeds can increase and new weed control techniques are usually required. The effects of conservation tillage on annual and perennial weeds, weeds that are spread by wind, plants from rangelands and pasture as weeds and volunteer plants as weeds arc reviewed. Current weed control methods with minimum tillage, herbicides, cover crops and other cultural practices in conservation tillage systems in North and South America are described. Some producers are successfully controlling weeds in continuous summer cropping systems in North America and in double cropping systems that include wheat in the winter and soybean or corn in the summer in Brazil, Argentina and southeastern United States. Successful conservation tillage systems usually involve cropping sequences of three or more crop types and several herbicides. In these cropping sequences, the ground is covered with a crop during most of the period in which the climate is favourable for weed growth. Perennial weeds are a problem in all tillage systems and there is a general dependence on glyphosate for perennial weed control. In successful conservation tillage systems, the amount and cost of herbicides used is similar to that for herbicides used in conventional tillage systems.
  • Authors:
    • Sweeney, D. W.
    • Moyer, J. L.
  • Source: Soil Science Society of America Journal
  • Volume: 58
  • Issue: 5
  • Year: 1994
  • Summary: With increased emphasis on conservation tillage, information is needed on the use of spring- or fall-seeded legumes as green manures for eastern Great Plains grain sorghum [Sorghum bicolor (L.) Moench] production. This study was conducted to determine whether legumes can be beneficial to subsequent grain sorghum crops grown in conservation tillage systems on prairie soil. Comparisons included the effects of (i) red clover (Trifolium pratense L.) and hairy vetch (Vicia villosa Roth) before grain sorghum vs. continuous grain sorghum, (ii) reduced or no-tillage, and (iii) fertilizer N rates on grain sorghum grown on two sites of a Parsons silt loam (fine, mixed, thermic Mollic Albaqualf). Surface soil at Site 1 was higher in pH (7.2 vs. 6.2), P (12 vs. 4 mg kg(-1)), and K (80 vs. 60 mg kg(-1)) than at Site 2. Yield of the first sorghum crop after legume kill-down in 1987 ranged from 79 to 131% more than for continuous grain sorghum. At the higher fertility Site 1, red clover residual increased yields to 3.7 from 2.7 Mg ha(-1) with continuous grain sorghum in the third year; at the lower fertility Site 2, the legume residual did not influence yield after the first year. First-year grain sorghum yielded 1.1 to 1.6 Mg ha(-1) more with reduced tillage than with no-tillage, but the difference was less in subsequent years. In 1987, yield was not affected by fertilizer N even following grain sorghum, but the response was significant in subsequent years. Low N response on this high organic matter prairie soil contributed to uncertain fertilizer N equivalencies and suggested other non-N benefits from the legumes.
  • Authors:
    • Deverel, S. J.
    • Rojstaczer, S.
  • Source: Geophysical Research Letters
  • Volume: 20
  • Issue: 13
  • Year: 1993
  • Summary: Historical and contemporary subsidence in the San Joaquin-Sacramento Delta, California indicates that subsidence rates associated with drainage of organic soils have declined over time. Contemporary measurements of carbon flux into the atmosphere can be used to predict contemporary rates of permanent subsidence. This correspondence indicates that most subsidence is caused by carbon oxidation. The current contribution of atmospheric carbon from the Delta is 2 x 10(12) gm C/yr. This estimate is a factor of 3-4 less than previous estimates and reflects the declining rate of CO2 production in the Delta over the last several decades. Estimates of current production of CO2 from other drained agricultural lands that are based upon time-averaged historical rates of subsidence are also likely to be too large.