• Authors:
    • Barra, J. D. E.
    • Carretera, A. R. H.
    • Oleschko, K.
  • Source: Soil & Tillage Research
  • Volume: 37
  • Issue: 1
  • Year: 1996
  • Summary: Little is known about the crop influence and the long-term effects of tillage systems on physical and morphological properties of Vertisols in the high plateau of the intertropical zone of Mexico. The purpose of the present study was to evaluate the impact of crop rotation on the development of structure and pedofeatures of Eutric Vertisols of Guanajuato, Mexico. Disturbed and undisturbed samples from the top soil (0-20 cm) of five agroecosystems differing in crop rotation (semipermanent alfalfa, two cereal crops per year, one vegetable and one cereal crop per year, three vegetable crops per year, and dryland maize) were collected. A second no-till soil sample was obtained from a nearby site under secondary vegetation and used as reference soil. Continuous and intensive cultivation lead to a significant reduction in Vertisol organic matter and total nitrogen contents. Tillage and crop rotations affected air-dried aggregate bulk densities, soil microstructure and development of pedofeatures. Calcite crystalline pedofeatures seemed to be more directly affected by the management systems used. Nodules, the most common crystalline pedofeatures in top soil of all cultivated Vertisols, were absent in no-till soils.
  • Authors:
    • Tyler, D. D.
    • Bock, B. R.
    • Thornton, F. C.
  • Source: Journal of Environmental Quality
  • Volume: 25
  • Issue: 6
  • Year: 1996
  • Summary: This study characterizes soil emissions of NO and N2O from banded applications of anhydrous ammonium (AA) and urea over the period from 6 May 1994 to 12 Sept. 1994 from a loess soil in western Tennessee. The N application rate for both sources was 168 kg ha-1. Fertilizer type strongly influenced emissions of N2O (F = 231; P = 0.0001) and NO (F = 69; P = 0.0001). During the 129 d measurement period, the AA treatment lost 12.33 kg of N2O-N or 7.33% of the applied N. The N2O-N loss from the urea treatment was about one-half that from AA; 6.34 kg ha-1 or 3.77% of the applied N. Loss of NO-N from both treatments was small compared with N2O-N loss. The urea treatment lost 0.27 kg ha-1 as NO-N and the AA treatment lost 0.2 kg ha-1 during the study period. While the measured loss rate of N2O-N from AA is similar to previous literature estimates, our values for urea are 20 to 40 times greater than the current literature reports of N2O-N loss of 0.1 to 0.2% of the urea applied. Higher N2O losses from urea in this study may be related to the fact that urea was banded below the soil surface, whereas urea has been broadcast on the soil surface in other N2O emissions studies.
  • Authors:
    • Harriss, R. C.
    • Narayanan, V.
    • Li, C.
  • Source: Global Biogeochemical Cycles
  • Volume: 10
  • Issue: 2
  • Year: 1996
  • Summary: The Denitrification-Decomposition (DNDC) model was used to elucidate the role of climate, soil properties, and farming practices in determining spatial and temporal variations in the production and emission of nitrous oxide (N[2]O) from agriculture in the United States. Sensitivity studies documented possible causes of annual variability in N[2]O flux for a simulated Iowa corn-growing soil. The 37 scenarios tested indicated that soil tillage and nitrate pollution in rainfall may be especially significant anthropogenic factors which have increased N[2]O emissions from soils in the United States. Feedbacks to climate change and biogeochemical manipulation of agricultural soil reflect complex interactions between the nitrogen and carbon cycles. A 20% increase in annual average temperature in °C produced a 33% increase in N[2]O emissions. Manure applications to Iowa corn crops enhanced carbon storage in soils, but also increased N[2]O emissions. A DNDC simulation of annual N[2]O emissions from all crop and pasture lands in the United States indicated that the value lies in the range 0.9 - 1.2 TgN. Soil tillage and fertilizer use were the most important farming practices contributing to enhanced N[2]O emissions at the national scale. Soil organic matter and climate variables were the primary determinants of spatial variability in N[2]O emissions. Our results suggest that the United States Government, and possibly the Intergovernmental Panel on Climatic Change (IPCC), have underestimated the importance of agriculture as a national and global source of atmospheric N[2]O. The coupled nature of the nitrogen and carbon cycles in soils results in complex feedbacks which complicate the formulation of strategies to reduce the global warming potential of greenhouse gas emissions from agriculture.
  • Authors:
    • Maclaren, P.
  • Source: Global Environmental Change
  • Volume: 40
  • Year: 1996
  • Authors:
    • Mikkelsen, R. L.
    • Parsons, J. E.
    • Gilliam, J. W.
  • Source: Buffer Zones: Their Processes and Potential in Water Protection
  • Year: 1996
  • Summary: riparian Buffers have been proven to be very effective in the removal of sediment associated nitrogen from surface runoff and nitrate from subsurface flows. In both surface and subsurface flows, hydrologic characteristics are the key to determining how effective the buffer will be in nitrogen removal. Even though buffers are extremely important to minimise entry of non-point source nitrogen into surface waters and removals of 90% are common, they do not work well in some hydrologic conditions.
  • Authors:
    • Yoo, K. H.
    • Shirmohammadi, A.
    • Yoon, K. S.
    • Rawls, W. J.
  • Source: Journal of Environmental Science and Health . Part A: Environmental Science and Engineering and Toxicology
  • Volume: 31
  • Issue: 3
  • Year: 1996
  • Summary: A Continuous version of distributed parameter model, ANSWERS (ANSWERS 2000) was applied to a field-sized watershed planted to cotton in the Limestone Valley region of northern Alabama. The field was cultivated for three years with conventional tillage followed by three years of conservation tillage. Overall, the ANSWERS model simulated runoff and nutrient losses in surface runoff within an acceptable range for the conventional tillage system conditions in continuous simulation mode. But the sediment losses predicted by ANSWERS were initially on the order of fifteen times or more higher than measured regardless of tillage systems. In order to duplicate measured data, the sediment detachment coefficient of rainfall and flow had to be reduced for calibration. The model poorly predicted soluble nutrient losses for the conservation tillage system due to the model's weakness in representing the surface application of fertilizer under this practice. The model simulates only one soil layer, in which soil moisture, nutrient concentration, and soil characteristics are assumed homogeneous. Currently, the model does not consider vertical nutrient concentration variation in soil profile. During the conservation tillage system, corn stalk and the residue of a winter cover crop were spread on the soil surface. However, the model did not properly represent surface spreading of crop residue, thus the model was unable to consider the organic-nitrogen contribution from crop residue to the erodible soil surface. This resulted in poor prediction of sediment-bound TKN, especially for conservation tillage system.
  • Authors:
    • Schulbach, K. F.
    • Jackson, L. E.
    • Wyland, L. J.
  • Source: The Journal of Agricultural Science
  • Volume: 124
  • Year: 1995
  • Summary: Winter non-leguminous cover crops are included in crop rotations to decrease nitrate (NO3-N) leaching and increase soil organic matter. This study examined the effect of incorporating a mature cover crop on subsequent N transformations. A field trial containing a winter cover crop of Merced rye and a fallow control was established in December 1991 in Salinas, California. The rye was grown for 16 weeks, so that plants had headed and were senescing, resulting in residue which was difficult to incorporate and slow to decompose. Frequent sampling of the surface soil (0-15 cm) showed that net mineralizable N (anaerobic incubation) rapidly increased, then decreased shortly after tillage in both treatments, but that sustained increases in net mineralizable N and microbial biomass N in the cover-cropped soils did not occur until after irrigation, 20 days after incorporation. Soil NO3-N was significantly reduced compared to winter-fallow soil at that time. A N-15 experiment examined the fate of N fertilizer, applied in cylinders at a rate of 12 kg N-15/ha at lettuce planting, and measured in the soil, microbial biomass and lettuce plants after 32 days. In the cover-cropped soil, 59% of the N-15 was recovered in the microbial biomass, compared to 21% in the winter-bare soil. The dry weight, total N and N-15 content of the lettuce in the cover-cropped cylinders were significantly lower; 28 v. 39% of applied N-15 was recovered in the lettuce in the cover-cropped and winter-bare soils, respectively. At harvest, the N content of the lettuce in the cover-cropped soil remained lower, and microbial biomass N was higher than in winter-bare soils. These data indicate that delayed cover crop incorporation resulted in net microbial immobilization which extended into the period of high crop demand and reduced N availability to the crop.
  • Authors:
    • De Jong, B. H. G.
    • Montoya-Gómez, G.
    • Nelson, K.
    • Soto-Pinto, L.
    • Taylor, J.
    • Tipper, R.
  • Source: Interciencia
  • Volume: 20
  • Issue: 6
  • Year: 1995
  • Summary: Although forestry and agroforestry are recognized as promising land-use alternatives for reducing the increasing levels of global atmospheric carbon, the viability of carbon sequestration projects at the land-user level has rarely been evaluated. We present the results of a feasibility study to: (1) evaluate the interest of local communities in a carbon sequestration project and how they would organize themselves for the proposed forestry project; (2) identify the carbon sequestration potential of the agroforestry/forestry systems that are both ecologically viable and preferred by local farmers (3) determine the social constraints of and potential for, such projects,- and (4) assess the economic potential of the carbon offsets estimated for such systems. This project was carried out by an interdisciplinary team of scientists and farmers in two ecological regions: the Tojolabal and Tzeltal zones of Chiapas, Mexico. Five systems with high carbon sequestration potential were considered technically and socially viable for each region. Initially, all participants will plant trees on an individual basis in their coffee plantation, fallow, and pasture lands, or in their maize fields. The estimated amount of carbon sequestered ranged from 46 7 to 236 7 tons of carbon per hectare (tC/ha). Net income benefits due to converting fields from maize cultivation to farm forestry ranged from $500-1000/ha depending on the value assigned to the sequestered carbon.1 Forests and farm woodlands that are sustainably managed have substantial economic and carbon sequestration potential. The principal barrier to communal forest management appears to be sociopolitical rather than economic. Because forest management requires long-term investments, good planning is essential and includes community control of projects, selection of appropriate tree species, and management techniques that are specific to the ecological and social conditions of the area. 1 All references to dollars in this report refer to U.S. dollars ($US).
  • Authors:
    • Li, C.
  • Source: Soil Management and Greenhouse Effect
  • Year: 1995
  • Authors:
    • Rowell, A. L.
    • Weinrich, K. B.
    • Barnwell, T. O.
    • Jackson, R. B.,IV
    • Patwardhan, A. S.
    • Donigian, A. S.
  • Source: Soil Management and Greenhouse Effect
  • Year: 1995