G. A. Peterson

Fungal translocation as a mechanism for soil nitrogen inputs to surface residue decomposition in a no-tillage agroecosystem

Additions of ( 15 NH4)2SO4 to the soil inorganic nitrogen (N) pool were used to measure rates of N flux from the mineral soil to surface-applied wheat straw decomposing in intact soil cores collected from a no-tillage (NT) field. Half of the soil cores were treated with a fungicide to reduce fungal populations. Fungicide application significantly reduced fungal biomass, decomposition rates, and net N immobilization in surface residues. Net N immobilization over the study period was estimated to be 1.5 and 0.9 gNm ˇ2 for untreated and fungicide-treated residues, respectively. The rate of 15 N transfer averaged 13.4 mg 15 Ng ˇ1 residue d ˇ1 for untreated wheat straw. Fungal inhibition reduced 15 N flux by 59‐78%, reductions of similar magnitude to those observed for fungal biomass. Nitrogen transfer in sterilized soil cores accounted for only 7.8% of the total upward N transport in control cores, indicating that abiotic processes did not contribute substantially to N flux. We estimate a total annual fungalmediated N flux of 2.4 g m ˇ2 , which is nearly equivalent to the N immobilization potential predicted, based on initial N and lignin content, for the wheat straw used in this study. We conclude that fungal N translocation is a significant mechanism for soil N input and can account for the observed net N immobilized by surface residues decomposing in the field. Both residue quality and N availability appear to be important controls on fungal biomass associated with surface residues and rates of soilto-residue N translocation. 7 2000 Elsevier Science Ltd. All rights reserved.

Cropping intensification in dryland systems improves soil physical properties: regression relations

Abstract Great Plains dryland agriculture is a risky venture because of large annual fluctuations in precipitation and high evaporation potentials. Water capture is limited by low water infiltration rates because many of our soils have relatively small aggregate size distributions, which limit infiltration, and are also susceptible to crusting and sealing. No-till management has permitted cropping intensification, which via improved water storage, has increased crop residue returned to the soil, decreased surface bulk density, and increased surface soil porosity. Our objective was to quantify the relationship between crop residue biomass generated by cropping system intensification and the physical properties of the surface soil (0–2.5-cm depth). This study was conducted within an existing long-term dryland experiment consisting of three sites in eastern Colorado that transect an evapotranspiration gradient. Each site transects a soil catena with three distinct soils arranged along a slope gradient. Only soils at the summit and toe slopes were sampled for this study. Soils are Argiustolls and Ustochrepts. Three no-till cropping systems, Wheat–Fallow (WF), Wheat–Corn–Fallow (WCF), and Continuous Cropping (CC), were sampled in the summer of 1998 after the cropping systems had been in place for 12 years. Bulk density, effective porosity, aggregate size distribution, sorptivity, and soil aggregate organic C content were measured at the surface 2.5 cm of the soil in each cropping system at the two soil positions at each site. Bulk density was reduced by 0.01 g cm −3 for each 1000 kg ha −1 of residue addition over the 12-year period. Each 1000 kg ha −1 of residue addition increased effective porosity by 0.3%. Increases in macroaggregation were associated with linear increases in the C content of the aggregates; each g kg −1 of organic C in the macroaggregates increased the proportion of macroaggregates by 4.4%. Implementation of no-till intensive cropping systems under this semiarid environment increased residue biomass, which has ultimately increased effective porosity, and thus water capture potential was increased.

Nitrogen transformations, utilization, and conservation as affected by fallow tillage method

While we know that fallow tillage method affects water conservation and soil erosion control, effects on soil and fertilizer N dynamics are less well known. In this paper, we summarize results from our investigations of N transformation and cycling for two experiments established in 1969 and 1970 near Sidney, NE. In this research, the effects of three fallow tillage methods (moldboard plow, sub-till, and no-till) on changes in soil properties, N dynamics, and subsequent crop growth were studied. One experiment was on land, that had been broken from sod before 1920, seeded to crested wheatgrass [Agropyron cristatum (L.) Gaertn.] in 1957, and cultivated again since 1967 with the above three fallow tillage methods. Wheat (Triticum aestivum) was grown in a wheat-fallow sequence with and without 45 kg fertilizer N ha ˇ1 . The second experiment was on land that was in native mixed prairie sod until 1969, and included a comparison of the three above tillage methods in a wheat-fallow system with plots remaining in native sod. During the first 13 years of study, total soil N loss from the surface 30 cm of soil (compared to the native sod) was only about 3% for the no-till system, compared to 8 and 19% for the sub-till and plow systems, respectively. For both experiments, considerable mineralization and nitrification of organic N occurred during the fallow period, with greatest rates for plowed fallow. No-till immobilized more labeled and total N in soil organic matter and in visible and partially decomposed crop residues on and near the soil surface. However, by harvest of the second crop grown after Nlabeled fertilizer N was applied, little if any of the labeled N was found in visible and partially decomposed crop residues or as soil inorganic N, suggesting that most of the fertilizer N applied was immobilized in soil organic N or was lost. Deep sampling (to 15 m) of the Native Sod plots showed that several hundred kg of NO3‐N ha ˇ1 had leached beneath the crop root zone, presumably during wet years after fallow. For plowed plots, the quantity of NO3‐N beneath the root zone approximated the loss in total soil N from the root zone, suggesting there was a little net loss of soil N by denitrification or ammonia volatilization. For no-till, quantities of NO3‐N beneath the root zone exceeded the loss of total soil N during cultivation, suggesting significant N-fixation occurred by unknown mechanisms. These results show that fallow tillage system does affect soil and fertilizer N cycling and transformations and the availability of N to crops. # 1998 Published by Elsevier Science B.V. All rights reserved.

Modelling climate, CO2 and management impacts on soil carbon in semi-arid agroecosystems

In agroecosystems, there is likely to be a strong interaction between global change and management that will determine whether soil will be a source or sink for atmospheric C. We conducted a simulation study of changes in soil C as a function of climate and CO2 change, for a suite of different management systems, at four locations representing a climate sequence in the central Great Plains of the US.Climate, CO2 and management interactions were analyzed for three agroecosystems: a conventional winter wheat-summer fallow rotation, a wheat-corn-fallow rotation and continuous cropping with wheat. Model analyses included soil C responses to changes in the amount and distribution of precipitation and responses to changes in temperature, precipitation and CO2 as projected by a general circulation model for a 2 × CO2 scenario.Overall, differences between management systems at all the sites were greater than those induced by perturbations of climate and/or CO2. Crop residue production was increased by CO2 enrichment and by a changed climate. Where the frequency of summer fallowing was reduced (wheat-corn-fallow) or eliminated (continuous wheat), soil C increased under all conditions, particularly with increased (640 μL L−1) CO2. For wheat-fallow management, the model predicted declines in soil C under both ambient conditions and with climate change alone. Increased CO2 with wheat-fallow management yielded small gains in soil C at three of the sites and reduced losses at the fourth site.Our results illustrate the importance of considering the role of management in determining potential responses of agroecosystems to global change. Changes in climate will determine changes in management as farmers strive to maximize profitability. Therefore, changes in soil C may be a complex function of climate driving management and management driving soil C levels and not be a simple direct effect of either climate or management.

Efficient and Environmentally Safe Use of Micronutrients in Agriculture

Abstract Micronutrient deficiencies in plants are common throughout the world. Many inorganic and organic fertilizers are manufactured and applied to soils and plants to correct micronutrient deficiencies. The objective of this article is to review current research and present our findings on factors that affect micronutrient fertilizer efficacy. In the United States, state laws regulate the “guarantee analysis” of fertilizers and essentially all states require that manufacturers meet a total analysis requirement. However, there is no consideration of micronutrient “availability” in this analysis. Our research has shown that Zn availability in granular fertilizer was related to water solubility (r2=0.92) and not total Zn content. The relative availability coefficients for organic and inorganic Zn fertilizers was also highly related to water solubility and independent of total Zn content. Because of this discrepancy, there are fertilizers in the marketplace that are very low in plant available micronutrients. In addition, some fertilizer manufacturing feedstocks contain heavy metals, which are transferred to the fertilizer and cause regulatory limits for heavy metal content to be exceeded. Canada has set limits for nine heavy metals: As, Cd, Co, Hg, Mo, Ni, Pb, Se, and Zn. Four states in the United States have also set limits for some heavy metals. Micronutrient fertilizer manufacturing and “guarantee analysis” should require certification to ensure that products reaching the marketplace are plant available and environmentally safe.

Continuous Dryland Cropping in the Great Plains

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Mobility of Organic and Inorganic Zinc Fertilizers in Soils

Abstract Zinc sulfate (ZnSO4 · H2O) has traditionally been the “reliable” source of zinc (Zn) fertilizer, but other sources of Zn are also available. Some are derived from industrial by‐products, varying from flue dust reacted with sulfuric acid to organic compounds derived from the paper industry. The degree of Zn mobility in Zn sources derived from these various by‐products is related to the manufacturing process, the source of complexing or chelating agents (organic sources), and the original product used as the Zn source. Many claims are made regarding the relative efficiency of traditional inorganic Zn fertilizers and complexed Zn sources. The objective of this column study was to compare the mobility of several commercial Zn fertilizer materials (organic and inorganic) that are commonly used to correct Zn deficiencies in soils. The sources included three granular inorganic Zn sources, two granular organically complexed Zn sources, and liquid ZnEDTA. Soil columns were leached five times with deionized water. Leaching events were separated by approximately 48 h. At the conclusion of the leaching phase, columns were analyzed for plant‐available Zn. Water solubility was the primary factor affecting Zn movement, not total Zn content or organic complexation of the fertilizers. The Zn sources evaluated can be separated into three groups: ZnEDTA, ZnLigno, and ZnSO4 were the most mobile Zn sources; the ZnOx55 was less mobile, but seemed mobile enough to meet crop needs; ZnOx26 and ZnSuc were relatively immobile Zn sources.

Climatic Gradient, Cropping System, and Crop Residue Impacts on Carbon and Nitrogen Mineralization in No‐Till Soils

Abstract In the West Central Great Plains of the United States, no‐till management has allowed for increased cropping intensity under dryland conditions. This, in turn, has affected the carbon (C) and nitrogen (N) mineralization dynamics of these systems. In this region, moisture stress increases from north to south due to an increase in evapotranspiration (ET), resulting in a climatic gradient that affects cropping system management. The objectives of this study were to determine the interaction of cropping system intensification and climatic gradient (ET) on C and N mineralization and to determine if the presence or absence of crop residue on the soil surface affects C and net N mineralization. Two cropping systems, winter wheat‐fallow (WF) (Triticum aestivium L.) and winter wheat‐corn (sorghum)‐millet‐fallow (WCMF) [Zea mays (L.), Sorghum bicolor (L.) Moench, Panicum milaceum (L.)] were studied at three locations across this aforementioned ET gradient. The treatments had been in place for 8 yrs prior to sampling in the study. These results showed that the more intense cropping system (WCMF) had a higher laboratory C mineralization rate at two of the three locations, which the study concluded resulted from larger residue biomass additions and larger quantities of surface residue and soil residue at these locations (Soil residue is defined as recognizable crop residue in the soil that is retained on a 0.6 mm screen). However, no differences in N mineralization occurred. This is most likely due to more N immobilization under WCMF as compared to WF. Presence or absence of crop residue on the surface of undisturbed soil cores during incubation affected potential C and net N mineralization more than either cropping system or location. Soil cores with the surface residue intact mineralized as much as 270% more C than the same soils where the surface crop residue had been removed. In laboratory studies evaluating the relative differences in cropping systems effects on C and N mineralization, the retention of crop residue on the soil surface may more accurately access the cropping system effects.

Conserving and Optimizing Limited Water for Crop Production

Proper soil and crop management systems are critical for sustainable production in semi-arid environments, and there are principles that apply to both dryland (non-irrigated) and limited-irrigation systems. In the non-irrigated environment, crop residue from no-till systems permits more diverse crop rotations with less frequent fallow, which leads to increased precipitation-use efficiency. Soil improvements from no-till systems include decreased soil erosion, increased soil organic matter, improved soil structure, and increased infiltration rate. These soil improvements create a positive feedback loop by making more water available to the crop, increasing yields, and returning more crop residues to the soil. In the Great Plains of the United States, we have found that annualized grain production from no-till systems with less frequent fallow can be increased by 75% with an increase in economic return from 13% to 36% compared with the traditional wheat-fallow cropping system. There is also a need to increase water productivity for irrigated crop production because of competition for water by municipal and industrial users, drought, and declining groundwater supplies. The adoption of cropping systems that use less water and insure economic sustainability must be developed. We have found that limited-irrigation practices that time irrigations with critical growth stages can reduce water use of corn by 50% while reducing yields by only 30%. Alfalfa was found to have great potential for limited irrigation because of its natural drought tolerance and perennial growth habit. Many of the principles of water-conservation practices identified in the United States are adaptable to Indias conditions. Soil and crop management systems that use less water and that are sustainable and economically viable in Indias limited water environment must be developed.

Productivity of Great Plains Soils: Past, Present, and Future

Soils are products of the interaction of climate, organisms, time, topography, parent material, and human intervention in management. Soil properties of the Great Plains have been predominantly shaped by the climate variable. However, cultivation and grazing pressures in the region have been significant factors in the most recent 150 years. Soil management, as it interacted with climate and soils, has affected the region’s productivity; the effects of those management practices will continue to affect soil productivity in the future.