Donald A. Graetz

Fate of phosphorus in Florida Spodosols contaminated with cattle manure

Abstract Phosphorus loading from dairies and beef ranches in the Lake Okeechobee watershed and the subsequent movement of the P into the drainage waters is a major factor influencing the eutrophication of Lake Okeechobee. The soils of this area are mainly Spodosols with the watertable lying between surface and spodic horizons for extended periods each year. In this study, the quantity of total P (TP) within the soil profile (A, E, Bh and Bw horizons) of dairies and beef ranches in the Lake Okeechobee basin was determined to evaluate the magnitude of P loading in these soils. The effect of cattle density was evident in TP concentrations throughout the soil profile. In the A horizon, mean TP concentrations were 1680, 165, and 34 kg P ha −1 for high, low, and nonimpacted areas, respectively. The same trend, although at lower concentrations, was evident in the E, Bh, and Bw horizons. The quantity of P considered to be potentially mobile under leaching conditions (water-soluble P, Mehlich I extractable or NH 4 Cl extractable), also followed similar trends as the TP concentrations. Based on chemical fractionation data, nearly 80% of TP in the A horizon of the highly impacted soils may be considered leachable. We calculated hat about 4000 kg P ha −1 would be available for leaching in the soil profile of the high intensity areas immediately adjacent to the dairy barns. This “labile P” appears to be solubilized slowly over a long period of time (likely several years). There seems to be no natural mechanism whereby the P is stabilized through the formation of minerals, and even if such processes do take place, a vast amount of P still remains in a form in which it is readily transported along with the drainage water. The A and E horizons had poor P retention capacities while the retention capacity of the Bh horizon varied with the soil type, Myakka ≥ Immokalee > Pomello. Due to the Low P-retention capacity in the upper horizons of these soils, there is potential for significant subsurface lateral P transport.

Agroforestry as an approach to minimizing nutrient loss from heavily fertilized soils: The Florida experience

Nutrient buildup in the soil caused by increased animal manure and fertilizer use in agricultural and forestry practices may increase the potential for their loss from the soil, leading to groundwater contamination and nonpoint source pollution. Studies in the tropics have suggested that agroforestry practices can reduce such nutrient (especially nitrogen) losses because of enhanced nutrient uptake by tree and crop roots from varying soil depths, compared to more localized and shallow rooting depths of sole crop stands. In temperate systems, such benefits have been well documented for riparian forest buffer practices. Currently, other temperate agroforestry practices are also being considered for their potential to reduce runoff and leaching of chemicals and thereby improve environmental quality within the agricultural landscape. In this regard, the ‘Florida P-Index,’ which considers both phosphorus transport characteristics and management practices, may be a useful tool in the evaluation of nutrient management practices and environmental benefits of agroforestry. Preliminary results from an alleycropping site and a silvopastoral site on two different soil types in Florida suggest that both of these agroforestry practices will likely reduce nutrient loss compared to conventional agricultural practices. The primary aspects of P-Index include consideration of transport factors such as soil erosion, soil runoff class, leaching potential, and distance from a water body along with management factors such as soil test P, P application method, and source and rate of P application. P-Index evaluation of these studies indicates that both agroforestry sites can be on a nitrogen-based nutrient management program. The relevance of some management practices such as application of manure vs. inorganic fertilizer is also discussed in light of the P-Index and the two agroforestry practices.

Denitrification potential of soils from constructed and natural wetlands

Laboratory experiments were conducted to determine the NO3− removal (reduction) potential of wetland soils currently used for the disposal of reclaimed wastewater. Surface soil samples were collected, incubated under anoxic conditions with two levels of NO3−, and the evolution of N2O was measured with time. Denitrification rates ranged from 0.06 to 0.92 g N m−2 day−1, when 10 mg NO3-N per kg oil was added. Complete reduction of NO3− to N2 occurred in the soils collected from two constructed wetlands (mineral soils) and one natural wetland (organic soil). However, the final reduction step was inhibited in the soils collected from two wetlands with organic soils. The inhibition was most likely the result of the lower pH of these soils. The results suggest that the addition of wastewater enhances the potential for denitrification in these soils.

Carbon and Nitrogen Dynamics in Wetland Soils

The pore space of wetland soils is generally filled with water rather than air as in well-drained soils. Oxygen is supplied to wetland soils by diffusion through the floodwater water and through transport via the vascular system of wetland plants. Under both conditions, oxygen is supplied more slowly than the potential consumption rate. These conditions result in the development of two distinctly different soil layers: (1) an oxidized or aerobic surface layer where oxygen is present, and (2) an underlying reduced or anaerobic layer in which no free oxygen is present (Pearsall, 1950).

Soil Phosphorus, Cattle Stocking Rates, and Water Quality in Subtropical Pastures in Florida, USA

Abstract Minimizing nonpoint source nutrient pollution is important to the sustainability of grazing lands. Increased nutrient loads have reduced water quality in Lake Okeechobee in south Florida, prompting establishment of a Total Maximum Daily Load (TMDL) that will require large reductions in phosphorus (P) runoff into the lake. A significant portion of this reduction must come from beef cattle ranches, the major land use in the region. A large-scale research project, consisting of a 420-ha array of 8 improved summer and 8 semi-improved winter pastures, was established from 1998–2003 to investigate the influence of beef cattle stocking rate on nutrient loads in surface runoff. Each pasture type had two replicates of four different cattle stocking rates including a control with no cattle and stocked pastures with low, medium, and high stocking rates (1.3, 1.0, 0.6 ha·AU−1 [animal unit] in summer pastures; 2.1, 1.6, and 0.9 ha·AU−1 in winter pastures). Cattle stocking rate did not affect nutrient concentrations or loads in surface runoff during the study period. Average annual P discharges were 1.71 kg·ha−1 from summer pastures and 0.25 kg·ha−1 from winter pastures. Average total P concentrations in runoff were 0.63 mg·L−1 for summer pastures and 0.15 mg·L−1 for winter pastures. Differences in runoff P were related to differences in soil P test results, a difference believed to be due to prior fertilization practices. Our findings show that reducing cattle stocking rates on beef cattle pastures is not an effective practice for reducing nutrient loads, and that accumulation of P in soil from historical fertilization has an overriding influence on P loads in surface runoff. Results indicate that reducing the overall volume of surface discharges would be a more effective strategy than altering cattle stocking practices to reduce nonpoint runoff of P from cattle pastures in this region.

SOIL DEVELOPMENT IN PHOSPHATE-MINED CREATED WETLANDS OF FLORIDA, USA

Soil characteristics of a wide variety of created wetlands were compared to those of native wetlands in phosphate-mined areas from central and north Florida, USA. Criteria selected for evaluation of soil samples from 184 sites included soil compaction, bulk density, organic matter (carbon) and nitrogen content, C∶N ratio, and available and total nutrient contents. Organic matter accumulation, one of the indicators of a functional wetland, increased across transects going from uplands toward the center of the wetlands, and with wetland age. The organic matter accumulation rate in the AO and A1 horizons was 320 g m2yr−1. Native wetlands had significantly greater organic matter accumulation, both in the litter and mineral soil surface. The C∶N ratio of the soil organic matter decreased with created wetland age and approached values commonly found in wetland soils (15–25). Bulk density decreased with increasing organic matter content in the created wetlands, and low bulk density soils appeared to support better vegetative growth. Based on the above-mentioned parameters, reclaimed wetlands are slowly developing into “typical” wetlands; the rate of development could possibly be increased by minimizing soil compaction, incorporation of organic matter, or by fertilization.

Uptake and distribution of metals by water lettuce (Pistia stratiotes L.)

Background, aim and scopeWater quality impairment by heavy metal contamination is on the rise worldwide. Phytoremediation technology has been increasingly applied to remediate wastewater and stormwater polluted by heavy metals.Materials and methodsLaboratory analysis and field trials were conducted to evaluate the uptake of metals (Al, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, and Zn) by an aquatic plant, water lettuce (Pistia stratiotes L.), and metal distribution in the plant.ResultsThe growth of water lettuce reduced Al, Fe, and Mn concentrations in water by >20%, K and Cu by >10%, and Ca, Mg, Zn, and Na to a lesser extent. A larger proportion of Ca, Cd, Co, Fe, Mg, Mn, and Zn was adsorbed or deposited on the external root surfaces while more Al, Cr, Cu, Ni, and Pb were absorbed and accumulated within the roots.DiscussionWater lettuce has a great ability in concentrating metals from its surrounding water with a concentration factor (CF) ≥102. The bio-concentration factor (BCF), which excludes the part on the root surfaces, is a more appropriate index than the CF for the differentiation of hyperaccumulation, accumulation, or non-accumulation plants for metals.ConclusionsWater lettuce is a hyperaccumulator for Cr, Cu, Fe, Mn, Ni, Pb, and Zn and can be applied for the remediation of surface waters.Recommendations and perspectivesFurther study on the bioavailability of metals in the water lettuce is needed for the beneficial use of metal-enriched plant biomass.

Development of compost-based media for containerized perennials

Growth of Bolivian sunset (Gloxinia sylvatica (HBK) Wiehler), Brazilian plume (Justicia carnea Lindl.), and golden globe (Lysimachia congestiflora) transplants was evaluated in media containing 25, 50, 75, or 100% compost (derived from biosolids and yard trimmings) as compared to commercial peat-based media. Compost-based media had higher pH, EC, bulk density, particle density and total porosity as compared to peat-based media. The effects of media composition on plant growth and development varied with each species tested. Gloxinia generally were smaller with reduced flower development when grown in compost-based media as compared to peat-based media. However, regardless of media composition, plants were of high visual color and quality. Justicia were similar in size or smaller when grown in compost-based media as compared to peat-based media but flower development was unaffected. However, the visual color and quality of the plants suffered when plants were grown in compost alone. Growth indexes of Lysimachia were similar among media or slightly reduced by 12%. Although, flower development was reduced by 16% in the second trial, plants were still acceptable in terms of visual color and quality, regardless of media composition.

Leaching of Nitrate, Ammonium, and Phosphate From Compost Amended Soil Columns

Compost amendment to agricultural soils has been reported to reduce disease incidence, conserve soil moisture, control weeds, or improve soil fertility. Application rate and placement of compost largely depends on the proposed beneficial effects and the rate may vary from 25 to 250 Mg ha−1 (N content up to 4 percent). Application of high rates of compost with high N or P levels may result in excessive leaching of nitrate, ammonium, and phosphate into the groundwater. Leaching could be a serious concern on the east coast of Florida with its inherent high annual rainfall, sandy soils and shallow water table. In this study, five composts (sugarcane filtercake, biosolids, and mixtures of municipal solid wastes and biosolids) were applied on the surface of an Oldsmar sand soil (in 7.5 cm diameter leaching columns) at 100 Mg ha−1 rate and leached with deionized water (300 ml day−1, for five days; equivalent to 34 cm rainfall). The concentrations of NO3-N, NH4-N, and PO4-P in leachate reached as high as 246, 29,...

Dicyandiamide as a nitrification inhibitor in crop production in the Southeastern USA

Abstract The yield response to N with and without the use of the nitrification inhibitor, dicyandiamide (DCD), was determined on the field crops, sweet corn, cotton, wheat, and grain sorghum; and on the vegetable crops, bell peppers, potato, and tomato, on a large number of different soils in Alabama, Florida, Kentucky, Mississippi, and Virginia. In general, DCD inhibited nitrification for several weeks but inhibiting nitrate formation was not often reflected in crop yield response to N. Increase in yield resulting from DCD use were found for sweet corn in Florida, grain sorghum in Alabama, and potato in one of two years in Florida. Results with cotton were erratic, sometimes resulting in increased, sometimes in decreased, yields. Use of DCD for wheat production in the Southeast did not prove to be advantageous. Generally when DCD use benefitted yields, this occurred at the lower N rates, indicating that preventing loss of N at the lower end of the response curve resulted in a measurable positive effect. ...