Thanh H. Dao

Soil physical properties and aggregate-associated C, N, and P distributions in organic and conventional cropping systems

Organic farming, which is growing in popularity, has been proposed as a sustainable alternative to conventional farming practices. However, it is not known how organic farming systems affect soil erosion risk and sediment-bound nutrient transport. Our objectives were to compare soil erosion risk and sediment bound nutrient transport potential for grain-based conventional and organic cropping systems by determining selected soil physical properties and distributions of carbon (C), nitrogen (N), and phosphorus (P) in soil aggregates of the 0-5 cm depth of a Christiana-Matapeake-Keyport soil association. We measured soil bulk density, aggregate stability, aggregate size distributions, and total C, N, and P associated with five soil aggregate size classes in no-till (NT) and chisel till (CT) systems and in an organic system (ORG). No-till soils had lesser bulk density and greater aggregate stability than did CT and ORG soils. Carbon, N, and P concentrations were greater in large (>2.00 mm) and small macroaggregates (0.21 to 2.00 mm) than in microaggregates (<0.21 mm) regardless of cropping system. When nutrient concentrations were combined with aggregate distribution data, the quantity of aggregate associated nutrients was greatest in microaggregates in ORG and CT soils but greatest in macroaggregates in NT soils. These results indicate an increased risk of sediment associated nutrient transport from ORG and CT soils compared with NT soils, since microaggregates in these soils are preferentially lost through sediment transport. The NT cropping system promoted macroaggregate formation and reduced the risk of particulate nutrient transport in this warm, humid region soil.

Comparison of release and transport of manure‐borne Escherichia coli and enterococci under grass buffer conditions

Aim:  To test the hypothesis that Escherichia coli and enterococci bacteria have similar release rates and transport characteristics after being released from land‐applied manure.

Mineralizable carbon, nitrogen, and water-extractable phosphorus release from stockpiled and composted manure and manure-amended soils

Much of land-applied manure is surface-broadcast or incorporated to a shallow depth of fields under crop Dissolved N and P transfer to runoff water may increase with residue and conservation tillage management because of surface applications and shallow soil incorporation of animal manure. Information is needed regarding water-extractable nutrient release the need to conserve stored soil water for crop producduring manure decomposition to quantify that potential transfer to tion (Unger and Parker, 1976; Dao and Nguyen, 1989; runoff in permanent pastures and conservation tillage systems. ReDao, 1993; Schwartz et al., 2002). Incorporating manure lease of net mineralizable C (MIN_C), net mineralizable N (MIN_N), using inversion or disk tillage may improve the effiand dissolved reactive P (DRP) was determined in stockpiled and ciency of manure nutrient utilization by crops, but intencomposted cattle (Bos taurus) manure and manure-amended soils at sive tillage often eliminates the benefits associated with 4, 20, and 35 C for 322 d at about 60% water-filled pore space. Flushes conservation tillage practices such as reduced soil water of CO2–C exceeding 100 mg kg 1 d 1, inorganic N, and DRP were evaporation, erosion and runoff control, and soil C storreleased rapidly from both manures when incubated alone or as soil age (Jones et al., 1985; Gilley et al., 1997; Dao, 1993, amendments. Dissolved P release varied inversely with sorption ca1998; Allmaras et al., 2000). Large quantities of stockpacity and degree of P saturation in an Aridic Paleustalf and Torrertic Paleustoll. Net mineralizable C, MIN_N, and DRP flux densities were piled cattle and poultry manure or manure mixed with lognormally distributed during the 322-d incubation. Results from the bedding materials are also applied to Conservation Relognormal modeling approach suggest that incubations needed to be serve Program grasslands and pastures (Marshall et al., performed only for as long as needed to attain the 50% maximal flux 2001). As a result, manure particulates remain on the density beyond the maximum to predict MIN_C, MIN_N, and DRP soil surface, causing elevated flow-weighted NH4–N and release flux density distributions. Significant nonlinear relationships DRP concentrations in runoff after manure applications exist between ln(cumulative CO2–C) and inorganic N or DRP and (Pierson et al., 2001). Water-extractable manure nutrihave an inflexion point between 14 and 20 d. The nonlinearity of the ents may directly discharge to surface waters or build C-to-N and C-to-DRP relationships indicates multiple substrate pools up at the soil surface and increase the potential for N and supports the use of lognormal distributions to describe MIN_C, and P losses and contamination of surface and ground MIN_N, and DRP release from manures and manure-amended soils and to shorten laborious incubations. water via surface and subsurface transport mechanisms (Liebhardt et al., 1979; Sharpley et al., 1993; James et al., 1996; Jansen et al., 2000; Zhang et al., 2002). Information regarding decomposition and particulate L production in the United States increasnutrient mineralization in manures at the soil surface is ingly occurs in confined animal feeding operations needed. Insights into the decomposition process may (CAFOs). Large numbers of animals are gathered in a be gained from the knowledge of transformations that relatively small land area where huge quantities of nutrioccur during the composting of animal manure to which ents in feeds are imported to support the operations of no additional materials (i.e., crop residues, wood chips, dairies and poultry (Gallus gallus domesticus), swine saw dust, etc.) have been added. The N fraction has been (Sus scrofa domesticus), and beef cattle production facilextensively studied to predict the N-supplying capacity ities (CAST, 1996). Large-scale CAFOs faced with inof composts (Gale and Gilmour, 1986; Hadas and Portcreasing pollution potential are managing manure as a noy, 1994; Thomsen and Olesen, 2000). Manure and waste product (Natl. Res. Counc., 1993; CAST, 1996). manure compost MIN_N is associated with proteins and This results in intensive land applications of manure in is thus strongly correlated with N released by digestion the immediate vicinity of the feedlots because of the with pepsin (Castellanos and Pratt, 1981). Composihigh cost of transporting manure for distances greater tional differences, primarily in total C and N between than about 30 km. Such management practices have manures and composts, result in significant differences resulted in nutrient-loaded soils in agricultural fields in manure and compost MIN_N. A linear relationship near CAFOs (Zhang et al., 2002). was observed between MIN_N and MIN-C as percentAnimal manure has long been used as an organic age of added total N and C during the first 4 wk of source of plant nutrients and organic matter to improve incubation of 10 manures and composts added to a Typic the physical and fertility conditions of agricultural lands. Abbreviations: CAFO, confined animal feeding operation; CM, comT.H. Dao, USDA-ARS, AMBL, BARC-East, Beltsville, MD 20705posted manure; DRP, dissolved reactive phosphorus; flux densitymax, 2350; and M.A. Cavigelli, USDA-ARS, SASL, 10300 Baltimore Ave., maximal flux density; MIN_C, net carbon mineralized; MIN_N, net Beltsville, MD 20705-2350. Received 14 June 2002. *Corresponding nitrogen mineralized; PDF, probability density function; S-1, first firstauthor (thdao@anri.barc.usda.gov). order reaction; S-2, second first-order reaction; SM, stockpiled manure; WFPS, water-filled pore space. Published in Agron. J. 95:405–413 (2003).

Response of coliform populations in streambed sediment and water column to changes in nutrient concentrations in water

As sediments increasingly become recognized as reservoirs of indicator and pathogen microorganisms, an understanding of the persistence of indicator organisms becomes important for assessment and predictions of microbial water quality. The objective of this work was to observe the response of water column and sediment coliform populations to the change in nutrient concentrations in the water column. Survival experiments were conducted in flow-through chambers containing sandy sediments. Bovine feces were collected fresh and introduced into sediment. Sixteen days later, the same fecal material was autoclaved and diluted to provide three levels - 1×, 0.5×, and 0.1× of nutrient concentrations - spike in water column. Total coliforms, Escherichia coli, and total aerobic heterotrophic bacterial concentrations were monitored in water and sediment. Bacteria responded to the nutrient spike with initial growth both in the water column and in sediment. The response of bacterial concentrations in water column was nonlinear, with no significant changes at 0.1 and .5× spikes, but a substantial change at 1× spike. Bacteria in sediment responded to the spikes at all added nutrient levels. Coliform inactivation rates both in sediment and in water after the initial growth occurred, were not significantly different from the inactivation rates before spike. These results indicate that introduction of nutrients into the water column results in nonlinear response of E. coli concentrations both in water and in sediments, followed by the inactivation with the same rate as before introduction of nutrients.

Insoluble Fe-associated inorganic and organic phosphates in animal manure and soil

Identifying and quantifying insoluble and reducible phosphorus (P) species in animal manure and soil are important issues in environmental P chemistry. We applied a joint dithionite-3-phytase incubation at pH 5.0 (100 mM acetate buffer) to investigate reducible P in animal manure and soil. After removal of soluble phosphate by 100 mM acetate (pH 5.0), poultry litter and dairy manure solids were incubated with the reducing agent sodium dithionite and/or 3-phytase to identify insoluble manure P species. Poultry litter contained most (spontaneous) labile P (9.7% insoluble P), reducible inorganic P (3.2% insoluble P), non-Fe-associated (reducible-irrelevant) organic P (5.7% insoluble P), and Fe-associated (reducible) organic P (16.2% insoluble P). In dairy manure, 51.5% insoluble P was most labile P, 28.1% non-Fe-associated organic P, and 20.4% Fe-associated organic P, but no reducible inorganic P. In a sandy loam soil, only Fe-associated inorganic P (682 mg kg−1 of dry soil) was observed compared to the original 2 mg soluble P kg−1 of dry soil). These data indicate that changes in the chemical and biological environments of manure and soil, such as anaerobic conditions and enzymatic activity, can influence manure P solubility and therefore bioavailability.

Short‐Term Effect of Lime, Phosphorus, and Iron Amendments on Water‐Extractable Lead and Arsenic in Orchard Soils

Abstract Lead arsenate was extensively used to control insects in apple and plum orchards in the 1900s. Continuous use of lead arsenate resulted in elevated soil levels of lead (Pb) and arsenic (As). There are concerns that As and Pb will become solubilized upon a change in land use. In situ chemical stabilization practices, such as the use of phosphate‐phosphorus (P), have been investigated as a possible method for reducing the solubility, mobility, and potential toxicity of Pb and As in these soils. The objective of this study was to determine the effectiveness of calcium carbonate (lime), P, and iron (Fe) amendments in reducing the solubility of As and Pb in lead‐arsenate‐treated soils over time. Under controlled conditions, two orchard soils, Thurmont loam (Hapludults) and Burch loam (Haploxerolls), were amended with reagent‐grade calcium carbonate (CaCO3), iron hydroxide [Fe(OH)3], and potassium phosphate (KH2PO4) and incubated for 16 weeks at 26°C. The experimental results suggested that the inorganic P increased competitive sorption between H2PO4 − and dihydrogen arsenate (H2AsO4 −), resulting in greater desorption of As in both Thurmont and Burch soils. Therefore, addition of lime, potassium phosphate, and Fe to lead‐arsenate‐contaminated soils could increase the risk of loss of soluble As and Pb from surface soil and potentially increase these metal species in runoff and movement to groundwater.

PHOSPHORUS FRACTIONS AND DYNAMICS AMONG SOIL AGGREGATE SIZE CLASSES OF ORGANIC AND CONVENTIONAL CROPPING SYSTEMS

Particulate nutrient transport from agricultural fields contributes to water quality degradation; however, insufficient data exist about potential bioactive P transport from erosion. Traditional measures of soil P such as total P or soil test P do not adequately assess the risk of water quality degradation due to particulate P because these measures do not necessarily reflect soil P bioactivity. To better understand the risk of water quality degradation due to sediment-associated P, we quantified the bioactive P fractions in five aggregate size classes from conventional no-till and chisel-till cropping systems as well as a tilled organic cropping system. Cropping system was not related to the concentration of bioactive P fractions in whole soils. However, aggregate size was related to bioactive P fraction concentrations. In general, macroaggregates had greater concentrations of bioactive P than did microaggregates and silt- and clay-sized particles across all cropping systems; the less than 0.053 mm aggregate size class had the lowest concentrations of bioactive P. To better understand the dynamics of these bioactive P fractions over time, we conducted an incubation study on the aggregates of the no-till system and quantified the bioactive P changes over time. Water-extractable P and complexed inorganic P concentrations did not change significantly during the incubation. However, complexed organic P concentrations increased up to 2-fold during the 56-day incubation. Using soil bioactive P fraction measurements from easily erodible aggregate sizes should aid in water quality degradation risk assessments. However, our research demonstrates the need to quantify the parameters that affect the dynamics of bioactive P fractions.

Mineralizable phosphorus, nitrogen, and carbon relationships in dairy manure at various carbon-to-phosphorus ratios

Phosphorus (P) in animal manure can be an important nutrient for crops or an environmental contaminant if in excess. Organic P in dairy manure may add to the environmentally bioactive P pools upon mineralization. A 353d incubation study of manures containing C:P between 83 and 130:1 was conducted to determine linkages between C and P transformations and the effects of C:P on the immobilization-mineralization of manure P. As C:P widened from 83:1 to 130:1, P mineralization increased and phosphate accumulated at rates between 0.013 and 0.021mgkg(-1) d(-1). Water-extractable C was positively correlated with N:P, particularly at narrow C:P (P<0.001). Absence of a negative feedback by phosphate suggested that P mineralization occurred with degradation of organic P-containing C substrates and appeared incidental to microbial P needs. Carbon content in manure may be managed to lower risks of elevated soluble P and C losses under non-limiting N conditions.

Loss of bioactive phosphorus and enteric bacteria in runoff from dairy manure applied to sod

Information on the concurrent release and interactions between manure-borne phosphorus (P) and enteric bacteria to runoff from a live or dead grass sod is limited. A study of simulated runoff and an enzyme-based fractionation of runoff P forms from dairy manure applied on grass-covered soil in runoff boxes was conducted to compare the detachment and potential edge-of-field movement of manure P, Escherichia coli, and enterococci in runoff. Concentrations and mass loads of bioactive P forms and bacteria in runoff were log-normally distributed over time during all simulations. Although P and enteric bacteria were simultaneously released to runoff, high correlations were found predominantly between water turbidity, concentrations of bacteria, and phosphohydrolase-labile P, a fraction associated with particulate manure. Delayed bacteria and particulate P concentrations and mass loads indicated live leaf and bacterial surface interactions that impeded their release to runoff. Resultant deviations in linearity between manure water-extractable P and bacteria distributions and the significant correlation between bacteria and the phosphohydrolase-labile P fraction suggested that manure-borne E. coli were released in association with manure particulates that contained organic P. The state of the grass cover determined the asymmetry of bacteria and bioactive P distributions. Given the micrometer size range of suspended particles, losses of colloidal particulate P and colloid-associated bacteria may extend well beyond the immediate vicinity of the deposited manure.

REMEDIATION TECHNIQUES FOR MANURE NUTRIENT LOADED SOILS

Many soils in the United States contain excessive levels of nutrients, especially phosphorus (P), due to repeated heavy applications of animal manure. Also, soils with a history of long-term poultry litter or swine manure applications have elevated levels of copper (Cu), zinc (Zn), selenium (Se), and arsenic (As). Runoff and eroded soils carry dissolved and sediment-associated nutrients to water bodies and degrade their quality. Manure-treated fields can also impair air quality by emitting odorous compounds and dust. Several best management practices (BMPs) have the potential to reduce nutrients in runoff water and loading to surface waters. The BMPs were grouped into two broad categories: (1) technologies to reduce excessive nutrient levels in the soil, and (2) technologies to reduce edge of field discharges of nutrients via runoff or sediment loss from overapplication of manure or other organic biosolids. Potential remedial approaches for nutrient-loaded soils include: • Phytoremediation (P, nitrate, metals) with plant species that preferentially bioaccumulate nutrients or metals and use of deep-rooted crops in novel rotations for subsurface nitrate-N recovery; • Soil and manure amendments with P immobilization chemicals and municipal or industrial byproducts to reduce dissolved reactive P and metal bioavailability (water treatment residuals, aglime, coal combustion by-products); • Addition of soil aggregation promoters, coagulants or flocculants such as polyacrylamide polymers to reduce sediment and particulate nutrient offsite discharges (organic matter, N, P, metals); • Deep mechanical tillage to dilute near-surface zone elevated nutrient concentrations and reduce odor emissions (P, metals, odor, trace greenhouse gases); and • Conservation buffer strips to remove dissolved reactive P from runoff and reduce edge-of-field losses of sediments and particulate nutrients and metals. Growing high biomass-yielding plants can remove large amounts of nutrients and may be a promising remedial strategy to export and reduce excess soil nutrients. Bermudagrass and certain warm-season annual grasses produce large dry matter yields, and thus take up large quantities of applied nutrients. Cool-season grasses and certain legumes have a higher uptake of certain nutrients, such as P, and may remove more specific nutrients than bermudagrass, although their yield potential is not as high. Various plant species, including Brassica, preferentially concentrate Cu, Se, and As from high metal soils. Using forage to extract P and specific metals in problem soils has been an effective approach, but is slow to lower soil levels. Grazed-only systems will not effectively remove nutrients from an over-application site since most of the applied nutrients, especially P and K, are redeposited on the land during grazing. Research using soil amendments has shown that land application of drinking water treatment residuals potentially reduces dissolved P in runoff water by up to 70% from land with excessive levels of soil test phosphorus. Other materials such as fly ash and flue-gas desulfurization products from coal combustion in electric power generation and aglime are readily available and also effec-tively reduce P solubility by up to 98% in manure and manured soils. The reactivity of fly ash components with manure P suggests that co-blending will result in reduced discharges upon land application of treated manure, and amending high P soils with coal combustion by-products can reduce soil P availability and the environmental impact of recycling manure on agricultural lands. Reducing particulate nutrient transport from nutrient-loaded fields depends heavily upon soil erosion control practices. The most widely studied and used methods to control erosion by water and wind involve a variety of conservation tillage and crop residue management methods for the wide range of soils and climatic conditions. When used in combination with metal salts, water treatment polymeric flocculants are a promising component of an effective set of management tools to decrease sediment and sediment-associated nutrient loss. Land management practices such as deep tillage and conservation buffers also provide relief from offsite discharges and reduce the ecological risks of the excessive nutrient levels. Many remedial technologies exist to reduce the environmental impact of agricultural land with excessive nutrient levels following repeated applications of manure or organic by-products. Emerging technologies for nutrient immobilization and alternative nutrient recovery using chemical barriers at the research and exploratory stages are being developed into practical BMPs. To further advance soil remediation research and technology transfer, we feel that areas of critical needs should include urgent efforts to: • Identify and develop efficient nutrient and metal accumulator plants and profitable crop rotations for efficient nutrient and metal removal, • Identify and develop efficient nutrient immobilizing chemicals and municipal or industrial byproducts for manure-derived P and metals, • Identify and develop soil treatment and recovery technologies to produce value-added specialty products, • Develop and apply geo-reference techniques to target remediation at the field and watershed scales, and • Develop and evaluate the effectiveness of specific BMP systems in reducing manure nutrient export to the surrounding environment. Integrated solutions are needed for managing excess manure nutrients in crop and livestock production systems. A combination of load reduction techniques and structural and cultural practices may be required to effectively balance the need to reduce soil nutrient levels and discharges from nutrient-loaded fields with the benefits of sustainable production of food and fiber.