Neutral Ammonium Citrate Extraction of Biosolids Phosphorus

Abstract

Matching biosolids application rates to crop phosphorus (P) needs requires quantifying the P fertilizer replacement value of biosolids. Neutral ammonium citrate (NAC) extraction of P, used for assessing available P in mineral fertilizers, was evaluated for 35 different biosolids. Biosolids NAC‐P was not statistically different (p=0.05) from total P using strong acid digestion (EPA 3051‐P). High P recovery by NAC was attributed to dissolution of P‐containing iron (Fe)/aluminum (Al) oxides under the aggressive extracting conditions (0.88 M citrate at 65°C). Citrate effectively dissolves P‐binding Fe/Al hydrous oxides, the very components that reduce phytoavailability when biosolids are land applied. Greenhouse studies with pasture grass (Paspalum notatum Flugge) grown in P‐deficient soils amended with biosolids revealed P phytoavailability was not correlated (r²=0.10) with biosolids NAC‐P. Phytoavailability was inversely correlated (r²=0.66) with biosolids total Al+Fe content. The NAC extraction, designed for commercial fertilizers, is inappropriate for quantifying biosolids phytoavailable P.     

Examining the integrity of soil metal bioavailability assays in the presence of organic amendments to metal‐spiked soils

This paper questions whether the presence of biosolids amendment in metal‐spiked soils alters the outcome of soil‐based assays of metal bioavailability. The effects of biosolids amendment on the efficacies of six soil metal bioavailability assays (total recoverable, EDTA, Ca(NO₃)₂, soil solution, diffusive gradient in thin films and free ion activity) were assessed against metal concentrations in wheat shoots (Triticum aestivum) germinated in three contrasting soils, each previously incubated for either 2 weeks or 6 months following treatment with Cd, Cu, Ni and Zn +/? biosolids amendment. Overall, Ca(NO₃)₂ was the most accurate method to predict Cd (r² = 0.62), Ni (r² = 0.73) and Zn (r² = 0.55) bioavailability in soils and therefore was used to compare variations in responses between biosolids and nonbiosolids‐amended soils. Comparisons between these two groups revealed no significant differences in linear relationships for all four metals and soil types assessed. These findings not only support Ca(NO₃)₂ as a robust and valid method for determining soil metal bioavailability across metal matrices and soil types, but also that the presence of biosolids does not compromise the predictive power of this assay or any of the others examined.     

Phosphorus‐31–nuclear magnetic–resonance spectroscopy to trace organic dung phosphorus in a temperate grassland soil

Cattle dung contributes to hot‐spot inputs of nutrients to grassland systems, but not much is known about its organic P (P_o) composition and fate in the grassland soils. We used ³¹Phosphorus (P)–Nuclear Magnetic–Resonance (NMR) spectroscopy of alkaline soil extracts to examine potentials for tracing of different functional P_o forms into a temperate grassland soil amended with dung. The proportion of monoester, DNA‐diester, and phospholipid+teichoic acid P were comparable in dung extracts, but the soil was dominated by monoester P. The temporal trends in the DNA‐diester P–to–monoester P (D_DNAM) and diester P–to–monoester P (DM) ratio of dung, native soil, and soil amended with dung were monitored in the 70 d field experiment. The D_DNAM and DM ratio in the dung‐amended soil (0–1 and 1–5 cm depth) were always intermediate between the dung and (unamended) control soil. Clearly, extracted soil P was a mixture of incorporated dung‐derived P and native soil P. The dung‐P contribution in the 0–1 cm samples peaked at 47% of the total extracted P at day 70 and at 15% after 42 d in the 1–5 cm soil depth (based on the DM ratio). The proportions of dung‐derived P and C in the soil were positively correlated with: 1) topsoil, using the D_DNAM ratio (r² = 0.975), and 2) top‐ and subsoil, using the DM ratio (r² = 0.656). We concluded that our D_DNAM and DM‐P ratios approach (obtained from solution‐³¹P NMR) did trace successfully the short‐term dynamics and fate of dung P_o in soil. It indicated that dung‐derived P_o varied as rapidly in soil as the dung‐derived C.     

Phosphorus retention in the soil matrix of constructed wetlands

Abstract

Constructed wetlands may be described as soil/plant systems for wastewater treatment in which pollutant removal is based on general principles of nutrient transformation in soils. Currently perceived as “black boxes”; by engineers, the design and operation of these systems may be greatly improved based on the knowledge gained from several decades of studying nutrient cycling in soil‐plant systems. This paper reports on an attempt to operate this linkage. Three pilot scale systems planted with reed, cattail, and water hyacinth were used to study the role of the soil matrix in phosphorus (P) removal over a period of five months. Phosphorus removal was superior in the soil‐based systems with a mean P reduction from the influent concentration (24 mg‐mL^‐1) of 80% compared with 54% in the soilless bed. Recycling the effluent into the system in order to increase the detention time did not contribute to improving removal, except in the soilless bed. This indicates that P removal in the soil‐based systems is rapid, and that an equilibrium value may be reached beyond which no further removal is possible. The effect of a lime amendment on the improvement of P removal was studied in batch tests in a decarbonated sand amended with 1.4%, 12.2%, 21%, 38%, and 49% calcium carbonate (CaCO₃). Phosphorus removal from solution can be significantly improved by the addition of small amounts of lime (2–4%). Fixation is also faster and sustainable in lime‐amended sands. These results suggest that P removal from wastewater can be greatly enhanced by the addition of small amounts of lime to the soil substrate.     

Relative Phosphorus Phytoavailability of Different Phosphorus Sources

Understanding differences in the phytoavailability of various phosphorus (P) sources should improve matching P additions to plant needs and minimize excessive buildup of bioavailable P, which can degrade aquatic systems. We evaluated relative P phytoavailability (RPP) of different P sources in glasshouse and field studies. Bahiagrass (Paspalum notatum Fluggae), ryegrass (Lolium perenne L.), and a second bahiagrass crop were grown in succession in a P‐deficient soil amended with four sources of P (triple superphosphate (TSP), Boca Raton and Pompano; biosolids, and poultry manure), each applied at two rates in the glasshouse study and to an established bahiagrass pasture in the field. The RPP values estimated from plant P uptake of each organic source of P relative to TSP in the glasshouse were similar for the three croppings and similar to the estimates derived from the field study, but varied for the different P sources. Values ranged from 30% for poultry manure to 85% for Boca Raton biosolids. Boca Raton biosolid P was as readily available as P in TSP and would be classified as a high RPP (>75% RPP) source, but Pompano biosolids and manure would be classified as moderate RPP materials (25–75% RPP). The RPP values observed in manure and Pompano biosolid treatments are consistent with 50% “effectiveness” suggested for biosolid P in U.S. Environmental Protection Agency (USEPA) guidelines, whereas P bioavailability of Boca Raton biosolids is similar to mineral fertilizer.     

Effect of organic residue amendment on mineralization of sulfur in flooded rice soils under laboratory conditions

Abstract

This study was undertaken to assess the mineralization of sulfur (S) in laboratory conditions of three rice soils (Joydebpur, Faridpur, and Thakurgaon), receiving the following treatments: 1) control, 2) rice straw (Oryza sativa L.), and 3) pea vine (Pisum sativum L.). The organic residue (25 mg g^‐1) was added and mixed with soil and glass beads (1:1, soil to bead ratio) and placed into a Pyrex leaching tube. The soils were flooded and incubated at 35°C, after which they were leached with deionized water at 1, 2,4, 8, and 12 weeks for analysis of SO₄ and other chemical properties in the leachates. Potentially mineralizable S (S_o) and C (C_o) pools and first‐order rate constants (K_s for S and K_c for C) in soils amended with rice straw and pea vine under flooded conditions were estimated using an exponential equation. The S_o and K_s varied considerably among the soils and types of added organic residues, and their values in rice straw and pea vine ranged from 8.70 to 29.55 and 0.124 to 0.732 mg S kg^‐1 wk^‐1, respectively. Except for the Thakurgaon soil, the S_o and K_s values in Joydebpur and Faridpur soils were higher in the unamended treatments. Higher S_o values in the unamended soils were probably due to less microbial activity to mineralize organic S from organic residues. The results indicate that the amount of SO₄ in flooded soils amended with organic residues are dependent on soil type, nature of organic residues, and time of incubation. The C_o and K_c values under flooded incubation were higher in residue amended soils than in unamended soils. Pea vine treated soils had higher C_o and K_c values than the soils treated with rice straw.     

Water Treatment Residuals and Biosolids Co‐applications Affect Phosphatases in a Semi‐arid Rangeland Soil

Co‐application of biosolids and water treatment residuals (WTR) land has not been extensively studied but may be beneficial by sorbing excess biosolid‐borne or soil phosphorus (P) onto WTR, reducing the likelihood of off‐site movement. Reduction of excess soil P may affect the role of specific P‐cleaving enzymes. The research objective was to understand the long‐term effects of single co‐applications and the short‐term impacts of repeated co‐applications on soil acid phosphomonoesterase, phosphodiesterase, pyrophosphatase, and phytase enzyme activities. Test plots were 7.5 × 15 m with treatments consisting of three different WTR rates with a single biosolids rate (5, 10, and 21 Mg WTR ha^?1; 10 Mg biosolids ha^?1) surface co‐applied once in 1991 or reapplied in 2002. Control plots consisted of those that received no WTR–biosolids co‐applications and plots that received only 10 Mg biosolids ha^?1. Plots were sampled to a 5‐cm depth in 2003 and 2004, and soil phosphatases and phytase enzyme activities were measured. Soil phosphodiesterase activity decreased in WTR‐amended plots, and pyrophosphatase activity decreased with increasing WTR application rates. In contrast, acid phosphatase and phytase activity increased with WTR addition, with WTR application possibly triggering a deficiency response causing microorganisms or plants to secrete these enzymes. Biosolids and WTR co‐applications may affect enzymatic strategies for P mineralization in this study site. Reductions in phosphodiesterase activity suggest less P mineralization from biomass sources, including nucleic acids and phospholipids. Increased acid phosphatase and phytase activities indicate that ester‐P and inositol‐P may be important plant‐available P sources in soils amended with WTR.     

Nitrogen transformation from three soil organic amendments in a sandy soil

Abstract

A sandy soil was amended with various rates (20 – 320 g air-dry weight basis of the amendments per kg of air-dry soil) of chicken manure (CM), sewage sludge (SS), and incinerated sewage sludge (ISS) and incubated for 100 days in a greenhouse at 15% (wt/wt) soil water content. At the beginning of incubation, NH₄-N concentrations varied from 50 – 280 mg kg^?1 in the CM amended soil with negligible amounts of NO₃-N. Subsequently, the concentration of NH₄-N decreased while that of NO₃-N increased rapidly. In soil amended with SS at 20 – 80 g kg^?1 rates, the NO₃-N concentration increased sharply during the first 20 days, followed by a slow rate of increase over the rest of the incubation period. However, at a 160 g kg^?1 SS rate, there were three distinct phases of NO₃-N release which lasted for160 days. In the ISS amended soil, the nitrification process was completed during the initial 30 days, and the concentrations of NH₄-N and NO₃-N were lower than those for the other treatments. The mineralized N across different rates accounted for 20 – 36%, 16 – 40%, and 26 – 50% of the total N applied as CM, SS, and ISS, respectively.     

Phosphorus fractions and profile distribution in newly formed wetland soils along a salinity gradient in the Yellow River Delta in China

Soil P availability has been identified as one of the key factors controlling wetland productivity, structure, and function. Soil P fractions at different depths in newly formed wetlands along a salinity gradient in Yellow River Delta (China) were studied using a modified Hedley fraction method. The total P (P_t) content ranged from 471.1 to 694.9 mg kg^–1, and diluted HCl‐extractable inorganic P (Dil‐HCl‐P_i) ranged from 324 to 524.2 mg kg^–1. The Dil‐HCl‐P_i is the predominant P form in all profiles, with on average 70% of the P_t extracted as P_i. Organic P (P_o) comprised (4.2 ± 2.0)% (mean ± SD) of the P_t, due to low organic‐matter content in coastal salt marsh ecosystems. The labile P (resin‐P, NaHCO₃‐P_i, and NaHCO₃‐P_o) and moderately labile P (NaOH‐P_i and NaOH‐P_o) concentrations were both low, ranged from 11.6 to 38.1 and 2.8 to 21.3 mg kg^–1, respectively, constituting (3.7 ± 1.1)% and (2.0 ± 0.7)%, respectively, of P_t, suggesting low availability of P to plants in these soils. Our results suggested that vegetation cover significantly influenced soil P dynamics and availability. In particular, the labile P content under Tamarix chinensis increased significantly by 23.2%–145.5% compared with adjacent soils. These findings have important implications for wetland conservation or restoration and long‐term sustainable management of newly formed wetland ecosystems in the Yellow River Delta.     

Sulfur mineralization in two soils amended with organic manures,crop residues,and green manures

The mineralization of sulfur (S) was investigated in a Vertisol and an Inceptisol amended with organic manures, green manures, and crop residues. Field‐moist soils amended with 10 g kg^—1 of organic materials were mixed with glass beads, placed in pyrex leaching tubes, leached with 0.01 M CaCl₂ to remove the mineral S and incubated at 30 °C. The leachates were collected every fortnight for 16 weeks and analyzed for SO₄‐S. The amount of S mineralized in control and in manure‐amended soils was highest in the first week and decreased steadily thereafter. The total S mineralized in amended soils varied considerably depending on the type of organic materials incorporated and soil used. The cumulative amounts of S mineralized in amended soils ranged from 6.98 mg S (kg soil)^—1 in Inceptisol amended with wheat straw to 34.38 mg S (kg soil)^—1 in Vertisol amended with farmyard manure (FYM). Expressed as a percentage of the S added to soils, the S mineralized was higher in FYM treated soils (63.5 to 67.3 %) as compared to poultry manure amended soils (60.5 to 62.3 %). Similarly the percentage of S mineralization from subabul (Leucaena leucocephala) loppings was higher (53.6 to 55.5 %) than that from gliricidia (Gliricidia sepium) loppings (50.3 to 51.1 %). Regression analysis clearly indicated the dependence of S mineralization on the C : S ratio of the organic materials added to soil. The addition of organic amendments resulted in net immobilization of S when the C : S ratio was above 290:1 in Vertisol and 349:1 in Inceptisol. The mineralizable S pool (S_o) and first‐order rate constant (k) varied considerably among the different types of organic materials added and soil. The S_o values of FYM treated soils were higher than in subabul, gliricidia, and poultry manure treated soils.     

Reclamation of Alkali Soils: Influence of Amendments and Leaching on Transformation and Availability of Phosphorus

Abstract

Phosphorus (P) availability to plants in reclaimed alkali soils was the main objective of this study, which was also focused on P transformations, decrease in Olsen‐P content, and magnitude of P lost in leachate in course of amendment application and leaching. Liquid sodium bicarbonate (NaHCO₃) was added to nonalkali soils to set up four ESP (exchangeable sodium percentage) levels (viz., 2.9, 25.0, 50.0, and 75.0), but actual ESP levels obtained were 2.9, 24.6, 51.2, and 75.3. Amendments (viz., gypsum and pyrites) and P treatments (viz., 0 and 50 mg P Kg^?1) were mixed with dry, sieved soil before filling into PVC (polyvinyl chloride) drainage columns, which were then compacted to uniform bulk density and leached with deionized water for 30 days. Results indicated that the pH and electrical conductivity (EC) of the soils increased with increase in ESP level of the soil but decreased with amendment application. Phosphorus addition to alkali soils decreased the pH on day 30, but it could not affect the EC of the soils. Successive increase in the ESP level of the soil increased the pH and EC off the leachate. Gypsum‐amended soils exhibited lower pH and EC values than pyrite‐amended soils. The EC of the leachate decreased sharply with time in amended soils, but the pH decreased slowly. Phosphorus addition affected the leachate pH earlier than the soil pH. Cumulative volume of leachate decreased with increasing ESP levels, but it increased with amendment and phosphorus application. Leaching of P increased with increase in ESP levels, and the maximum cumulative loss of P was 11.2 mg Kg^?1 in the 75.3 ESP soil. Cumulative P lost in the pyrite‐amended soils was higher than the gypsum‐amended soils. Phosphorus leaching in the gypsum‐amended soils stopped at day 10 and beyond, but it continued until day 30 in the pyrite‐amended soils. Part of the applied P in alkali soils was also lost along with the native P, whereas it was protected in the nonalkali soils. OlsenP increased with increasing ESP levels, and alkali soils invariably contained higher Olsen P than nonalkali soils. At day 30, alkali soils contained much higher Olsen P (12.6 mg Kg^?1) than nonalkali soils (5.9 mg Kg^?1). In general, there was a decrease in the Olsen P with both of the amendments, but it decreased more with pyrites than with gypsum. Phosphorus added through monopotassium phosphate (KH₂PO₄) remained extractable by Olsen's extractant up to day 30. Results also indicated that percent distribution of ammonium chloride (NH₄Cl)‐P, calcium (Ca)‐P, and unknown P increased with rising ESP levels but iron (Fe)‐aluminum (Al)‐bound P and residual P decreased. Percent distribution of Ca‐P and unknown P exhibited an increase with time also. Unamended alkali soils contained more NH₄Cl‐P than amended ones. Iron and Al‐ bound P and residual P increased more with pyrites, whereas formation of Ca‐P and unknown P was enhanced with gypsum. Applied P tended to convert more into NH₄Cl‐P, Ca‐P, and residual P than to Fe‐Al‐bound P or unknown P fractions. Models developed to estimate Olsen P and P concentration in leachate, through pH or EC, have application value for P management in alkali soils that are leached after application of amendments.     

Changes in Soil Phosphorus Fractions Resulting from Crop Residue Removal and Phosphorus Fertilizer

It is crucial to know how management factors influence soil test phosphorus (P) since non-point P sources for surface waters are becoming recognized as a problem throughout the USA. Phosphorus fertilizer and crop residue can impact the cycling of P in soils. An eight-year crop residue removal and P fertilization (0, 7.3, 14.7 and 29.4 kg P/ha) as triple superphosphate (TSP) experiment were conducted to determine the effect of P applications on soil P fractions. Significant differences in Bray-l extractable P were observed after only one year of P applications. Extractable P at the highest P rate was significantly higher than all the other rates. For each 5.6 kg P/ha added or removed, Bray-l P changed by 1 mg/kg. Fertilizer P applications did not significantly change the organic P (P_o) levels, microbial P (P_m) or soil pH, whereas residue treatments had effects on them except for soil pH. Residue-retained plots had significantly higher P_m in the last two years of study, and P_o in the 8^th year, compared with residue-removed plots.     

Effect of long‐term fertilization on the transformations of water‐extractable phosphorus in a fluvo‐aquic soil

Improved information on water‐extractable soil P (P_w) and its distribution in various forms is needed to assess its bioavailability and environmental impact. This study investigated P_w in a fluvo‐aquic soil solution in relation to the continuous application of inorganic fertilizer (NPK) and wheat straw–soybean‐based compost for 15 y. Phosphatase‐hydrolysis techniques were used to fractionate organic P (P_o) in water extracts of soil into phosphomonoester (P_om) and phosphodiester (P_od). In comparison with the noncomposted treatments, compost application significantly increased the levels of inorganic P (P_i) and P_o. P_om was the main form in water‐extractable soil P_o (71%–88%), in which sugar phosphate (P_os) occupied 48%–75%, inositol hexakisphosphate (P_op) comprised 13%–23%, and P_od only accounted for a small percentage (11%–26%). Long‐term compost application significantly increased the content of P_om, P_os, and P_od, but decreased the P_op content; the ratio of P_om to P_o increased significantly in compost‐treated soil, but the ratio of P_op to P_o and P_od to P_o significantly decreased. Thus, the equilibrium of phosphatase involved P transformations shifted to P_i in compost‐treated soil. The phosphomonoesterase and phosphodiesterase activities were significantly higher in compost‐treated soil, which favored the transformations of P_od into P_om and P_om into P_i. The ratio of P_o to P_w in water extracts of compost‐treated soil was similar to that of control soils with no fertilizer input (CK), but was significantly lower than in NPK treatment, which demonstrated that a larger increase occurred for soil P_i in water extracts of compost‐treated soil. Long‐term compost application in the fluvo‐aquic soil changed the composition of P_w, promoted the rate of P transformations in soil solution, and significantly increased soil P bioavailability.     

Phosphorus pools in soil after land conversion from silvopasture to arable and grassland use

Differences in soil P among silvopasture, grassland, and arable lands have been well established. Nevertheless, most of the reports compare soil properties under long‐term sites. Thus, there exists little information on the effect of the conversion of silvopasture to arable or grassland use on soil P pools. The objective of the study was to determine the impact of converting silvopasture system (SP) into arable cropping and grassland system on the distribution of P pools and potential P bioavailability. We compared the following systems: SP system, SP converted to arable cropland (SP‐AL), SP converted to grassland (SP‐GL), and for comparative purposes, a long‐term arable cropland (AL). The P fractionation was performed by a sequential extraction scheme, using acid and alkaline extractants on samples collected from the 0–10 and 10–20 cm soil layers. It was assumed that the large variations in soil‐P fractionations are caused by the different management practices associated with land conversion. The results of P fractionation showed a dominance of calcium‐bound P, HCl‐extractable Pi constituted up to 36% of the soil total P (TP). However, the type of land use did not affect this P fraction. On the other hand, the reduction in labile‐P_i and NaOH‐P_i fractions observed at the SP‐AL site may have led to the decline in readily available P. The soil total organic P (TP_o) content was 8% and 17% lower at SP‐AL compared to SP and SP‐GL site, respectively. Labile organic‐P (labile‐P_o) content was markedly higher at SP site compared to arable soils, and was ≈ 10% of TP_o. The NaOH‐P_o constituted the highest fraction of the organic‐P pool (55%–79% of TP_o) across all the study systems, and was positively correlated with TP_o (p < 0.01). The study indicates that conversion of SP system in temperate regions to arable cropping with conventional tillage seems to result in the reduction of P availability compared to SP, indicating SP as an important land‐use practice.     

Phosphorus exchange properties of European soils and sediments derived from them

The sorption and desorption of phosphorus (P) from eroding soil particles in land runoff are important processes contributing to agriculturally‐driven eutrophication. We investigated the P‐exchange properties and related chemical characteristics of contrasting European agricultural soils and sediment material eroded from them under indoor (small‐scale) and outdoor (larger‐scale) rainfall simulations. Quantity‐intensity (Q/I) relationships revealed large variation in equilibrium P concentrations at zero net P sorption (EPC₀) (0–10.3 mg l⁻¹) and instantly labile P (−Q₀, the amount of P to be desorbed to obtain a P equilibrium concentration of 0 mg l⁻¹) (2–75 mg kg⁻¹), both correlating closely with Al‐bound P and the P saturation degree of Al oxides (DPS_Alox). Maximum P sorption (Q_max) (43–515 mg kg⁻¹) also correlated most closely with Al_ox. The indoor and outdoor rainfall simulations produced sediments with different P sorption properties: in the indoor simulation (less kinetic energy, constant slope), the sediments had larger EPC₀ values, and usually larger −Q₀ values, than the sediments in the outdoor simulation (greater kinetic energy, variable slopes). Furthermore, the P exchange properties of the sediments differed from those of the bulk soil depending on the enrichment of soil P‐sorption components (Fe/Al oxides, clay). The outdoor simulation indicated that sites with gentle slopes produced sediments that were more enriched with Al_ox, Fe_ox, Mn_ox and organic C than those with steeper slopes. In this study, when the bulk soil had an initial EPC₀ greater than 1.3 mg l⁻¹, the outdoor rainfall simulation produced sediment with smaller EPC₀ and vice versa, indicating that, depending on the P status of the bulk soil, the sediment material was acting as source or sink for P during transport. However, on the basis of their EPC₀ values, most eroding sediments might be expected to desorb, rather than adsorb, P when entering surface water.     

Nitrogen release from air‐dried biosolids for fertilizer value

Mineral‐N production by air‐dried biosolids was measured in an Australian tenosol type soil with two moisture conditions over 70 days, using a controlled laboratory incubation procedure. The biosolids were from both air‐drying pans and stockpiles. Inorganic‐N components (NH₄‐N, NO₃‐N and NO₂‐N) were present in all biosolids, with higher concentrations in samples from air‐drying pans compared with stockpiles of 1 yr age. Nevertheless, significant production of NO₃‐N occurred in moist soil amended with all air‐dried biosolids. In contrast, saturated soil amended with air‐dried biosolids generally showed a net loss of inorganic‐N compounds during incubation, presumably owing to denitrification. In the saturated soil, only biosolids from air‐drying pans provided NO₃‐N production from existing NH₄‐N. The results indicated that biosolids from air‐drying pans provided the most robust production of NO₃‐N, compared with aged material from the stockpiles, owing to the reduced N content and increased stability of the organic fraction in stored biosolids. However, the rates of N‐mineralization in the tenosol soil were substantially lower than reported for more fertile soil types and most of the organic‐N content of the biosolids remained undegraded by day 70. The biosolids thus may substantially remain to provide improved properties of soil, such as structure and water‐holding capacity. The results suggest that anaerobically digested biosolids from air‐drying pans are potentially highly consistent products that could be effective replacements for inorganic‐N fertilizer in agricultural production.     

Soil phosphorus dynamics in a Vertisol as affected by cattle manure and nitrogen fertilization in soybean‐wheat system

Repeated application of phosphorus (P) as superphosphate either alone or in conjunction with cattle manure and fertilizer N may affect the P balance and the forms and distribution of P in soil. During 7 years, we monitored 0.5 M NaHCO₃ extractable P (Olsen‐P) and determined the changes in soil inorganic P (P_i) and organic P (P_o) caused by a yearly dose of 52 kg P ha^—1 as superphosphate and different levels of cattle manure and fertilizer N application in a soybean‐wheat system on Vertisol. In general, the contents of Olsen‐P increased with conjunctive use of cattle manure. However, increasing rate of fertilizer nitrogen (N) reduced the Olsen‐P due to larger P exploitation by crops. The average amount of fertilizer P required to increase Olsen‐P by 1 mg kg^—1 was 10.5 kg ha^—1 without manure and application of 8 t manure reduced it to 8.3 kg ha^—1. Fertilizer P in excess of crop removal accumulated in labile (NaHCO₃‐P_i and P_o) and moderately labile (NaOH‐P_i and P_o) fractions linearly and manure application enhanced accumulation of P_o. The P recovered as sum of different fractions varied from 91.5 to 98.7% of total P (acid digested, P_t). Excess fertilizer P application in presence of manure led to increased levels of Olsen‐P in both topsoil and subsoil. In accordance, the recovery of P_t from the 0—15 cm layer was slightly less than the theoretical P (P added + change in soil P — P removed by crops) confirming that some of the topsoil P may have migrated to the subsoil. The P fractions were significantly correlated with apparent P balance and acted as sink for fertilizer P.     

Organic and Inorganic Forms of Phosphorus in a Calcareous Soil Planted with Four Species of Eucalyptus in Southern Iran

The objectives of the present study were to evaluate the effect of four eucalyptus species on (i) selected surface soil properties and (ii) the distribution of inorganic and organic phosphorus (P_i and P_o) fractions. Soil samples were collected from soil 0–20 cm deep beneath and between trees. The P_i forms were determined by sequential extraction with sodium bicarbonate (NaHCO₃‐P), ammonium acetate (NH₄OAc) (OAc‐P), ammonium fluoride (NH₄F‐P), sodium hydroxide (NaOH)–sodium carbonate (Na₂CO₃) (HC‐P), citrate dithionite (CD‐P), and sulfuric acid (H₂SO₄) (H₂SO₄‐P). The P_o forms were sequentially extracted with NaHCO₃ (NaHCO₃‐P_o), NaOH (NaOH‐P_o), and H₂SO₄ (H₂SO4‐P_o). The NaOH‐P_o was subdivided into moderately stable (NaOH‐P_om) and highly stable P_o (NaOH‐P_os). Organic matter, clay and silt contents, total nitrogen, and available potassium of the soil beneath the trees increased. The OAc‐P and HC‐P forms beneath the trees were less than of that between them, which shows that these fractions probably are labile inorganic P pools. The NaHCO₃‐P_o and NaOH‐P_os forms were greater beneath the trees than those of interspaces, whereas NaOH‐P_om and H₂SO₄‐P_o were not affected by plantation.     

The effect of soil phosphorus on particulate phosphorus in land runoff

Accumulation of surplus phosphorus (P) in the soil and the resulting increased transport of P in land runoff contribute to freshwater eutrophication. The effects of increasing soil P (19–194 mg Olsen‐P (OP) kg⁻¹) on the concentrations of particulate P (PP), and sorption properties (Q_max, k and EPCo) of suspended solids (SS) in overland flow from 15 unreplicated field plots established on a dispersive arable soil were measured over three monitoring periods under natural rainfall. Concentrations of PP in plot runoff increased linearly at a rate of 2.6 μg litre⁻¹ per mg OP kg⁻¹ of soil, but this rate was approximately 50% of the rate of increase in dissolved P (< 0.45 μm). Concentrations of SS in runoff were similar across all plots and contained a greater P sorption capacity (mean + 57%) than the soil because of enrichment with fine silt and clay (0.45–20 μm). As soil P increased, the P enrichment ratio of the SS declined exponentially, and the values of P saturation (P_sat; 15–42%) and equilibrium P concentration (EPCo; 0.7–5.5 mg litre⁻¹) in the SS fell within narrower ranges compared with the soils (6–74% and 0.1–10 mg litre⁻¹, respectively). When OP was < 100 mg kg⁻¹, P_sat and EPCo values in the SS were smaller than those in the soil and vice‐versa, suggesting that eroding particles from soils with both average and high P fertility would release P on entering the local (Rosemaund) stream. Increasing soil OP from average to high P fertility increased the P content of the SS by approximately 10%, but had no significant (P > 0.05) effect on the P_sat, or EPCo, of the SS. Management options to reduce soil P status as a means of reducing P losses in land runoff and minimizing eutrophication risk may therefore have more limited effect than is currently assumed in catchment management.     

Drinking‐Water Treatment Residual Effects on the Phosphorus Status of Field Soils Amended with Biosolids,Manure, and Fertilizer

Abstract

Concerns about surface water pollution with phosphorus (P) from biosolids and manures are prompting land application guidelines that limit residual application rates to those based on crop‐P removals (typically, no more than 2 Mg ha^?1). Such rates are so low that the beneficial recycling of residuals is seriously threatened. Greater application rates [i.e., nitrogen (N) based] require judicious selection of residuals (low soluble P contents) and/or soil amendments, such as drinking‐water treatment residuals (WTRs) to control soluble P concentration. Although in the short term, WTR is effective in reducing soluble P levels, field studies to evaluate the stability of WTR‐immobilized P are scarce. The initial objective of this study was to determine the effects of WTR on P losses to surface and groundwater from Florida sand amended with different P sources (biosolids, manure, and inorganic fertilizer) applied at P‐ and N‐based rates. However, this objective could not be pursued to its logical conclusion because of severe flooding of the field 17 months after amendment application. The flooding appears to have compromised the treatments (moved soil and associated amendments across plots), which forced early termination of the experiment. Measurements taken after the flooding, however, provided a unique opportunity to assess the usefulness of WTR in controlling P solubility following severe flooding of WTR‐amended plots. Soluble P values measured from WTR‐amended A horizon plots were significantly lower than the plots without WTR amendment throughout the study. Phosphorus‐specific measurements in the Bh horizon suggest that excessive P leaching apparently occurred in the plots without WTR amendment and the control plots, whereas very little or no P leaching occurred in the WTR‐amended plots. Thus, despite extensive hurricane‐induced flooding of the fields, the WTR was able immobilize P and prevent excessive P leaching. We conclude that WTR could reduce offsite P transport, which will lower P loads into nutrient‐sensitive surface water systems, and that WTR‐immobilized P is stable even under severe flooding conditions.