Effects of long-term phosphorus fertilization and winter cover cropping on soil phosphorus transformations in less weathered soil

Information concerning sources and sinks of available P in soil is needed to improve soil P management and protect water quality. This study, conducted from 1989 to 1998 on a Sultan silt loam soil (Aquantic Xerochrept), determined the annual P removal rate by corn (Zea mays L.) and P transformation as affected by P rate and winter cover cropping. Treatments included two P rates (0 and 44 kg P ha^–1) applied to corn at planting each year. All cover crops received 19.6 kg P ha^–1 at seeding each fall. Also included was a control without any cover crop and with no P addition. Corn yield and P uptake were affected by P fertilizer additions, but not by cover crops. A fairly constant amount of P was supplied from indigenous soil P when no external P was added. When the amount of P added exceeded that removed by corn, the excess P was converted mainly to NaOH-extractable inorganic P (NaOH-P_i). When the amount of P applied was below that removed by corn, indigenous soil NaOH-P_i acted as a source of available P for the plant. With no reduction of organic P (P_o) extractable by NaOH or NaHCO₃, the contribution from P_o to the available P pool appeared limited. The role of NaOH-P_i in P availability in the soil was substantiated by its significant correlation with labile NH₄Cl-extractable P (NH₄Cl-P; r² =0.60, P <0.001) or NaHCO₃-P_i (r² =0.81, P <0.001) pools. The NaOH-P_i for the soil reflected the changes in soil P resulting from past fertilizer P input and P removal by the crops.Scientific Paper Number 0005-34     

Phosphorus fractions in an acid soil continuously fertilized with mineral and organic fertilizers

The effect of different treatments on the fate of applied P was investigated in a long-term field experiment started in 1972–1973 following a maize–wheat sequence. The soil samples were collected after 29 years of continuous addition of mineral fertilizers and amendments such as farmyard manure (FYM) and lime. The total P content of all the treatments increased compared to the original soil; NaOH-inorganic P (P_i) (NaOH-P_i) representing Fe and Al-bound P was the dominant P_i fraction. At the beginning of the experiment (1972–1973), the various P pools could be quantitatively ranked in the following order: residual P>NaOH-organic P (P_o)>NaOH-P_i>NaHCO₃-P_o>NaHCO₃-P_i>HCl-P>H₂O-P. As a result of continued P fertilization and cropping, the order changed as follows: residual P>NaOH-P_i>NaOH-P_o>NaHCO₃-P_i>NaHCO₃-P_o>HCl-P>H₂O-P. Compared to the imbalanced mineral fertilizer application, the balanced as well as integrated application of nutrients resulted in significantly lower P adsorption capacity of soils. The Olsen extractable-P fraction (plant-available P) increased from about 12 mg kg^–1 soil in 1972 to about 81 mg kg^–1 soil in the treatments receiving P for the last 29 years.     

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.     

Correlations between Phosphorus Fractions and Total Leachate Phosphorus from Cattle Manure– and Swine Manure–Amended Soil

Although many studies have examined the effect of different application rates of cattle manure, swine manure, and urea fertilizer on the distribution of phosphorus (P) fractions in soil, few studies have correlated P fractions in soil with inorganic P (P_i) and organic P (P_o) in leachates. As part of a long-term field study, cattle and swine manures were applied to a loamy soil based on a nitrogen (N) content equivalent of 100 (low) and 400 (high) kg total N ha^?1 yr^?1 and were compared to urea fertilizer at 100 kg N ha^?1 yr^?1 and an unamended control soil. Readily available P_i [resin and sodium bicarbonate (NaHCO₃)] was significantly greater in cattle manure– and swine manure–amended soil at a high application rate than in the control. With some exceptions, urea did not significantly affect P fractions in sequentially extracted P pools. Leaching of P_i and P_o was at levels of environmental concern when cattle and swine manures were applied at the high application rate but not at the low application rate. Cattle manure had significantly greater concentrations of P_i and P_o removed by leaching compared to swine manure, most likely because of its narrow N/P ratio and greater amount of P added. Positive correlations were observed between resin P_i and total leachate P_i and between NaHCO₃-P_i and total leachate P_i, indicating the value of these measurements in predicting P mobility. The results suggest that a threshold (40 μg P g^?1 of soil) must be exceeded before a positive correlation occurs.     

Long-Term Wheat Residue Management and Supplementary Nutrient Input Effects on Phosphorus Fractions and Adsorption Behavior in a Vertisol

The management of crop residues coupled with external nutrient inputs is important for improving and conserving soil fertility and productivity. We assessed the long-term effects of three wheat residue management options (RMO) (residue burning, incorporation, and surface retention) in combination with three supplementary nutrient inputs (SNI) [control, fertilizer, and farmyard manure (FYM)] on phosphorus (P) fractions and adsorption behavior of a Vertisol under soybean–wheat system. Wheat residue incorporation and retention improved the labile inorganic P [sodium bicarbonate (NaHCO₃-P_i)] by 3.2 and 5.0 mg kg^?1 and the labile organic P (NaHCO₃-P_o) by 2.4 and 4.2 mg kg^?1, respectively, as compared to residue burning. The soils under residue incorporation and retention had 38 and 26% more moderately labile organic P [sodium hydroxide (NaOH-P_o)], respectively, than the soil under residue burning. The SNI either as fertilizer or FYM further enhanced NaHCO₃-P_i, NaHCO₃-P_o, and NaOH-P_o. In contrast, less labile P fractions [hydrochloric acid (HCl)-P and residual-P] remained unaffected by RMO and SNI treatments. Residue retention or incorporation decreased P adsorption over the residue burning for all the three nutrient inputs. The P-adsorption data fitted well to the Langmuir equation (R² ranged from 0.970 to 0.994). The P-adsorption maximum (b), bonding energy constant (k), differential P-buffering capacity (DPBC), and standard P requirement (SPR) were lower with residue incorporation or surface retention than with residue burning. The SPR followed the order residue burning > incorporation > retention for RMOs and control > fertilizer > FYM for SNI treatments. The NaHCO₃-P_i, NaHCO₃-P₀, and NaOH-P_o had negative correlation with P-adsorption parameters and showed positive correlation with soybean P uptake. Wheat residue incorporation or retention plus FYM could be an effective strategy for enhancing the P fertility of Vertisols under a soybean–wheat system.     

Phosphorus fractions and phosphorus delivery potential of a luvisol derived from loess amended with organic materials

In a long-term field experiment, started in 1962, the fate of P applied with different organic materials [farmyard manure (FYM), compost and sewage sludge] in comparison to mineral fertilizer was investigated. Soil samples were collected after 38 years' continuous addition of these amendments to a luvisol derived from loess and cultivated to a cereal-root crop sequence. The total P (P_t) content of all treatments increased compared with the original soil; NaOH-inorganic P (NaOH-P_i) representing Fe- and Al-bound P was the dominant inorganic fraction. At the beginning of the experiment the various P pools could be quantitatively ranked in the following order: NaOH-P_i>residual P~NaHCO₃-P_i>H₂O-P>HCl-P. The order changed as follows: NaOH-P_i>NaHCO₃-P_i>residual P~H₂O-P>HCl-P, with transformations of non-labile residual P to the labile NaHCO₃-P_i pool with continued P fertilization and cropping. In addition, the content of organic P (P_o) forms (NaOH-P_o and NaHCO₃-P_o) increased. P_t delivery potential (desorbable P pool) increased between 35% and 185% compared to the P delivery potential in 1962. Compared to mineral fertilizer application, the application of organics resulted in a significantly higher, and FYM in a lower, P adsorption capacity of soils. The calcium lactate-extractable P (plant-available P) increased from 43.1 mg kg^-1 soil in 1962 to 175.9 mg kg^-1 soil in the treatment with 49 t compost ha^-1. The increase in the citrate-dithionate Fe-O ranged between 44% and 154% in the different treatments compared to the Fe-O content in 1962. In a pot experiment with soil from the field experiment, P removal by ryegrass was in the following sequence: FYM>compost=sewage sludge>mineral fertilizer.     

Quantifying microbial biomass phosphorus in acid soils

 This study aimed to validate the fumigation-extraction method for measuring microbial biomass P in acid soils. Extractions with the Olsen (0.5 M NaHCO₃, pH 8.5) and Bray-1 (0.03 M NH₄F–0.025 M HCl) extractants at two soil:solution ratios (1 : 20 and 1 : 4, w/v) were compared using eight acid soils (pH 3.6–5.9). The data indicated that the flushes (increases following CHCl₃-fumigation) of total P (P_t) and inorganic P (P_i) determined by Olsen extraction provided little useful information for estimating the amount of microbial biomass P in the soils. Using the Bray-1 extractant at a soil:solution ratio of 1 : 4, and analysing P_i instead of P_t, improves the reproducibility (statistical significance and CV) of the P flush in these soils. In all the approaches studied, the P_i flush determined using the Bray-1 extractant at 1 : 4 provided the best estimate of soil microbial biomass P. Furthermore, the recovery of cultured bacterial and fungal biomass P added to the soils and extracted using the Bray-1 extractant at 1 : 4 was relatively constant (24.1–36.7% and 15.7–25.7%, respectively) with only one exception, and showed no relationship with soil pH, indicating that it behaved differently from added P_i (recovery decreased from 86% at pH 4.6 to 13% at pH 3.6). Thus, correcting for the incomplete recovery of biomass P using added P_i is inappropriate for acid soils. Although microbial biomass P in soil is generally estimated using the P_i flush and a conversion factor (k _P) of 0.4, more reliable estimates require that k _P values are best determined independently for each soil. Received: 3 February 2000     

Carbon and phosphorus transformations during decomposition of pine forest floor with different phosphorus status

Summary Information on the mineralization of inorganic phosphate (P_i) from organically bound P (Po) during decomposition of forest floor and soil organic matter is vital for understanding P supply in forest ecosystems. Carbon (C) and phosphorus (P) fluxes were determined for forest floor samples from three Pinus radiata plots which had received no P (Control), 62.5 kg P ha^–1 (Low P) and 125 kg P ha^–1 (High P) 20 years before sampling. The P concentration of the forest floor samples had increased with fertilizer application, and the C:P ratio ranged between 585 and 1465. During a 9-week laboratory incubation 8.2–19.0% of the forest floor C was evolved as CO₂-C. The amount of CO₂ evolved from the forest floor of the Control plot was more than twice the amounts from the Low P and High P plots. There was little change in net P mineralization in the Control and Low P treatments throughout the incubation, but it increased slightly for the High P samples, suggesting a critical forest floor C:P ratio of 550 for net P mineralization. Changes in the ³²P-specific activities of the P_i and microbial P pools during incubation, and concurrent changes in microbial-³²P and ³²P_i, indicated internal P cycling between these pools. The rate of internal P cycling varied with forest floor quality, and was highest in the High P forest floor. The High P samples had microbial C:P ratios of 22 : 1 which remained constant during the incubation, suggesting the microorganisms had adequate P levels. Received: 2 July 1997     

Long-Term Effects of Tillage and Manure Applications on Soil Phosphorus Fractions

Application of manure on the basis of crop nitrogen (N) need increases the level of soil phosphorus (P), which is concern for deterioration of surface water quality. Soil samples were collected from a long-term field study to investigate the impact of crop N need–based manure application on soil P fractions and P adsorption and release kinetics. The field experiment was initiated in 1990. The soil was moderately well-drained Kennebec (fine silty, mixed, mesic Cumulic Hapludolls). No-tillage (NT) and conventional-tillage (CT) treatments were established in main plots, and subplots had five N treatments, including a control, and annual application of 84 or 168 kg N ha^?1 applied as ammonium nitrate (NH₄NO₃) or beef (Bos taurus) manure. Manure at the high N application rate had significantly greater Bray 1 P under NT than under CT at 0- to 5-cm soil depth. Nitrogen fertilizer treatments were not significantly different than the control for Bray 1 P. Continuous application of manure at the high N rate significantly increased all Hedley P fractions; however, the major increase was observed in high bioavailable P pools [iron oxide (FeO) P and sodium bicarbonate (NaHCO₃) P_i] and hydrochloric acid (HCl) P fractions. Soil organic P (P_o) pools, including both labile (NaHCO₃-P_o) and resistant [sodium hydroxide (NaOH) P_o], were increased by application of N from any source, suggesting biomass production and return of residue to soil surface was the responsible factor. Continuous application of manure based on N need also significantly increased FeO-P, NaHCO₃-P_i, and HCl-P fractions at lower soil depths (5–15 and 15–30 cm). Results from the P-adsorption study suggest that ability of soil to adsorb additional P was decreased by manure application and that EPC₀ was increased. Maximum desorbable P was observed for manure treatments under NT, although the release constant k (h^?1) was significantly less than with fertilizer N treatments.     

Changes in phosphorus fractions,sorption and release in Udic Mollisols under different ecosystems

A 20-year field trial was conducted to study the effects of ecological factors and fertilization on phosphorus characteristics of fertile Udic Mollisols under three ecosystems: (1) bare land ecosystem with no vegetation or fertilizers (BE), (2) natural ecosystem with native grasses but no fertilizers (NE) and (3) agroecosystem with a rotation of wheat–soybean–corn. The agroecosystem had received N and P fertilizers during 1985–1993 and had received no fertilizers (CK), N and P fertilizers (NP) or N and P fertilizers along with pig manure (NPM) during 1994–2005. While there was no P input or removal in the BE and NE, the CK had a net P loss of 174 kg ha⁻¹, whereas the NP had a net gain of 96 kg/ha and the NPM 504 kg ha⁻¹. Increasing net P input increased both Olsen P and total P in the 0–20-cm soil. The amounts of NaHCO₃-extractable (Ca₂–P) and NH₄Ac-extractable P fractions (Ca₈–P) were in an order of NPM > NP > CK > NE > BE. The H₂SO₄-extractable fraction (Ca₁₀–P) was lowest in the BE and highest in the NPM but was similar in the other treatments. The CK had the least, and the NPM had the highest amounts of NH₄F-extractable (Al–P) and NaOH–Na₂CO₃-extractable fractions (Fe–P). Among the inorganic P, the percentage of Ca₂–P, Ca₈–P and Al–P increased, whereas that of Fe–P, occluded P and Ca₁₀–P decreased with increasing P input into the system. Soil P adsorption was in an order of CK > NE > NP > BE > NPM, whereas P release was in an order of NPM ≫ NP > CK > NE > BE. The study concluded that soil P can be sustained under the natural ecosystem while annual applications of chemical fertilizers and animal manure increased both labile and non-labile P pools in the agroecosystem.     

低分子量有机酸对土壤磷活化影响的研究

研究两种低分子量有机酸（柠檬酸和苹果酸）对土壤磷活化影响,并用修正的Hedley法测定土壤磷活化前后磷组分的变化。结果表明,低分子量有机酸能持续活化土壤磷,活化强度随低分子量有机酸浓度的增大而增强,并且柠檬酸活化土壤磷的能力强于苹果酸。低分子量有机酸能促进作物有效态无机磷组分（H2O-P和NaHCO3-Pi）的释放;同时还促进有机磷组分（NaHCO3-Po和NaOH-Po）的矿化。在低分子量有机酸浓度达到0.5 mmol/L以上时,其对土壤磷组分的活化量的顺序为:NaOH-Pi HCl-P NaHCO3-Pi H2O-P,即铁铝结合态磷 钙结合态磷 作物有效态磷。低分子量有机酸活化土壤磷的过程中伴有大量铁、铝释放,且铁或铝的释放量与磷活化量之间显著正相关（P0.05）。说明铁、铝结合态磷是低分子量有机酸活化土壤磷的主要磷源,并且其活化机制可能与铁、铝结合态磷的螯合溶解有关。     

Transformation of added phosphorus to acid upland soils with different soil properties in Indonesia

Abstract

The transformation of added phosphorus (P) to soil and the effect of soil properties on P transformations were investigated for 15 acid upland soils with different physicochemical properties from Indonesia. Based on oxide-related factor scores (aluminum (Al) plus 1/2 iron (Fe) (by ammonium oxalate), crystalline Al and Fe oxides, cation exchange capacity, and clay content) obtained from previous principal component analyses, soils were divided into two groups, namely Group 1 for soils with positive factor scores and Group 2 for those with negative factor scores. The amounts of soil P in different fractions were determined by: (i) resin strip in bicarbonate form in 30 mL distilled water followed by extraction with 0.5 mol L^?1 HCl (resin-P inorganic (P_i) that is readily available to plant), (ii) 0.5 mol L^?1 NaHCO₃ extracting P_{i and} P organic (P_o) (P which is strongly related to P uptake by plants and microbes and bound to mineral surface or precipitated Ca-P and Mg forms), (iii) 0.1 mol L^?1 NaOH extracting P_i and P_o (P which is more strongly held by chemisorption to Fe and Al components of soil surface) and (iv) 1 mol L^?1 HCl extracting P_i (Ca-P of low solubility). The transformation of added P (300 mg P kg^?1) into other fractions was studied by the recovery of P fractions after 1, 7, 30, and 90 d incubation. After 90 d incubation, most of the added P was transformed into NaOH-P_i fraction for soils of Group 1, while for soils of Group 2, it was transformed into resin-P_i, NaHCO₃-P_i and NaOH-P_i fractions in comparable amounts. The equilibrium of added P transformation was reached in 30 d incubation for soils of Group 1, while for soils of Group 2 it needed a longer time. Oxide-related factor scores were positively correlated with the rate constant (k) of P transformation and the recovery of NaOH-P_i. Additionally, not only the amount of but also the type (kaolinitic) of clay were positively correlated with the k value and P accumulation into NaOH-P_i. Soils developed from andesite and volcanic ash exhibited significantly higher NaOH-P_i than soils developed from granite, volcanic sediments and sedimentary rocks. Soil properties summarized as oxides-related factor, parent material, and clay mineralogy were concluded very important in assessing P transformation and P accumulation in acid upland soils in Indonesia.     

Phosphate Amendments to Compost for Improving P Bio-Availability

This study was conducted to investigate changes in P-fractions, bio-available P (CAL-P), citric acid extractable P, acid phosphatase activity and microbial biomass C and N during incubation of mature biogenic compost (MBC), immature biogenic compost (IBC) or immature sheep manure compost (ISC) not amended with P or amended with rock phosphate (RP, 7.6% P) or triple-superphosphate (TSP, 19.5% P). Incubation was performed at 20?°C in darkness under aerobic conditions. Samples were collected for laboratory analysis at the start of incubation (D-0) and after one, six and 26?days during incubation (D-1, D-6, D-26). Addition of soluble P fertilizer (TSP) led to a threefold increase in all P fractions in comparison to compost without TSP; even a “priming effect” could be observed, promoting conversion of non-labile to labile P. Moreover, addition of TSP lowered biological activity, especially acid phosphatase activity (P-ase), due to already high concentrations of readily available P. In general, P fractions (bicarbonate extractable P_i (NaHCO₃-P_i) and bicarbonate extractable P_o (NaHCO₃-P_o) and sodium hydroxide extractable P_o (NaOH-P_o)) increased during incubation until day 6 at the expense of NaOH-P_i fraction, which decreased. Generally, RP-derived P showed little or no effect on P fractions during the entire incubation period and only led to slightly increased CAL-P and Citric-acid-P levels. Fertilizer effects on labile P fractions were most enhanced with ISC. IBC enhanced microbial growth and P-ase, thereby enhancing conversion of labile into moderate labile NaOH-P_o.     

Transformation and recovery of fertilizer phosphorus applied to five Quebec Humaquepts

Abstract

Improving phosphorus (P) fertilizer efficiency while minimizing environmental impacts requires better understanding of the dynamics of applied P in soils. This study assessed the fate of fertilizer P applied in Quebec Humaquepts. A pot experiment with five textural Humaquepts, each receiving 0 (P0), 10 (P10), 20 (P20) and 40 (P40) mg P kg^?1 soil was conducted under barley (Hordeum vulgare L.)-soybean (Glycine max L.) rotations. A modified Hedley procedure was used for soil P fractionation. The clayey soils reached a plateau of dry matter at less P applied than the coarser-textured soils. Plant P uptake, soil labile inorganic P (resin-P?+?NaHCO₃-P_i) and moderately labile inorganic P (NaOH-P_i) increased proportionally with P rate. The coarser-textured soils had lower contents of labile and moderately labile P_i, but a larger increase in labile P_i than the finer-textured soils after receiving P additions. The applied P was retained primarily as soil labile P_i, accounting for 43–69% of total soil recovery of applied P, compared to 20–30% recovered as moderately labile P_i, and 7–29% assumed to be sparingly soluble P (HCl-P?+?H₂SO₄-P). The labile P_i recovery of applied P was linearly depressed with clay content, compared to a quadratic relation for the moderately labile P_i recovery. The results suggest the importance of accounting for soil texture along with soil P adsorption capacity when assessing the efficiency of applied P, P accumulation in soils and subsequently P nutrient management.     

Effects of poultry manure amendment on phosphorus uptake by ryegrass,soil phosphorus fractions and phosphatase activity

Poultry manure (PM) contains a large proportion of phosphorus (P) in mineral-associated forms that may not be readily available for plant uptake. In addition, PM application influences both chemical and biotic processes, and can affect the lability of native soil P. To investigate the effects of PM on soil P availability, we grew ryegrass (Lolium perenne) in greenhouse pots amended with poultry manure. Biomass was harvested at 4, 8, and 16 weeks following PM application, with soil separated into rhizosphere and bulk fractions. Soil was sequentially extracted by H₂O, 0.5 M NaHCO₃, 0.1 M NaOH, and 1 M HCl, and inorganic P (P_i) and enzymatically hydrolyzable organic P (P_oe) were quantitated. Root P concentrations were 37% higher and total P uptake 59% higher with PM application than Control. At week 16, there was 30% more labile-P_i (H₂O- plus NaHCO₃-P_i) in the rhizosphere with PM than in Control. Phosphodiesterase activity increased with PM application. Furthermore, acid phosphomonoesterase, alkaline phosphomonoesterase, and phosphodiesterase activities were all higher in the rhizosphere than in bulk soil at week 16 with PM, indicating that increased labile-P_i was due primarily to stimulation of soil phosphatases to mineralize NaOH-P_oe. Soil pH increased with PM application and plant growth, and may have promoted P availability by decreasing sorption of Al- and Fe-associated inorganic and organic phosphates. These results demonstrate that whereas PM application may initially increase NaOH and HCl-P_i, these fractions can be readily changed into labile-P and do not necessarily accumulate as stable or recalcitrant P in soil.     

Changes in phosphorus fractions in three tropical soils amended with corn cob and rice husk biochars

ABSTRACT

The formation of phosphorus (P) compounds including iron-P, aluminum-P and calcium-P in highly weathered tropical soils can be altered upon biochar addition. We investigated the effect of corn cob biochar (CC) and rice husk biochar (RH) pyrolyzed at three temperatures (300°C, 450°C and 650°C) on phosphorus (P) fractions of three contrasting soils. A 90d incubation study was conducted by mixing biochar with soil at a rate of 1% w/w and at 70% field capacity. Sequential P fraction was performed on biochar, soil and soil-biochar mixtures. Increase in most labile P (resin-P_i, NaHCO₃-P_i) and organic P fraction (NaHCO₃-P_o + NaOH-P_o) in CC and RH biochars were inversely related to increasing temperature. HCl-P_i and residual P increased with increasing temperature. Interaction of CC and RH with soils resulted in an increase in most labile P as well as moderately labile P (NaOH-P_i) fractions in the soils. CC increased most labile P in the soils more than RH. The increase in most labile P fraction in soils was more significant at relatively lower temperatures (300°C and 450°C) than 650°C. However, the increase in HCl-P_i and residual P of the soils was more predominant at high temperature (650°C). The study suggested that biochar pyrolyzed at 300–450°C could be used to increase P bioavailability in tropical soils.     

Enhanced biological cycling of phosphorus increases its availability to crops in low-input sub-Saharan farming systems

Many soils in sub-Saharan Africa, which are farmed by smallholders, are P deficient and highly P fixing. Furthermore, P inputs supplied as farmyard manure (FYM) or inorganic P fertilizer are normally too small to replace P offtakes by crops. Consequently most soils are in a negative P balance, which is reflected in small, and often declining, crop yields. The obvious solution of simply applying adequate P is seldom an option due to shortages of manure, which is usually low in nutrients in any case, and the high cost of inorganic P fertilizer relative to the likely cash value of the harvest. Our aim was to see if we could devise practical methods to increase soil P availability in this situation and to investigate the mechanisms involved. Two approaches were adopted. Firstly, to attempt to saturate the P-fixing sites in the soils by applying a large annual application of P (75 kg P ha⁻¹), which should serve for several seasons. Secondly, to attempt to keep the fertilizer P in biological forms by supplying fertilizer P and cattle manure (FYM) in combination. Here, the aim was to promote the cycling of P through the soil microbial biomass and associated metabolite pools, with the expected result of decreasing P fixation and increased plant availability of this P. These treatments were investigated using two field sites on smallholder farms in Kenya: one, considered a ‘high P fixing’ soil at Malava (Kakamega District) and one considered a ‘low P fixing’ soil at Mau Summit (Nakuru District). The following treatments were applied in 1997 and 1998: nil; 75 kg P ha⁻¹ as super phosphate (P); 25 kg P ha⁻¹; FYM at 1.9 t ha⁻¹ dry matter; FYM+25 kg P ha⁻¹. All treatments also received 100 kg inorganic N ha⁻¹. Maize was the test crop. There was no significant correlation in either year at either site between soil P, measured as NaHCO₃-extractable P, resin P or NaOH-extractable P and maize yield. However, the different soil P fractions were closely correlated with each other. Yields at the high P rate (75 kg ha⁻¹y⁻¹) were often little better than the control. There was, however, a significant positive relationship (P<0.05) between soil microbial biomass P and crop yield, again at both sites and in both years. The treatment giving the best yield and the largest biomass P was always FYM+P. Our results indicate that the combined use of organic and inorganic fertilizers in these low input systems may promote increased biological cycling, enhanced availability and consequently improved plant uptake of soil and fertiliser P, to the advantage of the small scale farmer. The results also indicate that biomass P measurements may provide a better indicator of soil P availability in these soils than some more conventional chemical extractants. However, both findings require further evaluation.     

免耕和秸秆覆盖对黑垆土磷素形态组分的影响

[目的]探究免耕及添加秸秆条件下黑垆土土壤磷组分特征及其与AM真菌侵染的关系,了解雨养农业区农业系统磷素利用效率。[方法]在陇东黄土高原黑垆土区域,测定传统耕作、传统耕作+秸秆覆盖、免耕和免耕+秸秆覆盖4种处理小麦—玉米—大豆轮作系统中玉米阶段土壤全磷、速效磷组分及AM真菌菌根侵染率。[结果]水土保持耕作处理实施9a后,免耕和秸秆覆盖处理下0—5cm土壤磷素含量显著提高,活性磷组分H2O—Pi,NaHCO3—Pi,NaOH—Pi分别比对照提高84.6%,85.2%和56.6%;活性无机磷(H2O—Pi,NaHCO3—Pi之和)和潜在活性磷(NaOH—Pi)分别占总无机磷的11.4%和4.5%,全磷含量与磷组分、速效磷与磷组分呈显著正相关,2个免耕处理菌根侵染率分别比对照增加20.8%和16.5%。[结论]免耕和秸秆覆盖显著提高了土壤磷含量,免耕对AM真菌菌根侵染率有积极影响。     

Soil phosphorus dynamics in a long-term field experiment at Askov

Inorganic and organic soil P (P_i, P_o) fractions were followed monthly for 15 months in a 100-year-old, fertilization and crop-rotation experiment with the Rubaek-Sibbesen, macroporous resin method, the Olsen method, and the Hedley fractionation method. Resin P, and Olsen P had similar levels and variation patterns. They increased in spring after fertilization, decreased during summer and autumn, and increased again in winter after repeated slurry applications. Resin P_o decreased in spring and peaked in summer. The variation in time of the Hedley P_i and P_o fractions was relatively smaller and was neither related to season nor to fertilization. Unmanured soil contained much less total P than NPK and slurry-treated soils, but the differences in total P_i were greater than those in total P_o. Neither total P_i nor total P_o concentrations differed between NPK and slurry treatments, indicating that P_o in animal manure is quickly mineralized. All P_i and P_o fractions were smaller in unmanured than in fertilized treatments. These differences were relatively largest for resin P_i and resin P_o, i.e., the most labile fractions, and decreased for the medium and less labile P_i and P_o fractions. The reactions by resin P_i, Olsen P, and resin P_o to seasons and treatments indicate that these fractions are estimates of the most labile pools of P_i and P_o in soil, which make them relevant for shortterm studies. The medium and less labile P_i and P_o fractions of the Hedley fractionation method seem more relevant for long-term studies.     

Dynamics of phosphorus fractions in a savanna Alfisol under continuous cultivation

Abstract. We examined the dynamics of inorganic P (P_i) and organic P (P_o) pools of a savanna Alfisol under continuous cultivation complemented with nitrogen (N) and phosphorus (P) fertilizers with or without cow manure (D), using a modified Hedly fractionation. Continuous cultivation without P fertilizer decreased the concentration of P_i and P_o, pools including the residual P fraction compared with an unfertilized treatment with natural vegetation. Adding P fertilizers alone or in combination with D (P, NPK, D+P, D+NP and D+NPK) increased the concentration of labile P_i pools, but decreased the concentration of P_o, pools and the residual P fraction. The tendency of the residual P fraction to decrease along with P_o pools suggested that the residual P fraction was largely P_o. This is consistent with the correlation between the residual P fraction and total P_o (r=0.74^**) and the residual P fraction and organic carbon content (r=0.47^**). Analysis of the relationships between plant available P, estimated by resin-extractable P, and the more stable P_i and P_o, pools indicated that 85% of the variation in resin-P was atiributable to the hydroxide extractable P_i, (OH-P_i) and HCI-P_i which acted as sinks for fertilizer-P. The contribution of the residual P fraction to resin-P was indirect via bicarbonate-extractable P_o (HCO₃-P_o) and OH-P_o. These results also clarify why attempts to find P extraction methods which correlate well with response to P and to P uptake in the savanna have given inconsistent results.