Influence of soil phosphorus status and nitrogen addition on carbon mineralization from 14C-labelled glucose in pasture soils

 This study examines the effect of soil P status and N addition on the decomposition of ¹⁴C-labelled glucose to assess the consequences of reduced fertilizer inputs on the functioning of pastoral systems. A contrast in soil P fertility was obtained by selecting two hill pasture soils with different fertilizer history. At the two selected sites, representing low (LF) and high (HF) fertility status, total P concentrations were 640 and 820 mg kg^–1 and annual pasture production was 4,868 and 14,120 kg DM ha^–1 respectively. Soils were amended with ¹⁴C-labelled glucose (2,076 mg C kg^–1 soil), with and without the addition of N (207 mg kg^–1 soil), and incubated for 168 days. During incubation, the amounts of ¹⁴CO₂ respired, microbial biomass C and ¹⁴C, microbial biomass P, extractable inorganic P (P_i) and net N mineralization were determined periodically. Carbon turnover was greatly influenced by nutrient P availability. The amount of glucose-derived ¹⁴CO₂ production was high (72%) in the HF and low (67%) in the LF soil, as were microbial biomass C and P concentrations. The ¹⁴C that remained in the microbial biomass at the end of the 6-month incubation was higher in the LF soil (15%) than in the HF soil (11%). Fluctuations in P_i in the LF soil during incubation were small compared with those in HF soil, suggesting that P was cycling through microbial biomass. The concentrations of P_i were significantly greater in the HF samples throughout the incubation than in the LF samples. Net N mineralization and nitrification rates were also low in the LF soils, indicating a slow turnover of microorganisms under limited nutrient supply. Addition of N had little effect on biomass ¹⁴C and glucose utilization. This suggests that, at limiting P fertility, C turnover is retarded because microbial biomass becomes less efficient in the utilization of substrates. Received: 18 October 1999     

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.     

Land-use and fertilization effects on P forms in two European soils: resin extraction and 31P-NMR analysis

Macroporous anion-exchange resin extraction and ³¹P-NMR spectroscopy of dialysed NaOH extracts were used to investigate the effects of land use (Taubenberg, Bavaria: spruce forest, deciduous forest, permanent grass, arable) and fertilization (Askov, Denmark: unmanured, mineral fertilizer, animal manure) on forms of phosphorus in soil with emphasis on the potentially labile organic (P_o) and inorganic (P_i pools. Carbon content ranged from 12.5–118.1 g kg^?1 and total P (P_i) content from 511 to 2063 mg kg^?1. For all soils, the C:P_o ratios of SOM decreased in the order: whole soil, 150: 1–44:1; alkali extract, 57:1–41: 1; resin extract, 20:1–9:1; suggesting an increasing P functionality of the OM with increasing P_o lability. Analysis of functional relation showed a close relation between resin P_o and ³¹P-NMR estimates for diester-P including teichoic acid-P, indicating that these species contributed significantly to the labile P_o pool as determined by the resin method. The most marked effects of land-use were an increase in P_i under grass and arable, a concurrent sequestration of P_o and SOM under grass, and a depletion of P_o under arable. The amount of resin P_j appeared to be a function primarily of fertilization, and amounted to around 100 mg kg^?1 in the fertilized soils irrespective of the SOM content and P source. The forest soil and the unfertilized agricultural soil had much smaller resin P_j values. The soil under grass had the largest amounts in resin P_o and diester-P including teichoic acid-P, indicating a rapid turnover of P_o with build-up of a large potentially labile, microbially derived P_o pool. ³¹P NMR also showed large proportions of labile P_o species in soils where microbial activity is restrained by acidity (Taubenberg spruce forest, phosphonates) or where highly microbially altered OM is relatively enriched (Taubenberg arable, diester-P including teichoic acid-P). We conclude that the resin used in this study isolates a structurally and functionally reasonably uniform pool of potentially labile soil P_o.     

Sequentially extracted phosphorus fractions in peat‐derived soils

Phosphorus (P) forms were sequentially extracted from peat derived soils (Eutric Histosols and Gleysols) at eight sites in Saxony‐Anhalt (Germany) to disclose general differences in P pools between mineral and organic soils and to investigate effects of peat humification and oxidation in conjunction with land use and soil management on the P status of soils. Overall 29 samples providing a wide variety of basic chemical properties were subjected to the Hedley fractionation. The Histosol topsoils contained more total P (P_t) (1345 ± 666 mg kg^—1) than the Gleysol topsoils (648 ± 237 mg kg^—1). The predominant extractable fractions were H₂SO₄‐P (36—63 % of P_t) in calcareous and NaOH‐P_o (0—46 % of P_t) in non‐calcareous Histosols. These soils had large pools of residual P (13—93 % of P_t). Larger contents and proportions of P_o and of labile P fractions generally distinguished organic from mineral soils. Regression analyses indicated that poorly crystalline pedogenic oxides and organic matter were binding partners for extractable and non‐extractable P. Intensive management that promotes peat humification and oxidation results in disproportional enrichments of labile P fractions (resin‐P, NaHCO₃‐P_i, and NaHCO₃‐P_o). These changes in P chemistry must be considered for a sustainable management of landscapes with Histosols and associated peat derived soils.     

Effects of increasing periods under intensive arable vegetable production on biological, chemical and physical indices of soil quality

 The effects on soil condition of increasing periods under intensive cultivation for vegetable production on a Typic Haplohumult were compared with those of pastoral management using soil biological, physical and chemical indices of soil quality. The majority of the soils studied had reasonably high pH, exchangeable cation and extractable P levels reflecting the high fertilizer rates applied to dairy pasture and more particularly vegetable-producing soils. Soil organic C (C_org) content under long-term pasture (>60 years) was in the range of 55 g C kg^–1 to 65 g C kg^–1. With increasing periods under vegetable production soil organic matter declined until a new equilibrium level was attained at about 15–20 g C kg^–1 after 60–80 years. The loss of soil organic matter resulted in a linear decline in microbial biomass C (C_mic) and basal respiratory rate. The microbial quotient (C_mic/C_org) decreased from 2.3% to 1.1% as soil organic matter content declined from 65 g C kg^–1 to 15 g C kg^–1 but the microbial metabolic quotient (basal respiration/C_mic ratio) remained unaffected. With decreasing soil organic matter content, the decline in arginine ammonification rate, fluorescein diacetate hydrolytic activity, earthworm numbers, soil aggregate stability and total clod porosity was curvilinear and little affected until soil organic C content fell below about 45 g C kg^–1. Soils with an organic C content above 45 g C kg^–1 had been under pasture for at least 30 years. At the same C_org content, soil biological activity and soil physical conditions were markedly improved when soils were under grass rather than vegetables. It was concluded that for soils under continuous vegetable production, practices that add organic residues to the soil should be promoted and that extending routine soil testing procedures to include key physical and biological properties will be an important future step in promoting sustainable management practices in the area. Received: 18 November 1997     

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.     

Surface runoff phosphorus (P) loss in relation to phosphatase activity and soil P fractions in Florida sandy soils under citrus production

Phosphorus losses by surface runoff from agricultural lands have been of public concern due to increasing P contamination to surface waters. Five representative commercial citrus groves (C1-C5) located in South Florida were studied to evaluate the relationships between P fractions in soils, surface runoff P, and soil phosphatase activity. A modified Hedley P sequential fractionation procedure was employed to fractionate soil P. Soil P consisted of mainly organically- and Ca/Mg-bound P fractions. The organically-bound P (biological P, sum of organic P in the water, NaHCO₃ and NaOH extracts) was dominant in the acidic sandy soils from the C2 and C3 sites (18% and 24% of total soil P), whereas the Ca/Mg-bound P (HCl-extractable P) accounted for 45-60% of soil total P in the neutral and alkaline soils (C1, C4 and C5 soils). Plant-available P (sum of water and NaHCO₃ extractable P fractions) ranged from 27 to 61 mg P kg⁻¹ and decreased in the order of C3>C4>C1>C2>C5. The mean total P concentrations (TP) in surface runoff water samples ranged from 0.51 to 2.64 mg L⁻¹. Total P, total dissolved P (TDP), and PO₄³⁻-P in surface runoff were significantly correlated with soil biological P and plant-available P forms (p<0.01), suggesting that surface runoff P was directly derived from soil available P pools, including H₂O- and NaHCO₃- extractable inorganic P, water-soluble organic P, and NaHCO₃- and NaOH-extractable organic P fractions, which are readily mineralized by soil microorganisms and/or enzyme mediated processes. Soil neutral (55-190 mg phenol kg⁻¹ 3 h⁻¹) and natural (measured at soil pH) phosphatase activities (77-295 mg phenol kg⁻¹ 3 h⁻¹) were related to TP, TDP, and PO₄³⁻-P in surface runoff, and plant-available P and biological P forms in soils. These results indicate that there is a potential relationship between soil P availability and phosphatase activities, relating to P loss by surface runoff. Therefore, the neutral and natural phosphatase activities, especially the natural phosphatase activity, may serve as an index of surface runoff P loss potential and soil P availability.     

Sequential extractions and 31P-NMR spectroscopy of phosphorus forms in animal manures, whole soils and particle-size separates from a densely populated livestock area in northwest Germany

The solubility and forms of phosphorus (P) were investigated in manures from chicken and pigs, eight whole soil samples and clay-, silt-, and sand-size separates from an arable and a grassland soil. Total P (P_t) in liquid pig manure (16.2 g kg^–1) and dry chicken manure (26.2 g kg^–1) was distributed between residual P (39–41% P_t), H₂SO₄–P (17–27% P_t), labile resin- and NaHCO₃–P (24–39% P_t), and NaOH-P (3–10% P_t). Most soils had larger proportions of NaOH-P and residual P, indicating reactions of manure-derived P compounds with pedogenic oxides and humic substances. Clay-size separates had the highest P-concentrations in all fractions and were particularly enriched in exchangeable and labile P forms. Solution ³¹P-nuclear magnetic resonance (NMR) spectra of 0.5 M NaOH extracts from manures and some soil samples showed greater signal intensities for orthophosphate and monoester P than 0.1 M NaOH extracts. This can be explained by alkaline hydrolysis phosphate diesters at higher NaOH concentrations and/or by preferential extraction of diesters at lower concentrations. The ³¹P-NMR spectra showed differences between the two manures and confirmed that increasing proportions of ester-P can be expected if they are spread to soils. The NaOH extracts of soil samples were characterized by large proportions of orthophosphate-P (mean 77% of assigned P compounds), which seemed to be slightly enriched in clay fractions whereas the extracts from silt contained more ester-P. Sequential extractions and ³¹P-NMR spectroscopy both showed that these excessively manured soils are likely to lose large amounts of P. Received: 15 July 1996     

Fractionation of phosphorus in soils with different geological and soil physicochemical properties in southern Tanzania

ABSTRACT

Soil phosphorus (P) forms have been practically defined as chemically fractionated pools. A knowledge of the abundance and diversity of P forms in soil, and the factors affecting them, will lead to better soil management. However, little is known about the differences in P forms among soils with different geological properties in tropical Africa. The aim of this study was to investigate the P forms in soils with different physicochemical properties formed under different geological conditions in southern Tanzania and to identify the factors affecting the P forms in these soils. In total, 37 surface soil samples were collected from three geological groups; the plutonic (mainly granite) rock (PL) group, the sedimentary and metamorphic rock (SM) group, and the volcanic ash (V) group. Soil P was sequentially extracted by NH₄Cl, NH₄F, NaHCO₃, NaOH + NaCl, and HCl, and inorganic (P_i) and organic P (P_o) in each fraction were determined. The lowest total P was in the PL group (average, 360 mg P kg^-1) because of the high sand content. Iron (Fe)-P (NaOH-P_i) was the major form in this group, accounting for 8.4% of total P. In the SM group (average total P, 860 mg P kg^-1), Fe-P was the major form in most, accounting for 7.8% of total P. Soils in the SM group occasionally had high calcium (Ca)-P due to application of chemical fertilizer at the collection site. The V group had the highest total P (average, 1600 mg P kg^-1) and its major P form was Ca-P, which was possibly derived from primary minerals (i.e., apatite), accounting for 14% of total P. In addition, the high oxalate-extractable Al possibly caused the accumulation of Al-P in the V group. Oxalate-extractable Fe generally increased with increasing Fe-P_i, while oxalate-extractable Al increased with increasing organic P and Al-P_i in soils in all three geological groups. These results demonstrate that the soil P forms differ greatly among sites in southern Tanzania with different geological conditions and associated soil properties.     

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     

Components of organic phosphorus in soil extracts that are hydrolysed by phytase and acid phosphatase

 Extracts were prepared from soil using water, 50 mM citric acid (pH ∼2.3) or 0.5 M NaHCO₃ (pH 8.5), and were incubated with excess phytase from Aspergillus niger to determine the amounts of labile P. Two A. niger phytase preparations were used: (1) a purified form which exhibited a narrow substrate specificity and high specific activity against phytate; and (2) a commercial preparation (Sigma) with activity against a broad range of P compounds. A comparatively large proportion (up to 79%, or 5.7 μg g^–1 soil) of the organic P (P_o) extracted with citric acid was hydrolysed by the commercial phytase, while between 28% and 40% (up to 3.1 μg g^–1 soil) was hydrolysed using purified phytase. By comparison, only small quantities of the P_o in water and NaHCO₃ soil extracts were enzyme labile. While extractable P_o was increased both with increasing concentrations of citric acid (up to 50 mM) and increasing pH (pH 2.3–6.0), enzyme-labile P increased only with citric acid concentration. The labile component of P_o in citric acid extracts from soils with contrasting fertiliser histories indicated that enzyme-labile P_o is a relatively large soil P pool and is potentially an important source of P for plants. Received: 29 October 1999     

Basal organic phosphorus mineralization in soils under different farming systems

Soil organic P (P_o) mineralization plays an important role in soil P cycling. Quantitative information on the release of available inorganic P (P_i) by this process is difficult to obtain because any mineralized P_i gets rapidly sorbed. We applied a new approach to quantify basal soil P_o mineralization, based on ³³PO₄ isotopic dilution during 10 days of incubation, in soils differing in microbiological activity. The soils originated from a 20 years old field experiment, including a conventional system receiving exclusively mineral fertilizers (MIN), a bio-organic (ORG) and bio-dynamic (DYN) system. Indicators of soil microbiological activity, such as size and activity of the soil microbial biomass and phosphatase activity, were highest in DYN and lowest in MIN. In order to assess P_o hydrolysis driven by phosphatase in sterile soils, a set of soil samples was γ-irradiated. Basal P_o mineralization rates in non-irradiated samples were between 1.4 and 2.5 mg P kg⁻¹ day⁻¹ and decreased in the order DYN>ORG≥MIN. This is an amount lower, approximately equivalent to, or higher than water soluble P_i of MIN, ORG and DYN soils, respectively, but in every soil was less than 10% of the amount of P isotopically exchangeable during one day. This shows that physico-chemical processes are more important than basal mineralization in releasing plant available P_i. Organic P mineralization rates were higher, and differences between soils were more pronounced in γ-irradiated than in non-irradiated soils, with mineralization rates ranging from 2.2 to 4.6 mg P kg⁻¹ day⁻¹. These rates of hydrolysis, however, cannot be compared to those in non-sterile soils as they are affected by the release of cellular compounds, e.g. easily mineralizable P_o, derived from microbial cells killed by γ-irradiation.     

Land-use effects on phosphorus fractions in Cerrado oxisols

 Differently managed oxisols from Central Brazil were studied using a sequential P fractionation procedure because the effects of land use on the distribution of P forms in these soils are barely known. Therefore levels of labile and recalcitrant P forms in the natural Cerrado savanna were compared with those of crop, pasture and reforestation sites on differently textured oxisols. Under natural conditions of strong P deficiency, >60% of labile P was organic P (P_o), reflecting the high contribution of P_o to plant nutrition. Fertilisation after land-use change only increased levels of inorganic P (P_i) forms, the increase being most accentuated in the labile P_i fraction. At the crop and the pasture sites P tended to accumulate as recalcitrant P forms in the clayey soils, while in the loamy soils there was only a minor enrichment, probably due to the lower amounts of Fe- and Al-(hydr)oxides. In the reforestation sites, labile P was maintained at high levels, most likely through efficient recycling of the litter. The P fractionation procedure was also applied to particle-size fractions which reflected P transformations along an organic and mineralogical gradient. The clay fraction corresponded to 69–87% of total P and appeared to be both a sink for highly recalcitrant P and a source for labile P, especially labile P_o. The sequential P fractionation procedure was also an effective method with which to detect the presence of fertiliser-P residues in the coarse-silt fraction. Received: 18 June 1999     

Phosphorus sequestration in soil aggregates after long-term tillage and cropping

Cultivated soils in the Everglades are being converted to their historic use as pastures or seasonally flooded prairies as parts of restoration efforts, but long-term cultivation may have altered soil P distribution and availability which may pose eutrophication hazards upon change in land use. The objectives of this study were to determine the distribution of P in soil chemical and physical fractions for contrasting long-term land management practices. The distribution of P in labile, Fe–Al bound, Ca bound, humic–fulvic acid, and residual pools in five aggregate-size fractions were measured for fields under sugarcane (Saccharum sp.) cropping for 50 years and perennial pasture for 100 years. Both land uses were characterized by a high degree of macroaggregation, as aggregates >0.25 mm contained 76 and 83% of the total soil under cultivation and pasture, respectively. Soils under sugarcane sequestered a total of 77 kg ha⁻¹ more P than pasture at 0–15 cm. The distribution of P in chemical fractions significantly varied between land uses as cultivation increased P sequestration in Ca-bound fractions more for sugarcane (244 kg P ha⁻¹) than pasture (65 kg P ha⁻¹). Pasture sequestered more P in organic pools, as storage in humic–fulvic acid and residual fractions were 26 and 25%, respectively, higher than sugarcane. Labile P was 100% higher for pasture than sugarcane, but Fe–Al bound P storage did not differ between land uses. Aggregation increased P sequestration in humic–fulvic acid and residual fractions, and P storage in organic pools increased with increasing aggregate size. In contrast, cultivation decreased aggregation and increased P accumulation in inorganic fractions. Long-term cultivation altered the distribution of soil P from organic to inorganic pools. The P stored in inorganic pools is stable under current land use, but may be unstable and pose eutrophication hazards upon onset of future land use change to the seasonally flooded prairie ecosystem.     

Dissolution of Dominant Soil Phosphorus Fractions in Phosphorus-Responsive Soils of Bihar,India: Effects of Mycorrhiza and Fertilizer Levels

We investigated the effects of Arbuscular Mycorrhiza (AM) fungi and various phosphorus (P) levels on the distribution and availability of P in dominant soils of Bihar, India. Potassium chloride (KCl)-P (labile P), sodium hydroxide (NaOH)-P (Fe-Al-bound P), hydrochloric acid (HCl)-P (Ca-bound P), and residual P (Res-P) fractions were analyzed in the soils under maize plant. Ca-bound P was the most abundant P fraction in the alkaline soils (65% of the total P) followed by neutral soil (35% of the total P), whereas it was less abundant (<4%) in the acidic soil type. Fe-Al-bound P was found to be highest for acidic soil (65% of the total P). Soils under the inoculation with Glomus mossae and control gave the highest and lowest values (15.63 mg kg^?1 and 10.74 mg kg^?1 respectively) for the labile fraction which was similar to the organically bound residual fractions of P (200.17 mg kg^?1 and 193.66 mg kg^?1 respectively. Inoculation of the soils with AM fungi leads to the redistribution of P fractions in different soils which consequently helps in improvement of available P in soil conducive for plant uptake.     

Effects of repeated manure and fertilizer phosphorus additions on soil phosphorus dynamics under a soybean-wheat rotation

 Soil P availability and efficiency of applied P may be improved through an understanding of soil P dynamics in relation to management practices in a cropping system. Our objectives in this study were to evaluate changes in plant-available (Olsen) P and in different inorganic P (P_i) and organic P (P₀) fractions in soil as related to repeated additions of manure and fertilizer P under a soybean-wheat rotation. A field experiment on a Typic Haplustert was conducted from 1992 to 1995 wherein the annual treatments included four rates of fertilizer P (0, 11, 22 and 44 kg ha^–1 applied to both soybean and wheat) in the absence and presence of 16 t ha^–1 of manure (applied to soybean only). With regular application of fertilizer P to each crop the level of Olsen P increased significantly and linearly through the years in both manured and unmanured plots. The mean P balance required to raise Olsen P by 1 mg kg^–1 was 17.9 kg ha^–1 of fertilizer P in unmanured plots and 5.6 kg ha^–1 of manure plus fertilizer P in manured plots. The relative sizes of labile [NaHCO₃-extractable P_i (NaHCO₃-P_i) and NaHCO₃-extractable P₀ (NaHCO₃-P₀)], moderately labile [NaOH-extractable P_i (NaOH-P_i) and NaOH-extractable P₀ (NaOH-P₀)] and stable [HCl-extractable P (HCl-P) and H₂SO₄/H₂O₂-extractable P (resisual-P)] P pools were in a 1 : 2.9 : 7.6 ratio. Application of fertilizer P and manure significantly increased NaHCO₃-P_i and -P₀ and NaOH-P_i, and -P₀ fractions and also total P. However, HCl-P and residual-P were not affected. The changes in NaHCO₃-P_i, NaOH-P_i and NaOH-P₀ fractions were significantly correlated with the apparent P balance and were thought to represent biologically dynamic soil P and act as major sources and sinks of plant-available P. Received: 23 October 1997     

Heterotrophic nitrification is the predominant NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> production mechanism in coniferous but not broad-leaf acid forest soil in subtropical China

A study was carried out to investigate the potential gross nitrogen (N) transformations in natural secondary coniferous and evergreen broad-leaf forest soils in subtropical China. The simultaneously occurring gross N transformations in soil were quantified by a ¹⁵N tracing study. The results showed that N dynamics were dominated by NH₄⁺ turnover in both soils. The total mineralization (from labile and recalcitrant organic N) in the broad-leaf forest was more than twice the rate in the coniferous forest soil. The total rate of mineral N production (NH₄⁺ + NO₃⁻) from the large recalcitrant organic N pool was similar in the two forest soils. However, appreciable NO₃⁻ production was only observed in the coniferous forest soil due to heterotrophic nitrification (i.e. direct oxidation of organic N to NO₃⁻), whereas nitrification in broad-leaf forest was little (or negligible). Thus, a distinct shift occurred from predominantly NH₄⁺ production in the broad-leaf forest soil to a balanced production of NH₄⁺ and NO₃⁻ in the coniferous forest soil. This may be a mechanism to ensure an adequate supply of available mineral N in the coniferous forest soil and most likely reflects differences in microbial community patterns (possibly saprophytic, fungal, activities in coniferous soils). We show for the first time that the high nitrification rate in these soils may be of heterotrophic rather than autotrophic nature. Furthermore, high NO₃⁻ production was only apparent in the coniferous but not in broad-leaf forest soil. This highlights the association of vegetation type with the size and the activity of the SOM pools that ultimately determines whether only NH₄⁺ or also a high NO₃⁻ turnover is present.     

Nitrogen and phosphorus uptake by maize as affected by particulate organic matter quality, soil characteristics, and land-use history for soils from the West African moist savanna zone

 The impact of land use (unfertilized continuous maize cropping, unfertilized and fertilized alley cropping with maize, Gliricidia sepium tree fallow, natural fallow) on the soil organic matter (SOM) status and general soil fertility characteristics were investigated for a series of soils representative for the West African moist savanna zone. Three soils from the humid forest zone were also included. In an associated pot experiment, relationships between maize N and P uptake and SOM and general soil characteristics were developed. Soils under natural fallow contained the highest amount of organic C (1.72%), total N (0.158%), and had the highest effective cation exchange capacity (ECEC) [8.9 mEq 100 g^–1 dry soil], while the Olsen P content was highest in the fertilized alley cropping plots (13.7 mg kg^–1) and lowest under natural fallow (6.3 mg kg^–1). The N concentration of the particulate organic matter (POM) was highest in the unfertilized alley cropping plots (2.4%), while the total POM N content was highest under natural fallow (370 mg N kg^–1) and lowest in continuously cropped plots (107 mg N kg^–1). After addition of all nutrients except N, a highly significant linear relationship (R ²=0.91) was observed between the total N uptake in the shoots and roots of 7-week-old maize and the POM N content for the savanna soils. POM in the humid forest soils was presumably protected from decomposition due to its higher silt and clay content. After addition of all nutrients except P, the total maize P uptake was linearly related to the Olsen P content. R ² increased from 0.56 to 0.67 in a multiple linear regression analysis including the Olsen P content and clay content (which explained 11% of the variation in P uptake). Both the SOM status and N availability were shown to be improved in land-use systems with organic matter additions, while only the addition of P fertilizer could improve P availability. Received: 9 April 1999     

Growth,P uptake in grain legumes and changes in rhizosphere soil P pools

In soils with low P availability, several legumes have been shown to mobilise less labile P pools and a greater capacity to take up P than cereals. But there is little information about the size of various soil P pools in the rhizosphere of legumes in soil fertilised with P although P fertiliser is often added to legumes to improve N₂ fixation. The aim of this study was to compare the growth, P uptake and the changes in rhizosphere soil P pools in five grain legumes in a soil with added P. Nodulated chickpea (Cicer arietinum L.), faba bean (Vicia faba L.), white lupin (Lupinus albus L.), yellow lupin (Lupinus luteus L.) and narrow-leafed lupin (Lupinus angustifolius L.) were grown in a loamy sand soil low in available P to which 80 mg P kg⁻¹ was added and harvested at flowering and maturity. At maturity, growth and P uptake decreased in the following order: faba bean > chickpea > narrow-leafed lupin > yellow lupin > white lupin. Compared to the unplanted soil, the depletion of labile P pools (resin P and NaHCO₃-P inorganic) was greatest in the rhizosphere of faba bean (54% and 39%). Of the less labile P pools, NaOH-P inorganic was depleted in the rhizosphere of faba bean while NaOH-P organic and residual P were most strongly depleted in the rhizosphere of white lupin. The results suggest that even in the presence of labile P, less labile P pools may be depleted in the rhizosphere of some legumes.     

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.