The water–dispersible colloid (WDC) fraction (≤ 450 nm) has the potential to leach into groundwater, leading to a loss of nutrients from the bulk soil. The objective of this study was to determine the concentrations of organic carbon (C org ) and inorganic elements in the WDC fraction of volcanic (WDC–V) and arid (WDC–A) Chilean soils. The WDC fractions were isolated from three arid bulk soil at different meters above sea level (masl) from northern Chile and from five volcanic bulk soil from the southern Chile. The isolation of WDC was carried out according to Stokes’ Law, using Milli–Q water as the extracting agent. The WDC were simultaneously separated and analyzed into natural nanoparticles (NNPs; 0.06–30 nm), fine colloids (FC; 30–220 nm), and medium colloids (MC; 220–450 nm) fractions using flow field–flow fractionation coupled to inductively coupled plasma mass spectrometry or ultraviolet–visible detectors. The WDC–A fractions at 300 and 500 masl had higher concentrations of Fe, Al, Si, Ca, Mg, Mn, and Zn, ranging between 35 and 193,070 µg L −1 , compared to WDC–V, except for P. Additionally, C org in WDC–A at 300 and 500 masl ranged from 2600–3600 µg L −1 , which was higher than in WDC–V, except for Carilafquen and Lautaro soils. The concentration of Si, Al, Fe, and P increased with larger particle sizes in the WDC–V fraction, whereas in the WDC–A fractions, the concentrations of Si, Al, and Fe were higher in the FC and MC fractions, while P and Ca were higher in the NNPs and FC size fractions. The concentration of inorganic elements and C org in the WDC fractions differed between the arid northern soils and the volcanic soils of southern Chile, likely attributed to variations in the amount and stability of organic matter present in each soil type.

The aim of this study was to determine if three cereal crops differed in their behavior to take up soil and fertilizer P, with emphasis on the relationship between phosphatase activity and P fractionation. We used a vertical rhizobox experiment with wheat, oat, and barley sown on Chilean Andisol (Barros Arana Series) with low P availability under greenhouse conditions. Plants were fertilized with the equivalent of 100 kg P ha −1 of triple superphosphate (TSP) or rock phosphate (RP). Plant biomass was determined for each of the three cereal plant species. Additionally, phosphatase (P-ase) activity in roots, soil in presence of roots (soil+R), and soil in absence of roots (soil−R) after 60-day growth were evaluated, and soil P fractionation was determined using the Hedley procedure. Fertilizer increased both P uptake and biomass production, particularly in shoots. The P uptake efficiency (∆P uptake between fertilized and unfertilized treatment/P input) was low (4.6%) and similar for both fertilizers for oat, but RP was more efficient for wheat (> 30%) and even more so for barley (nearly threefold), due the higher shoot P concentration of RP fertilized plants, which could be attributable to a major P-ase activity in plants fertilized with RP. Despite, fertilizer P was most clearly identified in labile inorganic soil fractions with Olsen P being greater after TSP addition than RP. In particular, plants showed contrasting soil+R P-ase activity inducing differences in soil+R P speciation, increasing labile NaHCO 3 -P i and NaOH-P i fractions with TSP. Strong relationships were found between the sum of labile P i fractions and P uptake. We conclude that slower release of RP has a positive impact on P-ase activity and leads to better fertilizer efficiency than TSP, especially for barley.

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 degrees 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.

Volcanic ash derived soils represent between 50-60% of the total arable land area of southern of Chile, and they are the most important soils for pasture production. In these soils, high phosphorus (P) fixation and, in turn, low P availability and high aluminium (Al) soluble concentrations (at low pH) are the most limiting factors for pasture production. At the same time, the complexes between Al-or iron- (Fe) and organic matter as well as short-range order alumino-silicates (allophane) allow the retention of huge quantities of soil P. The aim of this work was to assess the status of P by both sequential extraction procedure (Hedley) and P-31-NMR analysis as influenced by Al and Fe in volcanic grasslands Andisols (Pemehue, Gorbea, Piedras Negras and Llastuco Soil Series) from Southern Chile. We applied Hedley chemical sequential fractionation to soils in order to examine the potential differences in extractable soil inorganic P (P-i) and organic P (P-o) fractions. We also determined total P and Olsen P in these grassland Andisols. Oxalate and pyrophosphate were employed to determine the active and organic matter complexed Al and Fe, respectively. Furthermore, we quantified Al and Fe in extracts of the Hedley P fractions. We found that Al extracted in oxalate was correlated positively with labile P-o concentration, specifically with both the NaHCO3-P-o (r=0.45, P <= 0.01), and the NaOH-P-o (r=0.43, P <= 0.01) fractions. This observation was reinforced by P-31-NMR analysis that showed higher monoester P and myo-IP6 content in soils with higher amounts of oxalate Al. Hedley sequential fractionation procedure confirmed the role of Al in the NaOH-P-o fraction for promoting P-o storage, as both fractions were correlated (r=0.33, P <= 0.05). In addition, Fe plays a substantial role in recalcitrant P accumulation as we found a high correlation between residual P and oxalate Fe (r=0.55, P <= 0.01).

Sequential fractionation has been widely used to study the nature and dynamics of soil P. Residual P – the recalcitrant P fraction remaining after sequential extraction with alkali and acid reagents – often constitutes the majority of the soil P, yet its nature and bioavailability is poorly understood. The objective of this study was to isolate, quantify, and characterize residual P following Hedley fractionation in a range of Andisols under grazed pasture by 31P nuclear magnetic resonance (NMR) spectroscopy. Residual P accounted for 45–63% of the total soil P, of which 53–77% was inorganic orthophosphate. Organic P accounted for 21–42% of the residual P, the majority of which occurred as phosphomonoesters including myo- (16% of the residual P) and scyllo-inositol hexakisphosphate (10% of the residual P). No phosphodiesters were detected in the residual fraction. We conclude that residual P in Andisols consists of a mixture of inorganic P and organic P. Our findings provide the basis for the development of new approaches to improve P use efficiency in agriculture.

The aim of this study was to compare P bioavailability in a Nothofagus rainforest Andisol (FS) and an adjacent clear-cut grassland soil (GS) in southern Chile to evaluate the effects of land use change on P chemical forms determined by chemical fractionation and 31P-NMR spectroscopy. Total phosphorus (P), Olsen P, microbial P, different soil P fractions (determined using a modified Hedley procedure), 31P-NMR spectroscopy results, acid phosphatase (P-ase) activity, pH and organic C were analyzed and compared. Forest samples were collected from the mineral soil at a depth of 2-20 cm and were compared with those collected from grassland soil at the same depth. Total P ranged from 2028 mg kg-1 (FS) to 2157 mg kg-1 (GL)and total organic P ranged from 829 mg kg-1 (FS) to 1176 mg kg-1 (GL). On the contrary, Olsen P, microbial P, labile P and P-ase activity were higher in the evergreen forest soil than in the grassland, with the predominance of the moderately labile (NaOH-Po) fraction, which ranged from 668 to 720 mg kg-1 in both soils. Phosphorus was mainly present in monoester-P form in the NMR extract in both soils (67 % on average). Other 31P-NMR signals were identified as C2-myo-inositol phosphate and scyllo-inositol hexakisphosphate. The results suggest that land use change from forest to grassland will reduce P bioavailability and P-ase activity.

Increased turnover of organic matter as a result of soil disturbance (e.g. by soil tillage) is described in principle, but the direct influence of soil disturbance on soil P turnover especially for organic farming systems has not been sufficiently proven. The objective of the study was to evaluate the short term effect of soil disturbance on different soil P fractions in a soil shaking experiment. Four soils were incubated for 10 days in the dark with three different disturbance treatments: 1) no disturbance, 2) overhead shaking for 2 h at the beginning of the experiment and 3) continuous overhead shaking at 5 r. p. m. The four investigated soils were: 1) a silty loam soil with long term bio-compost application and 2) the corresponding soil without bio-compost application, 3) a long-term organically managed clay loam soil and 4) a clay loam soil with long time application of pig manure, all not and from Baden-Württemberg, Germany. We determined NaHCO3-, NaOH- and H2SO4-extractable inorganic and organic P fractions (Pi and Po, resp.) in a sequential extraction. Furthermore, the potentially plant available P as Calcium-acetate-lactate-extractable P (CAL-P) and P extractable by electro-ultra-filtration (EUF-P), and aqua regia extractable total P (PT) were determined. Furthermore, we determined microbial biomass carbon (MBC), nitrogen (MBN) and phosphorus (MBP), and acid phosphatase activity in soil. The organically managed soil had the highest PT contents (1300 mg·kg-1). The soil with pig manure application had the smallest potentially labile P fractions (NaHCO3-Pi and -Po and NaOH-Pi). The ecologically managed soil had the biggest organic P fractions (114 mg·kg-1 NaHCO3-Po and 463 mg·kg-1 NaOH-Po), but, this soil was the lowest in CAL-P (5 mg·kg-1). Short term soil disturbance had effects on labile organic P fractions of two of the four analyzed soils, but inorganic P was rather unaffected. In the compost amended COMP(+) soil, there was an incorporation of P from the less available NaOH-P fractions into the more available NaHCO3-Po fraction. However, if taking all investigated soils and treatments into account, the effects of soil disturbance were limited and not consistent.

The effect of tillage systems and crop rotation on microbial biomass phosphorus (MBP) and acid phosphatase (P-ase) activity, and the amount of different phosphorus (P) forms measured by 31P-NMR spectroscopy were studied on a field experiment carried out in a temperate Ultisol from southern Chile. Two tillage systems, no tillage (NT) and conventional tillage (CT) and two crop rotations, oat-wheat (OW) and lupine-wheat (LW) were evaluated 4 yr after the start of the experiment to determine the effects of such management on some soil biological parameters and P forms at three depths (0-5, 0-10 and 10-20 cm). Microbial biomass P ranged from 6.5 to 22.6 mg/kg, whereas the mean total P (P(T)) was 1995 mg/kg for all treatments (OW and LW). Microbial biomass carbon (MBC) and surface P accumulation (at 0-5 cm depth), including Olsen P, MBP, orthophosphate monoesters (monoester-P), were larger under NT than CT. Tillage effects were greater than crop rotation effects in enhancing P availability. The LW rotation showed enhanced P-ase activity and increased monoester-P forms (57 vs. 30% of the total integral area of the spectra, in average) compared with OW. Nevertheless, OW rotation increased orthophosphate (ortho-P), especially at 10-20 cm. Microbial biomass carbon ranged from 532 to 2351 mg/kg, which represented 1.2-4.5% of total organic C (C(o)). Furthermore, MBP correlated positively with MBC (r = 0.80), Olsen P (r = 0.77), C(o) (r = 0.77), pH (r = 0.65), P(T) (r = 0.65) and P-ase activity (r = 0.57), suggesting the importance of the microbial biomass on soil P availability.

Total phosphorus (P), Olsen P, microbial P and different labile P fractions determined by a modified Hedley procedure, together to acid phosphatase (P-ase), pH and organic C, were analyzed in soil from four Nothofagus rainforest ecosystems differing in forest type and management. The sites included two evergreen (EF) and two secondary deciduous (DF) rainforests growing in an Andisol of southern Chile. Samples were collected from the organic layer and from mineral soil at 2-20 cm depth. Total P and phosphorus fractions were highest in evergreen forest soil with the predominance of moderate labile (NaOH-P.) fraction. Olsen P, microbial P, labile P fractions, inorganic P fractions and acid phosphatase were greater in the organic layer than the mineral soil, especially under evergreen forests. Evergreen forest litter affected P cycling enhancing P lability and P-ase activity in soil. Forest management affected P fractions in EF but not in DF forests, and generally did not modify soil properties. Results show that clearing of deciduous forest did not seriously affect the P cycle in this Andisol and confirm the fundamental role of the organic layer in P conservation of these ecosystems. (c) 2008 Elsevier B.V. All rights reserved.

The effect of management systems and crop rotation on soil phosphorus (P) fractions and selected soil properties were studied from 2002 to 2005 on an experiment established in 2001 in a volcanic derived Ultisol from southern Chile. Two tillage systems, no tillage (NT) and conventional tillage (CT), and two crop rotations, oat–wheat and white lupine–wheat were evaluated in order to determine the effects of such management in the lability of P in this soil. Seasonal additions of phosphate fertilizer at a rate of 80 kg P ha−1, to oat and lupine, and 200 kg P ha−1 to wheat were applied to the high P fixing soil used. Soil analyses were performed each year after growing season. Total P increased from 1643 to 2053 mg kg−1 after 4 years of cultivation but most of this added P (72%) became unavailable. The application of the Hedley P fractionation procedure indicated that P was mainly distributed as relatively labile P, extractable with NaOH (43.5% of the total P), and that only 9.6% was labile P, extractable with resin and NaHCO3. In NT soils the largest soil surface P accumulation was produced, mainly as inorganic P. In the oat–wheat rotation, the largest accumulation of moderate labile organic P was observed, preventing it from becoming unavailable; the lupine–wheat rotation left the greatest phosphatase activity in soil (738 μg PNFF g−1). Tillage and crop rotation exerted the same level of effects on labile+relatively labile P fractions (F-probabilities of 0.045 and 0.040, respectively), but cropping systems affected the soil properties much more. Over fertilization caused high levels of soluble P (66 mg kg−1 of resin extractable P in the last year), but also promoted the P accumulation under unavailable fractions, especially in CT systems. Wheat cropping resulted in a greater accumulation of soil total P and no labile P; whereas oat and particularly lupine cropping showed a reduction of no labile P and an increase of relatively labile P.

Entre las diversas practicas agrotecnologicas aplicables a sistemas agricolas de baja inversion, se consideran la adicion de residuos de plantas como una fuente de nutrientes, y el manejo adecuado de la simbiosis micorriza arbuscular (MA). Por ello, el objetivo general de este estudio fue determinar el efecto de la aplicacion de residuos de cosecha e inoculacion micorrizica sobre el desarrollo y nutricion mineral de plantas de trigo (Triticum aestivum L.) creciendo en un suelo acido (Typic Distrandepts) y algunos parametros quimicos y biologicos del suelo. Residuos de lupino (Lupinus albus L.) y de trigo se adicionaron al suelo a una dosis equivalente a 6 Mg ha-1, sobre el cual se sembro trigo cv. Otto, con y sin inoculacion de una cepa nativa de un hongo MA exogeno (Glomus etunicatum CH 110). La adicion de residuos organicos incrementaron significativamente el rendimiento de materia seca en un 50-60% y la adquisicion mineral de las plantas de trigo hasta en un 100%. Los residuos de trigo incrementaron el pH del suelo y la disponibilidad de P, mientras que los residuos de lupino incrementaron la colonizacion de hongos MA de 49 a 61%. La inoculacion de un hongo MA exogeno disminuyo la adquisicion de minerales, especialmente Al, lo que puede contribuir a la disminucion de problemas de toxicidad en suelos acidos.

A B S T R A C T Among the diverse agrotechnological practices appli- cable to low-input investment agricultural systems, the additions of plant residues to soils as a nutrient source, and the appropriate management of arbuscular mycorrhizal (MA) symbiosis, are being worldwide considered. Therefore, the main objective of this study was to test the application effect of two crop residues and mycorrhizal inoculation on wheat (Triticum aes- tivum L.) plants growing in an acidic soil (Typic Distrandepts) and some chemical and biological soil parameters. Both, lupine (Lupinus albus L.) and wheat residues were added to soil at equivalent rates of 6 Mg ha-1, and wheat cv. Otto was sown above, with and without the inoculation of a native strain of exogenous MA fungi (Glomus etunicatum CH 110). Organic amendments significantly increased dry matter yield by 50-60% and mineral acquisition by wheat plants near up to 100%. The wheat residues increased soil pH and P availability, while the lupine residues enhanced MA colonization from 49 to 61%. The inoculation of an exogenous MA fungus decreased the mineral ac- quisition, especially Al, which might contribute to reducing toxicity problems in acidic soils.