Advances in long‐term soil‐pollution monitoring of Switzerland

The Swiss soil‐monitoring network (NABO) was launched in 1984 and comprises currently 105 observation sites covering all characteristic land‐use types across Switzerland. So far, the sampling periodicity was 5 y, and the fifth sampling campaign will be accomplished by end of 2009. The concentrations of Cd, Zn, Pb, Cu, Hg, Ni, Cr, Co, and F were measured. The major results and conclusions are: (1) Even topsoils in remote areas are to some extent polluted, mainly by Pb, Cu, and Cd. However, elevated concentrations can also be of natural origin, e.g., for F, Ni, Cr, and Cd. (2) Land use alone is often a rather unreliable indicator to discriminate soil pollution. (3) After the 2nd campaign positive, negative, or no temporal concentration changes were measured. From the 3rd campaign onwards, nonmonotonous (zigzag) evolutions were frequently observed. Therefore, no certified trends can be stated after three measurement campaigns during a period of 10 y. (4) Soil monitoring is an environmental time‐series problem. The only way to detect reliable signals and trends earlier is to improve the overall measurement quality (precision and bias) and to shorten the measurement periodicity. (5) The causes of temporal soil concentration changes are complex and result from natural processes, anthropogenic processes, and methodological artifacts. Hence, not all soil concentration changes are due to anthropogenic inputs. Based on the state‐of‐art of our experience, some basic methodological requirements and recommendations can be supported for a “good soil‐monitoring practice”: (1) Assurance of long‐term continuity and consistency under changing boundary conditions as site conditions, methodologies, etc. (2) Implementation of a scientifically and politically appropriate spatial and temporal measurement resolution. (3) Long‐term assessment of reliability (quality assurance) by adequate quantification of precision, bias, and confidence intervals along the whole measurement chain. (4) Documentation of all potentially relevant boundary conditions by suitable metadata. Only soil‐monitoring results meeting these requirements are fit to support political decisions.     

Heavy‐metal displacement in chelate‐treated soil with sludge during phytoremediation

Heavy metals (HMs) in domestic sewage sludge, applied to land, contaminate soils. Phytoremediation is the use of plants to clean‐up toxic HMs from soil. Chelating agents are added to soil to solubilize the metals for enhanced uptake. Yet no studies report the displacement of HMs in soil with sludge following solubilization with chelates. The objective of this work was to determine the uptake or leaching of HMs due to a chelate added to a soil from a sludge farm that had received sludge for 25 y. The soil was placed in long columns (105 cm long; ?? 39 cm) in a greenhouse. Columns either had a plant (hybrid poplar; Populus deltoides Marsh. × P. nigra L.) or no plant. After the poplar seedlings had grown for 144 d, the tetrasodium salt of the chelating agent EDTA was irrigated onto the surface of the soil at a rate of 1 g per kg of soil. Drainage water, soil, and plants were analyzed for three toxic HMs (Cd, Ni, Pb) and four essential HMs (Cu, Fe, Mn, Zn). At harvest, extractable and total concentrations of each HM in the soil with EDTA were similar to those in soil without EDTA. The chelate did not affect the concentrations of HMs in the roots or leaves. With or without plants, EDTA mobilized all seven HMs and increased their concentrations in drainage water. Lower concentrations of Cd, Cu, Fe, Ni, and Zn in leachate from columns with EDTA and plants compared to columns with EDTA and no plants showed that poplars can reduce groundwater contamination by intercepting these HMs in the soil. But the poplar plants did not reduce Pb and Mn in the leachate from columns with EDTA. Concentrations of Cd and Pb in the leachate mobilized by EDTA remained above drinking‐water standards with or without plants. The results showed that a chelate (EDTA) should not be added to a soil at a sludge farm to enhance phytoremediation. The chelate mobilized HMs that leached to drainage water and contaminated it.     

Geochemical assessment,distribution, and dynamics of trace elements in urban agricultural soils under long‐term wastewater irrigation in Kano,northern Nigeria

A study was conducted to evaluate the distribution and origin of trace elements (Ti, Fe, Nb, Pb, Rb, Sr, Y, and Zr) in five representative long‐term wastewater‐irrigated urban vegetable gardens of Kano, Nigeria. Surface‐soil concentrations (0–15 cm) of Ti (4600–14 300 mg kg^–1), Fe (4000–31 800 mg kg^–1), Pb (96–355 mg kg^–1), and Y (33–98 mg kg^–1) were high compared to mean concentrations in comparable soils elsewhere. However, soil‐pollution assessment yielded no evidence of anthropogenic input of the trace elements studied. Indices such as the enrichment factor, the contamination factor, and the geoaccumulation index (I_geo) revealed little to no contamination with trace elements. The I_geo calculated for these metals varied across locations between 0.00 and 0.12 with Nb having the highest I_geo value. Similarly, the contamination factor was low for all metals with the exception of Fe reaching a contamination factor of 4.2 at one location. Geochemical‐balance evaluations showed depletion of all trace elements except for Fe which was 176% higher than in a natural uncultivated and unirrigated reference soil. Correlation and factor analyses showed that all determined trace elements likely originated from the same natural sources, which probably are the soil parent material and atmospheric depositions.     

应用DRAINMOD农田排水模型对地下水位和排水量的模拟

农田排水工程在防御涝渍灾害、促进农作物正常生长和改善田间耕作管理等方面起着积极的作用,合理的排水系统设计是保证排水系统正常运行、作物正常生长的关键。以水平衡理论为基础建立的DRAINMOD模型,可用于研究排水系统对作物生长和各水文要素的影响,适用于浅地下水位和较湿润地区。该文对模型的基本原理和参数输入要求进行了详细描述,采用加拿大安大略省南部Eugene F. Whelan试验站自由排水和控制排水-地下灌溉两种水位管理条件下,1992～1994年6～8月地下水埋深观测值、3年地表径流和地下排水水量观测值对模型进行了模拟验证。图形显示和统计参数指标分析表明,模拟值与观测值拟合较好,表明模型具有良好的水文模拟性能,可用于预测地下水埋深、地表和地下排水量,是农田排水工程设计和水管理的有效工具。     

Short‐term and residual availability of nitrogen after long‐term application of organic fertilizers on arable land

Knowledge on short‐term and long‐term availability of nitrogen (N) after application of organic fertilizers (e.g., farmyard manure, slurry, sewage sludge, composts) provides an important basis to optimize fertilizer use with benefits for the farmer and the environment. Nitrogen from many organic fertilizers often shows little effect on crop growth in the year of application, because of the slow‐release characteristics of organically bound N. Furthermore, N immobilization after application can occur, leading to an enrichment of the soil N pool. However, this process finally increases the long‐term efficiency of organic fertilizers. Short‐term N release from organic fertilizers, measured as mineral‐fertilizer equivalents (MFE), varies greatly from 0% (some composts) to nearly 100% (urine). The most important indicators to be used for predicting the short‐term availability of N are total and NH ‐N contents, C : N ratio (especially of the decomposable organic fraction), and stability of the organic substances. Processing steps before organic fertilizers are applied in the field particularly can influence N availability. Composting reduces mineral‐N content and increases the stability of the organic matter, whereas anaerobic fermentation increases NH ‐N content as well as the stability of organic matter, but decreases the C : N ratio remarkably, resulting in a product with a high content of directly available N. Nevertheless, long‐term effects of organic fertilizers rather slowly releasing N have to be considered to enable optimization of fertilizer use. After long‐term application of organic fertilizers, the overall N‐use efficiency is adequate to a MFE in the range of 40%–70%.     

Changes in uranium and thorium contents in topsoil after long‐term phosphorus fertilizer application

Archived soil samples from the beginning and end of three long‐term field trials conducted in central France were analysed for total uranium (U) and thorium (Th) contents to evaluate the effect of 15–30 yr of phosphorus (P) fertilizer treatments on the accumulation of these elements in the topsoil. For comparison, the soil samples were also analysed for total P. Three treatments were compared: no P application (P₀), 26 kg P/ha/yr (P₂₆) and 52 kg P/ha/yr (P₅₂). Significant effects of P fertilizer were observed on U content and, to a lesser extent, on Th content as a result of the P₅₂ treatment at two of the field trials. This effect was demonstrated both in the analyses at the end of the field trials [P₅₂–P₀: +0.25 and +0.44 mg U/kg soil, +0.58 (not significant) and +1.03 mg Th/kg soil] and when considering the changes in U and Th contents between the beginning and the end of the field trials (end–start: +0.18 and +034 mg U/kg soil, +0.35 and +0.45 mg Th/kg soil). The P fertilizer effect was also supported by the correlations of U and Th with total levels of P in the soil. However, in one of the three trials, no significant accumulation of U or Th because of fertilizer could be seen, suggesting either that less U and Th were applied using a different P fertilizer and/or that soil heterogeneity masked significant effects.     

Long‐lasting impact of biowaste‐compost application in agriculture on soil‐quality parameters in three different crop‐rotation systems

Soil‐quality parameters, such as soil organic matter (SOM) and plant‐available nutrient contents, microbial properties, aggregate stability, and the amounts of heavy metals were carried out in arable soils of different rotation schedules applied with a total of 50 Mg dry mass ha^–1 biowaste compost relative to an untreated control. This was investigated during a 10 y period from 1994 to 2004. Overall, soil‐quality parameters studied appeared to be promoted by biowaste‐compost application. This was evidenced for example by a remarkable increase of SOM and total N content of ≈ 15%–20% relative to the control. Subsequently, amounts of soil microbial biomass and alkaline phosphatase activity were significantly increased as well. In addition, biowaste‐compost application revealed an increase of plant‐available P and K contents and aggregate stability in soil. There was, however, no treatment effect for net N‐mineralization rates. Moreover, in soils of maize and sugar beet rotation schedule a slight decrease was found. Heavy‐metal contents of Pb and Zn were significantly increased in all compost‐treated soils, whereas no significant increase of Cd and Cu contents was measured. However, the investigated amounts were far below of the limits of the German Biowaste Ordinance. It is finally recommended, that biowaste compost may sustain and improve soil quality in agriculture when N nutrition will be considered.     

Forms of Al in soil and soil solution in a long‐term fertilizer application experiment

Under the conditions of a long‐term fertilizer experiment, this study aimed to determine the contents of total and exchangeable aluminium in soil as well as the Al concentration in the soil solution. Additionally, Al speciation was evaluated with the use of the MINTEQA2 software. The results obtained indicated that under the conditions of long‐term application of different mineral fertilizers or farmyard manure, the soil reaction changed to a great extent (pH 3.58–6.78). At the same time, the content of total Al in soil fluctuated from 18.85 to 22.13 g/kg and that of exchangeable Al ranged from 1.42 to 102.66 mg/kg. The concentration of Al in the soil solution was highly differentiated (5.19–124.07 μmol/L) as well as that of free aluminium ions (Al³⁺) (0–16.9 μmol/L). In acidic soils, aluminium complexes with organic matter are the predominant forms of Al in the soil solution. In soils with neutral soil reaction, there were no free aluminium ions. Soil liming and addition of organic amendment were the treatments that restricted the presence of toxic aluminium forms in soil.     

Using Moran's I and geostatistics to identify spatial patterns of soil nutrients in two different long‐term phosphorus‐application plots

The spatial variation of soil nutrients especially the soil test phosphorus (STP) in grassland soils is becoming important because of the use of soil‐nutrients information as a basis for policies such as the recently EU‐introduced Nitrates Directive. Up to now, the small‐scale spatial variation of soil nutrients in grassland has not been studied. The main aim of this study was to investigate the spatial patterns of soil nutrients in two grazed grassland plots with a long‐term (38 y) P‐application experiment, in order to better understand the spatial variation of soil nutrients and the correlation among soil nutrients in grasslands. Two small areas (one from a high‐P background and the other from a medium‐P background) were selected. Soil samples (304 per study area) were collected based on a 1 m × 1 m grid system. The samples were analyzed for STP, Mg, K, pH, and lime requirement (LR). The results were analyzed using conventional statistics, Moran's I, geostatistics, and a GIS. Based on the global Moran's I values, significant positive spatial autocorrelations were found for STP, Mg, pH, and LR in both study areas. Spatial clusters and spatial outliers were detected using the local Moran's I index. Clear linear‐shaped high‐high or low‐low value clusters of the studied variables except K were observed in the study areas due to long‐term usage of machine spreader or other agricultural‐management methods in the past. The corresponding linear patterns were further found in the spatial‐distribution maps. Small spatial patches were found for soil K revealing that it had a random spatial distribution related to the relatively uniform K fertilizer in the study areas. The spatial clusters revealed by local Moran's I were in line with the spatial patterns in the distribution maps.     

Water drainage and nitrate leaching under traditional and improved management of vegetable‐cropping systems in the North China Plain

Traditional irrigation and nitrogen (N) fertilization in North China may elevate water drainage and nitrate concentrations in soil and groundwater. A field experiment was conducted in an intensively irrigated vegetable (cauliflower, amaranth, and spinach) field for three consecutive years (1999–2002). The main objective was to test to what extent an improved water and fertilizer management, based on the maintenance of field capacity a defined range of the water content in the 0–50 cm soil layer and an N expert system, could reduce drainage and nitrate leaching without impairing vegetable yield. Rates of water drainage and related nitrate leaching were calculated based on measurements of soil water potential and soil‐water nitrate concentrations. Soil water potential was monitored with tensiometers at depths of 75 cm and 105 cm. Nitrate concentrations were analyzed in soil leachates collected at 90 cm soil depth using ceramic suction cups. The results revealed that the average annual drainage related to the cultivation season for cauliflower, amaranth, and spinach was reduced from 275 mm in the traditional system to 29 mm with improved management practice. The average annual cumulative nitrate leaching during the vegetable‐growing period amounted to 301 kg ha^–1 and 13 kg ha^–1 in the traditional and improved management practices, respectively. Vegetable yields were not significantly different under the traditional and improved management practices.     

Sorption of iron‐cyanide complexes on goethite investigated in long‐term experiments

The iron‐cyanide complexes ferrocyanide, [Fe^II(CN)₆]^4–, and ferricyanide, [Fe^III(CN)₆]^3–, are anthropogenic contaminants in soil. We investigated their sorption on goethite, α‐FeOOH, in batch experiments in a time range from 1 d to 1 yr, their desorption by phosphate and chloride as well as their surface complexes on goethite by Fourier‐transform infrared spectroscopy (FTIR). The sorption of both complexes continued over the whole time range. Percent desorption of ferricyanide by phosphate decreased, whereas that of ferrocyanide increased until it amounted to approximately 87% for both complexes. By FTIR spectroscopy inner‐sphere complexation of both complexes on the goethite surface was indicated. With both complexes, a Berlin‐Blue‐like layer (Fe₄[Fe(CN)₆]₃) was formed initially on the goethite surface which disappeared with increasing reaction time. After at least 30 d reaction time, ferricyanide was the only sorbed iron‐cyanide complex detected even when ferrocyanide was initially added. This resulted from slow oxidation of ferrocyanide, most probably by dissolved oxygen. Based on all results, we propose that ferricyanide forms monodentate inner‐sphere complexes on the goethite surface.     

Genetic variability for responses to short‐ and long‐term salt stress in vegetative sunflower plants

Sunflower (Helianthus annuus L.) has been rated as moderately salt‐resistant, and variability for salt resistance has been detected within this crop. However, variability in salt‐resistance mechanisms has not been assessed. Osmotic tolerance, the relation of salt resistance with whole‐plant Na⁺ and K⁺ distribution and tissue Na⁺ tolerance were investigated in several sunflower inbred lines. Plants were grown under controlled conditions, in pots with sand and perlite irrigated with salinized (NaCl, –0.65 MPa) nutrient solution. Osmotic tolerance was assessed from the initial effects of the salt treatment on plant elongation in eleven sunflower lines. Long‐term salinity responses were evaluated in four of those lines, by assessing whole‐plant growth. A principal components analysis (PCA) was run on relative‐to‐control growth data, and this information was used to establish a relative resistance ranking, which indicated lines HAR2 > HAR1 > HA64 > HAR5. Osmotic tolerance was observed in HA64 and HAR2. The lines showed different degrees of Na⁺ accumulation, it was very low in some of them, but relative salt resistance was not associated to this trait. Tissue Na⁺ tolerance was deduced by comparing the percentage of dead leaves as a function of leaf blade Na⁺ accumulation, and it was higher in HAR1 than in the rest. These results indicate that variability for salt‐resistance mechanisms exists in sunflower. Osmotic tolerance and tissue Na⁺ tolerance were detected in different lines, highlighting that such variability may be exploited for increasing salt resistance in this crop.     

Strategy for long term use of saline drainage water for irrigation in semi-arid regions

In arid and semi-arid regions, effluent from subsurface drainage is often saline and in the absence of a natural outlet, its disposal is a serious environmental threat. A field experiment was conducted for 7 years using drainage water of different salinity levels (EC_iw=6, 9, 12 and 18.8 dS/m) for irrigation of wheat during the dry winter season. The objective was to find whether crop production would still be feasible and soil salinity would not be increased unacceptably by this practice. The experimental crop was wheat during the winter season and pearl-millet and sorghum in the rainy season, grown on a sandy loam soil provided with subsurface drainage system. All crops were given a pre-plant irrigation with non-saline canal water and subsequently, saline drainage water of different salinity levels was used for the irrigation of wheat as per the treatment. On an average, the mean yield reduction in wheat yield at different EC_iw was 4.2% at 6, 9.7% at 9, 16.3% at 12 and 22.2% at 18.8 dS/m. Pearl-millet and sorghum yields decreased significantly only where 12 dS/m or higher salinity water was applied to previous wheat crop. The high salinity and sodicity of the drainage water increased the soil salinity and sodicity in the soil profile during the winter season, but these hazards were eliminated by the subsurface drainage during the ensuing monsoon periods. The results obtained provide a promising option for the use of poor quality drainage water for the irrigation of winter wheat without undue yield reduction and soil degradation.     

The role of non‐crystalline Fe in the increase of SOC after long‐term organic manure application to the red soil of southern China

Because of the important role of soil organic carbon (SOC) in nutrient cycling and global climate changes, there has been an interest in understanding how different fertilizer practices affect the SOC preservation and promotion. The results from this study showed that long‐term application of manure (21 years) could increase significantly the content of SOC, total nitrogen (N) and soil pH in the red soil of southern China. The chemical structure of SOC was characterized by using solid‐state cross‐polarization magic angle spinning (CPMAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy, and the aromatic C, ratio of alkyl C : O‐alkyl C, aromaticity and hydrophobicity of mineral fertilizers N, P and K plus organic manure (NPKM) and organic manure (M) treatments were less than those of mineral fertilizer nitrogen (N) and mineral fertilizers N, P and K (NPK) treatments. Both poorly crystalline (Fe_o) and organically complexed (Fe_p) iron contents were influenced significantly (P < 0.05) by different fertilizers, and it was observed that NPKM and M treatments increased the non‐crystalline Fe (Fe_o‐Fe_p) content. There was a significant (P < 0.01) positive correlation between soil organic C and non‐crystalline Fe in both the surface (0–20 cm) and subsurface (20–40 cm) soils. The results suggested that non‐crystalline Fe played an important role in the increase of SOC by long‐term application of organic manure (NPKM and M) in the red soil of southern China.     

Adaptation of Medicago sativa cv. Gabès to long‐term NaCl stress

The perennial Medicago sativa cv. Gabès is widely grown on saline soils in Tunisian oases. The mechanisms by which this NaCl‐tolerant cultivar maintains a positive growth balance were analyzed. In this plant of considerable agronomic interest, biochemical analyses were conducted in order to study the effects of salinity on mature leaves. Free‐radical detoxification mechanisms and changes induced by the accumulation of reactive oxygen species (ROS) in response to the NaCl stress were compared between the upper (young) and lower (old) carbohydrate source leaves. Long‐term NaCl (150 mM) treatment significantly reduced the size of source leaves supporting growth. Salinity damage was greater in the lower than in the upper leaves. This damage was associated with a high Na⁺ : K⁺ ratio and a decrease in the activity of H₂O₂‐scavenging enzymes, leading to lipid peroxidation. In lower source leaves that were mainly affected by ionic stress, superoxide dismutase (SOD) was overexpressed and guaiacol peroxidase (GPX) activity increased. In contrast, in upper source leaves that were mainly exposed to water deficit, catalase and ascorbate peroxidase (APX) activities increased whereas GPX activity was unchanged. The upper source leaves maintained adequate ionic and water status and an efficient ROS detoxification, allowing sinks to be supplied with photoassimilates and maintaining a positive growth balance in this cultivar of alfalfa.     

The role of antioxidative metabolism of tomato leaves in long‐term salt‐stress response

The antioxidative protection system as adaptation strategy to high soil salinity in the leaves of two tomato (Lycopersicon esculentum Mill.) hybrids (Buran F1 and Berberana F1) was investigated. Changes in the activity of superoxide‐dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7), ascorbate peroxidase (APX, EC 1.11.1.11), as well as total and oxidized ascorbate concentrations (AA and DHA) in the plant leaves subjected to three salinity levels (EC 3.80 dS m⁻¹, 6.95 dS m⁻¹, and 9.12 dS m⁻¹) relative to non‐saline control were analyzed during the fruiting phase. The obtained results clearly indicate a relation between SOD activity and AA concentration in the antioxidative protection without any peroxidase‐related H₂O₂ detoxification. Increased SOD activity accompanied by high AA concentration was noticed at all salinity levels, but the response of hybrids was specific for the particular salt concentration. The first salinity level (EC 3.80 dS m⁻¹) induced the highest level of AA in the Buran F1 (70%), while in Berberana F1 hybrid leaves the highest AA concentration (64%) was noticed at the third salinity level (9.12 dS m⁻¹). All salinity levels caused a decline in POD and APX activities in both hybrids. The possibility of a predominant role of ascorbate and SOD in the antioxidative protection of mature tomato leaves under long‐term salt stress is discussed.     

Soil‐ecological evaluation of willows in a floodplain

Willows (Salix spp.) were supposed to be suitable candidates for the phytoremediation of polluted floodplain soils, but it is unknown how willow growth alters concentrations and mobility of pollutants under the conditions of ongoing periodically flooding. Therefore, effects of willow cropping on total concentrations and mobility of As and heavy metals and soil microbial properties were determined after three and four growing seasons under willows in comparison to native grassland in a flood channel of a River Elbe floodplain (Central Germany). After 4 y of willow coppice, the heavy metal concentrations (mg kg^–1) were increased not only in the grass control plots (final Cu 274, Pb 276, Zn 935) but also under the willows (final Cu 248, Pb 251, Zn 779) compared to the initial concentrations (initial Cu 170, Pb 156, Zn 579). This increase might likely be caused by the ongoing sedimentation by flood events. The smaller increase under willows compared to grass might be related to an initial net effect of phytoextraction. The concentrations of the mobile fractions of Cd, Cu, Ni, and Zn were significantly lower under willows than under grass. Higher β‐glucosidase activities under willows than under grass might indicate a starting net decomposition of organic matter. Therefore, the study of long‐term and large‐scale effects are recommend before an appropriate evaluation of willow short‐rotation coppice for phytoremediation of polluted floodplains will be established.     

Fate of phosphorus in soil during a long‐term fertilization experiment in Finland

Accumulation and depletion of soil phosphorus (P) was studied in a long‐term (37 y) field experiment in Southern Finland. The loam soil had a high pH (7.5–7.7) due to an earlier liming. Spring barley, spring wheat, oat, and ryegrass, grown in rotation, were annually fertilized with 0, 32, or 67 kg P ha^?1 y^?1 (P₀, P₁, and P₂K) and sufficient N. The average dry matter grain yield 2,600 kg ha^?1 of the P₀ plots increased by about 500 kg ha^?1 at P₁ treatment and another 600 kg ha^?1 by P₂K. Soil samples were collected in 1978 (beginning), 1995, 2005, and 2015. According to the Chang and Jackson sequential extraction, the P₂K and P₁ treatments increased the inorganic soil P by 732 and 32 kg P ha ^?1 in 37 years, respectively, while the P₀ plots were depleted by –459 kg P ha ^?1. The P₂K treatment increased all four P fractions, extracted with NH₄Cl (easily soluble), NH₄F (Al‐P), NaOH (Fe‐P), and H₂SO₄ (Ca‐P). Continuous depletion (P₀) decreased the NH₄Cl‐P and NH₄F‐P pools, NaOH‐P and H₂SO₄‐P pools remaining stable. None of the P pools changed significantly at P₁. The remarkable gap between the measured change and the balance for the P₂K and P₁ treatments cannot be explained solely by lateral soil movement, meaning that a significant proportion of the applied P was lost either in surface runoff or transported below the investigated depth of 40 cm. Despite large P applications, the degree of P saturation reached only 20% in the P₂K topsoil, assuming a 50% reactivity of Fe and Al oxides. As derived from sorption isotherms, a high EPC₀ (i.e., equilibrium P concentration at zero net P sorption or desorption) of 1.30 mg L^?1 had been built up in the P₂K treatment, while in the P₁ treatment EPC₀ (0.33 mg L^?1) had remained unchanged and P depletion (P₀) had caused a decrease to 0.12 mg L^?1. These results demonstrate that P sorption and desorption properties respond strongly to both P fertilization and null fertilization treatments and that in a long‐term field experiment only a low proportion of the residual fertilizer P can be recovered from soil.     

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.