Assessing the effectiveness of manure export plus intensive silage cropping for lowering the Olsen‐P status of P‐enriched grassland

A substantial proportion of farmed grassland soils in Northern Ireland (NI) are overly enriched with P and pose a risk to water quality. To address this problem, manure could be exported rather than recycled to P‐enriched land and the latter intensively cropped with grass silage to deplete soil P. To assess the efficacy of such a strategy, a P‐ and K‐enriched grassland site was intensively cropped over a 6‐yr period with fertilizer N alone supplied to support silage growth. By year 6, soil P had declined from index 5 to index 3, and it was estimated that two more years of this management may bring it into the target index 2 range. Soil K, however, declined rapidly from index 4 to index 1 in just 4 yr, with the result that grass production became limited by K deficiency. It was concluded that nonrecycling of manure to P‐enriched grassland under silage management is probably the most effective strategy for lowering soil P status, but care must be taken to prevent K deficiency occurring.     

Critical Olsen P and CaCl2‐P levels as related to soil properties: results from micropot experiments

The usefulness of soil phosphorus (P) tests used in routine soil analyses is limited by the fact that a single measurement cannot encompass all P‐related factors potentially affecting plant performance. In this work, we performed micropot (15 mL) experiments to test the hypothesis that the predictive value of two common soil P tests (Olsen P and CaCl₂‐P) can be improved by considering properties commonly measured in laboratory analyses. Forty‐nine sets of soils ranging widely in properties were used for this purpose, each set consisting of samples with similar properties but differing in P status. Ryegrass and turnip were grown in a chamber for 30 days in two separate experiments and their yields at harvest recorded. The critical Olsen P and CaCl₂‐P levels, which were taken to be those corresponding to 95% asymptotic yield as calculated from data fitted to a Mitscherlich equation, were greater for turnip than for ryegrass, probably as a result of the difference in yield (49 and 160 mg dry matter/micropot on average for ryegrass and turnip, respectively) and hence in P requirements between the two species. Critical Olsen P spanned narrower ranges than critical CaCl₂‐P in both crops and is therefore seemingly the more robust of the two tests. Both critical P values exhibited moderate correlations with soil properties. Thus, critical Olsen P was (a) lower in soils with a medium pH – which is consistent with the fact that the bicarbonate solution method tends to overestimate plant‐available P in strongly acid and calcareous soils; (b) positively correlated with pH and carbonate content in calcareous soils; and (c) uncorrelated with soil properties in noncalcareous soils. On the other hand, critical CaCl₂‐P in some soil groups was negatively correlated with some properties increasing the P buffering capacity of soil (e.g. Fe oxide content). Taken together, our results suggest that routinely measured soil properties help to predict critical Olsen P better than critical CaCl₂‐P.     

Plant‐available soil phosphorus. Part I: the response of winter wheat and spring barley to Olsen P on a silty clay loam

Increasing fertilizer costs have prompted farmers to ask whether soils could be maintained at lower levels of plant‐available phosphorus (Olsen P) than currently recommended without losing yield. To help answer this question, we assessed the response to Olsen P by spring barley grown from 1986 to 1991, followed by winter wheat from 1992 to 2008, on a silty clay loam soil. Each year the curve relating grain yield to Olsen P was fitted statistically to determine the asymptotic yield and the Olsen P associated with 98% of that yield, that is, the critical level of Olsen P. The variance accounted for by the relationship ranged between 83 and 97% in all but two years, suggesting that the availability of soil P was the major soil factor affecting yield and that Olsen P was a reliable measure of plant‐available P in soil. Asymptotic annual yield of spring barley ranged from 2.34 to 7.12 t/ha and of winter wheat from 3.87 to 10.36 t/ha. In part, this range in yields was because of changes in the cultivar grown while the range of yields for any one cultivar was probably due to differences in weather, principally rainfall, between years. Critical Olsen P ranged from 7 to 18 mg/kg for both cereal crops (with one outlier at 26 mg/kg for winter wheat) most probably due to seedbed and soil structure conditions affecting root growth, and thus acquisition of available soil P, and the way these soil factors were affected by weather. Thus, a general recommendation for cereals grown on this silty clay loam, which is comparatively easy to cultivate, would be to maintain Olsen P at about 20 mg/kg in the plough layer to minimize the risk of losing yield in some years. This value, 20 mg/kg, equivalent to 20 mg/L, is the midpoint of P Index 2, the recommended P Index given in the Fertiliser Manual (RB209) (Defra 2010) for soils growing arable crops and grass in England, Wales and Northern Ireland.     

Plant‐available soil phosphorus. Part II: the response of arable crops to Olsen P on a sandy clay loam and a silty clay loam

The increasing cost of fertilizer has prompted farmers to ask whether soils could be maintained at lower levels of plant‐available phosphorus (Olsen P) than currently recommended, without limiting yield. To help answer this question, critical levels of Olsen P have been determined for spring barley, winter wheat, potatoes and sugar beet grown on a sandy clay loam and a poorly structured heavy textured silty clay loam. On each soil, there were plots with a range of well‐established levels of Olsen P and, in one experiment, two levels of soil organic matter (SOM). For each crop and each year, the response curve relating yield to Olsen P was fitted statistically to determine the asymptotic yield and the Olsen P associated with 98% of that yield, that is, the critical Olsen P. Maximum yield of all four crops varied greatly from year to year, in part due to applied nitrogen (N) where it was tested, and in part to seasonal variation in weather, mainly rainfall. The wide range in critical Olsen P, from 8 to 36 mg/kg, between years was most probably as a result of differences in soil conditions that affected root growth and thus acquisition of available soil phosphorus (P). Generally, a larger asymptotic yield was not necessarily associated with a larger critical Olsen P. Spring barley and winter wheat given little N required more Olsen P, 20–34 mg/kg, to achieve the asymptotic yield, compared to 10–17 mg/kg where ample N was given; presumably, more roots were needed to search the soil for the smaller amounts of available N and root growth is affected by the amount of plant‐available soil P. In a field experiment on one soil type, soil with little SOM required 2–3.5 times more Olsen P to produce the same yield as that on soil with more organic matter. Soil organic matter most probably improved soil structure and hence the ability of roots to grow and search for nutrients in field conditions because when these soils were cropped with ryegrass in controlled conditions in the glasshouse, the yields of grass were independent of SOM and there was the same critical Olsen P for both soils. Overall, the data confirm that, for these soil types, the current recommendations for Olsen P for arable crops in England, Wales and Northern Ireland are appropriate.     

The long‐term soil phosphorus balance across Chinese arable land

Quantifying temporal and spatial variation of soil phosphorus (P) input, output and balance across Chinese arable land is necessary for better P management strategies. Here, we address this challenge using a soil P budget to analyse the soil P balance in arable land across the whole of China, for the period 1980–2012. Results indicated that the total P input to soil increased from 22.5 kg P/ha in 1980 to 79.1 kg P/ha in 2012. However, the total P output from soil only increased from 17.9 kg P/ha in 1980 to 36.9 kg P/ha in 2012. Therefore, the average net soil P surplus in China increased from 4.6 kg P/ha in 1980 to 42.1 kg P/ha in 2012. Our research found great variation in soil P balances across different regions. Soil P balance varied between regions with the order of southeast (SE) > north central (NC) and the middle and lower reaches of Yangtze River (MLYR) > southwest (SW) > northwest (NW) > northeast (NE). Phosphorus that has accumulated in agricultural soil across China could theoretically meet crop P demands for approximately 4.8–12.0 yrs, depending on the bioavailability of P stored in soils. Increasing the return rates of manure and straw could substantially reduce the demand for fertilizer‐P. This paper represents a basis for more targeted, regionally informed P fertilizer recommendations in Chinese soils.     

An evaluation of Colwell‐P as a measure of plant‐available phosphorus in soils of volcanic and non‐volcanic origins in the highlands of Papua New Guinea

Various soil test methods including Olsen, Colwell, Bray and Truog have been used to assess the levels of plant‐available P (PAP) in soils situated in the highlands of Papua New Guinea (PNG). Up until now, though, there has been no guarantee that these tests provide valid assessments of PAP in these somewhat atypical organic matter‐rich tropical soils. Furthermore, the critical soil‐P concentrations associated with the tests have been based on studies conducted elsewhere in sub‐tropical and temperate latitudes and as such may or may not be valid for soils or cropping situations in PNG. Soil (Colwell)‐P and leaf‐P data collected during a recent survey of sweet potato gardens in the highlands of PNG were therefore used to determine if useful relationships existed between these variables for different groups of soils, and if they do, to use these relationships to evaluate critical soil Colwell‐P concentrations corresponding to a known critical concentration of P in sweet potato index leaf tissue. Separate, highly significant linear relationships were obtained between leaf‐P and Colwell‐P for soils of volcanic and non‐volcanic origins. Based on these relationships, the critical Colwell‐P concentration for volcanic soils was found to be four times greater than that for non‐volcanic soils, presumably because much of the P extracted from the former soils with alkaline sodium bicarbonate had been chemically ‘fixed’ via sorption and precipitation reactions with sesquioxides and rendered unavailable to plants at ambient soil pH. These critical Colwell‐P concentrations if adopted as benchmark values for the soil groups in question should ensure that the results of future soil fertility surveys involving Colwell‐P assessments are correctly interpreted.     

The ability of the DGT soil phosphorus test to predict pasture response in Australian pasture soils – a preliminary assessment

Diffusive gradients in thin‐films (DGT) technology provides an alternative assessment of available phosphorus (P) for a range of crops, suggesting a preliminary examination of the performance of the new DGT‐P test, compared to existing bicarbonate extractable Olsen and Colwell P tests, for pastures is justified. This study utilized historic data from the Australian National Reactive Phosphate Rock (NRPR) study (1992–1994) that included 25 experimental sites representing a wide range of soil types and climates used for pasture production. Stored (~19 yr) soil samples were analysed for DGT‐P, Olsen P and a single point P buffering index (PBI) and re‐analysed for Colwell P. Results showed the traditional bicarbonate extractable Colwell (r² = 0.45, P < 0.001) and Olsen P (r² = 0.27, P < 0.001) methods predicted relative pasture P response more accurately, compared to the novel DGT‐P test (r² = 0.09, P = 0.03) when all 3 yr of data were examined. We hypothesize that the harsher bicarbonate extraction used for the Olsen and Colwell methods more accurately reflects the ability of perennial pasture roots to access less labile forms of P, in contrast to the DGT‐P test, which does not change the soil pH or dilute the soil and appears unable to fully account for a plants ability to solubilize P. Further studies are needed to compare the capacity of DGT‐P to measure P availability in perennial pasture systems and to better understand the soil chemical differences between pasture and cropping systems.     

Limitations when quantifying microbial carbon and nitrogen by fumigation‐extraction in rooted soils

Soil microbial C and N (C_mic, N_mic) estimation by the chloroform fumigation‐extraction method is erroneous in densely rooted soils due to CHCl₃‐labile C and N compounds. The effect of a pre‐extraction with 50 mM K₂SO₄ and a pre‐incubation (conditioning at 25 °C for 7 days) on the flush in extractable, CHCl₃‐labile C (C‐flush) and N (N‐flush) was tested with reference to rooting density (0.3—75 mg root dry matter g^—1) in one arable and 3 grassland soils. In the arable soil and in the second horizon (10—20 cm) of a grassland soil, C‐flush values were not affected by the pre‐extraction. However, the pre‐extraction considerably reduced C‐flush values in the top soils of the grassland (above 10 cm). Only about 42 % was found in the pre‐extracted roots and the rest was lost during the pre‐extraction. The estimated concentrations of N_mic decreased due to pre‐extraction of soil samples with low root biomass. Clearly, the concentrations of N_mic were underestimated by introducing the pre‐extraction. Soil pre‐incubation reduced C‐flush values only slightly, whereas N‐flush values were not affected. It can be concluded that (1) CHCl₃‐labile root C and N is partly extracted with K₂SO₄ after pre‐incubation and (2) CHCl₃‐labile C and N removed with the roots during pre‐extraction is partly derived from microbial biomass. Soils with low rooting density (arable soils, grassland soils below approximately 10 cm depth) should therefore be fumigated and extracted without pre‐extraction. In densely rooted soils, fumigation extraction with and without pre‐extraction probably gives estimates for the minimum and maximum of C_mic and N_mic.     

Relationship of soil phosphorus with uranium in grassland mineral soils in Ireland using soils from a long‐term phosphorus experiment and a National Soil Database

Phosphorus fertilizer contains contaminants that may increase the content in the soil and in plants. The relationship between soil P and soil uranium (U) was investigated to determine potential effects of P‐fertilizer use. This study is based on a long‐term experiment (38 years with 0, 15, and 30 kg fertilizer P ha^–1 y^–1) for beef production on grassland at Teagasc, Johnstown Castle, Wexford, Ireland and also on soils from a National Soil Database (NSD). The NSD soils were taken at fixed locations on a predetermined grid system at the density of one sample every 50 km². Of the 1310 samples in the NSD, the 760 grassland mineral soils were selected for this study. The aim was to determine to what extent P fertilizer increases the content of U in the soil. The results showed that there was a small but significant increase in soil U in the high‐P treatments, which contained high levels of soil P, in the long‐term field experiment. The results from the NSD showed that there was not a significant relationship between extractable (Morgan's) soil test P (STP) and U. It is concluded that the use of chemical P fertilizer at normal rates used in agriculture in Ireland is not a major threat to U content of soil based on the results of this study. There was a significant relationship between total P and STP, in the NSD, with the latter making up approx. 1% of the former. Soil available P increased with soil pH, probably reflecting the use of chemical P fertilizer and lime on agricultural soils.     

Soil flooding and rice straw addition can increase isotopic exchangeable phosphorus in P‐deficient tropical soils

Soil flooding increases phosphorus (P) availability due to reductive dissolution of P‐bearing Fe(III) minerals. It is, however, unclear whether such processes also act in P‐deficient soils of the tropics that have large Fe/P ratios (dithionite‐ and oxalate‐extractable P and Fe). The objective was to identify the extent of P release induced by flooding in such soils and the soil characteristics involved. Six topsoils (0.4–5% Fe) from rice fields in Madagascar were incubated aerobically and anaerobically for 66 days amended with factorial combinations of (0, 50 mg P/kg); half of the flooded soils were also amended with 1 g rice straw/kg prior to flooding to stimulate soil oxygen depletion. The release of P after flooding was measured at day 40 with ³³P isotopic exchange, which detects both changes of labile P (exchangeable P) and changes in P solubility. Flooding increased labile P concentration in soil compared with aerobic soils by 1.4–60 mg P/kg, effects being significant in 6 of the 12 soil samples. Rice straw addition further increased the labile P in 5 of the 12 flooded soil samples by 2–27 mg P/kg. The release of labile P by flooding increased with soil oxalate‐extractable P concentration. Flooding combined with rice straw addition can increase the labile P in soil, even in soils with large amount of Fe; however, this release in unfertilized soils is likely insufficient for optimal nutrition of rice plants when evaluated against critical values for P solubility.     

Long‐term tillage effects on the distribution of phosphorus fractions of loess soils in Germany

Different tillage systems may affect P dynamics in soils due to differently distributed plant residues, different aggregate dynamics and erosion losses, but quantitative data are scarce. Objectives were to investigate the effect of tillage on the availability of P in a long‐term field trial on loess soils (Phaeozems and Luvisols) initiated from 1990 to 1997. Four research sites in E and S Germany were established with a crop rotation consisting of two times winter wheat followed by sugar beet. The treatments were no‐till (NT) without cultivation, except for seedbed preparation to a depth of 5 cm before sugar beet was sown and conventional tillage (CT) with mouldboard plowing down to 25–30 cm. Soil P was divided into different pools by a sequential extraction method, and total P (P_t) in the single P fractions was extracted by digesting the extracts of the fractionation to calculate the contents of organic P. The P_t content (792 mg [kg soil]^–1) in the topsoil (0–5 cm) of NT was 15% higher compared to CT, while with increasing depth the P_t content decreased more under NT than under CT. This was also true for the other P fractions except for residual P. The higher P contents in the topsoil of NT presumably resulted from the shallower incorporation of harvest residues and fertilizer P compared to CT, whereas estimated soil losses and thus also P losses due to water erosion were only small for both treatments. Contents of oxalate‐extractable Fe and organic C were positively related to the labile fractions of inorganic P, while there was a high correlation of the stable fractions with the clay contents and pH. Multiple regression analyses explained 50% of the variability of these P fractions. Overall, only small differences in the P fractions and availability were observed between the long‐term tillage treatments.     

Growth,phosphorus uptake,and rhizosphere microbial‐community composition of a phosphorus‐efficient wheat cultivar in soils differing in pH

In a pot experiment, the P‐efficient wheat (Triticum aestivum L.) cultivar Goldmark was grown in ten soils from South Australia covering a wide range of pH (four acidic, two neutral, and four alkaline soils) with low to moderate P availability. Phosphorus (100 mg P kg^–1) was supplied as FePO₄ to acidic soils, CaHPO₄ to alkaline, and 1:1 mixture of FePO₄ and CaHPO₄ to neutral soils. Phosphorus uptake was correlated with P availability measured by anion‐exchange resin and microbial biomass P in the rhizosphere. Growth and P uptake were best in the neutral soils, lower in the acidic, and poorest in the alkaline soils. The good growth in the neutral soils could be explained by a combination of extensive soil exploitation by the roots and high phosphatase activity in the rhizosphere, indicating microbial facilitation of organic‐P mineralization. The plant effect (soil exploitation by roots) appeared to dominate in the acidic soils. Alkaline phosphatase and diesterase activities in acidic soils were lower than in neutral soils, but strongly increased in the rhizosphere compared with the bulk soil, suggesting that microorganisms contribute to P uptake in these acidic soils. Shoot and root growth and P uptake per unit root length were lowest in the alkaline soils. Despite high alkaline phosphatase and diesterase activities in the alkaline soils, microbial biomass P was low, suggesting that the enzymes could not mineralize sufficient organic P to meet the demands of plants and microorganisms. Microbial‐community composition, assessed by fatty acid methylester (FAME) analysis, was strongly dependent on soil pH, whereas other soil properties (organic‐C or CaCO₃ content) were less important or not important at all (soil texture).     

Relation between soil P test values and mobilization of dissolved and particulate P from the plough layer of typical Danish soils from a long‐term field experiment with applied P fertilizers

Accumulation of phosphorus (P) in agricultural topsoils can contribute to leaching of P which may cause eutrophication of surface waters. An understanding of P mobilization processes in the plough layer is needed to improve agricultural management strategies. We compare leaching of total dissolved and particulate P through the plough layer of a typical Danish sandy loam soil subjected to three different P fertilizer regimes in a long‐term field experiment established in 1975. The leaching experiment used intact soil columns (20 cm diameter, 20 cm high) during unsaturated conditions. The three soils had small to moderate labile P contents, expressed by water‐extractable P (3.6–10.7 mg/kg), Olsen P (11–28 mg/kg) and degree of P saturation (DPS) (25–34%). Mobilization of total dissolved P (TDP) increased significantly (P < 0.05) from the intact soil columns with increasing labile P, whereas the increase in particulate P (PP) with increasing labile P content was modest and statistically insignificant. We found concentrations up to 1.5 mg TP/L for the plough layer of this typical Danish sandy loam soil. This highlights that even a moderate labile P content can be a potential source of TDP from the plough layer, and that a lower concentration margin of optimum agronomic P levels should be considered.     

Changes in soil phosphorus pools of grasslands following 17 yrs of balanced application of manure and fertilizer

Limiting the use of phosphorous (P) in intensive agriculture is necessary to decrease losses to surface waters. Balanced fertilizer application (P supply equals P offtake by the crop) is a first step to limit the use of P. However, it is questioned whether this balance approach is sufficient to maintain soil fertility. A long‐term field experiment (17 yr), on grazed grassland, has been conducted on sandy soil, marine clay soil and peat soil to obtain insight into the effects of balanced P fertilizer application on soil test P values and to explain the results by changes in P pools in the soil. The balance approach led to a gradual decline in plant available P, measured as P‐AL, in the topsoil (<0.10 m deep). This decline was accompanied by a decline in oxalate extractable P, dithionite extractable P and inorganic P (0.5 m H₂SO₄). The decline in these mineral P pools in the topsoil was (partly) compensated by an increase in the amount of organic P. There was evidence for the accumulation of P in an occluded form, especially at one of sites which received P as Gafsa rock phosphate [Ca₃(PO₄)₂].     

Application of low‐phosphorus‐containing legume residues reduces extractable phosphorus in a tropical Ultisol

Application of legume green manure (GM) is suggested to be effective in increasing the availability of native soil phosphorus (P) and the dissolution and utilization of phosphate rock (PR)‐P by food crops. Experiments were conducted to study the dynamics of extractable P (P extracted by Bray‐1‐extracting solution) of an Ultisol amended with or without GM residues of contrasting P concentrations in the absence of growing plants. In two separate experiments, GM residues of Aschynomene afraspera (a flood‐tolerant legume) and of Crotalaria micans (upland) with varying P concentrations were added to an acidic soil amended with PR‐P or triple superphosphate (TSP) in plastic bottles. Soil moisture was brought to field capacity of the soil in the upland experiment and saturated with distilled water in the lowland setup. This was done to simulate aerobic upland and anaerobic lowland soil conditions in the relevant plastic bottles. Only P concentration of the residues added varied, while lignin and C : N ratios were similar. A temperature of 25°C was maintained throughout the experiment. Changes in soil extractable Bray‐1‐P were measured at the end of the incubation period (60 or 80 d). In the aerobic soils, extractable P in the combined PR+GM or TSP+GM treatments was significantly lower than in the PR‐ or TSP‐ treated soils. The amendment with GM residues alone significantly increased Bray‐1‐P over the unamended control in the case of the inorganic P‐fertilized GM residues. The trend in extractable P was similar in the soils incubated under anaerobic conditions. However, in the case of PR, concentrations of P extracted by Bray‐1 solution did not significantly change in the presence or absence of GM. The results suggest that the incorporation of GM residues with low P concentration does not lead to a net P release in upland or lowland soils. These results have implications for nutrient cycling in farming systems in W Africa as most of the soils are poor and very low in available P.     

Improving minimum detectable differences in the assessment of soil organic matter change in short‐term field experiments

The demand for information on cropping system impact on soil organic matter (SOM) calls for efforts to improve the utilization of short‐term field experiments (e.g., to evaluate the parameterization of cropping systems in models). Those approaches have coped with the problem of determining small SOM changes within a large background mass. Thus, objectives of this survey are (1) the improvement of the minimum detectable difference (MDD) in SOM in the hudycrop short‐term field experiment by methods of sampling design and data treatment, (2) the verification to what extend the hudycrop short‐term field experiment allows for the determination of management induced effects on SOM, and (3) the investigation to what extent the obtained results may be suitable to evaluate the parameterization of a SOM balance model. The design of the hudycrop is suitable for excluding outliers plotwise. The estimation of plot means can be improved by the sampling design. Instead of determining a single plot mean in a mixed sampling procedure, the design provides multiple values for each plot, allowing for the identification of extreme values before calculating plot means. In consequence, minimum detectable differences decrease by a factor of 0.53 for soil organic C (SOC) and 0.63 for soil total N (STN) masses, allowing for detection of changes in the magnitude of 3.7 and 2.6% of background SOC and STN levels, respectively. Differences between treatments, however, are significant with corrected values (after outlier exclusion) for the crop production systems with the highest impact (potatoes and mulched red clover). Determining outliers based on Student's t‐test gives the lowest MDD and is therefore considered to be the most suitable method in this case. Correlations between apparent changes and SOM balances according to the HU‐MOD–2 model, used in this survey, indicate that the experimental design, in principal, is suitable for the evaluation of the parameterization of crop production systems in models. Still, an improved precision in SOM change detection is necessary. Reasonable options for that purpose are discussed in the paper.     

Phosphorus acquisition and wheat growth are influenced by shoot phosphorus status and soil phosphorus distribution in a split‐root system

Root proliferation and greater uptake per unit of root in the nutrient‐rich zones are often considered to be compensatory responses. This study aimed to examine the influence of plant phosphorus (P) status and P distribution in the root zone on root P acquisition and root and shoot growth of wheat (Triticum aestivum L.) in a split‐root soil culture. One compartment (A) was supplied with either 4 or 14 mg P (kg soil)^–1, whereas the adjoining compartment (B) had 4 mg P kg^–1 with a vertical high‐P strip (44 mg kg^–1) at 90–110 mm from the plant. Three weeks after growing in the split‐root system, plants with 4 mg P kg^–1 (low‐P plants) started to show stimulatory root growth in the high‐P strip. Two weeks later, root dry weight and length density in the high‐P strip were significantly greater for the low‐P plants than for the plants with 14 mg P (kg soil)^–1. However, after 8 weeks of growth in the split‐root system, the two P treatments of compartment A had similar root growth in the high‐P strip of compartment B. The study also showed that shoot P concentrations in the low‐P plants were 0.6–0.8 mg g^–1 compared with 1.7–1.9 mg g^–1 in the 14 mg P kg^–1 plants after 3 and 5 weeks of growth, but were similar (1.1–1.4 mg g^–1) between the two plants by week 8. The low‐P plants had lower root P concentration in both compartments than those with 14 mg P kg^–1 throughout the three harvests. The findings may indicate that root proliferation and P acquisition under heterogeneous conditions are influenced by shoot P status (internal) and soil P distribution (external). There were no differences in the total root and shoot dry weight between the two P treatments at weeks 3 and 5 because enhanced root growth and P uptake in the high‐P strip by the low‐P plants were compensated by reduced root growth elsewhere. In contrast, total plant growth and total root and shoot P contents were greater in the 14 mg P kg^–1 soil than in the low‐P soil at week 8. The two P treatments did not affect the ratio of root to shoot dry weight with time. The results suggest that root proliferation and greater P uptake in the P‐enriched zone may meet the demand for P by P‐deficient plants only for a limited period of time.     

Changes in soil aggregate‐associated enzyme activities and nutrients under long‐term chemical fertilizer applications in a phosphorus‐limited paddy soil

Paddy soils in subtropical China are usually deficient in phosphorus (P) and require regular application of chemical fertilizers. This study evaluated the effects of chemical fertilizers on the distribution of soil organic carbon (SOC), total nitrogen (N) and available P, and on the activity of the associated enzymes in bulk soil and aggregates. Surface soils (0–20 cm) were collected from a 24‐yr‐old field experiment with five treatments: unfertilized control (CK), N only (N), N and potassium (NK), N and P (NP), and N, P and K (NPK). Undisturbed bulk soils were separated into >2, 1–2, 0.25–1, 0.053–0.25 and <0.053 mm aggregate classes using wet sieving. Results showed that both NP‐ and NPK‐treated soils significantly increased mean weight diameter of aggregates, SOC, available P in bulk soil and aggregates, as compared to CK. Most SOC and total N adhered to macro‐aggregates (>0.25 mm), which accounted for 64–81% of SOC and 54–82% of total N in bulk soil. The activities of invertase and acid phosphatase in the 1–2 mm fraction were the highest under NPK treatment. The highest activity of urease was observed in the <0.053 mm fraction under NP treatment. Soil organic carbon and available P were major contributors to variation of enzyme activities at the aggregate scale. In conclusion, application of NP or NPK fertilizers promoted the formation of soil aggregates, nutrient contents and activities of associated enzymes in P‐limited paddy soils, and thus enhanced soil quality.     

Isolation and characterization of labile organic phosphorus pools in soils from the Askov long‐term field experiments

Isolierung und Kennzeichnung des labilen organischen Phosphor‐Pools in Böden des Langzeitdüngungsexperimentes Askov Labiler organischer Phosphor (P_o) im Boden spielt eine wichtige Rolle in der P‐Ernährung der Pflanzen und ist bedeutend hinsichtlich der Gewässereutrophierung. Im Rahmen dieser Arbeit werden neuere Ergebnisse zu den Eigenschaften des labilen P_o und seiner Reaktion auf unterschiedliche Düngungssysteme diskutiert. Die Untersuchungen fanden an Böden des Langzeitexperimentes zur organischen und anorganischen Düngung in Askov statt. Unser analytischer Ansatz basierte auf einer Kombination der Extraktion von labilem P_o mittels makroporösem Anionenaustauscher‐Harz und der Kennzeichnung von Struktur und Herkunft des NaOH‐extrahierbaren P_o mittels ³¹P‐NMR‐Spektroskopie. Die Analysen wurden an der Feinerde und an Korngrößenfraktionen durchgeführt. Die Ergebnisse zeigen, dass Harz‐Extraktion einen aktiven Pool an P_o isoliert, welcher v.a. aus mikrobiell synthetisierten Strukturen besteht. Die Größe dieses Pools variiert im Jahresgang und hängt von der P‐Düngung ab. Die Art des Düngers (NPK gegenüber Stallmist und Gülle) scheint demgegenüber den labilen P_o kaum zu beeinflussen. Der größte Teil des leicht verfügbaren P_o ist in der Tonfraktion lokalisiert. Es ist daher zu schließen, dass diese Fraktion wichtig im kurzfristigen Umsatz von P_o ist.