G. H. Rubæk

Particle leaching and particle-facilitated transport of phosphorus at field scale

Strongly sorbing compounds such as P, pesticides, and heavy metals can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals is relatively well understood, the particle-facilitated transport of highly sorbing chemicals such as P requires further investigation. The aim of this work was to study spatial variations in particle-facilitated transport of P at the field scale, and investigate which soil-physical or chemical parameters relate to the observed variations. Leaching experiments were performed in the laboratory on 42 undisturbed soil columns sampled in a grid covering 25 by 30 m of an agricultural field. The columns were equilibrated in the laboratory to a pressure head of −20 cm and irrigated at a rate of 10 mm h −1 with an artificial rainwater solution. The experiments exhibited considerable variation among the columns in the accumulated mass of particles and P leached during the 3.5 h of irrigation. Columns taken from the lower part of the field showed the highest mass of leached particles. These columns had higher clay contents and contained more continuous macropores. The mass of particles was negatively correlated to the average electrical conductivity of the effluent, and positively correlated to the macropore flow velocity. The accumulated masses of particulate organic and inorganic P were linearly related to the accumulated mass of particles leached. About 75% of the leached P was transported in a particle-facilitated manner. Overall, soil structure controlled to a large extent the leaching of particles and particle bound P.

Mitigation options to reduce phosphorus losses from the agricultural sector and improve surface water quality: a review

The EU Water Framework Directive (WFD) obliges Member States to improve the quality of surface water and groundwater. The measures implemented to date have reduced the contribution of point sources of pollution, and hence diffuse pollution from agriculture has become more important. In many catchments the water quality remains poor. COST Action 869 was an EU initiative to improve surface water quality that ran from 2006 to 2011, in which 30 countries participated. Its main aim was a scientific evaluation of the suitability and cost-effectiveness of options for reducing nutrient loss from rural areas to surface waters at catchment scale, including the feasibility of the options under different climatic and geographical conditions. This paper gives an overview of various categories of mitigation options in relation to phosphorus (P). The individual measures are described in terms of their mode of action, applicability, effectiveness, time frame, environmental side-effects (N cycling) and cost. In total, 83 measures were evaluated in COST Action 869.

Flax (Linum usitatissimum L.) depends on arbuscular mycorrhizal fungi for growth and P uptake at intermediate but not high soil P levels in the field

The contribution of indigenous arbuscular mycorrhizal fungi (AMF) to growth and phosphorus (P) uptake by oilseed flax (Linum usitatissimum L.) was examined in two field experiments covering soil P levels from 20–86 mg kg-1 NaHCO3-extractable P. The fumigant dazomet was applied to the soil in half of the plots to obtain control plants with reduced mycorrhiza formation. An extensive AMF colonization of up to 48% of the root length was established in untreated soil of both experiments, although P fertilization reduced colonization to 28–39% at the latest harvests. Fumigation markedly decreased or totally prevented AMF colonization throughout the experiments. Root growth responded to fumigation by increased total and specific root length. Shoot P uptake was decreased by fumigation at soil P levels lower than ca. 50 mg kg-1 whereas shoot growth was reduced by fumigation at soil P levels lower than ca. 40 mg kg-1. The effects of fumigation were ascribed to the suppression of mycorrhiza formation. The effect of the AMF increased with decreasing soil P levels. Phosphorus inflow through roots (based on shoot P uptake) was reduced more strongly by fumigation than total P uptake. The P inflow through fungal tissue in roots was estimated to 4 × 10-14 mol P cm-1 s-1. We conclude that AMF are essential to flax growth at soil P levels below ca. 40 mg P kg-1, which is representative of the conditions under which most flax is grown.

Stewardship to tackle global phosphorus inefficiency: The case of Europe

The inefficient use of phosphorus (P) in the food chain is a threat to the global aquatic environment and the health and well-being of citizens, and it is depleting an essential finite natural resource critical for future food security and ecosystem function. We outline a strategic framework of 5R stewardship (Re-align P inputs, Reduce P losses, Recycle P in bioresources, Recover P in wastes, and Redefine P in food systems) to help identify and deliver a range of integrated, cost-effective, and feasible technological innovations to improve P use efficiency in society and reduce Europe’s dependence on P imports. Their combined adoption facilitated by interactive policies, co-operation between upstream and downstream stakeholders (researchers, investors, producers, distributors, and consumers), and more harmonized approaches to P accounting would maximize the resource and environmental benefits and help deliver a more competitive, circular, and sustainable European economy. The case of Europe provides a blueprint for global P stewardship.

Critical evaluation of the implementation of mitigation options for phosphorus from field to catchment scales.

Nutrient regulations have been developed over the past decades to limit anthropogenic inputs of phosphorus (P) to surface waters. All of the regulations were promulgated in response to decreased water quality, which was at least partially associated with agricultural non-point source pollution. Improvements in water quality can take years, so the impacts of these regulations on water quality can not always be seen. Denmark has had nutrient management regulations aimed at achieving mass balance of P for 20 yr, and although great progress has been made, an average surplus of 11 kg P ha(-1) remains. Northern Ireland is also trying to move toward mass balance, but decreases in inorganic P fertilizer use have been undermined by an increase in the use of feed concentrates. In the Chesapeake Bay watershed, which covers several states in the USA, a variety of best management practices are starting to have an effect on P losses from agriculture, but water quality has only improved slightly. Impairment to the supply of drinking water to the City of Tulsa Oklahoma led to a lawsuit that has greatly affected the management of poultry litter in the supplying watershed. This paper discusses the different regulations that have developed in these four regions, evaluates the strategies used to prevent non-point source pollution of P, reports impacts on water quality, and looks for lessons that can be learned as we move forward.

International phosphorus workshop: diffuse phosphorus loss to surface water bodies--risk assessment, mitigation options, and ecological effects in river basins.

Agriculture is a major source of P to the aquatic environment in many countries. Although efforts have been made to improve the P utilization in agricultural production, which is reflected in modestly declining P surpluses in many countries, increasing agricultural P surpluses are still observed in some countries. The IPW5 Special Submission included in this issue addresses and discusses four key topics that emerged from the workshop: (i) managing agricultural P losses-effectiveness, uncertainties, and costs; (ii) P modeling at different scales; (iii) functioning of riparian buffers; (iv) ecological responses to P loadings and impacts of climate change. Each of these four topics interacts with each other as well as with the four tiers of the P Transfer Continuum (Source, Mobilization, Transport, and Ecological Effects). In this review paper we highlight the main outcomes of the workshop and the special collection of eight papers. Moreover, we identify the main gaps in our knowledge and future research directions on P, which are linked to important issues such as addressing scale effects, improved P models with the ability to quantify uncertainty, the linking of P losses with ecological effects, and climate change.

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

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

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

Labile soil organic phosphorus (Po) plays a crucial role in plant P nutrition and in environmental eutrophication. This paper discusses recent studies on the nature of labile soil Po and its response to different fertilization practices. Soil material was obtained from the Askov long-term experiment on animal manure and mineral fertilizers. Our analytical approach combined a macroporous anion exchange resin to isolate labile Po and 31P NMR spectroscopy to assess the chemical composition and origin of NaOH-extractable Po. The analyses were carried out on fine earth (<2 mm) and particle-size fractions. The results suggest that the resin extraction isolates an active pool of soil Po that consists primarily of microbially-derived compounds. The size of the active Po pool reflects seasonal variations and P fertilization. However, the source of added P (NPK vs. animal manure) appears to have only little influence on the labile Po. Our results further suggest that most of the active soil Po is associated with clay sized separates (<2 μm), indicating this fraction being important in the short-term turnover of Po.

Characterization of Leached Phosphorus from Soil, Manure, and Manure-Amended Soil by Physical and Chemical Fractionation and Diffusive Gradients in Thin Films (DGT)

We are challenged to date to fully understand mechanisms controlling phosphorus (P) mobilization in soil. In this study we evaluated physical properties, chemical reactivity, and potential bioavailability of P mobilized in soil during a leaching event and examined how the amounts and properties of leached P were influenced by surface application of cattle manure. Leaching experiments on manure itself, and on intact soil columns (14.1 cm inner dia., 25 cm height) before and after manure application, were carried out at an irrigation rate of 1 mm h(-1) for 48 h. High concentrations of dissolved reactive P (DRP) were found in manure leachates (up to 32 mg L(-1)), whereas concentrations of P in soil leachates were low both before and after manure application (around 0.04 mg L(-1) before application and up to 0.4 mg L(-1) afterward). This result indicates that the soil retained most of the P added with manure. Manure particles themselves were also largely retained by the soil. Combined physical (centrifugation) and chemical (molybdate reactiveness) fractionation of leached P showed that leachates in the manure treated soils were dominated by dissolved unreactive P (DUP), mainly originating from manure. However, centrifugation only removed a small fraction of total particles from the leachates, indicating that the so-called dissolved fraction may be associated with low density particulate matter. Deployment of Diffusive Gradients in Thin films (DGT) devices in the leachates proved to be a good approach for measuring reactive P in soil leachates. The results indicated that total reactive P (TRP) gave a better estimate of potentially bioavailable P than both total P (TP) and DRP in these experiments.

Risk assessment of replacing conventional P fertilizers with biomass ash: Residual effects on plant yield, nutrition, cadmium accumulation and mycorrhizal status.

Reutilizing biomass ashes in agriculture can substitute inputs of P from finite primary sources. However, recycling of ashes is disputed due to their content of toxic substances such as heavy metals. This study evaluates the potential risk of replacing easily soluble inorganic P fertilizer with P in biomass ashes in a barley crop grown on soil with adequate P status. Two contrasting doses of three different types of ashes were applied to an agricultural field with spring barley and compared to similar doses of triple-superphosphate fertilizer. In the second growing season after biomass ash application, grain, straw and root dry matter yield, and P and Cd uptake were determined. Resin-extractable P was measured in soil and the symbiotic arbuscular mycorrhizal fungal activity, colonization, and community composition were assessed. Crop yield was not affected by ash application, while P-uptake and mycorrhizal status were slightly enhanced with high ash applications. Changes to the mycorrhizal community composition were evident with high ash doses. Cadmium uptake in aboveground plant tissue was unaffected by ash treatments, but increased in roots with increasing doses. Consequently, we conclude that fertilization with biomass ashes can replace conventional fertilizers without risk to barley crops in the short term.