<p>We urgently need to wake up to the role that climate change will be playing in the phosphorus cycle.&#160; The paper will attempt to address the complexities, controversies and uncertainties of estimating the effects that climate change is having on the phosphorus cycle.&#160; Citing an example from three UK catchments, the effect of climate change on average winter phosphorus loads is predicted to increase by up to 30% by the 2050s, and these effects will only be off-set by large-scale agricultural changes (e.g. a 20&#8211;80% reduction in phosphorus inputs).&#160; Achieving phosphorus-related quality water in diverse and productive agricultural landscapes under a changing climate is going to be a massive challenge.&#160; It is less than a century since we started mining rock phosphate, but in the context of a 4.5-billion-year-old earth and the acceleration due to climate, we are living through a switching point for phosphorus in the earth system.</p>
This paper describes a research platform approach that has been developed in England to bring together researchers and stakeholders from a wide range of institutions to undertake multi-disciplinary, catchment-scale research on approaches to tackle agricultural water pollution.
Abstract Excessive phosphorus (P) concentrations can lead to conditions that limit the amenity of freshwater resources. This problem is particularly acute in agricultural catchments, where P fertilizer and manure amendments have been used to increase soil fertility and productivity. In these catchments, P indices are often used to help target critical source areas in order to reduce P exports. However, the overall impact of agricultural mitigation efforts on receiving waters has not always been consistent with declines in total P exports from catchments. In this paper we propose a model of dissolved P mobilization (i.e., entrainment) in surface runoff that accounts for this outcome and examine modifications to P indices that better accommodate dissolved P mobilization. We suggest that dissolved P mobilization commences near the soil surface and has two phases. When water is first applied, labile P is mostly mobilized by dissolution and advection. Subsequently, as the supply of readily accessible P is exhausted, diffusion and hydrodynamic dispersion mobilize P from other sources at a near constant rate for the remainder of the event. As most P exports occur in larger (i.e., longer) events, the second phase appears responsible for most dissolved P exports. Such a model of dissolved P mobilization is consistent with runoff monitoring data under natural and simulated rainfall, suggesting that on low (shallow) slopes where the interaction between surface soil and water may be prolonged, dissolved P concentrations are likely to be higher. Dissolved P mobilization from low‐slope areas is not well represented in P indices at present. We suggest that there needs to be a more complex, mechanistic structure to P indices that involves additional compartmentalization. Further, we suggest that this can be achieved without losing the simplicity of P indices or flexibility to integrate research data and experiential knowledge into tools that are relevant to specific regions.
The preparation of soil for measurement of properties such as microbial biomass P involves the removal of plant roots. Any soil attached to the roots (root‐attached soil) is also removed. In a very poorly drained silty clay loam under grassland we found that the root‐attached soil contained more than twice the quantity of bicarbonate extractable P than the bulk soil. Discarding this root‐attached soil could potentially result in underestimation of bicarbonate extractable P. We also showed that preferential inclusion of deeper soil due to variability of root density with depth is likely to result in underestimation of soil bicarbonate extractable P in fumigated and unfumigated soil samples. Additionally we investigated a conventional and alternative (rapid) soil preparation technique that might affect the accuracy of measurement of soil bicarbonate extractable P as part of a microbial biomass P measurement. Preparation technique made no significant difference to the quantity of P recovered.
ABSTRACT A measure of soil P status in agricultural soils is generally required for assisting with prediction of potential P loss from agricultural catchments and assessing risk for water quality. The objectives of this paper are twofold: (i) investigating the soil P status, distribution, and variability, both spatially and with soil depth, of two different first‐order catchments; and (ii) determining variation in soil P concentration in relation to catchment topography (quantified as the “topographic index”) and critical source areas (CSAs). The soil P measurements showed large spatial variability, not only between fields and land uses, but also within individual fields and in part was thought to be strongly influenced by areas where cattle tended to congregate and areas where manure was most commonly spread. Topographic index alone was not related to the distribution of soil P, and does not seem to provide an adequate indicator for CSAs in the study catchments. However, CSAs may be used in conjunction with soil P data for help in determining a more “effective” catchment soil P status. The difficulties in defining CSAs a priori, particularly for modeling and prediction purposes, however, suggest that other more “integrated” measures of catchment soil P status, such as baseflow P concentrations or streambed sediment P concentrations, might be more useful. Since observed soil P distribution is variable and is also difficult to relate to nationally available soil P data, any assessment of soil P status for determining risk of P loss is uncertain and problematic, given other catchment physicochemical characteristics and the sampling strategy employed.
Abstract This paper reports the use of a new technique, flow field‐flow fractionation (FlFFF), for the characterization of soil sampled under grassland. FlFFF can be used to determine the fine colloidal material in the <1 μm fraction obtained by gravitational settling of 1% m/v soil suspensions. The aim of this work was to determine the potential of FIFFF to characterize soil colloids in drained and undrained field lysimeters from soil cores sampled at different depths. Two different grassland lysimeter plots of 1 ha, one drained and one undrained, were investigated, and the soil was sampled at 20‐m intervals along a single diagonal transect at three different depths (0–2, 10–12, and 30–32 cm). The results showed that there was a statistically significant ( P = 0.05) increase in colloidal material at 30‐ to 32‐cm depth along the transect under the drained lysimeter, which correlates with disturbance of the soil at this depth due to the installation of tile drains at 85‐cm depth backfilled to 30‐cm depth with gravel. Laser sizing was also used to determine the particles in the size range 1 to 2000 μm and complement the data obtained using FlFFF because laser sizing lacks resolution for the finer colloidal material (0.1–1.0 μm). The laser sizing data showed increased heterogeneity at 30‐ to 32‐cm depth, particularly in the 50 to 250 μm size fraction. Therefore FIFFF characterized the finer material and laser sizing the coarser soil fraction (<2000 μm) at depth in drained and undrained grassland. This is of importance as colloidal material is more mobile than the larger material and consequently an important vector for contaminant transport from agricultural land to catchments.
Abstract Soil phosphorus drives food production that is needed to feed a growing global population. However, knowledge of plant available phosphorus stocks at a global scale is poor but needed to better match phosphorus fertiliser supply to crop demand. We collated, checked, converted, and filtered a database of c . 575,000 soil samples to c . 33,000 soil samples of soil Olsen phosphorus concentrations. These data represent the most up-to-date repository of freely available data for plant available phosphorus at a global scale. We used these data to derive a model ( R 2 = 0.54) of topsoil Olsen phosphorus concentrations that when combined with data on bulk density predicted the distribution and global stock of soil Olsen phosphorus. We expect that these data can be used to not only show where plant available P should be boosted, but also where it can be drawn down to make more efficient use of fertiliser phosphorus and to minimise likely phosphorus loss and degradation of water quality.
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