Philip M. Haygarth

Biogeochemistry. Phosphorus solubilization in rewetted soils.

Biogeochemical cycles are shaped by events that follow soil drying and rewetting. Here we show that the process of drying and rapidly rewetting soil increases the amount of water-soluble phosphorus present and that this is predominantly in organic form after having been released from the soil microbial biomass. This effect could not only significantly affect phosphorus pollution of waterbodies but might also corrupt results from analyses involving water extraction of dried soils.

Transfer of Phosphorus from Agricultural Soil

Publisher Summary The chapter identifies phosphorus transfer (PT) as a multidisciplinary issue and focuses areas where future research is required. Potentially mobile P (PMP) is a useful concept because it attempts to weight soil P status in terms of environmental significance rather than by means of conventional soil measurement. The chapter describes agronomic management strategies, but the approaches are orientated towards agronomic rather than environmental concerns. The most significant area for further developing understanding of P transfer is hydrology, with its complexities of spatial and temporal variability. The chapter also describes new model approach that allows improved integration, which is of particular relevance when assessments of management effects and options for mitigation strategies are required. The new model structure provides a simple means for classifying and rationalizing P transfer and a basis for a spatial land use model. Moreover, it provides a basis for discussion and a means for defining future research needs on diffuse P transfer and its effects.

β-Glucosidase activity in pasture soils

β-Glucosidase is involved in the degradation of cellulose in soils and has potential for monitoring biological soil quality. We assayed β-glucosidase activity in 29 permanent grassland soils from England and Wales with contrasting physico-chemical and biological properties (clay content 22–68%; total carbon 29–80 mg g−1; microbial carbon 412–3412 μg g−1). Substrate induced β-glucosidase activity ranged between 1.12 and 6.12 μmol para-nitrophenol g−1 soil h−1 and was positively correlated with concentrations of clay, total carbon and microbial carbon. This suggests that substrate-induced β-glucosidase activity is an integrative measure of physico-chemical and biological soil properties and may have applications in monitoring biological soil quality.

Impacts of Climate Change on Indirect Human Exposure to Pathogens and Chemicals from Agriculture

Objective Climate change is likely to affect the nature of pathogens and chemicals in the environment and their fate and transport. Future risks of pathogens and chemicals could therefore be very different from those of today. In this review, we assess the implications of climate change for changes in human exposures to pathogens and chemicals in agricultural systems in the United Kingdom and discuss the subsequent effects on health impacts. Data sources In this review, we used expert input and considered literature on climate change; health effects resulting from exposure to pathogens and chemicals arising from agriculture; inputs of chemicals and pathogens to agricultural systems; and human exposure pathways for pathogens and chemicals in agricultural systems. Data synthesis We established the current evidence base for health effects of chemicals and pathogens in the agricultural environment; determined the potential implications of climate change on chemical and pathogen inputs in agricultural systems; and explored the effects of climate change on environmental transport and fate of different contaminant types. We combined these data to assess the implications of climate change in terms of indirect human exposure to pathogens and chemicals in agricultural systems. We then developed recommendations on future research and policy changes to manage any adverse increases in risks. Conclusions Overall, climate change is likely to increase human exposures to agricultural contaminants. The magnitude of the increases will be highly dependent on the contaminant type. Risks from many pathogens and particulate and particle-associated contaminants could increase significantly. These increases in exposure can, however, be managed for the most part through targeted research and policy changes.

Potential contribution of lysed bacterial cells to phosphorus solubilisation in two rewetted Australian pasture soils

Soil drying renders considerable amounts of phosphorus soluble upon rewetting, which may be partly derived from lysed microbial cells. Using direct bacterial cell counting in water and tetra-sodium pyrophosphate extracts of two Australian pasture soils, we found that almost all extractable cells were lysed following the rewetting of dry soils. The amounts of phosphorus in the lysed cells corresponded closely to the increases in water-extractable phosphorus following soil drying, suggesting that bacterial cell lysis is a major source of the released phosphorus.

Ensemble evaluation of hydrological model hypotheses

It is demonstrated for the first time how model parameter, structural and data uncertainties can be accounted for explicitly and simultaneously within the Generalized Likelihood Uncertainty Estimation (GLUE) methodology. As an example application, 72 variants of a single soil moisture accounting store are tested as simplified hypotheses of runoff generation at six experimental grassland field-scale lysimeters through model rejection and a novel diagnostic scheme. The fields, designed as replicates, exhibit different hydrological behaviors which yield different model performances. For fields with low initial discharge levels at the beginning of events, the conceptual stores considered reach their limit of applicability. Conversely, one of the fields yielding more discharge than the others, but having larger data gaps, allows for greater flexibility in the choice of model structures. As a model learning exercise, the study points to a “leaking” of the fields not evident from previous field experiments. It is discussed how understanding observational uncertainties and incorporating these into model diagnostics can help appreciate the scale of model structural error.

Hydrological factors for phosphorus transfer from agricultural soils.

Understanding and managing against phosphorus (P) transfer from agricultural soils to receiving waters is a multidisciplinary task in which the role of hydrology is particularly difficult to simplify. We review current knowledge to define the spatial and temporal controls on P transfer from agricultural soils via the various hydrological pathways. Rainfall intensity and duration and the interval between rainfall events are key temporal variables which influence the discharge (and hence P load) to receiving waters. In terms of understanding mechanisms, we postulate that levels of hydrological activity may be nominally classified at two levels. Level 1 activity occurs during light or little rainfall for a high proportion of time; in contrast, level 2 activity occurs less frequently but is more energetic and has a large capacity for P transfer over a small time period. The range of potential hydrological pathways of P transfer creates confusion because terminology varies. Often, process terms such as leaching or generic terms such as drainage are confused with hydrological pathways per se. Here we define the spatial variation in the hydrological pathways responsible for P transfer at two scales-soil profile and slope/fieldwhich subsequently can help the understanding of P transfers into the wider catchment, where problems occur. Our aim is to provide a simplified basis for classifying the otherwise complex hydrochemical regimes which result in P transfer.

Assessing the potential for pathogen transfer from grassland soils to surface waters.

Contamination of surface waters with pathogenic micro-organisms is an area of growing importance in the context of diffuse agricultural pollution. Hydrological pathways linking farmed land to receiving waters may operate as vectors of disease transmission. Runoff from grassland systems may be particularly important. In this chapter, we synthesize and evaluate recent and contextual studies relating to the issue. The chapter is necessarily wide ranging and interdisciplinary but we have focused largely on the hydrological, soil-based, and microbiological perspectives. The potential for pathogen presence in livestock wastes is demonstrated through prevalence studies, and subsequent loading of grasslands with contaminated wastes generates a potential surface store of pathogens. These microbes may then be transferred to the wider environment when source and transport drivers are combined in, for example, precipitation events. The delivery of contaminated agricultural drainage waters into first order streams may impact the quality and ecological balance of watercourses if the micro-organisms of concern are still viable. This chapter evaluates both die-off and transfer processes operating from source through to the end point receptors in surface waters. Gaps in knowledge are identified and appear to be due to the contribution of heterogeneity and hydrological complexity of agricultural catchments and the complications of prevalence data derived via a range of methodologies.

Phosphorus Solubilization and Potential Transfer to Surface Waters from the Soil Microbial Biomass Following Drying–Rewetting and Freezing–Thawing

Abstract Drying–rewetting and freezing–thawing are two of the most common forms of abiotic perturbations experienced by soils, and can result in the solubilization of phosphorus (P). There is increasing interest in one particular component of soil P that may be especially susceptible to such stresses: the soil microbial biomass. We examine the evidence for the soil microbial biomass acting as a significant source of P in soils and surface waters by studying the literature on the processes responsible for its solubilization and transfer, resulting from abiotic perturbations. These perturbations have been shown to kill up to circa 70% of the total microbial biomass in some soils, and in some cases nearly all the additional P solubilized has been attributed to the microbial biomass. The degree to which the soil microbial biomass is affected by abiotic perturbations is highly dependent upon many variables, not the least degree, duration, and temporal patterns of stress, as well as the soil type. It is hypothesized that while abiotic perturbations can solubilize large quantities of P from the soil microbial biomass in some soils, only a small proportion is likely to find its way from the soil to surface waters. This is not to say that this small proportion is not significant with regard to surface water quality and nutrient loss from the soil, and may become more prevalent under future climatic change. We conclude that it is likely that only extreme conditions will elicit large responses with regard to the solubilization and transfer of phosphorus to surface waters.

Uncertainties in data and models to describe event dynamics of agricultural sediment and phosphorus transfer.

Mathematical models help to quantify agricultural sediment and phosphorus transfers and to simulate mitigation of pollution. This paper develops empirical models of the dominant sediment and phosphorus event dynamics observed at high resolution in a drained and undrained, intensive grassland field-scale lysimeter (1 ha) experiment. The uncertainties in model development and simulation are addressed using Generalized Likelihood Uncertainty Estimation. A comparison of suspended solids (SS) and total phosphorus (TP) samples with a limited number of manual repeats indicates larger data variability at low flows. Quantitative uncertainty estimates for discharge (Q) are available from another study. Suspended solids-discharge (SS-Q) hysteresis is analyzed for four events and two drained and two undrained fields. Hysteresis loops differ spatially and temporally, and exhaustion is apparent between sequential hydrograph peaks. A coherent empirical model framework for hysteresis, where SS is a function of Q and rate of change of Q, is proposed. This is evaluated taking the Q uncertainty into account, which can contribute substantially to the overall uncertainty of model simulations. The model simulates small hysteresis loops well but fails to simulate exhaustion of SS sources and flushing at the onset of events. Analysis of the TP-SS relationship reveals that most of the variability occurs at low flows, and a power-law relationship can explain the dominant behavior at higher flows, which is consistent across events, fields, and pathways. The need for further field experiments to test hypotheses of sediment mobilization and to quantify data uncertainties is identified.