The erodibility and phosphorus loss potential of a selection of Irish tillage soils

Abstract

Arable cropping, due to its intensive nature, can leave soil with reduced ground cover and impaired soil structure, making it vulnerable to erosion under heavy rainfall. Runoff containing suspended sediment (SS) and nutrients, particularly phosphorus (P), from agricultural fields is considered to be one of the main causes of water quality impairment. To date, in Ireland, no study has investigated erosion and associated P loss from tillage soils when subjected to high intensity rainfall, even though international research indicates significantly higher P export coefficients from this land use than from grassland. As a result, only agronomic nutrient advice is available and this has been adopted into current legislation. Research was therefore necessary to assess the potential P losses arising from complying with the legislation. This objective was addressed in the first part of the study using two simulated rainfall experiments. A related objective involving the development of a screening methodology to identify tillage fields with erosion risk and soil quality problems was addressed in the second part. The aim of the first part of the study was to quantify the amount of dissolved reactive phosphorus (DRP), total phosphorus (TP), particulate phosphorus (PP) and SS released in runoff from five tillage soils with varying soil test P (STP) when subjected to a rainfall intensity of 30 mm hr-1 applied in three successive events. Soil physical and chemical parameters, slope, and time interval between storm events were ranked in order of importance for the prediction of P and SS releases, and a runoff dissolved phosphorus risk indicator (RDPRI) was developed to identify the STP for Irish tillage soils above which there may be a potential threat to surface water quality. The effect of soil type on the flow weighted mean concentration (FWMC) of DRP (p = 0.013) depended on both slope and time between rainfall events. The effect of soil type depended only on surface slope for the FWMCs of SS (p = 0.044), TP (p = 0.014) and PP (p = 0.022) in surface runoff. Increasing the overland flow rate over the soil surface in the presence of rainfall had the effect of increasing the concentrations of SS, PP and TP (but not DRP) in surface runoff (p < 0.05) across all soils. An increase in extractable soil P resulted in an increase in concentrations of DRP in surface runoff (p < 0.05) across all soils. Of the five methods used to extract soil P in these experiments, water extractable P (WEP) was identified as having the greatest potential to be used as an indicator of the risk of P movement from soil into runoff water. However, despite its apparent advantage over Morgan¿s v Phosphorus (Pm) in determining environmental risk, it would appear to be impractical and costly to run two soil P tests side by side given that Pm gives a good approximation for both agronomic and environmental purposes. Combining the results of both experiments (rainfall only and rainfall and overland flow) indicated that if the current agronomic optimum Pm range for tillage soils of 6.1 - 10 mg L-1 is maintained by tillage farmers through adherence to recommended P application rates, then the risk of runoff with DRP concentrations in excess of the level at which eutrophication is likely to occur (0.03 mg molybdate reactive phosphorus (MRP) L-1) should be minimal. Ireland has a valuable resource in terms of its land and soil quality, and promoting sustainable soil management is one of the areas of action included in Food Harvest 2020, the national strategy for the development of the agri-food sector. Agricultural activities that can negatively impact on soil quality must be tackled if Ireland is to meet the ambitious growth targets set out in this vision. Therefore, in the second part of the study, the five soils above, and a sixth soil, were assessed in their natural field conditions to determine their erosion risk and soil quality status. At each study site, detailed soil classification results and visual soil assessments were used in conjunction with observed erosion levels to select the most appropriate indicators for assessing erosion risk and soil quality status. Parameters selected include: texture, slope, erosion features, structure, ponding, potential rooting depth, soil organic matter (SOM), average annual rainfall and current land use. Assessment of these indicators at each study site using the user friendly grading system developed here, made it possible to correctly identify the sites where the erosion risk was high. This work showed that adoption of an erosion and soil quality screening method by tillage farmers and advisory specialists in Ireland will enable the quantification of the extent of erosion risk and soil quality degradation on farms. This will allow remediation measures to be prioritised for the most vulnerable sites, which is likely to result in cost and resource savings for farmers and advisory services.