R.W. Mcdowell

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• PhD
• Principal Investigator at AgResearch

Reference conditions pertain to conditions without anthropogenic influence and serve to gauge the degree of river pollution and identify the best attainable water quality. Here we show estimates of the global human footprint of nitrogen and phosphorus concentrations and potential for related nuisance or harmful algal growth in rivers. We use statis...

Combining existing databases, we estimated global phosphorus stocks in croplands and grasslands that are not readily available to plants as 32–41% of the 2020 estimated geologic phosphorus reserves, representing 146–186 years of the 2020 mass of phosphorus fertilizer applied annually. Especially if accessed by more efficient crops, this stock could...

See the following link for the unreviewed preprint: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=5105393

Introduction In agroecosystems, phosphorus (P) applications over a long time have accumulated in soil as legacy P. This environmental challenge can be an agronomic opportunity as soil legacy P could be recovered in cropping systems using practices such as green manuring. We hypothesised that, at moderate soil available P levels, plant‐soil interact...

Simple models can help reduce nitrogen (N) loss from land and protect water quality. However, the complexity of primary production systems may impair the accuracy of simple models. A tool was developed that assessed the risk of N loss as the product of N source inputs and relative transport by leaching and runoff. A dynamic process‐based model was...

Good management practices (GMPs) on dairy farms have been shown to reduce contaminant losses and improve water quality. Few national long-term datasets exist globally on management practices on dairy farms over time and their effect on nutrient losses. Here, we examine 50 parameters across a 10-year period (from 2013 to 2022) thought to influence e...

A lake monitoring regime optimised to detect natural and anthropogenic variations in water quality is critical for supporting and understanding the impact of lake restoration actions. It can support efficient investment in monitoring by minimising the time required for change detection of lake water quality attributes. The objective of this study w...

Context Green manure crops have the potential to improve phosphorus (P) use efficiency in agroecosystems by enhancing the mobilisation of soil P reserves. Aims This study investigated and quantified the short-term mobilisation and uptake of soil P in the rhizosphere of several green manure crops. Methods Five plant species/varieties (Lupinus angu...

The full article is available free at https://doi.org/10.1016/j.mex.2024.102814 Risk index tools have the potential to assist farmers in making strategic decisions regarding their farm design to manage losses of nutrients. Such tools require a vulnerability framework, and these are often based on scores or rankings. These frameworks struggle to ta...

With the longevity of phosphorus reserves uncertain, distributing phosphorus to meet food production needs is a global challenge. Here we match plant-available soil Olsen phosphorus concentrations to thresholds for optimal productivity of improved grassland and 28 of the world’s most widely grown and valuable crops. We find more land (73%) below op...

The severity and frequency of droughts are projected to increase globally due to climate change, but the effects of this on water quality are uncertain. The Murray-Darling Basin (MDB) is the largest river system in Australia and has been impacted by droughts of varying severity within recent decades. In this study, we assessed the influence of hydr...

Critical source areas (CSAs) are small areas of a field, farm, or catchment that account for most contaminant loss by having both a high contaminant availability and transport potential. Most work on CSAs has focused on phosphorus (P), largely through the work in the 1990s initiated by Dr. Sharpley and colleagues who recognized the value in targeti...

Detecting change in water quality is key to providing evidence of progress towards meeting water quality objectives. A key measure for detecting change is statistical power. Here we calculate statistical power for all regularly (monthly) monitored streams in New Zealand to test the effectiveness of monitoring for policy that aims to decrease contam...

Pistachio is one of the most important and valuable orchard products in Iran and some other places in the world. Because it is adaptable to adverse environmental conditions, especially drought and salinity, more land is being used for pistachios. In an increasingly resource constrained world, producers, researchers and policy makers need to clearly...

Numerous drivers such as farming practices, erosion, land-use change, and soil biogeochemical background, determine the global spatial distribution of phosphorus (P) in agricultural soils. Here, we revised an approach published earlier (called here GPASOIL-v0), in which several global datasets describing these drivers were combined with a process m...

The application of Cd-contaminated phosphate fertiliser has enriched concentrations of this non-essential element in many agricultural soils. Consequently, concentrations of the metal in some agricultural products exceed the Maximum Limit in foods. Composts can reduce the transfer of Cd from soil to plants; however, it is unclear how long this bene...

To monitor and meet water quality objectives, it is necessary to understand and quantify the contribution of nonpoint sources to total phosphorus (P) loading to surface waters. However, the contribution of streambank erosion to surface water P loads remains unclear and is typically unaccounted for in many nutrient loading assessments and policies....

Management of groundwater quality is assisted by an understanding of reference conditions, which describe the concentration ranges expected for key substances in the absence of human impact. This study evaluates reference conditions for NO3–N in New Zealand groundwater based on three complementary methods: hierarchical cluster analysis, relationshi...

Abstract Local farm plans can be used to minimise the effect of food production on water quality and greenhouse gas emissions but must be integrated with global markets to incentivise farmers to implement plans. 1. The problem By 2050 we must feed an estimated 10 billion people. To meet this demand for food, existing agricultural land has intens...

Intensive land use, such as dairying, can impair water quality. Although many guidelines exist on how to mitigate the loss of dairy-associated contaminants from land to water through best management practices (BMPs), few datasets exist on the success of implementation on-farm. Five dairy-dominated catchments (from 598 to 2480 ha) in New Zealand wer...

Pistachio production is an economically important crop that grows in arid environments. To predict yield and sustainably manage the use of natural resources such as soil and water, we modelled the effect of soil properties by classification and regression tree, k-nearest neighbors, support vector machines and developed a new hybrid model of support...

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 sam...

Shallow subsurface pathways dominate dissolved reactive phosphorus (DRP) losses in grassland soils that are: poorly drained, shallow, or have a perched water table in wetter months causing saturation‐excess runoff. Saturated conditions can lead to anoxia, which can accelerate phosphorus (P) loss. Two scales of investigation were utilized in this st...

Dissolved reactive phosphorus (DRP) loss from agricultural soils can negatively affect water quality. Shallow subsurface pathways can dominate P losses in grassland soils, especially in wetter months when waterlogging is common. This study investigated the processes controlling intra- and inter-event and seasonal DRP losses from poorly drained perm...

Agricultural production has economic, environmental, social and cultural consequences beyond farm boundaries, but information about these impacts is not readily available to decision makers. This study applied the land use suitability concept by carrying out an assessment of a region that has the potential for intensification of agricultural produc...

Pasture-based and grass-fed branding are often associated with consumer perceptions of improved human health, environmental performance and animal welfare. Here, to examine the impacts of dairy production in detail, we contrasted global observational (n = 156) data for nitrogen and phosphorus losses from land by the duration of outdoor livestock gr...

Few data exist on the success of implementing strategies to improve water quality. The Best Practice Dairy Catchments project commenced in 2001 in five small dairy-dominated catchments and monitored the implementation of management strategies to mitigate declining water quality. Although ending in 2010 water quality data and farm practice data have...

Food production plays a central role in the health of humanity and our environment. New Zealand produces a large amount of food, but it is unknown if it can produce enough of the right crops in the places to better the health of New Zealanders, profitably, while maintaining New Zealand’s primary production exports and meeting ambitions to lower gre...

Water quality can be improved by fencing off streams from livestock. However, the remedial effect of fencing can fail as livestock trample near stream areas. Water samples were taken every 3–4 wk, and flow was monitored over time in a headwater stream from 2003 to 2019. In November 2005, the stream edge and an area near the stream used for wallowin...

Agricultural production has economic, environmental, social and cultural consequences beyond farm boundaries, but information about these impacts is not readily available to decision makers. This study applied the Land Use Suitability concept by carrying out an assessment of a region that has the potential for intensification of agricultural produc...

At moderate to high fertilization rates, sandy-textured soils can leach much phosphorus (P) threatening surface water quality. High rates are used to compensate for P leaching, but there is also potential to reduce P leaching by using different P fertilizers. We examined the effect of poultry manure (PM), sheep manure (SM), triple superphosphate (T...

Making optimal use of scarce resources in developing countries is a major challenge for sustainable crop production. Characterization of factors limiting crop production is an important step in meeting this challenge. We outline and test a new method to identify characteristics of land limiting maize production in 61000 ha of the Qazvin province, n...

Soil microbes and phosphatase enzymes play a critical role in organic soil phosphorus (P) cycling. However, how long‐term P inputs influence microbial P transformations and phosphatase enzyme activity under grazed pastures remain unclear. We collected top‐soil (0‐75 mm) from a grazed pasture receiving contrasting P inputs (control, 188 kg ha‐1 yr‐1...

International studies point out that some freshwater policy objectives are not achieved. This study describes that this is in part caused by shortcomings that include: the lack of targeted monitoring schemes to measure impact; a too small range of specific technologies rather than a wider suite of integrated multiple technologies; a too tight focus...

There are abundant opportunities to transition towards more sustainable phosphorus use. Taken collectively, these solutions unlock multiple environmental and societal benefits. Actions must be delivered cooperatively, as part of an integrated plan across sectors and scales. Indeed, coordinated action on phosphorus to support governments, existing c...

Streams can attenuate inputs of phosphorus (P) and therefore dampen the likelihood of ecosystem eutrophication. This P attenuation derives from many processes and remains poorly understood, particularly in reference to the geochemical mechanisms involved. We studied P attenuation in the form of (1) potential for mineral (co-)precipitation from the...

Soils irrigated with phosphorus (P)-rich wastewater can increase the risk of P losses from land to water. We investigated if wastewater-irrigated soils can leach P through the soil profile and enrich groundwater. Soils were sampled annually for 20 years to 7.5 cm depth and for one year to 2.2 and 15 m. A mass balance showed that topsoils >200 mg L−...

Nitrogen (N) and phosphorus (P) are critical to pasture productivity; however, limited information is available on how the single and combined additions of N and P affect soil P fractions and seasonal changes in microbial and biochemical processes linked to P cycling under pasture systems. A two-year field trial was conducted where N (0 or 250 kg h...

To decrease the concentration of the toxic metal cadmium (Cd) in topsoil, and the human food chain, many countries have limited the Cd concentration allowed in phosphorus (P) fertilisers. However, to inform those limits we need accurate estimates of Cd leaching from established farming systems. Different soil layers were sampled to 2000 mm depth of...

Context Soils irrigated with wastewater are generally phosphorus (P)-enriched. P losses from these soils may impair surface water quality. However, wastewater applications also alter soil pH and P availability. Aims We investigated if amending soils with aluminium (Al), iron (Fe) or calcium (Ca) sorbents could decrease the potential for P losses d...

Phosphorus (P) loss from agricultural soils can negatively affect water quality. Models and management to decrease losses increasingly focus on P that is available and transported from areas in the landscape that are regularly saturated – and periodically anaerobic. Current models use soil tests conducted in oxic conditions, which do not represent...

Applying phosphorus (P)-rich wastewater to land can significantly enrich P in topsoil and consequently increase the risk of P losses in surface runoff and leaching. We collected 654 samples (0–7.5, 7.5–15, and 15–30 cm depths) of wastewater-irrigated and non-irrigated soils at seven sites across New Zealand to assess the potential P mobility in irr...

Decreasing soil cadmium (Cd) is one method of removing Cd from the food chain. Phosphorus (P) fertilisers are a major source of Cd inputs into soil. Stopping P fertiliser should theoretically decrease Cd inputs and soil Cd accumulation, but there are few field data to show if this occurs. We examined three long-term grazed pasture trials in New Zea...

Understanding the lag time between land management and impacts on riverine nitrate–nitrogen (N) loads is critical to understand when action to mitigate nitrate–N leaching losses from the soil profile may start improving water quality. These lags occur due to leaching of nitrate–N through the subsurface (soil and groundwater). Actions to mitigate ni...

Full manuscript available: https://www.essoar.org/doi/abs/10.1002/essoar.10507536.1

In soils with a fragipan or poor permeability, water may remain in a soil profile long enough to make it anoxic and reductive. The reductive dissolution of iron (Fe)‐ and manganese (Mn)‐oxides can release associated phosphorus (P). Therefore, the dissolved P would be vulnerable to subsurface flow and could contaminate nearby streams. It was hypothe...

Assessing the effectiveness of mitigation measures for reducing contaminant losses to water from pastoral farming systems is a challenging task. Two important factors that contribute to this challenge are (i) the considerable spatial variability in landscape vulnerabilities to contaminant loss and (ii) differing land use pressures created by contra...

The Land Use Suitability (LUS) concept informs decision-making by stakeholders with information about the economic and environmental consequences of land use choices. LUS is composed of three indicators describing the inherent productive and economichttps://mail.niwa.co.nz/owa/- _msocom_1 potential of land parcels (productive potential), the contri...

Reducing agriculturally derived diffuse contaminant losses (via non-point sources) from land to water has proven difficult for decades. Owing to the diversity and complexity of factors controlling contaminant loss rates and pathways, regulation must be flexible and reflect farm- and region-specific variation. Environmental farm planning schemes are...

Contaminant loss from grazed pasture can negatively affect freshwater quality. There is, however, little data on the impact of different levels of grazing/treading on contaminant loss measured under field conditions. This study quantified phosphorus (P), sediment and Escherichia coli (E. coli) loss in surface runoff from plots grazed by cattle for...

Phosphorus (P) pollution of surface waters remains a challenge for protecting and improving water quality. Central to the challenge is understanding what regulates P concentrations in streams. This quantitative review synthesizes the literature on a major control of P concentrations in streams at baseflow—the sediment P buffer—to better understand...

Pastures are the most widespread land use, globally. The Winchmore trials were established in 1948–1949 in Canterbury, New Zealand and examined either different rates of phosphorus (P) fertiliser on the same irrigation schedule (Fertiliser trial), or different irrigation scheduling at the same rate of P application (Irrigation trial). About 96,000...

Environmental farm planning in New Zealand dates to the 1950s when soil conversation plans were first undertaken. Since then the extent and complexity of whole farm sustainability along with regulatory drivers and environmental compliance targets has increased. The public, and consumers, both domestically and internationally, are placing ever incre...

The United Nations Sustainable Development Goals (SDGs) are being integrated into bioeconomy strategies around the world, including the European Green Deal. We highlight how microbiome-based innovations can contribute to policies that interface with the SDGs and argue that international cooperation in microbiome science is crucial for success.

In New Zealand the primary sector together with central and local government agencies have been promoting measures to mitigate the adverse effect of farming practices on water quality over the last few decades. We assessed the effectiveness of some key measures such as stock exclusion, riparian protection, and nutrient and effluent management on re...

By 2050, elevated atmospheric CO2 (eCO2) could stimulate plant growth, but dwindling phosphorus (P) stocks in the soil could limit growth. However, little is known about how eCO2 could affect soil P availability and dynamics in P-poor soils. Here, we conducted a 6-week pot experiment where three plant species were grown in a low-P soil under ambien...

Dairy cow deposits on farm roadways are a potential source of contaminants entering streams. Phosphorus (P), suspended sediment (SS) and Escherichia coli (E. coli) loads in 18 runoff events over 12 mo from two-halves of a section of dairy farm roadway that spilt into an adjacent P-impacted stream were measured. The runoff from one half was untreate...

Human activities have increased the input of nitrogen and phosphorus into riverine systems. These inputs can increase algal growth that degrades aquatic ecosystems. We constructed a global database of diffuse loads (kg) and yields (kg ha −1 yr −1) of dissolved and total nitrogen and phosphorus forms for 7 years (centred around 2008) in 1,421 catchm...

To meet the water quality outcomes sought by catchment communities and regulators, the losses of nitrogen (N), phosphorus (P) and sediment from dairy and sheep/beef farms must be reduced across many catchments. We conducted a high-level desktop analysis of farm typologies and established that if mitigation actions were fully implemented across dair...

Elevated atmospheric carbon dioxide (eCO2) associated with climate change increases plant production and soil nutrient transformations. However, changes in soil phosphorus (P) availability and dynamics are unclear. We used the long-term New Zealand Free Air CO2 Enrichment (FACE) experiment to quantify changes in P availability and soil P fractions...

Benthic stream sediments interact strongly with phosphorus (P) and can buffer dissolved reactive P (DRP) concentrations. The sediment P buffer can be measured with the sediment equilibrium phosphate concentration at net zero sorption (EPC0), which often correlates well with DRP. Yet, it is unclear how much of this P affinity in sediments is attribu...

Achieving good water quality through output controls is difficult. The New Zealand Government recently proposed enforceable bottom lines to protect ecosystem health of 1 mg L–1 dissolved inorganic nitrogen (DIN) and 0.018 mg L–1 dissolved reactive phosphorus (DRP), but has now delayed considering them. In examining whether these bottom lines could...

Phosphorus (P) can leach from topsoil in inorganic and organic forms. While some evidence has shown inorganic P (orthophosphate) can leach to depth in some soils, less is known of dissolved organic P (DOP). This is not helped by a paucity DOP data for groundwater. We hypothesized that DOP species would leach in greater amounts to depth and at a fas...

Rhizosphere processes play a critical role in phosphorus (P) acquisition by plants and microbes, especially under P-limited conditions. Here, we investigated the impacts of nutrient addition and plant species on plant growth, rhizosphere processes, and soil P dynamics. In a glasshouse experiment, blue lupin (Lupinus angustifolius), white clover (Tr...

The extent of phosphorus (P) loss from soils under deep rooting crops such as lucerne is currently unknown. This study used large lysimeters (2.0 by 1.5 m) to quantify the amounts and forms of P in drainage under lucerne from two sites, a non-irrigated dairy system and an irrigated dairy system that also received farm dairy effluent (FDE). Results...

Cadmium (Cd) can accumulate in soil from the application of phosphorus fertilizer. However, there is little information on what happens to soil Cd concentrations when Cd inputs stop. This study used soil and pasture samples collected from a long‐term field trial to measure changes in Cd concentrations in soil for 22 yr after Cd inputs from fertiliz...

Reliable data on the rate of cadmium (Cd) accumulation in soils that receive phosphorus (P) fertiliser is vital to help manage the impact of Cd in agricultural systems in New Zealand. Studies of the long-term P fertiliser trial at Winchmore, where soils have received 0, 17, or 34 kg P ha⁻¹ yr⁻¹ for > 60 years, have reported that soil Cd concentrati...

Purpose High baseflow phosphorus (P) concentrations increase the likelihood of periphyton blooms. Several physical and chemical factors can control baseflow P concentrations such as hydraulic exchange with groundwater, particle size-sorting, redox chemistry and different sediment sources. We hypothesized that of these sources, anoxic sediments woul...

Phosphorus (P) stores in gravel-bed rivers are released for uptake by periphyton when pH levels exceed 8.5. The Tukituki River has low alkalinity water and frequently experiences periphyton blooms, and daytime pH > 9 during summer low-flows. We measured dissolved reactive P (DRP) and EPC0, the water concentration of DRP at which no net release or s...

Phosphorus (P) losses via surface runoff can be large. Soil water repellency (SWR) occurs in warm and dry conditions. If SWR coincides with P fertiliser applications, the risk of P loss by surface runoff may increase. Losses during hotter months can be particularly detrimental to surface water quality because warmer waters help algal growth. We hyp...

Periphyton (viz. algal) growth in many freshwater systems is associated with severe eutrophication that can impair productive and recreational use of water by billions of people. However, there has been limited analysis of periphyton growth at a global level. to predict where nutrient over-enrichment and undesirable periphyton growth occurs, we com...

The build-up of soil phosphorus (P) beyond plant requirements can lead to a long-term legacy of P losses that could impair surface water quality. Using a database of ∼4,50,000 samples collected from 2001–2015 we report the level of soil P enrichment by soil type, land use and region and the time it would take for Olsen P to decline to agronomic tar...

Abstract. Streams can attenuate inputs of phosphorus (P) and, therefore the likelihood of ecosystem eutrophication. This attenuation is, however, poorly understood, particularly in reference to the geochemical mechanisms involved. In our study, we measured P attenuation mechanisms in the form of (1) mineral (co-)precipitation from the water-column...

Since its discovery in 1669, phosphorus (P) in the form of fertilizer has become an essential input for many agroecosystems. By introducing a concentrated P source, fertilizers increase short‐term P export potential soon after their application and longer‐term export potential by increasing soil fertility (legacy P). The 4R concept was developed to...

The evolution of phosphorus (P) management decision support tools (DSTs) and systems (DSS), in support of food and environmental security has been most strongly affected in developed regions by national strategies (i) to optimize levels of plant available P in agricultural soils, and (ii) to mitigate P runoff to water bodies. In the United States,...

Quantifying environmental changes relative to ecosystem reference conditions (baseline or natural states) can inform assessment of anthropogenic impacts and the development of restoration objectives and targets. We developed statistical models to predict current and reference concentrations of total nitrogen (TN) and total phosphorus (TP) in surfac...

Unravelling sub-catchment scale nitrogen delivery to waterways  In large and heterogeneous catchments (100s-1000s of km 2), it is very difficult to link a nitrogen flux observed at the catchment outlet to the many past and present activities that collectively are responsible for it.  We therefore aim to establish cause-effect relationships at the...

Land suitability assessment can inform decisions on land uses suitable for maximizing crop yield while making best use, but not impairing the ability of natural resources such as soil to support growth. We assessed the suitability of maize to be produce in 12,000 ha land of Dasht-e-Moghan region of Ardabil province, northwest of Iran. Suitability c...