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 wallowing by farmed red deer was fenced off and planted in native trees, shrubs, and grasses. Contaminant (nitrogen and phosphorus species, sediment, and the fecal indicator bacteria Escherichia coli) concentrations decreased after fencing until at least 2013 (55–84%). However, nitrate concentrations more than doubled owing to the use of a portion of the catchment for a grazed winter forage crop in 2013. Most concentrations remained low after fencing, but sediment concentrations slowing increased (3% yr−1), which was attributed to animal tracking near the fence line. The presence of stock in the catchment enriched stormflow losses but prevented losses during baseflow. These data indicate that, when implemented well, fencing and planting could potentially sustain good water quality for decades in farmed red deer systems.
Core Ideas
Fencing decreased median contaminant concentrations by 55‐84%.
Decreases were sustained for at least 15 yr after fencing.
A small increase in sediment loss (3% yr−1) was noted due to animal tracking.
When implemented well, fencing could sustain good water quality for decades.
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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 a long-term trial that had applied 22.5 kg P ha−1 yr−1 for 67 years to grazed pastures that received either no irrigation or were irrigated when soil moisture fell below 10 or 20%. The annual yield of Cd leaching from the top 150 mm of soil to the 151–250 mm layer was between 1.1 and 1.8 g ha−1 with Cd leaching increasing with the frequency of irrigation. The rate of Cd accumulation measured to 2000 mm was within the mean and standard error estimated for treatments using a mass balance approach. Estimates of annual Cd leaching loss were like those established from field trials measuring leaching events over a year (0.3–1.8 g ha−1) with a similar rate of P application (9–24 kg P ha−1 yr−1). Using a Cd leaching rate of 1.8 g ha−1 yr−1 and P applications rates of 22.5 kg P ha−1, topsoil Cd concentrations may stop increasing if Cd concentrations in P fertiliser can be maintained at < 72 mg Cd kg−1 P.
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•To decrease topsoil cadmium (Cd) concentrations we need to make inputs < outputs.•Phosphorus fertiliser is the main input of Cd and leaching the main loss of Cd.•Cd leaching losses were between 1.1 and 1.8 g ha−1 over 67 years of increasing irrigation.•Topsoil Cd concentration may plateau if P fertiliser is applied <22.5 kg ha−1 yr−1 and contains <72 mg Cd kg−1 P.
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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 Zealand (Ballantrae, Winchmore and Whatawhata) where P fertiliser had been applied (from 10 to 100 kg P ha−1 yr−1) for up to six years and then stopped for 10 to 26 years. Stopping P fertiliser applications reduced soil Cd concentrations at Winchmore and Whatawhata where P had been applied at ≥34 kg P ha−1 yr−1. No reductions occurred below this rate nor at Ballantrae where only 10 years post P-application data were available. Decreases were ascribed to moderate rainfall (1630 mm at Whatawhata and 740 mm rainfall plus 770 mm irrigation at Winchmore) that enhanced Cd leaching and may have been aided at Winchmore by a decrease in soil pH over time (0.4 units). However, because stopping P fertiliser inputs may quickly impair pasture production, additional strategies may be required to decrease soil Cd quickly.
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•Stopping phosphorus (P) fertiliser can decrease topsoil cadmium (Cd) concentrations.•Stopping P fertiliser applications reduced soil Cd concentrations after 21 and 26 years.•Reductions only occurred where P had been previously applied at ≥34 kg P ha−1 yr−1.•Other strategies may be required to decrease soil Cd quickly as stopping P fertiliser may lower pasture production.
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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 faster rate through aquifer gravels than orthophosphate. We applied superphosphate with or without dung to a low P-sorption soil under pasture and irrigation. Between 0.7 (control) and 2.4 (dung +superphosphate) kg P ha−1 was leached through 30 cm with a mean ratio of DRP to DOP of 1.5. At 50 cm, 0.7 and 1.3 kg P ha−1 was leached with the DRP to DOP ratio decreasing to 1.1 due to greater DOP leaching (or DRP sorption). There was little difference in DRP losses measured at 50 and 150 cm depth. All DOP compounds except the monoester – inositol hexakisphosphate were leached at a faster rate than orthophosphate through aquifer gravels. These data suggest that where low P-sorption soils overlay similarly low P-sorption aquifers, DOP may reach groundwater at a faster rate than orthophosphate. Furthermore, as many DOP species are bioavailable to periphyton, our data suggest that DOP should be included in the assessment of the risk of P contamination of groundwater where connection to baseflow could be a long-term stimulant of periphyton growth.
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•Very few data are available on the leaching of dissolved organic phosphorus (DOP) from soil.•Losses of DOP increased relative to dissolved reactive P (DRP) with depth.•The load of P leached past 50 and 150 cm were similar.•Most DOP species were leached at a faster rate than orthophosphate through aquifer gravels.•DOP should be included as part of groundwater monitoring programmes.
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Stream sediments can act as a source or a sink of dissolved (filtered) phosphorus (P) via abiotic and biotic processes. The cumulative action and magnitude of abiotic processes has been quantified by ...the equilibrium P concentration at zero net sorption or desorption (EPC0). The EPC0 was determined in 76 large rivers of contrasting climate, topography, and geology across New Zealand. Measurements of EPC0 (0.004–0.065 mg L−1) indicated sediments were acting as a source of filtered reactive P (FRP) to the water column. The EPC0 was related to the proportion of intensive agriculture in the catchment, the concentration of readily available P in the sediment, sediment size, and catchment slope and elevation. Determination of EPC0 will yield a relative assessment of the sediment's ability to supply P to the water column especially at baseflow. Furthermore, the EPC0 may be less prone to short‐term variation (e.g., diurnal patterns) compared with grab samples. This information will help target efforts to mitigate FRP concentrations in rivers by managing sediment inputs. Additional work is required to determine, for instance, how long an EPC0 measurement remains valid before new sediment is deposited or existing sediment is scoured.
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Nitrate‑nitrogen (NO3-N) is a contaminant of concern in groundwater worldwide. Stakeholders need information on the ability to detect changes in NO3-N concentrations to prove that land management ...practices are meeting water quality aims. We created a database of quarterly to monthly NO3-N measurements in 948 sites across New Zealand; 186 of those sites had mean residence time (MRT) data. New Zealand has set a target of sufficient land use mitigations in the next 30 years to ensure steady state surface water concentrations do not exceed 2.4 mg L−1. Here we assess whether the current monitoring network could identify the impacts of these mitigations, assuming that the mitigations are successfully implemented at the source. Only 41 % of the network could detect statistically significant reductions with the current standard quarterly sampling after 30 years of monitoring. The percentage of sites increased to 60 % with increased monitoring frequency (often weekly) but this required a 100–300 % increase in monitoring costs. However, policy makers and stakeholders typically require information on policy and mitigation effectiveness within 5–10 years. Detection within 5–10 years was very unlikely (0–20 % of sites) regardless of the sampling frequency. Importantly, these analyses include the impacts of groundwater lag and temporal dispersion on the likelihood of detecting change, ignoring these impacts, incorrectly, yields a much higher likelihood of detecting reductions. We conclude that the current monitoring network is unlikely to be fit for the purpose of detecting NO3-N reductions within practical timeframes or budgets. Furthermore, we conclude that lag and temporal dispersion effects must be included in detection power calculations; we therefore recommend that MRT data is regularly collected. We also provide a python package to enable easy detection power calculations with lag and temporal dispersion impacts, thereby supporting the development of robust change-detection monitoring networks.
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•Detecting NO3-N reductions likely requires bespoke groundwater monitoring networks.•Excluding groundwater lag overestimates detection power.•40 % of New Zealand's network can detect policy-relevant changes after 30 years.•A 1–3-fold increase in funding allows detection in a further 20 % of NZ's network.
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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
= 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.
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 contaminant (phosphorus and nitrogen species, E. coli and visual clarity) concentrations to threshold levels in 5 or 20 years. While > 95% of all monitored sites had sufficient power and samples to detect change in nutrients and clarity over 20 years, on average, sampling frequency would have to double to detect changes in E. coli. Furthermore, to detect changes in 5 years, sampling for clarity, dissolved reactive phosphorus and E. coli would have to increase up to fivefold. The cost of sampling was predicted to increase 5.3 and 4.1 times for 5 and 20 years, respectively. A national model of statistical power was used to demonstrate that a similar number of samples (and cost) would be required for any new monitoring sites. Our work suggests that demonstrating the outcomes of implementing policy for water quality improvement may not occur without a step change in investment into monitoring systems. Emerging sampling technologies have potential to reduce the cost, but existing monitoring networks may also have to be rationalised to provide evidence that water quality is meeting objectives. Our study has important implications for investment decisions involving balancing the need for intensively sampled sites where changes in water quality occur rapidly versus other sites which provide long-term time series.
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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 were studied from 2001 to 2020. The first period, from 2001 to 2010, involved comprehensive “extension” advice to farmers consisting of workshops, stream water quality and flow monitoring, farm practice surveys, and identified solutions to address site-specific contaminant losses. In the second period (2011−2020), termed “post-extension”, only water quality monitoring and farm practice surveys were continued. Of the water quality contaminants (including dissolved reactive phosphorus (DRP), total phosphorus (TP), ammoniacal-nitrogen, nitrate-nitrite-nitrogen NNN, suspended sediment and E. coli), 83 % of water quality trend directions were either improving (n = 16) or showed no change (n = 9) during the extension period. Over the 20-year dataset, which included the post-extension period, 20 out of 30 contaminant-catchment combinations (67 %) were improving, but nine were degrading, dominated by NNN (n = 4), DRP (n = 2) and E. coli (n = 2). Abrupt decreases in contaminant concentrations, were correlated with on-farm practice changes mainly associated with transition from direct discharge of farm dairy shed effluent to waterways to land application, and the capture of effluent from off-paddock facilities (like stand off or feed pads). Best management practices reduced phosphorus (P) forms, E. coli and sediment concentrations. Increase in NNN concentrations was caused by transitioning from flood to spray irrigation and a commensurate increase in cow numbers and NNN leaching. These data indicate that extension advice and on-farm practice change have helped to improve overall water quality over time. Nevertheless, recent regulatory threshold values for some contaminant concentrations are not being met, meaning that more actions are required, over and above the BMPs implemented.
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•Water quality and BMP implementation was studied with (2001-2010) and without (2011-2020) extension.•During extension, 83% of water quality trends were improving or showed no change.•Over the 20-year period, including post-extension, 70 % of trends were improving or showed no change.•Abrupt decreases in contamination correlated with changes like shifting to land application of dairy shed effluent.•While extension advice helped to improve water quality, additional BMPs may be needed to meet recent regulatory thresholds.
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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 combined several databases to model and map global dissolved and total nitrogen (N) and phosphorus (P) concentrations, climatic and catchment characteristics for up to 1406 larger rivers that were analysed between 1990 and 2016. We predict that 31% of the global landmass contained catchments may exhibit undesirable levels of periphyton growth. Almost three-quarters (76%) of undesirable periphyton growth was caused by P-enrichment and mapped to catchments dominated by agricultural land in North and South America and Europe containing 1.7B people. In contrast, undesirable periphyton growth due to N-enrichment was mapped to parts of North Africa and parts of the Middle East and India affecting 280 M people. The findings of this global modelling approach can be used by landowners and policy makers to better target investment and actions at finer spatial scales to remediate poor water quality owing to periphyton growth.
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