James A. Hanly

Woodchips bioreactors are a proven end-of-drain treatment technology widely used to decrease the quantity of nitrate (NO3-) in agricultural drainage water. However, their application in municipal wastewater land treatment systems remains relatively underexplored. These systems often require large areas, but factors such as limited land availability, challenges in effluent application, high costs and land use constraints can result in undersized treatment sites, ultimately compromising treatment performance. Consequently, elevated NO3- concentrations are transferred to the drainage water of these sites. This study examines the land treatment system of a New Zealand town (Levin) with a population of approximately 20,000, which exemplifies the issue of high wastewater loading resulting in elevated NO3- levels in surface drains and contributing to NO3- enrichment in the Waiwiri Stream. Specifically, this study evaluated the ability of woodchip bioreactors to decrease NO3- concentrations in drainage water from a wastewater land treatment site, including the impact of soluble carbon (C) dosing. Pilot-scale bioreactors (5 m3) were evaluated during warm and cool seasons, and under varying hydraulic retention times (HRTs) and ethanol C:N ratios. Season, HRT and C dosing all had marked effects on NO3- removal efficiencies. At a 10-hour HRT the average bioreactor NO3- removal efficiency was 43% in the cool season and 59% in the warm season, which increased to 69 and 85%, respectively, at a 20-hour HRT. At 6.6-hour HRT operating under cool conditions, ethanol dosing at a C:N ratio of 0.75:1 achieved a NO3- removal efficiency of 93%, which was significantly higher that the NO3- removal efficiency of 24% achieved without ethanol dosing. These findings highlight that woodchip bioreactors can be an effective tool for mitigating NO3- in drainage water at municipal wastewater land treatment sites. Additionally, ethanol dosing further enhances performance, enabling shorter HRTs and treatment of greater daily volumes of drainage water, particularly during cooler periods, improving treatment capacity and efficiency under seasonal challenges.

Regenerative agricultural practices emphasize the use of diverse pasture species within sustainable agriculture production systems. The inclusion of a range of legume species in diverse pasture swards is likely to increase biological N fixation (BNF) across seasons, reducing the system’s reliance on synthetic N inputs. The present field study aims to quantify BNF in selected legume species within diverse pasture (combining 9 species) and standard pastures (ryegrass and clover combination) and assess their performance to identify the potential for improving N supply while maintaining year-round pasture quality. A year-round seasonal BNF was assessed by evaluating soil N status, nodulation patterns, plant composition, and conducting ¹⁵N natural abundance studies. The results revealed that the diverse pasture sward produced 5.4% more dry matter compared to the standard pasture, while soil mineral N (NO₃⁻, NH₄⁺) remained statistically similar between the two treatments. Nitrogen yield was 9.3% higher in the diverse pasture than in the standard pasture. ¹⁵N natural abundance analysis assessment revealed no substantial variation in BNF rates across treatments throughout the study. However, in contrast to standard pasture, the BNF rate in diverse pasture experienced a 3-fold increase from winter to summer, while the standard pasture exhibited a 1.5-fold increase. In both pasture systems, BNF increased with clover proportion up to 30%, indicating optimal fixation at moderate clover levels. The findings underscore the potential of diverse pastures when strategically managed to enhance seasonal BNF while sustaining pasture productivity.

This article reviews the literature on nitrate leaching under sheep grazing systems and focuses on identifying future research needs. Urinary nitrogen (N) is an important source of the nitrate leached from pastoral agriculture. Urinary N excretion can be measured or simulated using models and has been well characterised for dairy systems. It is difficult to continuously monitor the urinary N excretion of sheep under field conditions; consequently, measurements of N excretion in sheep urine are limited. Urination events by sheep vary greatly in volume (0.5 L to 6.9 L), concentration (3 to 13.7 g N/L), and frequency (8 to 23 events/day); this variation results in a corresponding variation in N loading rates in urine patches. The amount of nitrate leached under pastures grazed by sheep has typically varied between 1 and 50 kg N/ha/year, but rates as high as 300 kg N/ha/year have been reported. The quantity of nitrate leached under sheep depends on the season, climate, quantity and timing of drainage, the interaction between forage production and stocking rate, fertiliser applied, N fixation by legumes, forage type, and grazing management. The majority of studies examining nitrate leaching under sheep grazing systems are more than 20 years old; so, there is little recent information on nitrate leaching under modern pasture-based sheep production systems. Further research is required to quantify nitrate leaching levels under current sheep farming practices, to understand the impacts of this leaching on water quality, and to help identify effective strategies to reduce the transfer of N from grazed paddocks to receiving water bodies. This additional information will help provide information for decision support tools, including models and management practices, to help sheep farmers minimise their impact on the aquatic environment.

The use of plantain pasture in dairy systems can potentially reduce nitrogen (N) leaching losses via the lower N concentration in the urine (UNc) of cows. Reducing the urinary N load while cows graze pastures can reduce the risk of N leaching losses from urine patches. Research needs to demonstrate that these environmental benefits are not at the expense of milk production and farm profit. Three groups of 20 cows grazed in the following three pasture treatments: (i) plantain, (ii) plantain–clover mix (plantain, red [Trifolium pratense] and white clover), or (iii) ryegrass-white clover (wc) pastures, from spring to autumn for two years. Each year, pasture intake, diet quality, milk production and animal N (milk and urine) excretion were evaluated in spring, summer, and autumn. The cows grazing the plantain and plantain–clover mix pastures produced similar milk solids as cows grazing ryegrass–wc pasture but reduced their UNc during summer and autumn, when compared to those grazing the plantain–clover mix and ryegrass–wc pastures. Plantain reduced urinary N loads onto pastures by a greater number of urine patches with lower urinary N loading rates. The results demonstrate that plantain pastures do not diminish milk solids production from cows, and the lower UNc from summer to autumn could reduce N being lost to the environment.

Domestic sewage treatment plants ( STP s) that discharge partially treated wastewaters to surface waters contribute to eutrophication. A number of Allophanic soils have shown promise as a filter substrate to remove dissolved reactive phosphorus ( DRP ) from wastewaters. Once the phosphorus (P) sorption capacity of the soil is exhausted, it can potentially be applied to land as a soil amendment to recycle P for productive use. In this study, Allophanic soil that was P‐enriched by wastewater treatment ( WTS ) was evaluated for its agronomic effectiveness in a glasshouse pot experiment in which ryegrass ( Lolium multiflorum ) was grown on Ohakune silt loam. The ability of WTS to supply P for ryegrass growth was compared with a soluble phosphorus source (mono‐calcium phosphate, MCP ). The WTS had a total P ( TP ) concentration of 8128 mg TP/kg oven‐dried soil. As the initial P status of the Ohakune soil was very low, the cumulative dry matter yields increased significantly with increased additions of WTS and MCP . The WTS was highly effective at increasing available P in the soil, as measured by Olsen P and ryegrass yield and P uptake. The soluble fertiliser P value of WTS was estimated to be equivalent to 61% of MCP applied at the same rate. Therefore, the results show that WTS is an effective P source for plant growth and its application to soil has the potential to recycle both the soil and the P it contains.

Research was conducted in the Manawatu region, New Zealand, to investigate the ability of Papakai tephra to remove phosphorus (P) from dairy farm mole and pipe drainage waters. The capacity of this tephra to adsorb P was quantified in the laboratory using a series of column experiments and was further evaluated in a field study. In a column experiment, the P adsorption capabilities of 2 particle size factions (0.25–1, 1–2 mm) of Papakai tephra were compared with that of an Allophanic Soil (Patua soil) known to have high P adsorption properties. The experiment used a synthetic P influent solution (12 mg P/L) and a solution residence time in the columns of c. 35 min. By the end of the experiment, the 0.25–1 mm tephra removed an estimated 2.6 mg P/g tephra at an average P removal efficiency of 86%. The 1–2 mm tephra removed 1.6 mg P/g tephra at an average removal efficiency of 58%. In comparison, the Patua soil removed 3.1 mg P/g soil at a P removal efficiency of 86%. Although, the Patua soil was sieved to 1–2 mm, this size range consisted of aggregates of finer particles, which is likely to have contributed to this material having a higher P adsorbing capacity. A field study was established on a Pallic Soil, under grazed dairy pastures, to compare drainage water P concentrations from standard mole and pipe drainage systems (control) and drainage systems incorporating Papakai tephra. The 2 tephra treatments involved filling mole channels with 1–4 mm tephra (Mole-fill treatment) or filling the trench above intercepting drainage pipes with ‘as received’ tephra (Back-fill treatment). Over an entire winter drainage season, the quantity of total P (TP) lost from the control treatment drainage system was 0.30 kg P/ha. The average TP losses for the Mole-fill and the Back-fill treatments were 45% and 47% lower than the control treatment, respectively.

Low pasture magnesium (Mg) intakes by grazing dairy and beef cows is a main cause of hypomagnesaemia in these animals soon after calving. Magnesium fertiliser application to pasture can increase pasture mixed‐herbage (hereafter “herbage”) Mg concentration and reduce the risk of this disorder. However, the effectiveness of Mg fertilisers is variable depending on fertiliser type and soil properties. A 32‐month Mg fertiliser field trial was conducted on dairy pasture on an Immature Orthic Pumice Soil (near Taupo, New Zealand; exchangeable Mg 0.4 cmol _c kg ^–1 , pasture Mg 0.13%) to compare the effectiveness of a range of Mg fertilisers in elevating herbage Mg concentration. The fertilisers used were ground serpentine rock, three acidulated serpentine rock products, Epsom salts, and E‐mag (magnesium oxide). All fertilisers were applied at 100 kg Mg ha ^–1 at the start of the trial. The trial also had two treatments where Epsom salts was applied at 25 and 50 kg Mg ha ^–1 and a treatment where serpentine rock was reapplied during the second and third year at 100 kg Mg ha ^–1 . Magnesium fertiliser treatments did not affect pasture dry matter yield significantly at any harvest, however, the effect of treatments on herbage Mg concentration was highly significant ( P < 0.001) at all harvests. The single application of serpentine significantly increased herbage Mg concentration at 10 harvests, with five of these harvests having Mg concentrations ≥ 0.20%, which is generally considered to be the minimum “threshold level” for stock health. In comparison, the Mg concentration in pasture not receiving Mg fertiliser remained below the minimum threshold level throughout the trial. Application of serpentine rock annually further increased herbage Mg concentration and accumulation. The herbage Mg concentrations for this treatment were similar to those of the Epsom salts (50 kg Mg ha ^–1 ) treatment at the majority of the harvests. These results indicate that the annual application of serpentine rock may have potential as an alternative strategy to the application of more soluble Mg sources for raising herbage Mg concentration to levels considered adequate to meet stock health requirements. The more soluble Mg fertilisers—Epsom salts, the two serpentine‐superphosphate products, and an acidulated serpentine rock plus reactive phosphate rock product—had the greatest effect on increasing herbage Mg concentration and accumulation. These treatments increased herbage Mg concentration to ≥0.20% within 3–6 months after application and for the majority of harvests over the rest of the trial period. The effect of E‐mag on herbage Mg concentration was similar to that of serpentine rock during the first year, but the subsequent performance of E‐mag was similar to the more soluble Mg fertilisers.

Two field experiments were conducted in the Gisborne region of New Zealand to assess the effectiveness of four winter green‐manure crops (lupin, mustard/lupin mix, mustard, and annual ryegrass) for improving the short‐term nitrogen (N) availability of soils growing organic sweetcorn (Zea mays). Average soil mineral (0–15 cm) N in the control treatment (bare soil winter fallow) plots, measured at the time of sweetcorn emergence (late November 1997), was 50.4 kg N ha ^‐1 at Site A and 81.3 kg N ha ^–1 at Site B. Compared with the control (bare soil fallow) treatment, soil incorporation of the lupin and mustard/lupin mix treatments significantly increased soil mineral N by 30–45% at both trial sites at sweetcorn emergence. In contrast, the ryegrass treatment reduced soil mineral N levels by 33–43% at both sites. These treatment effects were related to green‐manure crop N concentrations just before soil incorporation. Sweetcorn N accumulation at final harvest was also significantly increased by soil incorporation of the lupin and mustard/lupin mix treatments and significantly reduced by the ryegrass treatment. Average sweetcorn ear yields in the control treatment plots at maturity were 16t ha ^–1 at Site A and 18 t ha ^–1 at Site B. However, ryegrass treatment significantly reduced sweetcorn ear yields by 64% at Site A and 48% at Site B, which was likely to have been caused by the lower soil mineral N levels attributed to soil incorporation of the ryegrass treatment. Although the lupin and mustard/lupin mix treatments increased soil N availability and uptake by sweetcorn, these treatments did not significantly improve sweetcorn ear yield. The lack of yield response was attributed to soil moisture limitations, which occurred in the latter part of the season and were likely to have restricted yield potential.