Warwick J. Dougherty

Farm-scale nitrogen, phosphorus, potassium and sulfur balances and use efficiencies on Australian dairy farms

Efficient and effective nutrient management decisions are critical to profitable and sustainable milk production on modern Australian dairy farms. Whole-farm nutrient balances are commonly used as nutrient management tools and also for regulatory assessment on dairy farms internationally, but are rarely used in Australia. In this study, nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) imports and exports were measured during a standardised production year on 41 contrasting Australian dairy farms, representing a broad range of geographic locations, milk production, herd and farm size, reliance on irrigation, and soil types. The quantity of nutrients imported varied markedly – with feed and fertiliser generally the most substantial imports – and were principally determined by stocking rate and type of imported feed. Milk exports were the largest source of nutrient exports. Nitrogen balance ranged from 47 to 601 kg N/ha.year. Nitrogen-use efficiency ranged from 14 to 50%, with a median value of 26%. Phosphorus balance ranged from –7 to 133 kg P/ha.year, with a median value of 28 kg P/ha. Phosphorus-use efficiencies ranged from 6 to 158%, with a median value of 35%. Potassium balances ranged from 13 to 452 kg K/ha, with a median value of 74 kg K/ha; K-use efficiency ranged from 9 to 48%, with a median value of 20%. Sulfur balances ranged from –1 to 184 kg S/ha, with a median value of 27 kg S/ha; S-use efficiency ranged from 6 to 110%, with a median value of 21%. Nitrogen, P, K and S balances were all positively correlated (P < 0.001) with stocking rate and milk production per ha. Poor relationship between P, K and S fertiliser inputs and milk production from home-grown pasture reflected the already high soil fertility levels measured on many of these farms. The results from this study demonstrate that increasing milk production per ha will be associated with greater nutrient surpluses at the farm scale, with the potential for greater environmental impacts. We suggest that simplified and standardised nutrient balance methodologies should be used on dairy farms in Australia to help identify opportunities for improvements in nutrient management decisions and to develop appropriate industry benchmarks and targets.

A quantitative assessment of phosphorus forms in some Australian soils

Solution 31P nuclear magnetic resonance (NMR) spectroscopy is the most common technique for the detailed characterisation of soil organic P, but is yet to be applied widely to Australian soils. We investigated the composition of soil P in 18 diverse Australian soils using this technique. Soils were treated with a mixture of sodium hydroxide–ethylenediaminetetra-acetic acid (NaOH-EDTA), which resulted in the extraction of up to 89% of total soil P. It was possible to identify up to 15 well-resolved resonances and one broad signal in each 31P NMR spectrum. The well-resolved resonances included those of orthophosphate, α- and β-glycerophosphate, phytate, adenosine-5′-monosphosphate, and scyllo-inositol phosphate, as well as five unassigned resonances in the monoester region and two unassigned resonances downfield (higher ppm values) of orthophosphate. The majority of 31P NMR signal in the NaOH-EDTA extracts was assigned to orthophosphate, representing 37–90% of extractable P. Orthophosphate monoesters comprised the next largest pool of extractable P (7–55%). The most prominent resonances were due to phytate, which comprised up to 9% of total NaOH-EDTA extractable P, and α- and β-glycerophosphate, which comprised 1–5% of total NaOH-EDTA extractable P. A substantially greater portion of organic P (2–39% of total NaOH-EDTA extractable P) appeared as a broad peak in the monoester P region; we propose that this is due to P found in large, ‘humic’ molecules. Orthophosphate diesters (1–5% of total NaOH-EDTA extractable P) and pyrophosphate (1–5% of total NaOH-EDTA extractable P) were minor components of P in all soil extracts. These results suggest that organic P in large humic molecules represents the second most abundant form of NaOH-EDTA extractable soil P (behind orthophosphate). Furthermore, small P-containing compounds, such as phytate, represent a much smaller proportion of soil P than is commonly assumed.

Phosphorus Fertilizer and Grazing Management Effects on Phosphorus in Runoff from Dairy Pastures

Fertilizer phosphorus (P) and grazing-related factors can influence runoff P concentrations from grazed pastures. To investigate these effects, we monitored the concentrations of P in surface runoff from grazed dairy pasture plots (50 x 25 m) treated with four fertilizer P rates (0, 20, 40, and 80 kg ha(-1) yr(-1)) for 3.5 yr at Camden, New South Wales. Total P concentrations in runoff were high (0.86-11.13 mg L(-1)) even from the control plot (average 1.94 mg L(-1)). Phosphorus fertilizer significantly (P < 0.001) increased runoff P concentrations (average runoff P concentrations from the P(20), P(40), and P(80) treatments were 2.78, 3.32, and 5.57 mg L(-1), respectively). However, the magnitude of the effect of P fertilizer varied between runoff events (P < 0.01). Further analysis revealed the combined effects on runoff P concentration of P rate, P rate x number of applications (P < 0.001), P rate x time since fertilizer (P < 0.001), dung P (P < 0.001), time since grazing (P < 0.05), and pasture biomass (P < 0.001). A conceptual model of the sources of P in runoff comprising three components is proposed to explain the mobilization of P in runoff and to identify strategies to reduce runoff P concentrations. Our data suggest that the principal strategy for minimizing runoff P concentrations from grazed dairy pastures should be the maintenance of soil P at or near the agronomic optimum by the use of appropriate rates of P fertilizer.

Stratification, forms, and mobility of phosphorus in the topsoil of a Chromosol used for dairying

The forms and stratification of soil phosphorus (P) and their relationship to mobile forms of P were investigated in soils collected from a subcatchment used for grazing of dairy cattle in the Adelaide Hills, South Australia. Phosphorus in the soils was highly stratified. The concentration of calcium chloride extractable P in the 0–0.01 m increment was, on average, 5.7 times greater than in the 0.05–0.10 m increment. Organic P (% of total P) in the top 0.01 m was significantly (P 50%) of dissolved unreactive P (DUP), whereas runoff from high P soils (low Po) had low proportions of DUP (<10%). Ultrafiltration of runoff samples revealed that 94 and 65% of the dissolved reactive P and DUP, respectively, was subcolloidal (i.e. <1 nm). These results highlight the relationship between soil fertility, the forms of soil P, and the concentrations and forms of P mobilised in runoff. Such relationships need to be considered in further studies of P mobilisation and the subsequent development of strategies designed to reduce runoff P concentrations.

Relationship between phosphorus concentration in surface runoff and a novel soil phosphorus test procedure (DGT) under simulated rainfall

There is a need to be able to identify soils with the potential to generate high concentrations of phosphorus (P) in runoff, and a need to predict these concentrations for modelling and risk-assessment purposes. Attempts to use agronomic soil tests such as Colwell P for such purposes have met with limited success. In this research, we examined the relationships between a novel soil P test (diffuse gradients in thin films, DGT), Colwell P, P buffering index (PBI), and runoff P concentrations. Soils were collected from six sites with a diverse range of soil P buffering properties, incubated for 9 months with a wide range of P additions, and then subjected to rainfall simulation in repacked trays growing pasture. For all soil and P treatment combinations, the relationship between DGT (0–10 mm) and runoff P was highly significant (P < 0.001, r2 = 0.84). Although there were significant curvilinear relationships between Colwell P and runoff P for individual soils, there were large differences in these relationships between soils. However, the inclusion of a P buffering measure (PBI) as an explanatory variable resulted in a highly significant model (P < 0.001, R2 = 0.82) that explained between-soil variability. We conclude that either DGT, or Colwell P and PBI, can be used to provide a relative measure of runoff P concentration.

Effect of variable soil phosphorus on phosphorus concentrations in simulated surface runoff under intensive dairy pastures

Intensive dairy operations in Australia regularly apply P fertiliser to maintain productive pasture species. However, extractable soil test P (STP) concentrations in this industry commonly exceed those required to maximise pasture production, a situation which can increase the risk of P loss to surrounding waterways. The current study examined relationships between STP (Olsen P and CaCl2 P) and surface runoff P concentrations from a red silty loam (Ferrosol), commonly used for pasture production in south-eastern Australia. Soil was mixed and re-packed into soil trays and a rainfall simulator was used to generate surface runoff. A wide range of soil Olsen P concentrations (0–20 mm, 15–724 mg/kg; 0–100 mm, 9–166 mg/kg) was created by surface-applying a range of P fertiliser rates 8 months before the rainfall simulations. A comparison of the 2 STP methods suggests that Australian soils have higher labile P concentrations for given Olsen P concentrations compared with those measured internationally, suggesting a greater likelihood of P loss in runoff. Furthermore, significant curvilinear relationships between STP and dissolved reactive P (DRP <0.45 µm) in surface runoff for both Olsen P depths (0–20 mm, r2 = 0.94; 0–100 mm, r2 = 0.91; P < 0.01) were determined, as well as significant linear relationships between DRP and both CaCl2 depths (0–20 mm, r2 = 0.83; 0–100 mm, r2 = 0.92; P < 0.01). This confirmed that the concentrations of P in surface runoff increased with increasing STP, providing further evidence of an urgent need to reduce excessive STP concentrations, to reduce the risk of P loss to the environment.

Small-scale, high-intensity rainfall simulation under-estimates natural runoff P concentrations from pastures on hill-slopes

Rainfall simulation is a widely used technique for studying the processes, and quantifying the mobilisation, of phosphorus (P) from soil/pasture systems into surface runoff. There are conflicting reports in the literature of the effects of rainfall simulation on runoff P concentrations and forms of P compared to those under natural rainfall runoff conditions. Furthermore, there is a dearth of information on how rainfall simulation studies relate to hill-slope and landscape scale processes and measures. In this study we compare P mobilisation by examining P forms and concentrations in runoff from small-scale, high-intensity (SH, 1.5 m2, 80 mm/h) rainfall simulation and large-scale, low-intensity (LL, 1250 m2, 8 mm/h) simulations that have previously been shown to approximate natural runoff on hill-slopes. We also examined the effect of soil P status on this comparison. The SH methodology resulted in lower (average 56%) runoff P concentrations than those measured under the LL methodology. The interaction method × soil P status was highly significant (P < 0.001). There was no significant effect of method (SH v. LL) and soil P status on P forms (%).The hydrological characteristics were very different between the 2 methods, runoff rates being c. 42 and 3 mm/h for the SH and LL methods, respectively. We hypothesise that the lower runoff P concentrations from the SH method are the result of a combination of (i) the P mobilisation being a rate-limited process, and (ii) the relatively high runoff rates and short runoff path-lengths of the SH method allowing for relatively incomplete attainment of equilibrium between P in the soil/pasture system and runoff. We conclude that small-scale, high-intensity rainfall simulation provides a useful tool for studying treatment effects and processes of mobilisation in pastures, but concentration and load data should not be inferred for natural conditions at larger scales without a clear understanding of the effects of the rainfall simulation methodology on the results for the system being studied.

Decrease in phosphorus concentrations when P fertiliser application is reduced or omitted from grazed pasture soils

Many intensively managed soils contain phosphorus (P) concentrations greater than required for optimum production. Soils with P concentrations in excess of the agronomic optimum can have unnecessary losses of P that can adversely affect water bodies. Reducing excessive soil-P concentrations is important for the economic and environmental sustainability of intensive agriculture, such as the Australian dairy industry. However, little is known of decreases in extractable soil-P concentrations when P fertiliser applications are reduced or omitted from soils with P concentrations and properties representative of intensive pasture grazing systems. Decreases in extractable P (calcium chloride (CaCl2), Olsen and Colwell) were monitored for up to 4.5 years for six Australian grazed pasture soils (Red Ferrosol, Brown Kurosol, Grey Dermosol, Brown Dermosol, Podosol and Hydrosol) with contrasting textures and P-buffering indices (PBI). Sixteen treatments consisting of four initial extractable-P concentrations (Pinit) paired with four ongoing P fertiliser rates (Pfert) were established for each of the six soils, except on an extremely low-PBI Podosol, where a range of Pinit concentrations could not be established. The resultant decreases in P were larger with higher Pinit concentration and lower rate of ongoing Pfert, except in the extremely low PBI Podosol where decreases in initially high CaCl2-P concentrations were large irrespective of ongoing Pfert. There was a greater proportional decrease in the environmentally extractable P compared with agronomically extractable P, with mean decreases in CaCl2-P of 57%, Olsen-P of 25%, and Colwell-P of 12%. The Pinit concentrations, which were well above agronomic optimum, remained above this target. This study advances scientific knowledge of extractable soil-P concentrations when P fertiliser inputs are withheld or reduced from grazed pasture soils, and aids land and catchment managers in estimating likely changes over time.

Nitrification (DMPP) and urease (NBPT) inhibitors had no effect on pasture yield, nitrous oxide emissions, or nitrate leaching under irrigation in a hot-dry climate

Modern dairy farming in Australia relies on substantial inputs of fertiliser nitrogen (N) to underpin economic production. However, N lost from dairy systems represents an opportunity cost and can pose several environmental risks. N-cycle inhibitors can be co-applied with N fertilisers to slow the conversion of urea to ammonium to reduce losses via volatilisation, and slow the conversion of ammonium to nitrate to minimise leaching of nitrate and gaseous losses via nitrification and denitrification. In a field campaign in a high input ryegrass–kikuyu pasture system we compared the soil N pools, losses and pasture production between (a) urea coated with the nitrification inhibitor 3,4-dimethyl pyrazole phosphate (b) urea coated with the urease inhibitor N-(n-butyl) thiophosphoric triamide and (c) standard urea. There was no treatment effect (P>0.05) on soil mineral N, pasture yield, nitrous oxide flux or leaching of nitrate compared to standard urea. We hypothesise that at our site, because gaseous losses were highly episodic (rainfall was erratic and displayed no seasonal rainfall nor soil wetting pattern) that there was a lack of coincidence of N application and conditions conducive to gaseous losses, thus the effectiveness of the inhibitor products was minimal and did not result in an increase in pasture yield. There remains a paucity of knowledge on N-cycle inhibitors in relation to their effective use in field system to increase N use efficiency. Further research is required to define under what field conditions inhibitor products are effective in order to be able to provide accurate advice to managers of N in production systems.

Stabilization of Molybdate Reactive Phosphorus in 0.5 M Sodium Bicarbonate Extracts of Soils

Molybdate-reactive phosphorus (MRP) in soil-free 0.5 M sodium bicarbonate (NaHCO3) extracts of 20 soils held at 22 °C increased 0–25% (median 11%) after 24 h and by 5–120% (median 43%) after 72 h. Addition of 2.5 mL L−1 of chloroform (1) or 0.25–1 g L−1 of thymol (2), phenyl mercury acetate (3), sodium cyanide (4), or sodium azide (5) showed that only (2)–(5) at 1 g L−1 stabilized the MRP for 72 h. Five of the soils were re-extracted with 0.5 M NaHCO3 containing 1 g L−1 of (2)–(5): all stabilized MRP for 72 h, and (2) increased MRP for three soils (P < 0.05). For 92 additional soils (0.5–200 mg MRP kg−1) extracted with 0.5 M NaHCO3 ± 1 g of sodium azide L−1, the azide had a negligible effect on MRP extraction, and in its presence, extracted MRP was unchanged after 72 h at 22 °C. We recommend this extractant for wider evaluation.