Agronomic Optimum Research Articles

Spatial distribution of available phosphorus in surface road and trackway surface materials on a sheep farm in Ireland

Farm roadway runoff is a high-risk source of pollution when connectivity with waters occurs. Nutrients in this runoff are dominated by fresh animal deposits, but recent dairy and beef farm studies showed that available phosphorus (P) accumulates in roadway surface material and can be lost in runoff. A current knowledge gap is to examine available P concentrations in unsealed roadway and trackway (non-maintained) network of a lowland sheep farm. The present study focused on a 45 ha farm stocked with 544 sheep in south-east Ireland. Ten locations were sampled along with the adjacent fields for available P (i.e., Morgan’s P) and ancillary parameters (e.g., pH, total P and heavy metals) in December 2022. The first sampling location was on an aggregate roadway and the other nine were on trackways representing an older aggregate roadway network used by the flock but now covered with soil and grass. Results showed a distinct difference in surface material pH between roadway and trackway locations. Trackways had a pH that mimicked adjacent fields around the agronomic optimum for grassland of ∼6.2. All sampling locations had elevated available P concentrations, ranging from 26.3 to 111.0 mg L−1 (mean 62.8 mg L−1), similar to the spatial distribution for dairy farms but above those found at beef farms previously studied. The highest available P concentrations were found in roadway and trackway sections adjacent to the farmyard. Other elevated sampling areas were on trackways (87.3 or 97.7 mg P L−1) away from the farmyard where sheep are funnelled to pasture, stop to seek shade, urinate and defecate but do not graze. By contrast the average available P concentration for the surrounding fields was 8.4 mg L−1 with a range of 2.7–15.9 mg L−1. Two sampling areas combine to create a critical source area where a high available P source becomes visibly mobilised as runoff during rainfall, discharges into an open drainage ditch, which is then connected to a local stream. Breaking the pathway before runoff enters the open ditch could be a cheap and effective way of mitigating nutrient losses at these two locations.

Soil phosphorus, hydrological risk and water quality carrying capacities in agricultural catchments

To support profitable agricultural production, nutrients, including phosphorus (P) are applied to soils. However, to avoid over-application and mobilisation of excess P, in-soil concentrations must be maintained at the agronomic optimum (crop requirement) through soil test P (STP) data. Areas above optimum STP (e.g., Olsen P) status have been linked to elevated instream soluble reactive P (SRP) concentrations. For example, when this status is combined with hydrologically sensitive areas (HSAs), excess P can be mobilised and transported directly to surface waters. Catchment carrying capacities for high STP are a possible management strategy to reduce these pressures. The aim of this study was to investigate the transferability of catchment carrying capacity approaches using primary and secondary datasets. Field by field STP status and LiDAR derived HSAs (2 m grid resolution) were compared with instream SRP concentrations using combinations of least squares regressions. The high range of STP catchment carrying capacities (15 % − 44 %, depending on the regression used) was influenced by the variation of instream SRP concentration thresholds (48 – 71 µg L-1) that are determined using altitude and alkalinity factors. However, a single SRP threshold of 35 µg L-1 reduced the catchment STP carrying capacity to a smaller range (10 % − 16 %), with a mean of 13 %. The analysis showed that instream particulate P concentrations were also related to above optimum STP but to a lesser degree and that all HSAs were vulnerable to P loss when soils were above optimum STP. Targeted management strategies should follow a “treatment-train” approach starting with reducing the catchment or farm area above agronomic optimum STP to a carrying capacity (proposed here as 13 %), followed by interception measures located at HSA breakthrough and delivery points to reduce both instream SRP concentration and load.

Weighted risk assessment of critical source areas for soil phosphorus losses through surface runoff mechanisms

In intensive livestock areas, soils commonly contain elevated nutrients above the agronomic optimum which increases the risk of nutrient losses and contributing to poor ecological status waterbodies. Large within-field variability in soil nutrient content exists, and at-risk phosphorus (P) hotspots are rarely quantified due to sub-optimal soil sampling regimes. This study aims to address this issue by developing and evaluating an improved classification of P transfer risk at a sub-field scale through a weighted risk assessment model that combines gridded soil sampling data with modelled in-field surface runoff pathways. Within-field soil P variability was quantified at six field-scale sites in Northern Ireland using two different sampling techniques; traditional bulked field soil sampling (i.e. bulk analysis of W pattern sampling) and gridded sampling (at 35 m resolution) alongside interpolation. Results show that traditional bulked sampling failed to account for the sub-field scale spatial variability in soil P content. This may contribute to the poor chemical and ecological status of surface waters by frequently under-predicting soil nutrient content, and failing to identify potential contributing sources of soil P losses. In contrast, higher intensity gridded sampling and interpolation revealed wide in-field spatial variability in soil P content, facilitating the identification of contributing sources of P losses to poor water quality and aiding in the characterisation of risk for nutrient losses to waterways. Hydrological modelling of in-field runoff pathways indicated several P sources potentially contributing to runoff-based P losses. Our weighted risk assessment model was successful in identifying P hotspots and transfer potential to water courses, illustrating that a similar approach could be applied anywhere in the world where excess P poses a problem for water quality. Model validation took place using instream water quality sampling data, which showed that higher risk weighting model results correlated to poorer water quality conditions. This methodology could be a useful management tool to help countries meet their national water quality targets.

Attainable yield and soil texture as drivers of maize response to nitrogen: A synthesis analysis for Argentina

• Attainable maize yield drives the agronomic optimum of initial N availability (AONav). • Soil texture modifies the curvature of the response models but not the AONav. • For yield environments from 9 to 14 Mg ha -1 , the AONav increases by 21 kg N Mg -1 . • Fine- and coarse-textured soils show the greatest and the lowest response rates to N. • We could maintain high N use efficiencies by correctly identifying yield environment. The most widely used approach for prescribing fertilizer nitrogen (N) recommendations in maize ( Zea Mays L.) in Argentina is based on the relationship between grain yield and the available N (kg N ha −1 ), calculated as the sum of pre-plant soil NO 3 - -N at 0−60 cm depth (PPNT) plus fertilizer N (N f ). However, combining covariates related to crop N demand and soil N supply at a large national scale remains unexplored for this model. The aim of this work was to identify yield response patterns associated to yield environment (crop N demand driver) and soil texture (soil N supply driver). A database of 788 experiments (1980−2016) was gathered and analyzed combining quadratic-plateau regression models with bootstrapping to address expected values and variability on response parameters and derived quantities. The database was divided into three groups according to soil texture (fine, medium and coarse) and five groups based on the empirical distribution of maximum observed yields (from Very-Low = <8.5 Mg ha −1 to Very-High = >13.1 Mg ha −1 ) resulting in fifteen groups. The best model included both, attainable yield environment and soil texture. The yield environment mainly modified the agronomic optimum available N (AON av ), with an expected increase rate of ca. 21.4 kg N Mg attainable yield −1 , regardless of the soil texture. In Very-Low yield environments, AON av was characterized by a high level of uncertainty, related to a poor fit of the N response model. To a lesser extent, soil texture modified the response curvature but not the AON av , mainly by modifying the response rate to N (Fine > Medium > Coarse), and the N use efficiencies. Considering hypothetical PPNT levels from 40 to 120 kg N ha −1 , the expected agronomic efficiency (AEN f ) at the AON av varied from 7 to 31, and 9–29 kg yield response kg fertilizer N (N f ) −1 , for Low and Very-High yield environments, respectively. Similarly, the expected partial factor productivity (PFPN f ) at the AON av ranged from 62 to 158, and 55–99 kg yield kg N f −1 , for the same yield environments. These results highlight the importance of combining attainable yield environment and soil texture metadata for refining N fertilizer recommendations. Acknowledging the still low N fertilizer use in Argentina, space exists to safely increasing N fertilizer rates, steering the historical soil N mining profile to a more sustainable agro-environmental scenario in the Pampas.

Quantifying N leaching losses as a function of N balance: A path to sustainable food supply chains

Growing public concern over agricultural nitrogen (N) pollution is now reflected in consumers’ food choices and shareholders’ resolutions, causing rapid changes in global food supply chains. Nitrate (NO3) leaching represents the primary N source for groundwater contamination and freshwater ecosystem degradation. However, simplified science-based indicators are still lacking to facilitate improved N management practices at the farm-level. We conducted a global analysis of published field studies to evaluate N balance (N inputs minus N outputs) as a robust predictor for NO3 losses. Using 82 studies (1110 observations) for rainfed cereal crops in temperate regions, we 1) quantified the response of NO3 losses to changes in N balance for major rainfed cereal crops while accounting for environmental and management variables; and 2) assessed the feasibility of improving water quality through lower N balance under different scenarios using the case study of maize (Zea mays L.) data from the US Corn Belt. Observations were grouped in studies from the US and non-US regions. Results show that NO3 losses increased exponentially as N balance increases for both the US and non-US regions, though they were 60% higher in the US at a given N balance. Scenario analysis revealed that reducing the N rate from the agronomic optimum to the lower point within the economic optimum N rate range decreased NO3 losses by 13% without impacting economic returns. The case study for maize showed that improvements in N use efficiency that increase grain yield at a given fertilizer rate can substantially reduce N balance and mitigate NO3 losses. This study provides an evidence-based foundation for food supply chain companies to mitigate global NO3 pollution, specifically by using the generalized relationships presented here to track progress in NO3 leaching mitigation. To further resolve uncertainties and improve region-specific estimates for NO3 losses, we propose a tiered monitoring and assessment framework similar to the IPCC (Intergovernmental Panel on Climate Change) methodology for N2O emissions, widely implemented in science and used for policy.

Farm- and community-level factors underlying the profitability of fertiliser usage for Ethiopian smallholder farmers

ABSTRACT While adoption rates for inorganic fertiliser are relatively high in Ethiopia, application rates are generally considered agronomically suboptimal. Using recent data on Ethiopian smallholder maize producers, we showed that maize response to nitrogen, and the profitability of fertiliser use depended on maize agronomy. The agronomic optimum ranged from 0 to 344 kg/ha with a mean value of 209 kg/ha. The actual nitrogen application rates were only about half the agronomic optimum, on average, and were less than the farm-specific economic optimum on 80% of maize fields. The average economic optimum level was 145 kg N/ha, but when we account for risk aversion, the resulting average optimum level is very close to the average observed usage level of 88 kg N/ha. Addressing risk aversion may help to induce greater levels of fertiliser investments at current prices and yield response rates. Our analysis also suggests that key pathways for increasing the economic returns to smallholder fertiliser investments include: complementing nitrogen inputs with phosphorus inputs and improved varieties, using lower levels of nitrogen under intercropping and manure inputs, enabling farmers to delay output sales beyond the immediate post-harvest period, and lowering the costs of accessing input and output markets.

What is the deal with the Green Deal: Will the new strategy help to improve European freshwater quality beyond the Water Framework Directive?

Agricultural land use covers almost half of the EU territory and reducing nutrient and pesticide losses to freshwaters is central to existing EU policy. However, the progress of improving freshwater quality and reducing eutrophication is slow and lags behind targets. The Green Deal is a key element of the EU plans to implement the United Nation's Sustainable Development Goals. Here, we discuss the opportunities that the Green Deal and associated strategies may provide for the achievement of the water quality goals of the Water Framework Directive in agricultural landscapes. We welcome Green Deal's aspirational stated goals. However, the reliance of mitigation of diffuse agricultural pollution on the reform of the Common Agricultural Policy represents grave risks for practical implementation and the achievement of the Green Deal objectives. We also argue that the new strategies should be targeted at tackling and understanding the sources of water quality problems along the full pollution continuum. To maximise the opportunities for tackling diffuse pollution from agricultural land use and achieving the delayed water quality targets, we stress that a range of targeted new instruments will be needed to close the gaps in the pollution continuum ‘from source to impact’. These gaps include: (I) smart and standardised monitoring of the impacts of proposed eco-schemes and agri-environment-climate measures, (ii) active restoration of agricultural streams and ditches and their floodplains to reduce secondary pollution sources, (iii) options to draw down nutrient levels to or below the agronomic optimum that reduce legacy sources, (iv) integrating farm-scale and catchment-scale analysis of trade-offs in reducing different pollutants and their combined effects, and finally (v) accounting for emerging pressures to freshwater quality due to climate change. Incorporation of the pollution continuum framework into tackling diffuse agricultural pollution will ensure that the European water-related policy goals are achieved.

Modelling Changes in Soil Phosphorus When Phosphorus Fertiliser Is Reduced or Ceases

In order for land managers and policy makers to manage excessive soil phosphorus (P) concentrations and reduce the risk of this particular source of P from impacting water bodies, models of soil P decline under various scenarios are needed. We modelled the decrease in calcium chloride-extractable P (CaCl2-P), and sodium bicarbonate-extractable P (Olsen-P and Colwell-P) using data from six Australian grazed pasture soils with contrasting P sorption properties, over a period of 4.5 years. Each soil had four initial soil P concentrations (Pinit), each of which received four on-going rates of P fertiliser (Pfert). The model predicts the final P concentration (Pfinal) by taking into account the P concentration previously measured (CaCl2-P, Olsen-P or Colwell-P), Pfert applied since measurement, and time since previous measurement: Final P concentration = (previously measured P concentration + ep x P fertiliser applied) exp (-dp x years since previous P concentration measurement). Where ep is the increase in soil P for each unit of applied P and dp is the decay constant representing how quickly the soil P decreased. The greatest decreases in proportion to Pinit occurred for CaCl2-P, followed by Olsen-P, and then Colwell-P. The model tended to fit the dataset well for Olsen-P and Colwell-P, with mean overestimation (modelled Pfinal concentration greater than actual Pfinal) of the Pfinal concentrations of 6.1 (32%) and 4.3 mg/kg (10%), respectively. Although there was less CaCl2-P data, the model successfully described it, with a mean overestimation of Pfinal CaCl2 of 3.1 mg/kg (26%). The overestimation of Pfinal CaCl2 was possibly due to the high CaCl2-P concentrations of the low P buffering index soils. The model predicted an average of 32 years (ranging from 26 to 49 years) for Olsen-P concentrations of between 55 and 96 mg/kg to decrease to an agronomic optimum of 17 mg/kg. Agronomic optimum was not a reliable indicator of environmental risk as some soils did not exceed the CaCl2-P environmental threshold until Olsen-P concentrations were twice the agronomic optimum, whereas low P sorbing soils tended to exceed the threshold before reaching agronomic optimum. Further work with more soils is required to examine the influence of soil properties – such as P sorption – on decreases in soil P.

Using a Crop Modeling Framework for Precision Cost-Benefit Analysis of Variable Seeding and Nitrogen Application Rates

A key goal of precision agriculture is to achieve the maximum crop yield while minimizing inputs and loses from cropping systems. The challenge of precision agriculture is these factors interact with one another on a subfield scale. The seeding density and the nitrogen (N) fertilizer application rate are two of the most important inputs influencing agronomic, economic and environmental outcomes including yield, return on investment (ROI) and nitrate (NO3-) leaching. A cropping system simulation framework is used to predict site-specific subfield optimum seeding density and (N) fertilizer application rates for the economic optimum (maximum ROI) versus agronomic optimum (maximum yield). The framework couples the process-based APSIM cropping system model with the SSURGO soils database, Daymet weather data service, land grant university estimates of crop production costs and commodity price estimates, and the R statistics software. The framework performance was evaluated using multiple years of precision yield monitor data obtained from a continuous maize (Zea mays L.) cropping system field experiment with varying N-fertilizer rates. Subfield model estimates of crop yield were sensitive to initial conditions related to historical management of the field and had an r2 = 0.65 and a root mean square error of 2356.0 kg ha-1. A site-specific application of the framework comparing economic optimum seeding density and N-fertilizer rates with agronomic optimum values estimated the average ROI benefit to be up to 12.1% with a NO3- leaching reduction of up to 15.2 kg ha-1 at the economic optimum. However, in a minority of cases NO3- leaching was greater at the economic optimum, indicating that managing to maximize ROI rather than yield may not always reduce environmental impacts. Our results suggest that managing cropping systems for the economic optimum is plausible using publicly available data with our framework and may likely lead to improved environmental outcomes.

A carrying capacity framework for soil phosphorus and hydrological sensitivity from farm to catchment scales

Agricultural fields with above optimum soil phosphorus (P) are considered to pose risks to water quality and especially when those areas are coincident with hydrologically sensitive areas (HSAs) that focus surface runoff pathways. This is a challenge to manage in areas of agricultural intensity in surface water dominated catchments where water quality targets have to be met. In this study, a soil P survey of 13 sub-catchments and 7693 fields was undertaken in a 220km2 catchment. HSAs were also determined as the top 25th percentile risk from a runoff routing model that used a LiDAR digital elevation model and soil hydraulic conductivity properties. Distributions of these spatial data were compared with river soluble reactive phosphorus (SRP) concentration measured fortnightly over one year. The results showed that 41% of fields exceeded the agronomic optimum for soil P across the sub-catchments. When compared with the available water quality data, the results indicated that the high soil P carrying capacity area of the sub-catchments was 15%. Combining high soil P and HSA, the carrying capacity area of the sub-catchments was 1.5%. The opportunities to redistribute these risks were analysed on fields with below optimum soil P and where HSA risk was also minimal. These ranged from 0.4% to 13.8% of sub-catchment areas and this limited potential, unlikely to fully reduce the P pressure to over-supplied fields, would need to be considered alongside addressing this over-supply and also with targeted HSA interception measures.

Selection of plant species for permanent ground cover in vineyards: Looking for an agronomic and environmental optimum

The installation of a permanent ground cover in vineyards offers numerous agronomic (soil protection, soil fertility) and environmental (reduced leaching of nutrients and of plant protection products, reduced use of herbicides, biodiversity) benefits. Nevertheless, such ground covers are not always free of drawbacks (competition for water and nitrogen, increased risk of frost, management). For the growers, the challenge is to manage the green ground covers in such a way as to preserve their advantages while limiting these drawbacks. Among the tools available to the winegrower is the sowing of a seed mixture composed of selected species: a choice depending of the soil and climatic conditions of the parcel, but also of the different, and sometime contradictory, objectives of the grower. This paper lists the agronomic and environmental issues addressed by the installation of a permanent ground cover in vineyards. It describes two concrete situations – ground cover for vineyards integrated in agro-ecological networks and green cover on the row of vines (as alternative to glyphosate) – for which, according to the objectives and the impacts, a choice of plant species is proposed. At last, it presents the trials now carried out to validate and optimize those selections, as well as the first observations and results gathered to date.

The efficacy of good practice to prevent long-term leaching losses of phosphorus from an irrigated dairy farm

Phosphorus (P) can be leached from intensive land uses, including grazed dairy farming. There is some evidence to suggest that P-leaching can enrich groundwater, especially where fertiliser or farm dairy effluent P (FDE) is applied to soils of low sorption capacity. We measured P fractions in leachate at 70 cm depth from two soils of low-P sorption capacity in an intensively grazed dairy farm, one a free-draining shallow soil and another a deep and moderately well-drained soil. As per industry good practice, the soils were maintained at an agronomic optimum and received P as either fertiliser or a lower rate of fertiliser plus FDE, applied according to regional rules and industry guidelines to avoid the FDE ponding on the soil surface and leaching to depth. Our hypothesis was that rules and guidelines were not sufficient to prevent P losses, especially in the free-draining soil. In response to annual applications of 40 kg P ha−1 yr−1 as fertiliser or 30 kg P ha−1 yr−1 as fertiliser and 10 kg P ha−1 yr−1 as FDE, dissolved and particulate P concentrations increased annually 4 to 7%. Mean total P load over the 14-yr period of measurement (2001–2015) from the FDE-treated, free-draining shallow soil was 1.46 kg ha−1 yr−1, much greater than the same soil without FDE (0.25 kg ha−1 yr−1) or the moderately well-drained soil with or without FDE applied (0.12 kg ha−1 yr−1, for both treatments). Leaching losses were attributed to a combination of high hydraulic conductivity enhanced by the presence of macropores and the increasing P saturation of macropore walls. An enrichment in dissolved reactive P was also detected in a well intercepting groundwater at 10-m depth. However, the source of the enrichment was unclear. These data suggest that despite following industry good practice, regional rules and industry guidelines significant P losses may occur when FDE is applied to soil at rates designed to maintain soil Olsen P in an agronomically optimal concentration. It is unclear if applying less FDE at lower rate, would decrease P losses. Therefore, less P must be applied, made less available for loss, or P-rich FDE not applied to this freely draining shallow stony soil (or similar soils) under irrigation.

Soil nitrate nitrogen buffer capacity and environmentally safe nitrogen rate for winter wheat-summer maize cropping in Northern China

Maintaining high crop yield and meanwhile reducing the risk of environmental pollution are of paramount importance for food security and sustainability. The aims of this study were (i) quantify soil NO3− -N buffer capacity under wheat-maize cropping system in Northern China, and (ii) establish agronomic optimum and environmentally safe N rates to guide N recommendation for the region. We compiled data of 76 sets of wheat and 73 sets of maize experiments spanning about 20 years, where N rate experiments conducted and soil NO3− -N content were measured immediately after crop harvest under conventional practice. By applying linear-plateau and plateau-linear models, we computed the soil NO3− -N buffer capacity, agronomic optimum N rate and environmentally safe N rates. Soil NO3− -N buffer capacity ranged from 10 to 221 kg ha−1 for wheat season and from 14 to 212 kg ha−1 for maize season, which were mostly less than 100 kg ha−1 in the region. The agronomic optimum N rates for wheat and maize were 46–221 and 44–250 kg N ha−1 with means of 110 and 127 kg N ha−1, respectively. The environmentally safe N rates were 19–248 kg N ha−1 with mean of 107 kg N ha−1 for wheat, and 36–209 kg N ha−1 with mean of 99 kg N ha−1 for maize. It was found that agronomic optimum N rates from 61% of wheat and 52% of maize were higher than the environmentally safe N rates, implying that current N recommendation had the high potential risk of nitrate leaching. Hence, it is suggested that N recommendation should consider not only the agronomic optimum N but also environmentally safe N rate, especially in areas where are vulnerable to nitrate leaching the latter should be placed at the top priority.

The Challenges of Managing Legacy Phosphorus Losses from Manure-Impacted Agricultural Soils

Historical application of manure to agricultural land in areas of intensive animal production, like the Delmarva Peninsula, has led to soil test phosphorus (STP) concentrations that far exceed agronomic optimum. While natural soils would typically serve as a sink for newly applied P sources, soils with P accumulation from long-term manure applications often serve as a source of P via a gradual release of dissolved P in runoff or leaching events. These losses of “legacy P” from manure-impacted soils are difficult to control and are linked to water-quality degradation in sensitive water bodies, like the Chesapeake Bay. In this review, we examine how long-term application of manure impacts soil P dynamics under P-buildup and -drawdown scenarios to better understand the behavior of legacy P in soils. We also examine the reasons that traditional best management practices (BMPs) fail to control legacy P losses from soils and discuss potential promising management strategies using the Delmarva Peninsula as a case study.

Past, present, and future use of phosphorus in Chinese agriculture and its influence on phosphorus losses.

Large inputs of phosphorus (P) in chemical fertilizers and feed supplements since 1978 have improved soil P status in arable land in China, but have also created challenges by increasing P concentrations in manure and exacerbating water quality degradation. Arable land in China can be divided into five management zones based on soil P chemistry, with 15–92 % of arable land having lower P status than the agronomic optimum and 0.3–7.2 % having severe risks of P leaching losses. A scenario analysis of soil P budget and agronomic P demand during 2011–2030 highlighted the great pressure China faces in sustainable P management and the need for drastic changes in current practices. This includes new policies to reduce P supplementation of feed and improved P use efficiency by livestock and programs to expand the adoption of appropriate fertilization, soil conservation, and drainage management practices to minimize P losses.

Effects of Overland Flow on Critical Soil Test Phosphorus Thresholds in Tillage Soils

In Regan et al. (Journal of Environmental Quality, 39, 18–192 2010), a runoff dissolved phosphorus risk indicator (RDPRI), based on rainfall alone, was developed for five Irish tillage soils. Results showed that tilled soils, subjected to simulated rainfall only (3 cm hr−1 inclined at 10° slopes), may produce surface runoff phosphorus (P) concentrations in excess of 0.03 mg L−1 (the value above which eutrophication of rivers is likely to occur) if their Morgan’s phosphorus (Pm), water extractable phosphorus (WEP) and Mehlich-3 phosphorus (M3-P) concentrations exceed 9.5 mg L−1, 4.4 mg kg−1 and 67.2 mg kg−1, respectively. The present study developed a modified RDPRI using the same soils, subject to two overflow run-on rates (225 and 450 ml min−1). Results of the 95 % confidence intervals suggested new critical soil test phosphorus (STP) thresholds for Pm, WEP, M3-P, Pcacl2 and Psatox of 7.83 mg L−1, 4.15 mg kg−1, 61 mg kg−1, 1.2 mg kg−1 and 17.1 %, respectively. Whilst these new values for Pm, WEP and M3-P are lower than those determined by Regan et al. (Journal of Environmental Quality, 39, 185–192 2010), the confidence intervals of the two sets of values overlap by more than 25 %, indicating no significant difference. The improved finding for Pm in this study is still in close agreement with the agronomic optimum (Pm = 6.1–10 mg L−1) used in Ireland for plant growth and crop yields, and fulfils the statutory requirements of the EU Good Agricultural Practice for the Protection of Waters, which prohibits fertiliser application to tillage soils with a Pm > 10 mg L−1.

A long‐term nitrogen fertilizer gradient has little effect on soil organic matter in a high‐intensity maize production system

Global maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affecting SOC within maize-based systems due to its effects on crop residue production and SOC mineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0-269 kg N ha(-1) yr(-1)) that created a large range in crop residue inputs (3.60-9.94 Mg dry matter ha(-1) yr(-1)), we provide the first agronomic assessment of long-term N fertilizer effects on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in physico-chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra-aggregate POM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition of POM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha(-1) yr(-1)) and an excessive N rate (269 kg N ha(-1) yr(-1)), there were no differences between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N2 O and NO3 emissions exponentially increase when agronomic optimum N is surpassed.

Soil Properties and Nutrient Export of a Duplex Hard-Setting Soil Amended with Compost

ABSTRACT Soils managed for field vegetable production often receive high nutrient inputs and are frequently cultivated. This can result in agronomically excessive soil P and soil structural degradation. In combination, these can lead to large losses of nutrients in runoff that can impact on aquatic environments. We investigated the effect of compost application at 62.5 and 125 t ha−1 in both 2005 and 2008 on soil nutrient and physical properties and runoff and nutrient export using simulated rainfall/runoff. The use of compost reduced (p < 0.01) the accumulation of available P in soils compared to conventional fertilizers but still resulted in excessively high soil P levels. The application of compost increased water stable aggregates, increased infiltration rates, and reduced the volume of runoff (all p < 0.01). Compost at either rate reduced runoff volumes by ∼50% compared to conventionally managed soils. As a consequence of reduced nutrient inputs/accumulation and improved soil physical properties, the use of compost at 62.5 t ha−1 resulted in a ∼50% reduction in N and P export in surface runoff, with no added benefit of using the higher compost rate. The rainfall simulations we reported only represent a ‘snap-shot’ of compost effects on runoff properties, and longer term monitoring of runoff would add further understanding of treatment effects. Nevertheless, we suggest that the use of compost may have environmental benefits although nutrient supply and status in the soil needs to be managed carefully to ensure adequate crop nutrition without unnecessarily increasing soil nutrients above the agronomic optimum.

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.

Nutrient losses from two contrasting dairy cow grazing systems

A key concern in farming systems that seek to maximise the grass content of livestock diets is the management of grazing so as to avoid damage to soil structure during periods with high antecedent soil moisture conditions. The aim of this study was to investigate, under the current codes of good agricultural practice, the impact of two contrasting grazing systems on soil structure and nutrient export from grazed grasslands. The two grazing treatments were (1) WinterCalf treatment, involving winter-calving Holstein-Friesian dairy cows, which had a mean ‘start of turnout’ date of 28 March, and were offered 5.0kg concentrate/cow/day throughout a 214-day grazing season; and (2) SpringCalf treatment, involving spring-calving Jersey Holstein-Friesian dairy cows, which had a ‘start of turnout’ date of 17 February, and were offered between 1.0 and 2.0kg concentrate/cow/day throughout a 260-day grazing season. Maximum reliance on grazed grass was a key objective of the latter system. Mean stocking rates during the grazing season were 5.1 and 4.3 cows per hectare with WinterCalf and SpringCalf systems, respectively. The experimental site was divided into four replicated blocks (A, B, C and D), with each block containing two grazing plots (425m and 500m, for the WinterCalf and SpringCalf systems, respectively). In addition to the two grazed treatments, an ungrazed treatment (UG) was established within each of the WinterCalf and SpringCalf treatment plots by fencing off a 12m exclusion sub-plots, which livestock could graze but not tread on. Changes in soil structure were recorded before and after grazing on all plots and with rainfall simulation to investigate the impact of soil structural changes on nutrient export to water. The results of the study demonstrated that over a complete grazing season, under the current code of good grazing practice, nutrient export from the SpringCalf treatment was not significantly greater than that from the WinterCalf treatment. However, results highlighted an increase in nutrient export from the SpringCalf treatment compared to the ungrazed treatment. These increases were as a result of a significant increase in overland flow volume from the SpringCalf treatment compared to the UG treatment. The time taken to initiate overland flow was a key factor in this relationship, with an increase from an average value of 536 seconds for the SpringCalf treatment to 1709 seconds for the Ungrazed treatment. Allowing time for soil to recover from compaction could be an effective mitigation strategy for reducing nutrient export from soil, particularly in areas where soil P is significantly above the agronomic optimum for grass production.