Helena Aronsson

Future agriculture with minimized phosphorus losses to waters: Research needs and direction

Abstract The series of papers in this issue of AMBIO represent technical presentations made at the 7th International Phosphorus Workshop (IPW7), held in September, 2013 in Uppsala, Sweden. At that meeting, the 150 delegates were involved in round table discussions on major, predetermined themes facing the management of agricultural phosphorus (P) for optimum production goals with minimal water quality impairment. The six themes were (1) P management in a changing world; (2) transport pathways of P from soil to water; (3) monitoring, modeling, and communication; (4) importance of manure and agricultural production systems for P management; (5) identification of appropriate mitigation measures for reduction of P loss; and (6) implementation of mitigation strategies to reduce P loss. This paper details the major challenges and research needs that were identified for each theme and identifies a future roadmap for catchment management that cost-effectively minimizes P loss from agricultural activities.

Nitrogen leaching and crop availability in manured catch crop systems in Sweden

Results are presented from five years (1990–1995) of a field leaching experiment on a sandy soil in south-west Sweden. The aim was to study N leaching, change in soil organic N and N mineralization in cropping systems with continuous use of liquid manure (two application rates) and catch crops. N leaching from drains, N uptake in crops and mineral N in the soil were measured. Simulation models were used to calculate the N budget and N mineralization in the soil and to make predictions of improved fertilization strategies in relation to manure applications and changing the time for incorporation of catch crops. In treatments without catch crops, a normal and a double application of manure increased average N leaching by 15 and 34%, respectively, compared to treatment with commercial fertilizer. Catch crops reduced N leaching by, on average, 60% in treatments with a normal application of manure and commercial fertilizer, but only by 35% in the treatment with double the normal application rate of manure. Incorporation of catch crops in spring increased simulated net N mineralization during the crop vegetation period, and also during early autumn. In conclusion, manured systems resulted in larger N leaching than those receiving commercial fertilizer, mainly due to larger applications of mineral N in spring. More careful adaptation of commercial N fertilization with respect to the amounts of NH4-N applied with manure could, according to the simulations, reduce N leaching. Under-sown ryegrass catch crops effectively reduced N leaching in manured systems. Incorporating catch crop residues in late autumn instead of spring might be preferable with respect to N availability in the soil for the next crop, and would not increase N leaching.

Simulations of soil carbon and nitrogen dynamics during seven years in a catch crop experiment

Abstract This study aimed, with the use of simulation models, to quantify the effect of several years incorporation of catch crop material into the soil on soil organic matter storage, N mineralisation capacity and risk for N leaching. C and N dynamics in crop and soil were simulated with the SOILN model (Version 9.2). The simulated results were compared with measurements of crop and soil N, crop biomass and N-leaching from a catch crop field experiment situated in southwestern Sweden. The generality of parameter values determining the long-term turnover of soil organic matter was tested. To reproduce measured soil mineral N and crop N, mineralisation had to be favoured by high C mineralisation. After 6 years of catch crop treatment, simulated soil organic matter content had increased by less than 2%, but the N-mineralisation capacity had increased by 25%, corresponding to 37 kg N ha−1. With a continuous annual use of catch crops only a few per cent of the extra mineralised N was leached. Without a succeeding catch crop, however, 30% of N from the increased mineralisation was leached. The results indicated that the decomposition rate increased immediately after frost events.

Turnover and Losses of Phosphorus in Swedish Agricultural Soils: Long-Term Changes, Leaching Trends, and Mitigation Measures

Transport of phosphorus (P) from agricultural fields to water bodies deteriorates water quality and causes eutrophication. To reduce P losses and optimize P use efficiency by crops, better knowledge is needed of P turnover in soil and the efficiency of best management practices (BMPs). In this review, we examined these issues using results from 10 Swedish long-term soil fertility trials and various studies on subsurface losses of P. The fertility trials are more than 50 years old and consist of two cropping systems with farmyard manure and mineral fertilizer. One major finding was that replacement of P removed by crops with fertilizer P was not sufficient to maintain soil P concentrations, determined with acid ammonium lactate extraction. The BMPs for reducing P leaching losses reviewed here included catch crops, constructed wetlands, structure liming of clay soils, and various manure application strategies. None of the eight catch crops tested reduced P leaching significantly, whereas total P loads were reduced by 36% by wetland installation, by 39 to 55% by structure liming (tested at two sites), and by 50% by incorporation of pig slurry into a clay soil instead of surface application. Trend analysis of P monitoring data since the 1980s for a number of small Swedish catchments in which various BMPs have been implemented showed no clear pattern, and both upward and downward trends were observed. However, other factors, such as weather conditions and soil type, have profound effects on P losses, which can mask the effects of BMPs.

Phosphorus leaching from loamy sand and clay loam topsoils after application of pig slurry

Appropriate management of animal waste is essential for guaranteeing good water quality. A laboratory leaching study with intact soil columns was performed to investigate the risk of phosphorus (P) leaching from a clay loam and a loamy sand. The columns (0.2 m deep) were irrigated before and after application of pig slurry on the surface or after incorporation, or application of mineral P, each at a rate of 30 kg P ha-1. The two soils had different initial P contents (i.e. the ammonium lactate-extractable P was 65 and 142 mg kg-1 for the clay loam and loamy sand, respectively), but had similar P sorption characteristics (P sorption index 3.0) and degree of P saturation (17-21%). Concentrations of dissolved reactive P (DRP) and total P (TP) before P application were significantly higher in leachate from the loamy sand (TP 0.21 mg L-1) than from the clay loam (TP 0.13 mg L-1), but only increased significantly after P application to the clay loam. The highest concentrations were found when slurry was surface-applied (DRP 1.77 mg L-1), while incorporation decreased the DRP concentration by 64% in the clay loam. Thus moderate slurry application to a sandy soil with low P saturation did not pose a major risk of P leaching. However, application of P increased the risk of P leaching from the clay loam, irrespective of application method and despite low P saturation. The results show the importance of considering soil texture and structure in addition to soil chemical characteristics in risk assessments of P leaching. Structured soils such as the clay loam used in this study are high risk soils and application of P to bare soil during wet periods, e.g. in autumn or spring, should be followed by incorporation or avoided completely.

Long-term measurements and model simulations of phosphorus leaching from a manured sandy soil

Cropping systems with high phosphorus (P) inputs may constitute a risk of P leaching, which contributes to eutrophication. The main objective of this study was to identify P leaching risks associated with three long-term fertilization regimes in separately tile-drained plots on a sandy soil in southwest Sweden. The three regimes resulted in different annual P surpluses of, on average, 16 kg P ha−1 (14 lb P ac−1) in mineral form and 18 kg P ha−1 (16 lb P ac−1) and 37 kg P ha−1 (33 lb P ac−1) as pig slurry. The importance of different soil characteristics (soil P, iron, aluminum, and calcium content, and degree of P saturation [DPS]) and processes (water flow and P sorption/desorption) was examined using 15 years (1989 to 2003) of P leaching measurements and simulations with the ICECREAM model. Measurements of high soil P content and DPS values in the topsoil, in combination with high precipitation and rapid water flow, indicated a high potential for P losses, which was confirmed by the model simulations. However, the model considerably overestimated total P leaching by a factor of 5 to 9 since measured P leaching was small for all treatments. Measured mean annual total P leaching and total P concentration ranged respectively from 0.14 kg ha−1 (0.12 lb ac−1) and 0.06 mg L−1 (3.75 × 10−6 lb ft−3) at a high rate of slurry application to 0.20 kg ha−1 (0.18 lb ac−1) and 0.08 mg L−1 (4.99 × 10−6 lb ft−3) in the mineral P treatment. The differences in concentration were statistically significant (p < 0.001). A main conclusion from this 15-year study was that annual pig slurry application rates of 37 to 58 kg P ha−1 (33 to 52 lb P ac−1) did not increase P leaching. High sorption capacity of the subsoil, caused by Fe, Al, and Ca, was obviously very important for controlling P losses. Thus, information on soil P content and fertilization must be supplemented with estimates of soil P sorption capacity when evaluating the risk of P leaching for different soils. This must also be considered in models used for assessment of P leaching from arable land. The current ICECREAM model does not include appropriate functions for describing P sorption/desorption processes in this type of soil and needs further development.

Reducing nitrate leaching after winter oilseed rape and peas in mild and cold winters

Nitrate leaching after winter oilseed rape and peas has not been studied at the most northern limits of oilseed rape cultivation where winters vary between being mild, with continuous drainage, and cold, with periods of frozen soil. Here, we studied the effect of N fertilisation to oilseed rape, catch crops after oilseed rape and peas and dired drilling of winter wheat after oilseed rape on N leaching in south-west Sweden. Nitrate leaching was determined in two field experiments, dated 2004–2006 and 2005–2007, respectively, on a sandy loam. Our results show that under oilseed rape nitrate leaching was low, at 16–23 kg N ha−1, in a mild winter with drainage from October to March. In the subsequent mild winter nitrate leaching under wheat was higher, amounting to 35–94 kg N ha−1. Nitrate leaching levels were similar, 32–58 kg N ha−1, for all crops in a cold winter with a long-lasting snow cover and main drainage occurring after snowmelt in March and April. Application of fertiliser N to oilseed rape at the optimum N rate, rather than 50 kg N ha−1 above optimum, reduced leaching in a following winter wheat crop by 25 and 27 kg N ha−1 in a cold and a mild winter, respectively. Spring undersowing of perennial ryegrass as a catch crop reduced leaching by 12 kg N ha−1 after optimally fertilised oilseed rape in a mild winter, despite only growing until mid-September when winter wheat was sown. An undersown catch crop of peas, then grown until November, reduced leaching by 15 kg N ha−1. Direct drilling of winter wheat after oilseed rape had no effect. These findings show that there are risks of enhanced leaching in early spring after a cold winter with a snow cover and superficially frozen soil. Optimising the spring N rate for oilseed rape was the most effective measure to decrease leaching in both mild and cold winters, and this effect was improved by an undersown catch crop in a mild winter.

The ability of cover crops to reduce nitrogen and phosphorus losses from arable land in southern Scandinavia and Finland

This review summarizes current knowledge from the literature and experimental studies on the role of cover crops (CCs) in reducing nitrogen (N) leaching and phosphorus (P) losses to waters under the marine and humid continental climate conditions of southern Scandinavia and Finland. Field leaching studies from 11 sites indicate that undersown ryegrass (Lolium spp.) CCs are robust, with average N uptake in aboveground CC biomass of 7 to 38 kg N ha−1 (6.2 to 34 lb N ac−1). Use of CCs sown at harvest (e.g., crucifers) is restricted to southern Scandinavia for climate reasons. The mean relative reduction in N leaching reported for all CCs investigated was 43%, but it ranged between 62% increase instead of a reduction after a red clover (Trifolium pratense) CC on a clay soil to a reduction of 85% to 89% with a perennial ryegrass CC on sandy soils in Denmark (36 to 51 kg ha−1 [32 to 46 lb ac−1]). The data indicate that CCs do not substantially reduce total P losses by runoff and leaching. The effects of CCs on total P leaching varied between a relative increase of 86% and a decrease of 43%. Climate conditions involving freezing-thawing during winter increased the risk of losses of dissolved P from CC biomass. CCs have been implemented to varying degrees into agri-environmental programs. They are mandatory in Denmark and subsidized in Norway, Sweden, and Finland. CCs are grown on 8% of arable land in Denmark, 5% in Sweden, 1% in Finland, and 0.5% in Norway, but CC area is now increasing dramatically in Finland due to a new subsidy program. In all countries there is a need, and potential, for increased use of CCs, but there are several constraints, particularly reduced interest among farmers. There is a clear need to identify CC systems and develop implementation strategies for appropriate distribution of CCs on different soils and regions with respect to required reductions in N leaching and P losses. For this, more knowledge is required, especially about the effect of CCs on P losses (e.g., the effect of species with different partitioning between shoot and root biomass and the effects of CC systems with harvesting of biomass). There is also a need to devise balanced solutions for maintaining and increasing the frequency of CCs in crop rotations to exploit the possible benefits of CCs in reducing nutrient losses.

Use Efficiency and Leaching of Nutrients in Organic and Conventional Cropping Systems in Sweden

In the past few years, organic farming has been proposed as a possible way of reducing N leaching from agricultural soils and improving the use efficiency of plant nutrients. This is, to a large extent, considered to be attributed to the fact that synthetic fertilisers are not allowed in such systems and the N inputs mainly originate in various types of organic manures. In this overview, results from a number of Swedish field studies are presented in which crop yields, nutrient-use efficiencies and leaching in organic and conventional systems are evaluated. Some studies were conducted in lysimeters and others in large tile-drained field plots. In two lysimeter experiments, leaching of N derived from either poultry manure or red clover (Trifolium pratense L.) green manure were compared with fertiliser N, all labeled with 15-N. In the lysimeters on which poultry manure was applied, 32% of N applied leached during three years, whereas only about 3% leached in ammonium nitrate fertilised lysimeters. In plots on a sandy soil, annual N leaching loads averaged over the whole 6-yr crop rotation reached 39,kg N ha-1 in the organic rotations and 25,kg N ha-1 in the conventional rotation. Phosphorus-leaching loads were overall small in all systems, whereas K leaching was highest in the conventional rotation (i.e., on average, 27,kg ha-1 yr-1). In terms of crop yields, they were reduced by 20–80% in the organic rotations compared to the same crops in the conventional rotations. This was explained in terms of N deficiency, weed competition, and infestation of crop diseases in the organic systems. These results suggest that organic crop production uses agricultural soils less efficiently, with no benefit for water quality.

Elymus repens biomass allocation and acquisition as affected by light and nutrient supply and companion crop competition

Background and Aims Competitive crops are a central component of resource-efficient weed control, especially for problematic perennial weeds such as Elymus repens. Competition not only reduces total weed biomass, but denial of resources can also change the allocation pattern – potentially away from the underground storage organs that make perennial weeds difficult to control. Thus, the competition mode of crops may be an important component in the design of resource-efficient cropping systems. Our aim was to determine how competition from companion crops with different modes of competition affect E. repens biomass acquisition and allocation and discuss that in relation to how E. repens responds to different levels of light and nutrient supply. Methods Greenhouse experiments were conducted with E. repens growing in interspecific competition with increasing density of perennial ryegrass or red clover, or growing at three levels of both light and nutrient supply. Key Results Elymus repens total biomass decreased with increasing biomass of the companion crop and the rate of decrease was higher with red clover than with perennial ryegrass, particularly for E. repens rhizome biomass. A reduced nutrient supply shifted E. repens allocation towards below-ground biomass while a reduced light supply shifted it towards shoot biomass. Red clover caused no change in E. repens allocation pattern, while ryegrass mostly shifted the allocation towards below-ground biomass, but the change was not correlated with ryegrass biomass. Conclusions The companion crop mode of competition influences both the suppression rate of E. repens biomass acquisition and the likelihood of shifts in E. repens biomass allocation.