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Enrichment of soluble P on the surface layer of long‐term no‐till (NT) soils, and consequent increase in dissolved P losses, is a concern for which occasional plowing has been suggested as a remedy. We measured the effect of such strategic tillage (ST) on surface and subsurface P losses from 0.5‐ha field plots on clay soil for 4 years. Two NT plots had discharged threefold dissolved molybdate‐reactive P (DRP) losses compared to annually plowed soil conventional tillage (CT). ST by plowing to 20‐cm depth was applied on one of the NT plots, whereas the other remained under NT. ST done in July was sown with canola (Brassica napusssp. oleifera) to establish plant cover before winter. Summed 4‐year DRP loss from ST treatment was 60% lower compared to NT (0.78 vs. 1.96 kg ha−1), accompanied with 11% higher particulate P (PP) loss (4.39 vs. 3.97 kg ha−1). CT plots produced slightly lower DRP losses (0.53–0.76 kg ha−1) than ST, but higher PP losses (6.02–7.96 kg ha−1). Bioavailable P (BAP) losses from ST were lower than from the other treatments if >7% of PP turns bioavailable. After ST, soil P stratification first vanished, but started to develop again when NT was resumed. Occasional tillage of NT soils mitigates DRP losses over several years, and it was at the study site the preferred mitigation option in reducing BAP losses. No‐tilled clay soil was plowed to 20 cm and P losses compared to those of continuous no‐till and annual plowing.Over 4 years, strategic tillage lowered dissolved reactive P loss by 60% and increased particulate P loss by 11% compared to continuous no‐till.Strategic tillage was associated with lower bioavailable P losses than no‐till or annual plowing.We assessed that under no‐till soil P stratification returns to the initial level in 7 years after one‐time plowing. No‐till is a very effective erosion control option and therefore a widely recommended water protection measure. However, categorical application of no‐till, at sites that are not very erosion‐prone, may be more detrimental than beneficial in eutrophication point of view, because dissolved P losses from no‐till tend to be higher than in regular inversion plowing. This study showed that occasional plowing of no‐till helps controlling dissolved P losses for several years, and it would be the preferred eutrophication control measure at the site of this study.

No‐till as a water protection measure is highly efficient in controlling erosion and particulate P (PP) loss but tends to increase dissolved reactive P (DRP) concentrations in runoff water. In a 9‐yr field study on a clay soil in Southwest Finland, the effects of no‐till and autumn plowing on surface runoff and subsurface drainage water quality were compared. The site had a 2% slope and was under spring cereal cropping, with approximately replacement fertilizer P rates. Vertical stratification of soil‐test P that had developed during a preceding 6‐yr grass ley was undone by plowing but continued to develop under no‐till. During the 9‐yr study period, no‐till soil had 27% lower cumulative total P losses than plowed soil (10.0 vs. 13.7 kg total P ha−1). Concentrations and losses of PP were clearly lower under no‐till than under plowing (5.6 vs. 12.3 kg PP ha−1), but DRP loss showed the opposite trend (4.3 vs. 1.4 kg DRP ha−1). There was an increasing trend in subsurface drainflow DRP concentration under no‐till, possibly because of development of a conductive pore structure from soil surface to drain depth. The potential benefit of no‐till in water protection depends on how much of the PP transported to water is transformed into a bioavailable form and used by aquatic organisms. The beneficial effect of no‐till in controlling P‐induced eutrophication at the study site would only be realized if the bioavailable share of PP exceeds 43%. Otherwise, no‐till would not be an efficient eutrophication control measure at this site. No‐till decreased total P losses by 27% compared with autumn plowing.No‐till produced 4.5‐fold higher DRP loss and 54% lower PP loss than plowing.When changes in DRP and PP are opposite, TP changes should be interpreted with caution.In this case, the effect on eutrophication largely depends on PP bioavailability.At the study site, increased DRP load is compensated if PP bioavailability is >43%.

Soil test P (STP) concentration indicates whether annual P applications can be expected to give yield increases and can also indicate an elevated risk of P mobilization and potential for P transfer to surface waters and groundwater from a particular field. Changes in STP with time thus project agronomic benefits and environmental risks of different P use strategies. To predict STP changes with time, we constructed a simple dynamic model for which the input variables are P balance and initial STP. The model parameters (soil type‐specific constants) were fitted using data originating from 44 P fertilizer experiments with different P rates. Model performance was evaluated using independent data sets that either had reasonably accurate input values (n= 103) or were obtained from farmers through interviews (n= 638). The simulations were in agreement with measured STP changes for both evaluation data sets when fittings were performed separately for four main soil types (clays, silts, coarse mineral soils, and organic soils). Statistical analysis confirmed that the model captured the trends in STP (NH4OAc test) with acceptable accuracy and precision, with r2of 0.83 and 0.66 for the data with more accurate input and for farmer interview data, respectively; the corresponding model efficiency statistics were 0.88 and 0.66. The model is not restricted to use with one soil test, as fittings for several different types of soil tests can be generated. In this study, we fitted the model for Olsen P data retrieved from the literature. Agronomic use of the model includes evaluation of P use strategies, e.g., when a certain STP level is targeted or when long‐term economy of P use is calculated. In an environmental context, the model can be used to predict STP changes with time under variable P balance regimes, which is essential for realistic assessment of changes in the potential for dissolved P losses. A simple model to project changes in STP was created.Model inputs are present STP and P annual balance.The model can be calibrated for several soil P tests.Tests with separate data showed the model's adequacy in predicting STP changes with time.

Runoff P associated with eroded soil is partly solubilized in receiving waters and contributes to eutrophication, but the significance of particulate phosphorus (PP) in the eutrophying P load is debatable. We assessed losses of bioavailable P fractions in field runoff from fine‐textured soils (Cryaquepts). Surface runoff at four sites and drainflow at two of them was sampled. In addition to dissolved molybdate‐reactive phosphorus (DRP) losses, two estimates of bioavailable PP losses were made: (i) desorbable PP, assessed by anion exchange resin‐extraction (AER‐PP) and (ii) redox‐sensitive PP, assessed by extraction with bicarbonate and dithionite (BD‐PP). Annual losses of BD‐PP and AER‐PP were derived from the relationships (R2= 0.77–0.96) between PP and these P forms. Losses of BD‐PP in surface runoff (94–1340 g ha−1) were typically threefold to fivefold those of DRP (29–510 kg ha−1) or AER‐PP (13–270 g ha−1). Where monitored, drainflow P losses were substantial, at one of the sites even far greater than those via the surface pathway. Typical runoff DRP concentration at the site with the highest Olsen‐P status (69–82 mg kg−1) was about 10‐fold that at the site with the lowest Olsen P (31–45 mg kg−1), whereas the difference in AER‐PP per mass unit of sediment was only threefold, and that of BD‐PP 2.5‐fold. Bioavailable P losses were greatly influenced by PP runoff, especially so on soils with a moderate P status that produced runoff with a relatively low DRP concentration.

Reduction‐induced phosphorus (P) release from particles transported by field runoff has been poorly studied for want of a method that could be used for large surveys. To rectify this shortcoming, we modified the bicarbonate–dithionite (BD) extraction step of a sediment P speciation scheme for analyzing redox‐sensitive P in runoff without sample preconcentration. The extraction comprised the addition of bicarbonate (pH buffer) and dithionite (reducing agent) into a runoff sample, 15 min of gentle shaking, filtration, and sample digestion. The samples were greatly reduced (Eh < −200 mV), and Fe and P were solubilized, but Al solubility was not increased. Phosphorus release from rock phosphates (calcium phosphates) was greater in the BD extraction than in water or bicarbonate solution, although no more than 0.2% of the total P was released. For runoff from a very fine Typic Cryaquept, the particulate phosphorus (PP) versus BD‐PP relationship was linear up to a PP concentration of about 1.0 mg L−1, but over the whole PP range studied (up to 2.6 mg L−1) somewhat better described by an exponential equation (BD‐PP = 0.297 × PP0.766; r2= 0.91, n= 79). The minimum detectable value given by the method was relatively low, 0.023 mg L−1, but reproducibility varied, with the coefficient of variation for 10 samples analyzed with 5 replicates ranging from 1.8 to 28.5%. Considering the variable reproducibility of the results and the lack of suitable reference material, the method needs further refinement and testing if it is to be used for quantitative determination of redox‐sensitive P in runoff.

Recent work has shown that a significant portion of the total loss of phosphorus (P) from agricultural soils may occur via subsurface drainflow. The aim of this study was to compare the concentrations of different P forms in surface and subsurface runoff, and to assess the potential algal availability of particulate phosphorus (PP) in runoff waters. The material consisted of 91 water‐sample pairs (surface runoff vs. subsurface drainage waters) from two artificially drained clayey soils (a Typic Cryaquept and an Aeric Cryaquept) and was analyzed for total suspended solids (TSS), total phosphorus (TP), dissolved molybdate‐reactive phophorus (DRP), and anion exchange resin–extractable phosphorus (AER‐P). On the basis of these determinations, we calculated the concentrations of PP, desorbable particulate phosphorus (PPi), and particulate unavailable (nondesorbable) phosphorus (PUP). Some water samples and the soils were also analyzed for 137Cs activity and particle‐size distribution. The major P fraction in the waters studied was PP and, on average, only 7% of it was desorbable by AER. However, a mean of 47% of potentially bioavailable P (AER‐P) consisted of PPi. The suspended soil material carried by drainflow contained as much PPi (47–79 mg kg−1) as did the surface runoff sediment (45–82 mg kg−1). The runoff sediments were enriched in clay‐sized particles and 137Cs by a factor of about two relative to the surface soils. Our results show that desorbable PP derived from topsoil may be as important a contributor to potentially algal‐available P as DRP in both surface and subsurface runoff from clayey soils.

Phosphorus losses by surface runoff and leaching were studied for three years in a 16-plot field on a heavy clay soil in south-west Finland to compare losses of total phosphorus (TP), particulate phosphorus (PP) and dissolved orthophosphate phosphorus (DP) from barley and grass ley plots. Barley was fertilized with 21 and 42 kg ha-1 y-1 P by placement technique. Ley was given P 42 and 84 kg ha-1 y-1, but split into two equal portions, by broadcasting. The mean losses of TP from the barley and ley were 1.2 and 1.6 kg ha-1 y-1, respectively. The PP losses (calculated as the difference between TP and DP) constituted 69% of the TP loss from barley, but only 35% of that from ley. In the case of barley, a considerable amount of PP was carried by drainage water. The mean DP losses were 0.4 kg ha-1 y-1 from barley and 1.0 kg ha-1 y-1 from ley. The higher DP loss from ley was due to both the higher fertilizer rate and the fertilizer broadcasting technique. During the second and third years of the experiment, fertilizer broadcasting on the ley plots was followed by high DP concentrations in surface runoff and drainage water. The peak DP concentrations in drainage water occurred soon after fertilizer broadcasting followed by only 7-15 mm of flow, indicating fast preferential movement of orthophosphate from the surface through the soil.