Risto Uusitalo

Phosphorus reserves and apparent phosphorus saturation in four weakly developed cultivated pedons

Abstract Phosphorus (P) reserves and related characteristics of soil horizons were studied in two clay (Inceptisols) and two coarse-textured (Inceptisol, Spodosol) cultivated soils in Finland. The soils were young and they all had been formed of multiple parent materials, the deposition of which were linked to the evolutionary stages of the Baltic Sea. In all C horizons, the content of total P and measured Chang and Jackson P fractions were very similar with 80–90% of the fractionable P in H 2 SO 4 -soluble form. The dominance of this fraction from the approximate depth of 70 cm down demonstrated the low degree of weathering of the soils. In the B horizons, soil formation had decreased total P and the proportion of H 2 SO 4 -soluble P while fertilizer P had accumulated in the Ap horizons, leading to an anthropic epipedon in one of the four soils. Accumulation of secondary P was studied by measuring the degree of phosphorus saturation (DPS) of the horizons as a molar ratio of P to the sum of Al and Fe in acid ammonium oxalate extracts. Oxalate extracted from 1.2 to 8.5 times more P than the sum of NH 4 Cl–, NH 4 F– and NaOH–soluble P fractions, suggesting that a substantial amount of the oxalate-extractable P was primary acid-soluble P. The method thus led to erroneously high DPS values in the poorly weathered soils. Phosphorus saturation calculated as a molar ratio of the sum of NH 4 Cl–, NH 4 F– and NaOH–soluble P fractions to oxalate-extractable Al and Fe did not indicate a high P saturation in any of the subsurface horizons studied. The result was in accordance with the very low amounts of water-extractable P in the respective horizons.

Phosphorus mitigation during springtime runoff by amendments applied to grassed soil.

Permanent grass vegetation on sloping soils is an option to protect fields from erosion, but decaying grass may liberate considerable amounts of dissolved reactive P (DRP) in springtime runoff. We studied the effects of freezing and thawing of grassed soil on surface runoff P concentrations by indoor rainfall simulations and tested whether the peak P concentrations could be reduced by amending the soil with P-binding materials containing Ca or Fe. Forty grass-vegetated soil blocks (surface area 0.045 m, depth 0.07 m) were retrieved from two permanent buffer zones on a clay and loam soil in southwest Finland. Four replicates were amended with either: (i) gypsum from phosphoric acid processing (CaSO × 2HO, 6 t ha), (ii) chalk powder (CaCO, 3.3 t ha), (iii) Fe-gypsum (6 t ha) from TiO processing, or (iv) granulated ferric sulfate (Fe[SO], 0.7 t ha), with four replicates serving as untreated controls. Rainfall (3.3 h × 5 mm h) was applied on presaturated samples set at a slope of 5% and the surface runoff was analyzed for DRP, total dissolved P (TDP), total P (TP), and suspended solids. Rainfall simulation was repeated twice after the samples were frozen. Freezing and thawing of the samples increased the surface runoff DRP concentration of the control treatment from 0.19 to 0.46 mg L, up to 2.6-3.7 mg L, with DRP being the main P form in surface runoff. Compared with the controls, surface runoff from soils amended with Fe compounds had 57 to 80% and 47 to 72% lower concentrations of DRP and TP, respectively, but the gypsum and chalk powder did not affect the P concentrations. Thus, amendments containing Fe might be an option to improve DRP retention in, e.g., buffer zones.

Characterization of Soil Phosphorus in Differently Managed Clay Soil by Chemical Extraction Methods and 31P NMR Spectroscopy

Changes in land use alter the natural cycling of phosphorus (P) in soil. Understanding the chemical nature of these changes is important when developing sustainable management practices for cultivated soils. In this study, we evaluated the ability of commonly used laboratory methods to characterize land use–induced changes in various P pools. Also, the characteristics of soil P revealed by different methods are discussed. Soil samples were taken from three differently managed field plots of the same clay soil: uncultivated grassland and organic and conventional crop rotations. Soil P reserves were characterized using Chang and Jackson and Hedley sequential fractionation procedures and by sodium hydroxide (NaOH)–ethylenediaminetetraacetic acid (EDTA) extraction followed by 31P NMR spectroscopy. Both of the tested fractionation methods identified differences in the P pools and provided evidence regarding land use–induced changes. However, the 31P NMR analysis suggests that the quality of organic P in this soil was not affected by the change in land use.

Labile organic carbon regulates phosphorus release from eroded soil transported into anaerobic coastal systems

Abstract Coastal eutrophication is expected to increase due to expanding and intensifying agriculture which causes a large amount of soil-associated P to be transported into aquatic systems. We performed anaerobic long-term incubations on field soil to mimic the conditions that eroded soil encounters in brackish sediments. The release of P from soil increased with the amount of labile organic C (acetate) addition and decreased with the soil/solution ratio. We deduce that in less-productive brackish systems, microbial Fe reduction allows for the maintenance of the coupled cycling of Fe and P and restricts the amount of P entering the oxic water. In more eutrophic systems, the formation of Fe sulfides as a result of SO4 reduction inactivates Fe, and leads to a higher release of P, thus generating an adverse feedback effect. The dependence of the fate of soil-bound Fe and P on the trophic status of the receiving water should be recognized in eutrophication management.

Accumulation and translocation of sparingly soluble manure phosphorus in different types of soils after long-term excessive inputs

Turkiselainten lanta sisaltaa runsaasti fosforia, joka onpaaosin niukkaliukoista. Maassa turkiselainlannan lannoitusvaikutus on siten aluksi heikompi kuin esimerkiksi naudan lannan lannoitusvaikutus. Turkiselainten lantaa hyodyntavassa viljelyssa on taman vuoksi suositeltu muihin lantalajeihin nahden suurempien fosforin kokonaismaarien lisaamista peltoon. Tassa tyossa tutkittiin maan fosforin muotoja 35 peltolohkolla, joille tiedettiin pitkan ajan kuluessa lisatyn suuria maaria turkiselainten lantaa. Pitkan ajan kuluessa kasvien fosforin ottoa suuremmat lisaysmaarat turkiselainten lantaa saaneilla lohkoilla olivat kasvattaneet maan fosforin kokonaispitoisuutta huomattavasti. Vaikka maarallisesti suurin osa ketun tai minkin lannan mukana maahan lisatysta fosforista oli maan fraktiointianalyysin tulkinnan mukaan kertynyt kasveille huonosti kayttokelpoiseen muotoon, alunperin hyvin niukkaliukoinen turkiselainten lannan fosfori oli kuitenkin kasvattanut myos kasveille kayttokelpoisen ja huuhtoutumiselle alttiina olevan fosforin pitoisuutta maassa. Pitkan ajan kuluessa maahan lisatyt suuret fosforimaarat olivat aiheuttaneet maan fosforinpidatyspaikkojen tayttymisen pintamaan lisaksi myos syvemmissa maakerroksissa. Talloin on uhkana fosforin kulkeutu mien pintavalunnan ja salaojavesien mukana vesistoihin. Maan fosforikyllastysaste ja helppoliukoisen fosforin pitoisuus olivat erittain suuria turkiselainten lantaa saaneilla lohkoilla 60 cm:n syvyyteen saakka. Vaikka maatalouden ymparistoohjelman puitteissa on turkiselainten lantaa hyodynnettaessa mahdollista kayttaa yli kaksinkertaisia fosforilisayksia kivennaislannoitteisiin verrattuna, lisayksia on syyta rajoittaa lohkoilla, joilla turkiselainten lannan mukana on pitkaan lisatty suuria maaria fosforia. Koska turkiselainten lanta on hyvin fosforipitoista ja sen kuiva-ainepitoisuus on suhteellisen suuri, ketun ja minkin lannan kuljettaminen on edullisempaa kuin useiden muiden lantalajien. Nain ollen sita kannattaa levittaa myoskin kauempana lannan syntypaikasta sijaitseville pelloille. Taman tutkimuksen aineistoon oli tietoisesti etsitty pitkaan turkiselainten lantaa saaneita peltoja, minka vuoksi tutkimusaineiston maiden fosforipitoisuus oli korkea. Vaikka tallaisten peltolohkojen osuus peltoalasta olisi suhteellisen pieni, maan fosforikyllastysta tulisi pyrkia alentamaan, silla yksittaiseen vesistoon paatyvasta kuormituksesta saattaa suuri osa olla peraisin muutamalta fosforilla kyllastetylta peltolohkolta.

Sediment from agricultural constructed wetland immobilizes soil phosphorus

Phosphorus (P) losses from agricultural soils impair the quality of receiving surface waters by enhancing eutrophication. This study tested the potential of using sediment from agricultural constructed wetlands (CWs) to immobilize soil P using two soils differing in texture and soil test P (STP). A silty clay soil (SIC) with high STP (24 mg ammonium acetate-extractable P [P] L) and a sandy loam soil (SL) with excessive STP (210 mg P L) were incubated with increasing amounts of clayey CW sediment. The soil-sediment mixtures were studied with the quantity/intensity (Q/I) technique, using chemical extractions, and by exposing the mixtures to simulated rainfall. In both Q/I and simulated rainfall tests, P solubility steadily decreased with increasing sediment proportion in the mixtures. However, in chemical extractions this effect was observed only at high sediment addition rates (10 or 50% [v/v] sediment). At a practically feasible sediment addition rate of 5%, dissolved reactive P (DRP) in percolating water from simulated rainfall decreased by 55% in SIC and by 54% in SL ( < 0.001 in both cases). Particulate P (PP) also showed a decreasing trend with increasing sediment addition rate. Upon prolonged simulated rainfall, the decreasing effect of sediment on DRP and PP declined somewhat. The effects of sediment addition can be attributed partly to increased salt concentrations in the sediment, which have a short-term effect on P mobilization, but mostly to increased concentrations of Al and Fe (hydr)oxides, increasing long-term P sorption capacity. Adding CW sediment at a rate of up to 5% of surface soil volume to soils could provide an alternative to chemical treatment (e.g., with metal salts) for immobilizing P in small, high-risk P leaching areas, such as around drinking troughs in pastures.

A Simple Dynamic Model of Soil Test Phosphorus Responses to Phosphorus Balances

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 ( = 103) or were obtained from farmers through interviews ( = 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 (NHOAc test) with acceptable accuracy and precision, with of 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.

Surface and Subsurface Phosphorus Discharge from a Clay Soil in a Nine-Year Study Comparing No-Till and Plowing

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). Concentrations and losses of PP were clearly lower under no-till than under plowing (5.6 vs. 12.3 kg PP ha), but DRP loss showed the opposite trend (4.3 vs. 1.4 kg DRP ha). 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.