Fate of phosphorus applied in slurry and mineral fertilizer: accumulation in soil and release into surface runoff water

Phosphorus (P) accumulation on the soil surface and its effect on the concentration of dissolved orthophosphate P (PO₄-P) in surface runoff water were studied after three years of surface application of slurry and mineral fertilizer to grass ley on a sandy soil, poor in P. The total amount of P applied was 107–143 kg ha^–1>, of which 72–119 kg ha^–1> was applied on the soil surface during two or three years without incorporation or mixing. The addition of slurry and mineral fertilizer resulted in an increase in inorganic P in the 0–5 cm but not the 5–25 cm soil layer, but organic P was not affected. The measured changes in inorganic P deviated only by 4–6 kg ha^–1> from the values derived from inputs and outputs of P (crop uptake + losses in surface runoff and drainage water). The increase in inorganic-P was accompanied by increases in the degree of P saturation (DPS) and in P extracted with acid am monium acetate (P_Ac ), sodium bicarbonate (P_Olsen) and anion-exchange resin (P_Resin). In surface runoff, 10–18 months after the last surface application of P, the mean flow-weighted concentration of PO₄-P was linearly increased with the values of DPS, P_Ac, P_Olson and P_Resin in the 0–5 cm soil layer. PO₄-P was lowest (0.033 mg l^–1> ) in the control plots and highest (0.62 mg l^–1>) in the plot where 143 kg ha^–1> P had been applied in slurry and fertilizer. On that plot, the corresponding values of DPS, P_Ac, P_Olson and P_Resin were 16%, 13 mg kg^–1>, 85 mg kg^–1> and 71 mg kg^–1 , even within a few years, and multiply the P loading to surface runoff from the site. A very shallow soil sampling (< 5 cm) is needed to assess P loading potential in a soil where P has been surface-applied.     

Phosphorus and nitrogen cycles in the vegetation of differently managed buffer zones

Vegetated buffer zones (BZs) between a cultivated field and a watercourse reduce erosion and load of particle-bound phosphorus (P), but decay of abundant vegetation increases the potential of BZs to act as a source of readily algal-available P. To quantify temporal variations in P and nitrogen (N) contents of the grassy vegetation of BZs on a clay soil (Vertic Cambisol) in south-western Finland, plant samples were collected six times between May 2005 and April 2006 from natural BZs, BZs grazed by cattle and BZs harvested by cutting and removal of the yield. The total dry weight biomass peaked in early August at 2,130–2,360 and 5,500–6,270 kg ha⁻¹ for the grazed and the other BZs, respectively. In August, 3,840–4,830 kg ha⁻¹ were removed from the harvested BZs while the entire biomass of the non-harvested BZs remained in the field. In October, total P and N contents varied from 2.4–10.2 to 19–72 kg ha⁻¹, respectively, the lowest amounts being for the young harvested BZ and the highest for the non-harvested BZs. A considerable decrease of P and N contents occurred in the biomass up to 6.1 and almost 30 kg ha⁻¹, respectively, after the first frosts. Harvesting of BZs is recommended to decrease the amount of P and N in the BZs and reduce the risk of P and N leaching outside the growing season.     

Selenium fractions in selenate-fertilized field soils of Finland

Depending on the soil environment, selenium (Se) can exist as several species differing greatly in bioavailability. Characterization of soil Se reserves is thus necessary in assessing the nutritional supply of this essential element. In low-Se areas, Se fertilization is an option for securing adequate Se nutrition. Fertilization is, however, challenged by the unknown fate of the residual Se. In this study, we aimed to clarify the Se status of selenate-fertilized field soils by fractionating soil Se into five pools: salt-soluble (KCl), adsorbed (KH₂PO₄/K₂HPO₄), organically associated (NaOH), elemental (Na₂SO₃) and recalcitrant Se (NaOCl). Changes induced in these fractions by repeated application of low selenate doses were examined by comparing samples collected from the same locations in 1992 and 2004. The distribution of Se among the five fractions was relatively similar in all soils. On average, 1% of the amount of Se acquired was salt-soluble, 17% adsorbed, 39% organically associated, 14% elemental and 29% recalcitrant organic Se or metal selenides. The added Se was distributed among several fractions, leading to a relatively small change in each fraction. However, in mineral soils (n = 5), a significant increase in soil Se concentration was found between 1992 and 2004 in adsorbed, organically associated and recalcitrant Se, as well as in the total amount of Se acquired as a sum of the fractions. Our findings provide the first field-scale experimental support for the theoretical assumption of accumulation of residual selenate in acidic mineral soil in insoluble form.