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A field experiment was established in 1978 on a loam soil (pH in CaCl2 7.1) to monitor gradual changes in the soil P status as response to different P fertilization regimes. For 18 years, cereals or grass were cultivated without P fertilization (P0) or with annual P application of 35 kg ha-1 (P1) or 70-79 kg P ha-1 and 71-83 kg K ha-1 (P2K). The effects of the treatments on the crop yield varied yearly. The Chang and Jackson fractionation analysis revealed that fertilizer P not taken up by the plant crops was mostly in the NH4F extract and to a lesser extent in the NaOH extract. The NH4F-extractable P proved also to be the main P source for plants. However, the changes in the reserves of inorganic and organic P did not agree very well with the calculated P balance in soil (applied P minus plant P uptake). This disproportion was partly explained by the soil movement from plots to the neighbouring ones during the experiment. Phosphorus extractable in acid ammonium acetate or water decreased gradually when no P was applied and increased with increasing P accumulation. The changes in the inorganic P reserves due to different P fertilization history were reflected a little more sensitively in the water extraction test than in the acid acetate test.

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Dissolved organic carbon (DOC) load in discharges from cultivated soils may have negative impacts on surface waters. The magnitude of the load may vary according to soil properties or agricultural management practices. This study quantifies the DOC load of cultivated mineral soils and investigates whether the load is affected by agricultural practices. Discharge volumes and concentrations of DOC and dissolved organic nitrogen (DON) were continually measured at three sites from surface runoff and artificial subsurface drainage or from combined total discharge over a two-year period (2012-2014). Two experimental sites in South-West Finland had clayey soils (with soil carbon contents of 2.7-5.9% in the topmost soil layer), and the third site in West-Central Finland had sandy soil (soil carbon contents of 4.3-6.2%). Permanent grassland, organic manure application, mineral fertilization, and conventional ploughing or no-till activities were studied. Furthermore, the biodegradable DOC pool of surface runoff and subsurface drainage water from no-till and ploughed fields was estimated in a 2-month incubation experiment with natural bacterial communities collected from the Baltic Sea seawater. The annual DOC and DON loads were affected by discharge volume and seasonal weather conditions. The loads varied between 25-52kgha and 0.8-3.2kgha, respectively, and were comparable to those from boreal forests with similar soil types. The DOC load increased with increasing topsoil carbon content at all sites. There were slightly higher DOC concentrations and DOC load from permanent grassland, but otherwise we could not distinguish any clear management-induced differences in the total DOC loads. While only 6-17% of the DOC in discharge water was biologically degraded during the 2-month incubation, the proportion of biodegradable (labile) DOC in surface runoff appeared to increase when soil was ploughed compared to no-till.

In this study, the main changes in the flows of nitrogen (N) and phosphorus (P) in the Finnish agricultural and forest sectors during the 20th century were identified and quantified and their future trends were considered.

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. Core Ideas 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%.

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