The time it takes to reduce soil legacy phosphorus to a tolerable level for surface waters: What we learn from a case study in the catchment of Lake Baldegg, Switzerland

The role of diffuse phosphorus (P) losses from agricultural land gained importance since technical and constructional measures in urban areas and industry have reached their potential in temperate regions. Phytoremediation strategies are a new suggestion to effectively reduce soil legacy P but until now, there is a lack of estimates on the time such strategies should take. With a rainfall-runoff model, spatial information on the hydrological risk of the catchment of Lake Baldegg (Switzerland) was generated and combined with a soil test P (CO2-saturated water extraction) map. Based on these results, two potential soil target test P values (2.0 mg P (kg soil)−1 target-P 1 and 1.6 mg P (kg soil)−1 target-P 2) were set. A simple nonlinear mixed effects model was used to compare different balance and cropping scenarios to decrease soil test P. The relationship between P-balance (input - output) and soil test P was found to be exponential. The confidence interval for the predicted time necessary to reach target-P 2 after a cessation of P-fertilization on intensively managed grassland was 2 to 9 years depending on the initial soil test P. If fertilization is completely ceased, the predicted P-decline times were longer (8 to 32 years). The decline-time for the balance that is recommended for farmers in the catchment of a P-fertilization that covers 80% of the demand was predicted to be 11 to 47 years. The study emphasizes that P phytoextraction can be an effective and time and resource efficient mitigation strategy for catchments with high legacy P.ISSN:0016-7061ISSN:1872-625

Soil quality and phosphorus status after nine years of organic and conventional farming at two input levels in the Central Highlands of Kenya

Under temperate climate conditions, organic farming systems show improved soil quality compared to non-organic systems, whereas little long-term research on the impact of organic farming on soil quality has been conducted in sub-Saharan Africa. Within the system comparison (SysCom) project, two long-term field experiments were set up in 2007 in the sub-humid Central Highlands of Kenya to compare organic and conventional farming at two input levels (high input systems with recommended rates of nitrogen (N), phosphorus (P) and pesticides and with irrigation vs. rain-fed low input systems with low N, P and pesticides), with a similar design at both sites. The two sites differ mainly in their inherent soil properties and in the amount and distribution of rainfall. At the end of each three-year crop rotation period, we analyzed a set of chemical, biological and physical soil quality parameters in 0–20 cm soil depth. After nine years, microbial parameters seemed to have reached a steady state, whereas chemical parameters were still changing. Most soil quality parameters were highest under the high input organic farming system. The high input conventional system performed well in preserving several soil quality indicators, but a trend for acidification and the lack of soil carbon build-up raise concerns about the long-term sustainability of the system. Low input organic and conventional farming systems did not improve soil quality and even showed decreasing trends in several chemical parameters. Total and available P accumulated over time, especially in both high input systems, suggesting increasing risks of losses to the environment. Pronounced site effects revealed strong interactions with pedo-climatic conditions, with soil quality under high input organic farming improving to a greater extent at the site with more favorable conditions. Besides effects on soil quality, important criteria for sustainable input levels are thus the general availability of inputs, resulting nutrient input–output budgets as well as interactions of inputs with inherent soil properties