Soil phosphorus (P) mining in agriculture – Impacts on P availability, crop yields and soil organic carbon stocks

Soil phosphorus (P) leaching is one of the major causes of diffuse P losses towards the environment in north-west Europe. In order to cut back these P losses over the long term, soil P stocks need to be drastically reduced. Greenhouse experiments have shown that a rapid reduction of the plant available P pool is attained by reducing the P fertilizer input to zero while cropping, but field trials confirming these findings are rare. Moreover, important additional questions arise in any P mining strategy: (i) to what extent will crop performance be affected in high P soils under 0 P fertilization, (ii) which crops reduce soil P stocks most efficiently and (iii) how can soil organic carbon (SOC) levels be maintained when organic fertilizers are no longer applied. In order to address these questions we set up two 0 P fertilization field trials at sites with a very high initial soil P status (i.e. a P saturation degree > 30%). On each site, we compared the effect of 0 P fertilization on crop yield, soil P status and SOC level to a business as usual scenario, for two crop rotations. At the first site, a comparison was made between a mixed crop rotation with or without the inclusion of a catch crop. At the second site, a comparison was made between an arable/grass and a vegetable crop rotation. Four consecutive years of 0 P fertilization had no effect on neither the crop yield nor the crop P uptake. Further, English ryegrass, silage maize, celeriac and Chinese cabbage made the largest contribution to P removal. Although 0 P fertilization reduced the P balance at field level with 102–121 kg P ha⁻¹ over four years, this had no significant (measurable) effect yet on the soil P stocks. Continuation of these experiments over a longer time will thus be needed to allow to see any such effects. Simulations of the evolution of the SOC levels over a thirty-year period showed that the use of grass as green manure or as main crop only allows to partially maintain the SOC levels in the absence of any organic fertilizers. These results indicate that P mining is a challenging task, that will take a long time to reduce the soil P stocks and that will come at a cost for the farmer.