ABSTRACT
The soil C, N, P and K content of agricultural soil were measured over the last 4 decades in NE Catalonia (NE Spain). Plant-available P and K increased by ca 109 and 105% respectively and total N decreased by 30%. The increases in plant-available P content are in accordance with the increasingly used pig slurry being very rich in P, and with P tendency to be retained in soils, since it is less mobile than N. The total soil N (Ntot) decrease occurred in the first decade (by 41%). The uptake and withdrawal of mineral N by crops and the leaching of mineral N into groundwater and rivers after torrential rainfalls were the two likely major pathways of N-loss from the soil. After the first decade, there has been no further decrease of Ntot as a result of the increasing fertilization of these fields, including the increasing applications of pig slurry. These results show an increasing P eutrophication in Mediterranean agricultural soils and will have several consequences for the next decades with (i) an increasing unbalance between N and P (and K) in soils that might affect crop productivity, (ii) an increasing leaching of N as nitrate to continental waters, both ground and surface waters, and (iii) a consequent need for the establishment of another fertilization strategy based on lowering the use of pig slurry and on increasing the use of fertilizers of slow mineralization that increase soil organic matter, and stabilise the soil N and P contents.