Dynamics of NPK nutrition and production of temporary grasslands in rotation with annual crops: lessons from 15 years of monitoring on the Lusignan observatory system

摘要

Over the last twenty years, we have observed a decrease in mineral fertilization practices grasslands on a national scale, particularly in P and especially in K. These decreases in mineral fertilization can lead to a risk of decreased production and loss of sustainability in a certain number of field situations. Diagnostic tools, based on grass analysis (N, P and K nutrition indices) or soil analysis, have been proposed for a number of years. The relevance of these tools is illustrated here on the ACBB long-term experimental set-up in Lusignan. This system explores the biogeochemical cycles of temporary grasslands and grassland-annual crop rotations under different management modes: duration of the grassland in rotation, mowing or grazing, level of N fertilization. The results obtained over 15 years illustrate that while P nutrition is maintained at a sufficient level, K nutrition can become limiting, leading to a decrease in production and sustainability. The results also show that P-K nutrition constraints are very much linked to the mineral balance in the field. These mineral balances are extremely variable in grassland depending on the management methods applied. Thus, the decrease in soil K fertility for vegetation is most rapid and strongest in mowed and N-fertilized grassland systems, intermediate in short grassland-crop rotations, and least rapid and weakest in annual rotation systems, in grassland with little N fertilization and in grazed grassland. During the transition phases from grassland to annual crop, the nutrition indices show singular behaviors. The relevance and limitations of vegetation and soil-based nutrition indicators used on grassland and annual crops are discussed.