Relationships between change in total P stock of the plough layer and P balance after 15 years of composts applications
Résumé
Understanding soil phosphorus (P) changes under continuous cropping over decades is an important agronomic and environmental issue. The objective of this study was to examine changes in total P stock of the plough soil layer of a cultivated ecosystem in relation to P balance. The long-term field experiment Qualiagro (INRA – Veolia Environnement) was established at Feucherolles, France, in 1998. Five treatments were designed and replicated in four blocks. Farmyard manure (FYM) and three types of composts (Municipal Solid Waste (MSW), Biowastes (BIOW) and Green Waste + Sewage Sludges (GWS)) were applied every two years at a rate of 4 t C ha-1. A control received no organic amendment. Changes of total P stock in the plough layer (0-28 cm) were compared to cumulated soil P balance since 1998, calculated as the difference between cumulated P input, as supplied through organic amendment and at seedling, and cumulated P output, by harvest of grains and crop residues if removed. Total soil P content and bulk soil density were determined at five soil sampling dates, crop yield and P content were measured annually. Cumulated P input for the 1998-2013 period ranged from 340 to 1695 kg P ha-1 for MSW and GWS respectively. Cumulated P export by crops was influenced by treatments. Soil bulk density increased less in amended soils than in control. A linear relationship between total soil P stock variation and soil P balance was found, close to the 1:1 line. These results confirm that P outputs through erosion, runoff and leaching were negligible. Contribution of subsoil to crop nutrition was estimated to be close to 0.5-1 kg P ha-1.