Targeting phosphorus loss with carbon farming practices? Results from an on-farm study.

Agricultural soils lose P to waterbodies and C to the atmosphere. Reversing the C trend requires change in management (carbon farming), but what is the effect of carbon farming practices on P transport from soils to waterbodies? This was empirically studied by analyzing the P loss risks from 20 farms participating in a 5-year on-farm carbon farming experiment. We evaluated the effect of C farming practices on soil P balance, P stocks, and potential P loss through surface runoff, subsurface drainage, and erosion. We integrated data from fertilizer application, yield, soil test P, site hydrology, and erosion into P loss estimation tools (P indices and annual phosphorus loss estimator model). Based on the results, carbon farming (cover crops, ley farming, improved grazing, soil amendments, and subsoiling) had only minor impacts on P loss compared with the current soil conservation practices (minimum tillage, vegetated buffers, and crop rotation) already applied by the farms. P fertilization was not adjusted in response to P availability, therefore resulting in weak P balances. Furthermore, only a small fraction of the field area (18%) was responsible for the majority (50%) of the estimated P loss, indicating the importance of P loss hotspots. C farming practices do not seem to improve water quality unless they are tailored to target key processes of P loss such as maintaining a negative P balance on high-P sites, reducing runoff, and focusing on local critical source areas.