Nutrient recycling in organic farming is related to diversity in farm types at the local level

Summary

Many authors have focused on the contribution of inner farm nutrient recycling to closed nutrient cycles, but little is known about the contribution of exchanges among farms to nutrient cycling. By using a network approach, we assessed the structure of farm mass flows networks and their consequences for nitrogen (N), phosphorus (P) and potassium (K) recycling in organic farming at the local scale. Organic farming was considered as a prototype of farming systems that aims at better closing nutrient cycles. Inflows and outflows were collected for two cropping years on 63 organic farms. The farms were located in three French agricultural districts with areas ranging from 520 to 1021 km2 and differentiated by their degree of specialization (specialized in crop production, animal production, or mixed). Local nutrient recycling was assessed at the district scale through: (i) the local supply, defined as the ratio of the amount of nutrients from exchanges among farms plus the amount of nitrogen from atmospheric sources, to the sum of inputs to organic farms; and (ii) the cycling index, defined as the fraction of nutrients flowing at least twice through the same farms. Results showed that exchanges among farms were mainly short-distance exchanges (<50 km on average) and contributed up to 70% of farm nutrient inflow. Mass flows among farms were two to four times greater in the mixed district than in specialized districts. As a consequence, both the local supply and the cycling index were greater in the mixed district than in the specialized districts. However, even if the local supply was generally high (85%, 52% and 54% for N, P and K inflows in the mixed district, respectively), the cycling index remained low (5%, 20% and 10% for N, P and K in the mixed district), indicating that most flows among farms were one-way and not actual nutrient recycling. This study contributes to the understanding of the magnitude, conditions and factors of nutrient recycling in agriculture at the local scale.