Higher flexibility in input N:P ratios results in more balanced phosphorus budgets in two long-term experimental agroecosystems

Inefficient phosphorus (P) use in intensive agriculture is common in both organic and conventional systems, resulting in P over-application and soil P build-up. Increasing crop P removal and P recycling within farming systems (e.g., via cover crops) and reducing P inputs lower P surpluses, resulting in more balanced P budgets. Lowering P inputs to reduce soil P surpluses is easier with mineral fertilizers for which nitrogen (N) and P inputs can be decoupled, whereas reducing inputs of organic amendments with a constrained N:P stoichiometry (manures, composts) often results in N under-fertilization and lower yields. We computed farm-gate P budgets for several vegetable and grain cropping systems in two long-term California agricultural experiments that vary in terms of inputs (mineral fertilizers, organic fertilizers, manure, yard compost), cash crops (corn, wheat, tomato, broccoli, lettuce), cover crops (type, frequency) and cropping intensity (biennially, annually or biannually). In organic systems, using manure or compost resulted in high P surpluses, whereas using pelleted or liquid organic fertilizers with higher N:P ratios resulted in smaller P surpluses. Systems receiving mineral fertilizers were often very close to P balance when fertilized regularly. Grain rotations generally had small P deficits whereas vegetable rotations had P surpluses due to lower crop P removal and higher output N:P in vegetables. Phosphorus uptake by cover crops was important (12–25kg Pha−1), but their benefits to soil fertility will depend on the magnitude and timing of P release during residue decomposition. Overall, using organic nutrient sources with a constrained stoichiometry and low N:P ratios resulted in significant P surpluses, confirming the need to use complementary N sources such as N-fixation or N-rich fertilizers to balance P budgets.