Nitrogen fertilization effects on soil phosphorus dynamics under a grass-pasture system

Nitrogen (N) and phosphorus (P) are critical to pasture productivity; however, limited information is available on how the single and combined additions of N and P affect soil P fractions and seasonal changes in microbial and biochemical processes linked to P cycling under pasture systems. A two-year field trial was conducted where N (0 or 250 kg ha^-¹ yr^-¹) and P (0 or 50 kg ha^-¹ yr^-¹) were applied in a full factorial design to an intensively managed grass-pasture system. Changes in plant growth and nutrient uptake, soil microbial biomass P, soil phosphatase activities, and soil inorganic and organic P fractions were assessed by regular sampling. Phosphorus addition increased Olsen P and shoot P uptake but not shoot biomass compared to the control. In contrast, N addition decreased Olsen P by 23% but increased both shoot biomass and P uptake by 1.6-fold, compared to the control. Microbial biomass P was irresponsive to N and P additions. Phosphatase enzyme activity significantly increased in summer under N addition, which was linked to labile organic P mineralization. After two growing seasons, N addition alone significantly decreased readily-available inorganic P, labile inorganic P, moderately labile inorganic P, and labile organic P by 75, 19, 7, and 28%, respectively, compared to the control. On the other hand, combined N and P addition significantly decreased readily-available inorganic P, labile inorganic P, and labile organic P by 39, 26, and 28%, respectively, but had no impact on moderately labile inorganic P compared to P addition alone. The findings of this study revealed that short-term N fertilization to N-limited grass-pastures can accelerate P cycling by mobilizing labile inorganic and organic P as well as moderately labile inorganic P pools. However, N fertilization combined with P applications exceeding plant requirements cannot mobilize moderately labile inorganic P, which accumulates under high P sorbing soils.