Nitrogen fertilization, grazing and timing of phosphorus and potassium replacement in a soybean-ryegrass production system

Abstract

The objective of this work was to investigate nitrogen balance in a soybean-ryegrass system, influenced by off-season N fertilization, sheep grazing, and timing of phosphorus and potassium replacement. A long-term four-year experiment was conducted in a randomized complete block design with four replicates. The present study was carried out in the fourth crop season, when data from three replicates were collected. The split-plot experimental design was adopted. The plots were arranged in a 2x2 factorial arrangement, consisting of two grazing levels (grazed and ungrazed) and two times of P₂O₅ and K₂O replacement (in summer or winter), whereas the subplots received different N rates (0, 50, 100, and 200 kg ha^-1 N). The analyzed variables were total dry mass yield, N accumulation in ryegrass, and soybean metrics such as biological N fixation (BNF), grain yield, and partial N balance (PNB). The highest rates of N in ryegrass increased grain yield by 14% in grazed areas. The replacement of P₂O₅ and K₂O in winter boosted grain yield by 11%. Off-season N fertilization increased N balance due to the increase in dry mass yield and N cycling, alleviating the negative PNB caused by soybean, regardless of the timing of P₂O₅ e K₂O replacement and grazing or not by sheep.