Soil CO2 emission in response to nitrogen fertilizer in Marandu grass pasture under grazing by beef cattle

Nitrogen fertilizer use is important for forage production in beef cattle production system. However, nitrogen use can affect the soil carbon pools. In this way, is important to evaluate the impact of fertilizer strategies on the soil CO₂ emissions. The aim of this study was to quantify soil CO₂ emissions in marandu palisade grass pastures (Brachiaria brizantha cv. Marandu) fertilized with 0, 90, 180 and 270 kg N ha^-1 year^-1 in Jaboticabal, SP, Brazil. The total dose was applied in three equal instalments (cycles) throughout the rainy season in 2015, and the source utilized was urea. The swards were grazed by beef cattle under continuous stocking management. The measurements of soil CO₂ emissions were performed using a flow chamber from LI-COR (LI-8100). Evaluations occurred according to fertilization cycles (intervals of 31, 42 and 30 days, respectively). Measurements were made daily in the first week after N application, and more sparsely with time, totaling ten to thirteen days of evaluation in each cycle. Data analysis were made with proc mixed from SAS, considering the effects of nitrogen level, management cycle and their interactions using orthogonal polynomial contrasts (P < 0.05). Soil CO₂ emission varied according a cubic effect as N level increased in first and third cycles (P < 0.05), but no differences were registered during the second cycle (P > 0.05). Total emissions for N levels 0, 90, 180 and 270 kg ha^-1 year^-1 were, respectively, 8.50, 9.45, 8.33 and 8.48 Mg CO₂ ha^-1, in first cycle; 8.14, 8.25, 8.55 and 8.20 Mg CO₂ ha^-1 in second cycle, and 8.20, 10.04, 7.45 and 8.49 Mg CO₂ ha^-1 in third cycle. Similar values among cycles were observed in the levels of 0 and 270 kg ha^-1 year^-1 while a quadratic and linear effect was registered, respectively for 90 and 180 kg N ha^-1 year^-1. The use of 90 kg N ha^-1 year^-1 resulted in the larger values for soil CO₂ emissions. Higher emissions are associated to higher root and microbial activities in soil, which in fact is related to CO₂ fixation by photosynthesis and to plant growth. These results shows the importance of accessing differences in soil carbon content in the long term as a way to understand the impact of N fertilization on the carbon pools and soil CO₂ emission.