Effects of Condensed Tannins on Greenhouse Gas Emissions and Nitrogen Dynamics from Urine-Treated Grassland Soil

Condensed tannins are a potentially important treatment option to mitigate N 2 O (nitrous oxide) and affect carbon dioxide (CO 2 ) and methane (CH 4 ) emissions; however, their effect has been poorly assessed. Here, we quantified the emissions of N 2 O, CH 4 , and CO 2 , soil N mineralization, and nitrification with increasing doses of condensed tannins added to the urine of cattle raised on pasture. The experiment consisted of incubation with doses of 0%, 0.5%, and 1.0% of condensed tannins added directly to the collected urine. The experimental design was completely randomized. Greenhouse gas fluxes were quantified for four weeks using static chambers and gas chromatography. The addition of condensed tannins increased N 2 O emissions ( P < 0.05), with total emissions averaging 95.84 mg N-N 2 O kg −1 , 265.30 mg N-N 2 O kg −1 , and 199.32 mg N-N 2 O kg −1 dry soil in the treatments with 0%, 0.5%, and 1% tannins, respectively. Methane emissions were reduced with the addition of tannins ( P < 0.05), with total emissions of 8.84 g CH 4 kg −1 , 1.87 g CH 4 kg −1 , and 3.34 g CH 4 kg −1 dry soil in the treatments with 0%, 0.5%, and 1% tannins, respectively. Soil respiration increased with the addition of condensed tannins ( P < 0.05), with total emissions of 3.80 g CO 2 kg −1 , 6.93 g CO 2 kg −1 , and 5.87 g CO 2 kg −1 in dry soil, in the treatments with 0%, 0.5%, and 1% tannins, respectively. The addition of condensed tannins reduced N mineralization and nitrification. We found evidence that the use of condensed tannins might not be a suitable option to mitigate N 2 O emissions. However, soil CH 4 emissions can be abated. The increases in soil respiration suggest that tannins affect soil microorganisms, and the effects on CH 4 and N 2 O could be related to the variation in the soil microbiome, which requires further clarification.