Changes in Soil Organic Carbon Contents and Nitrous Oxide Emissions after Introduction of No‐Till in Pampean Agroecosystems

We reviewed published results to estimate no‐till effects on SOC and denitrification in the Argentine Pampas and the potential of no‐till to mitigate the global warming effect. On an equivalent mass basis, 42 paired data sets were used for SOC comparisons of no‐till vs. plow till (moldboard plow or disk plow), 18 paired data for comparison of no‐till vs. reduced till (chisel plow or harrow disk), and 20 paired data for comparison of plow till vs. reduced till. Twenty‐six denitrification data sets were used for evaluation of tillage system and fertilization effects on N 2 O emission. Changes in SOC under no‐till were not correlated to time since initiation of experiments. Averaged over years a 2.76 Mg ha −1 SOC increase ( P = 0.01) was observed in no‐till systems compared with tilled systems, but no differences were detected between plow and reduced till. The SOC under tillage explained most of the SOC variation under no‐till ( R 2 = 0.94, P = 0.01). The model had a positive intercept and predicted a relatively higher increase of SOC in areas of low organic matter level. The conversion of the whole pampean cropping area to no‐till would increase SOC by 74 Tg C, about twice the annual C emissions from fossil fuel consumption of Argentina. Emissions of N 2 O were greater under no‐till with a mean increase of 1 kg N ha −1 yr −1 in denitrification rate for humid pampean scenarios. The increased emissions of N 2 O might overcome the mitigation potential of no‐till due to C sequestration in about 35 yr, and therefore no‐till might produce global warming.