Six years of nitrous oxide emissions from temperate cropping systems under real-farm rotational management
Josette Garnier, Antoine Casquin, Benjamin Mercier, Anun Martinez, Eric Gréhan, Abdel Azougui, S Bosc, Alexandre Pomet, Gilles Billen, Bruno Mary
Abstract
Nitrous oxide (N 2 O) emissions from agricultural soil were studied during 6 consecutive years on three plots of two real farms. We analyzed a total of 37 agricultural sequences: 20 sequences with crops including wheat, maize, and legumes and 17 fallow sequences with cover crops, volunteers or bare fallow soils. Controlling factors of N 2 O emissions were disentangled by using three approaches: (i) applying a random forest algorithm then extracting the most important predictors; (ii) analyzing all sequences separately as a function of the rain and soil mineral nitrate (SMN) content in soils; and (iii) experimentally, by watering soils with different SMN contents. The results showed that rainfall (and hence soil moisture) exerted the greatest control over N 2 O emissions, followed by daily maximum temperature and SMN. Regarding crop sequences, 6-year N 2 O cumulative emissions rates (g N 2 O N ha −1 ) ranked as follows: legumes (967) < maize (1192) < cereals (1375). Lowest cumulative N 2 O emissions at the Derrier conventional, no-till, farming plot (∼ 6000 kg N ha −1 ) was observed compared to ∼ 15000 kg N 2 O N ha −1 measured at the tilled organic farming Chantemerle and Barrière plots. Further, cumulative N 2 O emissions were rather important during fallow periods with or without cover crops. This study thus supports that N 2 O emissions need to be measured all year round, including both cropping and fallow periods to grasp the full rotation cycles. This long-term study of N 2 O emissions gathered a large number of data on N 2 O emissions over a variety of crops and fallow management and helped to initiate a typology of N 2 O emissions by crop.