Cover crop and weed biomass with their nitrogen and carbon content sampled in cover crop mixture according to mixture, irriguation and nitrogen fertilisation at sowing

This dataset contains a total of 1753 biomass samples (g dry biomass/m²) and their carbon and nitrogen content. These data were collected on a factorial experiment replicated on two field:years (one field in 2016 and another in 2017) at the INRAE U2E Bretenière experimental unit (47°14'11.2‘ N, 5°05’ 56.1’ E), CA-SYS platform, 15 km south-east of Dijon, France. Both fields were characterised by a clay (40%) - silt (50%) soil texture and a calcareous bedrock. The site is subject to a semi-continental climate, characterised by mean annual precipitation of 765 mm and an average daily temperature of 10.5°C, with strong variations between winter and summer. The present data was collected on a factorial experiment comparing four cover crop (CC) mixtures to a bare soil control, under different levels of nitrogen and water availability. Four CC mixtures (2leg-: 2 CC species without legumes, 2leg+: 2 CC species with legume, 8leg-: 8 CC species without legumes and 8leg+: 8 CC species with four legumes) including a wide range of species commonly used by French farmers (see species mix and density in the dataset: black oat, winter rye, sorghum, foxtail millet, common vetch, berseem clover, faba bean, brown hemp, brown mustard, field mustard, lacy phacelia, niger seed, buckwheat, linseed) were compared to a bare soil control. CC were sown in summer (August 10th and July 28th year 1 and 2, respectively) at their recommended pure stand seeding rate divided by the number of species in the mixture and majored by 15% (see tab in the dataset). Two levels of irrigation (0 mm or 40 mm) and nitrogen fertilisation (0N: 0 kg N ha-1 or 30N: 30 kg N ha-1 of ammonium nitrate 33.5%) were applied around CC sowing to mimic contrasted levels of soil resources. The experiment was set-up as a double entry split-plot in a randomised complete block design (3 blocks). Main plots were characterised by the intersection of the two irrigation and nitrogen fertilisation strips for practical reasons. CC mixtures and bare soil control were replicated 3 times within each main plot due to a forthcoming CC termination factor (not presented here). As a result, each combination of CC mixture (N=5), nitrogen level (N=2) and irrigation level (N=2) was replicated 9 times, resulting in a total of 180 plots (10 m² each). CC (all species combined) and weed (per species) aboveground biomass were sampled 90 days after CC sowing in each experiment, in all 180 plots, in one 1 m² and two 0.25 m² quadrats per plot in year 1 and 2, respectively. All biomass samples were oven dried at 80°C for 72 hours and weighed. The nitrogen concentration (g N / g aboveground biomass) in CC (all species summed) and weed (per species) was measured using the Dumas method (ThermoScientific™ FLASH 2000 CHNS/O Analyzer). The two quadrats were summed in year 2 so as to quantify one value of nitrogen concentration per plot for both CC (all species summed) and weeds (per species). The quantity of nitrogen in plants (g N m-1) was calculated by multiplying the nitrogen concentration (g N / g biomass) by biomass (g m-1) at the plot:year scale.