Effects of repeated root-zone application of biochar-fertilizer mixtures in the field on vegetable crop yields, plant N use and N2O emissions

Biochar produced via pyrolysis is increasingly used in agriculture, either applied pure in larger amounts or as part of fertilizer applications. Biochar production temperatures above 500 °C result in highly persistent aromatic carbon that sequesters C with mean residence times of centuries to millennia [1]. Research over the last decade has shown that its agricultural use has mainly positive effects [2] although these are not guaranteed. Biochar is discussed as one of the few available ready-to-go negative CO₂ emission technologies with agricultural benefits; its production and use can already be certified and traded as a carbon sink [3]. However, with regard to agricultural applications, the initial research approach to use large amounts of biochar per hectare (>10 t ha^-1) is usually not sufficiently incentivised by corresponding yield increases covering the expenses for biochar [1]. Hence, recent research focuses on biochar use as a mixing component of organic or mineral fertilizers. Here, we report the results of repeated smaller amount applications (2.5 t ha^-1 per crop cycle) of biochar to the root-zone with each cropping, i.e. to the same plots over four consecutive vegetable crop production sets during 2021 to 2023. The experimental set-up was a two-factorial randomized block design field study with five blocks (plot replications). Factor 1 was the use of mineral versus organic fertilizer, factor 2 was no biochar application (control) or two differently produced biochars from woody waste materials (i.e. 6 treatments overall). Fertilizers or biochar-fertilizer mixtures were applied to the root zone at seedling planting; for the mineral fertilization, part of the total fertilizer sum was added manually as fertigation irrigation during crop growth. In 2021, spring and summer 2022, celery root, salad and broccoli were grown, while in autumn-winter 2022/2023, spinach was grown. Greenhouse gas fluxes were measured weekly for the last crop cycle to investigate long-term (partly-aged) biochar effects with the closed-chamber method and will be reported. The biochar-organic fertilizer blends never increased yields significantly for either of the crop species. However, in the beginning, organically fertilized vegetable yields were generally lower than those with mineral fertilization which reversed over the third and fourth crop cycles (broccoli and spinach) for the controls. When combined with mineral fertilizer, biochar significantly increased the yield of celery (up to 20%), of broccoli (up to 70%) and spinach (up to 54%) while the biomass of salad was slightly reduced with biochar application. Soil mineral N concentrations before or after crop plantings / harvests and plant N uptake indicated an improved N use efficiency with biochar use. Our results demonstrate that lower amounts of biochar plus repeated applications with each crop cycle may be a way forward for agriculture in particular when combined with mineral fertilization.

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• EBC Certification of the carbon sink potential of biochar. (http://European-biochar.org). Version 2.1E of 1st February 2021, 2020. 35.