16 Dec 2022
16 Dec 2022
Status: this preprint is open for discussion.

High quality organic resources are most efficient in stabilizing soil organic carbon: Evidence from four long-term experiments in Kenya

Moritz Laub1, Marc Corbeels2,3, Antoine Couëdel2, Samuel Mathu Ndungu3, Monicah Wanjiku Mucheru-Muna4, Daniel Mugendi5, Magdalena Necpalova1,6, Wycliffe Waswa3, Marijn van de Broek1, Bernard Vanlauwe3, and Johan Six1 Moritz Laub et al.
  • 1Department of Environmental Systems Science, ETH Zurich, 8092 Zürich, Switzerland
  • 2CIRAD, Avenue d’Agropolis, F-34398 Montpellier, France
  • 3International Institute of Tropical Agriculture (IITA), c/o ICIPE Compound, P. O. Box 30772-00100, Nairobi, Kenya
  • 4Department of Environmental Sciences and Education, Kenyatta University, P.O. Box 43844-00100, Nairobi, Kenya
  • 5Department of Land and Water Management, University of Embu, P.O. Box 6-60100, Embu, Kenya
  • 6University College Dublin, School of Agriculture and Food Science, Dublin, Ireland

Abstract. In sub-Saharan Africa, long-term maize cropping with low external inputs has been associated with the loss of soil fertility. While adding high-quality organic resources combined with mineral fertilizer has been proposed to counteract this fertility loss, the long-term effectiveness and interactions with site properties still require more understanding. This study used repeated measurements over time to assess the effect of different quantities and qualities of organic resource addition combined with mineral N on the change of soil organic carbon concentrations (SOC) over time (and SOC stocks in the year 2021) in four ongoing long-term trials in Kenya. These trials were established with identical treatments in moist to dry climates, on coarse to clayey soil textures, and have been managed for at least 16 years. They received organic resources in quantities equivalent to 1.2 and 4 t C ha−1 per year in the form of Tithonia diversifolia (high quality, fast turnover), Calliandra calothyrsus (high quality, intermediate turnover), Zea mays stover (low quality, fast turnover), sawdust (low quality, slow turnover) and local farmyard manure (variable quality, intermediate turnover). Furthermore, the addition or absence of 240 kg N ha−1 per year as mineral N fertilizer was the split-plot treatment. At all sites, a loss of SOC, rather than gain, was predominantly observed due to a recent conversion from permanent vegetation to agriculture. The average reduction of SOC concentration over 19 years in the 0 to 15 cm depth ranged from 42 % to 13 % of the initial SOC concentration for the control and the farmyard manure treatments at 4 t C ha−1 yr−1, respectively. Adding Calliandra or Tithonia at 4 t C ha−1 yr−1 limited the loss of SOC concentrations to about 24 % of initial SOC, while the addition of saw dust, maize stover (in 3 of 4 sites) and sole mineral N addition, showed no significant reduction in SOC loss over the control. Site specific analyses, however, did show, that at the site with the lowest initial SOC concentration (about 6 g kg−1), the addition of 4 t C ha−1 yr−1 farmyard manure or Calliandra plus mineral N led to a gain in SOC concentrations. All other sites lost SOC in all treatments, albeit at site specific rates. While subsoil SOC stocks in 2021 were little affected by organic resource additions (no difference in 3 of 4 sites), the topsoil SOC stocks corroborated the results for SOC concentrations. The relative annual change of SOC concentrations showed a higher site specificity in high-quality organic resource treatments than in the control, suggesting that the drivers of site specificity in SOC buildup (mineralogy, climate) need to be better understood for effective targeting of organic resources. Even though farmyard manure showed the most potential for reducing SOC loss, our results clearly show that maintaining SOC with external inputs only is not possible at organic resource rates that are realistic for small scale farmers. Thus, additional agronomic interventions such as intercropping, crop rotations or strong rooting crops may be necessary to maintain or increase SOC.

Moritz Laub et al.

Status: open (until 27 Jan 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1416', Anonymous Referee #1, 15 Jan 2023 reply
  • RC2: 'Comment on egusphere-2022-1416', Anonymous Referee #2, 17 Jan 2023 reply

Moritz Laub et al.

Moritz Laub et al.


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Short summary
In sub-Saharan Africa, long-term low-input maize cropping threatens soil fertility. We studied how adding different-quality organic resources combined with mineral N fertilizer could counteract this. Of these, incorporating farmyard manure was most effective counteract soil carbon loss, mineral N fertilizer did not benefit soil carbon. Yet, only very high additions would completely offset soil carbon losses, so additional agronomic measures (e.g., crop rotations, intercropping) may be needed.