Managing Soil Organic Carbon in Tropical Agroecosystems: Evidence from Four Long-Term Experiments in Kenya
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Date
2023
Authors
Laub, Moritz
Corbeels, Marc
Couëdel, Antoine
Ndungu, Samuel Mathu
Mucheru-Muna, Monicah Wanjiku
Mugendi, Daniel
Necpalova, Magdalena
Waswa, Wycliffe
de Broek, Marijn Van
Vanlauwe, Bernard
Journal Title
Journal ISSN
Volume Title
Publisher
Copernicus Publications for EGU
Abstract
In sub-Saharan Africa, maize is one of the most important staple crops, but 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 nitrogen (N) on the change of soil organic carbon (SOC) contents over time (and SOC
stocks in the year 2021) in four ongoing long-term experiments in Kenya. These experiments were established
with identical treatments in moist to dry climates, on coarse to clayey soil textures, and have been conducted
for at least 16 years. They received organic resources in quantities equivalent to 1.2 and 4 t C ha−1 yr−1
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 of 240 kg N ha−1 yr−1
as mineral N
fertilizer or no fertilizer was the split-plot treatment. At all four sites, a loss of SOC was predominantly observed,
likely because the sites had been converted to cropland only a few decades before the start of the experiments.
Across sites, the average decline of SOC content over 19 years in the 0 to 15 cm topsoil layer ranged from
42 % to 13 % of the initial SOC content 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 contents to about 24 % of
initial SOC, while the addition of sawdust, maize stover (in three of the four sites) and sole mineral N addition
showed no significant reduction of SOC loss over the control. Site-specific analyses, however, did show that at
the site with the lowest initial SOC content (about 6 g kg−1
), the addition of 4 t C ha−1 yr−1
farmyard manure
or Calliandra with mineral N led to a gain in SOC contents. The 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 three of the four sites), the topsoil SOC stocks corroborated the results obtained from the SOC
content measurements (0–15 cm) over time. The relative annual change of SOC contents showed a higher site
specificity in farmyard manure, Calliandra and Tithonia treatments than in the control treatment, suggesting that the drivers of site specificity in SOC buildup (soil mineralogy, soil texture, climate) need to be better understood
for effective targeting management of organic resources. Farmyard manure showed the highest potential for
reducing SOC losses, but the necessary quantities to build SOC are often not realistic for smallholder farmers in
Africa. Therefore, additional agronomic interventions such as intercropping, crop rotations or the cultivation of
crops with extended root systems are necessary to maintain or increase SOC.
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Citation
Laub, M., Corbeels, M., Couëdel, A., Ndungu, S. M., Mucheru-Muna, M. W., Mugendi, D., ... & Six, J. (2023). Managing soil organic carbon in tropical agroecosystems: evidence from four long-term experiments in Kenya. Soil, 9(1), 301-323.