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

Abstract

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.

A robust DayCent model calibration to assess the potential impact of integrated soil fertility management on maize yields, soil carbon stocks and greenhouse gas emissions in Kenya

“…In our study, the low Sobol total sensitivity indices of the maize productivity parameters for the SOC stocks suggest a limited importance of root input in the soil system for the storage of SOC. These results align with a statistical analysis of SOC stocks across depth at the same experimental sites (Laub et al, 2022), which found limited differences between cropped and bare plots, and with another recent tropical long-term experiment (Cardinael et al, 2022), which estimated that less than 1 t of carbon input ha -1 per year came from maize roots.…”

“…For data used in model calibration and evaluation, the site-specific variance for each type of measurement was used as a measure of uncertainty of the measured data (specifically, the median variance per site and measurement type computed from the three replicates from each time point of each treatment). Site-specific variances were used because a statistical analysis of the data in earlier work had shown that variance heterogeneity existed only between sites, but not between treatments at the same site (Laub et al, 2022(Laub et al, , 2023.…”

“…In order to use DayCent to assess the potential of ISFM to contribute to closing the yield gap while minimizing environmental impact in Kenya and other SSA countries, the aim of this study was to use Bayesian calibration to derive robust DayCent parameters of SOM cycling and maize growth in Kenya. We used the experimental data of four long-term ISFM trials conducted in Kenya for almost two decades (Laub et al, 2022(Laub et al, , 2023. Of these, two sites were in humid western Kenya and two in subhumid to semi-arid central Kenya.…”

“…The goal was to evaluate to what extent DayCent can replicate the differences in yields and SOM development in response to different qualities and rates of organic resources combined with different rates of N fertilizer and between sites. Furthermore, because ISFM can simultaneously be a source of N 2 O to the atmosphere (Leitner et al, 2020) and mitigate CO 2 emissions from the soil (Laub et al, 2022), the second objective was to evaluate the total global warming potential of different rates of organic material in ISFM to find the optimal balance between limiting greenhouse gas emissions from the soil and optimizing the yield (that is, sustainable intensification).…”

Combining manure with mineral N fertilizer maintains maize yields: Evidence from four long-term experiments in Kenya

“…Recent results from another long-term experiment in western Kenya also showed that it is nearly impossible to maintain initial levels of SOM, even with high input rates of organic resources (Sommer et al, 2018;Nyawira et al, 2021;Kihara et al, 2020), probably because soils at the sites in the region were still fairly close to their high natural SOM levels at the onset of the experiments. In the same experiments than those of this study, it was further found that, except in Aludeka, the addition of Calliandra and Tithonia residues, even at high rates, did not counterbalance soil carbon losses (Laub et al, 2022), so their main benefit might be to supply N to crops. The absence of significant differences in maize yields between the CC4-N, TD4-N and CT+N treatments across sites towards year 18 is an indication of this beneficial N effect, as all these treatments supply similar amounts of N (about 300 kg N ha -1 yr -1 ).…”