Browsing by Author "Broek, Marijn Van de"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Depth effects of long-term organic residue application on soil organic carbon stocks in central Kenya
    (UoEm, 2025-06-17) Müller, Claude Raoul; Six, Johan; i Njiru, Daniel Mugend; Vanlauwe, Bernard; Broek, Marijn Van de;
    In arable soils, a substantial portion of soil organic carbon (SOC) is stored below the plough layer. To develop sustainable soil management strategies, it is important to assess how they affect the quantity of SOC stored in the subsoil. Therefore, we investigated the impact of organic and inorganic nutrient inputs on SOC stocks down to 70 cm depth in a long-term field trial in Embu, Kenya. There were three organic input treatments (manure, Tithonia diversifolia residues, and maize stover) and a control treatment, each with and without the application of mineral nitrogen. These different treatments were applied to a maize monoculture over 38 growing seasons (19 years). Our results show that manure application had the largest positive impact on SOC stocks compared to the control; this effect was observed down to 60 cm depth. In contrast, Tithonia diversifolia and maize stover application led to significantly larger SOC stocks compared to the control, although this was only within the top 20 cm and 40 cm, respectively. Among the three organic residue treatments, only the application of manure had a significant effect on the SOC stock of the subsoil (i.e. the 30–70 cm depth layer). However, when considering the whole measured profile (i.e. 0–70 cm), all treatments led to significantly higher SOC stocks compared to the 91 ± 12 t C ha−1 of the control: manure had the highest stocks (120 ± 24 t C ha−1), followed by maize stover (112 ± 17 t C ha−1) and Tithonia diversifolia (105 ± 11 t C ha−1). Mineral nitrogen application did not have a significant impact on SOC stocks down to 70 cm depth. Our findings demonstrate that SOC in the subsoil comprised 48.5 % ± 1.7 % of the total SOC stocks across the 0–70 cm soil profile; however, only manure application affected subsoil OC levels, whereas other organic amendments solely increased SOC in the surface layer. Our results imply that gathering knowledge on the soil below the typically studied 0–30 cm depth layer will improve the overall assessment of agroecosystem properties, which is necessary to optimize soil system resilience, limit organic matter losses, and improve crop productivity.
  • No Thumbnail Available
    Item
    Modeling integrated soil fertility management for maize production in Kenya using a Bayesian calibration of the DayCent model.
    (UoEm, 2024-07-04) Laub, Moritz; Wanjiku Mucheru-Muna, Monicah; Necpalova, Magdalena; Broek, Marijn Van de; Corbeels, Marc; Mathu Ndungu, Samuel; Mugendi, Daniel; Yegon, Rebecca; Waswa, Wycliffe; Vanlauwe, Bernard; Six, Johan
    Sustainable intensification schemes such as integrated soil fertility management (ISFM) are a proposed strategy to close yield gaps, increase soil fertility, and achieve food security in sub-Saharan Africa. Biogeochemical models such as DayCent can assess their potential at larger scales, but these models need to be calibrated to new environments and rigorously tested for accuracy. Here, we present a Bayesian calibration of DayCent, using data from four long-term field experiments in Kenya in a leave-one-site-out cross-validation approach. The experimental treatments consisted of the addition of low- to high-quality organic resources, with and without mineral nitrogen fertilizer. We assessed the potential of DayCent to accurately simulate the key elements of sustainable intensification, including (1) yield, (2) the changes in soil organic carbon (SOC), and (3) the greenhouse gas (GHG) balance of CO2 and N2O combined. Compared to the initial parameters, the cross-validation showed improved DayCent simulations of maize grain yield (with the Nash–Sutcliffe model efficiency (EF) increasing from 0.36 to 0.50) and of SOC stock changes (with EF increasing from 0.36 to 0.55). The simulations of maize yield and those of SOC stock changes also improved by site (with site-specific EF ranging between 0.15 and 0.38 for maize yield and between −0.9 and 0.58 for SOC stock changes). The four cross-validation-derived posterior parameter distributions (leaving out one site each) were similar in all but one parameter. Together with the model performance for the different sites in cross-validation, this indicated the robustness of the DayCent model parameterization and its reliability for the conditions in Kenya. While DayCent poorly reproduced daily N2O emissions (with EF ranging between −0.44 and −0.03 by site), cumulative seasonal N2O emissions were simulated more accurately (EF ranging between 0.06 and 0.69 by site). The simulated yield-scaled GHG balance was highest in control treatments without N addition (between 0.8 and 1.8 kg CO2 equivalent per kg grain yield across sites) and was about 30 % to 40 % lower in the treatment that combined the application of mineral N and of manure at a rate of 1.2 t C ha−1 yr−1. In conclusion, our results indicate that DayCent is well suited for estimating the impact of ISFM on maize yield and SOC changes. They also indicate that the trade-off between maize yield and GHG balance is stronger in low-fertility sites and that preventing SOC losses, while difficult to achieve through the addition of external organic resources, is a priority for the sustainable intensification of maize production in Kenya.