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.

中文翻译：

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优质有机资源最有效地稳定土壤有机碳：来自肯尼亚四次长期试验的证据

摘要。在撒哈拉以南非洲，外部投入少的长期玉米种植与土壤肥力的丧失有关。虽然已经提出添加优质有机资源与矿物肥料相结合来抵消这种肥力损失，但长期有效性和与场地特性的相互作用仍然需要更多的了解。本研究使用随时间推移的重复测量来评估不同数量和质量的有机资源添加与矿物 N 相结合对土壤有机碳浓度 (SOC) 随时间变化（以及 2021 年的 SOC 存量）的影响，持续了四个长期肯尼亚的长期试验。这些试验是在潮湿到干燥的气候、粗糙到粘性的土壤质地上采用相同的处理方法建立的，并且已经进行了至少 16 年。⁻¹每年以Tithonia diversifolia（高质量，快速周转），Calliandra calothyrsus（高质量，中等周转），玉米秸秆（低质量，快速周转），锯末（低质量，周转慢）和当地农场的形式肥料（可变质量，中间周转）。此外，添加或不添加 240 kg N ha ^-1每年作为矿质氮肥进行裂区处理。在所有地点，由于最近从永久性植被转变为农业，主要观察到 SOC 的损失而不是增加。^{对于 4 t C ha -1} yr ^-1的对照和农家肥处理，0 至 15 cm 深度的 SOC 浓度在 19 年内的平均降低范围分别为初始 SOC 浓度的 42% 至 13% 。在 4 t C ha ⁻¹ yr ⁻¹添加Calliandra或Tithonia将 SOC 浓度的损失限制在初始 SOC 的 24% 左右，而添加锯末、玉米秸秆（在 4 个地点中的 3 个）和添加单一矿物 N，表明 SOC 损失与对照相比没有显着减少。然而，特定地点的分析确实表明，在初始 SOC 浓度最低（约 6 g kg ^-1）的地点，添加 4 t C ha ^-1 yr ^-1农家肥料或Calliandra加上矿物质 N 导致 SOC 浓度增加。所有其他站点在所有处理中都失去了 SOC，尽管是在站点特定的速率下。虽然 2021 年的底土 SOC 储量几乎没有受到有机资源增加的影响（4 个地点中有 3 个没有差异），但表层土壤 SOC 储量证实了 SOC 浓度的结果。SOC 浓度的相对年度变化表明，高质量有机资源处理的地点特异性高于对照，这表明需要更好地了解 SOC 积累（矿物学、气候）中地点特异性的驱动因素，以有效瞄准有机资源. 尽管农家肥显示出减少 SOC 损失的最大潜力，但我们的结果清楚地表明，以对小规模农民来说现实的有机资源率，仅靠外部投入维持 SOC 是不可能的。