Soil geochemistry – and not topography – as a major driver of carbon allocation, stocks, and dynamics in forests and soils of African tropical montane ecosystems

The lack of field-based data in the tropics limits our mechanistic understanding of the drivers of net primary productivity (NPP) and allocation. Specifically, the role of local edaphic factors - such as soil parent material and topography controlling soil fertility as well as water and nutrient fluxes - remains unclear and introduces substantial uncertainty in understanding net ecosystem productivity and carbon (C) stocks. Using a combination of vegetation growth monitoring and soil geochemical properties, we found that soil fertility parameters reflecting the local parent material are the main drivers of NPP and C allocation patterns in tropical montane forests, resulting in significant differences in below- to aboveground biomass components across geochemical (soil) regions. Topography did not constrain the variability in C allocation and NPP. Soil organic C stocks showed no relation to C input in tropical forests. Instead, plant C input seemingly exceeded the maximum potential of these soils to stabilize C. We conclude that, even after many millennia of weathering and the presence of deeply developed soils, above- and belowground C allocation in tropical forests, as well as soil C stocks, vary substantially due to the geochemical properties that soils inherit from parent material

Soil carbon respiration in tropical forest soils along geomorphic and geochemical gradients

Tropical ecosystems and the soils therein have been reported as one of the most important and largest terrestrial carbon (C) pools and are considered important climate regulator. Carbon stabilization mechanisms in these ecosystems are often complex, as these mechanisms crucially rely on the interplay of geology, topography, climate, and biology. Future predictions of the perturbation of the soil carbon pool ultimately depend on our mechanistic understanding of these complex interactions. Using laboratory incubation experiments, we investigated if carbon release from soils through heterotrophic respiration in the African highland forests of the Eastern Congo Basin follows predictable patterns related to topography, soil depth or geochemical soil properties that can be described at the landscape scale and ultimately be used to improve the spatial accuracy of soil C respiration in mechanistic models. In general, soils developed on basalt and granite parent material (mafic and felsic geochemistry of parent material) showed significantly (p <0.05) higher specific respiration than soils developed on sedimentary rocks (mixed geochemistry) with highest rates measured for soils developed on granite. For soils developed on basalt, specific respiration decreased two-fold with soil depth, but not for soils developed on granite or sedimentary rocks. No significant differences in respiration under tropical forest were found in relation to topography for any soil and geochemical background. Using a non-linear, stochastic gradient boosting machine learning approach we show that soil biological, physical and chemical properties can predict the pattern of specific soil respiration (R2=0.41, p<0.05). An assessment of the relative importance of the included predictors for soil respiration resulted in 43 % of the model being driven by geochemistry (pedogenic oxides, nutrient availability), 12 % driven by soil texture and clay mineralogy, 34 % by microbial biomass, C:N, and C:P ratios and 11 % by topographic indices. We conclude that, in order to explain soil C respiration patterns in tropical forests, a complex set of variables need to be considered that differs depending on the local bedrock chemistry. Its effect is likely related to the varying strength of C stabilization with minerals as well as nutrient availability that might drive C input patterns and microbial turnover

Organic matter cycling along geochemical, geomorphic and disturbance gradients in forests and cropland of the African Tropics – Project TropSOC Database Version 1.0

The African Tropics are hotspots of modern-day land-use change and are, at the same time, of great relevance for the cycling of carbon (C) and nutrients between plants, soils and the atmosphere. However, the consequences of land conversion on biogeochemical cycles are still largely unknown as they are not studied in a landscape context that defines the geomorphic, geochemically and pedological framework in which biological processes take place. Thus, the response of tropical soils to disturbance by erosion and land conversion is one of the great uncertainties in assessing the carrying capacity of tropical landscapes to grow food for future generations and in predicting greenhouse gas fluxes (GHG) from soils to the atmosphere and, hence, future earth system dynamics. Here, we describe version 1.0 of an open access database created as part of the project &ldquo;Tropical soil organic carbon dynamics along erosional disturbance gradients in relation to variability in soil geochemistry and land use&rdquo; (TropSOC). TropSOC v1.0 contains spatial and temporal explicit data on soil, vegetation, environmental properties and land management collected from 136 pristine tropical forest and cropland plots between 2017 and 2020 as part of several monitoring and sampling campaigns in the Eastern Congo Basin and the East African Rift Valley System. The results of several laboratory experiments focusing on soil microbial activity, C cycling and C stabilization in soils complement the dataset to deliver one of the first landscape scale datasets to study the linkages and feedbacks between geology, geomorphology and pedogenesis as controls on biogeochemical cycles in a variety of natural and managed systems in the African Tropics. The hierarchical and interdisciplinary structure of the TropSOC database allows for linking a wide range of parameters and observations on soil and vegetation dynamics along with other supporting information that may also be measured at one or more levels of the hierarchy. TropSOC&rsquo;s data marks a significant contribution to improve our understanding of the fate of biogeochemical cycles in dynamic and diverse tropical African (agro-)ecosystems. TropSOC v1.0 can be accessed through the supplementary material provided as part of this manuscript or as a separate download via the websites of the Congo Biogeochemistry observatory and the GFZ data repository where version updates to the database will be provided as the project develops.</p

Assessment of Combined Effects of Human Faeces and Mineral Fertilizers on the Behavior of Okra (Abelmoschus esculentus L.) Cultivated in Lubumbashi, DR Congo

peer reviewedImproving soil fertility through organic fertilizers application has become a major factor that has enabled the world to feed billions of people. However, the required quantities of organic material are enormous, so it becomes necessary to combine different types of fertilizers to feed plants. The objective of this study was to evaluate the effects generated by the integration of human faeces to mineral fertilizers by bringing them to okra (Abelmoschus esculentus) crop. The trial was installed according to a 4×3 factorialdevice. Treatments in three repetitions, included four doses of human faeces (0, 1.75, 3.5 and 7 t haG1) and three doses of inorganic fertilizers (0 and 150 kg NPK+100 kg urea as mineral fertilizer popularized and 75 kg NPK+50 kg urea as mineral fertilizer popularized halved). These fertilizers were applied alone or in combination and were compared to an unfertilized control. Obtained results showed that emergence rate increases when human faeces are made at low doses. As for the vegetative parameters, the results are similar regardless of the type or dose of made fertilizers. Without mineral fertilizers the different doses of human faeces generate low yields of okra. The application of 7 t haG1 of human faeces combined with popularized mineral fertilizer dose halved (75 kg NPK+50 kg urea) afforded the highest yield (7.3 t haG1 or an increase of 348% compared to unfertilized control treatment). In the present study, where access to chemical fertilizers is very limited, these results allow to save a half of usually made mineral fertilizer dose for the cultivation of okra

Impact of Chicken Manure Integration with Mineral Fertilizer on Soil Nutriments Balance and Maize (Zea mays) Yield: A Case Study on Degraded Soil of Lubumbashi (DR Congo)

peer reviewedOne of the most important challenges to continuously maximize crop production on limited areas of agricultural land is to maintain or enhance soil fertility. Organic fertilizer application is needed to replace nutrient removed by crop from the fields in order to restore crop production potential of a soil. But application of organic fertilizer alone insufficiently increases crop yield per area because the nutrient content of organic fertilizer is unbalanced. A trial was conducted in the Lubumbashi region to investigate the combined effects of mineral fertilizer and chicken manure application on the balance of minerals and maize yield. Three mineral fertilizer doses (0 kg NPK+0 kg urea, 150 kg NPK+100 kg urea, 300 kg NPK+200 kg urea) and four chicken manure quantities (0, 1.75, 3.5, 7 t haG1) have been tried. The combination of these factors gave a total of 12 treatments. According to obtained results nitrogen and phosphorus content before maize sowing is higher than those obtained after flowering. The present work confirms that integrated application of chicken manure and mineral fertilizers is more effective in increasing nutrient availability and maize performance than mineral or organic fertilizer applied alone. In contrast, applied doses of chicken manure have not improved soil nutriment balance sheet. Combined with the low dose of mineral fertilizers (150 kg NPK+100 kg urea), the amount of 7 t haG1 of chicken manure resulted a better yield increase, which corresponds to 46% compared to the control, of which 16% only are due to mineral fertilizer application

Evaluation of Maize Response (Zea mays L.) to Various Modes and Moments of Chicken Manure Spreading in Lubumbashi, DR Congo

peer reviewedSoils of sub-saharan countries have low inherent fertility, this deficiency is main factor determining agricultural production. Most of previous studies showed beneficial effects of organic matter in improving physical and chemical properties of soil and increasing yields. However, itremains unclear about when and how to spread the organic matter to get maximum nutrients for plants. This study was conducted in conditions of Lubumbashi to determine the influence of spreading modes and moments of chicken manure on maize yield. A trial was installed following a 2×5 factorial design. The treatments, in 3 repetitions, included 2 methods of spreading (in localization and in coverage) and 5 spreading moments (0, 1, 2, 3 and 4 weeks before sowing). Chicken manure obtained at DAIPN farm (Kilobelobe) were used as organic manure. A sample of chicken manure was analyzed in the Laboratory of the Faculty of Agricultural Sciences (UNILU) showed high percentage of nitrogen and phosphorus. This study revealed that all treatments were statistically similar regarding all growth and yield parameters. Although, the result of the analysis of variance revealed that there is no significant difference between the treatment, it is observable that spreading in coverage to a week before sowing gave the best yield (4.4 t haG1). In the conditions of Lubumbashi, where maize is a very important aliment, yield increase generated by the application of chicken manure in coverage a week before sowing would be useful

Effects of Bradyrhizobium japonicum on Some Chemical Properties of Ferralsols under Soybean (Glycine max (L.) Merr.) Cultivation

peer reviewedThis study was conducted on acidic soils in two different agro-ecological zones in order to evaluate the influence of Bradyrhizobium japonicum on soil chemical properties in the Upper-Katanga (DR Congo). A split plot design with three replicates was installed in two sites. The main plots included three strains of Bradyrhizobium japonicum plus the untreated control and four soybean varieties in subplots. Seed inoculation was performed in the shade and sown on the same day. The results showed that Bradyrhizobium strains did not influence soil pH, Nitrogen, C: N ratio and organic matter neither at flowering nor at harvest. However, total and available phosphorus (P) were influenced by the different treatments at harvest in Kanyameshi site. The strain S1 induced the highest average of total and available P whereas, the strain S2 had the lowest value of total P, and S3 induced the lowest content in available P. By comparing the timing of soil sampling at flowering and harvesting, the Student test revealed significant differences in pH, total nitrogen, C: N ratio, organic matter and available P indicating that soil chemical properties was improved at harvest and are only partially influenced by applied Bradyrhizobium strains

Soil geochemistry – and not topography – as a major driver of carbon allocation, stocks, and dynamics in forests and soils of African tropical montane ecosystems

The lack of field-based data in the tropics limits our mechanistic understanding of the drivers of net primary productivity (NPP) and allocation. Specifically, the role of local edaphic factors - such as soil parent material and topography controlling soil fertility as well as water and nutrient fluxes - remains unclear and introduces substantial uncertainty in understanding net ecosystem productivity and carbon (C) stocks. Using a combination of vegetation growth monitoring and soil geochemical properties, we found that soil fertility parameters reflecting the local parent material are the main drivers of NPP and C allocation patterns in tropical montane forests, resulting in significant differences in below- to aboveground biomass components across geochemical (soil) regions. Topography did not constrain the variability in C allocation and NPP. Soil organic C stocks showed no relation to C input in tropical forests. Instead, plant C input seemingly exceeded the maximum potential of these soils to stabilize C. We conclude that, even after many millennia of weathering and the presence of deeply developed soils, above- and belowground C allocation in tropical forests, as well as soil C stocks, vary substantially due to the geochemical properties that soils inherit from parent material