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to the resulting higher temperatures during the thermophilic phase and to the faster decomposition of the organic matter (Awasthi et al. 2017; Chen et al. 2017; Zhang and Sun 2014).
Figure 1.18. The carbon cycle versus the biochar cycle (Roth 2014).
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excreta and cattle manure via thermophilic composting to produce a mature, nutrient-rich fertilizer, free of pathogens and phytotoxic substances, and with the least pile turning workload possible.
ii) Compare the composting process of human excreta with that of cattle manure. We hypothesized that under the same and suitable composting conditions humanure composting would behave differently than cattle manure, as the higher nutrient contents in human excreta compared to cattle manure could provide better and more balanced conditions for microbial activity, thereby yielding higher temperatures and accelerating the degradation process. Additionally, we expected that this higher nutrient load in human excreta compared to cattle manure would be reflected in the final composts.
iii) Given the benefits associated with biochar, we were particularly interested in studying the effect that biochar has during composting. We hypothesized that the use of biochar would reduce the losses of organic matter, organic C, and nutrients, and increase thermophilic temperatures and aeration conditions during composting as well as the pH and cation exchange capacity of the final composts.
2. Quantify CO2, CH4, N2O, and NH3 emissions of the appropriate-technology thermophilic composting process of the four composting treatments.
To further evaluate our thermophilic composting process, in Chapter 3, we analyzed the GHG and NH3
emissions occurring during thermophilic composting of human excreta and cattle manure with and without biochar addition. By quantifying and following these gas emissions, we aimed particularly to:
i) Assess the impact of our appropriate-technology composting process of human excreta on the atmosphere, and compare it with that of cattle manure composting. We hypothesized that humanure composting would result in higher NH3 emissions and in a faster decrease of CO2 emissions compared to cattle manure due to the higher thermophilic temperatures and faster degradation process as consequence of the better conditions for microbial activity provided by the higher nutrient contents in human excreta. In addition, due to the high presence of methanogens in the digestive tract of ruminant animal, we hypothesized that cattle manure would yield higher emissions of CH4.
ii) Evaluate the biochar’s climate change mitigation potential during the treatment of these excreta. In this regard, we hypothesized that biochar would improve aeration conditions inside the composting pile, and reduce the emissions of CH4, N2O and NH3 due to better oxygen supply and/or other biochar-mediated effects such as adsorption effects.
3. Investigate the use of human excreta and cattle manure compost, with and without biochar, and especially of large application rates, as fertilizers and soil amendments.
Besides investigating the composting process of human excreta and cattle manure, we also studied the use of the four types of compost produced (human excreta or cattle manure compost, with and without co-composted biochar) as soil fertilizer. For this, in Chapter 4, we explored in a 180-day incubation experiment the nutrient dynamics and GHG emissions of these four types of compost at two application rates (total compost N equaled 170 kg N ha-1, and three times this amount) to a sandy soil at 25°C,
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which was intended to resemble tropical temperatures of the Ethiopian highland. To elucidate the role of these composts as fertilizers and soil amendments, we specifically aimed to:
i) Evaluate the mineralization dynamics during the application of human excreta and cattle manure-derived compost in soil. In this regard, we hypothesized that under tropical conditions (in this study at an average soil temperature of 25°C) complete mineralization of the non-biochar-amended composts added would take place during the 180 days of incubation.
ii) Evaluate the potential that biochar-compost mixtures have in increasing C sequestration and reducing nutrient leaching in agricultural soils. Here, we wanted to test the hypotheses that the addition of relatively small amounts of co-composted biochar has the potential to stabilize soil organic matter and decrease C mineralization, reduce CH4 and N2O emissions and the risk of nutrient leaching from soil, mainly as nitrate and phosphate.
iii) Assess the application of large amounts of compost. We hypothesized that applying the maximum amount of compost allowed in Germany would not increase the risk of nutrient leaching from agricultural soils.
Following these three chapters, Chapter 5 seeks to connect their findings and discuss the implication that these have for the future of sustainable sanitation and soil nutrient management. In this work, Ethiopia is chosen as a model region; however, this integrated approach could be easily applicable to other regions worldwide.
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2
2 NUTRIENT DYNAMICS DURING COMPOSTING OF HUMAN EXCRETA, CATTLE MANURE AND ORGANIC WASTE AFFECTED BY BIOCHAR
Based on:
Castro-Herrera, D., Prost, K., Schäfer, Y., Kim, D-G., Yimer, F., Tadesse, M., Gebrehiwot, M., Brüggemann, N.
Nutrient dynamics during composting of human excreta, cattle manure and organic waste affected by biochar.
Manuscript submitted to Journal of Environmental Quality
Abstract
Lack of sanitation and limited access to fertilizers are global challenges, particularly in developing countries. Using compost made via thermophilic composting from human excreta could represent a strategy for increasing agricultural productivity and public health. We developed an appropriate-technology ecological sanitation concept by composting human excreta, and separately cattle manure, with kitchen scraps, teff straw, sawdust and biochar to produce a pathogen-free and nutrient-rich fertilizer and soil amendment. We followed the dynamics of the most important nutrients (N, P, K), as well as physical (temperature, moisture content), and chemical parameters (pH, electrical conductivity, cation exchange capacity, total organic matter, total organic C, Ca, Mg, and micronutrients) throughout the process. We implemented a well-running and hygienically safe thermophilic composting process, as assessed by the low N, P, K, Ca, and Mg losses, and the temperature profile. Average temperatures in the compost reached values above 60°C for 7, 6, 5, and 8 consecutive days for treatments containing human excreta, human excreta amended with biochar, cattle manure, and cattle manure amended with biochar, respectively. In the cattle manure with biochar treatment, biochar led to a significant temperature increase with a maximum value of 65.9°C reached at day 6. Biochar reduced losses of organic matter (18−23%), C (33−42%), N (49−100%), and decreased the extractable NO3–-N (32−36%) in the final compost. The tested ecological sanitation concept may thus represent a strategy to increase access to sanitation, food security, waste management and sustainable agricultural production, and to decrease N losses due to nitrate leaching.