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Technische Universität Berlin (2022)

Sustainable soil management in the semi-arid Caatinga of Brazil – how the application of soil amendments and grazing affect nutrient retention and soil organic carbon stocks

Beusch, Christine Hildegard

Titre : Sustainable soil management in the semi-arid Caatinga of Brazil – how the application of soil amendments and grazing affect nutrient retention and soil organic carbon stocks

Auteur : Beusch, Christine Hildegard

Université de soutenance : Technische Universität Berlin

Grade : Doktorin der Naturwissenschaften
- Dr. rer. nat. – 2022

Résumé partiel
The Caatinga is one of the largest seasonally dry tropical forests in the world. With an estimated area of 600 000 km2 to 900 000 km2 , it is located in the semi-arid north-east of Brazil. The climate is demanding with constant high temperatures, erratic rainfalls, and regular droughts, resulting in general water scarcity. Moreover, the soils in the Caatinga are usually only of low to medium fertility and often shallow and stony. The water-deficient conditions, paired with poor soils, are severely challenging agricultural practice in the region. As the backbone of agriculture, soils play a crucial role in food production. Therefore, the overall aim of this dissertation was to sustainably increase the fertility of Caatinga soils on a long-term basis to promote food security and livelihood of the local smallholders. Two important parameters that impact soil fertility were considered : nutrient retention and soil organic carbon (SOC). First of all, the effects of biochar and clay addition on the nutrient retention of an Arenosol were evaluated. Second, the influence of various factors on SOC stocks, in particular grazing, were studied. All research was conducted in the Itaparica region in the state of Pernambuco, Brazil. For the first thematic part, two locally available and inexpensive soil amendments were selected : a traditionally produced mid-temperature biochar made of the invasive tree species Prosopis juliflora (Sw.) DC and the clayey sediment of a temporarily dry lake. In a batch equilibrium experiment, the sorption of ammonium-N (NH 4+ -N), nitrate-N (NO3- -N), potassium (K+ ), and phosphate-P (PO43- -P) was quantified for substrate mixtures of an Arenosol with increasing shares of biochar and clay, respectively. In a corresponding field experiment using the same substrates, the leaching of NH4+ -N, NO3- -N, and K+ was quantified for two consecutive periods of eight months each by using self-integrating accumulators. Both experiments showed the same tendencies. Biochar addition induced marginal to medium retention of NO3- -N, medium retention for PO43- -P, and medium to strong retention of NH4+ -N. In the field experiment, the biochar showed medium retention of K+ , while it provoked strong K+ release in the batch equilibrium experiment. In contrast, clay addition resulted in the release of NO3- -N and medium to strong retention of NH4+ -N, K+ , and PO43- -P. Both soil amendments showed the potential to enhance the retention of nutrients and, thus, the fertility of an Arenosol. The nutrient retention capacity of the clay remained relatively stable for the 16 months of the field experiment, whereas the retention capacity of biochar significantly dropped for all nutrients by about half in the second observation period compared to the first. The reason was the comparatively low stability of this particular biochar, causing rapid decomposition under the given climatic conditions. Therefore, for future application, the long-term stability of biochar should be enhanced by higher pyrolysis temperatures.

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Page publiée le 31 octobre 2022