Impact of biochar and compost amendment on soil organic matter characteristics in kaolinitic tropical soils

Abstract

Increasing pressure on highly weathered tropical soils leads to land conversion from tropical forest or grassland to cropland, resulting in huge C and N losses. Soil organic matter (SOM) is the soil compartment most responsive to management yet bearing functions crucial to agronomic success. Both SOM storage as well as SOM functionality are considered aggregate-related parameters, since soil aggregation regulates the accessibility of SOM to soil biota, but also abiotic functions such as water uptake or nutrient retention. Circumtropical Anthropogenic Dark Earths on highly weathered soils exhibit atypical features such as high OM stocks and biological activity and an extraordinary resilience towards agricultural strain. This suggests profoundly altered SOM dynamics compared to adjacent sites, resulting in the inherent ability of OM accrual while maintaining high microbial activity. Owing to the high amount of pyrogenic C in these Dark Earths, biochar-compost mixtures are being discussed as alternatives to mineral fertilisers. Their richness in organic C and stabilised nutrients raises the question if these amendments can be used to mitigate conversion-induced SOM losses. The objective of this thesis was to investigate if biochar and compost amendment lead to a change in SOM dynamics that would support the assumption of a transformation towards an increase in soil-borne OM stocks in agricultural tropical soils, so as to halt cultivation-induced SOM losses. I conducted a meta-analysis to investigate, whether evidence from the literature would point towards an increase of OM stocks and a change in OM dynamics in biochar-amended tropical soils. I calculated the theoretically expected C increment and compared these values to the C contents after biochar amendment reported in the studies. The response appeared to be soil-specific, with more additional (non-biochar) C in soils with higher SOM and nutrient contents prior to amendment. Complementary, in highly weathered, nutrient-poor soils, biochar amendment occasionally led to a decrease in soil C stocks. The microbial response indicated higher substrate availabilities shortly after amendment, related to higher bacterial cell abundances and higher respiration. Studies surveying Anthropogenic Dark Earths suggested an alteration of the soil microbiome towards higher fungi:bacteria ratios and higher bacterial diversity in these soils compared to adjacent sites. Additionally, mineral depositions on and microbial colonisation of the biochar surface documented an in-situ alteration of the biochar itself. However, biochar was biodegradable to less than 10 %, suggesting its function as microbial substrate was minimal and transient. In a 2-year field experiment in the Adamaoua highlands in Cameroon, I studied the short-term effect of high biochar and compost application rates (25 t ha -1 year -1 biochar; 75 t ha -1 year -1 compost) on aggregate-related SOM characteristics of a recently converted grassland soil. On the bulk scale, C and N were lost without organic amendments. The C balance for all aggregate sizes was positive with biochar-compost, while it was neutral with compost alone and negative with mineral fertiliser (up to – 10 % C). Differences between aggregate size fractions were negligible after the first year, but intermediate-sized and microaggregate fractions (< 50 μm) were relatively N enriched after two years, independent of treatment. N losses were overcompensated by both organic amendments in all aggregate sizes, and N was preferentially lost from the largest and the smallest aggregate fractions in treatments without organic amendments. The latter points towards tillage-induced mineralisation (macroaggregates) and changes in N sorption features (microaggregates < 50 μm). The qualitative component-specific characterisation of tropical SOM is often challenging, since the strong mineral interaction complicates extraction-based analyses as well as infrared spectroscopy. Using thermogravimetry coupled to differential scanning calorimetry and mass spectrometry (DSC-TGA-MS), I corrected the exothermic signal resulting from OM combustion for endothermic pedogenic oxide and clay degradation features. Organic amendments lead to a depletion of the thermolabile fraction (~ 300 °C), perhaps as a result of a generally elevated microbial activity. The absence of additional organic input resulted in a reduction of the thermostable fraction (~ 400 °C), reflecting the progressive decomposition of thermostable substrate as a result of lacking fresh OM input. Biochar amendment rendered a clear black carbon peak (> 500 °C) which was distinguishable in all aggregate sizes. All samples showed a clear cellulose peak (~ 340 °C), reflecting the vegetation on site before and during the experiment. As no clear diffferences between aggregate sizes were evident, SOM characteristics as revealed by thermal analyses appeared to be input-regulated rather than aggregate-dependent. In conclusion, biochar and compost do have the potential to elevate SOM stocks in cultivated tropical soils. The evidence gathered in this thesis shows an immediate C and N enrichment by biochar-compost, while compost alone can merely halt cultivation-induced C losses. On the short term, labile C pools may be reduced upon addition of organic amendments. Results from the literature, however, suggest that this is just the beginning of a long-term shift in microbial dynamics. So far, it is not clear how this shift is initiated, if it is truly related to OM accumulation observed in Anthropogenic Dark Earths and how many amendment cycles at which dosage are necessary to stimulate soil-borne OM accrual.