Effective and efficient nutrient management is central to best-practice agriculture, facilitating sustainable intensification while reducing negative externalities. The application of biochar-based fertilizers (BBF) in tropical agronomy has the potential to improve nutrient management by enhancing nutrient availability and uptake. Here, we performed pot-trials with Theobroma cacao L. seedlings planted in an Oxisol with critically low phosphorus levels. Four fertilizer levels were deployed, including BBFs using micro-dosed biochar (16 g plant⁻¹ i.e., 0.3% soil amendment w/w) charged with mineral fertilizer. Three different fertilizer-placement levels (topsoil, root-zone hotspot and root-zone layer) were evaluated. The results from the topsoil application of mineral fertilizer (farmer practice) served as the reference data. The root-zone layer application of BBF increased the aboveground biomass, total leaf area and chlorophyll content index by 56%, 222%, and 140% respectively. Foliar phosphorus levels were also significantly elevated by 53%. The N:P ratio of the foliar tissue was improved, indicating the potential of BBF to ameliorate P limitations. Thus, low dosages of biochar, which is upgraded to BBF, can considerably improve plant nutrition. Small scale technology to produce biochar can be easily adopted and integrated in T. cacao systems. We suggest that BBF production and application within tropical, perennial systems can contribute to achieving a range of sustainable development goals (SDGs), including climate action.

This chapter aims at providing an overview of the diversity of agroecological conditions, features of main farming systems, agricultural land use, its dynamics and drivers during the last two decades as well as major threats in ten countries of southern Africa (SA10). Based on this, we attempt to identify the resultant challenges for sustainable land management and outline potential interventions with a focus on smallholder farmers. By analyzing cropland dynamics during 2000–2019, we show how land use has been shaped by climate, demographic development, economic imperatives and policy realities. Concrete examples of these complex interactions illustrate both considerable shrinkage in South Africa and Zimbabwe or expansion of cropland in Mozambique and Zambia. During the past 20 years, cropland increased by 37% on average across SA10 mainly at the expense of forestland—showing huge spatiotemporal heterogeneity among countries. Most smallholders face shrinking farm size and other resource limitations that have resulted in soil nutrient mining and low agricultural productivity—a highly unsustainable situation. We conclude with an outlook on potential transformation pathways (“TechnoGarden” and “AdaptiveMosaic”) for the near future and thereby provide a frame for further studies on sustainable land management options under given local settings. Keywords Agroecological Farming systems TechnoGarden AdaptiveMosaic

The rise in atmospheric carbon dioxide (CO ₂ ) concentrations requires scalable and effective carbon dioxide removal (CDR) strategies. pyrogenic carbon capture and storage relies on the pyrolysis of biomass and the non-oxidative use of biochar, e.g. in soils. Enhanced rock weathering (ERW) captures CO ₂ by forming dissolved bicarbonate. In addition to CDR, both methods may offer soil improvement as a co-benefit. However, their interaction and combined CDR potential remain largely unexplored. Here, we investigate their individual and combined effects on carbon dynamics in a temperate agricultural soil. Using the process-based LiDELSv2 model calibrated against data from the lysimeter experiment, we simulate 1000 year impacts of applying 4.2 wt% wood biochar, 2 wt% basanite rock powder (RP), their co-application, and co-pyrolyzed material (rock-enhanced biochar, RE-biochar) on soil organic carbon (SOC), net primary production (NPP), net CO ₂ ecosystem exchange (NEE), and calcium (Ca ²⁺ ) leaching in a northern German sandy soil. Biochar alone led to the highest increase in SOC and achieved a modeled NEE of −200 g C ha ⁻¹ yr ⁻¹ per ton of biochar throughout 1000 years, acting as a long-term carbon sink. Co-application and RE-biochar increased SOC too, but to a lesser extent. Rock powder alone reduced SOC by 7%. Although RP enhanced Ca ²⁺ leaching, this did not result in net CO ₂ removal. Ecosystem respiration and NPP remained stable in the long term. Our results suggest that, when accounting for assumed application rates, biochar is the primary driver of long-term soil-based CDR, while ERW provides only minor co-benefits. This highlights the need to tailor interventions to specific soil and climate conditions.

Biomass pyrolysis and a non-oxidative use of the produced biochar is widely acknowledged as a negative emission technology and part of Pyrogenic Carbon Capture and Storage (PyCCS). Biochar with a molar H/C ratio < 0.4, which is usually achieved by pyrolysis at 550°C or above, is highly persistent when applied to the soil. Still, the exact residence time remains subject to debate. Practical assessment tools and reliable models for carbon accounting are needed. Persistence of soil applied biochar is often assessed using soil incubation trials of rather short time horizons, lasting several month or years, with consecutive extrapolation of the observed degradation rate. Within such experiments, the decomposition rate of the biochar continues to decrease over time indicating that biochar consists of a broad range of carbonaceous compounds of different recalcitrance. Hydrogen pyrolysis, an analytical method used to determine the degree of aromatisation, suggests that 75% of the carbon in biochar with an overall H/C ratio 1000 years, (Bowring et al., 2022; Howell et al., 2022), independent of the soil type and climate. The remaining 25% of the biochar carbon (heteroaromatic, aliphatic, etc) are considered semi-persistent carbon (SPC), presenting a mean residence time (MRT) in soil of 50 to 100 years, depending on soil type and climate. Based on this, up to 99% of the PyC loss quantified in an incubation trial may be attributed to the spectrum of SPC compounds, while the occurring PAC decay is very small and a neglectable loss in the context of carbon sink accounting. To validate the PAC residence time, data on long-term dynamics of the global, natural PyC cycle provides new insights. Natural PyC is produced from incomplete combustion in e.g. forest fires and is introduced to ecosystems globally at a scale of 0.114-0.383 Gt year-1. Given the global deposits of natural PyC of 550-1,650 Gt a MRT of 1,440 to 14,500 years can be calculated (Bird et al., 2015; Santín et al., 2016; Bowring et al., 2022 ). Natural PyC is produced in an uncontrolled manner, thus achieving a lower degree of aromatisation, a smaller PAC pool and lower MRT compared to optimized PyC produced by controlled pyrolysis. Therefore, observations from the natural PyC cycle render the assumed PAC residence time conservative. Further research is necessary to enable empirical quantification of the PAC content of biochar across a broad range of feedstock material and pyrolysis conditions. Hydrogen pyrolysis is an elaborate, yet expensive tool not suitable for routine analysis e.g. in biochar and biochar C-sink certification. Thus, further methods for PAC quantification must be developed and standardized.Bird et al. (2015). Annual Review of Earth and Planetary Sciences, 43(1), 273–298. https://doi.org/10.1146/annurev-earth-060614-105038 Bowring et al. (2022). Nature Geoscience 15:2, 15(2), 135–142. https://doi.org/10.1038/S41561-021-00892-0 Howell et al. (2022). Science of The Total Environment, 849, 157610. https://doi.org/10.1016/J.SCITOTENV.2022.157610 Santín et al. (2016). Global Change Biology, 22(1), 76–91. https://doi.org/10.1111/gcb.12985

Worldwide agriculture operates under the threefold challenge of adapting to climate change and mitigating its effects while aiming for sustainable agricultural intensification to meet the food demands of a growing population. Conservation agriculture (CA), a combination of reduced tillage, diversified crop rotations, and mulching, claims to target all three challenges at the same time. However, major knowledge gaps regarding CA’s mitigation potential remain. This study used a mobile, closed chamber system to determine soilborne, greenhouse gas (GHG) emissions from rainfed, farmer-managed CA- and conventional agriculture (CONV), in northern Zimbabwe. Measurements were carried out in locations of contrasting soil fertility (Arenosols and Luvisols) and under contrasting environmental conditions (cold-dry, cold-moist, warm-dry, warm-moist). Additionally, a horizon-specific soil fractionation with consecutive soil carbon and nitrogen quantification was conducted. The GHG emissions from a total of 8 farms depended on soil temperature and moisture and tended to be higher in CONV fields, although differences were statistically not significant. Field emissions were highest under warm-moist conditions, which are prevailing for large parts of the growing season. Mean carbon dioxide (CO ₂ ) emissions from Luvisols were 3.0% lower in CA fields (583 mg CO ₂ m ² h ⁻¹ ) than under CONV (601 mg CO ₂ m ² h ⁻¹ ), respectively 7.6% lower in CA fields (464 mg CO ₂ m ² h ⁻¹ ) than under CONV (502 mg CO ₂ m ² h ⁻¹ ) in Arenosols. Conservation agriculture reduced mean nitrous oxide (N ₂ O) emissions by 17.5% from 0.27 mg N ₂ O m ² h ⁻¹ (CONV) to 0.23 mg N ₂ O m ² h ⁻¹ (CA) in Luvisols and by 54.7% from 1.16 mg N ₂ O m ² h ⁻¹ (CONV) to 0.53 mg N ₂ O m ² h ⁻¹ (CA) in Arenosols. The upper soil horizons of Luvisols had higher concentrations of particulate- and mineral-associated organic matter compared with Arenosols and lower soil horizons but no differences were noted between management systems. Our data indicate that the mitigation effects of CA are highly site-specific and that CA management practices can have unexpected negative effects on GHG fluxes. The unimodal rainfall distribution with a long dry winter period of 7 months and recurrent dry spells in northern Zimbabwe may prevent a net carbon sequestration under CA management that would have occurred in the humid tropics.

Combined application of biochar and silicate rock powder might have synergistic effects on carbon dioxide removal and soil improvement. However, it remains unknown how their combination affects mineralization and stabilization of soil organic carbon (SOC). We compared pure ¹³ C-labeled wheat-straw biochar, pure basanite rock powder, their co-application, and rock-enhanced biochar from co-pyrolysis of wheat and basanite. All amendments were mixed with three agricultural topsoils (temperate silty, temperate sandy, tropical sandy) and incubated for 66 days. The δ ¹³ C-signal of the respired CO ₂ was monitored to determine amendment-induced priming of native SOC. After incubation, a density fractionation was conducted to investigate the potential stabilization of native and biochar-derived SOC as free particulate organic matter (fPOM), occluded POM (oPOM), and mineral-associated organic matter (MAOM). Pure basanite application did not influence short-term SOC dynamics, probably due to limited weathering within the 66-day incubation. In combined applications (co-application and rock-enhanced biochar), the biochar effect was dominant. The effects of biochar-containing amendments (biochar, co-application, and rock-enhanced biochar) were mainly controlled by texture and pH. The sandy soils exhibited positive priming following the application of biochar-containing amendments. In the temperate soils, positive priming occurred in MAOM, while native fPOM was partly shifted to oPOM. Input of biochar-derived SOC compensated losses of native SOC in fPOM and MAOM, resulting in overall SOC gains in most fractions. While priming intensity of all biochar-containing amendments was comparable, rock-enhanced biochar most strongly increased SOC in the density fraction interpreted as MAOM and had a higher short-term stability. These findings demonstrate that short-term effects of combined applications on SOC dynamics were dominated by the biochar component, while highlighting the potential of co-pyrolysis for short-term SOC stabilization.