# Effect of Biochar Application Depth on Crop Productivity Under Tropical Rainfed Conditions
## 1 Introduction  
Biochar is charcoal produced by burning biomass under controlled temperature and oxygen conditions, mainly for the purpose of soil amendment [1; 2; 3; 4]. As a porous material with active functional groups, high pH and cation exchange capacity [5], biochar can serve as a natural pool of organic carbonand plant nutrient in the soil [6; 7; 8]. There is a widespread interest in biochar research because of its rediscovered potential to mitigate climate change, amend soils and increase crop productivity [9; 10]. However, this potential is strongly influenced by biochar production (temperature, oxygen supply, etc. ), application (rate, mode, time, soil type, etc.) and feedstock (plant, animal, domestic waste, etc.) conditions [11; 12; 13; 14; 15]. The highly fertile dark earths (the so-called _terra preta_) left behind centuries ago by the prehistoric Amerindians are characteristically rich in carbon [16; 17; 18]. The origin of the rich dark earths is traced to as far back as 1542 when [PERSON] first reported to the Spanish Court on the well-established networks of agricultural settlements along a tributary of the Amazon River [19; 20]. Unlike the predominantly _terra mulata_ (brown earths), which is the natural soil in the Amazon, _terra preta_ dark earths is anthropogenic or anthropic creation of humans [20; 21; 22]. Typically, _terra preta_ has three or more times total soil organic carbon and far more phosphorus, calcium and humus than the surrounding _terra mulata_ brown soils. The Amazonian _terra preta_ has remained highly fertile centuries after its formation. In addition to the potential to sequester carbon, the porous structure, large surface area, and high particle charge of biochar [23] interact with the soil [24; 25; 26] that results in beneficial effects on the ecosystem [27]. Among the countless benefits of the use of biochar are enhanced plant growth [28], improved soil water-holding capacity [29], reduced crop diseases [30], limited bioavailability of heavy metals [31], reduced soil nitrogen oxide emission [32], and reduced leaching of nutrients and fertilizers [29]. [PERSON] et al. [33] and [PERSON] [34] have documented a more exhaustive list of the beneficial uses of biochar. Biochar is hailed as an all-win solution to global energy, food, and environmental challenges [11]. In the tropics where soil fertility is depleted soon after cultivation in the predominantly slash-burn shifting cultivation [8; 20; 35], there is the need to switch to slash-char sedentary farming for sustainable crop production [32; 36]. Slash-burn shifting cultivation destroys up to 97% of soil organic carbon as against less than 50% under slash-char farming, of which most is retained as highly stable organic carbon [16; 24]. With an average residence time of over 1000 years [37], biochar can sustain crop productivity for as long as it remains active in the soil [38]. In a meta-analysis including 116 studies from 21 countries around the world, [PERSON] et al. [39] noted that biochar increases crop productivity by 11% on average. In another meta-analysis including 371 independent studies, [PERSON] and [PERSON] [40] noted considerable variations in the degree of enhancement of crop yield components and soil properties. Biochar application rates below 30 tons/ha typically result in 30% more productivity of legume and 29% more of vegetable crops than of cereal crops such as corn (8%) and rice (7%) [38; 40]. Studies, including literature data analyses, also show that biochar application rates above 55 tons/ha can lead to reductions in crop productivity [16; 40]. The _terra preta_ soil, which has so much reignited biochar research, occurs in varying depths and rates across the Amazon [20]. This forms the basis for investigation of the variations in soil/crop response to biochar application rates in terms of biochar-soil mixture, activation mode and/or application depth.