Biochar application to soils has been investigated as a means of improving soil fertility and mitigating climate change through soil carbon sequestration. In the present work, the invasive shrub "Eupatorium adenophorum" was utilized as a sustainable feedstock for making biochar under different pyrolysis conditions in Nepal. Biochar was produced using several different types of kilns; four sub types of flame curtain kilns (deep-cone metal kiln, steel shielded soil pit, conical soil pit and steel small cone), brick-made traditional kiln, traditional earth-mound kiln and top lift up draft (TLUD). The resultant biochars showed consistent pH (9.1 ± 0.3), cation exchange capacities (133 ± 37 cmol c kg -1 ), organic carbon contents (73.9 ± 6.4%) and surface areas (35 to 215 m 2 /g) for all kiln types. A pot trial with maize was carried out to investigate the effect on maize biomass production of the biochars made with various kilns, applied at 1% and 4% dosages. Biochars were either pretreated with hot or cold mineral nutrient enrichment (mixing with a nutrient solution before or after cooling down, respectively), or added separately from the same nutrient dosages to the soil. Significantly higher CEC (P< 0.05), lower Al/Ca ratios (P< 0.05), and high OC% (P<0.001) were observed for both dosages of biochar as compared to non-amended control soils. Importantly, the study showed that biochar made by flame curtain kilns resulted in the same agronomic effect as biochar made by the other kilns (P > 0.05). At a dosage of 1% biochar, the hot nutrient-enriched biochar led to significant increases of 153% in above ground biomass production compared to cold nutrient-enriched biochar and 209% compared to biochar added separately from the nutrients. Liquid nutrient enhancement of biochar thus improved fertilizer effectiveness compared to separate application of biochar and fertilizer.

Competing interests: The results of this paper were further part of the Asia Development Bank project TA-7984 NEP: Mainstreaming Climate Change Risk Management in Development, financed by the Nordic Development Fund and the Government of Nepal under the administrative lead of Landell Mills Ltd, UK. There are no patents, products in development or marketed products to declare. This does not alter our adherence to PLOS ONE policies on sharing data and materials.

Funding: This study was conducted with the main financial support from the Research Council of Norway (Fripro stipend 217918 to GCo). The results of this paper were further part of the Asia Development Bank project TA-7984 NEP: Mainstreaming Climate Change Risk Management in Development, financed by the Nordic Development Fund and the Government of Nepal under the administrative lead of Landell Mills Ltd, UK. The funders had no role in the study design, data collection and analysis, decision to publish and preparation of the manuscript.

Introduction

Biochar (BC) is the carbon-rich material produced by the pyrolysis of biomass i.e. heating in the partial or complete absence of oxygen [1]. Biochar is highly recalcitrant in nature unlike other forms of soil organic matter (SOM). Thus, biochar amendment to soils acts as a carbon sequestration technique which can also enhance soil fertility [1–3]. Agronomic benefits of biochar-amended soils can be the result of improved soil physical properties (bulk density, porosity, water holding capacity, permeability, aggregation), biological properties (improved environment for microbial populations such as mycorrhizae) and chemical properties (pH, CEC and nutrient retention capacity) [4–11].

Various pyrolysis technologies and various feedstocks can be used to produce biochar. This may result in a large variation in resulting biochar properties [12,13] which in turn may affect biochar effectiveness for increasing soil fertility [14,15]. Low temperature pyrolysis (300–500°C) has shown increased biochar yield and carbon content whereas high temperature pyrolysis (>500°C) has revealed lower biochar yield and higher surface area with increased adsorption capacities for various compounds [16]. Research on the effect of pyrolysis technology on agronomic biochar quality has up until now been scarce. Under rural (sub)-tropical conditions, biochar has mostly been produced with medium-sized traditional kilns made of bricks or simple earth mound heaps, improved retort kilns [17,18] or top-lit up-draft (TLUD) pyrolysis units [19]. Traditional kilns can be operated using all kinds of mixed biomass feedstocks. However, pyrolysis gases such as methane (CH 4 ), carbon monoxide (CO) and aerosols (PM 2.5 and PM 10) are released untreated, and this leads to greenhouse gas emissions, pollutant emissions and loss of energy [20]. Improved retort kilns have features to recirculate the produced syngases into the combustion chamber, resulting in up to 75% less toxic and greenhouse gas emissions as well as higher conversion efficiency (40–50%) compared to traditional brick kiln, due to less losses of energy-rich molecules [21]. On the other hand, improved retort kilns are more costly, difficult to operate and often consume a lot of start-up biomass materials [18]. TLUD kilns burn feedstock cleanly, thereby reducing gas emissions, as the syngases are combusted largely in the flame front. If used indoors this reduces negative health impacts [22]. There are some limitations with using relatively small TLUDs as they produce so little biochar (around 300 g per run) that they are mainly useful for small-scale kitchen gardening [20]. Larger TLUDs, while generating more biochar, require significant investments and expertise in order to be operated successfully.

To circumvent such challenges, the flame curtain, open pit "Kon-Tiki" kiln was recently developed [23]. It follows the principle of pyrolyzing biomass layer after layer in an open, conically built metal kiln that is easy to operate, fast, and results in low greenhouse gas emissions [20]. It thus allows biochar production in relatively large quantities (700 to 850 L volume biochar in 4–5 hours) [20–23]. The flame curtain kiln can even be operated as a simple conically shaped hole in the ground, leading to the same low emissions and similar biochar quality as the metal version, but essentially without any cost apart from the few hours of labour required to dig and prepare the soil pit [20].

Most studies on weathered soils have shown significant positive effects of biochar application on crop production; however, other studies have not shown any significant or even negative effects of biochar on crop yield [24,25]. Some examples from tropical countries on mostly acidic and weathered soils include the following. Radish yield increased significantly in biochar amended soils blended with mineral N fertilizers in pot trials, emphasizing the role of biochar in improving nitrogen use efficiency [2]. Moreover, conservation farming practice carried out with 4 tons/ha of biochar in a maize field in Kaoma, Zambia characterized by sandy acidic soils result in strong increases (0.9 ± 0.1 t ha- without biochar to 3.8 ± 0.5 t ha- with biochar) in crop yield [26]. Furthermore, application of biochar at 10 t ha-1 along with NPK mineral fertilizers (50g m-2) in maize, cowpea and peanut field showed an increase of 322%, 300% and 200% respectively compared with control plot (without biochar and NPK) in South Sumatra, Indonesia [7]. In contrast, field application of biochar did not show agronomic effects at four sites out of six in Zambia [26]. In seven field trials on five working farms in the UK, [27] observed positive yield effects in three trials, no effects in three trials and negative yield effects in one trial.

Recently, techniques for biochar nutrient enrichment, i.e. mixing nutrients with biochar before addition to the soil, have resulted in some promising increases in crop yield. Biochar enriched with cattle urine and amended to soil in Dhading, Nepal, increased the yield of pumpkin to 82.6 t ha-1 [28], more than 300% higher than that with only urine and 85% higher than the yield with the same amount of biochar without urine added. In another study, biochar enriched with compost nutrients by co-composting in the presence of biochar, was added to sandy soils and increased the yield of Chenopodium quinoa by 300% compared to non-enriched biochar treatments in the presence and absence of compost [29]. Biochar nutrient enrichment is probably effective due to penetration of nutrients in biochar micro- and nanopores. The pores of carbonaceous sorbents such as biochar are so narrow that water movement is restricted and an ice-like water structure is formed [30]. Earlier work has provided evidence of a relation between organic compound sorption and the nanopore volume of such matrices [30] and it is possible that a similar phenomenon could occur for nutrients in biochar. Nutrient addition to biochar has thus shown to be a promising method to enrich the biochar and render it a slow-release fertilizer. However, systematic studies on the optimal way to carry out such nutrient enrichments are lacking.

This is the first study to directly compare the agronomic effect of biochar produced from different kiln types and enriched in different ways (enriched hot biochar and enriched cooled-down biochar, as compared to non-enriched biochar where the same amount of nutrients was added separately). The study was carried out using a pot trial design in Nepal using a woody shrub as biochar feedstock. "Eupatorium adenophorum" is a promising feedstock as it is a naturally regenerating, ubiquitous, invasive woody forest shrub species locally named "Banmara" (forest killer) that is about 1–2 m high and stems up to 2 cm thick [31]. In this way, waste from an invasive species can be turned into a valuable resource for agronomic production and carbon sequestration. Biochar produced from Eupatorium feedstock has been found to meet all the requirements for premium quality based on European Biochar certificate [20]. In Nepal, average landholding size is very small and the soils can be acidic, exhibiting lower levels of C, N, P and exchangeable bases [32]. Overall, this study tested the following hypotheses: (1) Biochar produced from various kilns with different pyrolysis conditions exhibits different crop yield effects depending on kiln type, and (2) Nutrient enrichment improves the agronomic effect of biochar thereby increasing the maize biomass production.