Slash and Char as Alternative to Slash and Burn: soil charcoal amendments maintain soil fertility and establish a carbon sink
Christoph Steiner, www.biochar.org Summary of Dissertation, Faculty of Biology, Chemistry and Geosciences University of Bayreuth, Germany, Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany (email: Christoph.Steiner@uni-bayreuth.de)
ABSTRACT
Introduction
Tropical forests account for between 20 and 25% of the world terrestrial carbon (C). Soils under tropical forest contain approximately the same amount of C as the lush vegetation above it. The current conversion of Amazonian forest to agricultural land makes disturbance of this C stock important to the global C balance and net greenhouse gas emissions. Changes in land use, particularly by clearing forests, reduce organic C by 20% to 50% in the upper soil layers. Furthermore, this reduction of soil organic matter (SOM) is causing soil degradation. Thus agriculture is not sustainable without nutrient inputs beyond 3 years of cultivation. The efficiency of conventional fertilizers (such as nitrogen (N)) is limited by a low nutrient retention capacity conjoined with strong tropical rains. On the other hand, large amounts of phosphate fertilizers are needed to overcome the soils high P-fixation capacity.
To overcome these limitations, slash-and-burn agriculture (shifting cultivation) is practiced by about 300 to 500 million people, affecting almost one third of the planets 1500 million ha of arable land. This traditional agricultural practice is considered to be sustainable if adequate fallow periods follow a short time of cultivation. In most agricultural systems the tendency has been for population pressure to increase, leading to shorter fallow periods, and therefore agriculture is doomed to fail without soil fertility management.
The existence of an anthropogenic and C-enriched dark soil in different parts of the world and especially in Amazonia (Amazonian Dark Earths (ADE) or Terra Preta de Índio) proves that the predominant Ferralsols and Acrisols can be transformed into fertile soils. The ADEs fertility is most likely linked to an anthropogenic accumulation of phosphorus (P), calcium (Ca), and black C as charcoal. Charcoal persists in the environment over centuries and is responsible for the stability of the ADEs SOM. Today and as assumed also in the past,those soils have been intensively cultivated by the native population.
Charcoal formation and deposition in soils seems to be a promising option to transfer an easily decomposable biomass into refractory SOM pools. However, charcoal represents just 1.7% of the pre-burn biomass if a forest is converted by the traditional slash-and-burn technique. The production of charcoal for soil amelioration purposes (slash and char) out of the aboveground biomass (secondary forest and crop residues) instead of converting it to carbon dioxide (CO2) through burning (slash and burn) could establish a C sink and could be an important step towards sustainability and SOM conservation in tropical agriculture.
Objectives and Scope
The aim of this dissertation is to examine the use of charcoal in agricultural practice and management of a highly weathered Xanthic Ferralsol on terra firme north of Manaus (Brazil). This dissertation comprises field (chapters VI, VII, VIII, IX, X, and XI), greenhouse (chapter V) and laboratory experiments (chapter IV). In addition, data and information were gathered at local charcoal production sites (chapters II and III) and indigenous soil fertility management (chapter I) was observed and described. A socio-economic study on charcoal producers collected information on household economic activity, charcoal production technique, and efficiency. The feasibility of slash and char with and without carbon trade mechanisms for small farmers and the potential for carbon sequestration was discussed. The influence of charcoal and condensates from smoke (pyroligneous acid, PA) on the microbial
activity was assessed in a pot experiment via measurements of substrate induced respiration (SIR). The effectiveness of charcoal as slow-release nutrient carrier (N, P, and K) was studied in a greenhouse experiment. In a field trial 15 different amendment combinations based on 10 equal amounts of applied C in chicken manure, compost, charcoal and forest litter were tested during four cropping cycles with rice (Oryza sativa L.) and sorghum (Sorghum bicolor L.) in five repetitions. We assessed the efficiency of applied nutrients and the influence on the soil microbial population. The influence of charcoal, organic and inorganic fertilization on perennial crops (Musa sp., Paullinia cupana) was assessed by measuring soil respiration and soil chemical properties. One trial was carried out on an expanding banana plantation in order to test the suitability of charcoal application in the local farming context.
Most important research findings
Fire and organic matter are the main components of indigenous soil fertility management (chapter I). Small fires are used to create burned soil (Terra Queimada), and burned organic materials (ash and charred residues) are used to increase the fertility in patches for medicinal plants and vegetables. After a burn (Terra Queimada) the soil had a strong scent of pyroligneous acid (Terra Cheirosa) which is stimulating soil microorganisms (chapters I and IV). Although most total nutrient contents in newly created Terra Preta (TPn) are below the average Terra Preta (TPp) contents, most (Ca, K, Mg, Zn, Mn) are within the range.
The investigation of socioeconomic aspects of charcoal production and carbon conversion efficiency (chapter II) could show that access to markets in the city enables charcoal producers to earn seven times more money from charcoal production than producers without access to markets. The most important reason (77% of respondents) for making charcoal is the inability to sustain agricultural activities at a profitable level. The average C conversion efficiency of the brick kilns was 42% (C in the wood feedstock converted to charcoal C) and the charcoal recovery by wood weight was 25%. Only about 3.7% of the C in the wood would be transferred into a refractory soil carbon pool if just the waste were used for soil amelioration at the production site. A further big proportion of waste is generated during marketing and bagging of charcoal which is usually collected for agricultural purposes at the city market. Considering the low charcoal production costs (~ 48 USD per ton) in shifting cultivation systems, much more charcoal could be used as soil amendment if profit from C emission trading could be generated.
The uses of charcoal production residues are manifold and reach from chicken fodder amendment to direct applications and the creation of charcoal compost. The production of charcoal is practiced as an alternative clearing method (slash and char), although charcoal is not always used for soil amelioration purposes, and if so, only the accumulated waste (powder and pieces) is used (chapter III).
When charcoal was applied in unweathered condition the microbiological parameters (respiration, biomass, population growth, and efficiency) increased linearly and significantly with increasing charcoal concentrations (50, 100 and 150 g kg-1 soil). Application of pyrogenous acid caused a sharp increase in soil respiration, biomass, and reproduction. We suppose that the condensates from smoke contain easily degradable substances which could be utilized by the microbes for their metabolism (chapter IV).
In a greenhouse experiment, leaching of N was significantly (P < 0.05) reduced if ammonium sulphate was applied with charcoal (chapter V). In contrast, leaching of K was significantly (P < 0.05) increased if potassium chloride was applied with charcoal, due to the charcoals K content. At the end of the experiment soil N as well as soil K contents were significantly higher in the charcoal treatment. Charcoal simultaneously served as K fertilizer and increased the retention of N.
Long-lasting soil fertility improvement due to organic fertilization and a synergistic effect if both charcoal and mineral fertilizer were applied was observed in a field experiment(chapters VI, VII, VIII and IX). Chicken manure amendments resulted in the highest (P<0.05) cumulative crop yield (12.4 Mg ha-1) of four successive harvests. Most importantly,11 surface soil pH, P, Ca and magnesium (Mg) were significantly enhanced by chicken manure. Charcoal significantly improved plant growth and doubled grain production if fertilized with NPK in comparison to the NPK-fertilizer without charcoal (P < 0.05). Soil charcoal additions reduced exchangeable soil aluminium (Al) significantly.
The soil microbial population growth potential showed a significant positive correlation to nutrient availability in the soil and plant biomass production (chapter VII). Mineral fertilized soils amended with charcoal and Terra Preta soils had a significantly higher potential for microbial population growth coupled with a low microbial respiration in absence of an easily degradable C source (glucose). The soil respiration before substrate addition correlated positively with the population growth rate on the plots, whereas Terra Preta had a very low soil respiration and very high population growth after substrate additions. Forest soils had a higher respiration rate but a very low population growth. These results reflect the relatively high biodegradable OM content of primary forest topsoil but low available nutrients (requirement for microbial population growth), in contrast to refractory Terra Preta SOM with high available soil nutrient contents.
The 15N recovery (chapter VIII) in biomass was significantly higher on compost amended plots due to significantly higher biomass production. The retention in soil was significantly higher in the charcoal-amended plots after the second harvest due to higher N retention and cycling of crop residues which remained on the plots after harvesting. Total N recovery (in soil, crop residues and grains) was significantly (P < 0.05) higher on charcoal (18.1%), charcoal plus compost (17.4%), and compost (16.5%) treatments in comparison to only mineral fertilized plots (10.9%).
After abandonment of cropping, additions of inorganic fertilizer, compost, and chicken manure resulted in increases in weed ground cover of 40, 22 and 53%, respectively, and increases in species richness of 20, 48, and 63%, respectively (chapter IX). While charcoal additions alone did not significantly affect weed ground cover or species richness, a synergistic effect occurred when both charcoal and inorganic fertilizers were applied. The
percentage ground cover of weeds was 45% within plots receiving inorganic fertilizer, 2% within plots receiving charcoal, and 66% within plots receiving both amendments.
The comparison of mineral and organic fertilization in perennial plantations (chapter X) showed that charcoal increased pH, total N, availability of sodium (Na), zinc (Zn), manganese (Mn), copper (Cu), humidity, and decreased available Al and acidity only in the mineral fertilized plantation. Decreased acidity due to charcoal application was also found in a banana plantation at a farm (chapter XI).
Conclusions
Charcoal is influencing soil quality in manifold ways, most importantly by reducing available Al and reducing acidity. Furthermore, charcoal adds K to the soil and has the potential to reduce N leaching. Charcoal amendments increased the reproduction rate of the microbial population after substrate addition whether the plot was fertilized or not. The effects of charcoal on soil biological, chemical and physical properties are complex, making it difficult to isolate single significant charcoal effects, but added up they caused significantly increased
plant growth and crop production.
More information is needed on the agronomic potential of charcoal, the potential to use alternative biomass sources, and the production of by-products to evaluate the opportunities for adopting a slash and char system. The access to a global C trade mechanism would facilitate charcoal use for soil amelioration and thus would increase C sequestration and create a strong incentive to prevent further deforestation. Both of these actions would help
to mitigate global climate change.