Characterization of the residue (endocarp) of Acrocomia aculeate and its biochars as potential peat substitute in tomato cultivation

The South-American palm Acrocomia aculeata has great potential as a sustainable source for vegetable oils, but its industrialization implies the production of huge amounts of organic waste. Currently, this material and in particular the endocarp is mostly used for energy generation, but this traditional method is very inefficient because a considerable part of the energy is lost.  An environmentally more sustainable use may be its conversion into biochar, via pyrolysis. This material has recently gained considerable interest as a strategy to recycle agro-industrial waste by its conversion into a soil amendment with a high carbon sequestration potential. In addition, biochars derived from woody feedstocks show  high porosity and low biochemical degradability which may turn them into suitable alternative to peat as planting substrate in horticulture. Although the woody nature of the shells (endocarp) of Acrocomia represent a promising candidate for such porous biochars, this alternative has been widely neglected up to now. Therefore, in a first attempt a physical and chemical characterization of these residues and their biochars was performed and its suitability as growing substrate for tomato cultivation was evaluated. By analyzing biochars derived from feedstock with different particle size, we tested if aside from the pyrolysis conditions and the nature of the feedstock, the size of the latter may affect the nature of the pyrolyzed product.

Our results confirmed the increase of aromaticity with increasing pyrolysis temperature which has already been described for other organic feedstocks. The heat increase the pH only moderately (pH= 8.4 at 450°C). NMR spectroscopic analysis confirmed that this was caused mainly by the the selective enrichment of cations rather than by the loss of acid C groups. However, tomato plants prefer a soil pH around 6 to 6.8 which turns the biochar produced a 325°C with a pH = 7.2 into a more suitable growing substrate. Statistical analysis did not reveal a significant impact of particle size of the feedstock on chemical composition or pH of the resulting biochar. Comparably,  greater feedstock particle size did not affect the specific surface area of the biochars but considerably decreased the water holding capapcity. 

The Olsen-P increased from 39 mg kg^-1 for the natural sample to 81 mg P kg^-1 for the biochar produced at 450°C. K and Mg concentration were 2.6 g kg^-1 and 279 mg kg^-1 for the biochar yielded at 450°C.  For tomato plant cultivation, Sainju et al., (2003) recommended for P, K and Mg, 60 to 70 mg  kg^-1, 0.6 -0.7 g kg^-1. 0.4-0.7 g kg^-1.  Thus, with respect to those nutrients, the obtained biochar can provide sufficient macronutrients if used as a growing substrate for tomatos. However, due to the low N contents of the biochars, sufficient N fertilization – either by addition of mineral or organic fertilizers - is still required if such materials are intended to be used as growing substrate in tomato cultivation.

Acknowledgement: Financial support was provided by MINECO/FEDER (CGL2015-64811-P)

Sainju, U.M., Dris, R., Singh, B., 2003. Mineral nutrition of tomato. Food, Agric. Environ. 1, 176–184.