Enhancing Cation Exchange Capacity of Weathered Soils Using Biochar: Feedstock, Pyrolysis Conditions and Addition Rate

Rimena R Domingues,Carlos A Silva,Kurt A Spokas,Murilo Nunes Valenciano,Miguel A Sánchez-Monedero,Paulo F Trugilho,Leônidas C A Melo

doi:10.3390/agronomy10060824

Abstract

The addition of alkaline and high-cation exchange capacity (CEC) biochars is a suitable strategy to increase the CEC of weathered soils. The aim of this study was to evaluate the effect of biochar from different feedstocks and pyrolysis temperatures on the CEC of two contrasting Oxisols. Biochars produced from chicken manure (CM), eucalyptus sawdust (ES), coffee husk (CH) and sugarcane bagasse (SB),plus a control (without biochar), at 350, 450, and 750 °C were mixed with the soils at 2; 5; 10 and 20% (w/w) and incubated for 9 months. Feedstock, pyrolysis temperature and addition rate of biochar were key factors controlling the alteration of soil CEC. The CH biochar pyrolyzed at 350 °C was the most effective matrix at increasing soil CEC. In a rate-dependent way, ES and SB biochars increased C contents of both soils without improving soil CEC. The efficiency of high-ash biochars in enhancing soil CEC in both Oxisols was limited by the alkalization caused by high rates of CH and CM biochars. The increase in CEC is soil-dependent and modulated by high-ash biochar CEC and application rate, as well as by the original soil CEC.

Highlights

Brazilian weathered soils are characterized by high levels of Al3+, low availability of nutrients and a clay fraction dominated by low chemical activity minerals, resulting in extremely low cation exchange capacity (CEC) [1]
The ash content was highly variable among the biochar samples, ranging from 0.7% to 56% and following the order of chicken manure (CM) > coffee husk (CH) >
CEC of biochars was highly variable ranging from 1.3 cmolc kg−1 (SB at 750 ◦ C) to 72 cmolc kg−1

Summary

Introduction

Brazilian weathered soils are characterized by high levels of Al3+ , low availability of nutrients (mainly P) and a clay fraction dominated by low chemical activity minerals, resulting in extremely low cation exchange capacity (CEC) [1]. Under native “cerrado” vegetation, the median value for CEC of typical Brazilian soils is 1.1 cmolc dm−3 , and 96.5% of the samples present a CEC< 4 cmolc dm−3 [2]. In such conditions, most of the negative charges are developed on the surface of organic matter colloids, which explains the strong correlation between CEC and organic C in these soils [3]. Crop residues represent 80% of biomass produced worldwide (8201 Tg y−1 ) of which 38%, after pyrolysis, are eligible to act as nutrient sources to plants and inputs to improve fertility and C content of weathered tropical soils [5,6]. The motivation for producing biochar was stimulated by the high fertility status and high CEC of the Terra Preta de

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