Re-use of sugarcane residue as a novel biochar fertiliser - Increased phosphorus use efficiency and plant yield

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Highlights

  • A novel P rich biochar fertiliser with enhanced features is produced.

  • Biochar fertiliser and TSP are tested in sugarcane for different soils.

  • Mean soil pH, over 120 days, for biochar (∼7) is higher than in TSP (∼5).

  • Biochar fertiliser presents PUE 15% superior than TSP.

Abstract

With the advent of mechanized sugarcane harvesting in Brazil around 70 Mt of straw each year is left on the field, and ∼70% of its carbon (C) is decomposed and returned to the atmosphere on the same timeframe. The adoption of a stabilised C product such as biochar as a vehicle for nutrient delivery might address two ends of a problem: to increase nutrient use efficiency by plants while creating a viable and efficient strategy to increase stable C in soil. This study proposed a production route for a biochar fertiliser (BF) from sugarcane straw biochar (SSB), by activation of SSB with KOH and subsequent neutralisation with H3PO4. The phosphorus (P) content of BF (8.6% P) was superior to SSB. It was also noticed that structure of SSB matrix was altered, creating increased sorption capacity, with the fertiliser bound to the biochar rather than physically mixed. The BF was tested in a controlled condition experiment with sugarcane for 120 days using three low P soils with different clay contents (147, 326 and 528 g kg−1). BF led to higher biomass yield (15%) and P use efficiency (PUE) (∼10%) compared to standard P source (triple superphosphate – TSP) in the most clayey soil. The adoption of a fertiliser based on a nutrient enriched biochar can increase the effectiveness of crop production. Enhanced delivery of crop nutrients increases the attractiveness of increased soil C.

Introduction

Crop production is increasingly dependent on exogenous nutrient sources. Chemical fertilisers supplement the natural cycling of nutrients within the soil and replace nutrients removed in harvested products and crop residues. The proportion of fertiliser nutrients that is utilised by a crop in the same growing season is typically low, particularly for phosphorus (P) in tropical situations (Withers et al., 2018). Acidic, highly weathered, iron (Fe)-rich soils rapidly bind phosphates at mineral surfaces, limiting access to plant roots (Borges et al., 2019). The high P stock that can meet crop demand through natural turnover promotes higher leaching losses and increased application rates. The problem of increasing P use and decreasing use efficiency presents a major threat to sustainable crop production. Phosphorus leaching is becoming the leading cause of water pollution and water quality issues globally. Geographic imbalance in exploitable reserves of phosphate and the use of P fertiliser may accentuate the geopolitical tension around food, fibre and energy.

There are three general approaches to support sustainable intensification in relation to P: 1) to dramatically increase the recovery of P after crop processing and/or consumption; 2) to improve soil quality in a way that increases accessibility of soil P to crop plants, and 3) to develop P fertilisers that more directly supply the crop plant. Better supply and use efficiency could be realised by improving the functionality of fertiliser products and/or the traits of fertilised crops and/or the qualities of soils that improve soil–plant interactions. Changing soil and crop properties depend on wider developments that are more fundamental than P fertilisation alone. The association of higher carbon (C) content (as an indicator of organic matter) and improved P-use efficiency in highly weathered tropical soils is well known. On the other hand, increasing soil C content is challenging under tropical conditions. Approximately 70% of the C present in plant biomass is decomposed and returned to the atmosphere within one year of addition to soil (Sousa et al., 2017).

Biochar production could be an effective tool to increase soil C stocks, by adding C in an efficiently pre-stabilised form. Direct application of biochar to increase soil C has not been found attractive or efficient economically and sometimes controversial as a concept (Mukherjee and Lal, 2014). Biochar from crop residues can also be used as a vehicle for the delivering nutrients directly to plant roots (Hagemann et al., 2017). This involves small incremental increases in soil C, but with the primary purpose of supplying crop nutrient demand. This research converges on biochar as a way to address two related needs, namely better P delivery and increased soil quality.

Using biochar to directly deliver P to crops and to increase soil C can be realised as part of normal agronomic practice and according to the economics of fertiliser use (Qian et al., 2019). This strategy addresses two ends of the same problem, where gradual physicochemical improvements arise in bulk soil through gradual accumulation of stable C, benefiting nutrient acquisition, including P use efficiency. The conventional high cost and handling problems associated with biochar could be allayed in its use as a P-rich fertiliser, deploying the stable carbonaceous matrix as a viable technical solution to traditional issues related to P sources (Mukherjee and Lal, 2014; Kim et al., 2018). To decrease rather than increase dependence on exogenous resources and increase circularity within farming, it is essential that biochar drawn into the supply of fertiliser nutrients is drawn from residual biomass generated within the farming system.

This work seeks to develop these concepts in the context of sugarcane production and according to the following objectives: (i) to demonstrate that P can be intimately bound to a biochar C matrix, (ii) to assess the effect of sequential nutrient modification on the C matrix, and (iii) to assess the comparative efficiency of P delivery from biochar fertiliser and triple superphosphate to a crop plant.

Section snippets

Material and methods

A novel phosphorus- and potassium-enriched biochar product (biochar fertiliser, BF) was compared to conventional P–K fertilisers (triple superphosphate, TSP) using a plant-growth pot experiment with sugarcane (Saccharum officinarum). The sugarcane production system providing the specific context for this study and to reflect the driver of increased circularity of resource use, the source biomass for biochar was sugarcane straw. The straw comprises the leaves attached to the cane stem at harvest

Properties of sugarcane straw, biochar and biochar fertiliser

The results of the chemical and physical characterisations are presented in Table 1. Alkaline activation and acid neutralisation increased the native nutrient content of sugarcane straw biochar (SSB) by a factor of 30 for K and 40 for P, transforming it into the nutrient rich fertiliser (BF). The P content of BF was 8.6%, which compares to 21% for TSP, also 98% of total P in BF was soluble in H2O. The bulk density of SSB was markedly increased by the process of nutrient enrichment, increasing

Conclusion

Novel nutrient enrichment and pH adjustment of sugarcane straw biochar created a biochar fertiliser with plant-growth effects superior to conventional triple superphosphate fertiliser at equivalent P dose, especially in clayey soil. Biomass was ∼14% higher for biochar fertiliser than for triple superphosphate and P-use efficiency was ∼10% higher. The difference was sustained through two harvest cycles and distinguished mainly by different patterns of pH change. The addition of triple

Funding information

This project was funded by the FAPESP – São Paulo Research Foundation (Scholarship grant 2016/13813-8) and the National Council for Scientific and Technological Development – CNPq (Grant 404577/2016-4).

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Declaration of competing interest

The authors declare that they have no conflict of interest to this date.

Acknowledgements

The authors would like to acknowledge FAPESP – São Paulo Research Foundation for the Scholarship granted to the first author, Grant 2016/13813-8and CNPq for founding part of the research (CNPq 404577/2016-4). National System of Laboratories for Nanotechnology (SisNANO/MCTIC) is acknowledged for its financial support in infrastructure and equipment at the LNNano. Research was supported by facilities and equipment at LNBR – Brazilian Biorenewables National Laboratory (CNPEM/MCTIC).

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