Biochar in manure can suppress water stress of sugar beet (Beta vulgaris) and increase sucrose content in tubers

Highlights

• Biochar amendments to a drought prone soil enhanced soil moisture status.

• Nutrient leaching from soil following manure application was mediated by biochar.

• Combined biochar-manure amendment supressed water stress to sugar beet.

• Increased tuber sucrose content and yield resulted from biochar-manure blend.

Abstract

Increased soil drought events threaten the yields of sugar beet (Beta vulgaris L.) and other staples of arable production in central Europe. In this study we evaluated soil moisture and nutrients as impacted by a two and five % (wt) addition of biochar, manure and their blend to a loamy-sand Regosol. Cyclical soil drought was achieved by the controlled reduction of watering by 75% in pot experiments. Ongoing soil moisture and nutrient measurements were taken, and physiological parameters of sugar beet plants were analysed three weeks after the induced drought. At the end of the experiment (16 weeks) plants were harvested and their mass assessed, as well as their nutrient, pigment and sugar contents. In contrast to the addition of manure, soil volumetric water contents were two to three times greater after biochar amendment, compared to the control soil. Porewater analysis revealed that nutrient leaching (e.g., NO₃⁻, K⁺) from manure addition to soil was reduced when biochar was blended in (by ≤86% compared to manure alone). Crop analysis showed that leaf gas exchanges were moderated during drought following soil amendment, and leaf and tuber yields were increased furthest when combined biochar-manure blends were applied (> 2-times compared to the control). Perhaps most importantly, the advantageous soil conditions induced by the combined biochar and manure addition also resulted in significantly increased sugar contents in plants (2.4-times) pointing to immediate practical applications of these results in the field.

Introduction

Sugar beet (Beta vulgaris L.) is a traditional arable crop in Central Europe, with Czech Republic being one of the most prolific producers at 61,000 ha coverage and 63 t/ha yields (data for the years 2015–2017). Indeed, sugar is consumed worldwide with one-fifth derived from beet (Eggleston et al., 2017), which contains approximately 17% sugar. Climate change has had, and will increasingly have, a negative impact on crop yields. For example, since 1982, it is estimated that climate change induced a reduction of 70% in crop yields on average, with few agricultural areas now remaining unaffected by it (Raza et al., 2019). Drought events are one of the most important crop yield decreasing stresses (Ghaffari et al., 2021). This is partially because a reduction in soil moisture has deleterious effects on plant physiology, such as leaf water loss, growth inhibition, decreased photosynthetic activity, damaged organelle structures, induced chlorophyll degradation and even accelerated ageing processes (Fahad et al., 2017). Drought stress is particularly impactful to sugar beet yield and quality (Toscano et al., 2019) as demonstrated by Khaembah et al. (2021), who showed that reduced irrigation decreased sugar beet yield and nitrogen content. Similarly, sugar beet biomass production and pigment concentrations were shown to be reduced after a period of ten days without irrigation (Islam et al., 2020). To overcome water stress, increased irrigation is an increasing feature of modern agriculture to maintain yields. However, this incurs economic and time costs to farmers especially when additional fertilization is required. Over irrigation can induce soil salinization, while over fertilization can degrade soil in the long term, notably through nitrate leaching. Thus, it is important to investigate methods to retain moisture and nutrients in soils, in order to adapt the soil environment to changing climatic conditions.

Improvements in soil moisture holding capacity can be obtained through the application of organic amendments, such as biochars, manures, composts and digestates. Biochar is a carbon-rich material obtained from the thermochemical conversion of biomass under reduced oxygen conditions (Chen et al., 2019; Kumar et al., 2020). Biochar can be made from various lignocellulosic materials, such as plant residues, animal manure, food wastes, sludge etc. (Bolan et al., 2021; Sun et al., 2021), which are transformed into biochar via pyrolysis, hydrothermal carbonization, or gasification (Bolan et al., 2021; Sun et al., 2021). Biochar is frequently characterized by a porous structure, an alkaline pH and a high carbon content (Kumar and Bhattacharya, 2021). Due to these parameters biochars have various soil applications, such as to increase carbon storage and reduce the leaching of nutrients and contaminants (Bolan et al., 2021; Sun et al., 2021) demonstrably improving soil fertility, promoting enhanced microbial activity, plant growth and nutrient uptake (Jien et al., 2021; Rafael et al., 2019). This is due to various soil physico-chemical properties being typically modified by biochar amendments, i.e. bulk density, pH, electrical conductivity, cation exchange capacity, organic carbon, nutrient availability, soil moisture, and enzyme activity (Abideen et al., 2020; Adekiya et al., 2019; Agbede et al., 2020; Latini et al., 2019; Rafael et al., 2019; Rollon et al., 2020). The combined improvements in one or more soil properties have been shown to improve, for example, durum wheat, Phragmites karka, cocoyam and corn growth (Abideen et al., 2020; Agbede et al., 2020; Latini et al., 2019; Rollon et al., 2020). Some negative impacts on crop growth as a result of biochar application to soils have also been noted, such as decreased nutrient availability, due to biochars' high sorption capacity (Brtnicky et al., 2021; Zhao et al., 2019). For example, a study comparing three biochars showed that 1) pinewood biochar did not improve maize growth and, in fact, negatively affected nitrogen availability; 2) coconut husk biochar improved maize growth and had a reduced fertilizing quality; 3) orange bagasse biochar was able to improve maize growth and efficiently regulate nitrogen and phosphorus availability (Gonzaga et al., 2018). Similarly, Reed et al. (2017) found that biochar did not improve the growth and nutrient uptake of ryegrass one year after soil application. Likewise, out of four biochars produced from farmyard manure, poultry manure, wood chips and kitchen waste, their sole application to soil did not improve wheat yield and nitrogen uptake in all but one case, the poultry manure biochar (Sadaf et al., 2017). It also follows that, although biochars contain nutrients, largely governed by the source feedstock used to produce it, those nutrients may not be available to plants due to the biochar high stability (Galinato et al., 2011).

In contrast to biochars, manures inherently contain high amounts of labile nutrients, which are readily available to the plants. When amended into soils, manures can modify soil porosity, reduce bulk density, which increase soil available water content, increase organic matter content in soil, which, together with nutrients and soil structure improvement, increases crop yields (Wang et al., 2017). These materials can also be mixed and work synergistically for better outcomes when applied to soils. For instance, Banik et al. (2021) demonstrated that the application of biochar combined to manure could stabilize phosphorus and nitrogen released from manure, reducing leaching, concluding that biochar could act in a regulatory capacity to nutrients from manure. Similarly, Adekiya et al. (2019) applied a hardwood biochar and a poultry manure, either alone or combined, and observed that there were significant interactive effects between biochar and manure for the improvement of soil physical and chemical properties, i.e. bulk density, soil moisture, pH, organic matter and nutrient contents, and for nutrient uptake by plants. Finally, Bohara et al. (2019) showed that applying a combination of poultry litter and biochar to a very fine sandy loam soil improved the capacity of ryegrass to withstand drought stress. Despite its advantageous soil impacts, manures can contain pollutants, such as metals and metalloids (Zhang et al., 2012), which can be supplanted to soil following repeat amendments. Biochars have been shown to immobilize such potentially toxic elements emanating from other soil amendments if applied in tandem (Chen et al., 2019).

The aim of this study was to find the most advantageous soil amendment combinations from biochar and manure that could achieve a reduction in drought stress on sugar beet yield and quality. Specifically, it was hypothesised that combining biochar and manure could most significantly; (i) enhance soil moisture, (ii) regulate nutrient leaching from manure and (iii) maintain or enhance sugar beet physiology and yield during an induced soil drought.