(224b) Biochar Amendments for Increased Crop Yields: How Can Biochars Improve Crop Nutrient Availability?

Maintaining a sustainable food supply for the foreseeable future without excessive environmental degradation will require novel engineering approaches for remediating the human disruption of the nitrogen cycle. Over the past several decades, food production has been able to keep pace with human population growth only thanks to the development of new high-yielding crop varieties grown with the help of fertilizers. Thus, vast amounts of energy are consumed every year to produce the approximately 200 Mt of ammonia needed to make the nitrogen (N) fertilizers used by farmers around the globe.

Unfortunately, more than half of the N fertilizer applied to a field is not available for plant growth due to losses caused by surface runoff, leaching into surface and ground water, or volatilization. Because of these losses, the increased use of nitrogen fertilizers has been linked to a variety of water pollution problems ranging from the expansion of the hypoxic zone in the Gulf of Mexico caused by eutrophication to the contamination of wells and groundwater with nitrate N. Moreover, denitrification reactions convert nitrates to nitrous oxide, a major greenhouse gas and nitrogen oxide, a major atmospheric pollutant and significant contributor to the depletion of atmospheric ozone.

The amendment of soils with biochar has been heralded as a sustainable method for increasing crop yield and preventing pollution problems caused by fertilizer runoff. However, the available experimental results are highly variable [1-3]. While some studies reported significant beneficial effects with up to 60% higher yield after biochar addition, other studies reported that biochar amendments either had no effect or even resulted in up to 30% lower agricultural productivity [3]. One of the most important reasons for this variability is that biochar is not a single entity. Dozens of biomass feedstocks are used to produce biochars in multiple types of reactors under varying temperature and oxygen conditions, leading to thousands of biochars with widely varying properties. To complicate things even more, soil properties, climate conditions (like rainfall and temperature), plant requirements and many other parameters vary from application to application. As a result, such studies and meta-analyses have not provided much insight into the fundamental mechanisms that modulate the environmental performance of biochar amendments.

Our Previous Work on this Problem:

To begin bridging this knowledge gap, we focused our recent research on the mechanisms that control the ability of biochars to improve crop nutrient availability, one of the main mechanisms proposed to explain crop yield increases in biochar-amended soils [2].

As a first step in this direction, we have recently developed and tested a mathematical model that uses first principles to describe nutrient transport in soils amended with biochar [4]. Using a simple Langmuir isotherm to describe nutrient adsorption, our simulation results revealed that nutrient transport in biochar-amended soils is modulated by a complex interplay between the physical mechanisms governing flow in porous media and nutrient transport, and the chemical mechanisms modulating nutrient adsorption of the pore surface of biochar solids. Nutrient transport rates through the topsoil decreased when biochars with the proper adsorption capacity and affinity were mixed with the soil. This was due to the differential dynamics of the adsorption and desorption processes. Nutrient adsorption was significantly faster than desorption, with biochar essentially acting as a slow (or controlled) release medium for the adsorbed nutrient [4].

Current Work and Summary of Results:

The present study extends our previous work with a systematic study of the adsorption and desorption dynamics on a much broader class of biochars. We report the development of a mathematical model that describes the response of biochar particles to a pulse of nutrient that simulates a single fertilizer application. Nutrient adsorbs on the biochar particle when the fertilizer solution flows past the particle and starts desorbing when the fertilizer solution is replaced by pure water. Two nutrient transport mechanisms are considered: (a) diffusion through liquid-filled pores and (b) surface diffusion. We assume that nutrient adsorption follows a Sips isotherm whose parameters can be varied to describe adsorption on both homogeneous and heterogeneous solids. The Freundlich and Langmuir isotherms commonly used to describe adsorption of ammonium or nitrates on biochars are special cases of the Sips isotherm. The highly nonlinear partial differential equations describing this transient adsorption/desorption problem are discretized using high-order finite differences and the resulting system of first order differential equations are integrated with a variable-step, variable-order integrator based on numerical differentiation formulas.

As noted previously, a “good” biochar should be able to quickly adsorb the nutrient when it is present in the liquid flowing around it, but it should desorb it much more slowly in order to act as slow (or controlled) release medium. This is the mechanism through which a “good” biochar will improve crop nutrient availability [2, 4].

Our simulations clearly show that the ability of a biochar to slowly release the adsorbed nutrient depends on a complex interaction of (a) external mass transfer, (b) intraparticle diffusion (both pore and surface diffusion), and (c) adsorption dynamics (affinity constant and adsorption capacity).

The interaction between external mass transfer and intraparticle pore diffusion is a key factor for modulating the release of nutrient that has adsorbed on the biochar. Our system is characterized by low values of the Biot number, the dimensionless number that represents the ratio of external mass transport rate over the intraparticle diffusion rate. This is because of the low interstitial water velocities (10-90 cm/day) in biochar-amended soils. Low Biot numbers are one of the key factors responsible for the long desorption tails that characterize the slow release of nutrients from “good” biochars.

Our simulations reveal that all the processes mentioned above can significantly affect the nutrient release rates. For example, surface diffusion accelerates nutrient desorption from small (2 mm in diameter) and highly porous biochar particles (0.6 total porosity). But, surface diffusion will slow down desorption from larger (4-8 mm in diameter) and less porous particles, even though the latter biochar particles may have exactly the same adsorption isotherm parameters. Adsorption affinity and site heterogeneity can also interact with porosity and particle size to modulate nutrient desorption. Finally, surface diffusion again improves the ability of a biochar to act a slow release medium at high interstitial velocities.

By revealing that the response of biochar particle to a pulse of nutrient is governed by complex interactions among external mass transfer, intraparticle diffusion and adsorption, this study provides new insights into the fundamental mechanisms that influence the environmental performance of biochar amendments. It may also explain why biochars that share some key properties (e.g. carbon content) or produced at the same pyrolysis temperature can potentially have widely different impacts on crop yields, as has been repeatedly reported in the literature [2,3]. Much better characterization of biochar properties (porosity, adsorption isotherm, intraparticle transport mechanism) and more precise control of amendment or soil parameters (like particle size, biochar aging, soil permeability) are necessary before we can fully elucidate the mechanisms of biochar action.

References:

[1] Spokas, K. A.; Cantrell, K. B.; Novak, J. M.; et al. “Biochar: A Synthesis of Its Agronomic Impact beyond Carbon Sequestration.” J. Environmental Quality 41(2012) 973-989.

[2] Jeffery, S; F.G.A. Verheijena; M. van der Velde; A.C. Bastos. “A quantitative review of the effects of biochar application to soils on crop productivity using meta-analysis.” Agriculture, Ecosystems and Environment 144(2011) 175–187.

[3] Crane-Droesch, A; S. Abiven; S. Jeffery; M.S. Torn. “Heterogeneous global crop yield response to biochar: a meta-regression analysis.” Environmental Research Letters 8(2013) 044049.

[4] Sun, S; C. E. Brewer; C. A. Masiello; K. Zygourakis. “Nutrient Transport in Soils Amended with Biochar: A Transient Model with Two Stationary Phases and Intraparticle Diffusion” Industrial and Engineering Chemistry Research, 54 (2015) 4123–4135.