(60e) Dynamic Modeling of the Nitrogen Cycle in Biochar Amended Soils: A New Approach for Rapid Screening of Designer Biochars

Dynamic Modeling of the Nitrogen Cycle in Biochar Amended
Soils: A New Approach for Rapid Screening of Designer Biochars

Hao Sun^1,3, Caroline A.
Masiello² and Kyriacos Zygourakis¹

1.    
Department
of Chemical and Biomolecular Engineering, Rice University, 6100 Main Street,
Houston, TX 77005, USA

2.    
Departments
of Earth Science and Chemistry, Rice University, 6100 Main Street, Houston, TX
77005, USA

3.    
Currently
with Chevron Energy Technology Company, 3901 Briarpark
Dr., Houston, TX 77042

ABSTRACT

Maintaining a sustainable food supply in the 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 thanks to
the development of new high-yielding crop varieties optimally grown with the
help of fertilizers.

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. As a result,
the increased use of N as fertilizer 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. More importantly, denitrification reactions occurring in
saturated soils convert nitrate to nitrous oxide, a major greenhouse gas and
significant contributor to the depletion of atmospheric ozone

Several studies have reported that the addition of biochars
to weathered tropical or temperate region soils decreased leaching losses of
nutrients. In other cases, however, biochar amendments did not have the
expected beneficial effects. Such contradictory results generated a lot of
interest in the production of designer
biochars, which are engineered to provide specific services.  Although the idea of designer biochars
seems reasonable, their production becomes a daunting problem. There are dozens
of feedstocks from which biochars can be produced in multiple types of reactors
under varying temperature and oxygen conditions, quickly leading to thousands
of potential biochars with widely varying properties.  Then, each of these biochars must be
added to a wide range of soil types. 
For each biochar-soil type pair to be tested in a field trial, farmland
must be allocated in statistical replicates for multiple cropping seasons (ideally
for 3-5 years). Clearly, this is a very time-consuming process that may take
decades to implement.

In an effort to speed up this process, we have begun to
develop a computational framework that will allow us to rapidly assess the
environmental performance of biochars and guide their application as soil
amendments. The amendment of soils with biochar introduces a new stationary
phase that necessitates an extension to the classical dual porosity models that
considered pore diffusion but only one adsorbent (stationary) phase. Biochars
are highly porous materials with interconnected networks of pores that span
multiple length scales: from sub-nanometer micropores to macropores with sizes
of the order of 10 μm or larger. Moreover, the size and adsorption
capacity of biochar particles may be significantly different than the
corresponding properties of soil particles.

To properly account for all these factors, we developed a
dynamic convection-dispersion-adsorption model that considers nutrient
transport through beds with two porous adsorbent phases: soil and biochar.
Intraparticle diffusion of the nutrient in both stationary phases was also
considered and Langmuir isotherms were employed to describe the local
equilibria between the solute diffusing in liquid-filled pores and the solute
adsorbing on the pore surfaces of the biochar and soil particles. The resulting
system of PDEs was solved numerically using the method of lines.

Simulation results will be presented to demonstrate that
addition of biochar can effectively slow nutrient transport through the soil if
the biochar/soil ratio and crucial biochar properties (like its adsorption
capacity and affinity to the sorbate) are carefully matched to the soil
properties (water velocity, soil type) and the amount of rainfall or
irrigation. Simulations can also track the spatial and temporal evolution of
nutrient concentration profiles and identify the factors that modulate nutrient
partitioning between the soil and biochar phases.

By allowing us to isolate and analyze the interactions of
key components of our system, the mathematical model provides a systematic way
for adjusting the amount and properties of a biochar in order to maximize its agricultural
benefits. The amount of biochar applied to a field, for example, could be
matched to the hydrodynamic characteristics (permeability, pore water velocity,
soil type) of a specific field or to the amount of expected rainfall in order
to achieve the desired nutrient retention effects. Or, we could select a
feedstock and production method that would yield a biochar with optimal
adsorption capacity and sorbate affinity. At the same time, the model can tell
us when the application of a biochar will not work.  For example, simulations have identified
several cases where the addition of biochar would not provide significant
benefits either because the biochar has low adsorption capacity (e.g. cation
exchange capacity in the case of ammonium fertilizer), or weak affinity to the
nutrient/sorbate or because it has been applied to a highly permeable soil
characterized by high pore water velocities.