Reactive Transport of Manure‐Derived Nitrogen in the Vadose Zone: Consideration of Macropore Connectivity to Subsurface Receptors

Abstract

Core Ideas Manure N transformation was modeled with different soil pore connectivities to tile drains. Macropores facilitated NH4+ oxidation reaction products deeper in the profile. Nitrate losses to tile were best predicted by models using pores connected to tile drains. Macropores can be important conduits for surface‐derived nutrients to reach subsurface receptors. Accordingly, nutrient reactive transport processes in macroporous soils need to be well understood. In this study, steady‐state two‐dimensional reactive transport simulations with MIN3P‐THCm (version 1.0.519.0) were used to elucidate how soil macropore connectivity to tile drains can influence N transformations following liquid swine manure (LSM) applications to soil. Four different soil scenarios were considered: homogeneous sand, homogeneous clay loam, and clay loam with discrete macropores connected to or disconnected from the bottom boundary used to represent tile drain outflow. In relation to the homogeneous soils, macropores, overall, facilitated chemical diffusion into the adjacent soil matrix along their length and broadly augmented O2 ingress into the soil profile. These processes combined to critically control the spatial distribution of NH4+ oxidation reaction products. When used in transient simulation mode with field data observed at experimental tile‐drained plots that received LSM application, the model showed that simulated nitrate mass losses to tile are considerably higher and most realistic under the connected macropore scenario compared with the homogeneous or disconnected macropore scenarios.

Numerical and Analytical Modeling of Flow Partitioning in Partially Saturated Fracture Networks

“…When fractures are connected, they can form percolating clusters (Adler et al, 2013;Berkowitz & Scher, 1995) that can reach length scales far beyond the thickness of individual geological layers/units and potentially extend across the entire vadose zone. Similar features can be observed in soil systems, a type of material where the heterogeneity is commonly associated with macropores (wormholes), which can also form percolating clusters (Hussain et al, 2019;Jarvis, 1998;Nimmo, 2010).…”

The concept, approach, and future research of hydrological connectivity and its assessment at multiscales

“…Soil macropores, i.e., connected macropores and isolated macropores, in soils can form a complex network. Interconnected soil macropores can improve soil aeration and strongly affect water flow and chemical transport and reaction process (Hussain et al 2019 ; Pittman et al 2020 ). Those well connected, functional, and hydrologically active soil macropores with high connectivity are worthy to positively increase water infiltration (Jarvis et al 2017 ; Zhang et al 2018 ; Borges et al 2019 ; Ma et al 2021 ).…”

“…Soil macropores connectivity is directly implicated with groundwater discharge, waste disposal risk assessment, and heavy metal redistribution in soils (Beven and Germann 2013 ; Villamizar and Brown 2017 ). Despite the importance of soil macropores connectivity for environmental water quality, there are few studies that characterize soil macropore connectivity at larger scales (Hussain et al 2019 ).…”