Corn stover removal impacts on micro-scale soil physical properties

Abstract

Crop residues are a potential source for biofuel production. Yet, impacts of removal of corn (Zea mays L.) stover and other residues as biofuel feedstocks on micro-scale soil properties affecting the behavior of the whole soil are not well understood. Data on both macro- and micro-scale soil properties under different scenarios of stover management are needed to define the threshold levels of stover removal. Previous studies on stover removal impacts on soil properties have primarily focused on macroscale properties and little or not on microscale properties. Thus, this study was designed to assess impacts of annual stover removal for 3 consecutive years at rates of 0, 25, 50, 75, and 100% on soil physical properties at the aggregate level under no-tillage (NT) continuous corn systems in a Rayne silt loam (RSL) (fine-loamy, mixed, active, mesic Typic Hapludult) with 10% slope, Celina silt loam (CSL) (fine, mixed, active, mesic Aquic Hapludalfs) with 2% slope, and Hoytville clay loam (HCL) (fine, illitic, mesic Mollic Epiaqualfs) with < 1% slope in Ohio. Aggregates were sensitive to stover removal particularly in the 0- to 10-cm soil depth. Stover removal reduced aggregate stability, tensile strength (TS), water retention (WR), and subcritical water repellency, while it increased water sorptivity but had no effect on pore-size distribution within an aggregate. The interaction of aggregate stability, strength, and subcritical water repellency with aggregate water potential was highly significantly, indicating that the magnitude of changes in aggregate structural properties due to stover removal depended on the variable antecedent soil water conditions. The aggregate stability and strength decreased while water repellency increased with increasing water potential. Impacts of stover removal on the clayey soils were equal to or higher than those on silt loam soils. Removal at rates ≥ 25% reduced the raindrop kinetic energy (KE) required to break aggregates by 13 times at HCL, while removal at rates ≥ 50% reduced the KE by 2 to 3 times at RSL and CSL for air-dry aggregates. The KE and TS decreased with increasing soil water potentials. The TS was reduced by 10% to 30% at all water potentials at RSL, by 20% to 35% between − 0.1 and − 1.5 MPa, and by 2.2 times at − 166 MPa at CSL, and by 2 to 3 times between − 1.5 and − 166 MPa at HCL under complete stover removal. Removal of stover at rates ≥ 50% reduced subcritical water repellency by 2 to 10 times in all soils. Overall, stover removal altered micro-scale soil properties, and complete stover removal had the most detrimental effects. Based on the data from previous studies on macroscale soil properties and this study, stover removal adversely affects both macro- and micro-scale soil properties.

Introduction

Corn stover is one of the sources of lignocellulosic feedstock for biofuel production as an alternative to fossil fuel (Service, 2007). The impacts of such harvesting on macro- and micro-scale soil properties essential to the ecosystem services in general and soil properties in particular have not been, however, adequately addressed. Current discussion about stover harvesting is mostly focused on the amount of stover biomass available for biofuel production (Graham et al., 2007), harvesting protocols (Hoskinson et al., 2007, Shinners et al., 2007), and technologies for converting stover to ethanol (Demirbas, 2007, Kim and Lee, 2007) and not as much on the implications of stover removal on soil behavior (Wilhelm et al., 2004). Threshold levels of removal are being established strictly based on erosion T (tolerance) value, maximum amount of soil loss (approximately 11 Mg ha^− 1 yr^− 1) that will not significantly reduce the crop production (Troeth et al., 2004, SSSA (Soil Science Society of America), 2008), rather than on all the soil-related environmental and agronomic factors (Nelson, 2002, Graham et al., 2007). A priori and detailed assessment of implications on soil characteristics is needed to define the threshold levels of stover removal.

The importance of maintaining crop residues on the soil surface to control soil erosion and improve soil organic matter (SOM) dynamics is well recognized (Mann et al., 2002, Johnson et al., 2006, Wilhelm et al., 2007). Data on the implications of stover removal on soil physical properties specifically those at the aggregate or microscale level are not, however, well documented. Stover removal impacts on soil properties must be studied on both macro- and micro-scales to better understand the soil dynamics. Karlen et al. (1994) compared differences in physical, chemical, and biological properties of soils under 10-year NT continuous corn systems with and without stover removal. Recently, Blanco-Canqui et al. (2006) and Blanco-Canqui and Lal (2007a) assessed the one- and three-year impacts of stover removal on soil hydrological and structural properties in three NT soils receiving 0, 25, 50, 75, and 100% of stover mulch. These studies have primarily examined macroscale soil properties, but have not intensely scrutinized properties at the aggregate level.

Soil aggregates are the structural elements defining the macro-scale behavior of the whole soil (Horn et al., 1994, Blanco-Canqui et al., 2005). The architecture of the bulk soil, made up of aggregates, primary particles, and the inter-aggregate space, is directly influenced by the internal micro-scale or aggregate structure (Blanco-Canqui et al., 2005). Assessment of the implications of stover removal on discrete soil aggregates is important because aggregates influence root growth (Reuss et al., 2001), SOM dynamics (e.g., C protection and sequestration) (Six et al., 2000, Urbanek et al., 2007), water storage (Carminati et al., 2007), nutrient sorption and availability (Wang et al., 2001), and soil erosion (Blanco-Canqui et al., 2007). Soil erodibility is determined by the ability of intra-aggregate domains of surface aggregates to resist impacting raindrops and shearing forces of runoff (Legout et al., 2005). Aggregate structure also determines water, air, and heat flux through the soil matrix (Youngs and Leeds-Harrison, 1990), whereas the inter-aggregate space favors preferential or rapid flux of energy (e.g., macropore flow) through the whole soil (Leeds-Harrison and Youngs, 1997). Thus, characterization of properties of individual aggregates would allow a better understanding of the impacts of stover management on soil matrix behavior.

Aggregates may respond to stover removal differently from the whole soil because the mechanisms of development and turnover of these elemental units differ from the bulk soil (Horn, 1990). Soil aggregates tend to have higher tensile strength and density than the bulk soil because of their higher internal cohesiveness and friction forces (Munkholm and Kay, 2002, Blanco-Canqui et al., 2005). Thus, a study of individual aggregates may provide additional information to discern stover management implications. Such study must specifically assess the implications of stover removal on influential physical properties of individual aggregates such as density, stability, strength, pore-size distribution, and water repellency, sorptivity, and retention under NT across different soils.

Aggregate stability, a key soil structural property, is routinely determined on a group of aggregates or bulk soil (Nimmo and Perkins, 2002) rather than on single structural units or aggregates (Blanco-Canqui et al., 2007). Characterizing the stability of individual aggregates may help in better understanding of soil detachment dynamics. A related dynamic but little studied aggregate physical property is the subcritical water repellency, which refers to the ability of soil to slightly repel water due to the presence of hydrophobic organic substances (Goebel et al., 2004). The subcritical repellency is of paramount importance to soil processes in that it impacts water sorptivity and infiltration, aggregate slaking and dispersion, and runoff and soil erosion (Hallett et al., 2001, Goebel et al., 2004, Eynard et al., 2004). Removing stover cover may reduce aggregate subcritical water repellency by reducing the input of hydrophobic organic substances. To date, the extent to which stover removal affects the subcritical repellency of aggregates has not been assessed.

Sorptivity is another important property of discrete structural units. On well-aggregated soils, the water transmission characteristics between the inter-aggregate and intra-aggregate domains often differ (Gerke and Köhne, 2002). Hydraulic parameters are normally measured on the whole soil producing composite hydraulic conductivity values of inter-aggregate and intra-aggregate water flow. This common approach ignores the boundary values between the two domains. Knowledge of differences in water sorptivity between aggregates and the whole soil is essential to modeling preferential flow and soil matrix flow of water and solutes through the soil (Leeds-Harrison et al., 1994).

Data on the interactive effects of tillage-cropping-residue management systems on aggregate properties abound (Watts and Dexter, 1997, Munkholm and Kay, 2002), but experimental data on the independent effects of stover removal for expanded uses on physical properties of individual aggregates under the same tillage system such as NT are virtually unavailable. Thus, this study was undertaken to systematically assess the impacts of stover removal for alternate uses at various rates on physical properties of single aggregates in long-term NT continuous corn systems in three different soils in Ohio. The hypothesis tested is that stover removal even at small rates (e.g., 25%) significantly alters the aggregate properties. In this paper, the word “stover” refers to the aboveground residue (e.g., stalk, husk, leaves, cobs) of corn left on the soil surface after harvest (Wilhelm et al., 2004).