In-Field Corn Residue Management for Bioenergy Use: Potential Effects on Selected Soil Health Parameters

In the U.S. biofuel industry is using corn (Zea mays L.) residue mix (CRM) consisting of corncob and stover for cellulosic ethanol and biogas production. The field storage method left different depths of CRM on the field after its removal, where negative effects on plant growth were observed. The objective of this study is to evaluate the CRM effect on selected soil health indicators. The field study conducted with four different depths of CRM, two tillage systems (no-till (NT) and chisel plow (CP), and three nitrogen (N) rates (0, 180, and 270 kg∙N∙ha −1 ) in a randomized complete block design with split-split arrangements in three replications in a continuous corn system from 2010 to 2012 at the Agronomy Research Farm at Iowa State University. The findings of this study showed a negative effect on soil organic carbon (SOC) change across all treatments at 0 - 15 cm (−0.35 to −0.03 Mg∙ha −1 ∙yr −1 ), while at 15 30 cm there was an increase in SOC rate (0.13 to 0.40 Mg∙ha −1 ∙yr −1 ) after 2-yr. In addition, soil aggregate-associated C of macro-aggregates decreased by 8%, while micro-aggregates increased by 2%. Soil microbial biomass carbon (MBC) across tillage and N rates for 2.5 & 7.5 CRM treatments increased by 14% in June to July 2011, while in 2012 increased by 9%. However, at the 15 cm soil depth, soil bulk density (ρ b ), soil penetration resistance (SPR), and soil pH showed no significant differences among CRM treatments. The findings of this study showed that in-field CRM management can affect certain soil health parameters in the short term.


Introduction
In the past two decades the transformation of energy from fossil fuel base to bioenergy sources encouraged the establishment of a new industry; which is dependent on corn (Zea mays L.) grain as the main feedstock material for biofuel production in the Midwest, U.S. [1]. Recently, corn residue has been considered by the bioenergy industry as a viable feedstock source for cellulosic ethanol production [2]. The use of corn grain and crop residue for ethanol production and biogas has the potential to reduce fossil fuel consumption and greenhouse gas emissions [2] [3]. However, methods of collection and storage of feedstock for cellulosic ethanol production in the Midwest, U.S., can present significant challenges to soil health and plant growth.
The storage method consists of piling corn stover and corncob residue mix (CRM) at the edge of the field after harvest, where it is stored over winter until it is collected and transported for processing the following spring. Field observations of corn planted in CRM storage areas after removal exhibited poor plant development and yield reduction the following season. In Iowa, where soils are inherently fine-textured with poorly drained conditions, storing and handling CRM can have significant effects on soil moisture, nitrogen (N) availability, and tillage timing. These conditions present challenges for soil and crop residue management, which are essential to provide optimal seed conditions for germination, seedling development, and plant growth [4] [5].
In our initial field observations, CRM storage areas showed soil compaction due to heavy equipment traffic used to store and remove CRM; these soil compacted areas caused surface runoff, soil erosion, and subsequent decrease in plant aboveground biomass and grain production [2]. Careful management of CRM left after residue removal is essential to maintain healthy soil conditions to ensure healthy crops. Therefore, CRM on poorly drained soils, if not managed correctly, may cause autotoxic effects on seedlings, which slows plant growth and development [5] [6], and often leads to slow plant emergence and N mineralization [7] [8].
CRM on the soil surface can influence N availability early in the growing season, where cold soil temperatures slow SOC mineralization affecting plant N use and accumulation [5] [9]. Thus, CRM requires the integration of management practices that include tillage and N application to overcome initial N supply limitation. Adequate N fertilization can aid in soil C retention by increasing below and aboveground biomass production as a source for SOC input [10] [11].
However, the sustainability of CRM storage methods will depend heavily on the cropping system, N fertilization, climate, and soil type [12], for each region to minimize the potential negative impact on soil physical, chemical, and biological properties. The objectives of this study were to investigate the potential effects of CRM on the soil surface and its interaction with soil physical and biophysical properties. Also, to evaluate potential management practices such as tillage systems and N fertilization and their interaction effects on soil health. We hypo-

Soil Sampling and Measurements
Initial soil samples were collected in mid-October after harvest prior to the establishment of treatments in 2010. Those soil samples were analyzed to establish the baseline for the different parameters of the soil prior to the start of the study.
In the fall of 2011 and 2012, soil samples were collected and analyzed for soil organic carbon (SOC), soil total nitrogen (STN), soil bulk density (ρ b ), and soil pH. Soil sampling was done by collecting twelve 1.7 cm diameter soil cores from depths: 0 -7.5, 7.5 -15, and 15 -30 cm for each treatment plot. Collected soil cores for each sample were mixed and sieved through a 2 mm sieve, then air dried before analysis. The SOC and STN analyses were done by dry combustion using a CN analyzer (TRUSPEC, LECO Corporation, St. Joseph, MI). Soil pH Open Journal of Soil Science was measured by mixing 1:1 soil to water ratio using an AR15 pH meter (Accu-met® Research, Fisher Scientific International Inc.). Soil samples with pH values greater than 7.1, had a separate inorganic C analysis done to correct total soil C values, by subtracting them from the total soil C results to obtain SOC. The inorganic C determination was done by using a modified pressure calcimeter method [13].
Soil samples for ρ b were simultaneously conducted during the soil sampling for total C and N for each depth by taking three 1.

Water Stable Aggregates
Soil samples for water stable aggregates (WSA) were collected from all tillage, CRM, and N treatments in the fall of 2011 and fall of 2012. Soil samples collected in fall 2010 were analyzed to establish the baseline data of the soil physical properties prior to the start of the experiment. A single soil core was randomly taken Open Journal of Soil Science using a 7.6 cm diameter golf course hole-cutter to a soil depth of 15 cm; then the sample was brought back to the lab where it was sieved through an 8 mm sieve. Through this process, undesirable materials such as plant residue, rocks, corncob, and grains were removed. The soil samples were then air-dried and ready for analysis following the procedure by [15]. A 100 g of soil from each sample was placed at the top of a set of six sieves stacked top to bottom as follows: 4, 2, 1, 0.50, 0.25, and 0.053 mm. The set of six sieves were then submerged into a wet aggregate apparatus container filled with deionized water at ~21˚C and vertically oscillated for 5 minutes with a stroke length of 2 cm. The frequency of oscillation was maintained at 90 strokes min −1 . A wet aggregate apparatus is a custom-made machine in which the 20-cm diameter sieves could be fitted [16]. It was noted that soil passed through the last sieve after 5 minutes of constant stroking. This was considered as aggregate size of <0.053 mm, which was captured by a plastic tub at the bottom of the aggregate apparatus. Then, each aggregate size was washed into plastic tubs using deionized water and oven dried at 65˚C until all water in the tubs had evaporated. The weight of each aggregate size fraction was determined by weighing the plastic tub and dry aggregates and subtracting the plastic tub weight. The WSA for each fraction size is expressed as a percentage of the total sample weight on an oven-dry basis. Soil aggregate-associated total C was determined for each size fraction that was collected as described above, where oven-dried different aggregate size fractions were stored in a coin envelop for SOC analysis. The soil samples of each aggregate size fractions were ground with mortar and pestle and analyzed for SOC using a dry combustion method by using CN analyzer (TRUSPEC, LECO Corporation, St. Joseph, MI).

Soil Penetration Resistance
Soil penetration resistance (SPR) readings were collected following the application of CRM treatments. Measurements were taken two times during the study including spring 2011 and spring 2012. The SPR was determined using a Rimik CP-20 penetrometer (Soil Measurement System, Tucson, AZ) using a 30˚ cone with a 1.27 cm diameter base targeting an insertion speed of 1.3 m•min −1 , with a range of 0.01 to 2 m•min −1 . The SPR measurements were conducted for tillage, CRM treatments, and 180 kg/ha N rate, where three random insertion points per plot were recorded at 2.5 cm soil depth increments down to 60 cm.

Statistical Analysis
Data for soil health parameters were analyzed using the statistical analysis procedure of PROC MIXED [17] with repeated measurements. A compound symmetry-covariance structure was used for repeated measures. Tillage system was considered as the main plot treatment, which was split into different CRM levels as the split treatments, N rates as split-split-plot, and date of measurements as the repeated measure variable. Mean separation was determined using the PDIFF procedure and significant difference was determined at p ≤ 0.05.

Soil Organic Carbon and Total Nitrogen
During this study, SOC rate of change was significant at different depths (p = 0.006) for both tillage systems. Changes in SOC were mostly negative across CRM treatments, tillage systems, and soil depths (Figure 2(a) and Figure 2 Figure 2(d)). The increase in STN rate in this study with NT is largely due to less soil disturbance and greater soil moisture retention. However, low SOC and STN in the 2012 season may be due to the drought conditions in 2012 ( Figure 1). Yet, the increase in SOC at the 15 -30 cm soil depth for both tillage systems can be a result of CRM incorporation at lower depths for CP and greater soil moisture content with NT increasing root development and soil organic matter contribution [18]. The lack of significant differences in SOC and STN changes between CRM treatments and N rates in this experiment is most likely due to the short duration of treatments effect [19]. Also the changes in SOC and STN observed during this experiment can be attributed to the large amount of CRM applied resulting in greater raw organic materials input coupled with management effects (i.e., N rate and tillage), and a possible increase/decrease of SOC and STN mineralization rate [18] [20].

Soil Microbial Biomass Carbon
Results of soil MBC is presented as an average across tillage systems and N rates due to no significant differences between tillage systems and N rates within each  where MBC for 2.5 cm and 7.5 cm CRM treatments were significantly greater than that for control-2010 or removed CRM treatments. High soil MBC observed during June and July is consistent with findings by other studies, where this increase was attributed to an increase in organic matter input [21], and the decomposition of these organic materials throughout the growing season. The 2.5 cm and 7.5 cm CRM treatments greatly influenced MBC in this experiment over tillage systems and N rates, which was documented by another study [22] [23].

Water Stable Aggregates and Soil Aggregate-Associated Carbon
The WSA was measured to determine the potential change in soil structure at the aggregate level as affected by CRM treatments, tillage systems, and N rates.  The aggregate-associated C content was primarily affected by the relationship between aggregate fraction size and CRM treatments in 2011 and 2012. No differences were observed due to tillage systems and N rates (p = 0.0932), thus an average across these treatments is presented in Figure 5. During this study, CRM treatments showed no effect on aggregate-associated C in both years. An gate-associated C content can be explained by the aggregate hierarchy theory of soil aggregate fractions arrangement and accessibility of soil C to the microbial community, for loss or gain of soil organic matter [27]. In this arrangement, micro-aggregates were protected by macro-aggregates and a significant amount of fresh C (CRM), which further protect micro-aggregates from microbial activities. It is well documented that cultivation (i.e., tillage) reduces soil WSA percentage and its associated C content at the macro-aggregates level due to soil disturbance and change in its aggregate distribution [28]. However, in our study, no significant differences were found between tillage systems. This can be attributed to the length of treatment effects (2-yr) since the implementation of tillage systems and the application of CRM treatments offsetting any significant changes in the short term.

Soil Penetration Resistance and Bulk Density
The SPR measures treatments' effects on soil physical properties, the measure-  [29]. Likewise, we observed that CRM treatments had no effect on soil bulk density (ρ b ) in 2011 (Table 1) due to the short period of the experiment. However, we observed the following year (2012) that 2.5 cm and 7.5 cm CRM treatments showed slightly lower values of bulk density than the control and CRM treatments. The improvement in SPR and ρ b in plots covered with CRM treatments was most likely due to residue cover mitigating soil surface compaction from equipment traffic that can cause high ρ b [30].

Conclusions
The The combined effects of CRM and tillage treatments were evident in affecting soil physical properties, such as soil macro-aggregates and micro-aggregates fractions and their associated C content by increasing soil micro-aggregates stability and associated C content. Improvement in ρ b, SPR, and aggregate stability was associated with high amounts of CRM left on the soil surface. The findings of this study show that the process of CRM removal can affect certain soil health parameters in the short term differently, where some soil health indicators such as MBC, WSA, and SPR are most sensitive to management effects in the short term. However, long-term evaluation of CRM field management is essential to the development of best management practices that minimize the impact on soil health.

Conflicts of Interest
The authors have no conflicts of interest regarding the publication of this article.