Following the release of the 2005 BTS, a collaborative research team1 (Table 8.2) with mem­bers from the USDA-Agricultural Research Service (ARS) Renewable Energy Assessment Project (REAP) and several universities was established as part of the Sun Grant Regional Partnership (RP) to determine the amount of corn stover that could be harvested in a sus­tainable manner [31]. The core treatments included no tillage or the least amount possible for economic crop production [e. g. Coastal Plain soils near Florence, SC, have a naturally occurring hardpan (E horizon)], so in-row subsoiling is needed each year prior to planting], three residue removal rates (none, approximately half, and the maximum mechanically collectable amount), and four replications. Leveraging the Sun Grant Partnership funds with long-term ARS research expanded both the number of treatments being evaluated as well as the number of years of study. For example, at Mead, NE, the rainfed and irrigated studies were initiated in 1999 and 2001, respectively. At Morris, MN, the study was initiated in 2005, taking advantage of a tillage experiment established in 1995. At Ames, IA, two studies were initiated in 2005 and one in 2008. Additional management practices being evaluated at one or more of the locations include alternate tillage practices (e. g. chisel plow or strip-tillage), use of cover crops, rotation with soybean, harvesting of cover crops as well as the corn stover, and application of biochar.

For each experimental site, soil samples were collected to a depth of 1.0—1.5 m, divided into increments of 0-5, 5-15, 15-30, 30-60, 60-90 and 90-150 cm, and analyzed for several soil quality indicators [e. g. total organic carbon (TOC), total nitrogen, pH, bulk density, and soil-test phosphorus (P) and potassium (K)]. The Soil Management Assessment Framework (SMAF) was used to evaluate and combine the different indicators, and thus establish a baseline soil quality index that could be used to determine long-term effects of the various stover harvest rates [15]. To date, TOC and soil-test potassium have had the lowest indicator scores at several RP and other REAP sites [16]. Longer-term data leveraged from the REAP plots at Brookings showed that through the first eight years TOC decreased as residue removal rates increased (Figure 8.1). A more detailed examination of samples collected in 2008 showed higher organic carbon content in all aggregate size classes from the low removal treatment than in the high removal treatment (Figure 8.2). Higher total protein was also measured in soil samples from the low removal treatment than from the high removal treatment.

Whole plant samples were collected and fractionated into bottom, top, cob, and grain fractions. Plant parts lying on the ground within the sampling area (1.5 m[1] [2]) were also collected. Harvest index values and total nutrient uptake were collected using those samples. Stover was collected using a variety of mechanical harvesting techniques, all resulting in post-harvest soil surface cover differences, such as those shown for the Lamberton, MN,

Table 8.2 The Regional Partnership Stover team’s principle investigators, institutions, location, and site coordinates established to determine sustainable corn stover harvest strategies. Principle Investigators Institution Location Site Coordinates Dominant Soil or Soil Association Doug Karlen3 USDA-ARS Ames, IA 42 01′ 75.667 N Clarion-Nicolet-Webster Stuart Birrell Iowa State Univ. 93 76′ 44.830" W Shannon Osborne USDA-ARS Brookings, SD 44 20′ 20.30" N Kranzburg-Brookings Tom Schumacher South Dakota State Univ. 96 47′ 31.82" W Jeff Novak USDA-ARS Florence, SC 34 1 7′ 00.32" N Goldsboro-Lynchburg-Coxville Jim Frederick Clemson Univ. 79 44′ 30.37" W Jane Johnson USDA-ARS Morris, MN 45 68′ 26.44" N Barnes-Aastad Lowell Rasmussen Univ. of Minnesota — Morris 95 80′ 22.03" W John Baker USDA-ARS St. Paul, MN 44 42′ 57" N Waukegan John Lamb Univ. of Minnesota — St. Paul 93 05′ 59"W Normania-Ves-Webster 43 43′ 40" N 95 24′ 21" W 44 21’35" N 93 12′ 10" W Garwin Gary Varvel USDA-ARS Mead, NE 41 16′ N (irrigated) Tomek Richard Ferguson Univ. of Nebraska 96 41’W 41 1 5′ N (rain fed) 96 40′ W Aksarben Paul Adler USDA-ARS Univ. Park, PA 40 86′ N Opequon-Hagerstown complex Greg Roth Pennsylvania State Univ. 77 85’W ‘Team Leader

Figure 8.1 Eight-year residue removal effect on SOC in the top 15 cm (6 inches) near Brookings, SD. (Figure provided by Shannon Osborne, USDA-ARS).

site in the autumn (Figure 8.3) or the subsequent spring (Figure 8.4) following either conventional (chisel plow) or strip-tillage.

Additional data being collected at some but not all RP locations include greenhouse gas (GHG) emissions (CO2 and nitrous oxide, N2O), nitrate nitrogen (NO3-N) and phosphorus concentrations in water leaching through the soil profile, microbial biomass carbon, partic­ulate organic matter, glomalin-related soil proteins, the humic acid fraction of soil organic matter, aggregate stability, lignin, cellulose and other structural carbohydrates, and energy values for the various stover fractions. Collectively, these measurements are providing the data needed to develop the sustainable stover harvest strategies outlined through modeling in the 2011 BT2 report.

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g 1.5

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Low cut — (> 4.5 t/ha)

High cut — (~3.4 t/ha)

Strip-Tillage Treatment

Figure 8.3 Autumn (November 2010) soil cover following various corn stover harvest treatments and either conventional (chisel plow) or strip-tillage at the Lamberton, MN, research site. (Photos provided by John Baker, USDA-ARS).

Conventional (Chisel Plow) Treatment

Low cut — (> 4.5 t/ha)	High cut — (~3.4 t/ha)	No Removal

Strip-Tillage Treatment

Figure 8.4 Spring 2011 soil cover following various corn stover harvest treatments and either conventional (chisel plow) or strip-tillage in autumn 2010 at the Lamberton, MN, research site. (Photos provided by John Baker, USDA-ARS).

Conventional (Chisel Plow) Treatment

Figure 8.5 Soil CO2 flux versus soil temperature for all 2010 treatments at the Ames, IA, site. Each point represents the average of eight measurements (4 mid-row, 4 in-row). (Figure provided by Tom Sauer, USDA — ARS).

One example (Figure 8.5) of the information being gathered shows the dependence of CO2 flux on soil temperature. The relatively strong logarithmic relationship suggests that a temperature-based interpolation method (Q10) will be most effective for estimating annual CO2 fluxes. These results also suggest that management practices which result in warmer soil temperatures, for example, through residue removal, may lead to higher CO2 fluxes. However, this effect will likely be offset by lower amount of available carbon substrate, that is, residue, so that the overall effect of stover harvest on annual CO2 flux will likely be a reduction in treatment differences.

With regard to N2O, Figure 8.6 shows that precipitation strongly influences the flux by reducing oxygen availability and stimulating denitrification. The lag between precipitation and maximum emission is evident, and is consistent with reports in the literature suggesting that the nitrous oxide flux is not maximized when the soil is saturated, but rather when water-filled pores space (WFPS) is about 65%. Annual sums of net N2O emission at this site were highest for the non-removal treatment and lowest for the maximum collectable treatment. They were also positively correlated with cumulative soil respiration, indicating that carbon availability was a controlling factor with respect to denitrification.

As recognized in the 2011 BT2 report, crop yield is a major driver associated with the availability of stover as a potential cellulosic bioenergy feedstock. Corn produces the highest volume of residue of all the major crops grown in the U. S.A. and because of the approximate 1:1 relationship between grain yield and aboveground biomass, the volume of

Figure 8.6 Nitrous oxide and rainfall relationships at the Rice County, MN, site in 2010. The "Ch. 1 to Ch. 6" designations simply refer to the six chambers used for the measurements. (Figure provided by John Baker, USDA-ARS).

available residue is directly proportional to grain yield (Table 8.3). To date, the RP studies have shown variable crop yield responses associated with stover harvest. This includes (1) no detectable short-term (3-year) effects at the Brookings, Florence, Morris, or University Park sites; (2) trends for increased yield when stover is harvested from no-till treatments at Ames and Mead; and (3) inconsistent site-differences at the Lamberton, Bauer Farm, and Rosemount sites in Minnesota. Another five-year assessment of stover removal effects near Ames, IA [16], showed that the most consistent grain yield response was a 21% lower average for continuous corn than for rotated corn. That study also showed that harvesting corn stover increased the average NPK removal by 29, 3 and 34 kg ha-1 for continuous corn and 42, 3, and 34 kg ha-1 for rotated corn, respectively, when compared to harvesting only the grain. Furthermore, it showed that the lower half of the corn plant contributed very little to the total available feedstock biomass because of its high water content and that it was not a desirable feedstock because of its high potassium, chloride, and ligin content, as well as an increased amount of soil contamination that interferes with both biochemical and thermochemical conversion processes.

So, what is the bottom line with regard to harvesting corn stover as a cellulosic feedstock? Firstly, producers must know their land. Prior to initiating any harvest strategy they should have good soil-test and nutrient management records for any areas from which crop residues may be harvested. Obviously, any land with erosion problems must be excluded and efforts should be made to use available stover in those areas to restore and rebuild the soil. Harvesting stubble will remove additional nutrients and could affect long-term soil organic matter levels, erosion rates, and water conservation. Producers should have and be using

Table 8.3 Projected available stover as a function of corn grain yield, after accounting for the amount of crop residue needed to protect soil resources against erosion and to sustain soil organic matter levels as suggested by Wilhelm et al. (2007) [13]. (Based on [13]. With permission Copyright © 2007, American Society of Agronomy). Grain yield at 15.5% moisture Dry stover Total Stovera Availableb CC Stover Availablec CS Stover Total Available Bushels per acre kg ha 1 Mg ha-1 Million Mg Million Mg Million Mg Million Mg 150 9416 7.96 155 36.9 0.3 37 160 10 044 8.49 165 44.1 3.4 48 170 10 672 9.02 176 51.4 6.5 58 180 11 300 9.55 186 58.6 9.6 68 190 11 927 10.08 196 65.8 12.7 79 200 12 555 10.61 207 73.1 15.8 89 210 13 183 11.14 217 80.3 18.9 99 220 13 811 11.67 227 87.5 22.0 110 230 14 438 12.20 238 94.8 25.1 120 240 15 066 12.73 248 102.0 28.2 130 250 15 694 13.26 258 109.2 31.3 141 260 16 322 13.79 269 116.5 34.4 151 270 16 950 14.32 279 123.7 37.5 161 280 17 577 14.85 289 130.9 40.6 172 290 18 205 15.38 300 138.1 43.7 182 300 18 833 15.91 310 145.4 46.8 192 aAssuming stover collection from 60% of the 2005-2011 U. S.A. harvested corn area (32 460 000 ha) (i. e. 19 476 000 ha). This is approximately the area of corn production in Illinois, Iowa, Indiana, Nebraska, and Minnesota. bAvailable after subtracting 5.25 Mg ha-1 for maintaining soil organic matter in continuous corn (CC) on 70% of the harvested area. cAvailable after subtracting 7.90 Mg ha-1 for maintaining soil organic matter in a corn-soybean rotation on 30% of the harvested area.

long-term nutrient management and soil conservation plans. They should also be using the least amount of tillage possible. Again, avoid stover harvest from highly erosive areas and use routine soil-test and plant analyses to monitor the response on a routine basis. Finally, consider adopting other conservation practices, such as the inclusion of annual or perennial cover crops, buffer strips, and crop rotation, in order to enhance the sustainability of stover harvest.