Miscanthus Establishment and Overwintering in the Midwest USA: A Regional Modeling Study of Crop Residue Management on Critical Minimum Soil Temperatures

Miscanthus is an intriguing cellulosic bioenergy feedstock because its aboveground productivity is high for low amounts of agrochemical inputs, but soil temperatures below ‒3.5°C could threaten successful cultivation in temperate regions. We used a combination of observed soil temperatures and the Agro-IBIS model to investigate how strategic residue management could reduce the risk of rhizome threatening soil temperatures. This objective was addressed using a historical (1978–2007) reconstruction of extreme minimum 10 cm soil temperatures experienced across the Midwest US and model sensitivity studies that quantified the impact of crop residue on soil temperatures. At observation sites and for simulations that had bare soil, two critical soil temperature thresholds (50% rhizome winterkill at ‒3.5°C and ‒6.0°C for different Miscanthus genotypes) were reached at rhizome planting depth (10 cm) over large geographic areas. The coldest average annual extreme 10 cm soil temperatures were between ‒8°C to ‒11°C across North Dakota, South Dakota, and Minnesota. Large portions of the region experienced 10 cm soil temperatures below ‒3.5°C in 75% or greater for all years, and portions of North and South Dakota, Minnesota, and Wisconsin experienced soil temperatures below ‒6.0°C in 50–60% of all years. For simulated management options that established varied thicknesses (1–5 cm) of miscanthus straw following harvest, extreme minimum soil temperatures increased by 2.5°C to 6°C compared to bare soil, with the greatest warming associated with thicker residue layers. While the likelihood of 10 cm soil temperatures reaching ‒3.5°C was greatly reduced with 2– 5 cm of surface residue, portions of the Dakotas, Nebraska, Minnesota, and Wisconsin still experienced temperatures colder than ‒3.5°C in 50–80% of all years. Nonetheless, strategic residue management could help increase the likelihood of overwintering of miscanthus rhizomes in the first few years after establishment, although low productivity and biomass availability during these early stages could hamper such efforts

Simulated soil temperature trends across the Midwest US.

<p>(a) Total change (from linear regression) in annual average 10 cm soil temperatures for 1948–2007 for the MAIZE+TILL (bare soil) simulation; (b) total change (from linear regression) in annual extreme minimum 10 cm soil temperature for the MAIZE+TILL (bare soil) simulation from 1948–2007; (c) total change (from linear regression) in annual extreme minimum 10 cm soil temperature for the MAIZE+TILL (bare soil) simulation from 1981–2007. Regions bounded by solid white lines indicate trends with P<0.05.</p

Frequency of 10 cm soil temperatures reaching −3.5°C or colder for varied miscanthus residue thicknesses.

<p>Fraction of total years during the 1978–2007 time period that simulated annual 10 cm soil temperatures were at or below a −3.5°C threshold (based on a 3-day running mean) for (a) MAIZE+TILL (bare soil), (b) MISCAN+R_{1 cm}, (c) MISCAN+R_{2.5 cm}, and (d) MISCAN+R_{5 cm} simulations.</p

Plant residue biophysical values used to modify Agro-IBIS to simulate the effects of crop residue on soil surface energy balance and heat transfer.

<p>Plant residue biophysical values used to modify Agro-IBIS to simulate the effects of crop residue on soil surface energy balance and heat transfer.</p

Frequency of 10 cm soil temperatures reaching −6.0°C or colder for varied miscanthus residue thicknesses.

<p>Fraction of total years during the 1978–2007 time period that simulated annual 10 cm soil temperatures were at or below a −6.0°C threshold (based on a 3-day running mean) for (a) MAIZE+TILL (bare soil), (b) MISCAN+R_{1 cm}, (c) MISCAN+R_{2.5 cm}, and (d) MISCAN+R_{5 cm} simulations.</p

Comparison of observed and simulated soil temperatures in Illinois.

<p>(a) Comparison of 3-day running mean 10 cm soil temperatures for Agro-IBIS model simulations of miscanthus with no residue layer with observed values at the University of Illinois Energy Farm from 2009 to 2011. Observed values are the daily mean (n = 1 to 5) of observation for the miscanthus plots, n was less than five for periods when sensors were damaged, with n at least 3 for 80% of the days. (b) Comparison of monthly mean 10 cm soil temperatures for the same site and simulation. Data points are the mean (n = 3) of the 2009–20011 monthly values, error bars are ±1 S.E. for both simulated and observed values.</p

Comparison of observed and simulated soil temperatures in Iowa and Wisconsin.

<p>(a) Comparison of monthly average 10 cm soil temperatures for three Agro-IBIS model simulations with observational station data from Hu and Feng (2003) averaged over the state of Iowa for 1982–2002. Error bars are ±1 S.E. for both simulated and observed values. (b) Comparison of monthly average 10 cm soil temperatures for three Agro-IBIS models simulations compared with observational data at the University of Wisconsin-Madison Arlington Agricultural Research Station in tilled maize and switchgrass study plots for July 2009- June 2012. Observed data (maize and switchgrass) represent the monthly mean among 3 replicate plots (n = 3). Long-term averages (1988–2011) of monthly mean 10 cm soil temperature collected at the UW-Madison Automated Weather Observing Network (AWON) site at Arlington are plotted for comparison. Error bars are ±1 S.E. for all simulated and observed values.</p

Soil temperature differences between different residue layer thicknesses.

<p>Average annual (1978–2007) extreme minimum 10 cm soil temperature differences, based on a 3-day running mean, for the following paired simulations: (a) MISCAN+R_{1 cm} minus MAIZE+TILL (bare soil post harvest), (b) MISCAN+R_{2.5 cm} minus MAIZE+TILL, (c) MISCAN+R_{5 cm} minus MAIZE+TILL.</p

Impacts of soil surface residue management on annual average extreme minimum 10 cm soil temperatures.

<p>Average annual (1978–2007) extreme minimum 10 cm soil temperatures (based on 3-day running mean) for (a) MAIZE+TILL (bare soil post harvest), (b) MAIZE+NOTILL, (c) MISCAN+R_{1 cm}, (d) MISCAN+R_{2.5 cm}, and (e) MISCAN+R_{5 cm} simulations. The position of the 0.0°C, −3.5°C, −6.0°C, and −8.0°C 10 cm soil temperature isopleths are highlighted by labels on solid black lines; (f) differences in average annual extreme minimum 10 cm soil temperatures for MISCAN+R_{5 cm} – MAIZE+NOTILL (Fig. 6e–Fig. 6b results).</p

Comparison of observed and simulated annual extreme minimum 10 cm soil temperatures.

<p>(a) Agro-IBIS average annual extreme minimum 10 cm soil temperature (based on 3-day running mean temperatures) compared with observations from 125 observation sites (SI, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0068847#pone.0068847.s001" target="_blank">Table S1</a>) from the Midwest USA for original model values (open circles and dotted regression line), and statistically adjusted model values (filled circles and dashed regression line); (b) Agro-IBIS simulated frequency of occurrence of 10 cm soil temperatures (3-day running mean) reaching −3.5°C and (c) −6.0°C, respectively, compared with results from 125 observation sites in the Midwest USA. In these comparisons, Agro-IBIS was only simulated for a period from the beginning year that data was available for each observation station through 2007, which denotes the last year that gridded daily climate data was available.</p