Publication Type:
ThesisSource:
Agricultural and Biosystems Engineering, Volume Masters of Science (2022)Abstract:
<p>Prairie strips (PS) are an increasingly popular conservation strategy being implemented around Iowa and the Midwest, with over 14,000 acres of strips having been planted as of 2022 (nrem.iastate.edu/research/STRIPS). Foundational PS research (Phase I) occurred within the Neal Smith National Wildlife Refuge in Jasper County, Iowa. In recent years, efforts have expanded to tens of on-farm research sites around the Midwest (Phase II). By integrating native perennial vegetation within row crop (RC) fields that produce corn (Zea mays L.) and soybean [Glycine max. (L.) Merr.], prairie strips have increased pollinator and bird abundance, reduced field sediment and nutrient export, and have a favorable perception in farming and nonfarming populations. However, a thorough analysis of prairie strips’ impact on many soil properties has not yet occurred.</p>
<p>The first study in this thesis aimed to quantify and compare soil hydraulic properties between PS and RC across locations and establishment stages. We took measurements of unsaturated and saturated hydraulic conductivity and pore size distribution with a tension infiltrometer at two Phase I sites in 2010, 2011, and 2021. Field-saturated infiltration rate and sorptivity data were acquired from six Phase II sites containing six- to seven-year-old PS with the Cornell Sprinkle Infiltrometer system. Overall, between Phase I and II sites, we found few differences in saturated hydraulic conductivity and field-saturated infiltration rate between PS and RC. The most notable and decisive difference between treatments was observed at one Phase II site, where PS field-saturated infiltration rates were 3.6 times greater than RC across three sampling periods. This site’s soil type and history of topsoil degradation likely contributed to its relatively quick response to PS in saturated infiltration capacity. Comparisons of sorptivity between PS and RC treatments were more distinct than saturated infiltration capacity, as PS sorptivity was 26 and 38% greater than RC in fall sampling periods at three Phase II sites. Since sorptivity relates to early infiltration when capillarity controls water flow, this result implies that PS can limit runoff and protect regional soil and water quality. Greater sorptivity in PS is likely due to greater evapotranspiration compared to RC during the spring and fall.</p>
<p>The second study investigated soil health differences between PS and RC by employing soil physical, chemical, and biological analyses. We selected three Phase II sites for a full suite of soil health testing. Additionally, increased emphasis was placed on estimating wet-aggregate stability differences between PS and RC by analyzing multiple soil depth increments, utilizing two methodologies, and including three additional sites. Across twelve soil properties, several displayed clear treatment differences or lack thereof at each site, while it became apparent that others had differing responses depending on soil type and other site characteristics. Out of four physical properties tested, the most pronounced difference between PS and RC was found in measurements of wet-aggregate stability, as PS was consistently greater than RC across all sites, depths, and testing methods. For chemical properties, extractable potassium was significantly greater in PS than RC across all sites, as the mean values were 255 mg kg-1 and 192 mg kg-1 , respectively. Soil pH was also significantly greater in PS and RC, but only at sites located within the Southern Iowa Drift Plain landform region. Treatment differences in biological properties were also limited to Southern Iowa Drift Plain sites, as measurements of soil organic matter and carbon to nitrogen ratio were greater in PS than in RC. Overall, this study showed that soil health was not definitively greater in PS than in RC across sites at the current establishment stage; however, the treatment difference observed in wet-aggregate stability may signal changes to come, given its ability to enable other soil processes.</p>
<p>The final study utilized two soil health indices – Cornell’s Comprehensive Assessment of Soil Health (CASH) and the Soil Management Assessment Framework (SMAF) – to assess differences in PS and RC treatments. Additionally, we evaluated the utility of these scoring indices in the context of PS and RC treatment comparison. Both CASH and SMAF use scoring functions to transform observed values of soil health indicators into unitless scores ranging between 0 and 100. Scores generated for each indicator can then be integrated to produce an overall soil health score. CASH and SMAF agreed that PS had marginally greater overall soil health than RC across Southern Iowa Drift Plain sites. This difference was statistically significant for CASH, while lack of replication limited statistical analysis of SMAF scores. It was apparent that greater wet-aggregate stability in PS than in RC drove the treatment difference observed in CASH overall scores, and it is likely that wet-aggregate stability is a leading indicator of overall soil health improvement due to PS. Soil health scoring indices provided value to PS and RC soil health comparison by supplying a framework to integrate multiple soil properties into a single assessment and easing the interpretation of observed values. However, it was clear that overall soil health scores, individual indicator scores, and observed values should be used to supplement each other to perform the most accurate and complete assessment. Additionally, inherent soil quality must supplement soil health scores if productivity assessments are desired.</p>
