Potential of Cover Crop Influence on Water Repellency and the Sustainability of Southern U.S. Soils

April 2026 Report

There are 16 deliverables throughout the duration of this project (Figure 1).  The following have been completed:

• Plots were prepared on September 5^th, 2023, mapped, and flagged on September 8^th, 2023. (Deliverable #1)
• Cover crops were planted by hand in a way to mimic broad casting on September 13^th, 2023. (Deliverable #2) (Figure 2)
• September 14^th, 2023, we meet and interviewed a student from MANNRS on September 14^th, 2023, for part time undergraduate position. The student was hired for the part-time position on September 25^th, 2023.
• Pre-experiment tests including taking soil samples for aggregate stability and soil analysis were taken on September 8^th. The plots were too wet to measure infiltration and soil water repellency on September 8^th. Soil infiltration and soil water repellency were measured on September 15^th. (Deliverable #3) (Figure 3)
• Plots were maintained and weeds were hand-picked from plots weekly following planting. (Deliverable #4)
• November 7^th, 2023, radish, and mustard were harvested as they were affected by frost and starting to show signs of winter kill. Fresh biomass was weighed in the field (Fresh weight) and returned to the lab where the biomass was dried in the oven. (Deliverable #5)
• December 5^th, 2023, Collected 4-week-old residue samples of radish and mustard and returned the samples to the lab to dry in the oven. (Deliverable #5)
• December 13^th, 2023 took mid-experiment measurements for soil water repellency, and infiltration. Also took soil samples for aggregate stability. (Deliverable #6,7, & 8)
• March 12^th- 14^th 2024 Harvest and collect cover crop biomass and measure soil water repellency, and infiltration. Biomass samples were returned to the lab and placed in the oven to dry. Soil samples were also collected for aggregate stability and soil analysis. (Deliverable #5, 6, 7, & 8)
• March 19^th, 2024, contacted stakeholders and informed them of the project and upcoming workshops. (Deliverable #13)
• April 8^th, 2024, aggregate stability samples were taken from all plots. (Deliverable #7)
• April 11^th, 2024, collected 4-week-old biomass samples and returned them to the lab to dry in the oven at 60 °C. (Deliverable #5)
• April 12^th, 2024, measured SWR on soil cores from each plot. (Deliverable #8)
• April 15^th, 2024, measured infiltration rates in each plot. (Deliverable #6)
• April 18^th, 2024, presented preliminary results to farmers at a field day in Campobello, SC. (Deliverable #16)
• April 24^th, 2024, prepped plant tissues for analysis by grinding the samples using a Willey mill (<2 mm). (Deliverable #9)
• April 24^th, 2024, submitted an article on soil water repellency to the Minority Landowners Magazine. (Deliverable #15)
• May 2024, residue samples were sent to Spain for water repellency analysis at the Institute of Natural Resources and Agrobiology. (Deliverable #9) (Figure 4)
• June 4^th 2024, started processing the aggregate stability samples. (Deliverable #7)
• June 24^th, 2024 finished processing the aggregate stability samples. (Deliverable #7)
• September 2024: Started statistical analysis on the water drop penetration times of plant tissues by using a mixed model analysis. (Deliverable #10)
• October 16^th, 2024, presented preliminary results at the South Carolina Water Resources Conference in Columbia, SC. (Deliverable #16)
• November 10-13^th 2024, attended and gave an oral presented on the preliminary results of the study at the American Society of Agronomy, Crop Science Society of America, and Soil Science of America International annual meeting in San Antonio, Texas. (Deliverable #12)
• December 3^rd, 2024, presented preliminary results at the Certified Crop Advisors meeting in Santee, SC. (Deliverable #16)
• March 2025 finished statistical analysis for all parameters. (Deliverable #10)
• June 4^th, 2025, Land Grant Press article titled “An Introduction to Soil Water Repellency” was submitted for publication. (Deliverable #14)
• July 2025, a rough draft of the manuscript was completed, including results & discussion sections. (Deliverable #11)
• September 9^th, 2025, presented project results at the EuroSoils Conference in Seville Spain. (Deliverable #16)
• November 3^rd, 2025, received feedback on submitted Land Grant Press article and submitted corresponding edits. We are currently awaiting final approval. (Deliverable #14)
• December 22^nd, 2025, finished incorporating all edits and submitted the manuscript to be published in Soils Systems Journal. (Deliverable #11)
• March 11^th, 2026, Manuscript was accepted and published in Soils Systems Journal. (Deliverable #11) https://doi.org/10.3390/soilsystems10030040
• March 24th, 2026, Discussed project results and demonstrated how to measure soil water repellency at a farmer and stakeholders field day in Campobello, SC. (Deliverable #16)

Figure 1: Deliverables https://docs.google.com/document/d/16yOInYdfajKtF0NO-FSoJeH_pqe8r4oWZEh79FIt83M/edit?usp=sharing

Figure 2: Aerial view of plots. The yellow rectangle represents the plots for EXP C, while white rectangle represents the plots for EXP D. https://docs.google.com/document/d/1RIGO0XdKmra4ijeECq73qL9cwCJmM_Cw4_tTmBTL_eA/edit?usp=sharing

Figure 3: Example of how soil water repellency was measured in the field and what tools were used. https://docs.google.com/document/d/1dyFlsMEbAhYjWSwVoi-2E6a-RLWO0IlEC4qABp5eoRo/edit?usp=sharing

Figure 4: Example of water repellency exhibited by cover crop residues. https://docs.google.com/document/d/1b0HidWrgieSWlB7WNVezTjiaRoT4v7s0s4F0dXs7428/edit?usp=sharing

Project Results

Short-Term Changes in Aggregate Stability and Infiltration Rates

While the CC treatments did not have a significant influence on aggregate stability, aggregate stability was significantly influenced by timing. Soil samples taken midway through the CC growing period had significantly (p < 0.05) less water-stable aggregates (15–75%) compared to soil samples taken at the time of CC termination (44–88%) (Figure 5). Aggregate stability samples taken four weeks after termination were analyzed separately due to the exclusion of radish. Results for the samples taken four weeks post-termination showed that CC treatments did not have a significant influence on aggregate stability (60–85%) (Figure 6).

Infiltration rates varied between experiments and were affected by different factors. In EXP C, CC treatment and the interaction between CC and timing were statistically significant (Figure 7a). In EXP D, only timing was statistically significant. In EXP D, infiltration rates taken at the time of termination were significantly faster than infiltration rates taken during the middle of the experiment (Figure 7b).

Water Repellency of Soil and Cover Crop Residues

Soil water repellency was measured biweekly in each CC plot throughout the experiments until CCs were terminated. All soils were wettable, with WDPT < 5 s at all measured depths. As a result, no evidence of SWR was observed during the experiments.

The WR of the CC residues was measured using the WDPT method on dried, ground plant samples. The WDPTs varied from year to year with CC species and residue age. The mean WDPTs of the CC residues ranged as follows: 20–407 s (EXP A), 8–1908 s (EXP B), 157–2627 s (EXP C), and 60–4174 s (EXP D). Results indicate that treatment combinations (CC species and residue age) significantly influenced WDPT for EXP A (Friedman’s chi-squared statistic = 16.2; p-value = 0.02) and EXP B (Friedman's chi-squared statistic = 25.8; p-value = 0.002) but did not significantly influence WDPT for EXP C (Friedman’s chi-squared statistic = 15.7; p-value = 0.07) and EXP D (Friedman's chi-squared statistic = 12.8; p-value = 0.17).

Large WDPT differences between the experiments led to different levels of WR observed during each experiment. In EXP A, all fresh biomass (biomass collected at the termination) was strongly repellent, meaning it took at least 60 s for water to infiltrate and be absorbed by the residue samples (Figure 8a). In EXP B and C, fresh cereal rye and crimson clover residues were severely repellent, taking over 10 minutes for water to be absorbed by the residue sample (Figure 8b, c). Fresh crimson clover in EXP D was extremely repellent (>3600 s) (Figure 8d). Across all experiments, all CC residues exhibited some degree of WR, indicating that CC residues could influence the development of SWR.

In EXP A, the WDPTs of 4-week-old residues were statistically similar to the WDPTs of the fresh residues for three of the four CCs. The only 4-week-old CC in EXP A, significantly lower than its fresh residue, was radish (Figure 8a). In EXP B, more differences between fresh and 4-week-old residues were observed. For three out of the five CCs, the 4-week-old residues had lower WDPTs than the fresh residues, indicating that four-week-old residues had a lower level of repellency (Figure 8b). In EXP B, 4-week-old, crimson clover, radish, and red clover residues all have significantly lower WDPTs compared to their fresh residues. In contrast, 4-week-old and fresh fallow and cereal rye residues showed no difference in WDPTs. In EXP A and B, the 4-week-old residue never resulted in a significantly higher WDPT. Across EXP A and B, 4-week-old radish residues had one of the shortest WDPTs, indicating radish experienced one of the lowest levels of WR. However, in EXP C and D, while not significantly different from the other CC treatments, 4-week-old radish residues were severely repellent, indicating that variations in weather conditions may influence how radish residues break down (Figure 8c,d). The fallow treatment was generally low throughout all experiments and, therefore, may be the least likely to contribute to SWR.

Nutrient Content of Cover Crops

The nutrient content of the CCs varied across experiments. In EXP A, when there was a significant difference across CC treatments, the radish treatment contained higher levels of nutrients (calcium and magnesium) than the fallow treatment (Table 1). In EXP B, the crimson clover treatment contained significantly higher levels of nutrients (nitrogen, potassium, calcium, and magnesium) compared to red clover (Table 2), which was not the case in EXP C and D (Tables 8 and 9). In EXP C and D, the crimson clover and red clover treatments were similar across all nutrients measured (nitrogen, phosphorus, potassium, calcium, and magnesium), and both were consistently among the treatments with the highest nutrient levels (Tables 3 and 4). Across EXP B, C, and D, the cereal rye treatment had one of the lowest calcium contents.

Figure 5: https://docs.google.com/document/d/1dy6JuBXOqzsnXTp15VW3VbJHcKhDzz5QDOIzrfSzU28/edit?usp=sharing

Figure 6: https://docs.google.com/document/d/1MBCVWNM6mzZnFy4o8087othUByhq2l0HIQsVsacrlMo/edit?usp=sharing

Figure 7: https://docs.google.com/document/d/1NbV2vTq_XrteJJnZd0STDQWFh3vFa0kaoUZCIG1FIDY/edit?usp=sharing

Figure 8: https://docs.google.com/document/d/1V13oYjPnuZLIzBQuw4d0sjvyo9OeDUksxKYDZ0fxiIM/edit?usp=sharing

Table 1: https://docs.google.com/document/d/15SW3uDpN2RUy5dVKw1q-jpFeYiwWalKMo5GVrXePV-k/edit?usp=sharing

Table 2: https://docs.google.com/document/d/1YVKlFJ2Qb0u39ZLxhg-zlPchaxEXbxBCQAJtwKR_3fE/edit?usp=sharing

Table 3: https://docs.google.com/document/d/1A6fkFFudjEWsi38Ug_WJrjeNf4xggN3Ox_PtxhCnp5U/edit?usp=sharing

Table 4: https://docs.google.com/document/d/1kubrumrgN8_tlTGyxTfyOZKtbwLnzJeFG9xOYJCYv_I/edit?usp=sharing

Project Discussion

Short-Term Effects of Cover Crops on Aggregate Stability and Infiltration Rates

Our initial goal was to determine how SWR affects aggregate stability and infiltration rates. However, SWR was not found in this study. Therefore, SWR did not play a role in the short-term changes in aggregate stability and infiltration rates, and we failed to reject the null hypothesis that CCs with a higher degree of WR do not induce SWR throughout the CC growing season and do not influence short-term infiltration rate and aggregate stability. Instead, our findings indicate that short-term changes in aggregate stability were primarily influenced by timing. At the same time, infiltration rates were affected by timing, cover crop treatment, or their interaction, depending on the EXP.

Aggregate stability is a common measurement of soil structure (Kaspar et al., 2001; Haruna et al., 2018), which can change in a short amount of time compared to other soil health parameters (Blanco-Canqui and Ruis, 2020). Our results support this, as we observed an increase in aggregate stability when we compared samples taken in the middle of the experiment, when CCs were established, to samples taken at the time of CCs termination. The increase in aggregate stability over time is likely due to plant roots. Plant roots help improve aggregate stability through physical and chemical mechanisms (Bronick and Lal, 2005). Roots physically entrap and enmesh soil particles, enhancing the stability of the soil matrix (Bronick and Lal, 2005). Chemically, roots produce exudates that increase soil organic carbon, which helps to bind and hold soil particles together (Bronick and Lal, 2005). Plant roots also promote soil microorganism abundance and diversity (Locke et al., 2013), which can increase the production of soil organic carbon through fungi excretions. As the CC and fallow treatments increased aboveground biomass from the middle of the experiment to the time CCs were terminated, the below-ground biomass likely increased as well, thereby improving aggregate stability. Other studies, including those by Poirier et al. (2018) and Gould et al. (2016), have observed a positive correlation between root biomass and aggregate stability, further supporting the idea that the significant effect of timing is likely due to plant roots. The significant effect of timing was also found in a field study conducted in Illinois (Villamil et al., 2006). In a similar manner, the authors attributed the significant effect of timing to be related, in part, to the root morphology and the chemical composition of the root exudates (Villamil et al., 2006).

Our results also show no significant difference in aggregate stability between the fallow and CC treatments. While other studies, including the field study conducted in Illinois (Villamil et al., 2006) and a field study conducted in Georgia (McVay et al., 1989), have found the opposite, that CCs increase aggregate stability compared to a fallow treatment, our results are not uncommon (Blanco-Canqui and Ruis, 2020). Blanco-Canqui and Ruis (2020) reviewed 29 studies that looked at CCs’ effects on wet aggregate stability and found that 48% of the studies showed that CCs did not affect aggregate stability. The lack of difference between the CC treatments and the fallow treatment may be due to the aboveground biomass production and sampling time. Aboveground biomass helps to protect the soil from wind and rain erosion (Blanco-Canqui and Ruis, 2020; Ranaivoson et al., 2017), leading to less soil degradation (Ranaivoson et al., 2017). While the dry biomass of the fallow treatment was among the lowest in EXP C, it was statistically similar to other CC treatments, including radish and cereal rye. In EXP D, the fallow treatment produced a larger dry biomass than radish and cereal rye. Perhaps the biomass produced by the fallow treatment was sufficient to maintain a similar percentage of water-stable aggregates compared to the CC treatments. Generally, when aggregate stability increases under CCs, the increase is attributed to higher biomass productivity, which in turn increases soil organic carbon (Liu et al., 2005). This highlights the importance of timing. Soil organic carbon, while influenced by biomass inputs, can take time to have measurable changes. For example, Salisu et al. (2025) conducted a meta-analysis of 38 CC studies and found that short-term studies often reported no significant effects, or even adverse effects, of CCs on soil organic carbon. However, long-term studies showed an increase in soil organic carbon (Saliu et al., 2025). Therefore, short-term effects of CCs may not fully capture the influence of soil organic carbon on aggregate stability. Furthermore, a study investigating aggregate stability in a corn-soybean system without CCs reported a decrease in aggregate stability (Martens, 2000). Perhaps the aboveground biomass produced by all CC treatments in the current study was sufficient to provide physical protection from wind and rain erosion, thereby helping to prevent a decrease in aggregate stability.

In a similar manner, CCs are also known to influence infiltration rates. As CCs decompose, soil organic matter increases, reducing bulk density and increasing total porosity (Villamil et al., 2006; Cercioglu et al., 2018; Haruna et al., 2022). This, along with the channels left behind from the decomposed CC roots, helps to increase infiltration (Haruna et al., 2022). However, other factors such as chemical composition, soil texture, and management practices such as tillage may play a role in how CCs influence soil properties, including infiltration (Dai et al., 2024). Therefore, infiltration rates under CCs can have high variability and thus be site-specific (Blanco-Canqui and Ruis, 2020). For example, a study conducted in Nebraska found no differences among infiltration rates under different CC treatments and attributed the lack of difference to the silty clay loam soil texture (Blanco-Canqui and Jasa, 2019). On the contrary, studies in Georgia and Tennessee have found that CCs improve infiltration rates compared to fallow land (McVay, 1989; Nouri et al., 2019).

While we expected the fallow treatment to have one of the slowest infiltration rates, this was not always the case. In EXP B, the fallow treatment at the time of termination had one of the fastest infiltration rates. The inconsistency in our results may be due to field management. Before the experiment started, the field was tilled. Tilling a field alters soil structure (Sekaran et al., 2021), disrupting pore space, and thus can influence infiltration. Additionally, earthworms may have influenced our results. Previous studies have found a correlation between earthworms and increased infiltration rates (Blanco-Canqui et al., 2011; Korucu et al., 2018; Willoughby et al., 1997). Earthworms can modify the soil structure as they move and bury throughout a soil profile, creating bio pores and macropores, allowing water to move more quickly through the soil (Shipitalo and Korucu, 2002). It is also important to note that infiltration rates do not change as fast as aggregate stability under CCs. In fact, changes in infiltration rate due to CCs are often slow, sometimes taking more than 10 years (Blanco-Canqui, 2022).

Cover Crops as a Source of Essential Nutrients

As CCs decompose, they release nutrients back into the soil that can be utilized by the subsequent cash crop (Jahanzad et al., 2016). The nutrients released into the soil depend on the nutrient content of the CCs. Our results showed that the nutrient contents of each CC treatment varied between experiments. The variable results across experiments could be attributed to differences in environmental factors such as rainfall and air temperature, as environmental conditions influence biomass production, which subsequently influences nutrient content (Fageria et al., 2005). Additionally, a common trend in our nutrient content results is that both crimson and red clover generally had the highest nitrogen contents across all experiments. The high nitrogen content of the clovers is expected, as these species are in the legume family. Legumes can form symbiotic relationships with bacteria in the rhizosphere, allowing for nitrogen fixation where atmospheric nitrogen gets converted into plant-available forms of nitrogen within the nodules of the legume roots (Liu et al., 2011). This provides legumes with a plant-available form of nitrogen that other CCs may not have access to, thus leading to higher nitrogen contents within the plant tissues of legumes. Additionally, cereal rye was statistically similar to crimson clover in EXP B. Perhaps this result is due to the nature of cereal rye. Cereal rye is effective at scavenging nitrogen and reducing nitrogen leaching due to its fibrous root system (Quintarelli et al., 2022).

In the current study, resources were not available to analyze hydrophobic substances and organic matter content. However, based on previous research, we expect that CCs producing more biomass would lead to higher organic matter content (Miller et al., 2019). Greater biomass production would also likely increase the relative abundance of hydrophobic substances in the soil, as hydrophobic compounds are mainly derived from plant matter (Mao et al., 2015). Furthermore, the amount of hydrophobic material accumulated in the soil is expected to be influenced by CC species, as CC species differ in their physical and chemical compositions (Miller et al., 2019). For example, C3 plants, such as cereal rye in the current study, are known to contain more cuticular wax than C4 plants (He et al., 2016). Waxes are hydrophobic (Barthlott et al., 2017), thus likely increasing the amount of hydrophobic material within the soil and encouraging the development of SWR. Supporting this idea, Miller et al. (2019) found that grass species were among the most water repellent crops across 30 agricultural crop species.

To our knowledge, previous studies have not explored whether there is a correlation between the nutrients in CC tissue and the production of hydrophobic substances. However, previous studies have investigated the use of nitrogen fertilizer on the development of SWR. While some studies have found no significant effect of fertilizer on SWR (Bottinelli et al., 2017), other studies have seen that fertilizers increase soil hydrophobicity compared to no fertilizers (Hallett and Young, 1999). The increase in soil hydrophobicity due to fertilizers is attributed to the stimulation of microorganisms, which, in turn, leads to the production of hydrophobic materials as byproducts (Hallett and Young, 1999). In a similar manner to fertilizers, the nutrients provided by the CCs may stimulate and support soil organisms, leading to the production of more hydrophobic substances and, over time, may lead to the development of SWR.

Absence of Soil Water Repellency

Our study primarily aimed to determine the WR of CC residues; however, assessing the SWR underneath the CCs proved to be a crucial step in understanding the development of SWR within the cropping system. By evaluating the SWR of the underlying soil, we can identify if SWR is present and how SWR may change throughout the CC growing season. Particularly, root exudates and the byproducts of soil microorganisms are thought to influence soil hydrology as these organic substances can be hydrophobic and thus may impact the development of SWR (Moradi et al., 2012). The significance of SWR is evident as SWR has been documented in many cropping systems, including potatoes (Keizer et al., 2007; Kelling et al., 2003; Robinson, 1999), maize (Keizer et al., 2007; Li et al., 2018; Urbanek et al., 2007), wheat (Dekker et al., 1999; Urbanek et al., 2007; Ziogas et al., 2005), cereal crops (Cerdá and Doerr, 2007), tobacco (Ziogas et al., 2005), canola (Blackwell, 2000), and citrus orchards (Cerdá and Doerr, 2007; Jamison, 1943; Wallach et al., 2005). However, throughout our four field experiments, SWR was absent.

As discussed during previous experiments (Davis et al., 2024), this could be due to field operations such as tillage. Tillage helps to break down organic material mechanically, decreasing the amount of soil organic matter and soil organic carbon contributing to SWR (Blanco-Canqui and Does, 2010; González-Peñaloza et al., 2012). The mechanical breakdown of organic material also helps to speed up the decomposition rate (Lascalajr et al., 2008). A field study across Kentucky, Ohio, and Pennsylvania showed that no-till operations slightly increased SWR (Blanco-Canqui and Lal, 2009). Similarly, an experiment conducted in Spain found that all fields under conventional tillage were wettable, while some fields under no-till experienced slight repellency (González-Peñaloza et al., 2012). Additionally, Li et al. (2023) found similar results where no-till and reduced tillage increased SWR compared to conventional tillage. However, even though tillage generally decreases the occurrence of SWR, some studies suggest avoiding tillage operations in fields where SWR is an issue, as avoiding tillage may help preserve any infiltration paths present within the soil (Ward et al., 2015).

The absence of SWR during the experiments may also have to do with the season and weather conditions experienced during the time SWR was being measured. SWR was measured during the winter months when there was no extreme heat or extended dry periods. While some studies, such as a study conducted in Spain that measured SWR under different plant species, have found that SWR can be present in the winter months (Zavala et al., 2009), SWR is more likely to occur under extreme temperatures, which can drive out moisture or under prolonged dry periods (Mao et al., 2018; Sándor et al., 2021). Prolonged dry conditions can rearrange the orientation of organic materials, causing the hydrophilic heads to become tightly bonded to each other and to the soil surface, while the hydrophobic tails are exposed to the outer environment (Hallett, 2008). With the hydrophobic tails exposed to the outer environment, the soil acts as a hydrophobic material and repels water (Hallett, 2008).

While the weather and climatic conditions during our experiments did not favor the development of SWR, it is essential to consider the broader relationship between climate and SWR, as observed by other studies. Jaramillo et al. (2000) observed SWR in arid and humid climates and noted that the effect of climate on SWR may have more to do with how it affects the production of organic material within the soil. Humid climates favor the production of organic material, and therefore, soils in humid climates are more likely to develop SWR (Jaramillo et al., 2000). Additionally, more extreme SWR is expected to be found in humid climates compared to arid climates, which generate low amounts of organic materials (Jaramillo et al., 2000). While South Carolina does have a humid subtropical climate, it experiences short and mild winters (Griffin and Mogil, 2021).

A limitation of our study may be the timing of SWR measurements during the winter months. South Carolina experienced mild winter conditions, and prolonged dry conditions did not persist during the experiments. Instead, rainfall was observed throughout the experiment, which decreased the occurrence and severity of SWR and even made SWR disappear (Jaramillo et al., 2000). If we had extended our measurements into the summer months, when South Carolina experiences hotter and more extreme temperatures (Griffin and Mogil, 2021), we might have been able to observe SWR. During the summer months, the soil would be exposed to higher temperatures, which some studies have documented as affecting the degree of SWR (Dekker et al., 1998). On the controversy, however, other studies, such as Ward (2015), suggest either an opposite relationship or no effect of temperature on the degree of SWR. Therefore, continued measurements of SWR throughout the year could provide more insight and understanding into how SWR may develop and change within the soil.

While no SWR was found during our experiments, the absence of SWR is still a significant finding that agrees with studies in other regions where tillage was used as a management practice. The absence of SWR does not mean that SWR cannot develop under different conditions, including changes in management practices. Future research should investigate more sites within the Southern United States under various management practices, such as no-till, and measure SWR throughout the summer months. Investigating additional sites and measuring SWR during the summer would help to identify if the absence of SWR is influenced more by management practices, or site characteristics.

Water Repellency of Cover Crops and Their Potential Effects on Soil Water Repellency

While the benefits of CCs have been widely documented for many years, there is little information on how CCs may influence the development and severity of SWR. The focus of our study was to identify if CCs and their residues are water repellent and thus have the potential to contribute to the development or increase the severity of SWR within a cropping system. Our findings indicate that CCs and their residues exhibit WR, indicating that CCs may influence the development of SWR within a field. A similar finding was concluded from a study that measured the WDPTs of 30 agricultural crops, where all agricultural crops displayed WR (Miller et al., 2019).

Previous studies have identified that decomposition can significantly affect how plant residues influence the development of SWR. For example, in the Mediterranean maquis, six-month-old decomposing plant litter experienced a decrease in WR most of the time compared to fresh plant tissue, although both fresh and 6-month-old plant tis-sues were WR (Cesarano et al., 2016). While this study did not look at CCs but instead looked at shrubs, trees, and grasses, this trend was present in our study, where fresh and four-week-old residues exhibited WR. The older residues (4-week-old residue) in our study, while not always significantly different, were also found to exhibit lower levels of WR compared to the fresh residue for three out of four CCs in EXP A, all CCs in EXP B, four out of five CCs in EXP C, and three out of five CCs in EXP D.

Plant species have different biochemical properties (Miller et al., 2019; Cesarano et al., 2016), which can lead to varying levels of WR. This relationship is evident in the diverse WR levels observed in soils under different plant species. For instance, in the Mediterranean subhumid forest, soil under Heathland was found to be extremely repellent, while soil under olive trees was low or nonexistent (Martínez-Zavala and Jordán-López, 2009). The variation in WR levels is related to the type and amount of hydrophobic compounds within the plant tissues. Specifically, the proportion of hydrophobic coatings present determines the degree of WR, which can be derived from plant material (Doerr et al., 2006). Specific plant types and species are consistently associated with SWR in different regions of the world (Popović and Cerdá, 2023), with common examples including pine trees (Pinus sp.) (Phillips and Croteau, 1999; Jetter et al., 2007), eucalyptus (Eucalyptus sp.) (Khayet and Fernández, 2012; Hoffmann et al., 2013), and grasses such as needle-and-thread-grass (Stipa comata Trin. and Rupr.) (Miller et al., 2019). Additionally, a study conducted by Miller et al. (2019) measured the WR of 30 agricultural crops, revealing a large range of WDPTs from 8.3 to 2438 s. The wide range of WDPTs indicates that different crops have different levels of WR. Collectively, these studies demonstrate the crucial role of plant species and their biochemical properties in determining the degree of WR and the development of SWR, thereby supporting our findings that different CCs exhibit varying levels of WR.

Our results indicate that CC residues are water repellent and, therefore, have the potential to induce SWR, as previous studies have identified that SWR is caused by repellent, hydrophobic materials. The insights gained from this study can be used to help determine what CCs to plant in the Southern United States if SWR becomes an issue within a cropping system. Our study demonstrates that waiting a few weeks after termination may help reduce the development and severity of SWR, as four-week-old residues appeared to be less repellent than fresh residues, the majority of the time. Future research is needed to identify the WR of additional CC species and CCs terminated by chemical means. Additionally, chemical analysis of CCs may help to determine the specific compounds that induce SWR, which would help advance our knowledge.

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