Introduction
Allen et al. (Reference Allen, Pereira, Raes and Smith1998) observed that soil surface evaporation is a function of the mean water content in the topsoil (10–15 cm). However, several studies have reported that the evaporation process is characterized by multiple stages (Figure 1). The first stage (Stage I) is when the evaporation rate is constantly followed by a second stage (Stage II) characterized by a decreasing rate of evaporation (Metzger & Tsotsas, Reference Metzger and Tsotsas2005). As surface soil continues to dry, during Stage I, water from the deeper soil is supplied to the surface soil by capillary flow to maintain a constant evaporation rate (Lehmann et al., Reference Lehmann, Merlin, Gentine and Or2018; Shokri & Or, Reference Shokri and Or2011). As the soil becomes drier, capillary pathways will be disrupted and the evaporation rate drops significantly at Stage II (Shokri et al., Reference Shokri, Lehmann and Or2008). An investigation by An et al. (Reference An, Tang, Xu, Gong, Shi and Inyang2018) revealed that during Stage I, soil-water content decreased continuously with time; however, the ratio of actual to potential evaporation (AE/PE) remained stable. Subsequently, during Stage II, AE/PE decreased significantly. Reports showed that the evaporation rate during Stage II is driven by vapor diffusion across the dry soil profile, which increases at a rate inversely proportional to the square root of time (Brutsaert, Reference Brutsaert2014; Or et al., Reference Or, Lehmann, Shahraeeni and Shokri2013).
Figure 1. Schematic of the evaporation stages of a drying soil (adapted from Lehmann et al., Reference Lehmann, Assouline and Or2008).
Several studies have reported that biosolids and organic amendments enhance soil hydraulic properties, which in turn have positive impacts on soil and water relations including water retention and infiltration (Babalola et al., Reference Babalola, Adesodun, Olasantan and Adekunle2012; Bayabil et al., Reference Bayabil, Stoof, Lehmann, Yitaferu and Steenhuis2015; Brye et al., Reference Brye, Slaton, Norman and Savin2005; Mohamed Am et al., Reference Mohamed Am, Sekar and Muthukrish2010; Page-Dumroese et al., Reference Page-Dumroese, Ott, Strawn and Tirocke2018; Rahman et al., Reference Rahman, Dahab and Mustafa1996). However, there is no clear understanding of whether increased moisture retention due to organic amendments would lead to increased evaporation losses. It is expected that as more moisture is available within the topsoil due to organic amendment, the evaporation rate could potentially increase compared to non-amended control soils. A 75 g/kg rice barn or fish meal application to saline soils also reduced evaporation by 8–20% (Chang et al., Reference Chang, Ye, Shao, Zhang and Zhang2016). Adeyemo et al. (Reference Adeyemo, Akingbola and Ojeniyi2019) observed that the incorporation of 10 Mg/ha of poultry manure led to a reduction in cumulative infiltration rates for sandy soils. Milorganite is a commercially available product produced from sewage sludge treatment plants and is used as a soil amendment (Kebrom et al., Reference Kebrom, Woldesenbet, Bayabil, Garcia, Gao, Ampim, Awal and Fares2019; Staufenbeil, Reference Staufenbeil2019). Milorganite could potentially serve as a slow-release fertilizer as it contains more than 6% nitrogen and 4% phosphorous by mass. For example, the application of milorganite increased the content of soil inorganic nitrogen and maintained soil microbial biomass (Wang et al., Reference Wang, Felice and Scow2020). Hence, the objective of this study was to investigate the poorly studied effects of milorganite on soil moisture characteristics, hydraulic conductivity, and evaporation rates. The study hypothesis was that the addition of milorganite to the soil will affect the moisture characteristics, hydraulic conductivity, and evaporation rates of the soil.
Materials and methods
Soil and milorganite
Krome soil was collected from research fields at the Tropical Research and Education Center in Homestead, Florida, USA. Then the soil was sieved using a 2-mm sieve to remove coarse materials. The selected properties of both soil and milorganite are presented in Table 1.
Table 1. Selected properties of milorganite and Krome soil used in the study
Experimental design
The experiment was designed with three treatments in three replicates: control (no milorganite), 15% v/v milorganite, and 30% v/v milorganite. The study was conducted in a greenhouse using PVC columns (10-cm diameter and 1,178-cm3 volume). The 2-mm sieved soil was mixed uniformly with two rates of milorganite (15 and 30% v/v) and packed into the soil column at an average bulk density of 1.1 g/cm3. Water was slowly added to soil columns until the soil become fully saturated. In addition, columns filled with water only were used to measure potential evaporation rates from the free water surface. The experiment was replicated three times and continued until daily evaporation rates from soil columns were relatively negligible.
Data collection
Daily evaporation rates were recorded by measuring weight losses using a digital scale. Electrical conductivity measurements were conducted, after extraction of samples with demineralized water (w/v = 1:5), using HANNA Benchtop probe (HI5522-01; Hanna Instruments Inc., Smithfield, VA, USA). The swelling property of milorganite was calculated after full saturation of samples for 24 h and subsequently allowing free draining of gravitational water until the field capacity was reached. After drainage stops, the difference between the initial volume and the volume at the field capacity was regarded as the swelling capacity of milorganite.
Soil moisture characteristics and hydraulic conductivity
Soil hydraulic properties were measured using the HYPROP and WP4C equipment (METER Group, Inc., Pullman, WA, USA). Air-dried soil samples were uniformly packed in stainless-steel cylinders. After packing, any excess soil at the top was carefully removed with a saw blade and the top was left open. Sample preparation and processing were done following a similar approach by Lipovetsky et al. (Reference Lipovetsky, Zhuang, Teixeira, Boyd, May Pontedeiro, Moriconi, Alves, Couto and van Genuchten2020). Finally, the Hyprop-FIT software was used to develop soil moisture characteristics and hydraulic conductivity curves (Pertassek et al., Reference Pertassek, Peters and Durner2015).
Statistical analysis
The R statistical programming software was used for data analysis, and analysis of variance tests were performed using the Kruskal–Wallis rank-sum test. The significant effect of milorganite application rate was tested at a 5% significance level (p < .05).
Results and discussion
Evaporation rates
Daily evaporation rates from all soil columns followed a similar trend to the potential evaporation (ETo) rate from a free water surface at the beginning of the experiment (Figure 2a). However, evaporation rates from soils amended with milorganite sharply dropped after a week. There was an approximately 2–3-day lag in evaporation decline between the 15 and 30% milorganite treatments. On the other hand, evaporation rates from the control soil followed the rate from free water surfaces for a longer time. Comparing the ratios of soil evaporation with potential evaporation, as shown in Figure 2b, milorganite treatment reduced the length of Stage-I evaporation by more than half, leading to an extended period for Stage-II evaporation compared with non-amended control soils.
Figure 2. Daily evaporation rates from soils with and without milorganite and free water surface (a) and relative evaporation rates from soils compared with potential evaporation from a free water surface (b). The gray-shaded bars represent changes of evaporation from Stage I to Stage II for soils with milorganite and non-amended control soils.
At the end of the experiment, milorganite amended soils retained about half of the water that was initially added compared to the non-amended control soil (Figure 3). There was a 10% difference in final moisture retained between the two milorganite rates (15 and 30%). Wang et al. (Reference Wang, Felice and Scow2020) and Avery et al. (Reference Avery, Krzic, Wallace, Newman, Smukler and Bradfield2018) reported similar findings that milorganite increased soil-water retention. The reduction in the level of water in columns filled with water was linear, reaching near zero at the end of the experiment. It was apparent from this study that milorganite reduced evaporation rates while increasing moisture retention for extended periods, which suggests that milorganite could be used as a water conservation strategy in addition to being a source of nitrogen and phosphorous to plants.
Figure 3. Percentage of water retention of soils with and without milorganite, and water columns. Treatment values are averages of three replications.
Soil moisture characteristics and hydraulic conductivity
Soil moisture characteristic curves of soils amended with milorganite showed a shift with higher moisture retention both near saturation and in the dry regions (Figure 4).
Figure 4. Soil moisture characteristic curves (a) and hydraulic conductivity (b) from control and milorganite treated soils. Note: hydraulic conductivity readings of 15 and 30% milorganite treatments were multiplied by 200 to achieve the same scale as the control for plotting.
In most cases, milorganite amended soils had higher moisture content at a given pressure level. This could be due to increased absorption and retention of water by milorganite particles (Figure 4a). Results showed that the high moisture retention capacity of milorganite is associated with its high swelling capacity (Figure 5). The volume of a dry milorganite has increased by 54% at the field capacity, indicating the capacity of milorganite to absorb and retain water very well. A sharp decline in the moisture release curve was observed for the control soil. On the other hand, the soil hydraulic conductivity of milorganite amended soils was reduced by a factor of 200 compared with the control (Figure 4b). Hydraulic conductivity from 30% milorganite treatment showed a sharp decline with a small increase in pressure. This suggests that the swelling property of milorganite leads to the disruption of pore connectivity of soils, which limits the transportation of moisture from subsurface soil to surface soil, hence reducing the evaporation and hydraulic conductivity of soils.
Figure 5. Swelling capacity of milorganite with the addition of water.
Electrical conductivity
The finding from this study demonstrated a significant increment of electrical conductivity of soil after the addition of both 15 and 30% of milorganite (Figure 6). The electrical conductivity of 30% milorganite addition was also significantly more than that of 15% milorganite treatment. The increment of soil electrical conductivity after the addition of the milorganite is clearly due to the much higher salt content of milorganite compared to the soil used in this study (Table 1). Studies show that salt accumulation in soil negatively affects plants by making soil water less available for plant use regardless of soil moisture status. At high salt levels, the osmotic potential of soil water increases (Sheldon et al., Reference Sheldon, Dalal, Kirchhof, Kopittke and Menzies2017). The increase in the electrical conductivity could put much pressure on plants. This is in agreement with the finding of Romero-Aranda et al. (Reference Romero-Aranda, Soria and Cuartero2001), where it is reported that tomato yield and water uptake were reduced due to the increment in the salinity of the soil.
Figure 6. Effect of Milorganite on the electrical conductivity of the soil.
Conclusion
A study was conducted to investigate changes in evaporation and soil moisture characteristics of Krome soil with two rates of milorganite amendment. Milorganite amended soils had reduced evaporation rates compared with the control. Milorganite amendment reduced the length of Stage-I evaporation process, leading to an extended Stage-II evaporation period. This resulted in increased moisture retention and decreased hydraulic conductivity rates. The reduction of evaporation rates and hydraulic conductivity also suggests that milorganite incorporation leads to the disruption of the pore networks that are needed to transport water from subsurface depths. However, the effects of milorganite on salinity suggest that the increased moisture retention is less likely to be available for plant use as plants will have difficulty extracting water from soil with elevated salt levels. Therefore, the findings from our study suggest that irrigation management of milorganite amended soils should be optimized to avoid salt and water stress of plants grown on milorganite amended soil.
Acknowledgments
The authors would like to thank Christian Bartell, Syed Shaham Madni, and Gui Qin Yu for their assistance with field soil collection and sample processing and for their assistance in acquiring the necessary materials for the experiment.
Open peer review
To view the open peer review materials for this article, please visit http://doi.org/10.1017/exp.2022.25.
Data availability statement
The data used in this study are available upon request to the corresponding author.
Authorship contributions
H.K.B. conceived and designed the experiment. F.T.T., J.Z., and N.S.H. implemented the experiment and collected data. H.K.B. led the writing of the article with contributions from all the authors.
Funding statement
This publication is based on work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under the Award No. 2020-67019-31163 and the Hatch Project No. FLA-TRC-005751. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture.
Conflict of interest
The authors have no conflicts of interest to declare.
Open access
Comments
Comments to the Author: This paper is well revised following my suggestions， so it is acceptable to be published now。