4.1. Soil Moisture
In our perennial grass studies, soil moisture in a loam soil was increased significantly with Stockosorb 660 micro bands applied at a depth of 8 cm below the soil surface at rates of 1500 and 3000 kg ha−1 (0.1 and 0.2%). Soil moisture was increased over the control for up to seven weeks, with the largest differences occurring at the time of saturation and diminishing over time. The largest impact of soil moisture on seedling longevity occurred during approximately the last 12 days, when there was a significant soil moisture difference, despite the difference in soil moisture being near its lowest point.
Higher levels of soil moisture observed in this study as a function of the band-applied SAP are similar to other container studies with a variety of soil types, plant species, and SAP particle sizes, sources, and rates when mixed into the soil [
35,
40,
42,
52,
53,
54,
55,
56]. In general, such studies found SAP to increase soil moisture in sandy-textured soils at application rates between 0.04 to 1.0% (approximately 600–15,000 kg ha
−1). The effect decreased in finer-textured soils. In studies with both sandy and clayey soils, the sands had increased soil moisture, but the clays did not [
35,
52,
54,
55].
Our results were similar to El-Asmar et al. [
35]. In a soil moisture characteristic study with clay (C) or sandy clay loam (SCL) soils, they found numerically increased water content with increasing SAP application rates of 0.1, 0.2, 0.3, and 0.4% (approximately 1700–7100 kg SAP ha
−1) when Stockosorb 660 was blended into the full container of soil. However, water holding capacity was only increased significantly at the 0.4% rate at matric potentials of 30 and 200 kPa and the 0.3% rate at a matric potential of 100 kPa in SCL. In a further study growing maize (
Zea mays L.) in the same soils with the same SAP rates, they reported increases in evapotranspiration (ET), which is indicative of increased soil moisture [
57]. Banded SAP at rates of 0.2, 0.3, and 0.4% (approximately 3400–7100 kg ha
−1) resulted in increased ET in SCL. Mixing the SAP with the soil resulted in increased ET only for the 0.3 and 0.4% SAP rates in SCL. There were no differences in water content or ET in their C soil. In our studies, when the SAP was mixed with the soil, we saw an increase in soil moisture at SAP rates of 1500 and 3000 kg SAP ha
−1 (approximately the 0.1 and 0.2% rates used by El-Asmar et al. [
35]). We did not test higher rates in our studies. This difference could be explained by the soil texture differences. Our loam soil had only 21% clay compared to 45% clay in the El-Asmar C soil and 37% clay in their SCL, giving us a much larger percentage of the soil particles in the larger sand and silt proportions. As discussed above, SAP effects on soil moisture decrease in finer-textured soils.
Additionally, our studies were conducted over a much wider range of matric potentials. El-Asmar et al. [
35] measured water content at matric potentials no lower than 200 kPa with the lowest gravimetric water content (GWC) of 23%. A matric potential of 200 kPa is well within the range of plant available water, which is typically assumed to be between 30 kPa (field capacity) and 1500 kPa (permanent wilting point). We did not monitor matric potential in our studies. However, the soil in our studies was saturated and then allowed to dry down for 76–107 d—until all seedlings had wilted and died and GWC was measured as low as 2.2%. It may be that the SAP holds more water than the surrounding soil as the soil matric potential decreases, relatively increasing water content as the rest of the soil dries.
A third factor that may have increased soil moisture in our studies in treatments with the SAP mixed with the soil is the fact that the SAP was only blended into the top 8 or 15 cm of soil of 25 or 23 cm deep containers, respectively, not the full soil volume. When El-Asmar et al. [
35] included treatments of SAP mixed into the soil, the SAP was blended into the full container of soil. They [
35] suggest that banded SAP would allow water to diffuse into the soil below when it was saturated, but then act as a barrier to soil capillary rise as the soil dried. Concentrating the SAP into a portion of the container, even if not banded, could have a similar, though reduced, impact—slowing water movement from the soil below and increasing the overall moisture holding capacity.
Application of SAP increases plant available water not only by absorbing and holding water within the polymer, but it has also been shown to decrease bulk density and increase soil porosity [
28,
34,
35,
53,
58,
59]. Soil pores vary in size and shape and are categorized as macropores (>0.08 mm) and micropores (<0.08 mm). Macropores readily allow movement of air, water, and plant roots through the soil profile, while movement through micropores is more limited. Thus, an increase in soil macropore size can be beneficial. If, however, they are large enough to create large subsurface voids, moisture may be more easily lost due to drainage or evaporation [
23]. This may result in a reduction in seedling longevity. Sarvaš et al. [
60] found reduced transplant seedling survival with Stockosorb micro with Scotch pine (also known as “Scot’s pine”;
Pinus sylvestris). This was attributed to the swelling of the SAP that “pushed up” the seedlings, resulting in their demise, likely due to loss of soil moisture from large subsurface voids. No information is given on the size of the planting hole, but we speculate that their application rate (7 g tree
−1) was higher than ours. Depending on the transplant method, the planting holes could have ranged between approximately 8 and 25 cm in diameter. The applied rate of 7 g SAP mixed into each planting hole would be equivalent to approximately 1400 and 14,000 kg ha
−1.
Similarly, we frequently observed raised or cracked soil in our studies with SAP bands at ≥750 kg ha
−1. In general, the SAP broke through the soil surface, creating cracks, as it became saturated and swelled. Soil cracking did not occur with the deepest (15 cm) SAP placement. However, the entire soil surface in those pots was raised ~1 cm compared to the controls. Bakass et al. [
61] found that SAPs dry out ahead of the surrounding soil. Since it is the absorption of water that causes the SAP to swell, it shrinks as it dries. It is plausible that as the banded SAP swelled, it also spread to form a layer. This could have resulted in a void below the surface as it dried, and the displaced soil retained its shape.
We found that the soil in the pots was divided into separate layers at the level of the dried SAP after complete dry-down. This was similar to El-Asmar et al. [
35] who observed the formation of large air pores as individual SAP granules dried when mixed in the soil, as well as the formation of distinct soil layers after drying when SAP was applied as a band. The formation of large cracks and subsurface voids was likely exacerbated by both the placement in a concentrated band and the relatively high SAP rates in our studies. These cracks exposed the SAP and deeper layers of soil to air, light, and heat and could potentially lead to faster drying of the soil. However, this did not result in reduced soil moisture in these studies. As mentioned above, El-Asmar et al. [
35] suggest that banded SAP, wet or dry, prevents evaporative loss of soil moisture from beneath it, even when exposed to the elements, thus maintaining higher soil moisture levels.
Reducing the SAP rates to reduce the threat of soil cracking always resulted in a reduced impact on soil moisture level and longevity, if any impact occurred at all. Similarly, Bandak et al. [
62] found that the application of Super AB A-200 (Rahab Resin Co., Tehran, Iran or Iramont, Inc. Laval, Quebec, Canada) at rates of 1000 and 2000 kg ha
−1 increased soil moisture. However, no effect was measured at the low rate of 500 kg ha
−1. Hüttermann et al. [
42] suggest that there may be a minimum SAP concentration required to see an increase in soil moisture. Curiously, in our reduced rate study, the two lowest rates, 11 and 47 kg ha
−1, increased soil moisture for up to 11 d, but the 180 and 750 kg ha
−1 rates did not, possibly due to drying due to greater soil swelling. The 1500 kg ha
−1 rate also did not increase moisture in that study but did so in other studies. Increased soil moisture at the lowest rates could possibly be a result of the formation of relatively smaller air pockets created within the soil that still allowed for increased percolation and reduced evaporative loss without disturbing the soil surface.
The one exception to increased soil moisture with high-rate SAP bands was the SAP and Root Depth Study that showed no significant difference in soil moisture. Interestingly, unlike the other studies, there was no soil cracking, either. That study was conducted in round canisters with SAP placed in a band around the circumference of the canister, rather than square pots with the SAP band running across the middle. This may have been equivalent to reducing the amount of SAP at any one spot, thus reducing the soil cracking as the band swelled, but also reducing the positive impact on increased soil moisture.
Several other studies have also reported reduced SAP absorptive capacity when resaturated after complete dry-down of the soil [
38,
39,
40,
41,
55,
63,
64,
65,
66,
67]. Holliman et al. [
39] reported that the largest decrease was observed in the first 18 months of use. Bai et al. [
38] reported significant reductions of 73–99% in SAP water absorbency of four different SAPs after five rewet cycles over 5 months. Banedjschafie and Durner [
40] found that water retention of Super AB A-200 at application rates of 0.3, 0.6, and 1.0% (approximately 3400–15,000 kg ha
−1) thoroughly mixed into sandy soil reduced significantly after the first wetting/drying cycle but was significantly increased over soil alone. They also found plant available water decreased by about 50% compared to the initial saturation after six months of repeated wetting and drying. Zhang et al. [
37] found that the superabsorbent resin AG
101 (Formosa Plastic Corporation, Kaohsiung City, Taiwan) absorbed and reabsorbed water 30–50 times before degrading. The reduction in absorbency has been attributed to degradation due to weathering, microbial action, and exposure to salts [
27,
34,
38,
40,
43,
53,
55,
56,
63,
64,
65,
66].
The gradual replacement of the original structural SAP cations (usually sodium (Na
+) or K
+) with calcium (Ca
2+) and/or magnesium (Mg
2+) is one possible source of the reduced absorption efficiency of SAPs [
38,
40,
55,
56,
67]. Banedjschafie and Durner [
40] attributed the reduction in SAP absorption to specific salts, especially Ca
2+, in the soil solution, while water-soluble phosphorus (P) and K had only a moderate effect on the reduction in water uptake. Yu et al. [
56] attribute the reduction to the exchange in adsorbed cations within the SAP structure itself. They report that the strongest reduction is associated with bivalent cations, especially Ca
2+ and Mg
2+, compared to monovalent cations, especially Na
+ and K
+. We saw indications of this effect in our studies (
Figure 9). The UTTR soil used in this study was especially high in Ca
2+ and Mg
2+ (
Table 2), but GWC was increased with the use of a fertilizer high in K
+ applied to the band. If the application of monovalent cations in or near a subsurface SAP band would improve its absorptive capacity, lower SAP rates could provide desired establishment or soil moisture results while reducing soil disturbance.
4.2. Seedling Growth Parameters
The use of banded SAP in these studies resulted in the same or better seedling longevity and/or persistence without any apparent negative impact on rooting. Significant longevity increases were measured at times, but only consistently at the highest rate of 3000 kg ha−1 and only at placement depths of 8 and 15 cm, where the highest soil moisture was also measured. Despite soil cracking and subsurface voids, there was no negative impact on longevity at either the 1500 or 3000 kg ha−1 rate at any placement depth. When it was measured, we found no effect on seedling emergence, longevity, shoot or root biomass, shoot:root ratio, or root growth.
There has been very little research done on the effects of placing SAP in a concentrated band in the soil on plant growth, but it may have a positive impact. El Asmar et al. [
35] found similar longevity results which they attributed to the creation of soil water reservoirs in banded SAP. They applied SAP in clay (C) and sandy clay loam (SCL) soils at rates of 0.1, 0.2, 0.3, and 0.4% (approximately 1700–7100 kg SAP ha
−1), either mixed with the soil or placed in bands 25 cm below the soil surface to pine (
Pinus pinea) seedlings. They found an increase in seedling survival time with SAP rates of 0.2 and 0.4% when placed in bands. There was no difference relative to the control when the SAP was mixed in the soil. They also found increased corn shoot fresh and dry weights when the same SAP rates were banded and placed at a depth of 15 cm, but not when the SAP was mixed into the soil. We found no clear influence of SAP band application on plant height but did see an increase in the number of blades per plant. The extreme drought conditions of our studies prevented the seedlings from developing into mature plants. However, the increased blade count indicates the possibility that an increase in above-ground biomass may have resulted if they had persisted.
Other works have demonstrated that SAPs, in general, do have a positive impact on plant growth [
25,
42,
43,
68,
69]. Lucero et al. [
43] used both starch- and acrylic-based SAPs and found a significant effect on leaf biomass and area in black grama grass (
Bouteloua eriopoda), another long-lived, warm-season rangeland grass species. Additionally, Hüttermann et al. [
42] reported pronounced growth of Aleppo pine (
Pinus halepensis) seedling shoots and roots under drought conditions with Stockosorb K 400 mixed into the soil at a concentration of 0.4% (
w/w approximately 7000 kg ha
−1). Rezashateri et al. [
25] found similar results in a containerized study of wormwood (
Artemisia sieberi), a variety of sagebrush, with three different SAPs, including one from Stockosorb, at rates of 5 and 10 g kg
−1 soil (or approximately 1800–3700 kg SAP ha
−1). Yang et al. [
69] found that SAP rates between 30–45 kg ha
−1 increased biomass, grain number, and yield in corn grown in low-rainfall conditions. Coello et al. [
68] found that Aleppo pine growth generally increased with increased SAP application but noted a saturating effect.
Reducing the banded SAP rates below 1500 kg ha
−1 in these studies did not impact seedling longevity compared to the control. This is in contrast to Johnston and Garbowski [
70], who documented benefits to perennial grass establishment in field studies with in-season irrigation and application of two SAPs (Luquasorb 1280 RM and Tramfloc 1001) at rates of 310 and 450 kg ha
−1 blended with the seed. This contrast could be due to different SAPs, higher soil moisture, field conditions, and direct seed placement in their study. The swelling of the dispersed SAP and any resulting soil cracking or subsurface void formation would also have been diffused across the planting area.
Root growth affects seedling establishment [
25,
26,
71]. This may be especially true for seeded native species competing with invasive annuals in arid conditions [
26]. We saw no SAP influence on any seedling root parameters for the first 7 weeks (approximately 20 cm) of growth of bottlebrush squirreltail and Siberian wheatgrass in the SAP Depth and Root Growth study. In contrast, Garbowski [
72] saw higher root mass fraction in a field study using the same SAP as in our studies but at rates of 250 kg ha
−1 and blended to a depth of 10 cm. The increase in root mass was dependent on other treatments such as ambient precipitation and increased cheatgrass presence. Rezashateri et al. [
25] found increases in sagebrush root dry weight and root/shoot ratio with all tested SAPs and irrigation levels in sandy loam soil. Some treatments, including Stockosorb at 10 g kg
−1 (4000 kg ha
−1) at 75% average irrigation rate, resulted in increased root length, perimeter, area, and volume. Bandak et al. [
62] saw an increase in root length and weight of rain-fed wheat with the application of the superabsorbent polymer A200 SAP. Zhou et al. [
66] found shorter roots with more surface area in an irrigated summer maize (
Zea mays) using organic–inorganic composite superabsorbent polymers, which are SAPs incorporating inorganic materials such as clays. Hüttermann et al. [
42] found that the growth of Aleppo pine root tips stopped during water stress, but in the presence of SAP, the adventitious and side roots were able to continue to grow.
Our contrasting results may be a function of the length and the severe drought conditions of our studies. The impact of SAPs on seedling survival is diminished over multiple growing seasons [
4,
36,
72]. This could be a function of the degradation of the product [
39] or the plant roots growing beyond the placement location. Our trials were too short to observe SAP degradation, but we did not find any difference in root growth to and through SAP bands compared to the control in our study. However, the use of SAP has been shown to increase soil moisture in this and other studies, which may positively impact seedling survival in the short term [
35,
42,
43,
68,
70].
Some differences were observed between species in these studies, with Siberian wheatgrass responding more favorably to SAP than bottlebrush squirreltail. The number of days to emergence was not affected by SAP treatment in any study where it was measured. In most studies, the number of seedlings emerged was also not impacted. However, the number of Siberian wheatgrass seedlings that emerged was increased 1.6 times with SAP treatment in the Fertilizer study. Siberian wheatgrass persistence increased with SAP application in the Reduced Seeding Rate study. Bottlebrush squirreltail trended in the same direction, but this trend was not significant. This is similar to Minnick and Alward [
4], who found that Aquasorb (Ark Enterprises, Warsaw, MO, USA), a cross-linked Na-polyacrylate SAP, increased survival of rubber rabbitbrush (
Ericameria nauseosa) but not big sagebrush (
Artemisia tridentata) or four wing saltbush (
Atriplex canescens). The effects may be varied for different types of grasses as well. More research is needed to determine species that may benefit from banded SAP use more than others.
Though SAPs are reported to hold fertilizers in place to increase plant access and growth [
27,
73], we found the use of a low-N fertilizer, with or without SAP, had no positive impact on seedling emergence or longevity. In fact, application of the fertilizer at a depth of 8 cm negatively affected the number of seedlings that emerged, but not their longevity. This was despite an increase in soil moisture when SAP was also present at that depth. Applying the fertilizer at the soil surface negatively impacted seedling longevity but did not impact soil moisture. The use of fertilizers has been shown to negatively impact seedling establishment in restoration efforts in arid regions [
74,
75]. It is possible that rangeland species that have evolved in harsh conditions without additional nutrition inputs could be negatively impacted if SAPs held not only water, but also increased nutrient concentrations near their roots [
40,
56].
Drought severity may impact the effectiveness of SAP. In a containerized study using Aleppo pine, Del Campo et al. [
76] found that three SAPs (Aguaspon, Stockosorb, and Terracottem) mixed into the soil at rates of 0.01 and 0.1% (180–1800 kg ha
−1) were effective in moderate drought conditions, with effects diminished at suction tensions higher than 30 kPa. This would indicate that the effects of SAPs diminish beginning at about field capacity, much wetter than the dry soils in these studies or those found in the field during the summer. Garbowski et al. [
72] reported no establishment effects on first-year field-grown rangeland seedlings under drought conditions, in contrast to positive effects in the deficit-irrigated agriculture condition. They suggest there may be a soil moisture threshold for positive effects on soil moisture content. Their SAP application rates were 1/6–1/12 those used in these studies, and higher rates could allow for higher moisture holding capacity. Rezashateri et al. [
25] found that higher rates of SAP, 10 g kg
−1 compared to 5 g kg
−1 (or approximately 1800–3700 kg SAP ha
−1), had a greater impact on the growth of wormwood at 75% of the normal precipitation rate than 100 or 150% applied every 30 d to simulate natural precipitation patterns. In contrast, most of our studies were conducted under what should be considered severe drought conditions. It is entirely possible, though unlikely, that no further moisture inputs would occur during a growing season in the Great Basin. If the SAP was present and could increase soil moisture enough to allow seedlings to survive to the next precipitation event or allow the seedling roots to grow deep enough to find moisture lower in the soil profile, it could greatly impact seedling establishment rates. The benefits would need to be balanced against the need for higher SAP rates to capture and retain enough soil moisture as well as possible soil cracking and seedling death due soil displacement by swelling higher SAP rates.
Severe drought conditions may also explain why we did not see an impact on seedling longevity or other growth parameters when the SAP was mixed in the soil. The effects of mixing SAP into the soil were measured in only two of our studies. In the SAP Rate and Depth study, we observed a significant increase in soil moisture over the duration of the study but no increase in seedling longevity. This could be due to the trial conditions. SAPs have been used in agriculture when mixed in the soil to reduce the number of irrigations needed [
32,
33,
34]. They have also been shown to increase seedling growth in rangeland applications when mixed into the soil [
36]. In each of these situations, there are at least occasional additions of water through irrigation or precipitation. In our studies, the soil and SAP were saturated once and then allowed to dry down. No additional water inputs after initial saturation would have greatly increased the drought severity over the course of the studies relative to field conditions.
The utility of SAP use for seedling establishment will likely vary depending on location and project. Precipitation, temperature, terrain, and site size are just some of the factors that determine restoration success generally and SAP feasibility specifically. Priced between USD 5.00–6.18 kg−1 (USD 2.27–2.80 lb−1) (Ken Aguilar, Global Plastic Sheeting, San Diego, CA, USA; personal communication (2017); Scott Mecom, Creasorb, Omaha, NE, USA; personal communication (2017)), the cost per kg of Stockosorb 660 micro is low relative to restoration seed mixes. Seed budgets for reseeding projects in the Great Basin run between USD 185–865 ha−1 (USD 75–350 acre−1), with the average falling between USD 370–500 ha−1 (USD 150–200 acre−1) (Josh Buck, Granite Seed, Lehi, UT, USA; personal communication (2018)). To increase the likelihood of seeing a response, SAP was used in these initial studies at rates ranging from 11 to 3000 kg ha−1. At these rates and prices, the cost for Stockosorb 660 micro would range from USD 55–18,500 ha−1.
There are several mitigating financial factors to consider, however. It may be possible to procure SAP at a lower price if purchased in very large quantities. The soil in these studies only received one saturating irrigation at the time of planting. Having the ability to reabsorb water during multiple precipitation events, lower amounts of SAP may be required to increase establishment in the field. If used in a band, the SAP must be buried in the ground, presumably with an attachment to a range drill. As such, it may only be used with species and in areas with terrain and slope that permit that seeding method or something similar, limiting the amount needed to be purchased. Improved establishment rates could reduce the amount of seed, and thus the cost, needed each year. If establishment rates are successfully improved, the need and cost to reseed the same area multiple times may also be reduced or eliminated. Rather than being used on the entire reseeding area, SAP could be used facilitate assisted succession. Bio-islands or green firebreaks of established perennial grasses could provide a free seed source, invasive species control, fire protection, and erosion reduction for the larger landscape, allowing other species to establish. Invasive weed and fire suppression provided by perennial grasses could also help reduce the overall restoration costs for the site by reducing the amount of money spent to manually, chemically or mechanically do the same. Once SAP effectiveness is demonstrated in the field, future studies should be done to evaluate the SAP effect on establishment in the field, determine best SAP and seeding rates, and determine species best suited to SAP use to improve the seedling establishment rate while lowering restoration costs.