The in fluence of hydrogel soil amendment on the survival and growth of newly transplanted Pinus patula seedlings

Ada Mudhanganyi·Hilton G.T.Ndagurwa,2·Carlton Maravanyika·Robert Mwase

Introduction

In recent decades,exotic timber plantations in Zimbabwe have failed to meet planting targets resulting in a 25%decline intheplantedarea.This situationhasbeen worsened by an increase in temperatures and in the frequency and intensityofdroughts(Newetal.2006;Bokoetal.2007).Asa result,unplanted regeneration sites have increased in area,and afforestation of these sites is an important target for the plantationindustry.Thereisthusarealneedforafforestation techniquesthatextendtheplantingseasonintheseareas,and research is focused on this topic throughout southern Africa(e.g.,Oscroft et al.2000;Viero et al.2000,2002;Viero and Little 2006).One such approach is the application of amendmentssuchashydrogels,orsyntheticpolymerorganic combinations called soil conditioners that are capable of improving the water and nutrient retention of the soil.However,we are not aware of any study that has evaluated hydrogel applications in forest management in Zimbabwe.

Hydrogels are synthetic polymers or cross-linked co-or multipolymersthattakeontheconsistencyofagelwhenthey absorb water(American Soil Technologies Inc.2010).They haveahighwater-holdingcapacitywithpotentialtoabsorba volume of water 400 times their own weight(Bouranis et al.1995;Montesano et al.2015).Aqua Matrix Forestry®(ICAT Environmental Solutions)is a potassium-based waterabsorbent polymer,which contains nutrients(nitrogen:N,phosphorus:P,potassium:K),natural mycorrhiza and Trichoderma fungi(Goosen 2013).Thus,in addition to increasing soil–water retention capacity,hydrogels can also improve plantaccessto nutrients.Since the establishmentof newly transplanted seedlings is often restricted by low soil moisture and nutrient availability(Apostol et al.2009;Chirino et al.2011),application of hydrogels can enhance seedling survival and early growth particularly in dry conditions.However,if applied in excess,hydrogels can also have negative effects on plants(Sarvašet al.2007).Additionally,hydrogels may be phytotoxic and have been suggested to contribute to damage to root cell membranes(Alm and Stanton 1993;Sarvaš2003;Sloan 2004).Nevertheless,soil amendment with hydrogels is known to improve the survival and early growth of newly transplanted seedlings(Callaghan et al.1988,1989;Save et al.1995;Sarvaš2003),and they have been used successfully to improve seedling establishment in a number of species(Oscroft et al.2000;Vieroetal.2000,2002;VieroandLittle2006;Orikirizaetal.2009).Despite these various bene fits,no studies have evaluated the effect of hydrogel addition on the growth and survival of newly transplanted seedlings in Zimbabwe.

Pinus patula Schiede ex Schltdl.&Cham.is a native of the Mexican Highlands and was introduced to Zimbabwe’s Eastern Highlands in 1919(Crockford 1995).It is the dominant species in exotic plantations and of great socioeconomic importance in southern African forestry(Birks and Barnes 1991).The current project was carried out in response to the current plight of the forestry industry to reduce the area of unplanted regeneration sites in Zimbabwe’s pine plantations by extending planting beyond the rainy season.In this context,we tested the effects of hydrogel addition on the survival and growth of newly transplanted P.patula seedlings at different stages of the dry season.We hypothesised that adding hydrogel around the roots of seedlings at transplanting would improve seedling water status and nutrient access in the field.To test this hypothesis,we compared the survival and early growth of P.patula seedlings transplanted with or without a hydrogel at three stages of the dry season(early,mid and late).

Materials and methods

Study area

The study was carried out at Stapleford Forest(32°50′E,18°42′S;1745 m,a.s.l.)covering 26,000 ha in the Eastern Highlands of Zimbabwe(Childes and Mundy 1998).The mean annual rainfall is 1820 mm per year.Mean annual temperature is 15°C with ground frost during the winter,particularly in June and July.The soils are welldrained sandy clay loams derived from dolerite(Gwaze et al.2001).The predominant species in the plantation are Pinus patula Schiede ex Schltdl.&Cham.,Pinus taeda L.,Pinus elliottii Engelm.and hardwoods(Eucalyptus spp.).

Plant materials

Seedlings were raised in polythene pots from improved seedssupplied by Zimbabwe Forestry Commission Research Centre at monthly intervals from September 2013 to January 2014.Seeds were sown directly into polythene pots filled with sieved plantation soil and decomposed pine bark.The plantation soil ensures inoculation of the seedlings with mycorrhizal fungi.Once all seed had germinated,the seedlings were watered twice a day and then heavily once a week in the last 2 months before transplanting to the field.Pots were completely weeded by hand 3–4 times after watering during the growing period.In southern Africa,P.patula seedlings of 10–15 cm shoot height are suitable for transplanting into the field since this lessens the probability of root development problems(Dvorak et al.2000).Therefore,we selected seedlings that were 10 cm tall for the field experiments,and all seedlings were 7 months old at planting.

Experimental design and field measurements

A field experiment involving P.patula seedlings planted with or without hydrogel was conducted to investigate the effects of hydrogel on seedling survival and early growth at three stages during the dry season in 2014:early(April),mid(July)and late(October).At each time,three randomized complete blocks of 20 m×20 m were established 30 m away from the compartment boundary to avoid any edge effect.The planting site was pitted a week before planting by loosening the soil in an area approximately 25 cm in diameter to a depth of 25 cm at 3×3 m spacing.Thus,each plot had 44 pits.

On the morning of planting,seedlings were irrigated and transported to the field in containers.Seedlings were randomly divided into two equal treatment groups:with(hydrogel added)and without(no hydrogel added).The experiment was a randomized complete block design with three blocks,two hydrogel(with and without)treatments and 22 seedlings per hydrogel-block combination.Transplants receiving no hydrogel were planted with nursery soil at the centre of each pit into loosened soil.Seedlings receiving the hydrogel treatment were washed free of nursery soil before planting.

A hydrogel–water mixture,at 0.5%hydrogel concentration,was prepared by mixing 5 g of Aqua Matrix Forestry in 1000 mL water.At planting,600 mL of the hydrogel–water mixture was poured into the pit to cover the root plug.Then the root plug was covered with soil,avoiding mixing the soil and hydrogel–water mixture and not compacting the soil around the root plug(Viero et al.2002).We used this method to ensure that a greater portion of the root plug would be in contact with the hydrogel crystals;the degree of water availability to the roots of a transplant is directly in fluenced by root contact with or proximity to the hydrogel crystals(Fonteno and Bilderback 1993).Seedlings planted without hydrogel only received 600 mL of water at planting.Complete ring weeding in a 1-m radius was carried out for all transplants at planting and when necessary.

Seedling survival,shoot height,and root collar diameter were measured at planting and at monthly intervals for 3 months.Survival at each time was calculated for each plot as(number of live plants/total number planted)×100.Shoot height was measured from the top of the root plug to the top of the apical bud using a graduated ruler.Root collar diameter(RCD)was measured at the top of the root plug using a forest caliper.

Statistical analyses

All survival and root collar diameter data were logarithmically transformed log10(x+1)before analysis(Zar 1984).The effect of planting time(Pt:early,mid,late-dry season),treatment(T:with hydrogel,without hydrogel),and their interaction on seedling survival,height,root collar diameter and height to RCD ratio were tested using a repeated measures ANOVA followed by Tukey’s honestly signi ficant difference(HSD)test(P＜0.05).Where the assumption of sphericity was not met,the Greenhouse–Geisser correction was applied.An independent t test was used to compare seedling survival,height and root collar diameterbetween treatments.We used a one-way ANOVA,followed by Tukey’s HSD test(P＜0.05),to analyse differences in rainfall and temperature among the three sampling periods.Pearson’s correlation analysis was used to test the relationship between mortality and seedling variables with 0＜|r|＜0.3: weak correlation,0.3＜|r|＜0.7:moderate correlation and|r|＞0.7:strong correlation.Arcsine-transformed data were used when measurements were expressed as percentages.The effects were considered to be signi ficant at P＜0.05.All statistical tests were conducted in SPSS 21 for Windows(SPSS Inc.,2012,Chicago,IL USA).Evapotranspiration was calculated from weekly minimum and maximum temperatures using the Penman–Monteith method in CROPWAT 8.0 for Windows.

Results

Weather variables

Rainfall differed signi ficantly between the sampling periods,with the least rainfall in the mid-dry season followed by the early-dry season then the late-dry season(Table 1;Fig.1).Early-andlate-dryseasonrainfallwasgreaterbyafactorof3.1 and7.9thanthemid-dryseasonrainfall,respectively.Although themeanandminimumtemperaturesdidnotdifferbetweenthe seasons(P＞0.05;Table 1),maximum temperatures were greaterinthelatedry-season(Octoberon)thanintheearly-and mid-dry season(Table 1).Similarly,evapotranspiration was greatest in the late-dry season followed by the mid-dry season and least in the early-dry season(Table 1;Fig.1).

Planting time effects on seedling parameters

Seedling survival signi ficantly differed with planting time(Table 2),and seedling survival was greater in the late-and early-dry season than in the mid-dry season(Table 3).Similarly,seedling height signi ficantly differed among the seasons(Table 2).Seedlings planted in the late-dry season were taller than those planted in the early and mid-dry season,being greater by 15 and 26%,respectively(Table 3).However,the RCD and height/RCD ratio did not differ between the seasons(Table 2).

Hydrogel effects on seedling variables

Our findings show that the hydrogel treatment signi ficantly in fluencedseedlingsurvival,heightandtheheight/RCDratio(Table 2).Seedlings planted with hydrogel had higher survival than those planted without(Table 4).The hydrogel treatment increased seedling survival by 34 and 22%in the mid-and late-dry season,respectively.However,there was no difference in survival between hydrogel treated seedlings and the control in the early-dry season(Table 4).The hydrogel treatment also increased seedling height by 30,26 and 22%in the early-,mid-and late-dry season,respectively(P＜0.05;Table 4).The height to root collar diameter ratio signi ficantly differed between treatments(Table 2)and was greaterwith hydrogel than withouthydrogel only in the middry season(Table 4).Also,the interaction between planting time and treatment did not affect any of the measured variables(P＞0.05).

Relationship between mortality and seedling variables

Seedling mortality was moderately negatively correlated with height(r=-0.67,P=0.002)and height/RCD ratio(r=-0.51,P=0.015).However,there was no relationship between mortality and RCD(r=-0.004,P=0.988).

Table 1 Mean(±SE)monthly rainfall and daily temperatures(maximum,minimum and mean)during the three periods of the study at Stapleford Forest in the Eastern Highlands of Zimbabwe

Fig.1 Weekly rainfall and average daily evapotranspiration for each week during the early-,mid-and late-dry season of the study at Stapleford Forest in the Eastern Highlands of Zimbabwe(n=12 weeks in each season)

Table 2 F-ratio and the level of signi ficance of a repeated measures ANOVA to test the effect of planting time(Pt:early,mid,late),treatment(T:with hydrogel,without hydrogel),and their interaction on seedling survival,height,branch length,root collar diameter(RCD)and height/RCD ratio

Discussion

Seedling survival was greater in the early-and late-dry season than in the mid-dry season likely due to moisture differences between these sampling periods.The early-and late-dry seasons received more rainfall than the mid-dry season.In addition,unlike in the mid-dry season,rainfall was greater than evapotranspiration in the early-and latedry season.Therefore,we suggest that this rainfall reduced the effect of evapotranspiration and,consequently,may have improved soil moisture content,which also likely led to greater seedling survival and performance.The pattern was different for seedling height,however,with taller seedlings in the late-dry season than those planted in the early and mid-dry season.Seedlings planted in the late-dry season likely performed better because of the favourable temperatures and increased moisture levels.Temperatures were low in the early-and mid-dry-season,which could have affected plant growth through effects on plant growth regulators(Taiz and Zeiger 2010).However,we did not detect differences in the RCD and height to RCD ratio between the seasons possibly because of the constrained sampling period.

In support of our initial hypothesis,application of hydrogel improved the survival and early growth of newly transplanted seedlings similar to findings of other studies(Callaghan et al.1989;Burdett 1990;Oscroft et al.2000;Viero and Little 2006;Sarvašet al.2007;Orikiriza et al.2009;Chirino et al.2011).Water de ficit inhibits plant growth,stomatal conductance,and carbon dioxide assimilation leading to delayed root growth and is the major cause of transplant stress in forest tree seedlings(Burdett 1990).Hydrogels increase the water-holding capacity of the soil,and the stored water is released slowly(Bouranis et al.1995;Montesano et al.2015),extending the period of water availability to the plant,which helps seedlings to overcome soil moisture limitations,thereby reducing the risk of mortality during establishment.Similarly,the hydrogel apparently reduced damage during dry conditions by improving water availability and thereby enhanced seedling survival.The increase in survival reported here(22–34%)is within the range reported for seedlings of other Pinus species transplanted with hydrogels(Pinus pinea L.,Save et al.1995;Pinus halepensis Mill.,Hüttermann et al.1999;Pinus sylvestris L.,Sarvašet al.2007).However,we found no difference in survival between hydrogel-treated seedlings and the control planted in the early-dry season,perhaps because the seedlings may have bene fited from low evapotranspiration,residual moisture from the previous rainy season,or rainfall during this time may have maintained moisture levels in the subsoil.Considering that 90%survival of untreated seedlings in the early-dry season is acceptable,planting can be extended to this period without the need for soil amendments and associated costs.However,we did not measure soil moisture content during the study period,and therefore we remain conservative in the interpretations that hydrogel amendment improved soil water availability.In this regard,although findings show the in fluence of hydrogel amendments on seedling survival and growth,their implications for soil water availability are inferred from direct measurements on improved survival and growth of hydrogeltreated seedlings.

Table 3 Mean(±SE)survival,height,root collar diameter(RCD)and height/RCD ratio of P.patula seedlings planted in the early-,mid-and late-dry season in a pine plantation in eastern Zimbabwe

Table 4 Mean(±SE)survival,branch length,height and root diameter of P.patula seedlings planted with(+)and without(-)hydrogel in the early-,midand late-dry season in a pine plantation in eastern Zimbabwe

An increase in growth of up to threefold was reported for P.halepensis seedlings planted in hydrogel-amended sandy soils compared with unamended soils(Hüttermann et al.1999).Similarly,we found that seedling height and height to RCD ratio were greater in seedlings planted with hydrogel than those planted without hydrogel across all planting times.This result is ascribed to the additional reservoir of water that the hydrogel makes for the plant–soil system,which reduces transplant stress,thereby improving seedling field performance(Apostol et al.2009;Chirino et al.2011).In addition to water,hydrogels such as those used in this study can also improve plant access to nutrients.Firstly,because hydrogels have a greater capacity of storing water,it is possible that water-soluble nutrients may have been stored in the hydrogel and utilised by the transplants(Apostol et al.2009).Secondly,the hydrogel used in this study Aqua Matrix Forestry®contains nutrients(N,P,K),natural mycorrhiza and Trichoderma fungi(Goosen 2013).Therefore,seedlings planted with hydrogel had greater access to nutrients than those planted without hydrogel.The presence of mycorrhiza in the hydrogel likely increased the water-use ef ficiency and uptake of nutrients such as N and P,thereby enhancing seedling growth(Pineiro et al.2013).

Mycorrhizal fungi has been shown to improve soil aggregate stability in the immediate vicinity of seedling roots after planting(Caravaca et al.2002),which promotes root growth.Thus,we suggest that surrounding roots with the hydrogel containing mycorrhiza promoted root growth and thereby allowed the seedlings to withstand competition from surrounding vegetation after transplanting.However,seedlings planted without the hydrogel would have benefited from mycorrhiza in the substrate from the nursery.Therefore,the hydrogel is simply increasing the magnitude of the effect of mycorrhiza on early growth of the seedlings.Further,the presence of mycorrhiza and Trichoderma fungi makes the seedlings more resistant to soil-borne pathogens(Goosen 2013),further favouring seedling growth.While other seedling variables were affected by the hydrogel treatment,hydrogel addition had no effect on the root collar diameter similar to results of other studies(Pineiro et al.2013;Akhter et al.2004).This effect could be due to the restriction of root growth by the hydrogel surrounding the roots.Further,some hydrogels are phytotoxic and may lead to cell membrane damage in roots(Alm and Stanton 1993;Sarvaš2003).However,the effects of the hydrogel on root growth were not tested and should be the focus of future research.

The effect of the hydrogel on seedling survival was greater in the mid-dry season(34%)than in the late-dry season(22%)because the mid-dry season was relatively drier than the late-dry season.Additionally,considering that the difference of precipitation and evapotranspiration is an estimate of the ‘available water’to a system(Sumner and Jacobs 2005),the elevated daily evapotranspiration and low precipitation in the mid-dry season(Fig.1)suggest that soil moisture was likely limiting during this stage compared with the others.Therefore,the hydrogel may be the major source of moisture for seedlings in the mid-dry season,hence the great effect.In contrast,although evapotranspiration was high in the late-dry season,the earlyand late-dry season received signi ficant rainfall(Fig.1),which suggests that soil moisture was not limiting during these periods.Therefore,the high soil moisture could have reduced the effect of the hydrogel treatment during these periods.However,as indicated earlier,soil moisture was not measured and presents a future research direction.Combined hydrogel and water treatments generally are signi ficantly better for plant growth than hydrogel-only treatments(Viero et al.2002),as indicated here by seedling height,which was greater with hydrogel treatment in the late-dry season than in the early and mid-dry season.Thus,we suggest that the hydrogel likely ampli fied the effect of the rainfall received during the late-dry season;hence hydrogel-treated seedlings bene fited from the increased moisture availability.These findings indicate that combined hydrogel and water treatments are likely to improve seedling survival and performance than hydrogel-only treatments,which should be tested in the future.

Mortality was negatively correlated with height and the height/RCD ratio,which indicates that selection for taller seedlings should be aimed at increasing seedling field performance(Viero et al.2002).However,we did not find a signi ficant relationship between mortality and the RCD,yet mortality and the height/RCD ratio were correlated.This finding suggests that larger diameter stock should be used,but height must also be considered.Further,the signi ficant correlations between mortality and seedling height and height/RCD ratio indicated that grading of seedlings on these variables might improve survival as also suggested by other authors(Bayley and Kitzka 1996;Viero et al.2002).Therefore,seedlings of different sizes are likely to differ in survival and growth when planted with hydrogel amendment,which suggests a possible interaction between hydrogel amendments and seedling size and an area for future research.

Conclusions

In conclusion,the applied value of this study is two-fold.Not only do our findings con firm that the application of a hydrogel improves the survival and early growth of newly transplanted P.patula seedlings beyond the rainy season to include the dry months,a feat previously unattainable due to high seedling mortality,they also identi fied key seedling variables that can be used to grade seedlings in order to improve seedling survival and performance.However,further research is required on the effect of hydrogels on other seedling variables such as water-use ef ficiency,nutrient uptake and foliar and root biomass to draw conclusions on the wider bene fits of hydrogels in forest management.Additionally,the feasibility of using hydrogels in commercial forestry depends on the cost involved,which also requires further study.Given the current plight of the forestry industry in Zimbabwe’s pine plantations,hyrdogels offer great promise for extending the planting season to reduce the area of unplanted regeneration sites.

AcknowledgementsThe authors are indebted to Allied Timbers Ltd for allowing the research in their plantation and for providing the hydrogel Aqua Matrix Forestry®used in this study.At the time of the study,CM was employed by Allied Timbers Ltd.Special thanks also go to the reviewers for their time and effort that allowed us to increase the clarity and quality of our work.

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