Growth Efficacy of Sorghum and Rice Amended with Dried Versus Composted Aquatic Vegetation

Aquatic vegetation is a potential source of organic matter and nutrients for crop production and soil sustainability. However, its high water content and presence of toxic compounds have been major deterrents for commercial application. This split-pot study evaluated the application of Pistia stratiotes (PS) (water lettuce) and Lyngbya wollei (LW) (filamentous cyanobacteria) to grow rice and sorghum. The aquatic vegetation was applied as dried and composted amendments on sandy (<3% organic matter) and muck (>80% organic matter) soils. A completely randomized split-pot design evaluated the effect of the amendments on root dry weight (RDW), shoot dry weight (SDW), and nutrient content of above ground biomass. The application of dried PS and LW on sandy soil produced larger and heavier sorghum shoots than those grown under composted treatments. Soil type was not a determinant factor of plant nutrient content: total Kjeldahl nitrogen, phosphorus, potassium and silicon. Shoot dry weight of rice grown on sandy soils was significantly greater than grown on muck soils using dried LW and composted LW treatments. The allelopathic effects of PS and LW were more pronounced on sandy soil compared to muck soil, indicating the potential application for using aquatic vegetation as a soil amendment on sandy soil in the future.


Introduction
The presence of invasive aquatic vegetation in farm canals can impede drainage and irrigation and is cause for environmental concerns if not controlled properly (Alam et al., 1995;Ndimele et al., 2011).In the Everglades Agricultural Area (EAA) of south Florida, two most common aquatic vegetation species are Pistia stratiotes (water lettuce) and Lyngbya wollei (filamentous cyanobacteria).While several management approaches including chemical control and mechanical harvesting have been attempted, they may not be cost effective options for growers.However, if aquatic vegetation can be utilized as a soil amendment or bio-fertilizer it has the added advantage of offsetting some of the cost associated with mechanical harvesting, resulting in cleaner canals.Wilkie and Evans (2010) proposed three major concerns that have typically hampered the large-scale utilization projects for invasive aquatic vegetation.These are (i) the high upfront capital costs and complexity of utilization programs relative to operational cost associated with a control program; (ii) the "perceived" low value of products from aquatic plants relative to the expense of handling feedstock that is composed of almost 90% water; (iii) the possibility that any demonstration of value for invasive aquatic vegetation could have the perverse effect of speeding.While all these factors remain as important considerations, ongoing research into the beneficial uses of aquatic vegetation has the potential to reevaluate the current control strategies for many water bodies.
Numerous studies have been successful in showing the benefits of using aquatic plants as organic amendments, whether to control disease (Zhou & Everts, 2004), inhibit algal growth in waterways (Wu et al., 2013), or stimulate seedling growth (Ahn & Chung, 2000;Bhadha et al., 2014).Each of these studies was successful in showing the benefits of secondary metabolites (allelochemicals) produced by aquatic plants.Bhadha et al. (2014) were able to show that root length of rice significantly increased in response to Pistia stratiotes at the end of a two-week period compared to the control, illustrating the fact that Pistia stratiotes can be used as a potential bio-fertilizer to stimulate growth of rice.Alternatively, studies have also shown inhibitory effects of allelochemicals on plant growth under controlled laboratory settings.Alliota et al. (1991) isolated potential allelochemicals (α-asarone, steroid derivatives, hydroxyl fatty acids) from Pistia stratiotes, while Bagchi et al. (1990) found that Lyngbya wollei produced and released secondary metabolites which inhibited growth of other cultured cyanobacteria and algae.The best use of aquatic weeds from an agronomic perspective is to apply a layer of aquatic vegetation to the soil that would help suppress weeds and retain moisture.The application could either be during a fallow period or as mulch for a growing crop.Once decomposition occurs, the residues could be incorporated into the soil to add organic matter and nutrients.However, the environmental conditions and the use of composted rather than dried aquatic plants may influence the allelopathic activity of these compounds when applied to the soil for crop growth (Inderjit, 2005;Kalamdhad & Das, 2011).Riemer and Toth (1971) showed that some aquatic weeds, such as Myriophyllum heterophyllum and Elodea Canadensis may be composted for agricultural use without the addition of extra nitrogen.Singh (1963) conducted trials to evaluate the effect of five composted aquatic plants (Pistia sp., Hydrilla sp., Najas sp., Ottelia sp., and Eichhornia sp.) on vegetable yields.Crops of tomatoes and okra were grown on plots to which these composts had been added at the rate of 28 tons ha -1 .The study revealed that the yields of tomatoes and okra were increased by all aquatic plant composts except Eichhornia sp.The author concluded that composts derived from aquatic vegetation have the potential for manuring fish ponds or as agricultural fertilizer.Annually, in the United States professional growers alone purchase $250 million per year in compost products (EPA, 1997).This study attempts at preparing composts from a balanced mixture of aquatic plants having the necessary nutrients to grow rice and sorghum crops.
Rice (Oryza sativa L.) and sorghum (Sorghum bicolor L. Moench) are two common row crops grown in the EAA.Both crops are annual grass plants, which belong to Poaceae family; however, their establishment and adaptation to environmental conditions are different.Rice is adapted to aquatic habitats and can grow in several soil types such as saline, alkaline and acid soils.However, its growth is mostly influenced by the physical properties of the soil that will determine its ability to hold water (OECD, 1999).On the other hand, sorghum can grow in low fertility, moderately acidic and highly alkaline soils, but is best adapted to fertile, well drained soils and clay percentage between 10 and 30% (Dial, 2012).Bhadha et al. (2014) showed that both, P. stratiotes and L. wollei had a negative effect on the germination of five different varieties of plant seeds (snap bean, corn, sorghum, common lambsquarters, and rice) that were amended with various rates of dried application.The presence of numerous allelochemicals such as vicenen (apigenin-6,8-di-C-glycopyranoside), isovitexin (apigenin-6-C-glucoside), and lucenin (luteolin-5,8-di-C-glucoside) being released from the aquatic vegetation were identified as the cause of growth defects.This study evaluated the effect of applying Pistia stratiotes and Lyngbya wollei in dried versus composted form on sandy and muck soils to grow rice and sorghum.In Florida, sugarcane production on sandy soils has increased by nearly 85% in the past 25 years, and identifying a suitable soil enhancer for sandy soils has been gaining global interest due to concerns of soil loss via runoff, desertification and oxidation processes.

Method
The experimental design consisted of a split-pot experiment which allowed us to evaluate the root behavior under treatment and control settings.The split-pot was created by vertically dividing in half a 17 L rectangular pot (23 cm long, 23 cm wide, and 33 cm tall) with a PVC sheet (3 mm thick) secured to the inside pot corners and sealed with silicon to prevent lateral movement of water between compartments (Figure 1).The study evaluated four treatments which consisted of dried P. stratiotes, dried L. wollei, composted P. stratiotes and composted L. wollei applied to muck or sandy soils.The treatments were applied to one half of the pot while the other half was filled with control soil (either muck or sand).Both half-pot treatment and control were fertilized with a mixture of 37-0-0 slow release fertilizer (2.5 g), di-ammonium phosphate (1.0 g), potassium chloride (2.0 g), Sul-Po-Mag (0.95 g) and micromix (0.25 g).Pots were set up in a randomized complete block design with four replicates for the growth of rice and sorghum.

Plant Growth, Harvest and Analysis
Rice was transplanted to the split-pots one week after they germinated in the field and kept under these conditions throughout the experiment.Plants were grown within a small PVC cylinder (7.5 cm diameter) that was placed and filled with the selected soil type in the middle of the vertical barrier (Figure 1).Half-pot treatments contained 2 kg of fertilized muck or 4.5 kg of fertilized sand with composted PS (0.5 kg dry weight), composted LW (0.5 kg dry weight), dried PS (0.2 kg) or dried LW (0.2 kg).
Sorghum seeds were germinated directly in the PVC cylinder (Figure 1) and plants were irrigated manually on each side of the split-pot for adequate plant growth.Half-pot treatments contained 2 kg of fertilized muck or 4.5 kg of fertilized sand with composted PS (0.7 kg dry weight), composted LW (0.7 kg dry weight), dried PS (0.4 kg) or dried LW (0.4 kg).
Plants were harvested from the pots 30 days after planting.Shoot fresh weight was measured and air-dried at 110 °C for three days to obtain their dry weight.Shoots were grounded and tissue was analyzed for TKN, P, K, and Si.Roots from each half-pot were separated from the soil and cleaned.Root samples were air-dried at 110 °C for three days to obtain their dry weight.

Statistical Analyses
Data results were analyzed to determine the effect of four treatments (dried and composted, PS and LW) on two soil types (muck and sand) on shoot and root dry weights, as well as leaf nutrient content of rice and sorghum.The experiment was carried out as a split-pot technique, where the whole-pot factors were Soil (muck/sand), floating aquatic vegetation -FAV (water lettuce/filamentous algae), and Material (dried/composted), and the split-pot factor was treatment versus control.For each main effect in the model, Tukey multiple pairwise comparisons with letter grouping were displayed.For each model interaction, Tukey multiple comparisons of simple effects was conducted.A three factor linear model was fitted for each response variable using the GLIMIMIX procedure in SAS 9.3.

Sorghum Shoot Dry Weight and Plant Nutrient Content
The application of dried PS and LW on sandy soils produced larger sorghum shoots that weighed significantly (p < 0.05) higher than those grown under composted treatments.For sorghum plants, significant two-way interaction between soil and material type indicate that on average, dried amendments increase SDW in sandy soil (Table 3).Shoot dry weight (SDW) of sorghum grown on sandy soils was higher compared to the muck soils under all four treatments (Dried PS, Compost PS, Dried LW, and Compost LW) (Figure 2a).Moreover, separation of means for sorghum showed that the plant had significantly greater SDW when they contained half-pot treatment of dried FAV in sandy soils compared to composted LW in sandy soils, and dried and composted PS in muck soils; however, treatments in muck soils did not present significant differences among them.
Statistical analysis of soil and FAV main effects showed that on average P concentration in the plant was greater in muck soil, and when LW was used as an amendment compared to PS.However, a two-way interaction indicated dependence between soil and material type which shows that in general both half-pot dried and composted treatments increased P content in muck soils but decreased P content in sandy soils, while dried treatments also resulted in decreased P content compared to compost treatments in sandy soils.Moreover, separation of means shows that plants amended with half-pot treatments of dried LW and composted LW in muck soil, as well as composted LW in sandy soil had significantly more P content compared to dried PS and composted PS treatments in sandy soil (   4. ANOVA and Tukey-Kramer comparisons for mean shoot dry weight (SDW), total Kjeldahl nitrogen (TKN), phosphorus (P), potassium (K), and silicon (Si) content of rice plants with the application of four half-pot treatments in sand and muck soil   there was no interaction between factors, in this scenario, the separation of means showed that half-pot treatments of composted LW, composted PS and dried PS in muck soils resulted in significantly greater RDW of the treatment area compared to the control area (Table 6).Whereas half-pot treatment of dried LW in sandy soils resulted in greater RDW of the control area compared to the treatment area.Half-pot treatments did not present significant differences among them; therefore, differences in RDW between treatment and control areas were influenced by significant differences of the half-pot control.Mean separation indicated that the control area had significantly greater RDW when dried PS and dried LW was applied to the adjacent treatment sections in sandy soils, compared to any of the other treatments in either soil type.

Discussion
Both P. stratiotes (PS) and L. wollei (LW) have sufficient nutrient and micro-nutrients to be used as potential bio-fertilizers when applied to soils.However, theses aquatic plants can exude allelochemicals that can have both, negative and positive effect on the growth of sorghum and rice plant.

Sorghum
Silicon content was greater for plants grown in muck soil compared to sandy soils.Separation of means showed that sorghum plants had higher Si contents when half-pot dried LW was applied in muck soils, in comparison to dried FAV and composted PS treatments in sandy soils.According to Balakhnina and Borkowska (2013), plant Si concentrations range from 0.1% to 10%.However, previous research has determined that silicon applied to sorghum crops ensures better growth under environmental drought stresses (Hattori et al., 2005); this is not common for sorghum crop production in Florida during the wet summer months in which the plants were grown.No significant differences on RDW between control and treatments areas of sorghum root growth indicate that root inhibition was not observed with any of the treatments.It is possible that sorghum root exudates compounds, such as sorgoleone, potentially inhibited the allelopathic effect of FAV; or that the allelochemicals were decomposed once the treatments were added to the soil (Dial, 2012;Dayan et al., 2010;Inderjit, 2005).

Rice
In general, RDW of the control area was greater when rice was grown in sandy soil and when dried FAV was applied in the adjacent treatment section of the pot.Since every pot had a different control area, it is possible to infer that dried LW had a negative allelopathic effect on rice root growth; whereas composted PS, composted LW, and dried PS did not show root inhibition and could potentially be used as soil amendments in muck soils under the flooded environmental conditions of the rice experiment.Nevertheless, rate applications will have to be tested in order to determine to what extent these amendments can have a positive effect in root growth, since no significant differences were observed among treatments.Throughout the rice experiment, an inhibitory effect of dried PS and LW in sandy soils was only evident for RDW and not for SDW or plant nutrient content because the roots were separated by a physical barrier, while the above ground plant was product of the whole-pot which included the control area that also influenced foliar growth.It may also be possible that the negative effect of allelochemicals present in PS and LW only influences the site that had first contact with the allelopathic compounds, which in this case are the roots, and not the shoot of the plant.This happens because the phyotoxicity of the compounds can be different throughout the plant.Hence, it can negatively affect root development but there is a potential for the compounds to detoxify after entering the plant and not affecting the above ground biomass.Moreover, there is also a potential requirement for greater accumulation of phytotoxic levels on the above ground biomass to cause a negative physiological response (Inderjit & Duke, 2003).

In Summary
The study showed that utilization of the aquatic plants as dried versus composted amendment affects the behavior of sorghum and rice growth.For example, the application of dried PS and LW resulted in nearly twice as much increase in shoot dry weight in sorghum plant grown on sand compared to the composted treatment.Application of dried PS and LW clearly showed a significant negative treatment effect on rice root dry weight when grown on sandy soils compared to the control.Since sorghum root growth was not negatively affected by any treatment and the best RDW and SDW were obtained with the application of dried aquatic vegetation in sandy soils, then it is possible to consider the incorporation of dried FAV as a suitable amendment to enhance sorghum root and shoot growth in sandy soils.In general, the allelopathic effects (negative and positive) of PS and LW were more pronounced on sandy soils compared to muck soils, probably because organic soils have greater carbon molecules that bind the organic allelochemicals exuded by the aquatic vegetation rendering it unavailable for plant uptake.The fact that the application of aquatic vegetation such as P. stratiotes and L. wollei has a positive effect on crops grown on sandy soils is an interesting observation that warrants further investigation, especially for growers wanting to grow rice or sorghum on sandy soils.

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Table 3 )
. Potassium content did not indicate significant differences of