Effects of Organic Substrate Amendments on Selected Organic Fractions and Biochemical Parameters under Different Soils

The application of organic substrates can affect soil respiration, dehydrogenase (DH-ase) activity, dissolved organic carbon (DOC), and humic acid (HA) fractions. This study aimed to evaluate the effects of five organic substrates in the organic fractions of degraded alluvial soil, acid sulfate soil, and sandy soils and the physicochemical properties of the soil. Soil samples were amended at a rate of 5 tons ha−1 with (1) water hyacinth compost, (2) sugarcane filter cake compost, (3) biogas sludge-rice straw compost, (4) vermicompost, and (5) sludge. The results showed that soil respiration and DH-ase activity increased rapidly within the first 5 days of incubation, while the concentrations of DOC and HA decreased throughout the incubation period. The highest respiration and DH-ase activity occurred after the application of vermicompost. DOC was found to be the highest in soils amended with sugarcane. The highest concentration of HA was observed with the application of sugarcane residues, regardless of the type of soil. The application of water hyacinth and biogas sludge stimulated cumulative HA only in the acid sulfate soil, while vermicompost improved HA only in the degraded soil. The largest stimulation in respiration and DH-ase activity was observed in degraded and sandy soils, regardless of the type of amendment. In the acid sulfate soil (3.7 mg·C·g−1), larger amounts of DOC and HA were observed than in both degraded (1.7 mg·C·g−1) and sandy soils (1 mg·C·g−1). However, DH-ase activity was the lowest in acid sulfate soil.


Introduction
Te recycling of biological waste to organic fertilizers is a way to reuse nutrients and reduce environmental pollution.Various biological wastes such as biological municipal waste, animal excreta, and plant residues can be composted [1].Te application of composts or other organic substrates, such as sewage sludge is a current environmental and agricultural practice for maintaining soil organic matter, reclaiming degraded soils, and supplying plant nutrients [2,3].An adequate supply of nitrogen (N), phosphorus (P), potassium (K), and other essential nutrients is necessary to maintain crop productivity.
For many decades, farmers have used mature composts to maintain soil fertility and improve plant yields.After being engulfed in soils, organic substrates can be mineralized and then nutrients can be released in a more readily available form to plants [4].Immature or uncomposted organic substrates still have high microbial activity and might contain toxic compounds that can immobilize mineral nutrient elements in soils and harm plants.Tere are several ways to treat biological waste.Tey can be aerobically composted or anaerobic, such as by fermentation in a biogas reactor.Te composition of biological wastes signifcantly infuences their organic matter and nutrient content as a result of mineralization processes during the treatment process or after amendment in the soils.Immature compost or biogas sludge contains highly soluble organic carbon substances such as amino acids and carbohydrates [5,6].Tey can easily decompose after application to soils and make soils more anoxic.However, anoxic conditions can mobilize heavy metals in the soil.Immature compost or biogas sludge might also contain large amounts of intermediate organic products of decomposition, such as volatile fatty acids, alcohols, and phenols, which are toxic to plants.Terefore, in the usual agricultural practice, these organic substrates are applied some weeks before sowing to allow soil microorganisms to degrade labile organic matter and phytotoxic compounds and to release plant nutrients [7,8].
A reduction in carbohydrates, hemicellulose, and cellulose during the composting process is usually accompanied by increased humifcation.Terefore, humifcation is an index of compost maturity.Humic substances are valuable components of compost.In additions, the humifcation process increases the content of alkyl C, aromatic C, carboxyl (-COOH), phenolic (-OH), and carbonyl (-CO) groups, which are benefcial to the physical and chemical properties of the soil and plant productivity.When the C : N ratio of compost is lower than 20, the mineralization of N in compost is dominated by microbial N immobilization processes, which facilitates the release of mineral N for plant use [1,9].Te stability and maturity of organic substrates prior to soil application have shown to have strong efects on the biological and chemical properties of soils.Mature organic substrates can be defned as the stabilized and sanitized products of composting, which undergo a hot rotting process to decompose labile organic matter and are humifed [10], and are consequently benefcial to plant growth.
However, depending on the composting facility and compost demand, organic substrates applied to soil can have diferent degrees of maturity, i.e., they can be taken after the biooxidative phase before maturation and can be slightly transformed during the biooxidative phase or even during the thermophilic phase.Biological decomposition depends on the degradation rate of a wide range of C compounds present in organic substrates (carbohydrates, amino acids, fatty acids, and lignin), as well as on their nutrient content [11].Terefore, the efect of each organic substrate on soil properties depends on its composition.Te infuence of organic matter on soil properties depends on the amount, type, and size of added organic materials [12,13].Te decomposition of organic matter in soils is dependent on microbial activity.Te release of nutrients from applied organic substrate decreases exponentially with time as a result of decreasing carbon availability [14].
Microbial respiration, i.e., the amount of CO 2 released from compost, is often used as an indicator of microbial activity and the maturity (stability) of compost [15,16].Te high microbial activity of applied compost might also promote the degradation of soil organic matter (SOM).Tis is usually undesirable, as SOM is a very important component that determines the quality and fertility of soils [17][18][19].Te extent of biodegradation may also be refected in the amount of dissolved organic C available during the decomposable period.Te availability of carbon is essential for most microorganisms because carbon serves as an energy source for the synthesis of ATP.Te main components of the organic matter are carbohydrates, proteins, lipids, and lignin.Tey difer in the ease with which microorganisms can decompose them by aerobic (oxidation) or anaerobic (reduction) mineralization processes.
Rapidly and slowly mineralizable portions of dissolved organic carbon (DOC) are a measure of labile and stable DOC and the respective mineralization rate constants [20,21].Tese labile organic compounds mineralize quickly in soil and are oxidized during degradation.Components that are more resistant to mineralization, such as aromatic and hydrophobic structures, can be assessed by UV absorbance to quantify the amount of humic acid in the soil.Te biodegradability of DOC can be reduced due to its recalcitrance or inhibition of enzyme activity [22].Te capacity of microorganisms to assimilate organic matter depends on their ability to produce the enzymes needed for degradation of the substrate [23,24].Te more complex the substrate is, the more extensive and comprehensive the enzyme system requires source, which is incorporated in the cells.In particular, the presence of ester bonds, aromatic rings, quarterly C, and tertiary N in macromolecular carbon substrates increases their stability and recalcitrance to microbial decay processes and plays a key role in the formation of more stable soil organic matter.On the other hand, watersoluble (hydrophilic) carbon substrates of low molecular weight are easily and rapidly mineralized.Tey are part of the labile C pool and are important for the release of plant nutrients.Since many enzymes respond immediately to changes in soil fertility status, they can be used as potential indicators of soil quality for sustainable management.For example, dehydrogenase plays an important role in the initial oxidation of soil organic matter and occurs only in viable cells; therefore, dehydrogenase activity is a sensible indicator for measuring the metabolic activity of microorganisms in soils [25].
In this study, we hypothesized that the application of organic substrates to diferent soils would diferentially afect soil respiration (Res), dehydrogenase (DH-ase) activity, dissolved organic carbon (DOC), and humic acid (HA) fractions.To test this hypothesis, we evaluated fve organic substrates in three diferent soils under laboratory incubation conditions.

Soil and Organic
Substrates in Tis Study.Tree diferent soil types used for this experiment included old alluvial soil (degraded soil), acid sulfate, and sandy soil.Te soil was taken to a depth of 0-20 cm, air dried, homogenized, and sieved through a mesh of 2 mm before use.
Five organic substrates were used for the experiment: water hyacinth compost, sugarcane flter cake compost, biogas sludge-rice straw compost, vermicompost, and sludge (undigested slurry).Te characteristics of the experimental soils and organic substrates are shown in Table 1.Tese bottles contained the samples.Te samples were incubated for 90 days.Te change in internal pressure was recorded every day for each bottle.

Experimental Setup
Respiration activity was calculated from the pressure change using the following formula: where p is the pressure (hPa), V is the volume (L), n is the substance in mol, R is the universal gas constant, and T is the temperature ( °K).
Te following equation was used to calculate the daily oxygen consumption: where M R (O 2 ) is the molar mass of O 2 (mg mol −1 ), R is the universal gas constant; T is the temperature of incubation in ( °K), Δp is the decrease in pressure in the bottle (hPa), V fr is the volume of free gas (L), and m ts is the dry mass of the sample (g).

Biochemical Incubation.
In this study, 50 g of soilcompost mixture, soil without compost or compost without soil, were added to 200 ml bottles.After adding distilled water, the bottles were covered with caps to avoid gas tightness and placed into the climate chamber at 27 °C.Four parameters were analyzed: O 2 consumption, DHase, HA content, and DOC content on days 0, 5, 10, 20, 40, and 50 of incubation.Reincubation was started after water addition to reactivate biochemical activity, and samples were collected on days 51, 55, 60, 70, and 90.After measurement, the parameters were combined into two parts (0-50 days and 51-90 days) by including the collective days of two stages.

Analyses.
Dehydrogenase activity was determined by extraction of iodonitrotetrazolium chloride according to the method described by Von Mersi and Schinner [26].Humic acids were extracted with a slightly modifed procedure with HA, as recommended and used by the International Society for Humic Substances.Te sample was extracted with distilled water.Ten, 20 mL of distilled water was added to the 2 g sample by a pipette dispenser, giving a ratio between sample and water of 1 : 10.Ten, the sample was shaken for 24 hours.Te supernatant was removed by centrifugation for 30 minutes at 2000 rpm, followed by fltration through a 0.45 µm membrane.A stock solution of HA (200 mg•L −1 ) was prepared by weighing 0.1 g of HA stock in a 500 mL glass volumetric fask and bringing it to volume with a bufer solution.A series of HA standards were prepared from 0 to 100 mg•L −1 through dilution of the stock solution with the bufer solution.Te HA content of the samples was measured on a spectrophotometer at a wavelength of 280 nm.Te DOC content was determined by water extraction according to the methods described by Jones and Willett [27].A calibration curve for DOC was prepared from standard solutions that ranged in concentration from 0 to 10 mg•L −1 , all being diluted from a 100 mg L −1 stock solution.Te samples were diluted between the calibration series with distilled water, and their DOC content was measured with a DOC analyzer.

Data Analysis.
Te data were entered into a Microsoft Excel spreadsheet.SPSS version 14.0 was used for statistical analysis, namely, ANOVA one factor tests.Graphs were drawn in SigmaPlot 10, and data were analyzed using linear regression.

Efect of Organic Substrates and Incubation Time on
Respiration.In the degraded soil, among the treatments, the cumulative O 2 consumption of respiration varied between 1.14 and 6.64 mg O 2 g −1 soil during 90 days of incubation (Figure 1 top-left).From 0 to 50 days, respiration was the highest in the vermicompost treatment with 4.21 mg O 2 g −1 soil (p < 0.05).Te other treatments had lower respiration values, ranging from 0.8 to 1.17 mg O 2 g −1 soil, and they did not difer signifcantly from those of the unamended control.After the samples were dried and subsequently rewetted, microbial respiration was lower than that during the frst 50 days.Te vermicompost treatment resulted in the highest O 2 consumption with 2.43 mg O 2 g −1 soil (p < 0.05).Te treatments amended with biogas, water hyacinth, and sugarcane increased considerably, ranging from 1 to 1.17 mg O 2 g −1 soil compared to the unamended control (0.34 mg O 2 g −1 soil).From 0 to 90 days, all the treatments with amended soil had signifcantly higher O 2 consumption than the control.Te consumption of O 2 in the vermicompost treatment (6.64 mg O 2 g −1 soil) was considerably greater than that in the other treatments (p < 0.05).Te mean respiration rate in soil applied with biogas, water hyacinth, or sugarcane was between 1.68 and 2.24 mg O 2 g −1 soil In the acid sulfate soil, during the incubation period, the cumulative consumption of O 2 ranged from 1.61 to 4.48 mg O 2 g −1 soil (Figure 1-top-right).After 50 days of incubation period, the cumulative microbial respiration increased substantially in response to vermicompost treatment to 3.25 mg O 2 g −1 soil (p < 0.05).No signifcant diferences were observed in the cumulative respiration between the unamended soil and other amendments.After rewetting (51-90 days), the O 2 respiration rate ranged between 0.74 and 1.23 mg O 2 g −1 soil.Tere was a signifcant increase (p < 0.05) in O 2 consumption in the vermicompost-amended soil compared to that in other treatments.Te diference in microbial respiration between other treatments and the unamended control was not signifcant.As an exception, the respiration of water hyacinth added to the soil (0.74 mg O 2 g −1 soil) was signifcantly lower than that of the soil amended with biogas (0.96 mg O 2 g −1 soil).In general, the vermicompost-amended soil had the highest microbial respiration (4.48 mg O 2 g −1 soil) (p < 0.05).When comparing the other amendments (supplements) to the control, no signifcant change in soil microbial respiration was observed.Te consumption of O 2 ranged between 1.61 and 1.84 mg O 2 g −1 soil.Te respiration rate of the second incubation phase showed a strong decrease in the rate of O 2 consumption in vermicompost-amended soil.Tere were no   4 Scientifca diferences in microbial respiration among soils amended with water hyacinth, sugarcane, or biogas.In sandy soil, microbial respiration ranged from 1.13 to 5.81 mg O 2 g −1 soil during 90 days of incubation (Figure 1 bottom-left).Tere was a higher consumption of O 2 in the vermicompost treatment (4.49 mg O 2 g −1 soil) than in other amendments (p < 0.05).Respiration in the water hyacinthamended soil was signifcantly higher than that in the control treatment (p < 0.05).Te sugarcane and biogas amendments did not show apparent diferences in respiration compared to the control.Following the rewetting period of the air-dried soil, the respiration levels of the fve amendment treatments difered from the unamended control.Microbial respiration was the highest in vermicompost-amended soil (1.32 mg O 2 g −1 soil, p < 0.05).Te application of sugarcane to the soil increased respiration, distinguishing it from both biogas and water hyacinth, and yet it remained similar to that of the slurry.Generally, the respiratory activity in amended soils was considerably diferent from that of an unamended control.Microbial respiration was stimulated in vermicompost soil (5.81 mg O 2 g −1 soil), followed by sugarcane application (1.67 mg O 2 g −1 soil, p < 0.05).Te application of water hyacinth, biogas, and slurry produced similar microbial respiration rates with values ranging from 1.38 to 1.46 mg O 2 g −1 soil (p < 0.05).When comparing the two phases of incubation, days 0-50 and 51-90, the microbial decomposable activity of the frst time phase had a much higher rate than that of the second phase.
In organic substrates, during 90 days of incubation, the respiration activity of all organic substrates ranged between 37 and 408 mg O 2 g −1 substrate (Figure 1 bottom-right).Te amount of O 2 consumption obtained from the treatment applied with vermicompost, sugarcane, biogas, and water hyacinth varied from 274 to 408 mg O 2 g −1 , 37.2-57.8mg O 2 g −1 , 21.6-26.2mg O 2 g −1 , and 13.2-31.5mg O 2 g −1 , respectively.Te consumption of O 2 from the vermicompost substrate was the highest than that of other substrates (p < 0.05).

Efect of Organic Substrates and Incubation Time on DOC.
Te highest concentrations of DOC were found in soils treated with sugarcane, biogas, and vermicompost.After rewetting on day 51, most treatments began to show slight increases in the level of DOC, indicating renewed activity of soil microorganisms.Again, during second incubation period, the DOC decreased during incubation.
In the degraded soil, the sums of the DOC concentrations ranged from 0.99 to 1.72 mg•C•g −1 soil during the entire 90 days of incubation (Figure 2 top-left).Sugarcane treatment signifcantly increased the quantity of DOC (1.35 mg•C•g −1 soil) compared to the control (0.97 mg•C•g −1 soil) in the frst 50 days (p < 0.05).Te other treatments did not difer signifcantly from the DOC in the control.However, the cumulative DOC in the slurry treatment was lower than that in the biogas and vermicompost treatments (p < 0.05).After reincubation, there were no signifcant diferences between all treatments with small variations of 0.2 to 0.37 mg•C•g −1 soil.Te accumulated DOC was lower in the second phase of the incubation than in the frst phase (approximately 3 times lower).For 0-90 days, in vermicompost, biogas, and sugarcane treatments, the sum of DOC signifcantly increased from 1.43 to 1.72 mg•C•g −1 soil, whereas treatments amended with slurry and water hyacinth did not afect the content of DOC compared to the control (1.17 mg•C•g −1 soil).Tere were no signifcant diferences in the amount of DOC among the water hyacinth, biogas, and vermicompost treatments.
In acid sulfate soil, the cumulative amount of DOC varied from 1.68 to 3.69 mg•C•g −1 soil during the 90-day incubation period (Figure 2 top-right).For 0-50 days, all treated soils showed a signifcant increase in DOC concentration of 1.59 to 2.6 mg•C•g −1 soil compared with that in the unamended control (p < 0.05), except for the slurryamended soil, which reached 1.4 mg•C•g −1 soil DOC accumulation.When comparing organically modifed soils, the cumulative amount of DOC reached a substantial maximum (p < 0.05) in both sugarcane and biogas treatments with values of 2.47 and 2.6 mg•C•g −1 soil, respectively.Furthermore, the vermicompost treatment had a signifcantly higher DOC concentration than water hyacinth and slurry treatments (p < 0.05).After rewetting of air-dried soils (51-90 days), cumulative DOC values in vermicompost, sugarcane, and biogas treatments resulted in signifcantly higher than those of the water hyacinth, slurry, and control treatments (p < 0.05).Generally, compared to the control, the addition of sugarcane, biogas, and vermicompost caused a substantial increase in the sum of the DOC concentration from 0 to 90 days (p < 0.05).While vermicompost-amended soil had a less signifcant efect on DOC production compared to both sugarcane-and biogas-amended soils (p < 0.05), there were no signifcant diferences in the level of DOC for water hyacinth and slurry-amended soil.Furthermore, at a rate similar to that of degraded soil, the mineralization rate of DOC increased from acid sulfate soil during the second part of the experiment than during the frst 50 days.
In the sandy soil, the DOC value changed between 0.36 and 0.98 mg•C•g −1 soil during 90 days of the incubation experiment (Figure 2 bottom-left).Te results showed that there were no signifcant diferences in the DOC concentration in all soil modifed with the treatments from 0 to 50 days after incubation.Te soil treated with sugarcane, biogas, and water hyacinth had considerably higher DOC concentration than the unamended control soil (p < 0.05).Similarly, there was no signifcant change among the treatments in the DOC concentration after rewetting the dry soil.Unlike what occurred in primary incubation, there was a decrease in the percentage of DOC during secondary incubation.Compared to those in the amended treatments and unmodifed control, the DOC concentration increased signifcantly in all the organic substrate-treated soils with values ranging from 0.77 to 0.98 mg•C•g −1 soil (p < 0.05), and the only exception was observed in the slurry-treated soil (0.54 mg•C•g −1 soil).

Scientifca
In organic substrates, there was a signifcant increase in DOC production on the vermicompost substrate to 62 mg•C•g −1 substrate at 0-50 days after incubation (Figure 2 bottom-right).However, there was no diference in DOC concentration between the biogas substrate (8.89 mg•C•g −1 substrate) and the sugarcane and water hyacinth substrates.On the other hand, the cumulative DOC on the sugarcane substrate (37.38 mg•C•g −1 substrate) was signifcantly higher than that in the water hyacinth (3.99 mg•C•g −1 substrate).For 51-90 days and 0-90 days, the vermicompost substrate exhibited the highest DOC concentration, with values of 24 mg•C•g −1 substrate and 62 mg•C•g −1 substrate, respectively (p < 0.05).Te DOC concentration of the water hyacinth substrate was lower than that of other substrates at 1.56 mg•C•g −1 for 51-90 days and 5.55 mg•C•g −1 substrate for 0-90 days.Tere were no diferences in DOC concentration for biogas and sugarcane substrates, which exhibited similar values of 5.01 mg•C•g −1 substrate for 51-90 days and respective values of 13.9 and 17.6 mg•C•g −1 substrate for 0-90 days.

Efect of Organic Substrates and Incubation Time on
Humic Acid (HA).According to DOC production, humic acid (HA) production was the highest at the beginning of incubation (day 0) and decreased dramatically over the next 20 days.Soon after the rewetting on day 51, the HA content increased in all soils before again decreasing as HA production decreased during the following 50 days.
In the degraded soil, during the three months of incubation, the HA concentration ranged from 0.76 to 1.1 mg•C•g −1 soil (Figure 3 top-left).At 0-50 days and 0-90 days, the amount of HA was 0.51 or 0.76 mg•C•g −1 soil for the water hyacinth treatment, 0.73 or 1.06 mg•C•g −1 soil for sugarcane treatment, 0.74 or 1.07 mg•C•g −1 soil for biogas treatment, and 0.71 or 0.91 mg•C•g −1 soil for vermicompost treatment, respectively.Te results indicated that there were signifcant increases in HA concentration between the other soil modifed with other treatments and the control, except for the slurry treatment.At 51-90 days, there were no signifcant diferences observed in HA concentration after rewetting as the HA concentration ranged from 0.2 to 0.33 mg•C•g −1 soil among all the treatments.
In the acid sulfate soil, the sum of the HA concentrations ranged from 1.18 to 3.62 mg•C•g −1 soil during the whole experiment (Figure 3 top-right).In the frst 50 days, the HA concentrations in sugarcane-and biogas-modifed soils signifcantly increased to 1.91 and 2.14 mg•C•g −1 soil, respectively.No signifcant changes in HA concentration were 6 Scientifca found for other treatments as they ranged from 0.73 to 1.09 mg•C•g −1 soil.Te concentration of HA decreased in the following rewetting period from 51 to 90 days.In water hyacinth, sugarcane, and biogas treatments, the sum of HA was signifcantly higher than in the control with 0.73, 1.48, and 1.35 mg•C•g −1 soil for 0-50 days and 1.82, 3.27, and 3.62 mg•C•g −1 soil for 0-90 days, respectively.In the sandy soil, the HA concentration varied between 0.54 and 0.94 mg•C•g −1 soil during 3 months of laboratory incubation (Figure 3 bottom-left).Te sugarcane treatment had the highest HA concentration with 0.62 mg•C•g −1 soil in the frst 50 days (p < 0.05).Tere were signifcantly higher amounts of HA concentration in the water hyacinth, biogas, and vermicompost treatments than in the slurry treatment and the control.However, no signifcant diferences in the HA concentration were observed among the treatments after rewetting for 51-90 days.Te amount of HA from the sugarcane treatment increased signifcantly (0.94 mg•C•g −1 soil; p < 0.05) compared to that from both the slurry treatment and the control at 0-9 days.Te other treatments did not difer in their sum of HA.
Following the subsequent rewetting stage on day 51, the rate of HA release decreased, as the maximum rate of HA production occurred mainly during the previous stage.Te highest concentration of HA was found in vermicompost (p < 0.05) at 17.2 mg•C•g −1 substrate.Sugarcane (4.82 mg•C•g −1 substrate) and biogas (5.06 mg•C•g −1 substrate) had signifcantly increased the HA concentration compared to water hyacinth (1.73 mg•C•g −1 substrate).

Efect of Organic Substrates and Incubation Time on
Dehydrogenase (DH-Ase) Activity.In the degraded soil, the sum of the DH-ase activity ranged from 0.37 to 3.05 mg•g −1 soil during 90 days of incubation (Figure 4-top-left).In the frst 50 days, there were signifcant diferences in DH-ase activity in all treatments except for the sugarcane and biogas treatments.Most treatments with amended soil had signifcantly higher DH-ase activity than the unamended control (p < 0.05).Compared with that in the amended treatments, the DH-ase activity showed a signifcant increase Scientifca in all treatments in a sugarcane and biogas < water hyacinth < slurry < vermicompost.After exposure to a drying and rewetting period, most of the soils modifed by treatments signifcantly increased DH-ase activity compared to that of the control.Vermicompost treatment showed the highest DH-ase activity with a value of 1.16 mg•g −1 soil (p < 0.05).Te DH-ase activity of the sugarcane treatment (0.52 mg•g −1 soil) was signifcantly higher than those of biogas and slurry treatments (0.41 and 0.36 mg•g −1 soil, respectively).Water hyacinth treatment signifcantly increased the DH-ase activity (0.48 mg•g −1 soil) in comparison to slurry treatment only.At 0-90 days, among organic soil amendments, vermicompost-treated soil exhibited the highest DH-ase activity (3.05 mg•g −1 soil), followed by soil treated with sugarcane (1.19 mg•g −1 soil), slurry (1.32 mg•g −1 soil), and water hyacinth (1.34 mg•g −1 soil).Te biogastreated soil had the lowest DH-ase activity (1.01 mg•g −1 soil) but this value was signifcantly higher than that of the control (p < 0.05).
In the acid sulfate soil, the DH-ase activity varied between 0.04 and 0.17 mg•g −1 soil during 90 days of incubation (Figure 4 top-right).Te result indicated that the DH-ase activity did not difer between the soil treated with vermicompost or slurry and the control in the frst 50 days.Te sugarcane treatment (0.06 mg•g −1 soil) resulted in signifcantly lower DH-ase activity than water hyacinth treatment (0.08 mg•g −1 soil).However, biogas treatment (0.07 mg•g −1 soil) was not signifcantly diferent in DH-ase activity compared to sugarcane and water hyacinth treatments.After rewetting, the highest DH-ase activity was found in the water hyacinth treatment (0.09 mg•g −1 soil).Te DH-ase activity in the sugarcane treatment (0.07 mg•g −1 soil) was signifcantly higher than that in the biogas treatment (0.04 mg•g −1 soil).Te treatments amended with vermicompost and slurry were close to 0.01 mg•g −1 soil and showed no signifcant diferences compared to the control (0.02 mg•g −1 soil).
In the sandy soil, the DH-ase activity ranged from 0.36 to 2.71 mg•g −1 soil during the 90 days (Figure 4-bottom-left).At 0-50 days, all amended treatments signifcantly increased in DH-ase activity compared with that of the unamended control.Te DH-ase activity in the soil treated with slurry and sugarcane (0.73 and 0.75 mg•g −1 soil, respectively) was signifcantly higher than that in the soil treated with biogas (0.52 mg•g −1 soil).Te DH-ase activity of the water hyacinth and sugarcane-amended soils (0.55 mg•g −1 soil) was signifcantly higher than that in the biogas-amended soil (0.27 mg•g −1 soil).However, slurry-amended soil did not signifcantly increase the activity of the DH-ase enzyme.Most treatments with amended soil had signifcantly greater 8 Scientifca DH-ase activity in comparison to the control at 0-90 days.
In the organic substrate, the DH-ase activity varied between 3.0 and 5.2 mg•g −1 substrate during 90 days of incubation (Figure 4 bottom-right).Vermicompost had the highest DH-ase activity (3.88 mg•g −1 substrate in the frst 50 days).On the other hand, sugarcane (2.7 mg•g −1 substrate) has signifcantly increased the DH-ase activity compared to water hyacinth and biogas (2.00 and 2.02 mg•g −1 substrate, respectively).At 51-90 days, the highest level of DH-ase activity reached 2.51 mg•g −1 substrate in sugarcane (p < 0.05).Vermicompost with a value of 1.01 mg•g −1 substrate and biogas with 1.01 mg•g −1 substrate showed signifcantly lower DH-ase than in the water hyacinth (2.22 mg•g −1 substrate).In general, the highest DH-ase activity occurred in vermicompost and sugarcane, which had values of 5.13 and 5.21 mg•g −1 substrate, respectively (p < 0.05).Te DH-ase activity of biogas (3.01 mg•g −1 substrate) was signifcantly higher than that of water hyacinth (4.24 mg•g −1 substrate).

Efect of Organic Substrates and Incubation Time on
Respiration.Te increase in O 2 consumption after three soils were amended with organic substrates in the experiment could be explained by the improved microbial decomposition of the soil organic matter [28,29].Likewise, the application of organic substrates to soils accelerated microbial activity and therefore increased substrate decomposition with the release of CO 2 [30][31][32].
In general, the rate of consumption of O 2 from organic substrate-treated soils, which occurred rapidly during the initial stages of incubation on day 10 and after rewetting on day 70, followed by a relatively linear utilization of O 2 until the end of both periods.Te increase in the respiration rate of O 2 on day 10 may have been primarily related to the supply of available C and easily decomposable polysaccharides [33,34].
In fact, the CO 2 evolution rate is also an indicator of organic matter turnover and soil microbial activity, which are related to the efect of organic substrate amendments [35][36][37].Te rapid increase in respiration observed in the three soils treated with vermicompost demonstrated higher organic C decomposition than in those treated with the other amendments, most likely due to the high C content (86%) and high C : N ratio of 30 :1 that may favor microbial activity.Te vermicompost substrate alone had the highest O 2 consumption.As a result, the readily decomposable organic C fraction in the vermicompost residue was higher than that in the water hyacinth, sugarcane, and biogas residues.
Furthermore, for degraded and sandy soils, high amounts of O 2 consumption were observed during the incubation stages for treatments with water hyacinth, sugarcane, biogas, and slurry.Tis indicated a readily decomposable organic C fraction in these substrates.Furthermore, microbial respiration varied between modifed organic soils and other soils included in this study.For example, when sugarcane was incorporated into sandy soil, the amount of O 2 consumed was (1.7 mg O 2 g −1 soil) for degraded soil incorporated with sugarcane.Vermicompost applied to the degraded soil led to the highest rate of O 2 consumption than the other treatments and the other soils.Although O 2 consumption in the degraded soil amended with slurry (1.1 mg O 2 g −1 soil) showed low O 2 consumption compared to the acid sulfate soil (1.6 mg O 2 g −1 soil).
Te amount of CO 2 released from organic substrates in soil varies according to the material used [2,3], as well as the interactions between the organic amendment and the soil [38].In this study, a calculation of CO 2 -C oxidization for 90 days relative to organic C showed that the stimulation of microbial activity through the application of organic material was stronger for degraded soil (61-80%) than for the other two soils (acid sulfate soil and sandy soil, 44 to 66%).Terefore, the diferences in soil properties afected the amount of CO 2 -C emissions.
Air drying and subsequent rewetting during the second stage were expected to stimulate microbial activity to physically breakdown the protected organic fraction.Nevertheless, the decline in microbial respiration in the second stage of the incubation observed in this study was probably caused by depletion of polysaccharides during the incubation, which is consistent with the conclusions of Špaldoňová and Frouz [39].
Except for the vermicompost amendment, the other organic substrate treatments had little or no efect on microbial respiration activity in acid sulfate soil, suggesting that low soil pH and high Fe, Al, and other heavy metals' availability in the acid sulfate soil may be dominant constraints.Decreased soil pH may have reduced soil microbial respiration, as was found by Xiao et al. [32], or the decline in CO 2 evolution was probably caused by the accumulation of toxic metabolites during incubation [40,41].However, this study clearly showed that the addition of organic substrates to acid sulfate soil increased the soil pH (0.3-1.2 units).Some researchers have supported the idea that the application of organic material can increase soil respiration in acidic soils, by increasing the soil pH and soil fertility through improving the nutrient supply for crop production [42][43][44].However, respiration in acid sulfate soil was not common in some organic amendments, so soil acidity problems could be corrected by applying limestone or gypsum in the experiment.

Efect of Organic Substrates and Incubation Time on DOC.
Dissolved organic carbon is considered to be composed of a wide range of organic compounds, ranging from shortchain acids to large molecules such as fulvic and humic acids.Te results demonstrated that increasing the content of organic C can enhance the release of DOC or reduce sorption on the soil.Te application of organic substrates to soil had an efect on DOC accumulation.Scientifca Sugarcane, biogas, and vermicompost amendments improved the accumulative DOC, exceptions occurred for soils amendment with slurry, and water hyacinth.Terefore, this can be explained by the higher carbon content in soils treated with sugarcane, biogas, and vermicompost, which ranged from 43.8 to 86.2% compared to that slurry (0.3%) and water hyacinth (25.6%), which controlled the efect of the DOC concentration.In most soils, the DOC concentration increased considerably after being supplied with sugarcane, biogas, and vermicompost substrates.Similar results were obtained for increasing the OC content without decreasing the DOC, and there was a positive relationship between the organic C content and DOC release [45][46][47].
Furthermore, the DOC concentration was the highest at the start of the experiment (day 0) and gradually decreased with time in the later stages.Tis decrease is mainly attributed to the mineralization of DOC by microorganisms [48].Furthermore, decreasing DOC resulted in a decrease in release or an increase in the retention of DOC.Te reduction in DOC could be explained by the sorption of the soil material during the experimental period [49][50][51].
In addition, retention is generally thought to be mostly caused by physicochemical processes such as sorption and/ or precipitation.Te clay texture of the soil property is ideal for maximizing organic C sorption into the solid phase [52].In addition, both the clay content and Fe oxides control organic C sorption [53,54].Te decrease in the DOC concentration is perhaps strongly infuenced by adsorption and precipitation of Fe and Al oxides [54] and reduced solubility of DOC at lower pH [55].In contrast, the concentration of DOC increases with increasing acidity or the addition of sulfate as a result of the cleavage of metal ionorganic matter bonds [56,57].Terefore, the result could be due to this dual efect of mineral soil properties, on the one hand, favoring primary production in DOC and, on the other hand, because of the increase in soil C content rates with lower pH.In summary, the accumulated DOC concentration increased in the order of sandy soil < degraded soil < acid sulfate soil.
Moreover, DOC production from organic substrates appears to follow the same regulation as DOC production from soil organic matter [58].However, this study showed that the DOC content of acid sulfate treated with vermicompost was lower than that of acid sulfate treated with sugarcane or biogas.Te DOC content in the organic substrates alone was not similar, indicating that the soil properties could afect the changes in DOC as mentioned above.Terefore, organic supplements could alter many biogeochemical processes and afect the retention of DOC due to changes in soil chemical properties, as well as increasing the concentration of DOC [58].

Efect of Organic Substrates and Incubation Time on
Humic Acid.Humic acid (HA) from diferent sources may difer in its biodegradability.Tey are heterogeneous mixtures of a variety of organic compounds with varying molecular sizes and properties [59].Te incorporation of vermicompost caused slight increases in the accumulation of HA in the degraded soil.Te addition of sugarcane increased the HA content in the three soils, while water hyacinth and biogas enhanced the HA concentration only in acid sulfate and degraded soils, demonstrating that the application of diferent organic substrates afected a variety of chemical, physical, and biological reactions.Similarly, the characteristics of HA may vary greatly depending on the type of material [60].
Furthermore, the mineralization rates of HA in sandy soils tended to be lower than those in the other two soils, while acid sulfate had the highest mineralization rate among the three soils.Tis shows that HA from diferent soils has very diferent chemical compositions and structures [60].
It is difcult to release HA because it binds to colloidal soil surfaces [61].During the 3 months, the HA concentration reached its highest value on day 0. Afterward, the rate of HA concentration underwent a lag phase, and during the fnal period after rewetting, the constant trend of HA also declined slightly.Tis could be explained by the fact that the HA fraction in the soil slowly decomposed, and the undecomposed residues were protected by adsorption to the components of the mineral soil.Similarly, HA is a relatively stable product of organic matter decomposition.Terefore, no increases in HA were observed for some amendments such as water hyacinth, biogas, and vermicompost when added to sandy soil.
Generally, the removal of HA, which may have inhibitory efects on the mineralization of other associated soils can also increase the mineralization rates of the other isolated components.HA formed from organic substrates was diferent from soil HA because HA produced from organic substrates has comparatively lower concentrations of total acidity and carboxyl and phenolic functional groups than soil HA [62].HA from organic substrates could be more easily biodegradable than soil HA because of its lower molecular size and aromatic condensation content, which caused organic sole substrates to strongly increase the HA concentration compared to organic soil amendments.

Efect of Organic Substrates and Incubation Time on
Dehydrogenase Activity.In this study, it was found that the incorporation of organic amendments into soil stimulated DH-ase activity due to the possible presence of intracellular enzymes in the added material, and it may have also stimulated microbial activity in the soil [63,64].For degraded soils, DH-ase activity was estimated by evaluating efective organic addition [65].Te highest level of DH-ase activity for vermicompost amendment in sandy and degraded soils suggested the availability of a large quantity of biodegradable substrates (which was in agreement with the higher content of labile C observed in these soils), and therefore improved microbial activity.
In addition, the application of organic substrates to these soils, e.g., water hyacinth, sugarcane, biogas, and slurry amendments, also led to an enhancement of DH-ase activity.Hence, the activity of soil DH-ase depended on the condition and intensity of the biological conversion of the organic compounds.DH-ase is considered to exist in soils as 10 Scientifca integral parts of intact cells that functions as a biological catalyst for specifc reactions depending on the type of amendment or soil C [66,67].For both the degraded and sandy soils, the DH-ase activity reached a maximum at 5 days and sharply decreased after 20 days.Te high DH-ase activity in the soils could be explained not only by the neutral pH conditions but also by the organic amendments.However, a small increase in DH-ase activity was observed at 51 days, after which the rate of decrease in DH-ase activity continued until the end of the experiment, probably due to the decreasing availability of more easily decomposable substrates.
Soil properties such as EC, heavy metal toxic compound, and pH can have a negative efect (either jointly or individually) on enzyme activity [68].However, unlike in degraded soil and sandy soil, the stimulation of DH-ase activity was strongly limited during the incubation period in acid sulfate soil (less than ten times as compared to the other two soils), indicating that the greater inhibitory impact on DH-ase enzyme activity was largely due to the efect of the low soil pH.Tus, pH may be the best explanation for the overall variance in DH-ase activity.Similarly, pH is the best indicator of DH-ase activity [69], and very little enzyme activity is observed below pH 6.6 and above pH 9.5 [70].However, in acid sulfate soil, DH-ase activity was enhanced after the addition of water hyacinth, sugarcane, and biogas during the experiment.Tis indicated that the positive efect of organic amendments on soil biological quality is due to stimulation of microbial growth and/or the addition of microbial cells or enzymes with the amendments that counteract soil deterioration caused by some toxic compounds [71].

Conclusions
Te application of organic substrates to soils infuences the accumulation of DOC and HA and can stimulate respiration and DH-ase activity.However, diferent efects can result from the incorporation of organic substrates into soils and vice versa.In addition, organic substrates alone tend to induce greater respiration, DOC, HA, and DH-ase activity compared to soil organic amendments.Generally, O 2 consumption, DH-ase enzyme, DOC, and HA parameters of both degraded soil and sandy soil exhibited similar patterns, difered strongly from those of the acid sulfate soil.In particular, acid sulfate soil is capable of producing larger amounts of DOC and HA than both degraded and sandy soils, but the DH-ase enzyme occurs in lower quantities in acid sulfate soils than in the other two soils.Te application of sugarcane, biogas, and vermicompost in the three soils increased the DOC concentration.Furthermore, sugarcane increases HA production in all three soils, whereas the addition of water hyacinth and biogas stimulates cumulative HA in degraded and acid sulfate soils, and the application of vermicompost enhances HA only in degraded soils.Almost all organic substrates (water hyacinth, sugarcane, biogas, vermicompost, and slurry) applied to degraded and sandy soils increased O 2 consumption and DH-ase activity, and vermicompost amendment had the greatest efect.However, for acid sulfate soil, only vermicompost amendment increased O 2 respiration, in contrast to the combined application of water hyacinth, sugarcane, and biogas which efectively increased DH-ase activity.Unlike the production of DOC and HA, the greater inhibitory impact on microbial respiration and DH-ase enzyme activity is probably due to the pH of the soil in acid sulfate soil.

Table 2 )
. To set up the experiment, a hole was made into the caps of the glass bottles using a drilling machine.Five grams of pure soil (control), pure compost, or 5 g of soil amended with a compost equivalent to 50 Mg•ha −1 was added to each bottle.After the addition of water, the soil or substrate mixtures were homogenized by using a stainless steel spoon.A 2-ml tube was inserted vertically into each bottle, into which approximately 2 pellets of NaOH were placed.Te bottles were covered with rubber septa, sealed, airtight, and placed in a climate cabinet at a constant temperature of 27 °C.
2.2.1.Respiration Experiment.Te respiration experiment was designed with 22 treatments and four replications to

Table 1 :
Characteristics of the soil and organic substrates.

Table 2 :
Summary time and parameters of the experiment.Exp Number of treatments t 0 t 1 t 2 t 3 t 4 t 5 Reincubation t 6 t 7 t 8 t 9 t 10 Collected and analyzed