Assessing the Increase in Soil Moisture Storage Capacity and Nutrient Enhancement of Different Organic Amendments in Paddy Soil

Increasing soil moisture storage capacity is a strategy that can be implemented to minimize the use of water in paddy rice cultivation. Organic materials from different sources have the potential to increase soil moisture storage and nutrient enrichment. An incubation study was conducted to evaluate the incorporation of five selected organic amendments—as follows: rice husk biochar (RHB), oil palm empty fruit bunch biochar (EFBB), compost (COMP), rice husk ash (RHA), and oil palm bunch ash (PBA), with a control (no amendment) on soil moisture storage and some chemical properties of soil. The soil was incubated with five amendments for 60 days and sampled at 15-day intervals. After completion of the incubation, a greater extent of gravimetric water content was observed from RHB (0.46 g g−1) and EFBB (0.45 g g−1) followed by compost (0.40 g g−1). The addition of organic amendments significantly influenced soil chemical properties. Maximum soil pH was altered by PBA followed by EFBB compared to its initial value (5.01). The inclusion of EFBB finally contributed to the highest amount of total carbon (7.82%) and nitrogen (0.44%). The addition of PBA showed the highest available P and exchangeable K followed by RHB when compared with the amendments. The results indicated that RHB, EFBB, and compost retain more soil moisture compared to ash sources and added soil nutrients, indicating their potential to improve the chemical and hydrological properties of paddy soil.


Introduction
Rice is one of the most widely grown cereals in the world and serves as a staple for half of the world's population, particularly in developing countries. In 2017, approximately 748 million tons of rice were produced over the world, requiring more than 160 million ha of land [1]. Rice is the largest consumer of water and it consumes about 34-43% of irrigation water over the world [2]; producing one kilogram of rice requires 3000 to 5000 L of water [3]. Furthermore, water for agricultural purposes becoming scarce due to climate change and rapid industrialization and urbanization presents a challenge to farmers regarding the production of more rice per unit of land with limited water [4]. Continuous flooding irrigation systems require large quantities of water for rice, and a larger amount of water is lost through evaporation, percolation, and seepage [5]. Many modifications in rice cultivation, such as aerobic rice, direct seeding, alternate wetting, and drying, etc., have been made to save water and are used because of the changing climatic conditions of the earth [6]. Under the circumstances, the approach of using different organic amendments

Characterization of Organic Amendments
The rice husk biochar used in the study was purchased from Sendi Enterprise (Sungai Burong, Selangor), produced by pyrolyzing rice husk at 300 • C. Oil palm empty fruit bunch biochar was collected from Parkar Go Green SdnBhd (Sri Kenari, Kajang), pyrolyzed at 450 • C. Rice husk ash and commercial compost were directly bought from D Syria Enterprise (Taman Sri Serdang, Seri Kembangan). Oil palm bunch ash was supplied by Odorata Enterprise (Kota Tinggi, Johor). The pH of organic amendments was determined in a 1:10 ratio (w/w) of solid and water [28]. A CNS analyzer was used for analyzing the total C, N, and S of different organic amendments; total P, Ca, and Mg were determined by dry ashing (Cotennie, 1980) followed by ICP-OES determination described earlier.
Total phosphorus, potassium, calcium, and magnesium in different amendments were determined by dry ashing followed by ICP-OES (Perkin Elmer, Massachusetts, USA). Chemical properties of different organic amendments are shown in Table A2.

Gravimetric Water Content
At each incubation sampling campaign, fresh soil was collected in an aluminum container and weighed in a balance with ±0.001 g precision and then oven-dried at 105 • for 24 h to find the gravimetric water content following Cooper [29]: Gravimetric water content (g g −1 ) = (weight of wet soil−weight of dry soil)/(weight of dry soil).

Scanning Electron Microscopy (SEM) of Organic Amendments
The organic amendments used in the experiment were dried and metallized using BAL-TECB sputter coater system (SCD 005, BALZERS) to achieve an ideal conductive surface. After metallizing, the samples were magnified by 1000× and analyzed using a LEO 1455VP (Oxford instrument and INCA software, London, UK) scanning electron microscope (SEM) at 15 kV from the microscopic unit of Institute of Biosciences (IBS), Universiti Putra Malaysia.

Incubation Study
An incubation experiment was conducted in the laboratory of the Land Management Department of Universiti Putra Malaysia (UPM), Serdang, from 15 May to 13 July 2019 (60 days). The experiment was conducted by a completely randomized design (CRD) with three replications. A composite soil sample (0-15 cm) was collected from a wetland paddy field of Tanjung Karang, air-dried, and sieved by 2 mm mesh. The soil (350 g) was Agriculture 2021, 11, 44 4 of 15 placed in each of the plastic pots with Whatman no.42 filter paper placed at the bottom of the containers to check loss of soil particles through the 2 mm hole at the bottom of each pot which facilitated excess water drainage. Five organic amendment treatments (rice husk biochar, oil palm empty fruit bunch biochar, compost, rice husk ash, and oil palm bunch ash) were used at a rate of 4% (weight/weight) and mixed thoroughly with soil; soil without amendment was considered as the control. Soil was saturated by adding 150 mL of deionized water. Four sets of pots for each treatment with three replications (4 × 6 × 3 = 72) were incubated at 26 ± 2 • C temperature for 60 days. Soil sampling was performed for each set at 15, 30, 45, and 60 days of incubation followed by a destructive sampling method. After the harvest of each set, the remaining sets were saturated by adding 150 mL of deionized water. The collected sample was used to determine moisture content, pH, percentage of total C and %N, available P, and exchangeable K, Ca, and Mg by the method used for the initial soil mentioned above.

Statistical Analysis
Analysis of variance (ANOVA) was used to calculate statistical difference among various treatments using the R statistical software and to detect significant differences between the treatment means; Tukey's test at a 5% level of confidence (p < 0.05) was considered.

Scanning Electron Microscopy (SEM) Visualization of Organic Amendments
SEM micrograph analysis was conducted to characterize the morphological structure of various organic amendments used in the study (Figure 1). Micropores observed in rice husk biochar (RHB) and oil palm empty fruit bunch biochar (EFBB) were absent in the rice husk ash (RHA), compost (COMP), and oil Palm Bunch Ash (PBA). RHA and PBA exhibited irregular structures, though both were also from plant origin, akin to RHB and EFBB. Biochar produced from rice husk exhibited pores marked by cell wall structures ranging from 0.5 to 10 µm. Oil palm EFBB also showed the same origin of pore structure similar to rice husk but the size of the pore ranged between 1 and 10 µm. The number of micropores sized < 1 µm were found to be higher in the case of rice husk biochar. Compost showed a large surface area and intra spaces between different particles but the porous structure was missing.

Soil pH
Soil pH was significantly affected (p < 0.05) by organic matter amendment but not with the incubation time (except two biochars and PBA). Between different organic amendments-compost, RHA, and control-showed significant changes in soil pH with incubation time (Table 1). A sharp increase in soil pH was observed in all the treatments including control at 15 days of incubation (DOIs) compared to soil initial pH (5.01) ( Table  A1). The highest increment of soil pH was observed in PBA (6.91) followed by EFBB

Soil pH
Soil pH was significantly affected (p < 0.05) by organic matter amendment but not with the incubation time (except two biochars and PBA). Between different organic amendments-compost, RHA, and control-showed significant changes in soil pH with incubation time (Table 1). A sharp increase in soil pH was observed in all the treatments including control at 15 days of incubation (DOIs) compared to soil initial pH (5.01) ( Table A1). The highest increment of soil pH was observed in PBA (6.91) followed by EFBB (6.16); at amendments at 15 DOI and then on further DOIs the pH was quite stable. A small increase in soil pH was also observed in compost and RHA amended soils compared to initial pH at 15 DOI and remained unaffected at the later incubation campaigns. At the end of incubation, PBA produced the highest pH value (6.95) followed by EFBB (6.26), while others showed statistically similar values.

Total Carbon, Total Nitrogen and Carbon Nitrogen (CN) Ratio
Application of different organic materials significantly (p < 0.05) changed total carbon, total nitrogen, and CN ratio in soil when incubated for 60 days (Table 2), but there was no significant variation within their incubation time except for the CN ratio. Values are presented as mean ± standard error with different uppercase letters in the same column and lowercase letters in the same row indicating significant difference (p < 0.05) among the organic amendments and incubation days, respectively. RHB, Rice husk biochar; EFBB, Oil palm empty fruit bunch biochar; COMP, Compost; RHA, Rice husk ash; PBA, Oil palm bunch ash, and control, no amendment.
Soil treated with EFBB showed the maximum value of total carbon in soil followed by COMP and RHB. During all days of incubation (DOIs), EFBB treated soil exhibited the maximum value of total carbon (TC) which was significantly higher than in COMP and RHB treated soils; further, these two materials produced statistically similar values. The lowest total C% was observed in PBA treated soil which was statistically similar to the control. After 60 DOI, EFBB produced 7.82% of total C in soil which is significantly higher Agriculture 2021, 11, 44 7 of 15 than other treatments, while the lowest value was obtained from PBA (4.84%), which is identical to the control (5.31%).
Total N content in soil was also significantly (p < 0.05) affected by different organic matter management methods but changes between the amendments were insignificant at 15, 20, 45, and 60 DOI for all soils except the control (Table 2). Similar to total C, total N was also the highest in the EFBB treated soil at 15, 30, 45, and 60 DOI (0.43%, 0.43%, 0.44%, and 0.44% respectively), which was statistically similar over the COMP treated soil at 30, 45, and 60 DOI (0.42%, 0.43%, and 0.43% respectively). The lowest total N content was found in PBA over all intervals excluding 45 DOI and the values were statistically identical to the control.
The CN ratio of the soil was calculated using the ratio of total carbon and total nitrogen, and the addition of organic amendment increased the CN ratio of the soil; the effect of incubation time was also significant (p < 0.05) for EFBB and RHA while others were unaffected (Table 2). Soil treated with EFBB showed the maximum CN ratio (19.22) at 15 DOI and slightly decreased over time (17.92 at 60 DOI). Incorporation of RHB, COMP, and RHA exhibited statistically identical CN ratios at distinct DOIs, although these were different from EFBB. Throughout the DOIs, the PBA and control treatments produced the lowest and statistically similar CN ratios among other organic materials.

Available Phosphorus
Phosphorus availability in soil was significantly affected by different organic amendments and DOIs (p < 0.05). For all the organic amendments, P availability curves peaked at 30 DOI, and then there was a sharp drop at 30 DOI and then they flattened (Table 3). In the PBA amended soil, the highest P availability was 68.56 mg kg −1 at 30 DOI and the lowest was 33 mg kg −1 at 45 DOI which was more than three-fold higher compared to the inherent P availability observed in the control. Among the organic amendments, the highest P availability was observed with PBA followed by RHB while the lowest P availability was observed in the control (soil without amendment). COMP and EFBB amendments almost had the same pattern of P availability between the DOIs which is almost similar to the P availability pattern in the control. After 60 DOI, maximum available P (41.41mg kg −1 ) was obtained by PBA treatment and minimum available P from the control (8.39 mg kg −1 ), which is similar to compost (9.03 mg kg −1 ).

Exchangeable Potassium
Exchangeable potassium of soil under study was significantly (p < 0.05) influenced by the DOI and type of organic amendment ( Table 4). The K availability pattern followed the same as observed for P availability. Exchangeable K with PBA amendment showed a sharp increase at 30 DOI compared to that at 15 DOI and then the availability was relatively stable at 45 and 60 DOI. The highest exchangeable K with PBA amendment was observed at 45 DOI (8.20 cmol c kg −1 ) and the lowest at 15 DOI (4.71). An eight times higher exchangeable K availability was observed at all the sampling days when compared to control. EFBB amendment followed the PBA amendment for K availability and the K availability was relatively stable at all the sampling days maintaining a 4-7 fold higher compared to control. The other three amendments-RHB, RHA, and COMP-almost had the same levels of exchangeable K across the DOIs. Among the organic amendments overall, PBA exhibited the highest value of exchangeable K in all DOIs.

Exchangeable Calcium (Ca) and Magnesium (Mg)
Incubation time significantly (p < 0.05) affected the exchangeable Ca and Mg in soil when amended with organic material (Table 5). Initially, exchangeable Ca and Mg were high at 15 DOI for all treatments and then decreased in 30 and 45 DOI. Both exchangeable Ca and Mg peaked at 60 DOI and were statistically significant (p < 0.05) over the rest of the DOIs (except 15 DOI). Organic materials significantly altered (p < 0.05) the exchangeable Ca concentration of the soil over 15 and 45 DOI (Table 4); the addition of PBA maximized exchangeable calcium during these DOIs (19.66 and 14.08 cmol c kg −1 , respectively). However, at 60 DOI, the five amendments produced statistically similar values of exchangeable Ca in soil.
Magnesium concentration exhibited a small but statistically significant (p < 0.05) variation among different organic amendments. Soil amended with PBA showed the highest exchangeable Mg concentration at 60 DOI (16.30 cmol c kg −1 ) which was statistically similar to EFBB, RHB, and RHA, and the minimum value was obtained in the control (11.03 cmol c kg −1 ).

Gravimetric Water Content
The gravimetric water content in soil was significantly influenced (p < 0.05) by DOI and type of organic matter amendments. The highest water content was observed with RHB amendment while the lowest water content was observed in the control (Figure 2) and the variation was about two-fold higher between them. A gradual increasing trend of water content was observed with increasing DOIs in all the organic amendments with the exceptions of RHA amendment and control, while the water content trend was quite unchanged for PBA amendment. Water content under the control condition showed a decreasing trend with as the days of incubation increased; highest gravimetric water (0.29 g g −1 ) was obtained at 15 DOI, while the lowest was obtained at the end of incubation (0.17 g g −1 ). For RHB, maximum gravimetric moisture (0.46 g g −1 ) was obtained at 60 DOI followed by 0.44 g g −1 at 45 DOI, while a sharp drop was observed at 30 and 15 DOI, with moisture values of 0.36 and 0.35 g g −1 . Fairly good and stable moisture contents were observed for COMP amendment with moisture content variation between 0.36 and 0.40 g g −1 across the whole incubation period. The values of gravimetric water content of soil treated with organic amendments at different DOIs given in Supplementary Table  (Table S1).
Values are presented as mean ± standard error with different uppercase letters in the same column and lowercase letters in the same row indicating significant difference (p < 0.05) among the organic amendments and incubation days, respectively. RHB, Rice husk biochar; EFBB, Oil palm empty fruit bunch biochar; COMP, Compost; RHA, Rice husk ash; PBA, Oil palm bunch ash, and control, no amendment.
Organic materials significantly altered (p < 0.05) the exchangeable Ca concentration of the soil over 15 and 45 DOI (Table 4); the addition of PBA maximized exchangeable calcium during these DOIs (19.66 and 14.08 cmolc kg −1 , respectively). However, at 60 DOI, the five amendments produced statistically similar values of exchangeable Ca in soil.
Magnesium concentration exhibited a small but statistically significant (p < 0.05) variation among different organic amendments. Soil amended with PBA showed the highest exchangeable Mg concentration at 60 DOI (16.30 cmolc kg −1 ) which was statistically similar to EFBB, RHB, and RHA, and the minimum value was obtained in the control (11.03 cmolc kg −1 ).

Gravimetric Water Content
The gravimetric water content in soil was significantly influenced (p < 0.05) by DOI and type of organic matter amendments. The highest water content was observed with RHB amendment while the lowest water content was observed in the control (Figure 2) and the variation was about two-fold higher between them. A gradual increasing trend of water content was observed with increasing DOIs in all the organic amendments with the exceptions of RHA amendment and control, while the water content trend was quite unchanged for PBA amendment. Water content under the control condition showed a decreasing trend with as the days of incubation increased; highest gravimetric water (0.29 g g −1 ) was obtained at 15 DOI, while the lowest was obtained at the end of incubation (0.17 g g −1 ). For RHB, maximum gravimetric moisture (0.46 g g −1 ) was obtained at 60 DOI followed by 0.44 g g −1 at 45 DOI, while a sharp drop was observed at 30 and 15 DOI, with moisture values of 0.36 and 0.35 g g −1 . Fairly good and stable moisture contents were observed for COMP amendment with moisture content variation between 0.36 and 0.40 g g −1 across the whole incubation period.

Relationship between Different Soil Properties Influenced by Organic Amendments during the Incubation Period
Pearson's correlation analysis was performed to determine the relationship between soil properties. From the analysis (Table 6), it was observed that gravimetric water content (GWC) significantly positively correlated with the TC, TN, and CN ratio and soil pH had a significant negative relation with the TC, TN, and CN ratio, while pH exhibited a significant positive correlation with available P and exchangeable. K. Furthermore, significant positive correlations were observed between available P and exchangeable. K, and also between exchangeable. Ca and Mg.

Impact of Biochar on Nutrient Release
At the end of incubation, pH was remarkably increased in the soil receiving PBA and EFBB treatments compared to the control; the change in EFBB was less compared to PBA. pH increases by PBA and EFBB were 1.1 and 1.1 and 0.41 units, respectively, compared to control. This may be due to the high pH of PBA and EFBB, which accelerates soil pH toward neutral. Other organic amendments did not make significant changes in soil pH because they were neutral to slightly acidic. Oil palm bunch ash contains a large amount of calcium; it showed alkaline properties and improved soil pH [30]. EFBB was also alkaline and alleviates soil acidity by improving soil pH, as reported by Abdulrahman et al. [31]. EFBB causes a reduction in Al 3+ activity and increased floodwater pH of low land paddy soil [15]. Generally, rice husk biochar incorporated soil increases in pH did not occur in our study because the rice husk biochar used in this study exhibited a low pH which also agrees with the findings of Wang and Liu [32].
At the end of incubation, the maximum carbon added by EFBB was 47% higher compared to the control, whereas RHB and compost increased carbon by 24% and 28%, respectively. PBA incorporation decreased the total carbon by 8% compared to control, which may be due to the very low carbon (2.10) content; an increase in carbon in soil is directly proportional to the extent of carbon present in the organic amendment. Incorporation of biochar and compost in soil increased soil total carbon, which is also reported by Mensah and Frimpong [33]. Wang and Liu [32] mentioned that biochar has unique properties to enhance long-term carbon storage in soil. Organic amendments such as EFBB and compost rich in nitrogen increased total nitrogen by 16% and 13%, respectively, compared to control, whereas PBA incorporation reduced it by 7% in due to low N content (0.02%). An increase in the total nitrogen of soil occurred due to the N-rich amendment reported by Bakar et al. [15]. Lehmann [34] also highlighted soil nitrogen enrichment by biochar and compost.
The increased available phosphorus in soil until the first thirty days of incubation in this study is similar to the findings of Schefe et al. [35]. Microbial mineralization of organic amendments may have flushed this increased available P in soil [36,37]. Later, the concentration of available P reduced with time due to the fixation of available P by the hydrous oxides of Al and Fe; this was the same in all the amendments. Tropical soil rich in soluble Fe fixes available P [38]. A decrease in available P in the latter days contradicts the findings of Opala et al. [39], who reported increased P availability with organic amendments. We also found the same for PBA-increased available P compared to the control even at the final days (60 DOI) of incubation. This biochar is very rich in P compared to other biochars and composts, which happens to increased available P all along the incubation time (similar to the findings of Wang et al. [40]; Glaser and Lehr [41]. Among different organic amendments (except RHB), P content was comparatively higher in lower C and N materials consecutively; P content was lower in the highest C material, EFBB (Table A2), which was reflected after the materials were incorporated into the soil; this is a possible cause for the negative correlation between total C, N, and available P.
At 60 DOI, the K contents of PBA and EFBB increased by about 690% and 300%, respectively, compared to the control; increased exchangeable K with incubation time was due to high potassium content in PBA and EFBB (Table A2). The addition of biochar increased soil exchangeable K; these findings are well supported by many researchers [16,42]. Generally, biochar increases K availability due to high ash content; further, it absorbs K ions on its surface and reduces leaching loss [43,44]. Moreover, biochar application promotes K solubilizing bacterial growth and enhances K release from K-rich clay minerals [44]. Compost contributed a little increase in available K due to its poor K content; however, the same result was also obtained by Lashermes et al. [45]. A slight drop in K availability occurred from 45 to 60 days, which may be due to K fixation in the clay interlayers [46].
Available Ca 2+ in soil increased due to the calcium-rich amendments, PBA, and compost. All amendments raised exchangeable calcium in soil compared to initial soil. The addition of calcium-rich amendments increased available Ca 2+ reported by Ch'ng et al. [36]. Except for PBA, amendments did not show any remarkable change in exchangeable Mg 2+ concentration in the soils; these findings are similar to the study of Hirzel et al. [47], who also reported small changes in magnesium from a short incubation study. From correlation analysis, a negative relation was observed between total C and N with exchangeable Ca 2+ and Mg 2+ ; in this study, biochars were carbon-rich materials though their low Ca contents. Further, biochar has a unique surface chemistry and different functional groups [48], which were possible causes for initial nutrient absorption to its surface and reduced their availability at early DOIs. At the end of incubation, increased availability may be due to the microbial decomposition. In correlation analysis, we considered the data of all DOIs, and the relationship was weak and nonsignificant.

Impact of Biochar on Water Retention
At the DOIs, biochars and compost increased up to 170% and 135% compared to the control. Biochar amendment in the soil increases the water retention capacity of soil due to its very high surface area. Biochars and compost amendment increased the soil moisture content over the DOIs but PBA did not, as the carbon content in PBA was very low. From correlation analysis (Table 6), total C and N of soil has a positive and significant relation with gravimetric water content. The application of biochar improved soil water retention, due to its high surface area and carbon-rich properties that facilitate enhanced soil porosity by accelerating soil macroaggregate formation and stability, but reduced bulk density [49,50]. The leading two biochars in terms of water retention, used in this experiment, originated from rice husk and oil palm. Głąb et al. [51] and Chen et al. [42] also noted that the application of biochar improves soil water storage. To determine why there is a tendency of biochar to retain water, biochars were analyzed for SEM micrographs. The SEM micrographs showed that, among the biochars used in this study, rice husk and oil palm empty fruit bunch had cellular structure pores as well as the highest surface areas, leading to very high water retention capacities [52]. Micropores in RHB and EFBB observed from the SEM micrograph boosted the water retention capacity. Improved moisture retained by biochar increased soil moisture storage directly by its large surface area and high quantity of pores that act as a capillary reserves of water [53]. Wang et al. [54] further elaborated on the improved soil porosity with biochar by raising soil inter-and intraparticle porosity. Therefore, it can be summarized that biochars with very high surface areas and pore volumes can improve soil water retention capacity through reducing the soil bulk density [51] and increasing surface area [55] as well as soil porosity [50]. All these improved soil properties through biochar amendments lead to improved soil water retention [56].

Conclusions
After this 60-day incubation study, it was observed that the incorporation of selected organic amendments-as follows: RHB, EFBB, COMP, RHA, and PBA-at rates of 4% (weight/weight) in soil improved the chemical properties and moisture retention of a clay textured paddy soil. The initial properties of the amendments played a major role in changing the chemical properties of the soil. Among the amendments, two biochars (RHB and EFBB) conserved high gravimetric water contents. From the structural analysis of biochars by scanning electron microscopy, the presence of porous structures and the large surface areas helped retain soil moisture by storing water in micropores. Amendments such as RHB and EFBB biochars enrich plant nutrients and increase the water holding capacity of paddy soil. The results of this study are material, dose, soil, and lab-environment specific; field validation of the results is imperative to demonstrate the soil improvement effects of the two biochars.

Conflicts of Interest:
The authors declare no conflict of interest.