Environmentally friendly bioameliorant to increase soil fertility and rice (Oryza sativa) production

1 Introduction

The increase in greenhouse gases in the atmosphere causes global climate change. This causes several environmental problems, such as long droughts and increased rainfall intensity [1]. In agriculture, drought or high rainfall intensity can reduce soil nutrients and reduce crop production. Indonesia, a country with rice as its main staple, is affected by the current climate change [2].

Indonesian rice production in 2018 will reach 33.94 million tons. In 2019, there were 31.31 million tons of rice, down by 7.75% compared to rice production in 2018, equivalent to 2.63 million tons of rice. Global rice production in 2019 is largely unchanged, month on month, at 512 million tons (milled basis), down 0.5% from 2018 [3]. Climate change has aggravated food insecurity conditions due to reduced harvests. This situation occurs due to bad weather (drought and flood) that results in disturbed and not optimal growth and development of rice [4].

Climate change affects the intensity of rainfall and the dry season. In the humid tropics and rainy season, increasing the intensity of rainfall events and total rainfall will increase the rate of leaching in the soil. Flooding induced by intense rainfalls may affect the soil erosion, finally resulting in its infertility [5].

Reduced soil nutrition due to climate change certainly needs to be addressed so that the nutritional needs of plants remain met. Currently, the effort that is often made is the use of inorganic fertilizers. However, excessive use of inorganic fertilizers is certainly bad for the environment. Excessive nitrogen fertilizer on the ground can lead to lakes and rivers, causing eutrophication. This is because excessive nitrogen fertilizer is generally not absorbed and can disturb groundwater and surface water [6]. Artificial fertilizers can also cause greenhouse gas emissions such as nitrous oxide and ammonia. Although an increase in agricultural yields meets national food needs, it is at the same time threatening the environment with its adverse effects. These impacts include the degradation of soil fertility, water hardness, an increase in toxic residues, and the development of insect resistance [7]. To overcome this, efforts need to be made so that the fulfillment of plant nutrients and the environment are maintained. This can be done with adaptation and mitigation efforts.

Adaptation is done by using superior varieties that have high yields and are environmentally specific [8]. The variety that can be used is Ciherang, which is a popular variety widely planted by farmers. According to data from the Indonesian Rice Research Center, in 2019, Ciherang rice varieties became the most widely planted by farmers, with a total land area of 5,011,968 ha (30.8% of the total land area) [9]. The Ciherang variety was chosen for control treatment because the majority (53%) of farmers used Ciherang [10]. That is because Ciherang has wide adaptability and high productivity, so many farmers plant it. Besides, Ciherang is easy to find and obtain, so it is not difficult for farmers to get it [11]. The Mendawak variety is also a superior rice variety because, based on the research results, the variety has high productivity [12].

In addition to adaptation efforts, mitigation efforts can also be one of the things pursued, namely through the use of bioameliorants. Bioameliorant is a mixture of organic matter that can increase soil fertility by improving physical and chemical conditions. The criteria for a good bioameliorant are having a complete nutrient content and being able to improve soil structure. The application of bioameliorants based on manure and green manure can increase soil fertility, improve soil structure, and reduce methane emissions in paddy fields [13].

The use of inorganic fertilizers is very popular among farmers in Indonesia, despite the fact that they are not recommended because they can reduce soil fertility and their evaporation emits greenhouse gases such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) [10]. Therefore, an effort is needed to reduce the use of inorganic fertilizers among farmers by promoting bioameliorants as a substitute. Some of the bioameliorants that are quite good at increasing soil fertility are goat manure, Azolla pinnata, and Sesbania rostrata [10]. The use of bioameliorants can be a solution to replace inorganic fertilizers. In addition to the relatively cheap price, because it is obtained directly from privately owned farms (goat manure) or you can grow your own (A. pinnata and S. rostrata), it also has a better impact on the environment.

Goat manure has higher nitrogen and potassium content compared to chicken and cow manure, but the nitrogen content is still lacking in supporting the growth and development of rice plants. Therefore, as a bioameliorant ingredient, goat manure needs to be combined with nitrogen-rich green manure such as A. pinnata and S. rostrata. Azolla can be symbiotic with cyanobacteria viz Anabaena azollae. Under good growing conditions, the symbiosis between Azolla and Anabaena can fix free nitrogen from air up to 100–170 kg N ha⁻¹ per year [14]. Sesbania rostrata for rice production can reduce the problem of environmental pollution because it can reduce the need for chemical nitrogen and the spread of N in the environment. Sesbania aged 40–60 days in one hectare is able to provide 5–6 tons of dry biomass where the nitrogen content in the biomass can supply 50 to 100% of the nitrogen requirement for rice plants [15].

The combination of goat manure and A. pinnata bioameliorant can increase soil organic carbon content by up to 122% and also increase soil nitrogen content by 90%, while the combination of goat manure and S. rostrata bioameliorant can increase soil organic carbon content by up to 85% and also increase soil nitrogen content by as much as 50%. However, the test results revealed that the two bioameliorant combinations did not provide a significant difference in increasing rice productivity. This is presumably due to mixing during field testing [15]. In a previous study [15], it was suspected that there was mixing between each bioameliorant treatment because the research was carried out directly in the field, and at that time, there was an overflow due to the high intensity of rainfall. Therefore, it is necessary to carry out further testing in a more controlled environment, which is done in a greenhouse by planting each clump of rice in a bucket, so that the growth environment is more controlled to determine the effect of the combination of bioameliorant goat manure, A. pinnata, and S. rostrata on increasing rice productivity. The purpose of this study was to determine the effect of bioameliorants (goat manure and a combination of goat manure with Azolla and/or Sesbania) on increasing soil fertility and rice production on infertile soils due to excessive use of inorganic fertilizer.

2 Materials and method

This study aims to obtain the best bioameliorant technology and rice varieties to support the practice of rice cultivation. This research begins with resource mapping, namely the available bioameliorant raw materials and potential rice varieties. After that, the next activity is field laboratory testing (Figure 1). Observations have been made at the JAMTANI Field Laboratory, which is located in Pangandaran Regency, West Java Province, Indonesia, from May to August 2018. The field trial has been designed as a factorial randomized block design with three replications. The green manure used as the first factor consists of P1 = goat manure 10 tons ha⁻¹; P2 = goat manure 10 tons ha⁻¹ + A. pinnata 10 tons ha⁻¹; P3 = goat manure 10 tons ha⁻¹ + S. rostrata 2 tons ha⁻¹; and P4 = goat manure 10 tons ha⁻¹ + A. pinnata 5 tons ha⁻¹ + S. rostrata 1 tons ha⁻¹. The average population of rice plants per hectare in Indonesia is 250,000 clumps. Based on this, the use of bioameliorant for one clump of rice in each treatment was as follows: P1 = goat manure 40 g per plant; P2 = 40 g of goat manure per plant + A. pinnata 40 g per plant; P3 goat manure 40 g per plant + S. rostrata 8 g per plant; and P4 = P3 goat manure 40 g per plant + A. pinnata 20 g per plant + S. rostrata 4 g per plant. The rice varieties used as the second factor consist of V1 (Ciherang variety) and V2 (Mendawak variety).

Figure 1

Testing implementation stages.

Planting is carried out in buckets with a diameter of 30 cm, and each bucket is filled with 5 kg of soil. The soil that is used as a planting medium was first analyzed for content in the soil fertility laboratory of Universitas Padjadjaran. Furthermore, post-harvest land from each treatment (P1–P4) was re-analyzed to detect whether there was an increase in soil fertility. The soil fertility parameters that are analyzed are organic carbon and total nitrogen soil. The parameters of rice varieties observed were plant height, number of productive tillers, number of grain amount, percentage of grain content, the weight of 100 seeds, and weight per plant. Plant height observed at 82 days after planting (before harvest), while other parameters were observed at harvest, which is approximately 100 days after planting.

Observation characters were analyzed based on a factorial randomized block design with the formula H ijk = π + K i + P j + P k + ( P j × P k ) + eijk , where H _ijk are the results of the jth treatment and the kth treatment in the ith group, π is grand mean, K _i is the effect of the ith group, P _j is the effect of the jth treatment, P _k is the effect of kth treatment, P _j × P _k is the interaction effects of jth and kth treatments, eijk is experimental error, i is 1, 2, …., k (k = group), j is 1, 2, …., p to −1 (p = first treatment), k is 1, 2,…. p to −2 (p = second treatment). The nutrient uptake efficiency (NUE) is calculated using a formula as follows: E e = Y d f − Y ef F × 100 % , where E _e is NUE (%), Ydf is the yield of plants that have received additional green manure, Yef is the yield of plants that have not received additional green manure, and F is the additional amount of green manure applied. Differences between the varieties were tested via an analysis of variance (ANOVA) post hoc analysis using Tukey’s honestly significant difference (HSD). The data were computed using the PKBT Stat 4.0 software developed by the Center for Tropical Horticulture Studies, Bogor Agricultural Institute, located in Bogor City, Indonesia.

3 Results

Table 1 shows the results of the soil analysis that was given several treatments of goat manure and a combination of bioameliorants. The results of the laboratory analysis showed an increase in organic carbon content and nitrogen content when compared to the initial soil without treatment. The content of organic carbon has increased in treatments P2, P3, and P4, whereas in treatment P1, the organic carbon content has decreased by 0.07%. The nitrogen content in all treatments has increased. The C/N ratio in all treatments decreased compared to the initial C/N ratio. This indicates that the treatment has a good influence on soil health.

The results of the ANOVA test in Table 2 show that rice varieties showed significance on the weight of 100 seeds, while other characters showed non-significant. The bioamelioran treatment resulted with significant difference in plant height, in the number of productive tiller, the number of grain amount and weight per plant. Meanwhile no noticeable difference was found in the percentage of filled grain and weight of 100 seeds. The results of the interaction between varieties and treatments showed significant results only on plant height, whereas other characters showed non-significant results.

The coefficient of variation (CV) value for each character has a range of 3.35–14.95%. The CV values in this range are still relatively good because they are still below 20%. The CV value indicates the level of accuracy of the treatment. A CV of less than 20% is considered good [15]. It indicates that the trial error for the observed characters is relatively small. The greater the CV value, the greater the uncertainty of a study. With a CV of less than 10%, trial errors and uncertainty levels of this study were relatively small.

Variance results for plant height showed no differences between varieties but had differences between bioameliorants. In Table 3, the results of further tests revealed that the average of plant height did not show any significant differences between the Ciherang and Mendawak varieties. While in the bioameliorant treatment, the highest average plant height was found in P3 (88.20 cm), which was not significantly different from P4 (84.45 cm). At plant height, the results of the statistical analysis showed an interaction between varieties and bioameliorants.

The variance results for the average number of productive tillers showed no difference between varieties but had differences between bioameliorants. In Table 4, the results of further statistical tests revealed that the average number of productive tillers did not show any difference between the varieties of Ciherang and Mendawak. Whereas the bioameliorant that affects the highest number of productive tillers was found in treatment P4 (22.53), it was not significantly different from P1 (19.03) and P2 (21.27). At the average number of productive tillers, the results of the statistical analysis showed no interaction between varieties and bioameliorant.

The variance results for an average of the total grain amount showed no difference between varieties but had differences between bioameliorants. In Table 5, the results of further statistical tests revealed that the average total grain amount did not show any significant difference between the Ciherang and Mendawak varieties. While the bioameliorant that affected the highest average of total grain amount was found in treatment P3 (141.78), it did not differ significantly from P2 (140.59). In the average total grain amount, statistical test results showed no interaction between varieties and bioameliorants.

The variance results for an average percentage of grain content showed no differences between varieties but did reveal differences between bioameliorants. In Table 6, further test results revealed that the average percentage of grain content did not show any significant difference between the Ciherang (67.90%) and Mendawak (64.79%) varieties. Whereas the bioameliorant that influenced the highest average percentage of grain content was found in treatment P3 (68.70%), it was not significantly different from P1 (66.23%), P2 (68.51%), and P4 (61.95%). In the average percentage of grain content, statistical test results showed no interaction between varieties and bioameliorant.

Variance results for an average weight of 100 seeds showed differences between varieties and bioameliorants. In Table 7, the results of further statistical tests with an average weight of 100 seeds show that the Mendawak variety (2.75 g) has higher weight than the Ciherang variety (2.66 g). Whereas the bioameliorant that affected the highest average weight of 100 seeds was found in treatments P3 (2.73 g) and P2 (2.73 g), it was not significantly different from P1 (2.64 g) and P4 (2.72 g). In the average weight of 100 seeds, the statistical test results showed no interaction between varieties and bioameliorants.

The variance results for the average weight per plant showed no differences between varieties but had differences between bioameliorants. In Table 8, the results of further statistical tests revealed that the average weight per plant did not show any significant difference between the Ciherang and Mendawak varieties. Whereas the highest bioameliorant that affected the weight per plant was in P2 (63.44 g), it was not significantly different from P3 (63.25 g) and P4 (58.20 g). At the average weight per plant, the results of statistical tests showed no interaction between varieties and bioameliorants.

NUE from the addition of green manure as a bioameliorant ranged from 1.17 to 5.77%. Based on the results of statistical tests, there was a significant difference in each bioameliorant treatment with the addition of green manure. The highest NUE value was found in bioameliorant P3 (goat manure 10 tons ha⁻¹ + S. rostrata 2 tons ha⁻¹) with a value of 5.77% (Table 8). The addition of 2 tons ha⁻¹ of S. rostrata was able to increase rice yields by 5.77% compared to using only 10 tons ha⁻¹ of goat manure.

4 Discussions

The highest increase in C-organic was found in the bioameliorants P2 (4.13%) and P4 (4.02%), where the bioameliorants contained A. pinnata. This is because A. pinnata biomass is easily decomposed, whereas S. rostrata biomass undergoes a slower decomposition process, causing the bioameliorant P3 (3.74%) to have a not-so-high C-organic content. The decomposition process of Azolla takes place quite quickly, namely within 3–6 weeks [16], while the decomposition of Sesbania takes place slowly, which is around 8–12 weeks [17]. Sesbania rostrata biomass undergoes a slower decomposition process because it has a high lignin content. Sesbania rostrata plants have 2.5% lignin content and 27.30% cellulose, while A. pinnata has 1.5% lignin content and 14.08% cellulose [15]. That causes the structure of A. pinnata to be more easily decomposed compared to S. rostrata. The slow process of decomposition will slow down the supply of nutrients for plants so that it does not have an immediate effect on plants [17].

The slow decomposition of S. rostrata also causes a low C/N ratio in bioameliorant P2 (8.79%) but a high ratio in P3 (9.84%). A higher C/N ratio indicates that the organic material has not completely decomposed, while a lower ratio indicates that the organic material has further decomposed into humus [18]. High C/N in P3 treatment is due to S. rostrata’s high lignin content, which makes the decomposition process run more slowly. Lignin degradation is a limiting step for the speed and efficiency of decomposition because lignin is a barrier that prevents the penetration of the enzyme solution. The complexity of the structure, high molecular weight, and nature of the solution in water make lignin degradation very difficult. Lignin is also a barrier to cellulolytic enzyme access during the decomposition process, inhibiting the decomposition process [19].

High nitrogen content was found in the bioameliorant P2 (0.47%) with A. pinnata green manures, while the bioameliorant P3 with S. rostrata had a low nitrogen content (0.38%). Azolla pinnata biomass is quite effective in increasing total nitrogen in soil because Azolla can tether nitrogen from the symbiotic air with cyanobacteria A. azollae. Azolla decomposition process in the soil will increase the availability of nitrogen in C-organic soil [20]. Under good growing conditions, the Azolla–Anabaena symbiosis can fix up to 100–170 kg N ha⁻¹ per year [14]. Sesbania produces 5.2 tons ha⁻¹ of dry matter, which is equivalent to 135 kg N ha⁻¹ [21]. The high nitrogen content in the soil analysis test results is because green manures contain high nitrogen in the biomass. The bioameliorant P1, which only uses 10 tons of goat manure per hectare, contains the lowest organic carbon of 3.36% and the nitrogen content is also not high as 0.40%. The low content of organic carbon and nitrogen in the soil in the P1 treatment might affect disruption of plant growth. Lack of organic carbon and nitrogen causes sub-optimal plant growth [22].

Based on the results of data analysis on the character of plant height, there was an interaction between bioameliorants and rice varieties. This shows that the combination of bioameliorant and variety affects plant height. Plant height is one of the characteristics used as an indicator of growth, which is used to measure the influence of the environment or the treatment applied [10]. In Ciherang varieties, P2 and P3 bioameliorants have a different effect on plant height compared to P1 bioameliorants. The plant height at P2 and P3 was higher than that in P1. Meanwhile, in the Mendawak variety, the plant height in bioameliorant P3 was higher than that in P1. This is thought to be due to Azolla and Sesbania as a sources of bioameliorants.

Bioameliorant P3 also provides quite good results in supporting the development of productive tillers. Productive tillers are tillers that produce panicles. Rice plants with many productive tillers indicate a high yield [12]. Based on the results of the analysis, there was no interaction between varieties and bioameliorants, besides that there was no significant difference between the Ciherang and Mendawak varieties. This indicates that the productive tiller’s character is influenced by the bioameliorant independent factor. The average productive tillers in P3 reach 22.53, which is different from P1, but not different from P2 and P4 bioameliorants. The addition of Sesbania as a bioameliorant source can support the development of productive tillers in rice plants.

No significant difference was found in total grain amount between the bioamelioran treatment P2 and P3. The average total grain amount in P2 reached 140.59 while in P3 it reached 141.78. When compared with the P1 and P4 treatments, the combination of P2 and P3 bioameliorants has a higher total grain amount per panicle. The process of forming paddy grain is strongly influenced by environmental factors, one of which is the availability of nutrients in the soil. The available nutrients support the process of forming grain [23]. Azolla has nitrogen (N) 3–5%, phosphorus (P) 0.5–0.9%, and potassium (K) 2–4.5%, while Sesbania has nitrogen (N) 2.5–4.5%, phosphorus (P) 0.7–1.0%, and potassium (K) 3–5% [15]. The content of N, P, and K in P2 and P3 treatments is thought to be able to support the needs of rice plants in forming grain. Meanwhile, in the P1 and P4 treatments, the content of N, P, and K was thought to be insufficient, so that the formation of grain in the two treatments was not optimal. One of the weaknesses in this study is that there is no content test for each bioameliorant material, so the content value approach is only carried out based on a literature study. However, in future research, this content test should be carried out to improve the quality of the research.

The different results were in the percentage of the characters of filled grain and weight of 100 seeds. In both characters, each bioameliorant treatment did not show any significant differences. On the character of the percentage of filled grain, the analysis showed that there was no significant difference between the bioameliorant treatments and between varieties. Thus, bioameliorant and variety did not influence the percentage of filled grain character. Meanwhile, in the weight of 100 seeds character, although there was no significant difference between bioameliorants, there were significant differences between varieties, where the Mendawak variety had a greater weight of 100 seeds than the Ciherang variety. The weight of 100 seeds character correlates with the shape and size of the grain and is influenced by the genetic traits of rice varieties. The Mendawak variety genetically has a larger grain size than the Ciherang variety [11]. This is thought to affect the character of the weight of 100 seeds so that the Mendawak variety has an average weight of 100 seeds reaching 2.75 g, which is greater than the Ciherang variety, which is only 2.66 g.

In the weight per plant character, the results of the analysis showed no interaction between bioameliorants and varieties, and there was no significant difference between varieties. This indicates that the weight per plant character is influenced by the bioameliorant factor, and it is strengthened by the results of the statistical analysis, which show that there is a significant difference between bioameliorants. The Tukey’s HSD test results showed that the bioameliorants P2 and P3 had a higher average weight per plant compared to P1 but were not significantly different from P4. The difference in weight per plant observed between treatments, especially the low value for P1 is consistent, as goat manure in P1 had low organic carbon and nitrogen content, while the addition of Azolla and Sesbania in P2, P3, and P4 treatments raised the nutrients content enough to support rice production.

In general, the bioameliorant with the composition of goat manure, which is added with Azolla and Sesbania biomass, has a better effect in supporting the growth and development of rice plants. Therefore, to determine the best bioameliorant, a NUE analysis is performed. The NUE value of a treatment indicates how efficiently the nutrients in that treatment are utilized by plants [15]. NUE was calculated by comparing each treatment with the control, where, in this study, the control used was a positive control, namely bioameliorant P1 (goat manure 10 tons ha⁻¹). The results of the NUE analysis showed that bioameliorant P3 (goat manure 10 tons ha⁻¹ + S. rostrata 2 tons ha⁻¹) had a value of 5.77%, the highest compared to P2 treatment (goat manure 10 tons ha⁻¹ + A. pinnata 10) tons ha⁻¹) and P4 (goat manure 10 tons ha⁻¹ + A. pinnata 5 tons ha⁻¹ + S. rostrata 1 ton ha⁻¹). Thus, bioamlerioan P3 (goat manure 10 tons ha⁻¹ + S. rostrata 2 tons ha⁻¹) is the most efficient bioamleriorant in increasing rice production with an increase in production reaching 5.77% compared to P1 (goat manure 10 tons ha⁻¹).

NUE is calculated based on the additional amount of biomass given compared to the production yield. Thus, although the P2, P3, and P4 treatments were not significantly different in the weight character per plant, efficient use of P3 was more efficient because less Sesbania biomass was used, which was only 5 tons ha⁻¹ or equivalent to 20 g per plant. Therefore, a bioameliorant from a mixture of goat manure and S. rostrata is recommended to increase soil fertility and rice production in the study area (Pangandaran Regency). This research was conducted based on the potential of existing resources in the location. Goat manure, Azolla, and Sesbania are potential resources with abundant availability, so their availability can be guaranteed if all three are used as biochemical ingredients and used extensively.

5 Conclusions

The bioameliorant can increase the content of C-organic up to 9.04–20.41% and increases the soil nitrogen content by 11.76–38.24%. High nutrition in the soil is certainly good for plant growth, especially for the yielding character. The effect of the addition of bioameliorant did not cause a difference in weight per plant between Mendawak (61.34 g) and Ciherang (56.96 g). Based on the results of the study, the most efficient bioameliorant to be added to the rice crop was the P3 treatment (goat manure 10 tons ha⁻¹ + S. rostrata 2 tons ha⁻¹), with a NUE value reaching 5.77%.