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Journal of Agricultural Science and Food Research

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Evaluation of Biofertilizers in Cultured Rice

Nino Paul1, Pompe C Cruz2, Edna A Aguilar2, Rodrigo B Badayos3 and Stephan Hafele1*

1International Rice Research Institute, Los Baños, Laguna, Philippines

2Crop Science Cluster, College of Agriculture, University of the Philippines at Los Baños, Laguna, Philippines

3Agricultural Systems Cluster, College of Agriculture (CA), University of the Philippines at Los Baños, Laguna, Philippines

*Corresponding Author:
Stephan Hafele
International Rice Research Institute
Los Baños, Laguna, Philippines
E-mail: [email protected]

Received date: May 28, 2013; Accepted date: June 18, 2013; Published date: June 21, 2013

Citation: Paul N, Cruz PC, Aguilar EA, Badayos RB, Hafele S (2013) Evaluation of Biofertilizers in Cultured Rice. J Biofertil Biopestici 4:133. doi:10.4172/2155-6202.1000133

Copyright: © 2013 Paul N, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Biofertilizers are becoming increasingly popular in many countries and for many crops, but very few studies on their effect on grain yield have been conducted in rice. Therefore, we evaluated three different biofertilizers (based on Azospirillum, Trichoderma, or unidentified rhizobacteria) in the Philippines during four cropping seasons between 2009 and 2011, using four different fertilizer rates (100% of the recommended rate [RR], 50% RR, 25% RR, and no fertilizer as Control). The experiments were conducted under fully irrigated conditions in a typical lowland rice environment. Significant yield increases due to biofertilizer use were observed in all experimental seasons with the exception of the 2008/09 DS. However, the effect on rice grain yield varied between biofertilizers, seasons, and fertilizer treatments. In relative terms, the seasonal yield increase across fertilizer treatments was between 5% and 18% for the best biofertilizer (Azospirillum-based), but went up to 24% in individual treatments. Absolute grain yield increases due to biofertilizer were usually below 0.5 t·ha−1, corresponding to an estimated additional N uptake of less than 7.5 kg N ha−1. The biofertilizer effect on yield did not significantly interact with the inorganic fertilizer rate used but the best effects on grain yield were achieved at low to medium fertilizer rates. Nevertheless, positive effects of the biofertilizers even occurred at grain yields up to 5 t·ha−1. However, the trends in our results seem to indicate that biofertilizers might be most helpful in rainfed environments with limited inorganic fertilizer input. However, for use in these target environments, biofertilizers need to be evaluated under conditions with abiotic stresses typical of such systems such as drought, soil acidity, or low soil fertility.

Keywords

Azospirillum; biofertilizer; grain yield; inorganic fertilizer; PGPR; plant growth-promoting rhizobacteria; rice; Trichoderma

Introduction

Biofertilizers are becoming increasingly popular in many countries and for many crops. They are defined as products containing active or latent strains of soil microorganisms, either bacteria alone or in combination with algae or fungi that increase the plant availability and uptake of mineral nutrients [1]. In general, they contain free-living organisms associated with root surfaces but they may also include endophytes, microorganisms that are able to colonize the intercellular or even intracellular spaces of plant tissues without causing apparent damage to the host plant. The concept of biofertilizers was developed based on the observation that these microorganisms can have a beneficial effect on plant and crop growth (e.g.) [2]. Consequently, a range of plant growth-promoting rhizobacteria (PGPR) has been identified and well characterized. Direct beneficial effects can occur when the microorganisms provide the plants with useful products.

The best known case of this are microorganisms that can directly obtain N from the atmosphere and convert this into organic forms usable by plants. Such biological nitrogen fixers (BNF) include members of the genus Rhizobium, Azospirillum, and blue-green algae. Rhizobia are symbiotically associated with legumes and nitrogen fixation occurs within root or stem nodules where the bacterium resides [3]. The genus Azospirillum also has several N-fixing species, which are rhizobacteria associated with monocots and dicots such as grasses, wheat, maize and Brassica chinensis L. [4,5]. Azospirillum strains have been isolated from rice repeatedly, and recently the strain Azospirillum sp. B510 has been sequenced [6,7].

Considerable N fixation by Azotobacter spp. and Azospirillum spp. in the rice crop rhizosphere was reported repeatedly [6,8], but others [9] questioned such high amounts of non-symbiotic N fixation in agriculture. Instead, it was hypothesized that the beneficial effect of Azospirillum inoculums may not derive from its N-fixing properties but from its stimulating effect on root development [2], probably often triggered by phytohormones [10]. This view was confirmed by Okon and Labandera-Gonzales [11], who concluded that the main effect of Azospirillum spp. is the stimulation of the density and length of root hairs, the rate of appearance of lateral roots, and the root surface area. Phytohormone production and a beneficial effect on plant growth were also shown for a range of other microorganisms [12,13].

Another important genus for biofertilizer producers is Trichoderma, a fungus present in nearly all soils. Trichoderma spp. thrives in the rhizosphere and can also attack and parasitize other fungi. Trichoderma spp. have been known for decades to increase plant growth and crop yield [14-16], to improve crop nutrition and fertilizer uptake [16,17], to speed up plant growth and enhance plant greenness [18], as well as to control numerous plant pathogens [19-21]. A part of these effects may also be related to the fact that some Trichoderma spp. seem to hasten the mineralization of organic materials [22], thus probably releasing nutrients from soil organic matter. Positive effects on plant nutrition were also described for other organisms, and many soil bacteria may enhance the mineral uptake of the plant, as for example by the increased solubility of phosphate in the soil solution [23].

There is a wide range of reports on the effect of biofertilizer application in crops grown in non-flooded soils (unlike lowland rice), and the technology for Rhizobium inoculation of leguminous plants is well established. A review on results from Azospirillum inoculation experiments across the world and covering 20 years was conducted by Okon and Labandera-Gonzales [11]. They found a success rate of 60- 70% with statistically significant yield increases on the order of 5-30%. However, the vast majority of these trials were on wheat, maize, sorghum, or millet, and only one of the experiments included in the analysis was on rice. Consequently, results from biofertilizer use in rice are still rare. Some reports from groups promoting the use of biofertilizers indicated considerable yield increases upon their use. Trichoderma harzianum, used as a coating agent for rice seed, was reported to result in a 15-20% yield increase compared with rice plants receiving full inorganic fertilizer rates only [22]. As already mentioned above Sison [8], reported enhanced growth and development of rice and maize after the use of biofertilizer containing Azospirillum spp, and asserted the biofertilizer would provide 30-50% of the crop’ N requirement. Similarly, Razie F and Anas I [6] claimed that the inoculation of rice seedlings with Azotobacter spp. and Azospirillum spp. was able to substitute for the application of inorganic N fertilizer, and that this technology enabled rice yields of 3.9 to 6.4 t·ha−1 (yield increases in comparison with the control were about 2.0-3.0 t·ha−1). Another study tested the effect of rice root inoculation with Azospirillum spp. under different N fertility levels, and found a more pronounced yield response at lower levels of inorganic N fertilization [24]. Generally, rice yield increases in this study were lower, and ranged around 0.5 t·ha−1. A yield-increasing effect on rice by inoculation with Azospirillum sp. strain B510 was also shown by Isawa et al. [25] but the experiment was conducted in pots only.

Based on these reports, it can be assumed that biofertilizers could offer an opportunity for rice farmers to increase yields, productivity, and resource use efficiency. And, the increasing availability of biofertilizers in many countries and regions and the sometimes aggressive marketing brings ever more farmers into contact with this technology. However, rice farmers get little advice on biofertilizers and their use from research or extension because so little is known on their usefulness in rice. Necessary would be recommendations describing under which conditions biofertilizers are effective, what their effect on the crop is, and how they should best be used.

To start addressing these issues, we conducted this study, testing different biofertilizers in an irrigated lowland rice system in the Philippines during four seasons. The objectives of the study were (1) to evaluate the effects of different biofertilizers on irrigated rice grain yield, (2) to investigate possible interactions of the effect of these biofertilizers with different inorganic fertilizer rates, and (3) to determine, based on the results, whether biofertilizers are a possible option to improve the productivity of rice production and under which conditions they give good results.

Materials and Methods

Site Description

The experiments were conducted during two dry seasons (DS) and two wet seasons (WS). In the 2008/09 DS and the 2009 WS, an experimental site at the Central Experimental Station of the University of the Philippines at Los Baños (CES-UPLB) was used, whereas the experiment in the 2010 WS and the 2010/11 DS was conducted at the Experimental Station of the International Rice Research Institute (IRRI) in Los Baños (ES-IRRI). Both experimental sites were located in close vicinity (about 1 km apart) in Laguna Province, Philippines (14°11’ North, 121°15’ East, 21 masl), in a typical lowland rice production area with the dominant soil type “anthraquic Gleysols” [26].

Detailed soil characteristics were analyzed only for the field at ES-IRRI (Table 1) but the soil at CES-UPLB was similar. The soil at both sites had a fine texture (clayey loam) and a high cation exchange capacity (CEC). Topsoil pH values at CES-UPLB in the 2009 DS and WS were 6.9 and 6.8, respectively, while pH values of 6.9 (2010 WS) and 6.5 (2011 DS) were observed at the ES-IRRI site. The soil organic carbon concentrations at both farms were relatively high, ranging between 1.5% and 1.9%. Related to this, organic N concentrations were also high at both farms (0.15-0.27%). The high soil organic matter content also caused high P availability as indicated by high Olsen P values, which were far above the critical low level of 10-15 mg·kg−1 [27]. Similarly, the exchangeable K was adequate for both experimental sites at the start of the cropping seasons [27].

Site
Soil type
  UPLB
Anthraquic Gleysols
IRRI
Anthraquic Gleysols
    2008/2009 DS 2009 WS 2010 WS 2010/2011 DS
pH (1:1) - 6.9 6.8 6.9 6.5
Total organic C g kg−1 18.6 15.9 16.2 15.0
Total soil N g kg−1 2.7 1.6 1.5 1.5
Olsen P mg kg−1 55 40 35 30
Avail K cmol kg−1 - - 1.26 1.32
Exch K cmol kg−1 1.50 1.06 1.50 1.50
Exch Ca cmol kg−1 - - 18.9 18.1
Exch Mg cmol kg−1 - - 13.5 13.3
Exch Na cmol kg−1 - - 1.01 1.00
CEC cmol kg−1 - - 33.6 33.0
Clay g kg−1 - - 441 445
Silt g kg−1 - - 332 355
Sand g kg−1 - - 227 200

Table 1: Average top-soil characteristics (0–15 cm depth) for all experimental seasons and both experimental sites.

Experimental Treatments and Design

In all four seasons, the experiment was a two-factor experiment arranged in a randomized complete block design (RCBD) with three replications. Main plots were assigned to four different fertilizer levels: i) the full recommended rate (100% RR) of inorganic fertilizer; ii) 50% RR, 25% of RR, and the Control treatment in which no inorganic fertilizer was applied. However, the recommended rate changed between seasons and was 120 kg N ha−1, 60 kg P2O5 ha−1, and 60 kg K2O ha−1 in the DS, and 90 kg N ha−1, and 30 kg P2O5 ha−1, 30 kg K2O ha−1 in the WS. The exact N, P, and K amounts applied are given in Table 2.

Fertilizer Rate Unit Dry Season Wet Season
0% RR N-P2O5-K2O in kg·ha−1 0-0-0 0-0-0
25% RR N-P2O5-K2O in kg·ha−1 30-15-15 22.5-7.5-7.5
50% RR N-P2O5-K2O in kg·ha−1 60-30-30 45-15-15
100% RR N-P2O5-K2O in kg·ha−1 120-60-60 90-30-30
or      
0% RR N-P-K in kg·ha−1 0-0-0 0-0-0
25% RR N-P-K in kg·ha−1 30-7-13 22.5-3-6
50% RR N-P-K in kg·ha−1 60-13-25 45-7-13
100% RR N-P-K in kg·ha−1 120-26-50 90-13-25

Table 2: Inorganic fertilizer treatments in all four experimental seasons as ratio of the Recommended Rate (RR) and as actual nutrients applied in the dry and wet season.

Subplots (30 m2 each) were assigned to the different biofertilizers tested in the experiment. Three different biofertilizers available in the Philippines were used, and an overview of their characteristics is given in Table 3. The products were Bio-N® (BN), BioGroe® (BG), and BioSpark® (BS; the same product was called BioCon in 2009). In addition, a Control treatment was used in which no biofertilizer was applied. Thus, the total number of treatment combinations tested was 16.

Product ID BN BG BS
Product name Bio-N® BioGroe® BioSpark®
Active ingredient Azospirillum lipoferum, A.brasilense Plant growth-promotingrhizobacteria (not defined) Trichoderma parceramosum,T. pseudokoningii, andUV-irradiated strain ofT. harzianum
Active organism Bacteria Bacteria Fungi
Product type Dry powder in200-g pack Dry powder in100-g pack Dry powder in250-g pack
Carrier medium Sterile charcoal/soilmixture Sterile charcoal/soilmixture Dry organic medium (ricehull)
Producer declared cell number 108 cfu g−1 - 109 cfu g−1
Shelf life 3 months 6 months 24 months
Product amount 1000 g 40 kg−1  seed 400 g 40 kg−1 seed 200 g 40 kg−1 seed
recommended and used      
(for 1 ha) 2011 biofertilizer costs US$6.82 US$3.64 US$6.36
needed for 40 kg seed      
Elemental contents*      
N % 0.13 0.34 1.27
P % 0.091 0.063 0.687
K % 0.22 0.24 0.72
Supplier BioTech UPLB BioTech UPLB BioSpark Corp.

Table 3: Characteristics of the three biofertilizer used and tested.

BN was developed in the early 1980s by Paredes JC [28]. According to the distributor (BIOTECH, UPLB), it contains Azospirillum lipoferum and A. brasilense, isolated from Saccharum spontaneum (local name is Talahib). BN is available in dry powder form in a 200-gram package, which can be used for seed inoculation, direct broadcasting on seeds, or mixed with water as a root dip. The BN product has a shelf-life of 3 months and the package we used was well before its expiry date. BN is specifically targeted at rice and corn.

The second product tested was BG, developed by Paterno of BIOTECH at UPLB. It contains unknown plant growth-promoting bacteria (rhizobacteria) that influence root growth by producing plant hormones and providing nutrients in soluble form [28].

The last product tested was BS, developed by Cuevas [29]. According to personal information from her, it contains three different species of Trichoderma isolated from Philippine forest soils (including Trichoderma harzianum), and is mass-produced using a pure organic carrier [29]. The product can be used for seed coating or for soil application in the seedbed.

Crop Establishment and Management

In all experiments, rice variety PSB Rc18, a modern-type variety with 120 days duration, was used. Seed for the BN and Control treatments was soaked for 24 h, incubated for another 24 h, and sown using the modified dapog (mat) method. BN was prepared in a slurry solution and applied by dipping the roots of the seedlings into the slurry, 1 h before transplanting in the field. For the BG and BS treatments, seeds were initially also soaked for 24 h. The biofertilizers BG and BS were then applied by mixing the seeds with the biofertilizer product, thus coating the seeds. BG and BS were applied at 400 g 40 kg−1 seed and 200 g 40 kg−1 seed, respectively. The seed-biofertilizer mixture was then incubated for 10 hours in an open jute sac to allow cooling, followed by 14 hours incubation in the closed sack like the control. Seeds were sown using the modified dapog method. In all treatments, 14-day-old seedlings were transplanted at 2-3 seedlings per hill with a planting distance of 20 cm. Missing hills were replanted within 7 days after transplanting (DAT).

Inorganic fertilizers used for the fertilizer treatments were urea (46-0-0 N-P2O5-K2O) and compound (14-14-14 N-P2O5-K2O) fertilizer. Compound fertilizer was applied basal just before transplanting according to the treatment. The remaining N was applied in equal splits at 10 DAT and at 55 DAT. A water depth of 3-5 cm was aimed for at every irrigation from early tillering until 1-2 weeks before physiological maturity. To control insect pests and diseases in the 2010 WS and 2010/11 DS, granular Furadan was applied 20 DAT at a rate of 33 kg·ha−1 and Hopcin was applied at a rate of 0.8 L·ha−1 at flowering. Molluscicide was applied right after transplanting to control golden apple snails in the field. Post-emergence herbicide was applied once at the 2-3-leaf stage of emerging weeds. Hand-weeding was done thereafter as needed. Application rates were based on the recommended rate of the specific pesticides that were used.

Sampling and Statistical Analysis

Grain yields were determined in the study for a 5-m2 (2.5 m-2.0 m) designated sampling area, which was strategically located at the center of each subplot, leaving at least two border rows. Grain moisture content was determined immediately after threshing (Riceter grain moisture meter, Kett Electric Laboratory, Tokyo, Japan) and all grain yields are reported at 14% moisture content. The data gathered in the study were statistically analyzed using the procedures described by Gomez and Gomez [30]. Analysis of variance was conducted using SAS (Version 9.0) and treatment means were compared by the least significant difference (LSD) and were considered significant at p ≤ 0.05.

Results and Discussion

In all four seasons and across the biofertilizer treatments, grain yield increased with increasing amounts of applied fertilizer (Table 4). However, this increase was not always statistically significant and the yield increase varied considerably between seasons. Overall, the lowest grain yields occurred in the 2009 WS, ranging only from 1.9 to 2.7 t·ha−1. Generally, low yields in that season were due to a typhoon that caused considerable damage through flooding of the experimental field and lodging of the crop. For this reason, the crop was harvested prematurely by about 1 week, which further reduced attainable yields.

Season Bio-fertilizertreatment*** Inorganic fertilizer treatment** Mean*
0% RR 25% RR 50% RR 100% RR
Grain yield (kg·ha−1)
2008/09 DS BG 4016 4421 4569 5134 4508 a
  BN 4163 4753 4900 5081 4683 a
  BS 4351 4569 4375 5173 4610 a
  Control 4062 4440 4630 4799 4534 a
  Mean* 4158 c 4548 b 4617 b 5034 a  
2009 WS BG 1963 1975 2502 2383 2206 bc
  BN 2149 2417 2420 2604 2398 a
  BS 2005 2179 2287 2674 2286 ab
  Control 1902 2000 2038 2165 2026 c
  Mean* 2005 c 2143 bc 2038 ab 2456 a  
2010 WS BG 4326 4303 4670 4596 4482 ab
  BN 4197 4529 5131 4794 4663 a
  BS 3952 4336 4578 4732 4399 bc
  Control 3389 4245 4274 4716 4219 c
  Mean* 3965 c 4353 b 4659 a 4710 a  
2010/11 DS BG 4145 4665 4926 5556 4825 a
  BN 4009 5049 5262 5519 4960 a
  BS 3955 4876 5175 5492 4861 a
  Control 3801 4420 4707 5265 4548 b
  Mean* 3977 c 4751 b 5014 b 5458 a  

Table 4: Grain yield of the variety PSB Rc18 as affected by inorganic fertilizer level and biofertilizer treatments in all four experimental seasons and both sites.

Grain yields in the other three experimental seasons were similar and ranged from 4.0 to 5.2 t·ha−1 in the 2009 DS, from 3.4 to 5.1 t·ha−1 in the 2010 WS, and from 3.8 to 5.6 t·ha−1 in the 2010/11 DS. These ranges already indicate a relatively low yield increase due to fertilizer application in the 2008/09 DS (up to 1.2 t·ha−1 for the full fertilizer rate of 120-60-60 kg N-P2O5-K2O ha−1) and the 2009 WS (up to 0.8 t·ha−1 for the full fertilizer rate of 90-30-30 kg N-P2O5-K2O ha−1). A higher response to inorganic fertilizer was achieved in the 2010 WS (up to 1.7 t·ha−1 for the full fertilizer rate of 90-30-30 kg N-P2O5-K2O ha−1) and the 2010/11 DS (up to 1.8 t·ha−1 for the full fertilizer rate of 120-60-60 kg N-P2O5-K2O ha−1).

The effects of biofertilizer treatments on grain yield, depending on the inorganic fertilizer treatment, are shown in Table 4. Significant yield increases due to biofertilizer use were observed in all experimental seasons with the exception of the 2008/09 DS. In the 2010/11 DS, no significant difference between the three biofertilizers tested was detected, but all three achieved better yields than the Control. The biofertilizer achieving the highest average grain yields across all four inorganic fertilizer treatments and in all four seasons was BN. Statistically significant interactions between biofertilizer treatment and inorganic fertilizer treatment could not be detected in any season (at p ≤ 0.05), suggesting that the effect of the biofertilizer was independent of the inorganic fertilizer rate. However, there was a trend of higher yield increases due to biofertilizer use at low to medium inorganic fertilizer rates (Table 4). This trend was most obvious for the BN biofertilizer whereas the performance of the BS and BG biofertilizers was less consistent.

Season Biofertilizertreatment*** 0% RR Inorganic fertilizer treatment** Mean
25% RR 50% RR 100% RR
Relative yield increase (%)*
2008/09 DS BG -1 0 -1 7 1
  BN 2 7 6 6 5
  BS 7 3 -6 8 3
  Control - - - - -
2009 WS BG 3 -1 23 10 9
  BN 13 21 19 20 18
  BS 5 9 12 24 13
  Control - - - - -
2010 WS BG 28 1 9 -3 8
  BN 24 7 20 2 12
  BS 17 2 7 0 6
  Control - - - - -
2010/11 DS BG 9 6 5 6 6
  BN 5 14 12 5 9
  BS 4 10 10 4 7
  Control - - - - -

Table 5: Relative yield increase over the Control treatments with the same inorganic fertilizer rate for all biofertilizers tested, in all seasons and at both experimental sites.

The grain yield increase due to biofertilizer only (0% RR inorganic fertilizer treatment) usually ranged from 200 to 300 kg grain ha−1 for the best biofertilizers with the exception of the 2010 WS, when the BN treatment had an almost 800 kg·ha−1 better grain yield than the Control. In relative terms (Table 5), the seasonal yield increase across fertilizer treatments was between 5% and 18% for the BN biofertilizer (up to 24% for individual treatment combinations), between 3% and 13% for the BS biofertilizer (up to 24% for individual treatment combinations), and between 1% and 9% for the BG biofertilizer (up to 28% for individual treatment combinations). For the calculation of the relative yield increase, only average values could be compared and no statistical analysis could be conducted.

The effect of biofertilizer on the agronomic efficiency of N fertilizer (AEN) is shown in Table 6. For these calculations, the yield of each treatment was compared with the grain yield baseline (the Control treatment in which no biofertilizer and no inorganic fertilizer were used) and the yield increase was divided by the N rate applied. Again, only average values could be compared and no statistical analysis was possible. The results (Table 6) indicate considerably higher overall AEN values in the 2010 WS and the 2010/11 DS. Also, the AEN values are generally higher at low N rates and decrease with higher N application rates. The biggest AEN increase caused by biofertilizer occurred at the lowest N fertilizer rate (25% RR treatment), and, among the different biofertilizers tested, the BN biofertilizer resulted in the highest and most consistent AENs.

Season Bio-fertilizertreatment** 0% RRReferencegrain yield(kg·ha−1) Inorganic fertilizer treatment *
25% RR 50% RR
AEN***
100% RR
(kg grain yield increase kg−1 N applied)
2008/09 DS BG   12 8 9
  BN   23 14 8
  BS   17 5 9
  Control 4062 13 9 6
2009 WS BG   3 13 5
  BN   23 12 8
  BS   12 9 9
  Control 1902 4 3 3
2010 WS BG   41 28 13
  BN   51 39 16
  BS   42 26 15
  Control 3389 38 20 15
2010/11 DS BG   29 19 15
  BN   42 24 14
  BS   36 23 14
  Control 3801 21 15 12

Table 6: Estimated agronomic efficiency (AEN) of applied N depending on the inorganic fertilizer treatment and the biofertilizer used.

In our experiments, the selected biofertilizers were used as recommended by the producers but we could not check the viability or the contents of the products. Thus, we did not verify whether the biofertilizers contained the declared organisms (Table 6; the contents of BG remained unidentified) or the required number of living cells in the inoculate. The importance of quality control and regulation for biofertilizer production was emphasized by Reddy and Giller [31], who also pointed out that the frequent absence of such mechanisms can cause non-functional products. Maintenance of high standards for Azospirillum inoculants with proven efficient strains and cell numbers on the order of 1-109 to 1-1010 colony-forming units (cfu) g−1 or mL−1 was also requested by Okon and Labandera-Gonzales [11].

But, the fact that the products in our study caused a significant effect on grain yield in three out of four seasons (only two out of four seasons for BG) indicated that the biofertilizers tested had sufficient active ingredients and that the producers maintained a good quality over the four seasons (or 2.5 years). Theoretically, the effect of the biofertilizers could also have been caused by non-living ingredients but the applied amount was so small that even micronutrients could not explain the observed effects. Also, no micronutrient deficiencies are known from either of the two experimental sites.

The general effect of inorganic fertilizer was as expected, and grain yields increased continuously with increasing fertilizer rates (Table 4). However, the response to inorganic fertilizer was low in the 2008/09 DS and the 2009 WS, as also indicated by the low AEN (Table 5). Good and economic values for AEN are usually 15-20 kg grain yield per kg N applied, and, at AEN < 10, inorganic fertilizer use may give negative economic returns depending on the input and output prices [32,33]. Low response in the 2009 WS can be explained by the negative effects of a typhoon and the early harvest. The low response in the 2008/09 DS could be due to the combination of a very fertile soil (high grain yield in the 0% RR treatment) and a limited yield potential in that season (low maximum yields in the 100% RR treatment).

The tested biofertilizers did increase grain yield significantly, and especially the BN biofertilizer did so consistently. Even in seasons in which no significant effect could be detected due to the yield variability between plots, the grain yield with biofertilizer was usually better than without. The seasonal yield increase across fertilizer treatments was between 5% and 18% for the BN biofertilizer (up to 24% for individual treatments; Table 5), which is within the 5-30% range reported for Azospirillum inoculums and non-rice crops by Bashan and Levanony and Okon and Labandera-Gonzales [4,11]. Similarly, the here-observed yield increase for the Trichoderma-based BS (3-13%) was close to the 15-20% rice yield increase described by Cuevas [22].

The trend of yield increases between the different inorganic fertilizer treatments was not so clear across seasons but yield increases were often lower at higher inorganic fertilizer rates, which were also reported by Rajabamamohan et al. [24]. Absolute grain yield increases due to biofertilizer were usually below 0.5 t·ha−1 (Table 1), corresponding to an estimated additional N uptake of less than 7.5 kg N ha−1 (based on 0.5% N in straw, 1.0% N in grain, and harvest index 0.5). Both values are far below grain yield increases and additional N uptake reported by Razie and Anas and Sison [6,8], but similar to the rice grain yield increases reported by Rajabamamohan et al. [24].

The calculated AEN values (Table 6) suggested higher N use efficiency for treatments with biofertilizer use. Increased nutrient uptake and fertilizer use efficiency were also reported for Trichoderma spp. [16,17,34] and for Azospirillum spp. [11]. But, the results could be explained in several ways. One possibility is that the biofertilizer stimulated root growth and thereby increased the uptake of indigenous N from the soil (the higher AEN would then be only an artifact of the calculation method). Second, the increased root growth could reduce N fertilizer losses, and the third option could be biological N fixation (which could explain the superior performance of the BN biofertilizer, supposedly containing organisms capable of biological N fixation). But, our experiment cannot answer the question of which process or combination of processes is at work here, if that is possible at all under field conditions [9].

Summary and Conclusions

The study was conducted to evaluate the effect of different biofertilizers on the grain yield of lowland rice, and investigate possible interaction effects with different inorganic fertilizer amounts. The results showed significant yield increases for all products tested in some seasons but the most consistent results were achieved by the Azospirillum-based biofertilizer. In most cases, the observed grain yield increases were not huge (0.2 to 0.5 t·ha−1) but could provide substantial income gains given the relatively low costs of all biofertilizers tested. The positive effect of the tested biofertilizers was not limited to low rates of inorganic fertilizers and some effect was still observed at grain yields up to 5 t·ha−1. However, the trends in our results seem to indicate that the use of biofertilizers might be most helpful in low- to mediuminput systems. The results achieved can already be used to develop better advice for farmers on biofertilizer use in lowland rice, but several important questions remain. In particular, biofertilizers need to be evaluated under conditions with abiotic stresses typical for most lowto medium-input systems (e.g., under drought or low soil fertility) and with a range of germplasm because their effect might depend also on the variety used. More upstream-oriented research would be needed to better understand the actual mechanisms involved, which in turn could also contribute to making the best use of biofertilizers in ricebased systems.

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