Divergence of compost extract and bio-organic manure effects on lucerne plant and soil

Introduction

Preserving and restoring native grasslands and agricultural production systems by improving soil quality have been common goals worldwide (Henwood, 2010). However, long-term inappropriate management and large inputs of chemical fertilizers and pesticides into soils have led to severe soil problems. Commercial soil conditioners or manures are used in agricultural production systems for amending nutrient-deficient soils by providing multiple nutrients (Soumare, Tack & Verloo, 2003; Hu & Qi, 2013), including, but not limited to, bio-organic manure (BOM), effective microorganisms (EM), and compost extract (CE). However, farmers are sometimes reluctant to use organic fertilizers because the effects of organic nutrients on plant growth are not as quickly seen as with inorganic fertilizers. Thus, new materials and new techniques for increasing the efficiency of organic materials are needed.

Introducing exogenous microorganisms into agricultural environments has been applied to accelerate bioremediation and improve agricultural productivity (Paluch, Thomsen & Volk, 2013). Although less is known about the practical effectiveness and mechanisms of these products, the assessment of their impacts has focused on their potential to increase soil organic matter content, promote nutrient availability to crops (Khan, Zaidi & Wani, 2007), enhance the proliferation of beneficial bacteria (Priyadi et al., 2005; Javaid, 2010), and prevent infection by pathogens (Compant et al., 2005). However, additional microorganisms from commercial products have to be incubated formerly and propagated in the lab and further applied to the fields. Their survival rate and their influences on plant and soil have been viewed skeptically (Watanabe, Miyashita & Harayama, 2000; Chen et al., 2015).

In contrast, indigenous microorganisms may be optimally suited for survival and multiplication in significant amounts (Paluch, Thomsen & Volk, 2013). However, there is less information available on the ways to activate naturally occurring soil microbial populations. The use of CE produced by plant extraction technology may be one method to activate indigenous soil microbial populations. CE derived from endophytes extracted from different plant species such as soybean (Glycine max (L.) Merr.), lucerne (Medicago sativa L.), the brown algae (Ascophyllum nodosum) (Bestfarming systems Co.), and has been widely used recently. A new bio-molecular method for determining the presence of nifH, the gene harboured in endophytic bacteria for nitrogenase reductase from nodulated plant species has been developed (http://www.amazingcarbon.com) (Gao et al., 2015; Le et al., 2015). This study demonstrates that CE activates indigenous microorganisms in the soil, particularly N-fixing bacteria or archaea that are not able to be cultured in the laboratory. Besides, increasing evidence suggests that is of lavonoids and flavonoids exuded from the root of many legumes can activate rhizobium genes, which helps in the nodulation process (Peters, Frost & Long, 1986; Brunetti et al., 2013). Combing microbial remediation with legume remediation is expected to be an economically and environmentally appropriate approach for soil amendment.

This study was undertaken to verify if: (1) there are positive effects of adding CE on plant and microbial biomass, microbial activity, plant and soil nutrient contents; (2) combining CE and BOM achieve the highest impacts on soil nutrient contents and lucerne growth; (3) CE stimulates the nodulation of legume as effectively as by inoculating rhizobacteria. Results are expected to reveal the interactive effects of CE and BOM application on plant species growth and soil properties, and aid in the selection of optimal fertilization approaches for soil quality improvement. In addition, the experiments will enable exploring the stimulative effect of CE on activating and enriching indigenous microorganisms from plant rhizosphere.

Materials and Methods

Results

Biomass and nutritional value of lucerne

Shoot, root and total biomass of inoculated lucerne significantly increased (P < 0.05) under CE application by 73.3%, 38.3% and 63.8%, respectively (Fig. 1). Compared with the control, the three biomasses of lucerne did not change under CEBOM treatment, and decreased by17.5% to 26.6 % when BOM was added. For non-inoculated lucerne, all three biomasses showed highest values with CE addition alone by increasing from 46.0% to 85.3%, then were second with CEBOM treatment. BOM addition did not affect the biomass of non-inoculated lucerne. The effects of amendment treatments on soil characteristics and plant nutritional values were further analyzed by ANOVA (Table S1). CE and BOM significantly affected the nutritional values (Table 2). The CP concentration of lucerne was significantly higher in BOM and CE treatments than in the control. CDBOM and ME showed the same trend in response to treatments. CE and BOM significantly decreased lucerne NDF from 66.18 to 52.45 g kg⁻¹ DM and CE alone decreased NDF by 36.3%.

Table 2:

Effect of compost extract (CE), bio-organic manure (BOM) and CE + BOM (CEBOM) application on shoot nutrient values of lucerne.

Treatment	CP (g DM/kg)	NDF (g DM/kg)	CDOM (g DM/kg)	ME (MJ DM/kg)
Control	17.7c	66.18a	68.61b	9.84b
CE	23.3a	58.24c	77.77a	10.87ab
BOM	21.4b	61.48ab	72.42b	10.09b
CEBOM	24.6a	52.45bc	79.99a	11.01a

DOI: 10.7717/peerj.3775/table-2

Notes:

Abbreviations are:

DM

dry matter

CP

crude protein

NDF

neutral detergent fibre

CDOM

cellulose digestible organic matter

ME

metabolisable energy

MJ

joules per mole

Different letters within a column indicate significant differences (P < 0.05) between treatments. LSD multiple comparisons were used.

Comparison between rhizobium nodulation among soil amendment treatments

The highest number of nodules and total nodule weight occurred in inoculated lucerne with the CE treatment (Fig. 2, Tables S1 and S2). The number of nodules and the total nodule weight of the inoculated lucerne showed no significant difference with non-inoculatedlucerne under CE application. Under BOM application, the nodulation of lucerne decreased significantly. Table S3 showed the effects of treatments on plant N and soil N of inoculated lucerne.

Discussion

The shoot, root and total biomass and the nutritional value of inoculated and non-inoculated lucerne were significantly increased with the application of CE alone and CE with BOM together. It indicates that CE acts better than BOM for improving legume growth, because nitrogen- fixation can be suppressed by the addition of external nitrogen (Goergen & Chambers, 2009). However, it does not imply that BOM can substitute for CE, because highly synergistic effects on plant nutrient and soil properties were observed. Both soil biotic (MBC, MBN, MR) and abiotic (soil water holding capacity, organic matter, alkaline N, available P and K) factors were improved, and thus plant growth was enhanced.

When CE was applied, 164% higher plant biomass occurred, which could be mainly attributed to the stimulation of beneficial microorganisms (increased numbers of bacteria and fungi, Table 4) by accelerating the decomposition of organic materials and increasing the release of nutrients (Javaid & Bajwa, 2011). In contrast, BOM provided additional microorganisms and external nutrients to the soil (Higa, 2001). These inoculated microorganisms have to compete with naturally occurring bacteria (Sherr, Sherr & McDaniel, 1992). The interactions of different microorganisms may complement each other and further influence plant growth due to microbial diversity (Van der Heijden, Bardgett & van Straalen, 2008). By stimulating the proliferation of beneficial bacteria and restraining harmful microorganisms through CE addition, some essential substances such as nucleic acids, amino acids and bioactive substances (e.g., hormones and enzymes) are synthesized (Javaid & Bajwa, 2011). They can accelerate the decomposition of lignin materials in the soil and the mineralization of organic material as well as the control of soil diseases, resulting in a change in soil microbial parameters and physical and chemical soil characteristics as seen in this study (Kim et al., 2004; Javaid & Bajwa, 2011). Besides, lucerne has been reported to produce root-derived antimicrobials in dole, terpenoid, benzoxazinone, flavonoid and isoflavonoid, which are used for inhibiting soil-borne pathogenic bacteria (Dixon, 2001) and thus enhance the symbiosis between plant and beneficial microorganisms. In soil organic amendment systems, beneficial microbes activated by CE and added by BOM accelerate the mineralization of soil organic matter (Javaid, 2011), which releases more nutrients for plant uptake (Flores, Vivanco & Loyola-Vargas, 1999).

Higher uptake of N, P and K by plant and higher soil content of N, P and K (Table 2) indicate that amendment treatments increased the availability of nutrients in the soil. In this study, the soil had a low pH value indicating acidic conditions, which resulted in low nutrient availability and low inorganic fertilization efficiency (Chen et al., 2015). The observed increase in soil pH with applying CE and BOM together demonstrated that a combination of soil conditioner and BOM could amend an acidic soil by accelerating the rate of microbial processes and thus releasing nutrients more quickly, as shown by other studies (Hati et al., 2006; Lee et al., 2009). Separately, they had no significant effects. CE includes less nutrient elements but has greater effects on soil and plant nutrient in comparison with BOM.

The identical numbers of nodules as well as nodule weight in inoculated and non-inoculated lucerne added with CE indicates the ability of CE for activating indigenous rhizobia (Le et al., 2015). Mechanisms for the maintenance of root-soil contact have been attributed to root exudation (Le et al., 2015). The plant-soil feedback driven by root exudates could initiate and manipulate a signal exchange between roots and soil organisms (Keymer & Lankau, 2017). CE may stimulate root exudates, which could initiate one-way signals informing roots on chemical and physical soil properties, and regulating the symbiotic and protective interactions with microbes (Jones, Farrar & Giller, 2003; Johnson et al., 2015). The exudates accumulate mainly inducible antimicrobial compounds gathered in the roots (Flores, Vivanco & Loyola-Vargas, 1999; Dixon, 2001). Many indigenous microorganisms in the soil are not able to interact synergistically with plant species; therefore, a number of antimicrobial root exudates are secreted. Le et al. (2015) confirmed that some endophytic actinobacteria had the potential to enhance the growth of lucerne and its interactions in rhizobial symbiosis. The mechanism by which CE produces a stimulatory effect on properties soil and plant growth needs to be further explored.

Conclusions

This study demonstrated that CE application alone and in combination with BOM enhanced lucerne biomass and nutritional value by improving nodulation and soil biotic and abiotic properties. The similar effects of CE and BOM on plants and soils does not necessary mean that they can be treated as substitutes for one another. Rather, their combination provides a mode of soil amendment which improves soil quality more efficiently. CE synthesizes some kinds of active substrates that stimulate the activity of indigenous rhizobia in the soil rather than adding extra microorganisms into the soil as BOM does. Root exudation may help in regulating soil properties and root-microbe-soil interactions. However, the nature of their reactions in the soil need to be further explored.

Supplemental Information