A Meta-Analysis and Review of Plant-Growth Response to Humic Substances- Practical Implications for Agriculture.pdf

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  CHAPTER TWO A Meta-Analysis and Review of Plant-Growth Response to HumicSubstances: Practical Implicationsfor Agriculture Michael T. Rose * , † , Antonio F. Patti * , Karen R. Little * , † , { ,Alicia L. Brown † , W. Roy Jackson * , Timothy R. Cavagnaro † , } * School of Chemistry, Monash University, Clayton, Victoria, Australia † School of Biological Sciences, Monash University, Clayton, Victoria, Australia { School of Applied Sciences and Engineering, Monash University, Churchill, Victoria, Australia } SchoolofAgriculture, FoodandWine,TheUniversityofAdelaide,WaiteCampus,PMB1GlenOsmond,SA,Australia Contents 1.  Introduction 38 2.  Meta-Analysis 41 2.1  Methods 41 2.2  Results 45 3.  Plant-Growth Response to HS: Moderating Factors 51 3.1  General plant-growth response 51 3.2  Application rate 52 3.3  HS properties 54 3.4  Environmental conditions 55 3.5  Plant type 56 4.  Practical Use of HS in Agriculture 57 4.1  Direct application 57 4.2  Application as synergists 59 5.  Knowledge Gaps and Research Needs 62 6.  Conclusions 63Acknowledgments 63Appendix A. Study References for SDW Data used in the Meta-Analysis 64Appendix B. Study References for RDW Data used in the Meta-Analysis 77References 85 Abstract  The breakdown products of plant and animal remains, extracted in an alkaline solution,are commonly referred to as humic substances (HS). They can be extracted from a wide  Advances in Agronomy , Volume 124  #  2014 Elsevier Inc.ISSN 0065-2113 All rights reserved. 37  variety of sources, including subbituminous coals, lignites (brown coals), peat, soil, com-posts, and raw organic wastes. The application of HS to plants has the potential toimprove plant growth, but the extent of plant-growth promotion is inconsistent andrelatively unpredictable when compared to inorganic fertilizers. The goal of this reviewwas to determine the magnitude and likelihood of plant-growth response to HS and torank the factors contributing to positive growth promotion. These factors included thesource of the HS, the environmental growing conditions, the type of plant beingtreated, and the manner of HS application. Literature reports of exogenously appliedHS – plant interactions were collated and quantitatively analyzed using meta-analyticand regression tree techniques. Overall, random-effects meta-analysis estimated shootdry weight increases of 22  4% and root dry weight increases of 21  6% in response toHS application. Nevertheless, actual responses varied considerably and were mainlyinfluenced by the source of the HS applied, the rate of HS application, and to a lesserextent, plant type and growing conditions. HS from compost sources significantly out-performed lignite and peat-derived HS in terms of growth promotion, while HS appli-cation rate nonlinearly moderated the growth response under different circumstances.Our results demonstrate the difficulty in generalizing recommendations for the use of HS in agriculture; however, some specific suggestions for maximizing the efficacy of HSunder certain conditions are offered. We also outline some recent developments in theuse of HS as synergists for improving fertilizer use efficiency and the activity of microbialinoculants. Finally, we identify a number of research gaps, which, when addressed,should clarify how, when, and where HS can be best applied for the greatest benefit. 1. INTRODUCTION Humic substances (HS) are a category of naturally occurring organiccompounds that arise from the decomposition and transformation of plant,animal, and microbial residues (MacCarthy, 2001). They are a natural com-ponent of practically all soils, but levels vary and there is considerable evi-dence that modern agriculture involving practices such as soil tillage hasresulted in their decline (Novotny et al., 1999; Shepherd et al., 2001).The loss of humic material, together with overall reductions in soil organicmatter, is of concern because they play important roles in maintaining keysoil functions and plant productivity (Lal, 2004; Sparling et al., 2006). Con-sequently, there is interest in the application of HS-based amendments toagricultural systems in order to reverse this trend (Piccolo and Mbagwu,1997; Quilty and Cattle, 2011).HS are chemically complex with no clearly defined chemical structure,although generalized models have been proposed (Bruccoleri et al., 2001).While traditionally viewed as complex macromolecules, they have morerecently been described as mixtures of smaller molecules, containing 38  Michael T. Rose et al.  aromatic rings, aliphatic chains, and ionizable functional groups that interactwith each other to form aggregated colloids (Piccolo, 2001; Pinton et al.,2009;SuttonandSposito,2005).Thereissignificantevidencethattheexog-enous application of HS can help improve soil fertility, primarily throughtheir complex chemistry which facilitates interactions with a variety of min-eral and nonmineral organic soil components. Some of the documentedbenefits of soil amendment with HS include improved soil aggregationand structure, increased pH buffering and cation exchange capacity,increased water retention capacity, increased bioavailability of immobilenutrients (such as P, Fe, and Zn), and decreased toxicity of aluminumand heavy metals (Chen et al., 2004a; Imbufe et al., 2005; Peiris et al.,2002; Piccolo and Mbagwu, 1989; Piccolo et al., 1997; Tan andBinger, 1986).As well as indirectly influencing plant productivitythrough modificationof soil characteristics, HS can also directly impact on physical and metabolicplant processes. A recent review by Muscolo et al. (2013) reviews evidencefor the hormone-like effects of HS and how these relate to the chemicalstructural features of these materials. The authors highlight a predominanceof auxin-like effects and that nonlignin structures are the principal contrib-utors. These effects can be elicited through an interaction with either rootsor shoots. For example, hormonal-like responses on plant roots were dem-onstrated by Trevisan et al. (2010) and HS may also stimulate H þ -ATPaseand ion transporter activity in the root plasma membrane (Mora et al., 2010;Pinton et al., 1997, 2009). Both these effects can enhance nutrient acquisi-tion, the former through increased soil exploration, and the latter by accel-erating nutrient uptake. These effects appear to be especially prominent for cases involving HS derived from compost and vermicomposts, which maycontain auxin-related compounds (Muscolo et al., 1999; Quaggiotti et al.,2004), including indole-acetic acid derivatives and other low molecular weight organic acids (Russell et al., 2006). In contrast, effects on leaf func-tion have been less well documented and appear somewhat contradictory(Nardi et al., 2002). Foliar application of HS may increase leaf chlorophyllconcentration (Sladky´, 1959), but it is also recognized that HS contain arange of functional groups which are able to interfere with photosynthesis(Pflugmacher etal., 2006).Foliarapplicationshavealsobeenshowntoinflu-ence transpiration, though the mechanism is unclear and both increases anddecreases in water loss and leaf gas exchange have been observed.Despite numerous publications on the potential positive effects of HS onplant growth and productivity over more than five decades (Billingham, 39 Plant-Growth Response to Humic Substances  2012; Chen et al., 2004b; Quilty and Cattle, 2011) and substantial interest intheir potential for improving nutrient-use efficiency and contributing toC sequestration in the soil, the use of commercial products containingHS in agriculture varies and there is scepticism about their effectiveness(Billingham, 2012). Part of the reason for this is no doubt related to the widerange in physicochemical properties of HS, which vary according to themethod of extraction and the environmental matrix from which they aresourced. HS are formed under a variety of environmental conditions andare, therefore, highly heterogeneous and structurally difficult to define(Senesi, 1994). Commercial products often contain mixtures of humicmaterials and added plant nutrients; hence, the cause of any observed ben-eficial effect cannot be easily attributed to the HS themselves. In addition,the recommended rates of application of commercial products are generallyvery small in relation to the natural levels of HS present in the soil. As a con-sequence, the effect of an HS product is substantially less predictable thanother plant or soil amendments of a known chemical structure, such asinorganic fertilizers or synthetic organics including pesticides and growthregulators. Moreover, because of the multiple chemical functional groupsof HS, a particular HS product may behave completely differently under different environmental conditions, or when applied to different plantspecies. Finally, as with many chemical fertilizers, the timing, location,and rate of application will play a crucial role in determining whether beneficial or harmful effects will evolve and whether or not any beneficialeffects are economically worthwhile. This is particularly important becauserecent publications have pointed out potential negative effects and havequestioned the economic viability of applying HS for improved cropproduction (Asli and Neumann, 2010; de Santiago et al., 2010; Hartz andBottoms, 2010).In light of the potential benefits of HS, together with their inconsistentperformance under field conditions, we sought to improve the understand-ing of theeffects of HSon plant growth by conductinga meta-analysis of thepublishedliterature.Morespecifically,ourobjectiveswere(i)toquantifythemagnitude and likelihood of plant-growth promotion, in terms of shoot androot biomass, resulting from HS application; (ii) to determine the influenceof environmental conditions, plant type, HS properties, and the manner of application on plant-growth response to HS; (iii) to identify gaps in our understanding of the interaction of HS with plants; and (iv) to provide somegeneral recommendations for the practical use of HS in agronomic systemsand suggestions for future work. 40  Michael T. Rose et al.
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