External agricultural inputs such as mineral fertilisers, organic amendments, microbial inoculants, and pesticides are applied with the ultimate goal of maximising productivity and economic returns, while side effects on soil organisms are often neglected. We have summarised the current understanding of how agricultural inputs affect the amounts, activity, and diversity of soil organisms. Mineral fertilisers have limited direct effects, but their application can enhance soil biological activity via increases in system productivity, crop residue return, and soil organic matter. Another important indirect effect especially of N fertilisation is soil acidification, with considerable negative effects on soil organisms. Organic amendments such as manure, compost, biosolids, and humic substances provide a direct source of C for soil organisms as well as an indirect C source via increased plant growth and plant residue returns. Non-target effects of microbial inoculants appear to be small and transient. Among the pesticides, few significant effects of herbicides on soil organisms have been documented, whereas negative effects of insecticides and fungicides are more common. Copper fungicides are among the most toxic and most persistent fungicides, and their application warrants strict regulation. Quality control of organic waste products such as municipal composts and biosolids is likewise mandatory to avoid accumulation of elements that are toxic to soil organisms.

Impact of agricultural inputs on soil organisms—a review

Effects of dissolved organic matter (DOM) on the bioconcentration of organic chemicals in aquatic organisms--a review.

Four commercial composts were added to soil to study their effect on plant growth, total rhizosphere microflora, and incidence of plant growth-promoting rhizobacteria (PGPR) in the rhizosphere of tomato plants. Three of the compost treatments significantly improved plant growth, while one compost treatment significantly depressed it. Compost amendments caused only small variations in the total numbers of bacteria, actinomycetes, and fungi in the rhizosphere of tomato plants. A total of 709 bacteria were isolated from the four compost treatments and the soil control to determine the percentage of PGPR in each treatment. The PGPR tests measured antagonism to soilborne root pathogens, production of indoleacetic acid, cyanide, and siderophores, phosphate solubilization, and intrinsic resistance to antibiotics. Our results show that the addition of some composts to soil increased the incidence in the tomato rhizosphere of bacteria exhibiting antagonism towards Fusarium oxysporum f. sp. radicis-lycopersici, Pyrenochaeta lycopersici, Pythium ultimum, and Rhizoctonia solani. The antagonistic effects observed were associated with marked increases in the percentage of siderophore producers. No significant differences were observed in the percentage of cyanogens, whereas the percentages of phosphate solubilizers and indoleacetic acid producers were affected, respectively, by one and two compost treatments. Intrinsic resistance to antibiotics was only marginally different among the rhizobacterial populations. Our results suggest that compost may stimulate the proliferation of antagonists in the rhizosphere and confirm previous reports indicating that the use of composts in container media has the potential to protect plants from soilborne root pathogens.

Effect of compost on rhizosphere microflora of the tomato and on the incidence of plant growth-promoting rhizobacteria.

SUMMARY 1. This review focuses on direct and indirect interactions between dissolved humic substances (HS) and freshwater organisms and presents novel opinions and hypotheses on their ecological significance. Despite their abundance in freshwaters, the role of HS is still inadequately understood. These substances have been considered too large to be taken up by freshwater organisms. On the contrary, here we present evidence that dissolved HS are indeed taken up and interact directly and/or indirectly with freshwater organisms. 2. We show that dissolved HS exert a mild chemical stress upon aquatic organisms in many ways; they induce molecular chaperones (stress shock proteins), induce and modulate biotransformation enzymes and modulate (mainly inhibiting) the photosynthetic release of oxygen by freshwater plants. Furthermore, they produce an oxidative stress, which may lead to membrane oxidation. HS modulate the multixenobiotic resistance activity and probably other membrane-bound pumps. This property may lead to the increased bioaccumulation of xenobiotic chemicals. Furthermore, they can modulate the numbers of offspring in a nematode and feminise fish and amphibians. The ecological consequences of this potential remain obscure at present. HS also have the potential to act as chemical attractants (as shown with a nematode). 3. In some macrophytes and algae we show that HS interfere with photosynthesis and growth. For instance, the presence of HS suppresses cyanobacteria more than eukaryotic algae. By applying a quantitative structure activity relationship approach, we show that

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2427.2006.01571.x

Dissolved humic substances – ecological driving forces from the individual to the ecosystem level?

....................................................................^ 2 Introduction.....................................................^ 2 Physical and Chemical Characteristics of the Okefenokee Swamp...................................................................................... 3 Geology...................................................................................................................................................................... 3 Peat Deposits.................................................................^ 4 Hydrology............................................................................................................................................................^ 5 Water Chemistry........................................................................................................................................................ 6 Major Habitats in the Okefenokee Swamp............................................................................................................................ 7 Forested Upland................................................................................................................................................^ 7 Forested Wetland................................................._ 7 Scrab-Shrub Wetland................................................................^ 8 Prairie......................................................................................................................................................................... 8 Successional Processes and Disturbances in the Okefenokee Swamp.................................................................................. 8 Formation of Tree Islands.......................................................................................................................................... 8 Episodic Fires............................................................................................................................................................. 9 Logging...................................................................................................................................................................... 9 Animal Life in the Okefenokee Swamp................................................................................................................................ 9 Sunimary...........................................................»^ 11 References Cited.................................................................................................................................................................... 11

/pdf/humic-substances-in-the-suwannee-river-georgia-interactions-iwlwa80y3p.pdf

Humic substances in the Suwannee River, Georgia; interactions, properties, and proposed structures

By incorporating molecular weight fractions of humic acids of various origins into selective substrates designed for the enumeration of physiological groups of microorganisms, it was found that the presence of humic acids at concentrations of up to 30mgl −1 normally resulted in increased numbers of soil microbes active within a particular physiological group. Observed increases could be as much as 2000-fold. Microbes in an organic humus-rich soil were more stimulated by humic substances than organisms from a sandy soil. In certain microbes humic substances appeared to induce a change in metabolism, allowing the organisms to proliferate on substrates which previously they could not utilize. Indications were obtained that within the 10–30 mgl −1 concentration range lower molecular weight humic fractions (approx. 5500 dallons) were more effective than higher molecular weight material. At higher concentrations the reverse was sometimes noticed. Similarly, fulvic acids at concentrations of up to approximately 50mgl −1 would appear to have a more pronounced physiological effect than humic acids, whereas the latter might be more effective at higher concentrations. The response of certain physiological groups to humic products of natural origin appeared to be comparable to that of surfactants such as Tween and Brij. This would suggest that the physiological action of humic substances is, at least partly, the result of their surface activity, making the membrane one of the prime targets of the physiological action of humics on living cells.

Physiological action of humic substances on microbial cells

The effects of humic acids (HA) on the membrane permeability of potato cells and on the phase transition in liposomes of dipalmitoyl phosphatidylcholine (DPPC) were studied. A HA concentration of 40 mg 1−1 was sufficient to produce a maximum K+ efflux from potato cells. The effect was independent of external Ca2+ and would therefore appear to be unrelated to the ability of HA to chelate apoplastic Ca2+. In potato cells with cholesterol-enriched membranes, the above-mentioned effect of HA (40 mg 1−1) on K+ efflux was reduced by 60%. Dilatometric measurements of phase transition in DPPC liposomes showed that both HA and Triton X-100 were able to cancel the sudden increase of liposome volume at phase transition as normally occurring for liposomes in pure water. From these observations, it would appear that HA by means of their surface activity, can alter such cell membrane properties as membrane fluidity.

S.A. Visser

Papers

Physiological action of humic substances on microbial cells

Surface-active effects of humic acids on potato cell membrane properties