6. Biological Characteristics of Soil. ENVS 334: Applied Soil Science and Land Management INSTR.: R.M. Bajracharya

Transcription

1 6. Biological Characteristics of Soil ENVS 334: Applied Soil Science and Land Management INSTR.: R.M. Bajracharya 1

2 Soil biological properties Organisms, both animals (fauna/micro-fauna) and plants (flora/micro-flora) are important in the overall quality, fertility and stability of soil. They are responsible for the formation of humus, a product of OM degradation and synthesis. Moreover, organisms aid in the physical manipulation, mixing, and formation of soil & its structural stabilisation. Soils contain a vast number and wide range of organisms A greater proportion of these belong to the plant family Organisms are important in the myriad of biochemical reactions and intricate biological processes that take place within the soil. 2

3 Soil Organisms Organisms (biological component) of the soil play major roles in: Nutrient cycling & release (breakdown of organic compounds) Biochemical weathering of minerals & soil development Ameliorating soil physical & chemical properties Without this living component, the mere accumulation of the mineral fraction would not be soil. Soil organisms include plants and animals. Majority of soil organisms are plants (microflora), but animals are equally important (have more physical role) Most are microscopic, i.e., microflora and microfauna. 3

4 Animals Major groups of organisms of common occurrence in soils. Macro Micro Live mainly on plant materials Mostly predatory Roots of higher plants Predatory, parasitic or live on plant tissue Small mamals; insects; millipedes; mites; slugs; snails; earthworms Moles; Insects; mites; centipedes; spiders Nematodes Protozoa Rotifers Plants Algae Fungi Green; Blue-green; Diatoms Mushrooms; Yeasts; Molds Actinomycetes of many kinds Bacteria Aerobic Anaerobic Autotrophic Heterotrophic 4

5 Group of Organisms Biomass (g/m 2 ) Grassland Oak Forest Spruce Forest Herbivores Detritivores: Large Small Predators TOTAL Organisms Values commonly found in surface soil No./sq. meter No. per gram Biomass (kg/hfs) Microflora: Bacteria Actinomycetes Fungi Alage Microfauna: Protozoa Nematoda Earthworms Others

6 Earthworms Earthworms are the most important of the soil macro-animals. More than 1800 species known worldwide. Length ranges from few cm to 3 m long (topics) They ingest soil along with OM & create pores/ channels (improve aeration & drainage). Numbers range from <1/m 2 to >500/m 2 & turnover as much as 250 mt/ha/year of soil. Casts are higher in bacteria, OM & available plant nutrients than the soil itself. Hence, earthworms ameliorate soil in many ways, physically and biochemically 6

7 Termites and Ants Important in OM breakdown & soil manipulation Turnover 10s to 100+ t/ha/y of soil ~2000 species of termites found mostly in tropics & subtropics (savannahs and forests). Termites ( white ants ) build extensive & large mounds upto 6 m in height and >6 m deep. Mounds abandoned after years Effects of termites variable; may disrupt crop production soil less fertile, but aid in soil formation in the tropics. Ants have less widespread & more local influence on soils. Some species have exceptional ability to breakdown woody materials. 7

8 Soil Microfauna Of the many microscopic animals that live in the soil, 3 groups are of some importance: Nematodes ( threadworms or eelworms ): Found in relatively large numbers in nearly all soils >1000 species known May be beneficial (saprophytic) or detrimental to crops Parasitic nematodes, such as Heterodera sp. infest roots Protozoa among simplest form of animal life Most varied and numberous of the soil microfauna Unicellular, but much larger (5-100 μm) than bacteria >250 species isolated; include cilliates, flagellates, amoeba Rotifers thrive under moist conditions (swampy) About 100 species; active mainly in organic soils or wet areas of mineral soils. 8

9 Microscopic soil animals Nematode [magnified ~120 times] Rotifers: (L) Rotaria rotatorial; (R) Philodina acuticornus Ciliated protozoan (Glaucoma scintillaus) 9

10 Soil Flora Roots of Higher Plants Plant roots are constantly growing & dying in soil, thereby, supplying soil organisms w/ food+energy. Roots also physically modify soils by creating stresses & strains enhancing aggregation; create channels. They constantly release exudates, mucilages, enzymes, as well as, dead cells materials leading to the formation of humus to significant depths in the soil. Upon harvesting of crops, 15-40% of above ground weight of plants is left in the ground as root mass. Rhizosphere is the zone immediately surrounding active young roots. Here microorganisms are ~100 time greater in number Organic acids solubilize plant nutrients making them readily available for plant uptake. 10

11 The Rhizosphere Bacteria associated with roots of wheat [magnified 3900 X] 1-simple exudates; 2-secretions (from metabolic processes); 3-plant mucilages; 4-mucigel (gelatinous mucilage-soil mixture); 5-lyzates (cmpds released through digestion of cells by bacteria) 11

12 Soil Alage Soil algae are found near the surface of soil (need light for photosynthesis). They are active mostly under wet conditions. Several hundred species identified; four major groups in soils: Blue-green: most common; grow in grasslands and wet soils such as paddy fields Green: survive under non-flooded conditions and under low ph (acidic) soils Yellow-green: less common Diatoms: tend to occur in old gardens under drier conditions. Blue-green alage growing within leaves of Azolla (aquatic fern) in paddy fields fix nitrogen. 12

13 Soil Fungi There are over 690 species of soil fungi identified. Important in transforming soil constituents (all types of organic comps cellulose, starches, gums and lignin). Fungi include yeasts, molds & mushrooms: Yeasts are uncommon in soil habitats. Molds: filamentous and microscopic to semi-microscopic Develop in all ranges of soil ph (acidic to alkaline) Especially thrive in acidic surface soils (e.g., forests) Common genera: Penicillium, Mucor, Fusarium, Aspergillium Mushroom fungi: found in moist areas with high OM Common in forests and grasslands Decompose woody tissue (lignins) Have mass of hyphae below the ground surface Significant impacts locally 13

14 Mycorrhizae Symbiotic fugus-root assoc. markedly increases avail. Of certain essential nutrients to plants, esp. in infertile soils. P, Zn, Cu, Ca, Mg, Mn & Fe Of economic importance in production of wheat, potato, beans, sugarcane, cassava & dryland rice. Ectomycorrhiza: assoc. more with tree roots (pine, birch, oak, spruce, fir, etc.) Endomycorrhiza: assoc. w/ annual & perennial crops + trees (apple, maple, poplar, coffee, rubber, etc.) VAM: vesicular arbuscular mycorrhizae (most important) 14

15 Actinomycetes These are filamentous and similar to molds, but mycelial threads are smaller than fungal hyphae. Unicellular and similar in size to larger bacteria. Grow best in moist but well aerated soil conditions. Most suitable ph is (near neutral) Actinomycetes remain active even under low moisture conditions. Actinomycete threads 15

16 Bacteria in Soils Bacteria are single-celled & among the simplest of life forms; 4-5 μm in length; approach clay-size Rapid reproduction & response to envir. changes Variable shapes: round, spiral, rod-like Rod-shaped most common in soils. May be autotrophic or heterotrophic; in soils, most are heterotrophs (get energy & C from OM) Bacteria participate vigorously in all organic transactions in the soil; they are most numerous and often exceed both fungi & actinomycetes. They have nearly exclusive action in nitrification, sulfur oxidation & N-fixation in soil. 16

17 General pathway of OM breakdown and role of organisms Energy and CO2 Input Energy and CO2 loss Detritivores Carnivores Detritus Primary consumers Secondary consumers Tertiary consumers Soil humus Microphytic feeders Feces and dead bodies Soil microflora 17

18 OM decomposition in soil Detritivores (ie. Primary consumers-micro-flora/fauna) initiate the decomposition of detritus (organic debris) They utilise the energy in plant residues as main food source Microflora mainly act through chemical decomposition Action of fauna is both physical and chemical Secondary consumers join in: Mites, snails, beetles, springtails, earthworms, etc. These include carnivores that feed on animals and microphytic feeders that consume bacteria, fungi, algae Tertiary consumer further prey upon the primary or secondary consumers Even in the latter cases, however, microflora still play a major role they are active within the digestive tract of many animals. Moreover, microflora are mainly responsible for further decomposing organic material in animal feces and dead bodies of animals. Hence, they are known as the ultimate decomposers. 18

19 Composition of organic matter in soil 40% 8% 8% H Ash C 75% 25% O 44% Water Dry matter 25% 5% 10% Sugars and Starches: 1-5% Hemicelluloses: 10-30% Cellulose: 20-50% 60% Fats, waxes Proteins Carbohydrates Lignins 19

20 Rate of OM decomposition Much of OM undergoes enzymatic oxidation with carbon dioxide, water and heat released as the main products [C, 4H] + 2O 2 CO 2 + 2H 2 O + energy (heat) Essential elements (N, P, S) are released/immobilised by series of specific reactions for each element Compounds resistant to microbial action are ultimately formed through the decomposition processes (SOM or humus) Rate of decomposition of different compounds: Sugars, starches & simple proteins rapidly decomposed Crude proteins Hemicelluloses Cellulose Lignins, fats, waxes, etc. Very slowly decomposed 20

21 Distribution of SOM in different soil types Forest soil Grassland soil Well drained Poorly drained Well drained Poorly drained Soil OM (%) Soil OM (%) Soil depth (cm) Soil depth (cm) 21

22 SOM (humus) and C:N ratio of soils Soil organic matter is a stable, complex mixture of dark amorphous and colloidal substances modified from original tissue or synthesized by soil organism called humus. Humus is not a single substance, but a mixture of complex compounds. It is thought to consist of: Fulvic acid low molecular weight; light coloured; acid & alkali soluble Humic acid medium molecular weight & colour; alkali soluble, acid insoluble Humin highest molecular weight; darkest colour; acid & alkali insoluble 22

23 Humus & C:N Ratios of Soils Humus has high CEC ( cmol c /kg), low plasticity & cohesion. The carbon-nitrogen ratio of soils range from 8:1 to 15:1, being most frequently between 10:1 and 12:1 compared to higher values for plant material and slightly lower for microbes (4:1 to 9:1). C:N ratio is important because of competition for N among organisms when plant residues are added, leading to temporary nitrate depression; and also because N influences the maintenance of soil OC levels (soil C:N ratios remain relatively constant). 23

24 Organism activity, N availability and humus formation High C:N residues added Organism activity Soil N level Low C:N residues remain Increase N-depression period Compounds in original tissue Synthesized by organisms Soil humus New soil humus level TIME 24