CUET UG Biology Booster Test 3-Nutrient Cycling and Ecosystem Services -

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CUET UG Biology Booster Test 3-Nutrient Cycling and Ecosystem Services

⏱️ 30 Minutes

❓ 20 Questions

📌 Answers are locked once submitted — results and explanations appear at the end.

CUET UG Biology Booster Test 3-Nutrient Cycling and Ecosystem Services

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QUESTION 1 OF 20

Analytically speaking, how does the "storage" phase in nutrient cycling fundamentally differ from the retention of energy within an ecosystem's biomass?

Answer Review

1 Analytically speaking, how does the "storage" phase in nutrient cycling fundamentally differ from the retention of energy within an ecosystem's biomass?

A Nutrients can be stored long-term in environmental reservoirs for repeated use, whereas trapped energy is continually lost as heat and cannot be cycled.

B Energy can be stored indefinitely in environmental reservoirs, while nutrients cannot.

C Nutrients are only stored in the living biomass, whereas energy is stored in the Earth's crust.

D Both nutrients and energy follow identical closed-loop storage mechanisms.

💡 Short Explanation

Nutrients cycle through reservoirs (crust, atmosphere). Energy is unidirectional and lost as heat (thermodynamics). Nutrients are reused; energy is not.

✅ Correct Answer Explanation

Matter (nutrients) is conserved and reused within the biosphere via cycles. Energy, however, follows the laws of thermodynamics; it is used for metabolic work and dissipated as heat. Once lost as heat, it cannot be recovered or recycled, requiring constant solar replenishment.

❓ Why Other Options Are Incorrect

Option B → Energy cannot be stored indefinitely; it is entropy-prone.
Option C → Nutrients are stored in both biomass and abiotic reservoirs; energy is not "stored" in the crust.
Option D → They follow fundamentally different thermodynamic paths.

🎯 Strategy

Used: Elimination

Application: Identifying the distinct thermodynamic fates of matter vs. energy.

Final Logic: Nutrients cycle/reuse; Energy dissipates/lost.

🧠 Memory Trick

"Matter cycles, Energy flies."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Nutrient Cycling

2 Evaluate the following analytical statements:
I. Because energy flow is unidirectional and dissipative, the repeated usage of nutrient elements is the only way an ecosystem can sustain its biological structure over time.
II. Decomposers represent the critical intersection where the unidirectional flow of energy terminates, but the closed-loop repeated usage of nutrients is renewed.
III. Nutrient cycling depends entirely on the inflow of deep-sea hydrothermal energy. Which of the statements are correct?

A I, II, and III

B II and III only

C I and III only

D I and II only

💡 Short Explanation

I is correct: Recycling nutrients allows sustainability given the non-recyclable nature of energy. II is correct: Decomposers release inorganic nutrients back into the cycle. III is incorrect: Nutrient cycling is driven primarily by solar energy and biogeochemical processes, not just deep-sea vents.

✅ Correct Answer Explanation

Statements I and II correctly identify the necessity of nutrient recycling and the role of decomposers as the "link" between the unidirectional energy flow and the cyclical nutrient flow. Statement III is overly specific and incorrect as a general principle.

❓ Why Other Options Are Incorrect

A, B, C → All include the false statement III.

🎯 Strategy

Used: Elimination

Application: Testing the relationship between energy flow and nutrient cycles.

Final Logic: Recycling is essential for sustainability.

🧠 Memory Trick

"Decomposers = Recycling Bridge."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Ecosystem Function

3 The rapid compensation of localised carbon deficits within a terrestrial ecosystem is primarily facilitated by:

A The slow weathering of crustal rocks over millennia.

B The vast and highly mobile nature of atmospheric and hydrospheric reservoirs associated with gaseous cycles.

C The absolute cessation of soil microbial activity.

D The strict limitation of primary productivity to aquatic zones.

💡 Short Explanation

Carbon is a gaseous cycle. Atmospheric/Hydrospheric reservoirs are globally mobile. Allows rapid redistribution compared to slow sedimentary weathering.

✅ Correct Answer Explanation

Because carbon is part of a gaseous cycle, it is held in the atmosphere and hydrosphere. These reservoirs are highly dynamic and mobile, allowing for rapid exchange and compensation of local carbon deficits.

❓ Why Other Options Are Incorrect

Option A → Weathering is too slow for rapid compensation.
Option C → Cessation of activity would worsen deficits.
Option D → Productivity occurs in terrestrial zones too.

🎯 Strategy

Used: Substitution

Application: Recognizing the property of "gaseous" cycles (mobility).

Final Logic: Gaseous cycle = High mobility.

🧠 Memory Trick

"Gaseous = Fast/Mobile."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Gaseous Cycle

4 Trace the functional path of a carbon atom through a complete ecosystem cycle, highlighting productivity and decomposition:
1. Carbon dioxide is released back to the atmospheric reservoir.
2. Heterotrophic decomposers catabolise the dead biomass.
3. Atmospheric carbon is fixed by plants via Gross Primary Productivity (GPP).
4. Organic carbon is transferred to primary consumers.

A 1, 3, 2, 4

B 3, 4, 2, 1

C 3, 2, 4, 1

D 4, 2, 1, 3

💡 Short Explanation

Step 1: Atmospheric fixation (3). Step 2: Consumption (4). Step 3: Decomposition (2). Step 4: Return to atmosphere (1).

✅ Correct Answer Explanation

Carbon begins as CO2(fixed in GPP, 3), moves to consumers (4), is broken down by decomposers (2), and is finally released back as CO2to the atmosphere (1).

❓ Why Other Options Are Incorrect

B, C, D → Incorrect logical sequences regarding trophic flow.

🎯 Strategy

Used: Substitution

Application: Tracing the carbon pathway through biotic and abiotic components.

Final Logic: Atmosphere → Producer → Consumer → Decomposer → Atmosphere.

🧠 Memory Trick

"Fix → Eat → Rot → Release."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Carbon Cycle

5 Which of the following features is analytically NOT characteristic of a sedimentary cycle (like phosphorus) when contrasted with a gaseous cycle?

A Its primary reservoir is located in the Earth's crust.

B It depends on the release of elements through the weathering of rocks over time.

C It features rapid, global, and uniform mixing through the atmosphere.

D It involves the repeated use of nutrient elements.

💡 Short Explanation

Sedimentary cycles (Phosphorus) do not have a major atmospheric phase. They are slow and localized compared to the rapid mixing of gases.

✅ Correct Answer Explanation

Sedimentary cycles are slow, localized, and lack a significant gaseous (atmospheric) phase. Only gaseous cycles (like Carbon, Nitrogen) feature rapid, global atmospheric mixing.

❓ Why Other Options Are Incorrect

Option A, B → These are classic characteristics of sedimentary cycles.

Option D → All nutrient cycles involve repeated use.

🎯 Strategy

Used: Elimination

Application: Differentiating cycle types based on mixing rates and reservoirs.

Final Logic: Sedimentary = Slow/No Atmospheric phase.

🧠 Memory Trick

"Sediment = Slow; Gas = Fast."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Sedimentary Cycle

6 Match the deep concepts regarding nutrient pathways:

Column 1	Column 2
1. Rapid exchange reservoir	i. Mineralisation
2. Sedimentary cycling	ii. Atmosphere/Hydrosphere
3. Release from humus	iii. Earth's crust as reservoir
4. Unidirectional factor	iv. Energy dissipation

A 1-i, 2-iv, 3-ii, 4-iii

B 1-iii, 2-ii, 3-iv, 4-i

C 1-ii, 2-iii, 3-i, 4-iv

D 1-iv, 2-i, 3-iii, 4-ii

💡 Short Explanation

Rapid exchange = Atmosphere/Hydrosphere (ii). Sedimentary cycling = Earth's Crust (iii). Release from humus = Mineralisation (i). Unidirectional = Energy dissipation (iv).

✅ Correct Answer Explanation

Matches: Rapid gaseous exchanges use the atmosphere (ii). Sedimentary cycles are rooted in crustal reservoirs (iii). Humus degradation to inorganic form is mineralisation (i). Energy loss is always unidirectional (iv).

❓ Why Other Options Are Incorrect

A, B, D → Incorrect pairings of concept and mechanism.

🎯 Strategy

Used: Option Grouping

Application: Mapping ecological terms to their definitions.

Final Logic: Match definitions.

🧠 Memory Trick

"1-ii, 2-iii, 3-i, 4-iv."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Nutrient Cycling

📄 Passage

Humification and mineralisation occur during decomposition in the soil. Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients. The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation

Analytically, why is mineralisation a functionally critical step following humification?

A It rapidly converts highly resistant organic humus back into usable inorganic nutrients for autotrophs.

B It turns inorganic salts into colloidal humus to prevent leaching.

C It permanently traps nutrients in an amorphous state.

D It allows humus to photosynthesise independently.

💡 Short Explanation

Humus is resistant (slow decay). Mineralisation is the process that finally unlocks those nutrients. Vital for autotrophs.

✅ Correct Answer Explanation

Humification stores nutrients in a stable form. Mineralisation is the essential step where microbes break that stable form down to return inorganic nutrients (N, P, etc.) to the soil, where producers can take them up again.

❓ Why Other Options Are Incorrect

Option B → Mineralisation is the opposite of forming humus.
Option C → It releases, not traps, nutrients.
Option D → Humus cannot photosynthesize.

🎯 Strategy

Used: Substitution

Application: Defining the necessity of nutrient release.

Final Logic: Mineralisation = Unlock nutrients for plants.

🧠 Memory Trick

"Humus = Bank; Mineralisation = ATM."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Decomposition

📄 Passage

What physical and chemical properties of humus necessitate a slow rate of microbial degradation?

A Its high solubility in water and lack of carbon.

B Its colloidal nature and high resistance to microbial action.

C Its complete lack of any inorganic nutrients.

D Its immediate dissipation as heat into the environment.

💡 Short Explanation

Humus is colloidal. Its complex organic structure makes it difficult for microbes to break down quickly.

✅ Correct Answer Explanation

Humus is a dark, amorphous substance resulting from humification. Its colloidal nature and complex chemical bonding make it highly resistant to microbial breakdown, ensuring it degrades slowly.

❓ Why Other Options Are Incorrect

Option A → It is not highly soluble.
Option C → It acts as a nutrient reservoir; it is not devoid of them.
Option D → It is organic matter, not heat.

🎯 Strategy

Used: Substitution

Application: Recalling properties of humus.

Final Logic: Colloidal/Resistant = Slow.

🧠 Memory Trick

"Colloidal = Stable."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Decomposition

9 Analyze the interplay of factors affecting decomposition:
I. A detritus rich in nitrogen and sugars decomposing in a warm, moist climate will exhibit the highest rate of mineralisation.
II. A detritus rich in lignin decomposing in a cold, dry climate will result in a rapid depletion of soil organic matter.
III. Soil moisture influences the rate at which bacterial and fungal enzymes can perform catabolism. Select the correct analytical statements:

A I and II only

B I and III only

C II and III only

D I, II, and III

💡 Short Explanation

I is correct: Nitrogen/sugars + Warm/Moist = Fast. II is incorrect: Lignin + Cold/Dry = Very slow (opposite of rapid depletion). III is correct: Moisture affects enzymatic activity.

✅ Correct Answer Explanation

Statement I describes ideal conditions for fast decomposition. Statement III correctly identifies moisture as a regulator for microbial enzymes. Statement II is false because lignin and cold/dry conditions dramatically slow (not rapid depletion) decomposition.

❓ Why Other Options Are Incorrect

A, C, D → All include the false statement II.

🎯 Strategy

Used: Elimination

Application: Identifying factors that accelerate vs. inhibit decomposition.

Final Logic: Warm+Moist = Fast; Cold+Dry = Slow.

🧠 Memory Trick

"N-Sugar = Fast; Lignin = Slow."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Decomposition

10 If a highly productive terrestrial ecosystem experiences a sudden, long-term shift to severe anaerobiosis, what dual effect will this have on its structural dynamics?

A A massive increase in GPP and rapid nutrient cycling.

B A severe decline in mineralisation rates and an extensive build-up of un-decomposed organic materials.

C The immediate transition of the carbon cycle into a sedimentary cycle.

D The acceleration of the grazing food chain at the expense of the detritus food chain.

💡 Short Explanation

Anaerobiosis = No oxygen for aerobic microbes. Microbial decomposition stalls. Organic matter builds up (e.g., peat).

✅ Correct Answer Explanation

Anaerobiosis prevents aerobic decomposition. This halts mineralisation (nutrient release) and causes organic matter (detritus/humus) to accumulate because it cannot be broken down, drastically changing the ecosystem's nutrient availability.

❓ Why Other Options Are Incorrect

Option A → Mineralisation would decline, not increase.
Option C → The carbon cycle remains gaseous.
Option D → The detritus food chain (driven by microbes) would actually be suppressed.

🎯 Strategy

Used: Elimination

Application: Understanding the impact of environmental changes on decomposition.

Final Logic: Anaerobic = No decomposition = Build-up.

🧠 Memory Trick

"No Air = No Decay = Pile-up."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Factors affecting Decomposition

11 The ultimate efficiency and functional boundary of an ecosystem's inorganic-to-organic conversion rate (Gross Primary Productivity) is strictly governed by the fact that:

A Plants only capture 2-10 per cent of the Photosynthetically Active Radiation (PAR).

B All incident solar radiation is 100% PAR.

C Decomposers fix solar energy faster than plants.

D The earthworm completely bypasses photosynthesis.

💡 Short Explanation

Most sunlight is not PAR (Photosynthetically Active Radiation). Of the PAR that is available, plants only capture a very small fraction (2–10%) to convert into biomass. This determines the overall productivity limit of the ecosystem.

✅ Correct Answer Explanation

The conversion of radiant energy to chemical energy (biomass) is limited by the amount of light that is actually usable (PAR) and the efficiency with which plants can capture it. Only 2–10% of PAR is successfully used, setting the "functional boundary" for GPP.

❓ Why Other Options Are Incorrect

Option B → Only about 50% of incident solar radiation is PAR.
Option C → Decomposers do not perform photosynthesis.
Option D → Earthworms are heterotrophs; they do not perform photosynthesis, but they also don't "bypass" it—they depend on it.

🎯 Strategy

Used: Substitution

Application: Applying knowledge of photosynthetic efficiency.

Final Logic: Low Capture Efficiency = Limit on GPP.

🧠 Memory Trick

"PAR 2-10%."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Productivity

12 Which of the following processes does NOT contribute to the universal tendency of ecosystems toward increasing disorderliness (entropy) and energy loss?

A Respiration losses (R) by autotrophs

B Dissipation of heat to the environment during trophic transfers

C The cycling of inorganic nutrients from decomposers back to autotrophs

D The death of organisms and subsequent microbial catabolism

💡 Short Explanation

Respiration, trophic transfer, and decomposition all involve energy dissipation (entropy). Nutrient cycling is the reuse of matter, which does not inherently contribute to the loss of energy (it is a matter cycle).

✅ Correct Answer Explanation

Nutrient cycling involves the conservation and reuse of elements (atoms), which is separate from the dissipative flow of energy. Options A, B, and D all involve the degradation of high-quality energy into low-quality heat (entropy).

❓ Why Other Options Are Incorrect

Option A, B, D → All are processes where chemical energy is lost as heat, increasing disorder.

🎯 Strategy

Used: Odd One Out

Application: Distinguishing between matter cycling (non-dissipative) and energy flow (dissipative).

Final Logic: Energy dissipates; Matter cycles.

🧠 Memory Trick

"Cycle ≠Entropy."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Energy Flow/Laws of Thermodynamics

13 Match the specific organismal actions to their analytical functional representations:

Column 1	Column 2
1. Sparrow digesting a worm	i. First trophic level (Producer)
2. Phytoplankton fixing PAR	ii. Detritivore (Fragmentation)
3. Fungi degrading detritus	iii. Secondary Consumer
4. Earthworm breaking down bark	iv. Saprotroph (Decomposer)

A 1-iii, 2-i, 3-ii, 4-iv

B 1-iii, 2-i, 3-iv, 4-ii

C 1-iv, 2-iii, 3-i, 4-ii

D 1-i, 2-ii, 3-iii, 4-iv

💡 Short Explanation

Sparrow eating worm = Secondary consumer (iii). Phytoplankton = Producer (i). Fungi = Saprotroph/Decomposer (iv). Earthworm = Detritivore (ii).

✅ Correct Answer Explanation

Matches: Sparrow eating a primary consumer (worm) is a secondary consumer (iii). Phytoplankton are producers (i). Fungi break down organic matter as saprotrophs (iv). Earthworms perform physical fragmentation (ii).

❓ Why Other Options Are Incorrect

A, C, D → Incorrect mapping of roles.

🎯 Strategy

Used: Option Grouping

Application: Classifying organisms by their ecological function.

Final Logic: 1-iii, 2-i, 3-iv, 4-ii.

🧠 Memory Trick

"1-iii, 2-i, 3-iv, 4-ii."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Trophic Structure

14 Arrange the following components to demonstrate a complete flow from the Grazing Food Chain (GFC) into the Detritus Food Chain (DFC) based on nutrition sources:
1. Zooplankton (Primary Consumer)
2. Fungi and Bacteria (Saprotrophs)
3. Dead Biomass (Detritus)
4. Phytoplankton (Producer)
5. Fish (Secondary Consumer)

A 4, 5, 1, 2, 3

B 4, 1, 5, 3, 2

C 3, 2, 4, 1, 5

D 1, 4, 5, 3, 2

💡 Short Explanation

GFC start: Phytoplankton (4) → Zooplankton (1) → Fish (5). Transition: Death leads to Dead Biomass (3). DFC: Fungi/Bacteria (2) break down biomass.

✅ Correct Answer Explanation

Energy flows from producers (4) to consumers (1, 5). When these organisms die, they become Detritus (3), which then fuels the DFC decomposers (2).

❓ Why Other Options Are Incorrect

B, C, D → Incorrect logical flow of energy from GFC to DFC.

🎯 Strategy

Used: Substitution

Application: Tracing energy transfer across two connected food chains.

Final Logic: GFC → DFC (via death).

🧠 Memory Trick

"Eat → Eat → Die → Decay."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Food Chains/Detritus vs Grazing

15 If the economic value of an ecosystem were solely judged by its tangible structural biomass (standing crop), which crucial functional component would be disastrously ignored?

A The physical dimensions of the ecosystem.

B The ecosystem services, such as atmospheric purification and nutrient regeneration.

C The species composition of top carnivores only.

D The incident solar radiation before it enters the atmosphere.

💡 Short Explanation

Standing crop is just the "weight" of the organisms. Services are the "functions" nature performs. Ignoring functions ignores the true value of an ecosystem.

✅ Correct Answer Explanation

Ecosystem services (like clean air, nutrient cycling, pollination) are critical to planetary health. Focusing only on "biomass" (standing crop) values only the product, not the functional, life-sustaining services the ecosystem provides.

❓ Why Other Options Are Incorrect

Option A, C, D → These are secondary to the broad functional utility provided by ecosystem services.

🎯 Strategy

Used: Substitution

Application: Recognizing the difference between structural (biomass) and functional (services) metrics.

Final Logic: Services = True functional value.

🧠 Memory Trick

"Value ≠Weight."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Ecosystem Services

16 The ability of a forest to continually purify air and water (an ecosystem service) relies fundamentally on the integration of:

A Anaerobic decomposition and strict detritus buildup.

B Its abiotic components interacting seamlessly with its producers, consumers, and decomposers as a functional unit.

C The absolute isolation of its food webs from abiotic factors.

D Only its secondary consumers.

💡 Short Explanation

Ecosystem services require a functioning ecosystem. Ecosystem definition: Biotic + Abiotic interaction.

✅ Correct Answer Explanation

Purification occurs because the entire forest system functions as one. Plants (producers) cycle gases, soil/water (abiotic) store and filter substances, and decomposers break down pollutants. It is the integration of all parts that creates the service.

❓ Why Other Options Are Incorrect

Option A → Anaerobic buildup is usually detrimental to forest services.
Option C → Ecosystems rely on biotic-abiotic integration.
Option D → Services are system-wide, not just secondary consumers.

🎯 Strategy

Used: Substitution

Application: Defining the basis of ecosystem functionality.

Final Logic: Service = Integrated Ecosystem.

🧠 Memory Trick

"Integration = Purification."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Ecosystem Structure

17 Why must crop fields, still be analyzed under the principles of ecosystem structure and function?

A Because they completely defy the laws of thermodynamics.

B Because they still require solar input, feature primary productivity, and involve abiotic-biotic interactions (e.g., soil microbes).

C Because they naturally contain more vertical stratification than a tropical forest.

D Because humans act as the sole primary producers in these systems.

💡 Short Explanation

Man-made ecosystems still follow the same laws of physics and biology. They have producers, energy flow, and microbial soil interactions.

✅ Correct Answer Explanation

Even artificial ecosystems like crop fields follow fundamental ecosystem principles: solar energy drives productivity, and they rely on the interplay between plants and soil organisms (microbes).

❓ Why Other Options Are Incorrect

Option A → No system defies thermodynamics.
Option C → Forests have more vertical stratification.
Option D → Plants are the primary producers; humans manage them.

🎯 Strategy

Used: Substitution

Application: Applying ecological principles to man-made systems.

Final Logic: Man-made = Still an Ecosystem.

🧠 Memory Trick

"Farm = Ecosystem."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Artificial Ecosystems

18 When evaluating a small home aquarium, which of the following natural components is typically NOT inherently self-sustaining and usually requires human intervention?

A The constant input of solar/radiant energy and balanced nutrient cycling

B The presence of water as an abiotic component

C The biological existence of algae

D The physical boundary of the system

💡 Short Explanation

In nature, energy and nutrients are self-regulating. In an aquarium, humans must provide light and perform cleaning/filtering to maintain the balance.

✅ Correct Answer Explanation

Aquariums require human intervention (feeding, light control, filtration) to mimic the self-regulation found in natural, large-scale ecosystems. Without this, nutrient imbalances (e.g., waste buildup) or light deficits would collapse the system.

❓ Why Other Options Are Incorrect

Option B, C, D → These are physical/biotic components inherent to the setup itself, not the maintenance processes.

🎯 Strategy

Used: Elimination

Application: Differentiating between natural self-regulation and artificial maintenance.

Final Logic: Man-made → Human intervention needed.

🧠 Memory Trick

"Aquarium = Manual care."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Artificial Ecosystems

19 Which of the following statements presents a functionally INCORRECT summary of energy dynamics in an ecosystem?

A A constant input of solar energy is the basic requirement to counteract the universal tendency toward increasing disorderliness.

B Energy flow is unidirectional from producers through to decomposers.

C Plants capture a maximum of 10% of the photosynthetically active radiation.

D The amount of energy increases at successive trophic levels due to biomass accumulation.

💡 Short Explanation

Energy decreases at successive trophic levels (10% law). Biomass generally decreases. Statement D is the only false one.

✅ Correct Answer Explanation

This statement is incorrect because energy flow follows the 10% law; there is a significant loss of energy as heat at each transfer level. Energy never increases as it moves to higher levels.

❓ Why Other Options Are Incorrect

Option A, B, C → These are all functionally correct descriptions of ecosystem energy dynamics.

🎯 Strategy

Used: Elimination

Application: Testing the "10% law" of energy transfer.

Final Logic: Energy decreases, it never increases.

🧠 Memory Trick

"10% = Decrease."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Energy Flow

20 In the context of ecosystem functioning, if decomposition processes were perfectly 100% efficient without any humification delay, what would be the analytical impact on nutrient cycling?

A Nutrients would be locked away permanently, destroying the cycle.

B There would be no colloidal reservoir of nutrients in the soil, leading to rapid fluctuations in inorganic nutrient availability for autotrophs.

C The ecosystem would shift entirely to a gaseous cycle.

D Ecosystem services would cease entirely.

💡 Short Explanation

Humus is the colloidal reservoir (slow release). Without humus, nutrients would be flushed out (leaching) or consumed too quickly, causing unstable spikes and crashes.

✅ Correct Answer Explanation

The humification process creates a stable nutrient reservoir (colloid). Removing this "storage" phase would mean nutrients are immediately mineralized and prone to leaching or rapid uptake, leading to highly unstable nutrient levels in the soil for plants.

❓ Why Other Options Are Incorrect

Option A → Mineralisation releases, not locks away.
Option C → The cycle type is determined by the element, not the decomposition rate.
Option D → Services would change, but not necessarily cease.

🎯 Strategy

Used: Substitution

Application: Analyzing the functional role of humus as a buffer.

Final Logic: No Humus = No Buffer = Unstable nutrients.

🧠 Memory Trick

"Humus = Stable reservoir."

📚 Class XII – Biology – Chapter: Ecosystem – Topic: Decomposition/Humus role

CUET UG Biology Booster Test 3-Nutrient Cycling and Ecosystem Services

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