Gersmehl diagrams, soil fertility, energy flows and trophic levels

Topic 7.3 – Nutrient Cycling, Soil Fertility, Energy Flows & Trophic Levels in Tropical Rainforests and Savannas

Learning Objectives

• Construct and interpret Gersmehl diagrams for rainforest and savanna ecosystems.
• Describe vegetation succession (climatic climax, sub‑climax, plagioclimax) and explain how it influences nutrient fluxes.
• Identify the main soil orders found in each biome, relate their formation to climate and parent material, and explain their effect on nutrient retention.
• Explain the four core nutrient‑cycling processes (inputs, recycling, outputs, Δ‑store) and calculate trophic efficiencies.
• Analyse how climate, fire, grazing and human land‑use modify nutrient cycles and evaluate management options.

1. Overview of Nutrient Cycling

Essential elements (C, N, P, K, Ca, Mg…) continuously move between the biotic (plants, animals, microbes) and abiotic (soil, water, atmosphere) components of an ecosystem. In tropical rainforests and savannas the cycle is driven by four linked processes:

1. Primary production – photosynthesis fixes CO₂ into organic matter.
2. Decomposition – microbes and detritivores break down dead material, releasing nutrients.
3. Mineralisation & immobilisation – organic N, P, etc. are converted to inorganic forms (mineralisation) or taken up again by microbes (immobilisation).
4. Leaching & runoff – soluble nutrients are lost from the soil profile, especially during intense rains.

2. Vegetation Characteristics & Successional Stages

Vegetation type determines the timing and magnitude of nutrient inputs and outputs. The three successional categories required by the Cambridge syllabus are defined below.

Disturbance alters the balance between Input (litter, N fixation) and Output (leaching, volatilisation), thereby influencing soil fertility.

3. Soil Types, Profiles & Spatial Variation

Soil order reflects the combined influence of climate, parent material, topography and time. The table below summarises the three orders most common in tropical rainforests and savannas, adds a column on spatial variation, and links each to nutrient‑retention properties.

4. Gersmehl Diagrams – Quantifying Nutrient Fluxes

A Gersmehl diagram balances the four components of a nutrient budget:

Legend

• Input_i – atmospheric deposition, biological fixation, weathering of parent material.
• Recycling_i – mineralisation of litter, excretion, animal carcasses, mycorrhizal uptake‑release cycles.
• Output_i – leaching, gaseous loss (e.g., N₂O, NH₃), harvest/removal.
• ΔStore_i – net change in soil or biomass nutrient pool over the accounting period.

5. Soil Fertility & Nutrient‑Limiting Factors

For nitrogen, the change in the plant‑available pool can be expressed as:

Rainforest – Phosphorus Limitation

• High rainfall → intense weathering of silicate parent rock releases Al³⁺ and Fe³⁺.
• Al‑ and Fe‑oxides bind PO₄³⁻ strongly (chemical fixation), making P unavailable despite high total P.
• Biological adaptations: arbuscular mycorrhizae, specialised phosphatase enzymes, and P‑recycling via leaf litter.

Savanna – Nitrogen Limitation

• Seasonal heavy rains cause rapid nitrate (NO₃⁻) leaching from the shallow, often iron‑rich soils.
• Biological N fixation by leguminous grasses (e.g., Acacia spp.), cyanobacteria on grass stems, and some herbaceous legumes supplies most internal N.
• Fire releases organic N as NH₄⁺, but repeated burns also volatilise N as N₂, creating a net deficit over time.

6. Human & Climate Impacts on Nutrient Cycles

• Deforestation – removes litter layer, reduces recycling, increases leaching of N and P, and raises soil temperature, accelerating mineralisation.
• Fire & grazing – short‑term nutrient release (ash, volatilisation) but long‑term loss through erosion and soil compaction.
• Agriculture – synthetic N & P fertilizers boost inputs; excess leads to runoff, eutrophication, and alteration of downstream Gersmehl balances.
• Dust deposition – Sahel dust supplies ≈10 % of the P budget to West African savannas; dust pulses can temporarily relieve P limitation.
• Climate change – projected increases in temperature and altered precipitation patterns intensify leaching in savannas and may shift the balance between climax and plagioclimax states.

7. Case Studies

7.1 Amazon Basin – Post‑fire Nutrient Loss (2019‑2022)

• Two consecutive years of extensive forest fires released ~2 × 10⁶ t of carbon.
• Surface‑soil organic matter declined by 15 %.
• Gersmehl analysis showed P leaching rose to ~30 % of the pre‑fire P store; N fixation dropped 40 % because leguminous understory species were lost.
• Modelled recovery without active re‑forestation exceeds 30 years.

7.2 Southern African Savanna – Fire‑dust Interaction (2018‑2021)

• Annual prescribed burns (dry season) released 0.8 t ha⁻¹ of N as NH₄⁺, boosting short‑term plant growth.
• During the same period, dust storms from the Kalahari deposited ~5 kg P ha⁻¹ yr⁻¹, partially offsetting the strong P fixation of the underlying lateritic soils.
• Gersmehl budgets indicated that when fire frequency exceeded a 3‑year return interval, net N loss (>10 % yr⁻¹) outweighed the benefits of ash fertilisation.
• Management implication: integrate fire‑rotation with strategic planting of N‑fixing trees (e.g., Acacia mellifera) to maintain a positive ΔN.

8. Energy Flows & Trophic Levels

Energy available to herbivores (secondary consumers) using the 10 % rule:

• Rainforest: E_herbivores ≈ 120 g C m⁻² yr⁻¹.
• Savanna: E_herbivores ≈ 30 g C m⁻² yr⁻¹.

Trophic Structure Comparison

• Rainforest – 5–6 trophic levels: primary producers → primary herbivores → primary carnivores → secondary carnivores → apex predators → top‑level scavengers. High niche specialisation.
• Savanna – 3–4 trophic levels: primary producers → large grazing herbivores → apex carnivores (e.g., lions, cheetahs) → occasional scavengers. Dominance of generalist feeders.

9. Comparative Summary – Nutrient Cycling & Energy

10. Classroom Evaluation Activity

Think‑Pair‑Share – “Re‑forestation of degraded savanna with nitrogen‑fixing trees (e.g., Acacia spp.)”

1. Individually list two potential benefits and two possible drawbacks of the intervention.
2. Discuss with a partner and combine the lists.
3. Share with the class. The teacher records points and guides a brief evaluation focusing on:
   • How the Gersmehl nutrient balance changes (increase in Input_N via fixation, possible change in Output_N through altered leaching).
   • Effects on soil CEC and long‑term fertility (addition of organic matter, root‑exudate‑driven microbial activity).
   • Impacts on trophic structure (new woody habitat for browsers, potential increase in predator numbers).

11. Key Concepts Sidebar

12. Summary

Both tropical rainforests and savannas recycle nutrients, but they do so in fundamentally different ways. Rainforests rely on rapid decomposition, thick organic layers and strong internal recycling, leading to phosphorus limitation despite high total P. Savannas depend more on external inputs (dust, atmospheric N fixation) and suffer greater nitrogen loss through leaching and fire‑induced volatilisation. Energy flow follows the classic 10 % rule, yet the absolute energy available to higher trophic levels is far greater in rainforests because of their higher NPP. Human activities—deforestation, fire, agriculture—disrupt these balances, but informed management (e.g., planting N‑fixing trees, adjusting fire frequency) can restore more sustainable nutrient budgets and support richer trophic structures.