Agricultural soils have a C:N ratio of around 10-13:1. What would happen to the available N in these soils if straw (C:N of 40:1) were added to them?

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Agricultural soils have a C:N ratio of around 10-13:1. What would happen to the available N in these soils if straw (C:N of >40:1) were added to them?

BlackTom AI · Accepted Answer

Question restatement: The soil has a typical agricultural C:N ratio of about 10–13:1. If straw, which has a high C:N ratio (>40:1), is added to such soil, what happens to the soil’s available nitrogen (N)?

Option 1: Nitrogen from the soil available N pool will be temporarily immobilized. This is the core concept: when a carbon-rich material with a high C:N ratio is added, soil microbes decompose it but require additional nitrogen to build their own biomass. Since the straw provides relatively little N, microbes scavenge N from the soil's mineral N pool, converting it into microbial biomass and thereby reducing the pool of readily available inorganic N temporarily. This immobilization means less N is available for plant uptake in the short term.

Option 2: There is an increase in soil available N pool due to mineralization of straw and release of mineral N to soil. This would be true if the substrate had a favorable C:N ratio (low enough) or if microbial activity released N; however, with straw’s very high C:N, microbial metabolism tends to immobilize N rather than mineralize it, at least in the short term. Therefore this statement contradicts the typical immobilization process observed with high-C substrates.

Option 3: There is no effect on the mineral N pool. In reality, the addition of a high-carbon material often causes a measurable shift in the N pool due to microbial uptake for biomass synthesis. Saying there is no effect ignores the well-documented immobilization response that accompanies high C:N inputs, making this option inaccurate.

Additional context: The balance between carbon inputs and nitrogen availability governs whether mineralization or immobilization dominates. High C inputs relative to N (as with straw) generally drive immobilization, slowing N mineralization temporarily until microbial turnover and straw decomposition progress.

Summary of why option 1 is correct and the others are not: The introductory C:N mismatch (straw >40:1 vs soil 10–13:1) creates demand for soil N by microbes, leading to temporary immobilization, whereas option 2 describes mineralization that is unlikely under these conditions, option 3 ignores the expected microbial N uptake dynamics under high-C input.

MERGE_2025_0322 Quiz 4 Week 7 and 8 revision

Agricultural soils have a C:N ratio of around 10-13:1. What would happen to the available N in these soils if straw (C:N of >40:1) were added to them?

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