Passage:
In temperate agroforestry systems, intercropping leguminous trees such as *Alnus glutinosa* alongside grain crops is often advocated to reduce reliance on synthetic nitrogen fertilizers. Proponents highlight that *Alnus* roots form symbiotic associations with *Frankia* actinomycetes, fixing atmospheric nitrogen into organic compounds that eventually enrich the surrounding soil through leaf litter decomposition. However, recent biogeochemical studies reveal a critical temporal mismatch in this mechanism. While synthetic fertilizers release plant-available nitrate () instantaneously during the grain crops' peak vegetative growth phase in early spring, the mineralization of organic nitrogen from *Alnus* leaf litter requires microbial activity that peaks only when soil temperatures rise in late summer—weeks after the grain crops have completed their primary nitrogen uptake.
Consequently, excess unabsorbed nitrate mineralized late in the season frequently leaches into subsoil aquifers rather than benefiting the intended grain crops. Furthermore, long-term soil monitoring demonstrates that prolonged *Alnus* cultivation significantly accelerates soil acidification due to the net release of hydrogen ions during nitrification. This drop in soil pH subsequently immobilizes available phosphorus, a nutrient essential for root development in grain species. Thus, while leguminous intercropping elevates total soil nitrogen content, it fails to improve crop yield unless paired with lime applications or supplementary inorganic phosphorus, offsetting the net cost savings initially promised by biological nitrogen fixation.
Based on the passage, which of the following best describes why unsupplemented *Alnus* intercropping fails to enhance grain crop productivity?
- The temporal misalignment between nitrogen mineralization and crop absorption requirements, combined with subsequent soil acidification that restricts phosphorus availability, prevents crop yield improvements.Answer
- BThe symbiotic *Frankia* actinomycetes deplete available soil phosphorus during early spring to support atmospheric nitrogen fixation.
- CSynthetic nitrogen fertilizers accelerate leaf litter decomposition, causing nitrate to leach into subsoil aquifers before grain crops can absorb it.
- DThe primary benefit of *Alnus* intercropping is negated because synthetic fertilizers release nitrate too slowly during the early spring growth phase.
- ESubsoil aquifers become saturated with hydrogen ions, directly damaging the root structures of grain crops during late summer.