Impact of green manure crop species on rhizosphere soil phosphorus

Context

Green manure crops have the potential to improve phosphorus (P) use efficiency in agroecosystems by enhancing the mobilisation of soil P reserves.

Aims

This study investigated and quantified the short-term mobilisation and uptake of soil P in the rhizosphere of several green manure crops.

Methods

Five plant species/varieties (Lupinus angustifolius (lupin – early and late flowering varieties), Pisum sativum (pea), Cicer Arietinum (chickpea), and Fagopyrum escolentum (buckwheat)) were grown in two contrasting soils, pumice (1100 mg total P kg⁻¹, anion storage capacity 39%) and volcanic ash (2800 mg total P kg⁻¹, anion storage capacity 95%) in rhizosphere study containers. After 40 days, rhizosphere (0–5 mm) and bulk (>5 mm) soils were sampled and subjected to P fractionation. Organic anions were collected from the rhizoplane using an anion exchange membrane.

Key results

Dry matter yield, P uptake, and rhizoplane organic anion exudation were affected by plant species, soil type, and their interaction. Rhizosphere P changes of labile inorganic organic P and stable inorganic P were influenced by plant species and soil type, while moderately labile inorganic P was affected by only plant species. Interaction between plant species and soil type had no effect on rhizosphere P depletion or accumulation. The quantities and composition of organic anions determined in rhizoplane exudates were highly variable (0.01–0.1 μmol cm⁻² h⁻¹). However, significant correlations were observed between the depletion of moderately labile and stable soil inorganic P and concentrations of malate in exudates.

Conclusions

The findings of this study clearly demonstrated the capacity of green manure crops (especially blue lupin) to rapidly mobilise and deplete different forms of soil P across the soil types.