Inclusion of Leguminous Green Manures Enhances Crop Biomass, Nutrient Uptake, Soil Phosphorus Dynamics and Bioavailability

Introduction

In agroecosystems, phosphorus (P) applications over a long time have accumulated in soil as legacy P. This environmental challenge can be an agronomic opportunity as soil legacy P could be recovered in cropping systems using practices such as green manuring. We hypothesised that, at moderate soil available P levels, plant-soil interactions under green manures can mobilise soil legacy P and promote cereal crop P uptake and growth.

Methods

Alongside a fallow treatment, three green manure treatments that included two legume treatments (narrow-leaf lupin [Lupinus angustifolius], pea [Pisum sativum L.]) and one cereal treatment (wheat [Triticum aestivum] and barley [Hordeum vulgare]) were rotated with the main crops of wheat and barley in two phases on a pumice soil (27 mg kg⁻¹ Olsen P) in a microcosm experiment. Plant roots and shoots and end-of-experiment soil samples were collected for analysis.

Results

Over two crop rotations, inclusion of narrow-leaf lupin and pea green manures significantly increased main crop biomass (27%–35%) and P uptake (15%–29%) relative to control, while the cereal green manure decreased the following crop's yield (−13%) and P uptake (−19%). Relative to fallow, microbial biomass P and soil organic P pools increased under all green manures yet total inorganic P decreased under leguminous green manures. This depletion (35 mg P kg⁻¹) under narrow-leaf lupin was equivalent to ~47 kg P ha⁻¹. Phosphatase enzyme activities relevant to P cycling increased particularly under leguminous green manure treatments.

Conclusions

Leguminous green manures such as narrow-leaf lupin could mobilise soil P to crops in field conditions, suggesting that drawdown of soil legacy P while sustaining crop yield can be tenable.