Contrasting effects of different cover crops on soil P cycling via residue nutrient content and rhizosphere properties

Background/Question/Methods

Phosphorus (P) use in modern intensive agriculture is often inefficient, with low recovery of P inputs in yields, excess P stored in soils, and high leaching/runoff losses. Reducing soil P losses and enhancing soil P cycling could increase P use efficiency, maintain soil fertility and reduce P inputs. Cover crops can reduce P losses via plant uptake and mobilize soil P via acidification and higher enzyme activity in the rhizosphere. However, these effects may vary based on cover crop type/diversity and soil P levels.

I conducted a greenhouse experiment comparing the effects of seven different cover crops: three legumes (Vicia faba, Vicia benghalensis, Pisum sativum), three cereals (Triticum aestivum, Avena sativa, Secale cereale), and one mustard (Sinapis alba). All plants were grown in monocultures, and cereals and legumes were also grown in two mixtures. Phosphorus was added as yard compost and bone meal only (30 mg P kg^-1 soil) or compost and bone meal supplemented with mineral fertilizer or poultry manure (80 mg P kg^-1soil). I sampled aboveground biomass and soils (resin P, pH, enzyme activity) to determine the effects of different cover crops on residue nutrient content and rhizosphere properties.

Results/Conclusions

Legumes had lower plant %P but higher biomass, P uptake and plant C:P than cereals and mustard, with larger differences among plants at high P supply. Legumes in mixtures had lower biomass but higher plant %P compared to monocultures, whereas cereals in mixtures had higher biomass but similar plant %P vs. monocultures. Legumes had more N (higher %N and lower C:N) than cereals and mustard, and plant N was generally higher in mixtures.

Legumes depleted resin P (especially at high P supply) and reduced soil pH to a greater extent than other plants, with mixtures showing intermediate P depletion and acidification. Legumes and mixtures had higher monoesterase activity than cereals and mustard, whereas diesterase was higher in legumes and mixtures only at low P supply. Chitinase and B-glucosidase activity did not vary among plants, although manure increased activity.

Overall, mixtures produced residues that have higher N and P, with potentially higher mineralization rates than monocultures. In contrast, legumes generally had higher P uptake – and thus a higher potential to reduce soil P losses – and stronger effects on rhizosphere properties. Mixtures with a high proportion of legumes might therefore maximize cover crop benefits for P retention and availability.