Influence of cover crop residue traits on phosphorus availability and subsequent uptake by plants

Cover crops are typically thought to increase the P nutrition of crops. However, there are mixed reports on this with some studies reporting a negative effect. An improved understanding of cover crop residues and their P release dynamics could offer new insight with the benefit of improved management for optimal P availability in cropping systems. Here, we examined the influence of cover crop residue traits for six different crop types on soil P availability and subsequent plant (ryegrass) P uptake over a four-month period in a soil with moderate P availability. Among the residue traits examined (residue P concentration, N concentration, C:P ratio, C:N ratio, N:P ratio and specific leaf area), only residue P concentration and C:P ratio were related to soil P availability and subsequent crop P uptake. Important short-term effects of residue C:P ratio on P availability were highlighted. Strong to moderate negative correlations between residue C:P ratio and subsequent crop P uptake (R 2 between 0.4 and 0.8) were observed. Decreases in subsequent crop uptake of up to 43% compared to unfertilized pots occurred for residues with high C:P ratios for the first cut, strongly suggesting microbial P immobilization. Effects faded with time, with most cover crop residue additions having little to no influence on ryegrass P uptake over a four month period. Residues with the highest C:P ratio nonetheless resulted in a 22% decrease in cumulative P uptake compared to unfertilized pots. Our study highlighted that cover crop C:P ratio should be managed in order to ensure minimized adverse effects of microbial P immobilization. The observed low effects of cover crop residues on P uptake in a subsequent crop suggest that improving P availability in context with moderate P limitations via cover cropping may require relying on other services provided by cover crops such as mobilization of sparingly available P pools.