Differences in cover crop contributions to phosphorus uptake by ryegrass in two soils with low and moderate P status

Growing evidence has emerged that cover crops may be able to improve phosphorus (P) cycling and contribute to the P nutrition of the subsequent crop. This could be particularly important in farming systems, with limited access to inputs and where low soil P availability has been identified. In a pot experiment using soils labelled with radioactive P-33, we examined how a range of cover crop residues directly contribute to P uptake of ryegrass, as well as how they affect P uptake of ryegrass from the soil P pool. Two soils with a low and a moderate P status (6.3 and 15.3 mg Olsen-P kg(-1) soil) were chosen for the pot experiment as models for soils that are gradually becoming depleted in available P. Residues from five cover crop species (buckwheat (Fagopyrum esculentum Moench), oilseed radish (Raphanus sativus L.), garden sorrel (Rumex acetosa L.), white lupine (Lupinus albus L.) and hairy vetch (Vicia villosa Roth)) showed a wide species-dependent variability in P concentration (3.4-8.8 mg P g(-1) DM) and other quality traits. Cover crop residues contributed less to ryegrass growth and P uptake than the water-soluble P fertilizer. At the same P application dose, cover crops contributed 0.4-2.3 mg P kg(-1) soil (18-42 %) to the total P uptake of ryegrass depending on species and soil P status, whereas mineral P fertilizer contributed up to 5.2 mg P kg(-1) soil (46 %). In the low P soil, application of sorrel and radish significantly increased the P uptake of ryegrass compared to the control without P (by 93 and 75 %, respectively), whereas in the moderate P soil, sorrel and vetch increased the uptake (by 61 and 43 %, respectively). The cover crop effects on P uptake of ryegrass correlated significantly but only moderately well with their P concentration, content of water-extractable P and C:P ratio (R-2 = 0.4, R-2 = 0.4 and R-2 = 0.5, respectively). As expected, the contribution of mineral fertilizer to P uptake of ryegrass was lower in the low P soil with a higher P sorption capacity compared to the moderate P soil, whereas the contribution of cover crops residues in these two soils was species-dependent. Addition of mineral fertilizer resulted in a greater uptake of soil P compared to the control whereas buckwheat and lupin with highest C:P ratios gave rise to a substantially smaller uptake of soil P, which is an indication of microbial P immobilization. Our study demonstrated that cover crop residues may contribute to the P nutrition of the subsequent crop, in some cases substantially, depending on the quality of the residues and the soil P status.