Long-term alfalfa (Medicago sativa L.) establishment could alleviate phosphorus limitation induced by nitrogen deposition in the carbonate soil

Phosphorus (P) limitation is a widespread problem of primary production in dryland submitted to persistent nitrogen (N) deposition. The legume alfalfa (Medicago sativa L.), which can fix N2, might potentially strengthen P limitation in dryland ecosystems and is widely distributed as forage. However, there is still unclear how alfalfa grassland mobilizes the soil P to meet its demand. In this experiment, alfalfa introduction was used for long-term revegetation to evaluate the P uptake of plants from deep soil and assess the P limitation induced by N deposition compared with fallow. Our results showed that alfalfa introduction increased the soil P storage significantly at 0-2.4 m soil depth (+0.74 Mg ha -1), whereas it decreased at 2.4-4.8 m soil depth (-0.21 Mg ha -1) after 15-year establishment. Alfalfa establishment increased soil organic P concentration (180.9 mg kg -1 vs. 67.2 mg kg -1) and its relative contribution to total P (19.64% vs. 8.08%) at 0-4.8 m. Alfalfa establishment also increased the concentration and proportion of labile and intermediate P fractions at 0-4.8 m (9.12 mg kg -1 vs. 6.87 mg kg -1, 1.12% vs. 0.98%; 16.06 mg kg -1 vs. 8.39 mg kg -1, 1.69% vs. 1.17%). Alfalfa introduction decreased the concentrated HCl-Pi (250.66 mg kg -1 vs. 229.32 mg kg -1, 36.81% vs. 28.91%) in 2.4-4.8 m soil depth. These results indicated that the deep root system of alfalfa grassland could promote the P mobilization from deep to shallow soil. The concentrated HCl-Pi may be the main potential P source of alfalfa from 2.4-4.8 m to 0-2.4 m of soil depth, and long-term establishment of alfalfa can alleviate P limitation caused by N deposition in carbonate soil. Our results suggested that species with deep roots (such as alfalfa) could be selected as an economical way to mitigate nitrogen deposition in drylands.

Automated summary

The introduction of alfalfa for long-term revegetation significantly increased soil phosphorus storage at 0-2.4 m depth but decreased at 2.4-4.8 m depth. Alfalfa establishment enhanced soil organic phosphorus concentration and its relative contribution to total phosphorus, alleviating phosphorus limitation induced by nitrogen deposition.