Effects of Legume Cover Crops on Soil Nitrogen Availability, Biomass and Foliar N of Young Grapevines under Simulated Warming and Reduced Precipitation

In Mediterranean viticulture, climate change is reshaping management practices by increasing water scarcity and temperatures, challenging productivity and wine quality. The establishment of new vineyards is particularly vulnerable at early stages. In this context, legume cover crops (CCs) may enhance soil resilience and vineyard establishment through increased biological activity and biological N fixation.

This study evaluated the potential of two legume CCs to improve soil N availability and early grapevine development under a simulated warming scenario (+2 °C) and contrasting precipitation regimes. A microcosm experiment (12 kg soil per pot) was conducted under semi-controlled greenhouse conditions (Madrid, Spain) using a multifactorial design including three soil management treatments (bare soil, cover crop of Medicago truncatula Gaertn., and cover crop of Trifolium subterraneum L.), two precipitation levels (current and 15 % reduced), and two grapevine cultivars (white cv. Airén and red cv. Tempranillo). Cover crops were mowed 75 days after sowing, and their residues were left on soil surface as mulch. After one growing cycle, soil total N and extractable NO₃⁻ were measured, and grapevine foliar biomass, as well as foliar N content, were determined.

Under warming conditions, legume CCs did not increase soil total N or extractable NO₃⁻ compared with bare soil. In contrast, reduced precipitation increased both parameters. Moreover, reduced precipitation decreased total foliar N amount by a 14 %. This suggests that reduced precipitation limited N uptake by the grapevine and in consequence increased the soil NO₃^-. These results may be explained by decreased water availability, given that N assimilation is an active, energy-dependent process regulated by the water status of grapevine and CCs.

Foliar biomass showed significant interaction between soil management and precipitation level. Under bare soil conditions, reduced precipitation decreased leaf biomass by 22 % relative to current precipitation. In contrast, under current precipitation, CCs reduced leaf biomass by 20 % compared with bare soil.  However, under reduced precipitation CCs did not decrease foliar biomass respect to bare soil. This interaction indicates that cover crop competition is significant under current precipitation but not under reduced precipitation. A reduction in foliar biomass under CCs, when not accompanied by reduced precipitation, would indicate that factors other than water competition are involved. One such factor could be N uptake by the cover crops, which reduces N uptake by the grapevine and consequently limits its foliar development.

In conclusion, legume CCs did not increase soil N availability grapevine N status, or foliar growth in grapevines during their first growing cycle. However, they were not detrimental to grapevine foliar biomass under water-restricted conditions compare to the bare soil. Overall, the results highlight water availability as a key factor modulating of the soil–plant N balance. These results support the use of legume CCs as sustainable soil management for climate-resilient viticulture at the first year of grapevine establishment. Further research is needed to optimize legume CCs management to enhance soil N availability and grapevine performance under future climate change scenarios.

Acknowledgements: proyecto CUBIC. PID2023-147576OB-C21 y PID2023-147576OB-C22, financiadas por MICIU/AEI/10.13039/501100011033.