Warming has limited effects on plant growth through nutrient release: evidence from sub-Antarctic Marion Island.

Background and aims: Cold ecosystem plant productivity is nutrient-limited, largely due to temperature-limited soil decomposition rates. Climate warming is predicted to indirectly stimulate productivity by stimulating microbial activity and thus nutrient release. However, these trends are not consistent across cold systems, and the predictions require empirical testing. Here, we investigated whether soil warming on sub-Antarctic Marion Island (49.9°S, 37.8°E) indirectly stimulates grass productivity through increased nutrient release.

Methods: Four grasses (native Polypogon magellanicus and Poa cookii and alien Poa annua and Agrostis stolonifera) were subjected to soil warming (ambient +3 °C) in a potted experiment for 5 months. A second experiment with a fertilizer (NPK) treatment tested for nutrient limitation under warming. Additionally, soils with varying organic content were incubated at 5 °C (control), +3 °C and +6 °C for 42 d to determine changes in soil and microbial C, N and P.

Key results: Warming consistently increased plant growth for only one species (the invasive alien Poa annua), but increased leaf N overall. Warming increased soil NH4+ but NO3-, organic N and PO43- remained unchanged, and warming had small or non-significant effects on microbial biomass, N and P. In contrast to warming alone, NPK fertilization stimulated growth at least two-fold and increased leaf N, showing nutrient limitation to growth despite soil warming.

Conclusions: It is important to empirically test the assumption of nutrient release with cold-ecosystem warming, and we show that warming-induced nutrient release should not be assumed. Only the ruderal and phenotypically plastic Poa annua increased above-ground biomass with soil warming, indicating that nutrient release on Marion Island is limited even with short-term warming (<1 year). Nutrient release with warming is likely not a major driver of vegetation change on Marion Island.