Regional Mountain Conference

In an earlier experiment above the Engadine valley (2200 m a.s.l., Canton Grisons) we demonstrated the climate sensitivity of subalpine pasture carbon (C) stocks, resulting in a negative net ecosystem C balance of ca. 10 t C ha^-1 in five years with + 3°C warming, despite unaffected plant productivity. A follow-up study, using a long-term (32 yrs.) grassland mineral fertilisation experiment above the Albula valley (1200 m a.s.l., Canton Grisons), indicated that under strongly limiting nitrogen (N) supply, the uniformly low N-availability set allometric limitations for microbial decomposition, leading to uniformly declining soil organic C (SOC) stocks.

In the study presented here, we analysed another long-term grassland fertilisation experiment in the Swiss Jura mountains (920 m a.s.l., Canton Solothurn), providing 22 years of climate, yield and SOC stock data. We attempted to isolate the effects of plant yield gradients (2.8 – 8.3 t dry matter ha^-1 yr^-1) and N-availability (0 – 150 kg N ha^-1 yr^-1) in driving SOC stock change.

We hypothesised that N-fertilisation driven plant productivity affects both organic matter availability for SOC storage and interacts with N availability driven C use efficiency levels to generate different, steady-state SOC stocks. But if SOC stocks would not differ between treatment combinations, we assumed that the climate factor, consisting of a distinct pattern of temperature and precipitation, would act evenly on effective SOC storage capacity and would result in the same SOC stocks in all treatment combinations.

We found that while plant productivity was highly responsive to different mineral fertilisation levels, SOC stocks were not responsive to either plant productivity or N input, even though they usually constitute limiting factors for soil C storage. This led us to assume that we observed, on a decadal scale, the decline from an earlier effective SOC storage capacity to a new, lower effective storage capacity. We deduce that the SOC stock decline is driven by increased temperatures, that have increased OM decomposition rates. This implies, that under the given edaphic conditions and including a wide range of agricultural management options, the warming climate constitutes an increasingly limiting factor for SOC storage.