Nutrient accumulation by biologically active soil cover in a dry, high-altitude glacier foreland in the NE Pamir (Tajikistan) in the context of ongoing climate changes

The foreland of the Uisu Glacier, located in NE Pamir, is characterized by a combination of processes typical for periglacial and hyperarid areas. Therefore, the development of vegetation and soils is significantly hampered there, with soil organic carbon pools being among the lowest reported worldwide (1.4 kg m^-2 in the layer 0-50 cm). Given the extreme environmental conditions in the investigated area, we expected that (1) biological soil crusts (BSCs) of different developmental stages comprise the dominant part of the biologically active soil cover; and hence (2) play an important role in the accumulation of C, N and P, with the microbial biomass and total nutrient retention patterns positively related to their developmental stage. To assess the potential importance of BSCs for nutrient accumulation processes in soils of the study area, we compared the C, N, and P enrichment in soils under the BSCs with the same parameters in soils under vascular plants. Subsequently, we studied C, N and P accumulation in the soils from six distinct plant communities recorded in the foreland.

Our study showed that BSCs dominated in five out of ten study plots with recorded biologically active soil cover. Among them prevailed poorly developed morphotypes (no lichens, no bryophytes) with a mean coverage of 14%. Compared to advanced crusts (mean coverage 1.1%), they accumulated less total C, N and available P in their biomass. Yet, they were still the main biological soil-forming factor next to the plants (mean coverage of 8.6%) in our study site, given that stones and non-crusted bare soils covered most of the area (joint mean coverage over 80%). Soil-forming properties of both poorly developed and advanced crusts were confirmed by the observed enrichment of their sub-crust soils in total C, N and available P in comparison to non-crusted bare soils (on average, there were ~1.5 times more nutrients in soils under poorly developed crusts and ~2.5 more nutrients in soils under advanced crusts than in non-crusted bare soils). Moreover, the average enrichment observed for soils under advanced crusts was similar to the results obtained for soils under individual vascular plants from the same study plot (~2.5 times more than in non-crusted bare soils).

The large-scale studies of soils from the identified plant communities showed that only in three of them C and N amounts were higher than in the respective bare soils. Phosphorus content was similar in all the studied communities, with values approximately two times higher than for bare soils. Statistical analyses showed that the amount of C, N and P was strongly positively correlated with the percentage coverage of plants and with species diversity of the community (expressed as Shannon Wiener index and as a number of species).

As further aridification is projected for the Pamir, leading to the limitation of ecological niches available for vascular plants, BSCs could potentially become the most important or even the sole player in the accumulation of soil nutrients in many areas.

This work was supported by the Polish National Science Centre Grant No 2017/25/B/ST10/00468.