Impact of parent material, pedogenesis and plant cover on various P forms in soils developed in the foreland of the Uisu glacier in the Eastern Pamir, Tajikistan

Global models of ecosystem limitation maintain that in the early stages of pedogenesis, low nitrogen availability limits the earliest stages of primary succession. However, high-altitude arid and hyperarid areas are underrepresented in these models. Significantly, the areas combining aridity with glaciation/deglaciation processes (i.e. Himalayas, Eastern Pamir, dry Antarctic), where soil development and ecological succession are still challenging for research. Therefore, our studies focused on the deposition of various forms of phosphorus in soil chronosequence developed in the foreland of the Uisu glacier in the Eastern Pamir in relation to soil physiochemical properties and vegetation cover. Our previous studies performed on a sequence of terraces, alluvial cones, and terminal moraines developed later from the late Pleistocene in the foreland of the Uisu glacier showed extreme cold and dryness noticeably slowed down soil development in the area, even if permafrost was not preserved in the soil profiles. Thus, soil development, manifested in the transformation of physicochemical soil properties and diagnostic horizons, had very low intensity and led to relatively little spatial soil differentiation in the foreland. In the presented research, soils from the locations mentioned above were sampled, and a modified Hedley fraction extraction technique was used to separate phosphorus into (1) an easily bioavailable fraction extracted with NaHCO₃ (NaHCO₃- P_t), (2) a moderately bioavailable fraction extracted with NaOH (NaOH-P_t) and (3) a fraction unavailable for plants extracted with HCl (CHCl- P_t).

Moreover, total nitrogen (TN), total carbon (TC) and total organic carbon (TOC) contents were analyzed in the samples with standard analytical methods. We found shallow P content in all studied soil samples. For the samples from the terraces with high vegetation cover, we found significant solid positive correlations between both bioavailable phosphorus fractions and TN (r²=0.78 for NaHCO₃- P_t and r²=0.80 for NaOH-P_t, in both cases p<0.001);  and between these fractions and TOC (r²=0.49 for NaHCO₃- P_t and r²=0.53 for NaOH-P_t, in both cases p<0.001). For the samples from the moraine located 14 kilometres from the glacier and covered with sparse desert plants, we recorded no significant correlations between any bioavailable fractions of phosphorus and TN or TOC. However, the fraction of phosphorus unavailable for plants (CHCl- P_t) was strongly positively correlated with TOC (r²=0.70, p<0.01). It seems that the availability of P depends more on the decomposition process of organic matter than on the biochemical mineralization of minerals. 

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