C:N:P stoichiometry and nutrient limitation of stream biofilms impacted by grassland degradation on the Qinghai-Tibet Plateau

Abstract

Grassland degradation, a process of retrogressive succession, is an extensive global threat, especially on the Qinghai-Tibet Plateau. However, the influences of grassland degradation on carbon (C): nitrogen (N): phosphorus (P) ratios of streamwater and biofilms, as well as their implications for nutrient limitation, are not well studied. Assessing nutrient limitation and stoichiometry could facilitate understanding and mitigating the effects of nutrient input variations on stream ecosystems. Using a “space for time” substitution, contemporary grassland status quantified by landscape-level normalized difference vegetation index (NDVI) were used to study potential impacts of grassland degradation on nutrient cycling in streams feeding Qinghai Lake (China). C, N, and P concentrations and ratios of streamwater and benthic biofilms were assessed. The results showed that grassland degradation was associated with lower DOC, TN, DOC:TP, and TN:TP of streamwater, especially in September. DOC:TN:TP ratios of streamwater demonstrated relatively high variability with an average of 387:169:1 (molar). C, N, P biomass per unit area of stream biofilm also demonstrated considerable variability but their ratios were relatively constrained, with an average of 471:32:1. Moreover, biofilm C, N, and P concentrations and ratios were not closely associated with streamwater nutrients or grassland degradation. The complex chemical and biological composition of stream biofilms may make their stoichiometry difficult to interpret. However, the non-isometric relationships between biofilm C and P, as well as the large imbalance between C:N:P ratios of streamwater and biofilms, suggest that stream biofilms in this catchment are sensitive to P variation and potentially limited by DOC and P. By differentially impacting DOC and nutrient supplies, grassland degradation may alleviate P limitation but aggravate carbon limitation in stream biofilms.