Responses and sensitivity of N, P and mobile carbohydrates of dominant species to increased water, N and P availability in semi-arid grasslands in northern China

Aims We aimed to improve the understanding of the carbon and nutrient physiological responses and adaptation of semi-arid grassland plants to environmental changes. Methods We investigated plant leaf non-structural carbohydrate (NSC = soluble sugars + starch), nitrogen (N) and phosphorus (P) levels in an Inner Mongolian semi-arid grassland community treated with water, N and P additions for 8 years. Two dominant grasses (Agropyron cristatum (L.) Gaertn., Stipa krylovii Roshev.) and two forbs (Artemisia frigida Willd., Potentilla bifurca L.) were analyzed. Important Findings Water addition decreased plant leaf N and P concentrations, whereas N and P addition increased them, indicating that the semi-arid grassland studied suffers from a shortage of N and P supply. Both N and P addition decreased the levels of soluble sugars, starch and thus also NSC in plant leaves, which may be attributed to (i) increased carbohydrate consumption associated with a higher growth rate, and (ii) a dilution effect of greater plant size under N and P addition. Water addition tended to increase the leaf NSC levels both in the grasses (+9.2%) and forbs (+0.6% only), which may be a result of increased photosynthesis of plants with increased water availability. Under conditions of ambient and increased water supply in the present study, N addition resulted in an N/P ratio of > 16 in the grasses but a significantly lower N/P ratio of < 11 in the forb species. This finding implies that growth of the two grass species will be limited mainly by P availability but the forbs will still be mainly limited by N supply if N deposition, alone or in combination with summer precipitation, continues to increase as predicted in Inner Mongolia.