Variations in the natural 13C and 15N abundance of plants and soils under long-term N addition and precipitation reduction: interpretation of C and N dynamics

Background: The nitrogen isotope natural abundance (delta N-15) provides integrated information on ecosystem N dynamics, and carbon isotope natural abundance (delta C-13) has been used to infer how water-using processes of plants change in terrestrial ecosystems. However, how delta C-13 and delta N-15 abundances in plant life and soils respond to N addition and water availability change is still unclear. Thus, delta C-13 and delta N-15 abundances in plant life and soils were used to investigate the effects of long-time (10 years) N addition (+ 50 kg N.ha(- 1).yr(- 1)) and precipitation reduction (- 30% of throughfall) in forest C and N cycling traits in a temperate forest in northern China. Results: We analyzed the delta C-13 and delta N-15 values of dominant plant foliage, litterfall, fungal sporophores, roots, and soils in the study. The results showed that delta N-15 values of foliage, litterfall, and surface soil layer's (0-10 cm) total N were significantly increased by N addition, while delta N-15 values of fine roots and coarse roots were considerably decreased. Nitrogen addition also significantly increased the delta C-13 value of fine roots and total N concentration of the surface soil layer compared with the control. The C concentration, delta C-13, and delta N-15 values of foliage and delta N-15 values of fine roots were significantly increased by precipitation reduction, while N concentration of foliage and litterfall significantly decreased. The combined effects of N addition and precipitation reduction significantly increased the delta C-13 and delta N-15 values of foliage as well as the delta N-15 values of fine roots and delta C-13 values of litterfall. Furthermore, foliar delta N-15 values were significantly correlated with foliage delta C-13 values, surface soil delta N-15 values, surface soil C concentration, and N concentrations. Nitrogen concentrations and delta C-13 values of foliage were significantly correlated with delta N-15 values and N concentrations of fine roots. Conclusions: This indicates that plants increasingly take up the heavier N-15 under N addition and the heavier C-13 and N-15 under precipitation reduction, suggesting that N addition and precipitation reduction may lead to more open forest ecosystem C and N cycling and affect plant nutrient acquisition strategies.