作者：Xu, Z., H. Ran, M. Li, I. Brunner, J. Yin, H. Liu, D. Kong, X. Lü, T. Sun, J. Cai, R. Wang, Y. Zhang, P. He, X. Han, S. Wan, and Y. Jiang.
Background and aims
Evidence for impacts of environmental changes on belowground net primary production (BNPP) from long-term experiments is rather scarce. We aimed to understand how long-term changes in water and nitrogen availability affect production and vertical allocation of roots in semi-arid grasslands and its consequence on carbon (C) and nitrogen (N) cycles.
We investigated changes of BNPP and its vertical allocation along the soil profile to 40 cm in depth in response to simultaneous increases in water and N availability over 11 years in an old-field grassland in northern China.
Water addition increased BNPP in all soil layers (0–10 cm, 10–20 cm, and 20–40 cm), and enhanced the percentage BNPP in the upper soils but decreased that in 10–20 cm soil layer. Nitrogen addition decreased BNPP in 10–20 cm and 20–40 cm soil layers as well as total BNPP in 0–40 cm, and increased the percentage BNPP in the upper soil layer but decreased that in 10–20 cm soil layer. Water addition increased soil total C and N concentrations in 0–10 cm and 10–20 cm soil layers, while N addition only marginally decreased soil C: N ratio in 0–10 cm and 20–40 cm soil layers. Both soil total N concentration and soil C: N ratio were closely related to BNPP.
Our results highlight the importance of environmental factors, especially water availability, in determining BNPP, and in turn controlling soil nutrient accumulation in semi-arid grasslands, although the specific mechanisms remain unclear. The projected increase in precipitation in those semi-arid grasslands would enhance soil C and N sequestration. The increased allocation of BNPP in upper soils with long-term precipitation increment and N deposition may accelerate the cycles of C and N in these ecosystems, and thus increase the risk of soil C and N loss.