Karst rocky desertification restoration increases soil inorganic N supply to reduce plant N limitation
Dongni Wen, Yuanyuan Huang, Yuanfei Huang, Nana Ding, Kang Ni, Hua Wang, Ahmed S. Elrys, Lei Meng, Tongbin Zhu, Alena Gessert, Christoph Müller
Abstract
Nitrogen (N) limitation of plant growth following vegetation restoration is widespread in global terrestrial ecosystems, especially in karst rocky desertification areas. However, neither the temporal changes in plant N limitation during the restoration of those areas nor the mechanisms underlying N availability are well understood. In this study, several indicators reflecting soil N availability, N transformation rates, and plant communities were investigated in four areas in southwest China differing in their grade of rocky desertification. Our results showed that plant growth was severely N limited in the intense rocky desertification areas. The plant community-level foliar N content, 15 N values, and N:P ratio increased significantly as the rocky desertification grade decreased, indicating a decrease in plant N limitation. This was attributed to increased soil N availability, evidenced by the higher soil δ 15 N values as well as total N and inorganic N contents along the rocky desertification grade. With the decreasing rocky desertification grade, the rates of organic N conversion to ammonium (NH 4 + ) and nitrate (NO 3 – ), the adsorption of NH 4 + on cation-exchange sites, and the release of adsorbed NH 4 + increased significantly, which could enhance soil inorganic N supply capacity and accelerate NH 4 + turnover to increase N availability. Noticeably, the sharp decrease in the rate of NH 4 + oxidation to NO 3 – with the decrease in the rocky desertification grade led to a shift in inorganic N from NO 3 – -dominated to NH 4 + -dominated. The increased contents of soil organic matter, calcium, iron-aluminum oxides, and sand, the proportion of aggregates > 2 mm, as well as the greater abundances of fungi and bacteria were the primary drivers of the N transformation rates along the rocky desertification grade. Overall, our study highlights the importance of N cycling in controlling N availability and thus in determining plant N limitation in karst rocky desertification areas. The results of the study provide a scientific basis for the ecological restoration of rocky desertification in karst ecosystems.