Abstract
Invasive plants can impact nutrient cycling, potentially creating positive feedbacks for further invasion. We studied the impact of three woody nitrogen (N)-fixing invasive plant species on soil N-cycling and phosphatase activity in a montane tropical forest-grassland mosaic in the Nilgiri Biosphere Reserve in southern India, an ecosystem with > 100-year history of biological invasions. Soils were collected over a year from patches invaded by Acacia mearnsii, Cytisus scoparius, and Ulex europaeus, and from uninvaded grasslands, to assess inorganic N-availability and N-mineralization rates (using in situ open-top mineralization tubes). Phosphatase activity was measured from soils collected at the beginning of the growing season. Soils of invaded areas had higher inorganic N-availability and phosphatase activity than soils of uninvaded areas. Whilst net N-mineralization rates were unchanged between invaded and uninvaded sites, net nitrification rates were higher and net ammonification rates lower in invaded areas, particularly in the dry season. Impacts of C. scoparius and U. europaeus on these variables were similar to each other, and lower than the impacts of A. mearnsii. These results show that invasive N-fixers are significantly altering nutrient availability and cycling, and also changing the proportion of the forms of inorganic N available, in the Nilgiri grasslands. Restoration activities in these invaded grasslands should explore soil N management strategies such as soil C amendments and planting of specific native species, in conjunction with other strategies, to control invasive plants.
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Data availability
The datasets generated during and/or analysed during the current study are available in electronic form in the attached zip “Supplementary_Data.zip”.
Code availability
All analysis was done in the R programming environment using openly available functions.
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Acknowledgments
We thank Tamil Nadu Forest Department for permits to conduct this project in the Nilgiris Forest Division, and the Tamil Nadu Electricity Board for providing us with residential quarters during this project. We are grateful to Atul Joshi, Balavignesh Baliah, Chandan Pandey, Chintan Sheth, Dayani C, Joyshree Chanam, Kaikho D, Karthik Teegalapalli, Manjunatha H Chandregowda, Rajat Rastogi, Rasikapriyaa Sriramamurthy, Sandeep Pulla, Shashank Ongole, Yadugiri V T for their insights in different phases of the project and comments on the manuscript. We thank Dr. Jayashree Ratnam and Dr. Deepak Barua for their comments during the initial stages of the project. We thank Selva, Susilan, Kamal, and Saravanan who provided invaluable assistance with fieldwork throughout the project. We also thank the anonymous reviewers for their comments on the manuscript. National Centre for Biological Sciences, Bengaluru, provided core funding for this study.
Funding
This work was supported by the National Centre for Biological Sciences, Tata Institute of Fundamental Research, Bengaluru, by the Department of Atomic Energy, Government of India [Grant Number 12-R&D-TFR-5.04-0800]. Manaswi Raghurama was awarded an intra-mural travel Grant by National Centre for Biological Sciences to attend, and present the results of this study, in the 18th International Savanna Science Network Meeting, Kruger National Park, South Africa.
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Both authors contributed to the study conception and design, and statistical analysis. Sample collection and analysis was performed by Manaswi Raghurama. First draft of the manuscript was written by Manaswi Raghurama and both authors commented on previous versions of the manuscript. Both authors read and approved the final manuscript.
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Raghurama, M., Sankaran, M. Invasive nitrogen-fixing plants increase nitrogen availability and cycling rates in a montane tropical grassland. Plant Ecol 223, 13–26 (2022). https://doi.org/10.1007/s11258-021-01188-4
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DOI: https://doi.org/10.1007/s11258-021-01188-4