Effects of resource addition on recovery of production and plant functional composition in degraded semiarid grasslands
Degradation of semiarid ecosystems from overgrazing threatens a variety of ecosystem services. Rainfall and nitrogen commonly co-limit production in semiarid grassland ecosystems; however, few studies have reported how interactive effects of precipitation and nitrogen addition influence the recovery of grasslands degraded by overgrazing. We conducted a 6-year experiment manipulating precipitation (natural precipitation and simulated wet year precipitation) and nitrogen (0, 25 and 50 kg N ha−1) addition at two sites with different histories of livestock grazing (moderately and heavily grazed) in Inner Mongolian steppe. Our results suggest that recovery of plant community composition and recovery of production can be decoupled. Perennial grasses provide long-term stability of high-quality forage production in this system. Supplemental water combined with exclosures led, in the heavily grazed site, to the strongest recovery of perennial grasses, although widespread irrigation of rangeland is not a feasible management strategy in many semiarid and arid regions. N fertilization combined with exclosures, but without water addition, increased dominance of unpalatable annual species, which in turn retarded growth of perennial species and increased inter-annual variation in primary production at both sites. Alleviation of grazing pressure alone allowed recovery of desired perennial species via successional processes in the heavily grazed site. Our experiments suggest that recovery of primary production and desirable community composition are not necessarily correlated. The use of N fertilization for the management of overgrazed grassland needs careful and systematic evaluation, as it has potential to impede, rather than aid, recovery.
KeywordsNitrogen Overgrazing Water availability Plant functional group Recovery
This work was supported by the National Natural Science Foundation of China (NSFC, 31300386/41071207), the Deutsche Forschungs Gemeinschaft (DFG, project DI 546/3-2), and the Innovation Team Training plan of the Tianjin Education Committee (TD12-5037). We thank the Botany Institute for providing working facilities, Drs. Yingzhi Gao and Marc Giese for their help with field work, Dr. Holger Brueck for his constructive suggestions on experimental design, and Drs. Peter Adler, Yongfei Bai, Kimberly La Pierre, David Schimel, Chris B. Zou and two anonymous reviewers for useful comments on previous drafts of this manuscript.
Author contribution statement
SL, QC and KD designed study; QC, HL, XYG, FP and HW performed the research; QC, SL and DUH analyzed data; QC, DUH, and SL wrote the paper.
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