Water and nitrogen limitation on net primary production along topographical gradient of boreal steppe: In the perspective of climate change

Background/Question/Methods

In grasslands, net primary production (NPP) is limited by water as well as nitrogen (N) because of a strong feedback between water and N cycles. Topography affects availability of the resources, and results in spatial variability in NPP.Responses of NPP of grasslands to precipitation change and N deposition have been frequently reported, but there are few attempts to test the responses while taking into account topography. We carried out water and N manipulation experiment along topographical gradient of boreal steppe of Northern Mongolia, which is one of the hot spots of climate change.

Results/Conclusions

The responses of total ANPP to changes in experimental water addition (H₂O) and rainfall decreasing using rainout shelter (–H₂O) were proportional at the upper slopes: 35% increase in H₂O treatment and 35% decrease in –H₂O treatment. At the lower slope, H₂O treatment did not significantly affect the total ANPP, whereas –H₂O treatment diminished total ANPP by 30%.Nitrogen addition significantly increased total ANPP at the both slopes. Interestingly, the N addition overcame the effects of –H₂O treatment; in the –H₂O × N treatment, total ANPP was higher than those of–H₂O treatment as well as control plots at both slopes signifying high N limitation on ANPP of the ecosystem. Strong positive effect of interaction of water and N (H₂0 × N treatment) on the ANPP at the both slopes; 3.5 times at the upper and 2.5 times higher at the lower slope. These increases in total ANPP due to N addition were much higher than those observed in other grasslands. The results suggested that ANPP of boreal steppe is co-limited by water and N along topographical gradient, but the degree of the co-limitation is high in the upper slopes. Traditional seasonal movement of Mongolian nomads might lessen the high N limitation in the Mongolian grasslands through redistribution of soil nutrients by livestock animals.In the perspective of climate change, although increase in N deposition is projected, it is hard to say the effect of the N deposition can offset the effect of future precipitation change.