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