Root growth rates and nitrogen accumulation in a highly productive temperate peatland

Background/Question/Methods

Belowground productivity of coastal wetland vegetation is important because root and rhizome growth drives peat accumulation, which can reverse subsidence and reduce sea level rise impacts. We used root ingrowth cores to quantify seasonal belowground growth rates of hardstem bulrush (Schoenoplectus acutus) and cattail (Typha spp.) in a U.S. Geological Survey experimental wetland site with peat substrate in the Sacramento-San Joaquin River Delta. We tracked root growth with seasonal total N and ¹⁵N concentrations in root samples. Our main questions were: 1) What is the root production potential in a cattail/bulrush-dominated peatland, and 2) How does total root N vary with uptake of ¹⁵N over one year? In April 2011 we deployed 12 mesh polypropylene cores (7.62 cm diameter x 28 cm long) filled with peat moss at 8 plot locations distributed across a peat accumulation gradient for a total of 96 cores. Every 4 months (August 2011, December 2011 and April 2012) 4 cores were retrieved from each plot. Live and dead roots and rhizomes were separated, dried and weighed. Composite root samples from each plot and date were analyzed for total N and ¹⁵N concentration with a mass spectrometer connected to a Carlo Erba NA 1500 elemental analyzer. 

Results/Conclusions

Seasonal root turnover was low, based on the minimal presence of dead roots in core samples. The maximum belowground biomass recorded was 7165 g/m² measured after one year. Four plots displayed continual root accumulation over the year, and were predominately located in the middle of the marsh away from inflows or outflows. Average total N concentrations ranged from 0.87 in August to 0.66 in December to 0.79 in April, with the December value being significantly lower than the other time periods (F_2,69 = 10.43, P=0.001, n=72).  Total N concentration was negatively related to belowground biomass in August (R² = 0.79) though this relationship did not hold for the other seasons. Active N recycling and competitive plant N uptake was supported by the light ¹⁵N signature of roots, as compared to wetland source water. This was especially observed in the backwater environments where surface water nitrate was below detection.  Further, in the two backwater plots with continual root growth (#1 and #4), total root/rhizome N was negatively related to δ¹⁵N, with δ¹⁵N decreasing with time (R² = 0.87 and R² = 0.92, respectively).