Relationships between seasonal flooding and emergent plant production in prairie whitetop (Scolochloa festucacea) marshes

The mechanisms linking spring flooding to changes in plant primary production were examined in seasonally flooded shallow prairie marshes dominated by whitetop (Scolochloa festucacea) at Delta, Manitoba, Canada. This study tested the hypotheses that flooding influences aboveground production by: (1) altering biomass allocation between above- and belowground structures, (2) changing soil salinity, and (3) by changing availability of plant-limiting nitrogen. Experimentally manipulated water levels provided comparisons between marshes with spring flooded and nonflooded hydrologic regimes. Spring flooding increased aboveground production (950 g/m$\sp2)$ compared with nonflooded marsh (360 g/m$\sp2).$ Flooding increased the number of flowering shoots and mid-summer shoot recruitment. Ingrowth mesh bag methods for estimating belowground production showed consistently greater estimates and lower sampling variability compared with soil coring methods. Below ground production measured by mesh bags was greater in flooded marsh (415 g/m$\sp2)$ than in nonflooded marsh (309 g/m$\sp2).$ Changes in total production between flooding regimes indicated that differences in aboveground production could not be explained solely by changes in biomass allocation. Flooding reduced pore water salinity compared with nonflooded marsh but mean growing season soil salinities at 15-cm depth over two years in both flooded marsh (12 mS/cm) and nonflooded marsh (17 mS/cm) were greater than the salinities (approximately 8 mS/cm) that reduced whitetop growth by more than 50 percent in greenhouse trails. Irrigation of plots with fresh water in nonflooded marsh reduced pore water salinity and increased whitetop growth. Flooding appeared to increase whitetop growth by providing plants access to fresh water in the top 10 cm of the soil profile. Net nitrogen mineralization rates were greater in flooded marsh (261 kg N/ha/y) than nonflooded marsh (114 kg N/ha/y). Net nitrification was significant in nonflooded marsh (60 kg N/ha/y) but not in flooded marsh ($\u3c$1 kg N/h/y). Net nitrification increased levels of soil nitrate during summer and early spring in nonflooded marsh. Net mineralization rates in flooded and nonflooded marsh approximately balanced whitetop nitrogen uptake requirements. However, irrigation of nonflooded marsh did not increase net mineralization rates while increasing whitetop growth, indicating that soil salinity and not nitrogen availability provided the most immediate control over plant growth