Biogeochemistry

Once considered purely as nutrient sinks, floodplains are now known to play multiple roles from a geochemical perspective. Based on the type of floodplain, associated vegetation, and the degree and nature of disturbance, floodplains may also serve as sources or transformation zones for nutrients. The widely held perception of flood-plains as fertility hot spots belies the complexity associated with input-output budgets as well as the bio-geochemical processes within the floodplain ecosystem. In particular, the impact of hydroperiod on biogeochemi-cal processes sets floodplain biogeochemistry apart from that of non-wetland ecosystems. Periodic flooding makes possible nutrient exchange across the aquatic-terrestrial ecotone and controls the nature of decomposition, nutrient uptake and release by vegetation, and many other processes. As an example, the process of denitrification or the anaerobic conversion of nitrate to gaseous forms of nitrogen is very important on floodplains. In addition, the interaction of hydrology and biogeochemistry necessitates the development of unique approaches to the study of nutrient cycling in these ecosystems.

As previously mentioned, floodplains may serve as sinks, sources, or transformation zones for geochemical inputs of nutrients derived from inflow, precipitation, nitrogen fixation, and soil weathering. Multiple roles may proceed simultaneously on the same floodplain if spatial heterogeneity in hydrology, vegetation, disturbance, and nutrient influx so dictate. The use of a geochemical budget allows net inputs to be compared to net outputs and is based on the perspective of the ecosystem as an integrated system.

In general, the factors that promote nutrient sink activity on floodplains include (1) presence of aggrading vegetation; (2) wide carbon: nutrient ratios in living vegetation and detritus; (3) topographic positions conducive to somewhat frequent, short duration, and low-energy flooding; (4) basin geomorphology that promotes significant sediment loads in streams (e.g., redwater, brownwater, or whitewater based on the color of suspended clay); (5) high occurrence of nitrogen-fixers; and (6) until nutrient saturation is approached, association with a river subjected to high anthropogenic nutrient loadings.

Alternatively, rivers draining low gradient basins with sandy soils are often referred to as blackwater systems because their waters are stained with organic substances (Figure 4). These tend to carry low sediment loads and, consequently, alluviation (i.e., sink activity) is less pronounced. Also, floodplains occupied by mature vegetation communities may act as transformers of nutrients (e.g., inorganic inputs of nitrogen converted to organic outputs) rather than a sink or source. The latter is a key facet ofthe 'kidney function' of these systems and has great significance for maintenance of water quality. Sink activity, such as the filtration and accumulation of sediments (and associated nutrients) from sheetflow also plays a major role in cleansing water (Figure 5). Finally, floodplains that have been altered in some way by disturbance may function as nutrient sources. The longevity of the source activity could be short-term (e.g., a well-planned forest harvest followed by rapid forest regeneration) or long-term (e.g., conversion to agricultural or urban uses, impoundments, or climate change).

Similarly, all biogeochemical processes within flood-plain ecosystems reflect the overriding influence of hydrology. As an example, the timing of litterfall is heavily affected by hydroperiod because different vegetation communities occur under different hydrologic regimes. In

Figure 5 Flint River - sediment accumulation on the Flint River floodplain near Ft. Valley, GA during floodwater drawdown.

Figure 4 Amazon River: (a) a broad and (b) a close-up view. The formation of the Amazon River at the 'o encontros das aquas' or mixing of the Rio Negro and Rio Solimoes nearManaus, Brazil. The blackwater Rio Negro is contrasted with the sediment-laden Rio Solimoes.

the southeastern United States, forest species associated with Nyssa may grow under wetter conditions than communities dominated by some species of Quercus. On wetter sites, Nyssa foliage tends to senesce earlier in the autumn than other floodplain tree species and, consequently, the senesced foliage is exposed to a different microenvironment than litter that falls later in the year. As a result, nutrient release and immobilization sequences are likely to differ among sites.

Mass loss and nutrient dynamics during decomposition are a function of both litter quality and the decomposition microenvironment. Litter quality (the biochemical composition of detritus) is defined by the conditions under which a plant is growing as well as genetics and has been shown to be closely linked to variation in hydroperiod. Also, the frequency and duration of flooding play a

Figure 5 Flint River - sediment accumulation on the Flint River floodplain near Ft. Valley, GA during floodwater drawdown.

dominate role in determining biomass and composition of microbial populations. Key determinants of shifts between nutrient mineralization and immobilization include hydroperiod and nutrient inflow. In the southeastern United States, mass loss rates of foliar litter (with litter quality held constant) are maximized by moderate durations of flooding followed by several months of noninundation.

In general, rates of litter mass loss in forested flood-plains exceed those of uplands. Globally, decay constants for temperate floodplain forests average approximately 1.00 while the mean for all temperate deciduous forests is less than 0.80. This differential is partly due to the greater availability of soil moisture (better habitat for microbial populations) during parts of the year. However, mass loss, as measured by disappearance of confined litter, includes both mechanical disintegration as well as metabolic conversion of organic carbon and, consequently, periodic inundation offers greater opportunities for disintegration and export.

The general perception that floodplains are very fertile has led to misconceptions regarding the degree to which insufficient nutrient availability may constrain floodplain NPP. In many cases, it is true that floodplain soils are more fertile than upland counterparts. However, vegetation species found in many floodplains often have higher annual nutrient requirements compared to species adapted to uplands. Consequently, forest vegetation on many floodplains is likely to be nitrogen deficient and, in some cases such as blackwater systems, deficient in phosphorus and base cations as well. An example would be the nutrient-demanding Populus deltoides Batr. plantations that grow in extraordinarily fertile soils of the Southern Mississippi Alluvial Valley, USA. In spite of fertile soils and high aboveground NPP (20-25 tha- yr~ ), those systems would increase in NPP if supplied with additional nitrogen.

The degree to which a floodplain ecosystem is deficient or nondeficient for particular nutrients is critical in regard to that system's potential to act as a nutrient sink. As previously mentioned, the kidney function is enhanced if floodplain vegetation can assimilate incoming nutrients from sources such as polluted water or atmospheric inputs. Once a deficiency is eliminated, it is still possible for floodplain vegetation to assimilate particular nutrients such as nitrogen through luxury consumption. However, a level may be reached after which the vegetation's capacity to retain nutrients is saturated. The latter condition reflects a high degree of biotic stress and is a serious threat to floodplain vegetation associated with eutrophic streams.

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