Effects of 10 yr of nitrogen and phosphorus fertilization on carbon and nutrient cycling in a tidal freshwater marsh

Abstract

Tidal freshwater marshes can protect downstream ecosystems from eutrophication by intercepting excess nutrient loads, but recent studies in salt marshes suggest nutrient loading compromises their structural and functional integrity. Here, we present data on changes in plant biomass, microbial biomass and activity, and soil chemistry from plots in a tidal freshwater marsh on the Altamaha River (GA) fertilized for 10 yr with nitrogen (+N), phosphorus (+P), or nitrogen and phosphorus (+NP). Nitrogen alone doubled aboveground biomass and enhanced microbial activity, specifically rates of potential nitrification, denitrification, and methane production measured in laboratory incubations. Phosphorus alone increased soil P and doubled microbial biomass but did not affect microbial processes. Nitrogen or P alone decreased belowground biomass and soil carbon (C) whereas +NP increased aboveground biomass, microbial biomass and N cycling, and N, P, and C assimilation and burial more than either nutrient alone. Our findings suggest differential nutrient limitation of tidal freshwater macrophytes by N and microbes by P, similar to what has been observed in salt marshes. Macrophytes outcompete microbes for P in response to long-term N and P additions, leading to increased soil C storage through increased inputs of belowground biomass relative to N and P added singly. The susceptibility of tidal freshwater marshes to long-term nutrient enrichment and, hence their ability to mitigate eutrophication will depend on the quantity and relative proportion of N vs. P entering estuaries and tidal wetlands.

Effect of long-term differentiated fertilisation regimes on greenhouse gas emissions from a subtropical rice-wheat cropping system

“…The combined use of N and P (i.e., NP and NPK treatments) also stimulated CH 4 emission to various extents when compared to separated use of N or P (i.e., N, NK and PK treatments). The stimulatory effect of combined application of fertilisers may result from high soil organic carbon decomposition with increasing microbial biomass and activity and/or more photosynthesised carbon released into soils as exudates due to concurrent N and P addition (Atere et al 2019, Herbert et al 2020. Small residual effects of N and P addition appeared to stay during the fallow period.…”

From the headwaters to the sea: The role of riparian, alluvial, and tidal wetlands to filter nutrients and ameliorate eutrophication

“…On the basis of such low N:P, mineral soil wetlands of riparian and floodplain likely exhibit strong N limitation as they remove large amounts of sediment and associated P coming off the adjacent terrestrial landscape. N:P of tidal fresh and brackish marshes are considerably higher but also are limited by N based on long‐term (10 year) additions of N, P, and N + P to tidal fresh marshes of the Altamaha River (Herbert et al, 2020) and most other coastal wetlands (Morris et al, 2013). N:P was strongly correlated with soil N ( r = 0.95) but not P, suggesting that soil N and not P regulates plant productivity and possibly other ecosystem processes of wetlands along the waterway.…”

“…The effects of N:P on denitrification, another mechanism in addition to burial, are not clear. However, it is not expected to affect denitrification to the same extent as external N loading which is known to increase coupled nitrification-denitrification (Herbert et al, 2020). Carbon sequestration and N burial were greatest in tidal fresh and brackish marshes (Figure 5) where plant productivity (Więski et al, 2010) and soil accretion were greatest (Figure 3).…”

Mass removal efficiencies in water and consequences after a river diversion into coastal wetlands: second thoughts

“…The soil bulk density in the Davis Pond Diversion is 0.08 ± 0.1 g cm 3 (Delaune et al, 2013) which defines them as organic soils. Organic soils are negatively impacted as plants and soils are subjected to enhanced nutrient availability in tundra, salt marsh, brackish marsh and fresh marsh (Mack et al, 2004;Bragazza et al, 2006;Darby & Turner, 2008;Turner, 2011;Deegan et al, 2012;Wigand et al, 2018;Herbert et al, 2020;Krause et al, 2020). The belowground biomass of roots and rhizomes decreases because plants put more resources into the above-ground biomass when access to nutrients is facilitated; root mass may increase in the upper few decimeters but decline at lower depths.…”