Carbon storage in seagrass soils: long-term nutrient history exceeds the effects of near-term nutrient enrichment

A R Armitage,J W Fourqurean

doi:10.5194/bg-13-313-2016

Abstract

Abstract. The carbon sequestration potential in coastal soils is linked to aboveground and belowground plant productivity and biomass, which in turn, is directly and indirectly influenced by nutrient input. We evaluated the influence of long-term and near-term nutrient input on aboveground and belowground carbon accumulation in seagrass beds, using a nutrient enrichment (nitrogen and phosphorus) experiment embedded within a naturally occurring, long-term gradient of phosphorus availability within Florida Bay (USA). We measured organic carbon stocks in soils and above- and belowground seagrass biomass after 17 months of experimental nutrient addition. At the nutrient-limited sites, phosphorus addition increased the carbon stock in aboveground seagrass biomass by more than 300 %; belowground seagrass carbon stock increased by 50–100 %. Soil carbon content slightly decreased ( ∼ 10 %) in response to phosphorus addition. There was a strong but non-linear relationship between soil carbon and Thalassia testudinum leaf nitrogen : phosphorus (N : P) or belowground seagrass carbon stock. When seagrass leaf N : P exceeded an approximate threshold of 75 : 1, or when belowground seagrass carbon stock was less than 100 g m−2, there was less than 3 % organic carbon in the sediment. Despite the marked difference in soil carbon between phosphorus-limited and phosphorus-replete areas of Florida Bay, all areas of the bay had relatively high soil carbon stocks near or above the global median of 1.8 % organic carbon. The relatively high carbon content in the soils indicates that seagrass beds have extremely high carbon storage potential, even in nutrient-limited areas with low biomass or productivity.

Highlights

Increases in anthropogenic nutrient supply can alter coastal intertidal and subtidal plant communities by increasing aboveground biomass, lowering belowground biomass, or both (Deegan et al, 2012; Darby and Turner, 2008; Herbert and Fourqurean, 2009; Turner et al, 2009). Such changes in plant community structure are closely linked to the carbon storage potential of vegetated coastal ecosystems – a topic of key interest in emerging carbon markets (Russell et al, 2013; Couto et al, 2013; Alongi, 2014)
We evaluated the influence of long-term and near-term nutrient history on aboveground and belowground carbon accumulation in seagrass beds, using a nutrient enrichment experiment embedded within a naturally occurring, long-term gradient of nutrient availability within Florida Bay
The total carbon stock in aboveground seagrass tissue, which is a function of seagrass biomass, was 3 to 10× higher in Paddition treatments, but only at the three most P-limited sites in eastern Florida Bay (Site × P p < 0.001; Table 1; Fig. 2a)

Summary

Introduction

Increases in anthropogenic nutrient supply can alter coastal intertidal and subtidal plant communities by increasing aboveground biomass, lowering belowground biomass, or both (Deegan et al, 2012; Darby and Turner, 2008; Herbert and Fourqurean, 2009; Turner et al, 2009). Such changes in plant community structure are closely linked to the carbon storage potential of vegetated coastal ecosystems – a topic of key interest in emerging carbon markets (Russell et al, 2013; Couto et al, 2013; Alongi, 2014). Microbial communities are but inversely influential, with microbial decomposition and respiration generally causing net efflux of carbon from the soils in a process known as mi-

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