Abstract
Abstract. Ecosystems limited in phosphorous (P) are widespread, yet there is limited understanding of how these ecosystems may respond to anthropogenic deposition of nitrogen (N) and the interconnected effects on the biogeochemical cycling of carbon (C), N, and P. Here, we investigate the consequences of enhanced N addition for the C–N–P pools of two P-limited grasslands, one acidic and one limestone, occurring on contrasting soils, and we explore their responses to a long-term nutrient-manipulation experiment. We do this by combining data with an integrated C–N–P cycling model (N14CP). We explore the role of P-access mechanisms by allowing these to vary in the modelling framework and comparing model plant–soil C–N–P outputs to empirical data. Combinations of organic P access and inorganic P availability most closely representing empirical data were used to simulate the grasslands and quantify their temporal response to nutrient manipulation. The model suggested that access to organic P is a key determinant of grassland nutrient limitation and responses to experimental N and P manipulation. A high rate of organic P access allowed the acidic grassland to overcome N-induced P limitation, increasing biomass C input to soil and promoting soil organic carbon (SOC) sequestration in response to N addition. Conversely, poor accessibility of organic P for the limestone grassland meant N provision exacerbated P limitation and reduced biomass input to the soil, reducing soil carbon storage. Plant acquisition of organic P may therefore play an important role in reducing P limitation and determining responses to anthropogenic changes in nutrient availability. We conclude that grasslands differing in their access to organic P may respond to N deposition in contrasting ways, and where access is limited, soil organic carbon stocks could decline.
Highlights
Grasslands represent up to a fifth of terrestrial net primary productivity (NPP) (Chapin et al, 2011) and potentially hold over 10 % of the total organic carbon stored within the biosphere (Jones and Donnelly, 2004)
The model calibration selected parameter values for PWeath0 and PCleaveMax that indicate contrasting use of P sources by the two simulated grasslands, with the acidic grassland capable of acquiring more P from organic sources having a PCleaveMax value of 0.32 g m−2 per season compared to the limestone, with a value 10 times smaller at 0.03 g m−2 per season
Inorganic P availability was greater in the limestone grassland due to the larger weatherable pool of P, PWeath0, at 300 g m−2 compared to 150 g m−2 in the acidic grassland
Summary
Grasslands represent up to a fifth of terrestrial net primary productivity (NPP) (Chapin et al, 2011) and potentially hold over 10 % of the total organic carbon stored within the biosphere (Jones and Donnelly, 2004). Since the onset of the industrial revolution, human activity has doubled the global cycling of N, with anthropogenic sources contributing 210 Tg of fixed N yr−1 to the global N cycle, surpassing naturally fixed N by 7 Tg N yr−1 (Fowler et al, 2013). Much of this additional N is deposited on terrestrial ecosystems from atmospheric sources. This magnitude of N deposition results in a range of negative impacts on ecosystems (including grasslands) such as reductions in biodiversity (Bobbink et al, 2010; Southon et al, 2013), acidification of soil, and the mobilisation of potentially toxic metals (Carroll et al, 2003; Horswill et al, 2008; Phoenix et al, 2012).
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