Functional Relationships with N Deposition Differ According to Stand Maturity in Calluna-Dominated Heathland

Abstract

Plant and soil bio(chemical) indicators are increasingly used to provide information on N deposition inputs and effects in a wide range of ecosystem types. However, many factors, including climate and site management history, have the potential to influence bioindicator relationships with N due to nutrient export and changing vegetation nutrient demands. We surveyed 33 heathlands in England, along a gradient of background N deposition (7.2-24.5 kg ha -1 year -1 ), using Calluna vulgaris growth phase as a proxy for time since last management. Our survey confirmed soil nutrient accumulation with increasing time since management. Foliar N and phosphorus (P) concentrations in pioneer-and maturephase vegetation significantly increased with N deposition. Significant interactions between climate and N deposition were also evident with, for example, higher foliar P concentrations in pioneer-phase vegetation at sites with higher temperatures and N deposition rates. Although oxidized N appeared more significant than reduced N, overall there were more, stronger relationships with total N deposition; suggesting efforts to control all emissions of N (i.e., both oxidized and reduced forms) will have ecological benefits.

Time- and age-related effects of experimentally simulated nitrogen deposition on the functioning of montane heathland ecosystems

“…Finally, young Calluna plants had significantly higher shoot N and P and litter N contents than mature ones across all N treatments, very likely indicating that (1) young plants may acquire high amounts of nutrients to support their greater and faster annual growth rates (Gimingham, 1972;Jones and Power, 2015), while (2) mature plants with lower annual growth rates may store the acquired nutrients more evenly in their higher above-and below-ground plant biomass.…”

“…As a result, excess N accumulation in heathland ecosystems promotes enhanced rates of nutrient uptake by Calluna plants and subsequent increases in foliar tissue N and P contents (Calvo et al, 2007;Jones and Power, 2012;Pilkington et al, 2005b;Rowe et al, 2008;von Oheimb et al, 2010), as well as increases in litter N and P contents (Pilkington et al, 2005b). Age-related differences in Calluna nutrient uptake and growth rate are expected to influence the impacts of N deposition on heathlands (Jones and Power, 2015;Meyer-Grünefeldt et al, 2015), but till now only a limited number of studies have assessed these effects (i.e., Britton et al, 2008;Jones and Power, 2015). European heathlands have traditionally been managed to create pastures for breeding livestock and their nutrient poor status has been preserved through practices as mowing, sod cutting and prescribed burning (Fagúndez, 2013;Härdtle et al, 2006Härdtle et al, , 2009, resulting in the periodic rejuvenation of heathland vegetation (Gimingham, 1972;Henning et al, 2017).…”

“…In recent decades, however, land use abandonment has led to heathland management cessation and to Calluna plants reaching the mature or degenerate phase of development (sensu Gimingham, 1972;Calvo et al, 2007;Henning et al, 2017). As time progresses since the last management (e.g., prescribed burning, mowing, sod-cutting, and grazing), ageing heathland ecosystems accumulate N in soils and in the vegetation biomass (Härdtle et al, 2009;Jones and Power, 2015). Therefore, specific measures to compensate for atmospheric N deposition are required to remove the excess of N stored in the ecosystem, and, thus to keep a low-N status (Calvo et al, 2005;Härdtle et al, 2006Härdtle et al, , 2009Marcos et al, 2009).…”

“…The reported increase in Calluna tissue N and P contents may be related to the observed increases in litter N and P contents (Jones and Power, 2012), likely due to the inputs of the N and P enriched shoots to the litter layer (Pilkington et al, 2005b). However, the increase in Calluna tissue and litter P contents did not alter soil available P content, since all newly-mineralised soil P may have been immediately taken up by the plants or incorporated into the soil microbial biomass to satisfy their enhanced P demands in response to the addition of N (Friedrich et al, 2011;Johnson et al, 1998Johnson et al, , 1999Jones and Power, 2015;Rowe et al, 2008); or even immobilized by iron and aluminium (hydr)oxides in acid soils (Kooijman et al, 1998). Particularly, the enhanced P demand of Calluna plants in the chronic high N treatment (N56) in low available P status of heathland soils was probably satisfied by: (1) the observed increase in the activity of soil acid phosphatase enzyme that is necessary for the mineralization of soil organic P (López-Poma and Bautista, 2014;Pilkington et al, 2005a;Rowe et al, 2008); and by (2) the observed increase in the degree of Calluna root mycorrhizal colonization by ericoid fungi that might enhance plant nutrient uptake from the widely extended hyphae network (Díaz et al, 2006;Jones and Power, 2012;Rowe et al, 2008), including the nutrient uptake of organic forms (Johnson et al, 2005).…”

Enhanced shoot investment makes invasive plants exhibit growth advantages in high nitrogen conditions

“…Concerns about how invasive plants will interact with N deposition have been articulated (Bradley et al, 2010). Until recently, there were limited studies comparing invasive and native responses to predicted N deposition (He et al, 2012;Jones and Power, 2015). Here, we provide data on how N addition influences the growth of invasive and native species.…”