Impacts of chronic N input on the carbon and nitrogen storage of a postfire Mediterranean‐type shrubland

Abstract

Mediterranean‐type shrublands are subject to periodic fire and high levels of nitrogen (N) deposition, but little is known how chronic N deposition affects carbon (C) and N storage during succession. We conducted a long‐term experiment in Californian chaparral to test the hypothesis that chronic N enrichment would increase postfire C and N accumulation. The experimental layout consisted of a randomized design where four 10 × 10 m plots received 5 g N m−2 annually since 2003 and four 10 × 10 m plots served as controls. Aboveground and belowground C and N pools and fluxes were measured seasonally (every 3 months) for a period of 10 years. Added N rapidly increased soil extractable N pools and decreased soil pH; however, total soil C and N storage were not affected. Added N plots initially had significantly lower C and N storage than control plots, presumably because of nutrient losses from leaching and/or higher belowground C allocation. However, rates of aboveground N and C storage became significantly higher in added N plots after 4–5 years of exposure, thus increasing fuel buildup, which has implications for future fire intensity. This recovering chaparral stand is not yet “N saturated” after 10 years of chronic N input. However, N leaching continues to be higher in added N plots, indicating that postfire chaparral stands in high‐N deposition areas can be important sources of N to groundwater/aquatic systems even if productivity is stimulated by N input.

Nitrogen cycling and export in California chaparral: the role of climate in shaping ecosystem responses to fire

“…). The role of climate in driving plant recovery and N immobilization may be even more pronounced in watershed subject to high rates of N deposition, where N export can outstrip plant recovery and N uptake for prolonged periods following fire (Vourlitis and Hentz ).…”

“…These findings are consistent with streamflow observations in the region, which show that following an initial postfire flush, N export can rapidly return to baseline conditions (Coombs andMelack 2013, Verkaik et al 2013). The role of climate in driving plant recovery and N immobilization may be even more pronounced in watershed subject to high rates of N deposition, where N export can outstrip plant recovery and N uptake for prolonged periods following fire (Vourlitis and Hentz 2016).…”

Hydroseeding increases ecosystem nitrogen retention but inhibits natural vegetation regeneration after two years of chaparral post-fire recovery

“…In contrast, extractable NO 3 concentrations were more similar in burned and unburned sites even though the burned stands had consistently higher rates of nitrification than the unburned stand. Presumably, the burned stands lost more NO 3 from leaching and/or gaseous efflux (Riggan et al 1994;Meixner et al 2006;Vourlitis and Hentz 2016), thus reducing concentrations of soil extractable NO 3 .…”

“…These impacts are strongly affected by fire severity, which in turn affects rates of vegetation and microbial recovery (Neary et al 1999;Knicker 2007). Nutrient losses are positively related to fire intensity (DeBano and Conrad 1978); however, nutrient availability may exhibit an increase immediately after fire due to the rapid decomposition and mineralization of ash (Fenn et al 1993; Carreira et al 1994;Vourlitis and Pasquini 2008;Vourlitis and Hentz 2016). This nutrient pulse may lead to high rates of nutrient loss from leaching (Riggan et al 1994;Meixner et al 2006), but leaching losses typically decline over time as plants recover (Vourlitis and Hentz 2016).…”

“…Nutrient losses are positively related to fire intensity (DeBano and Conrad 1978); however, nutrient availability may exhibit an increase immediately after fire due to the rapid decomposition and mineralization of ash (Fenn et al 1993; Carreira et al 1994;Vourlitis and Pasquini 2008;Vourlitis and Hentz 2016). This nutrient pulse may lead to high rates of nutrient loss from leaching (Riggan et al 1994;Meixner et al 2006), but leaching losses typically decline over time as plants recover (Vourlitis and Hentz 2016). Similarly, microbial populations typically decline after fire (Hanan et al 2016), but low intensity fires may have a small effect on microbial populations while moderate-high intensity fires significantly reduce microbial population size (Neary et al 1999;Knicker 2007;Fonturbel et al, 2012).…”