Experimental nitrogen and phosphorus additions increase rates of stream ecosystem respiration and carbon loss

Abstract

Nitrogen (N) and phosphorus (P) enrichment reduces organic carbon (C) storage in detritus‐based stream ecosystems, but the relative effects of N and P concentrations and ratios on stream metabolic rates have not previously been tested. We tracked changes in whole‐stream ecosystem respiration (ER) and gross primary productivity (GPP), particulate organic matter (POM) standing stocks, fungal biomass, and POM‐specific respiration rates before and during 2 yr of experimental N and P enrichment in five forest streams. Nutrient additions (∼ 96 μg N L −1 to ∼ 472 μg N L −1 and ∼ 10 μg P L −1 to ∼ 85 μg P L −1 ) targeted dissolved N : P molar ratios of 2, 8, 16, 32, and 128. Whole‐stream ER was positively related to standing stock of wood, a seasonably stable POM compartment that varied by up to 2× among streams. Nutrient enrichment generally increased ER but had no effect on low‐level GPP. Prior to nutrient enrichment, ER was higher at lower N : P, but during enrichment ER increased with increasing N : P. Respiration rates on leaf litter and wood increased with enrichment but decreased with increasing P, and the quantity of leaf litter generally declined with increasing N. Respiration rates on fine benthic organic matter (FBOM) were higher with increasing N : P, and FBOM standing stocks decreased with increasing N. Fungal biomass did not change with nutrient enrichment. Compared to pre‐enrichment conditions, nutrients increased seasonal variation in leaf litter standing stocks and whole‐stream respiration rates. Our results demonstrate how nutrient‐stimulated loss of C from detritus‐based ecosystems occurs through the maintenance of enhanced respiration rates on detrital resources that are particularly sensitive to N inputs.

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