A review of the importance of mineral nitrogen cycling in the plant-soil-microbe system of permafrost-affected soils—changing the paradigm

• Published in: Environmental research letters Vol. 17; no. 1; pp. 13004 - 13037
• Main Authors: Ramm, Elisabeth, Liu, Chunyan, Ambus, Per, Butterbach-Bahl, Klaus, Hu, Bin, Martikainen, Pertti J, Marushchak, Maija E, Mueller, Carsten W, Rennenberg, Heinz, Schloter, Michael, Siljanen, Henri M P, Voigt, Carolina, Werner, Christian, Biasi, Christina, Dannenmann, Michael
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.01.2022
• Subjects: Ammonification; Cold weather; Cycles; Depolymerization; global change; gross N turnover; Meta-analysis; Microorganisms; Mineralization; Nitrification; Nitrogen; Nitrogen cycle; Nutrition; Organic carbon; Organic matter; Organic soils; Permafrost; Plant nutrition; plant-soil-microbe system; Soil layers; Stoichiometry; Tropical environments; Tropical soils; Turnover rate
• ISSN: 1748-9326; 1748-9326
• DOI: 10.1088/1748-9326/ac417e

The paradigm that permafrost-affected soils show restricted mineral nitrogen (N) cycling in favor of organic N compounds is based on the observation that net N mineralization rates in these cold climates are negligible. However, we find here that this perception is wrong. By synthesizing published data on N cycling in the plant-soil-microbe system of permafrost ecosystems we show that gross ammonification and nitrification rates in active layers were of similar magnitude and showed a similar dependence on soil organic carbon (C) and total N concentrations as observed in temperate and tropical systems. Moreover, high protein depolymerization rates and only marginal effects of C:N stoichiometry on gross N turnover provided little evidence for N limitation. Instead, the rather short period when soils are not frozen is the single main factor limiting N turnover. High gross rates of mineral N cycling are thus facilitated by released protection of organic matter in active layers with nitrification gaining particular importance in N-rich soils, such as organic soils without vegetation. Our finding that permafrost-affected soils show vigorous N cycling activity is confirmed by the rich functional microbial community which can be found both in active and permafrost layers. The high rates of N cycling and soil N availability are supported by biological N fixation, while atmospheric N deposition in the Arctic still is marginal except for fire-affected areas. In line with high soil mineral N production, recent plant physiological research indicates a higher importance of mineral plant N nutrition than previously thought. Our synthesis shows that mineral N production and turnover rates in active layers of permafrost-affected soils do not generally differ from those observed in temperate or tropical soils. We therefore suggest to adjust the permafrost N cycle paradigm, assigning a generally important role to mineral N cycling. This new paradigm suggests larger permafrost N climate feedbacks than assumed previously.