The main focus of this study was to evaluate the effects of soil moisture and temperature on temporal variation of N 2 O, CO 2 and CH 4 soil-atmosphere exchange at a primary seasonal tropical rainforest (PF) site in Southwest China and to compare these fluxes with fluxes from a secondary forest (SF) and a rubber plantation (RP) site. Agroforestry systems, such as rubber plantations, are increasingly replacing primary and secondary forest systems in tropical Southwest China and thus effect the N 2 O emission in these regions on a landscape level. The mean N 2 O emission at site PF was 6.0 ± 0.1 SE lg N m -2 h -1 . Fluxes of N 2 O increased from <5 lg N m -2 h -1 during dry season conditions to up to 24.5 lg N m -2 h -1 with rewetting of the soil by the onset of first rainfall events. Comparable fluxes of N 2 O were measured in the SF and RP sites, where mean N 2 O emissions were 7.3 ± 0.7 SE lg N m -2 h -1 and 4.1 ± 0.5 SE lg N m -2 h -1 , respectively. The dependency of N 2 O fluxes on soil moisture levels was demonstrated in a watering experiment, however, artificial rainfall only influenced the timing of N 2 O emission peaks, not the total amount of N 2 O emitted. For all sites, significant positive correlations existed between N 2 O emissions and both soil moisture and soil temperature. Mean CH 4 uptake rates were highest at the PF site (-29.5 ± 0.3 SE lg C m -2 h -1 ), slightly lower at the SF site (-25.6 ± 1.3 SE lg C m -2 h -1 ) and lowest for the RP site (-5.7 ± 0.5 SE lg C m -2 h -1 ). At all sites, CH 4 uptake rates were negatively correlated with soil moisture, which was also reflected in the lower uptake rates measured in the watering experiment. In contrast to N 2 O emissions, CH 4 uptake did not significantly correlate with soil temperature at the SF and RP sites, and only weakly correlated at the PF site. Over the 2 month measurement period, CO 2 emissions at the PF site increased significantly from 50 mg C m -2 h -1 up to 100 mg C m -2 h -1 (mean value 68.8 ± 0.8 SE mg C m -2 h -1 ), whereas CO 2 emissions at the SF and RP site where quite stable and varied only slightly around mean values of 38.0 ± 1.8 SE mg C m -2 h -1 (SF) and 34.9 ± 1.1 SE mg C m -2 h -1 (RP). content could be demonstrated for all sites, thus, the watering experiment revealed significantly higher CO 2 emissions as compared to control chambers. Correlation of CO 2 emissions with soil temperature was significant at the PF site, but weak at the SF and not evident at the RP site. Even though we demonstrated that N and C trace gas fluxes significantly varied on subdaily and daily scales, weekly measurements would be sufficient if only the sink/ source strength of non-managed tropical forest sites needs to be identified.

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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 (SOC) and total nitrogen (TN) 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.