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

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.

337 Efficient RFI Bulls Presented Lower Fractional Synthesis Rates of Plasma Proteins When Fed Corn but Not Grass Based Diets

Protein turnover (PT), the continual synthesis and degradation of body proteins not leading to protein gain, is an essential high energy-demanding process. We assumed that PT might explain the between-animal variations of residual feed intake (RFI). The objective was to measure PT in extreme RFI cattle fed two contrasted diets (grass or corn-based). We conducted a RFI test for 84 days with 100 Charolais bulls and we selected the 32 most extreme (8 per diet and RFI group) for PT measurements using 1) the urinary 3-methyl-histidine to creatinine ratio, as a biomarker of the fractional protein degradation rate (FDR) of skeletal-muscle and 2) the isotopic N turnover rate measured in urine and plasma, as a proxy, respectively, of the whole-body FDR and the fractional protein synthesis rates (FSR) of plasma proteins. The 3-methyl-histidine and creatinine were determined from 10 d total urine collection. Isotopic N turnover in urine and plasma was evaluated by modelling the 15N depletion rate over 112 d following an isotopic N dietary change. Higher plasma FSR and higher skeletal-muscle and whole-body FDR were observed with corn-vs-grass diets (≥11%; P ≤ 0.03), in line with higher metabolizable protein and net energy intakes (≥10%, P = 0.001). Differences between extreme RFI animals were noted with the corn diets only, where efficient animals presented significant lower plasma FSR (-10%; P = 0.04) and numerically lower skeletal-muscle and whole-body FDR (-13% and - 8.9%; P > 0.16 respectively) than non-efficient. Non-significant differences were probably due to an insufficient size of our experimental setup. Plasma FSR is related to the PS of hepatic exportation, hence the lower plasma FSR observed in efficient RFI animals fed corn diets may reflect a lower organs to carcass ratio. Altogether results suggests that efficient RFI bulls fed corn diets had a lower hepatic PT with no-significant changes of whole-body and skeletal muscle PT.

Fate of nitrate during groundwater recharge in a fractured karst aquifer in Southwest Germany

Over the past decades, fractured and karst groundwater systems have been studied intensively due to their high vulnerability to nitrate (NO3−) contamination, yet nitrogen (N) turnover processes within the recharge area are still poorly understood. This study investigated the role of the karstified recharge area in NO3− transfer and turnover by combining isotopic analysis of NO3− and nitrite (NO2−) with time series data of hydraulic heads and specific electrical conductivity from groundwater monitoring wells and a karstic spring in Germany. A large spatial variability of groundwater NO3− concentrations (0.1–0.8 mM) was observed, which cannot be explained solely by agricultural land use. Natural-abundance N and O isotope measurements of NO3− (δ15N and δ18O) confirm that NO3− derives mainly from manure or fertilizer applications. Fractional N elimination by denitrification is indicated by relatively high δ15N- and δ18O-NO3− values, elevated NO2− concentrations (0.05–0.14 mM), and δ15N-NO2− values that were systematically lower than the corresponding values of δ15N-NO3−. Hydraulic and chemical response patterns of groundwater wells suggest that rain events result in the displacement of water from transient storage compartments such as the epikarst or the fissure network of the phreatic zone. Although O2 levels of the investigated groundwaters were close to saturation, local denitrification might be promoted in microoxic or anoxic niches formed in the ferrous iron-bearing carbonate rock formations. The results revealed that (temporarily) saturated fissure networks in the phreatic zone and the epikarst may play an important role in N turnover during the recharge of fractured aquifers.