Denitrification Enzyme Activity

We evaluated patterns of denitrification and factors effecting denitrification in the upper Mississippi River. Measurements were taken over 2 years, during which river discharge ranged from record flooding to base flow conditions. Over the period of study, average denitrification enzyme activity was highest in backwater lakes and lowest in the main channel. Throughout the study reach, highest denitrification enzyme activity occurred during fall and lowest occurred in winter. Rates during spring floods (2001) were only slightly higher than during the preceding winter. Mean unamended denitrification rates ranged from 0.02 (fall 2001 in backwaters) to 0.40 µg N·cm -2 ·h -1 (spring 2001 in backwaters). Laboratory experiments showed that denitrification rates increased significantly with addition of NO 3 regardless of sediment C content, while rates increased little with addition of labile C (glucose). Denitrification in this reach of the upper Mississippi River appears to be NO 3 limited throughout the growing season and the delivery of NO 3 is strongly controlled by river discharge and hydrologic connectivity across the floodplain. We estimate that denitrification removes 6939 t N·year -1 or 6.9% of the total annual NO 3 input to the reach. Hydrologic connectivity and resultant NO 3 delivery to high-C sediments is a critical determinant of reach-scale processing of N in this floodplain system.

Maize inoculation by Azospirillum stimulates root growth, along with soil nitrogen (N) uptake and root carbon (C) exudation, thus increasing N use efficiency. However, inoculation effects on soil N-cycling microbial communities have been overlooked. We hypothesized that inoculation would (i) increase roots-nitrifiers competition for ammonium, and thus decrease nitrifier abundance; and (ii) increase roots-denitrifiers competition for nitrate and C supply to denitrifiers by root exudation, and thus limit or benefit denitrifiers depending on the resource (N or C) mostly limiting these microorganisms. We quantified (de)nitrifiers abundance and activity in the rhizosphere of inoculated and non-inoculated maize on 4 sites over 2 years, and ancillary soil variables. Inoculation effects on nitrification and nitrifiers (AOA, AOB) were not consistent between the three sampling dates. Inoculation influenced denitrifiers abundance (nirK, nirS) differently among sites. In sites with high C limitation for denitrifiers (i.e. limitation of denitrification by C > 66%), inoculation increased nirS-denitrifier abundance (up to 56%) and gross N 2 O production (up to 84%), likely due to increased root C exudation. Conversely, in sites with low C limitation (<47%), inoculation decreased nirS-denitrifier abundance (down to −23%) and gross N 2 O production (down to −18%) likely due to an increased roots-denitrifiers competition for nitrate. The rhizosphere provides a peculiar environment where a huge variety of positive, negative and neutral interactions between roots and microorganisms occur 1. Such interactions can significantly influence plant growth as well as the functioning, the abundance and the diversity of rhizospheric microbial communities 2. Beneficial interactions are known to be established by plant growth-promoting rhizobacteria, PGPRs, with host plants through several mechanisms, including associative N 2 fixation, phosphate solubilization or phytosiderophore production 3, 4. This can result in improved root growth 5, 6 , increased number and length of lateral roots 7 , as well as an increased root and shoot biomass 8, 9 and physiology 10. The better root development induced by inoculation can consequently enhance nutrient 11 and water 12 uptake by plant, stimulate ion transport systems in root 13 and increase the amount of root carbon, C, exudation 14, 15. Azospirillum spp. are well-known PGPRs that are able to colonize the roots of many crop plant species including maize 16, 17. These PGPRs produce phytohormones that can promote root growth and improve nutrient and water absorption by plants 18-21. In particular, inoculation of cereal crops by PGPRs like the well-studied Azospirillum lipoferum CRT1 is often expected to improve crop capacity to retrieve mineral nitrogen, N, from soil. This could pave the way for improving the sustainability of these cropping systems under low N inputs conditions 8. However, inoculated plants could differently affect N dynamics in their rhizosphere, thus influencing the levels and types of mineral N forms available and possibly N losses from soil through leaching of nitrate, NO 3 − , or emission of nitrous oxide, N 2 O, a potent greenhouse gas 22 .

Sediments in the backwaters of the Upper Mississippi River (UMR) are highly organic and provide an optimal environment for N removal. We monitored an 8.6-ha UMR backwater site near La Crosse, Wisconsin, for nearly 3 y to assess temporal variability, seasonal trends, and the factors regulating denitrification. We measured rates of unamended denitrification (DEN) and denitrification enzyme activity (DEA) rates at ambient temperature and DEA at 308C (DEA30). Seasonal mean (61 SE) DEN rates ranged from 0.041 6 0.015 to 0.47 6 0.23 lg N cm À2 h À1 and were highest in winter and lowest in autumn. Seasonal rates of DEA exhibited a different pattern with the highest rates in summer (25.6 6 3.4 lg N cm À2 h À1 ) and the lowest rates in winter (10.6 6 2.1 lg N cm À2 h À1 ). The overall mean DEA30 rate was 31.0 6 1.9 lg N cm À2 h À1 but showed no significant seasonal pattern. Short-term (weekly) and seasonal variability exhibited by rates of DEN and DEA were best explained by water-column NO 3 À concentration and temperature, respectively. No environmental variables explained a significant amount of variability in DEA30. Our results suggest that nutrient (i.e., NO 3 À ) availability and temperature are both regulators of denitrification, with NO 3 À concentration being the most important limiting factor in this system. The high DEN rates during winter were in response to elevated NO 3 À concentrations resulting from a chain reaction beginning with algal blooms creating oxic conditions that stimulated nitrification. Increasing hydrological connectivity in large rivers as a river management tool to reduce N flux to downstream areas may be beneficial.

Soil texture plays a key role in belowground C storage in forest ecosystems and strongly influences nutrient availability and retention, particularly in highly weathered soils. We used field data and the Century ecosystem model to explore the role of soil texture in belowground C storage, nutrient pool sizes, and N fluxes in highly weathered soils in an Amazonian forest ecosystem. Our field results showed that sandy soils stored approximately 113 Mg C ha -1 to a 1-m depth versus 101 Mg C ha -1 in clay soils. Coarse root C represented a large and significant ecosystem C pool, amounting to 62% and 48% of the surface soil C pool on sands and clays, respectively, and 34% and 22% of the soil C pool on sands and clays to 1-m depth. The quantity of labile soil P, the soil C:N ratio, and live and dead fine root biomass in the 0-10-cm soil depth decreased along a gradient from sands to clays, whereas the opposite trend was observed for total P, mineral N, potential N mineralization, and denitrification