Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Abstract

Immobilization of ammonium (NH + 4 ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of 15 NH + 4 solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH + 4 , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old, 15 N enriched nitrate (NO − 3 ) was applied in the 0-150 mm soil layer. After 24 h, > 30 times more 15 NH + 4 was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH + 4 -N present at this time in the soil with roots had been converted from NO − 3 . We estimated rates of conversion of NO − 3 to NH + 4 , and rates of NH + 4 immobilization by plants and microbes, by simulating N-flow of 14+15 N and 15 N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO − 3 reduction to NH + 4 (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO − 3 uptake, plant NH + 4 uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO − 3 to NH + 4 via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH + 4 than generally assumed.

Plant Colonizers Shape Early N-dynamics in Gopher-mounds

“…However, the rapid accumulation of aboveground biomass in Aegilops suggests that the species can compete for N with microbes through its early growth and nutrient uptake. Plant effects on microbial N uptake are critical since microbial N represents a stock of biologically active N that can be mineralized and used by plants (Burger and Jackson, 2005). Aegilops is an aggressive invasive species in northern California grasslands (DiTomaso, 2000), and its reductions of microbial N stocks may partly explain its tendency to out-compete and displace other plant species.…”

Assessment of the importance of dissimilatory nitrate reduction to ammonium for the terrestrial nitrogen cycle

“…However, the transfer of 15 N from NO − 3 to NH + 4 could in fact be due to three different pathways (Fig. 1b): (1) DNRA, (2) plant N efflux and (3) remineralisation by microorganisms (Burger and Jackson, 2004). Using data from a microcosm 15 N study and simulation models Burger and Jackson (2004) provided evidence that each of the three pathways was on its own able to explain the observed 15 N dynamics.…”

“…In the above mentioned Nothofagus study these alternative pathways (plant N efflux and remineralisation) could be ruled out as no roots were present in the laboratory incubation and the 15 N enrichment in five organic N fractions was too low to explain the 15 NH + 4 enrichment by remineralisation Huygens et al, 2007). More detailed studies are needed to investigate the importance of the alternative pathways proposed by Burger and Jackson (2004), i.e. DNRA, plant N efflux and remineralisation by microorganisms, by combining 15 N labelling studies with numerical data analysis.…”

In-Season Prediction of Nitrogen Use Efficiency and Grain Protein in Winter Wheat (Triticum aestivumL.)

“…Hirel et al (2007) argued that NUE is a function of factors such as climate, soil texture, the interaction between soil and microbes (Hirel et al 2007;Walley et al, 2003;Burger and Jackson 2004) and attributes related to the available N pool (Hirel et al, 2007;Schulten and Schnitzer, 1998).…”