Plant–microbe competition: does injection of isotopes of C and N into the rhizosphere effectively characterise plant use of soil N?

Abstract

Despite considerable attention over the last 25 yr, the importance of early protein breakdown products to plant nitrogen (N) nutrition remains uncertain. We used rhizosphere injection of N-, C- and C-labelled inorganic N and amino acid (l-alanine), with chase periods from 1 min to 24 h, to investigate the duration of competition for amino acid between roots (Triticum aestivum) and soil microorganisms. We further investigated how microbial modification of l-alanine influenced plant carbon (C) and N recovery. From recovery of C isotopes, intact alanine uptake was 0.2-1.3% of added. Soil microbes appeared to remove alanine from soil solution within 1 min and release enough NH to account for all plant N recovery (over 24 h) within 5 min. Microbially generated inorganic or keto acid C accounted for< 25% of the lowest estimate of intact alanine uptake. Co-location of C and N labels appears a reasonable measure of intact uptake. Potential interference from microbially modified C is probably modest, but may increase with chase period. Similarly, competition for l-alanine is complete within a few minutes in soil, whereas NO added at the same rate is available for > 24 h, indicating that long chase periods bias outcomes and fail to accurately simulate soil processes.

Newly depolymerized large organic N contributes directly to amino acid uptake in young maize plants

“…The 15 N uptake in maize after 48 h reached up to 12% of added 15 N with the > 100 kDa organic N. This is in the same range as the 0–26% of added 15 N in alanine and tri‐alanine in grass after 2.5 h (Wilkinson et al ., ), in the lower range of the 13–28% of added 15 N in an amino acid mixture recovered in grass after 48 h (Sauheitl et al ., 2009b), and somewhat lower than the 30% of added 15 N in alanine recovered in wheat after 24 h (Hill & Jones, ). The lower level of 15 N uptake in the present study compared to studies with similar chase periods is probably due to the expected delay in production in bioavailable organic N after depolymerization.…”

“…This turnover of organic N sources is both plant‐ and microbial‐driven. It is established that small peptides and amino acids in soil solution are turned over rapidly (within minutes to hours) (Jones et al ., 2005a; Czaban et al ., 2016b; Hill & Jones, ), whereas larger organic N like proteins are decomposed at slower rates (Jan et al ., ).…”

“…In fertile soil, crop N uptake is viewed as being dominated by the inorganic N forms (Schimel & Bennett, ; Hill & Jones, ), although most plants have the capacity for organic N uptake (Chapin et al ., ; Kielland et al ., ; Näsholm et al ., ; Paungfoo‐Lonhienne et al ., ). Most knowledge about plant organic N uptake comes from studies based on the addition of individual amino acids including glycine (Näsholm et al ., ), alanine (Hill & Jones, ), asparagine (Czaban et al ., 2016a, ), glutamate (Jones et al ., ), mixtures of amino acids (Forsum et al ., ; Jämtgard et al ., ; Sauheitl et al ., 2009b) or short peptides (Paungfoo‐Lonhienne et al ., ; Hill et al ., 2011a; Soper et al ., ). These studies confirm direct plant uptake of organic N, but further indicate that the microbial competition for these small organic N compounds may reduce the importance of organic N uptake for crop N nutrition (Biernath et al ., ; Jones et al ., ; Hill & Jones, ).…”

“…Most knowledge about plant organic N uptake comes from studies based on the addition of individual amino acids including glycine (Näsholm et al ., ), alanine (Hill & Jones, ), asparagine (Czaban et al ., 2016a, ), glutamate (Jones et al ., ), mixtures of amino acids (Forsum et al ., ; Jämtgard et al ., ; Sauheitl et al ., 2009b) or short peptides (Paungfoo‐Lonhienne et al ., ; Hill et al ., 2011a; Soper et al ., ). These studies confirm direct plant uptake of organic N, but further indicate that the microbial competition for these small organic N compounds may reduce the importance of organic N uptake for crop N nutrition (Biernath et al ., ; Jones et al ., ; Hill & Jones, ). However, studies with the addition of a single pulse of one or a few amino acids may not reflect natural conditions in soil (Hill & Jones, ), where the release of amino acids and peptides via depolymerization of proteins would most likely result in lower and more constant concentrations over time.…”

“…These studies confirm direct plant uptake of organic N, but further indicate that the microbial competition for these small organic N compounds may reduce the importance of organic N uptake for crop N nutrition (Biernath et al ., ; Jones et al ., ; Hill & Jones, ). However, studies with the addition of a single pulse of one or a few amino acids may not reflect natural conditions in soil (Hill & Jones, ), where the release of amino acids and peptides via depolymerization of proteins would most likely result in lower and more constant concentrations over time. Furthermore, the release of N from protein‐sized organic N would give a broader profile of small organic N compounds (i.e.…”