Rapid root to leaf uptake of inorganic and amino acid nitrogen in three dryland plant species.

Our aim was to quantify inorganic and organic nitrogen (N) uptake
and compare short-term nutrient acquisition patterns among three
dryland plant species: Bouteloua eriopoda, Achnatherum hymenoides,
and Gutierrezia sarothrae collected from a mixed grassland community
in the Northern Chihuahuan Desert to better understand how
asynchronous resource availability may influence biotic interactions
and nutrient retention in these ecosystems. We collected living
plants from two locations within the Sevilleta National Wildlife
Refuge and transplanted them into pots maintained in the greenhouse
with supplemental light and water for two months. We then conducted
a greenhouse experiment using these species to compare nutrient
uptake of 15N-labeled ammonium (NH4+), nitrate (NO3-), and glutamate
(an amino acid) over 12 to 48 hours. Our study examined three main
questions: (1) How rapidly do these dryland plants take up available
soil N?, (2) Does leaf uptake differ among inorganic and amino acid
N forms?, and (3) Do plant species differ in the speed or form of
short-term N uptake?. In the greenhouse, we applied one of three
isotopic 15N tracers directly to plant roots and quantified N uptake
and recovery in leaves after 12, 24, and 48 hours. We found that
plants took up inorganic and amino acid N to leaves as rapidly as 12
h following application, and N uptake more than doubled between 24
and 48 h. Inorganic N uptake was 3-4x higher than organic N uptake
in all three species, and plants took up ammonium and nitrate at
2-3x faster rates than glutamate. On average, B. eriopoda had higher
inorganic N recovery and uptake speeds, while G. sarothrae had the
highest organic N uptake over time. A. hymenoides root to leaf
uptake was ~50% lower than the other two species after 48 h. Plants
showed similar patterns of short-term foliar uptake and recovery
indicating a lack of niche partitioning by N form among the three
dryland species measured. Our results suggest that soil inorganic N,
particularly nitrate, may comprise a greater proportion of plant N
nutrition than amino acid-N in this habitat and may be more widely
exploited following a precipitation pulse.