Nitrogen rather than water availability limits aboveground primary
productivity in an arid ecosystem: substantial differences between
grasses and shrubs
Abstract
Changes in water and nitrogen availability can affect the structure and
function of arid ecosystems. How these resources affect aboveground
primary productivity (ANPP) remains far from clear. We examined the N
and water limitation of ANPP from the species to the community level and
the response of ANPP to annual precipitation in a Patagonian steppe. We
conducted a 7-year field experiment with water addition (+W), nitrogen
addition (+N), and +NW. Destructive methods for grasses and allometric
relationships for shrubs were used to assess ANPP and vegetation indices
(NDVI and MSAVI2) to estimate community ANPP. An increase in ANPP of one
grass species ( Papposstipa humilis) and a decrease of the grass
Poa ligularis under +N were observed. Some shrubs species
exhibited mortality under nitrogen addition. Nitrogen exerted a positive
effect on grass ANPP and amplified the sensitivity of grass ANPP to
annual precipitation. However, +N had not effects on the shrub ANPP and
shrub ANPP-precipitation relationship. Water addition by itself had no
effect on ANPP for either shrubs or grasses. However, shrubs responded
positively to an unusually wet year regardless of treatment and were
also more sensitive to changes in annual precipitation than grasses.
Total ANPP increased significantly in +N relative to the C and +W, but
without changes in the sensitivity to annual precipitation. The results
suggest that the responses of grasses and shrubs to water inputs is
driven by soil moisture redistribution and rooting depth and that grass
and community ANPP is more limited by N than by water.