Interactions of water and nitrogen on primary productivity across spatial and temporal scales in grassland and shrubland ecosystems

EPSTEIN, H.E.; PARUELO, J.M.; PIÑEIRO,G.; BURKE, I.C.; LAUENROTH, W.K.

Dryland ecohydrology

Springer

Lugar: New York; Año: 2006; p. 201 - 216

At the regional scale, it has been thought that water limitations to primary production decrease, and nitrogen limitations to primary production increase, with increasing mean annual precipitation in dryland ecosystems.  Hooper and Johnson (1999), in a comprehensive review of fertilization experiments across precipitation levels, found that the relative response of primary productivity to N fertilization does not increase with increasing precipitation, suggesting that sub-humid ecosystems are no more N-limited than semiarid ecosystems.  They reasoned that if net primary productivity, N availability and plant nitrogen demand all increase similarly along precipitation gradients, then the relative response of plants to N fertilization will not vary along these gradients.         Since the Hooper and Johnson study in 1999, however, several studies have added to the evidence that N-availability either does not vary or increases to a lesser extent than net primary productivity along precipitation gradients (Austin and Sala 2002, Barrett et al. 2002, Barrett et al. 2002); this begs for an alternative explanation of why the NPP response to N fertilization does not vary along precipitation gradients.  One potential explanation is that plant nitrogen demand either does not vary or increases to a lesser extent than NPP along these gradients, an idea already supported by greater C:N ratios in wetter areas along moisture gradients (e.g. Murphy et al. 2002).  Greater nitrogen-use efficiencies, in general, might not necessarily be indicative of nitrogen limitation, but may indicate less of a demand for nitrogen by plants.         Both Paruelo et al. (1999b) and Hooper and Johnson (1999) found that primary productivity responses to changes in water and N were greatest at intermediate levels of mean annual precipitation (~400-700 mm yr-1), areas that are likely to be composed of a mixture of shorter and taller plant species.  These results also suggest constraints to primary productivity responses at lower and higher levels of annual precipitation; structural and functional properties of the vegetation may be the culprits at both ends of gradient, although more work is clearly necessary here.  The interactions between water and nitrogen over space and time, and their effects on plant productivity, are quite complex, and past research efforts have only scratched the surface of underlying mechanisms.