Arid and semi-arid ecosystems are among the most impoverished terrestrial systems in terms of water and nitrogen (N) availability. Productivity (NPP) is generally low, soil N pools are small and N loss through percolation is assumed to be negligible. Increased water availability can stimulate both N plant uptake (and hence NPP) and microbial N mineralization, but controls on N cycling are still poorly understood because of confounding temporal effects. We conducted an experiment with five levels of water input (-80% reduced PPT, -50% reduced, ambient PPT, +50% increased, and 80% increased PPT) and 2 levels of N fertilization (ambient N and 10 g.m-2.yr-1 NO3NH4) in the Northern Chihuahuan desert (NM, USA) and estimated inorganic N pools, in situ net N mineralization and N leaching loss from the upper-root zone of N-NO-3 and N-NH-4.
Results/Conclusions
Our results showed that the water and fertilization treatments affected N loss and N immobilization in different ways, but did not affect N net mineralization (0.013 μg N.g-1.day-1 ambient N treatment; -0.476 μg N.g-1.day-1 in the fertilized treatment). Soil mineral N increased 2-4 times in the drought treatments compared to the irrigated ones, leading to an accumulation of both inorganic N forms. Moreover, nitrate was the most abundant ion in both water interception treatments. On the other hand, the addition of ammonium nitrate interacted with water availability significantly increasing nitrate loss by leaching (4.15 μmol N-NO-3.cm-2 in the +80% PPT vs. 1.27 μmol N-NO-3.cm-2 in the -80% PPT). Our experiments showed that an accumulation of inorganic N during drought might be the result of decreased plant uptake, making the system susceptible to large nitrate losses when wet conditions arise. Loss of N during wet periods may lead to a nitrogen limitation phase for the plant community. In a future with increasing interannual rainfall variability and droughts, the interactions among the water, N and carbon cycle may result in important feedbacks in these ecosystems.
