Print PageLong-term manipulative experiments of water and nutrient availability are a unique tool for studying the functioning of arid and semiarid ecosystems. We might expect that ecological systems have hierarchical responses depending on the duration of resource availability. For example, aboveground net primary productivity (ANPP) is expected to respond rapidly to a change in precipitation (PPT), but changes in species composition may take several years to occur. These changes in vegetation may affect soil properties and nitrogen availability with feedbacks to plant growth, establishment, and species diversity. Our objective was to assess ecosystem responses to long-term changes in precipitation and nitrogen availability in the Northern Chihuahuan Desert (NM, USA), using rainfall manipulations (80% reduced PPT, ambient, 80% increased) and fertilization additions (with and without ammonium nitrate) for five consecutive years.
Results/Conclusions
Our results show that differences in ANPP among treatments increased with time since the beginning of the manipulation. After year one, ANPP decreased 20% with drought and increased 30% with irrigation. But after the fourth year, drought treatments were 50% less productive than ambient PPT treatments, and irrigated plots were 50% more productive than ambient. This response was mostly driven by changes in grass productivity because shrub and forb ANPP did not change among treatments or with time. After 5 years of manipulations, species richness, diversity, and evenness were higher in ambient PPT treatments. Drought and irrigated plots had similar species richness but differed in species identity. Fertilization did not have a significant effect on ANPP across years, but had higher diversity compared to ambient nitrogen plots. Soil nitrogen availability decreased with increasing PPT after the first year of manipulations, and this response was similar across all years. Soil organic carbon increased with PPT but decreased with fertilization. Long-term manipulations revealed time-lags in the ecosystem response to changes in PPT and N. We found hierarchical responses ranging from changes in ANPP and functional group cover to species composition and soil properties that were not manifest in a short-term study.
