Climate models predict that arid and semi-aridland ecosystems will experience increases in temperature and shifts in the timing and magnitude of seasonal precipitation. These water limited systems may be especially sensitive to changes in precipitation amount and seasonality, because aridland biogeochemistry is governed to a large extent by seasonal rainfall. In this study, we asked how the biogeochemistry and soil microbial functions of semi-arid grasslands would respond to changes in either the amount or seasonal distribution of precipitation. In 2012, a precipitation manipulation experiment was established in two southwestern semi-arid grasslands dominated by blue and black grama grasses. Three treatments were established at each site: ambient precipitation (control), chronic drought (2/3 reduction in rainfall) from April to September, and a delayed monsoon in which natural precipitation quantity was delayed by 6 weeks. All treatments were maintained for five years. In 2017, we collected soil samples from each replicate of each treatment (N=10) at three time points: pre-monsoon, mid-monsoon, and post-monsoon. We measured soil available nitrate (NO3-), phosphate (PO43−) and ammonium (NH4+), extractable organic carbon (EOC) and total nitrogen (ETN) and soil microbial biomass nitrogen (MBN) and carbon (MBC).
Results/Conclusions
In both grasslands, only the available soil nutrients NO3- and PO43− differed between treatments with the highest concentrations found in chronic drought plots. There was a sampling time by site interaction for all available nutrients (NO3-, NH4+ ,PO43− ), but in general NO3- and PO43− increased over the summer while NH4+ decreased. Soil EOC and MBC were higher in the delayed monsoon treatment for both sites. EOC also varied by site and was lower in the blue grama grassland. MBC showed a site by date interaction, but was highest mid-monsoon at both sites. In contrast to EOC and MBC, ETN and MBN showed no treatment effects. ETN decreased 3x by the post monsoon period while during the same post-monsoon period MBN was highest. In this study, we found clear effects of precipitation manipulations on soil properties, with responses depending on variables measured, sampling time, and ecosystem type (blue vs. black grama grasslands). C and N cycling in semi-arid grasslands are suggested to be closely tied to precipitation events and we show strong effects of the timing and amount of precipitation on a variety of biogeochemical and microbial responses. It is imperative that we continue studying aridland precipitation, nutrient and microbial dynamics to heighten our understanding of their sensitivities to global change.