Distinguished Professor University of New Mexico Albuquerque, NM, United States
Abstract: Drylands play a significant role in the global carbon cycle due to the influence of high precipitation variability on aboveground net primary production. However, impacts of precipitation variability on belowground net primary production (BNPP) remain unclear due to a lack of long-term empirical data. Given BNPP comprises a significant portion of total NPP in drylands, it is crucial to understand the patterns and controls of BNPP. To explore how precipitation variability affected BNPP, we conducted a rainfall manipulation experiment in a northern Chihuahuan Desert grassland in central New Mexico, USA over a nine-year period (2012-2020). This experiment consisted of thirteen 8 m by 13 m plots, all of which received ambient precipitation throughout the year. From July to September during the summer monsoon, five “small frequent” plots received 5 mm of added rainfall per week (n = 12) and five “large infrequent” plots received 20 mm of added rainfall per month (n = 3), totaling 60 mm of added rainfall by the end of each summer monsoon. Root ingrowth donuts that were harvested annually were used to estimate annual BNPP in shallow (0-15 cm) and deep (15-30 cm) soil layers within each plot. We hypothesized that total BNPP would be greatest under the large infrequent rainfall treatment and that this difference would result from greater BNPP in the deeper soil layer.
Total BNPP (0-30 cm) ranged from 12.0 ± 1.8 g m2 y-1 under ambient conditions in 2020 following a relatively dry monsoon to 442.9 ± 52.3 g m2 y-1 under the small frequent rainfall treatment following a strong monsoon in 2013. Temporal variability in total BNPP, although substantial, was lowest under large infrequent rainfall (64.0%) and highest under ambient conditions (121.1%). That is, increased precipitation reduced interannual variability of BNPP. Total BNPP was positively correlated with soil volumetric water content in all three treatments (p ≤ .05; .54 ≤ R2 ≤ .69) as well as precipitation received over the monsoon (p ≤ .1; .37 ≤ R2 ≤ .80). Contrary to our expectations, mean BNPP under large infrequent and small frequent rainfall addition did not significantly differ, and mean total BNPP was highest under small frequent rainfall addition, particularly in comparison to ambient conditions (p ≤ .05). These results contribute to a growing body of research on the significance of small rain events in arid and semi-arid grasslands, further advancing our understanding around drivers of belowground production in dryland ecosystems.